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Dispersal, survival, and population regulation of the vole Microtus townsendii Beacham, Terry D. 1979

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DISPERSAL, SURVIVAL, AND POPULATION REGULATION OF THE VOLE MIGBOTUS TOWNSENDII by TERRY DALE BEACHAM B. Sc.(Hons.), U n i v e r s i t y of Manitoba, 1974 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) He accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA March 1979 c Te r r y Dale Beacham, 1979 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f , Zoology  T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 W e s b r o o k P l a c e V a n c o u v e r , C a n a d a V6T 1W5 D a t e 2 A p r i l 1979 i i A b s t r a c t A number of f i e l d s t u d i e s ou v o l e s have suggested t h a t d i s p e r s a l i s an important mechanism of p o p u l a t i o n r e g u l a t i o n . To i n v e s t i g a t e i f v o l e s disperse from d e c l i n i n g p o p u l a t i o n s , I made two v o l e - p r o o f e n c l o s u r e s , each having an area i n t o which the v o l e s c o u l d d i s p e r s e . These two fenced p o p u l a t i o n s and two unfenced c o n t r o l p o p u l a t i o n s were trapped from Hay 1976 u n t i l June 1978 near Vancouver, Canada. A l l f o u r M i c r o t u s townsendii p o p u l a t i o n s were low d u r i n g the 1976 summer, and the p o p u l a t i o n sampled by l i v e - t r a p s i n c r e a s e d d u r i n g the f a l l and winter non-breeding season, owing to delayed capture i n l i v e - t r a p s . Male and female minimum s u r v i v a l r a t e s d u r i n g the 1976 winter were 0.92 per two weeks. a l l f o u r vole p o p u l a t i o n s d e c l i n e d f o r t h r e e months a f t e r breeding began i n s p r i n g 1977, the d e c l i n e s being c o n c u r r e n t with d i s p e r s a l of the s m a l l e r v o l e s . More males d i s p e r s e d than females dur i n g t h i s s p r i n g d e c l i n e , and male minimum s u r v i v a l dropped to 0.76 per two weeks, whereas female minimum s u r v i v a l remained high at 0.90. Voles were 2-15% h e a v i e r i n the peak 1977 s p r i n g than d u r i n g the d e c l i n e s p r i n g of 1978. Mounding i n c r e a s e d s h a r p l y when breed i n g began and reached a peak s i x weeks l a t e r . D i s p e r s a l of s u b a d u l t v o l e s was common i n the peak p o p u l a t i o n s of summer 1977, with these i n d i v i d u a l s more o f t e n i n r e p r o d u c t i v e c o n d i t i o n than were r e s i d e n t s u b a d u l t s . I suggest t h a t a g g r e s s i v e b e h a v i o u r a l i n t e r a c t i o n s between smaller animals a t t a i n i n g s e x u a l maturity and l a r g e r sexually-mature a d u l t s prompted the d i s p e r s a l of the s m a l l e r i n d i v i d u a l s . A l l f o u r vole p o p u l a t i o n s d e c l i n e d f o r a second time i n October 1977 f o r e i g h t months, with the d e c l i n e spanning both the non-breeding and subsequent breeding season. Most of the l o s s e s o c c u r r e d i n the non-breeding season, when the v o l e s were n e i t h e r wounded nor d i s p e r s i n g . The s m a l l e r males s u r v i v e d e s p e c i a l l y p o o r l y and both sexes l o s t body weight. Minimum s u r v i v a l of males was 0.81 per two weeks, while female minimum s u r v i v a l was 0.82. The r a t e of d e c l i n e a c c e l e r a t e d r a p i d l y when breeding began i n s p r i n g 1978, and w i t h i n f o u r weeks the p o p u l a t i o n s v i r t u a l l y disappeared, with minimal wounding and no d i s p e r s a l . Male minimum s u r v i v a l dropped t o 0.47 per two weeks, while female minimum s u r v i v a l was 0.65. I suggest t h a t i n t e r f e r e n c e c o m p e t i t i o n between l a r g e and s m a l l voles f o r a l i m i t e d food supply may have accounted f o r some of the non-breeding season l o s s e s . The known l e n g t h of time spent on the g r i d s ( l i f e t i m e ) and d i s p e r s a l tendency were non-randomly d i s t r i b u t e d among l i t t e r s i n i n c r e a s i n g and peak p o p u l a t i o n s , which i n d i c a t e s a h e r i t a b l e b a s i s f o r these' t r a i t s . S p r i n g and summer-born d i s p e r s e r s l e a v e at the same age, when they become s e x u a l l y mature. In the f a l l of 1977, avian p r e d a t o r s accounted f o r a minimum of 15% of the l o s s from the tagged v o l e p o p u l a t i o n s , and t h i s percentage d e c l i n e d d u r i n g the winter of 1977. The p r e d a t o r s s e l e c t e d males and small i n d i v i d u a l s , and m o r t a l i t y accounted f o r by avain p r e d a t i o n was density-dependent i n the v o l e p o p u l a t i o n s ; denser v o l e p o p u l a t i o n s s u f f e r e d higher p r e d a t i o n r a t e s . J u v e n i l e s u r v i v a l was highest i n i n c r e a s i n g p o p u l a t i o n s and lowest i n d e c l i n i n g ones, whereas n e s t l i n g s u r v i v a l was lowest i v i n peak p o p u l a t i o n s . N e s t l i n g m o r t a l i t y c o n t r i b u t e d l i t t l e or nothing t o p o p u l a t i o n d e c l i n e s . Apparent a d u l t s u r v i v a l was dependent upon the t r a p p i n g technique. The concurrent use of l i v e - t r a p s and p i t f a l l s showed t h a t l i v e - t r a p s f a i l e d to enumerate completely any s i z e or sex category of v o l e s . Moreover, the est i m a t e s of demographic parameters based s o l e l y on l i v e - t r a p data may be i n a c c u r a t e owing to d i s p e r s a l o f young, sexually-mature i n d i v i d u a l s before they e n t e r l i v e - t r a p s . L i v e - t r a p s sample the dominant i n d i v i d u a l s p r e f e r e n t i a l l y , p i t f a l l s the subord i n a t e s . T h i s study i n d i c a t e s that although d i s p e r s a l from i n c r e a s i n g and peak vole p o p u l a t i o n s may stop them f rota i n c r e a s i n g i n d e f i n i t e l y , i t can not account f o r the subseguent d e c l i n e . According t o the C h i t t y h y p o t h e s i s only one e x p l a n a t i o n i s r e g u i r e d to account f o r these d e c l i n e s : v o l e s at the beginning of the two d e c l i n e s have been subjected to d i f f e r e n t s e l e c t i o n p r e s s u r e s , and d i f f e r e n c e s between the two d e c l i n e s are due to changes i n the p r o p e r t i e s of the i n d i v i d u a l s . The c h a r a c t e r i s t i c s of the 1978 d e c l i n e p a r t i a l l y support the C h i t t y hypothesis and a l s o the view that changes i n food q u a l i t y or q u a n t i t y were i n v o l v e d i n p o p u l a t i o n r e g u l a t i o n a t l e a s t i n t h i s p a r t i c u l a r i n s t a n c e . V TABLE OF CONTENTS Ab s t r a c t i i Table of c o n t e n t s ......................................... v L i s t o f t a b l e s i x L i s t of f i g u r e s x i v Acknowledgments ................................. .......... x v i i General i n t r o d u c t i o n ................. 1 L i t e r a t u r e c i t e d 4 S e c t i o n 1. D i s p e r s a l d u r i n g p o p u l a t i o n f l u c t u a t i o n s o f the vole Mierotus townsendii 6 I n t r o d u c t i o n 6 Methods .. 7 H €S UX tS * • • • • * • • • * * • • »* • • • * * . * * • • • * • ' *<• • ; * • * * * * * * * * * * * . ^ ^ T r a p p a b i l i t y ••......... 12 P o p u l a t i o n d e n s i t y 15 P o p u l a t i o n d e n s i t y and d i s p e r s a l .................... 24 S u r v i v a l and d i s p e r s a l .............................. 32 D i s p e r s a l and time of b i r t h 36 Sex r a t i o s of d i s p e r s i n g v o l e s ...................... 39 Sexual maturity and d i s p e r s a l ....................... 42 Body weight and d i s p e r s a l ........................... 47 D i s c u s s i o n 53 L i t e r a t u r e c i t e d ....................................... 61 S e c t i o n 2. D i s p e r s a l tendency and d u r a t i o n of l i f e of l i t t e r m a t e s d u r i n g p o p u l a t i o n f l u c t u a t i o n s of the vo l e M i e r o t u s t o w n s e n d i i .................................... 66 Intiroductxon * • • • « * • . • « • - • * • • • . * * • * • • - * • . • • • • • - * • • • • • * • • • . * • * • * ' • • 66 v i Methods .......... .. . 67 R e s u l t s 70 P o p u l a t i o n d e n s i t y and d i s p e r s a l .................... 70 L i t t e r m a t e l i f e s p a n 71 L i t t e r m a t e d i s p e r s a l tendency 76 D i s c u s s i o n ............................................. . 79 L i t e r a t u r e c i t e d ....................................... 85 S e c t i o n 3. S e l e c t i v i t y of Avian P r e d a t i o n i n D e c l i n i n g P o p u l a t i o n s o f the v o l e Microtias townsendii ............ 87 I n t r o d u c t i o n ..................................... - 87 Hethods .................. ..... ................ 87 R e s u l t s ...................... ^ ....... . ... ........... ... 89 Vole abundance ................. « r . .-. 89 Incidence of predators .............................. 91 Predator impact ..................................... 92 Sex r a t i o of v o l e s eaten by p r e d a t o r s , 95 Body weight d i s t r i b u t i o n s ........................... 96 P r o b a b i l i t y of p r e d a t i o n 98 D i s c u s s i o n ....................... .................... 100 L i t e r a t u r e c i t e d .•«•• .................................... 104 Secti o n 4 . / S u r v i v a l i n f l u c t u a t i n g p o p u l a t i o n s of the vo l e M i c r o t u s townsendii .................................... 106 I n t r o d u c t i o n ........................................... 106 M 6 tllO&S • ••••••• • • • • * • • • • • m • • • * • • • A 106 R6SllX tS • • • * « • • • • * • • • * • • • • «.« * ••<• • • • m * • » m • • • • • • -108 T r a p p a b i l i t y 108 P o p u l a t i o n d e n s i t y 109 J u v e n i l e s u r v i v a l .................................., 111 v i i Preweanling s u r v i v a l ......................... «^ • 114 a d u l t and subadult s u r v i v a l ......................... 124 D i s c u s s i o n 128 L i t e r a t u r e c i t e d 136 S e c t i o n 5. Demography of d e c l i n i n g p o p u l a t i o n s o f the v o l e Mierotus townsendii .................................... 139 I n t r o d u c t i o n ........................................... 139 Methods 139 R e s u l t s ... 140 P o p u l a t i o n d e n s i t y ................................. 140 Body weights 150 Hates of d e c l i n e .....................155 S i z e - s e l e c t i v i t y of s u r v i v a l .......,................. 159 G l T O Wt- tl IT t i t Q S m m • * m m m m: • m m m. m m • • m m m m m m »m'^\ m • m • • ^  • • • • 9 161 Wounding r a t e s •••••••••••••••••••••••••>•••••••••••» 162 D X S C U S S X O n <D * * * • m • • • mm* • m • • • • m m m m m • • • ,164 L i t e r a t u r e c i t e d 173 S e c t i o n 6. P i t f a l l versus l i v e - t r a p enumeration of f l u c t u a t i n g p o p u l a t i o n s of the vole Mierotus townsendii 177 I n t r o d u c t i o n ........................................... 177 M©tliocls ••••••«.•••••*••• ••.•-••••••••••.*.•*.•••.*••••••••••.•• 178 R e s u l t s .................. 179 T r a p p a M l i t y 179 P o p u l a t i o n d e n s i t y .................................. 180 F i r s t c a p t u r e of v o l e s i n l i v e - t r a p s and p i t f a l l s ... 189 Body weight d i f f e r e n c e s a t f i r s t capture i n l i v e - t r a p s and p i t f a l l s .......................... 193 D i s p e r s a l 199 v i i i J u v e n i l e s u r v i v a l 200 A comparison of a d u l t males caught i n l i v e - t r a p s and p i t f a l l s 202 Why are some males not caught i n l i v e - t r a p s ? ....... 206 Growth r a t e s of v o l e s caught i n p i t f a l l s and l i v e - t r a p s and of those caught only i n p i t f a l l s . . 2 1 0 Discussion. • •••••*•.••••••.•••••*••••••*•••••*•••••**•••**•• 211 L i t e r a t u r e c i t e d 220 General summary ........................................... 223 i x LIST OF TABLES Table 1.1. T r a p p a b i l i t y estimates f o r M i c r o t a s townsendii i n Longworth l i v e - t r a p s on c o n t r o l g r i d s A and D. T i s t r a p p a b i l i t y , N i s sample s i z e . .......................... 12 Table 1.2. T r a p p a b i l i t y estimates f o r Mierotus townsendii i n Longworth l i v e - t r a p s on experimental g r i d s B and C. T i s t r a p p a b i l i t y , N i s sample s i z e . .......................... 14 Table 1.3. P o p u l a t i o n d e n s i t y , r a t e of i n c r e a s e , and d i s p e r s a l r a t e s f o r p o p u l a t i o n s of Mierotus townsendii. A l l data are mean value s f o r the p e r i o d . ................. 30 Table 1.4. Number of tagged Mierotus townsendii d i s a p p e a r i n g from combined l i v e - t r a p and p i t f a l l p o p u l a t i o n s on both experimental g r i d s and number o f tagged d i s p e r s e r s caught. Table. 1.5. Mean weight (• 1 SB) of s u c c e s s f u l and u n s u c c e s s f u l r e c r u i t s t o c o n t r o l g r i d s d u r i n g the 1977 s p r i n g d e c l i n e . .......................................... 35 Tab l e 1.6. P r o b a b i l i t y of an i n d i v i d u a l f i r s t caught from s p r i n g t o winter d i s p e r s i n g before the s t a r t of the next breeding season. ......................................... 37 Table 1.7. P r o b a b i l i t y of an i n d i v i d u a l d i s p e r s i n g d u r i n g 1977 given i t s month of f i r s t c a p t u r e i n 1976. Both sexes and g r i d s B and C are combined. .......................... Table 1.8. Sex r a t i o s ( p r o p o r t i o n of males) i n r e s i d e n t and d i s p e r s i n g p o p u l a t i o n s of Mierotus townsendii on both c o n t r o l and experimental g r i d s . Sample s i z e i s i n parentheses. .......... .... ... . . . . . .. ...... .... ..... . .... . . 38 40 X Table 1.9. P r o p o r t i o n of males a c c o r d i n g t o age group i n d i s p e r s i n g and r e s i d e n t p o p u l a t i o n s of M i c r o t u s t o w n s e n d i i . Sample s i z e i s i n parentheses. ............... 41 Table 1.10- Percentage of subadult v o l e s i n breeding c o n d i t i o n i n r e s i d e n t and d i s p e r s i n g p o p u l a t i o n s . Sample s i z e i s i n parentheses. .................................. 43 Tab l e 1.11. Median weight at s e x u a l maturity f o r g r i d B male and female r e s i d e n t s and d i s p e r s e r s . 9555 co n f i d e n c e l i m i t s cLITQ X Q p c l X r ©lit }XQ S6S« •. * • • • • • • • • • •. • • • • • • • • • 45 Table 1.12. Median weight at s e x u a l maturity f o r g r i d C male and female r e s i d e n t s and d i s p e r s e r s . 95% c o n f i d e n c e l i m i t s are i n parentheses. ...................................... 46 Table 1.13. P r o p o r t i o n of males and females t h a t are j u v e n i l e o r s u b a d u l t i n d i s p e r s i n g and r e s i d e n t p o p u l a t i o n s d u r i n g d i f f e r e n t seasons. Sample s i z e i s i n parentheses. ........ 50 Table 2.1. L i t t e r s i z e s from the c o n t r o l and experimental p o p u l a t i o n s i n 1976 and 1977. ............................ 72 Table 2.2. A n a l y s i s of v a r i a n c e t a b l e s to t e s t non-random l i f e t i m e s among H i c r o t u s t o w n s e n d i i l i t t e r s . . L i f e t i m e was d e f i n e d as the time between f i r s t and l a s t c a p t u r e s . ..... 74 Table 2.3. A n a l y s i s of v a r i a n c e t a b l e s to t e s t non-random l i f e t i m e s w i t h i n l i t t e r s with d i s p e r s e r s removed. A l l r e s u l t a n t l i t t e r s of s i z e 1 were elimimated. These data are thus a subset of those i n Table 2.2 .................. 75 Table 2.4. Analyses of v a r i a n c e t a b l e s f o r d i s p e r s a l tendency w i t h i n l i t t e r s born i n experimental p o p u l a t i o n s d u r i n g 1976 and 1977. ........... ............................ 77 Table 2.5. A n a l y s i s of v a r i a n c e t a b l e t o t e s t whether x i d i s p e r s i n g M. townsendii s i b l i n g s have the same l i f e s p a n . ..................... ^ .......^..... 79 Table 3.1. Number of tagged v o l e s d i s a p p e a r i n g from f o u r Mierotus townsendii p o p u l a t i o n s and percentage recovered from a v i a n predator p e l l e t s . R e s u l t s from a l l g r i d s were pooled i n each p e r i o d . ................................... 93 Table 3.2. Percentage of vole m o r t a l i t y accounted f o r by avian p r e d a t o r s and average M. townsendii d e n s i t y f o r 1977-1978. ...... ............. 94 Table 3.3. Sex r a t i o s (males:females) o f eaten and r e s i d e n t Mierotus townsendii p o p u l a t i o n s . Sample s i z e s are i n parentheses. P e r i o d i s from s p r i n g 1977 to s p r i n g 1978. .. 96 Table 3.4. Number of l a r g e and s m a l l male and female Mierotus townsendii i n eaten and r e s i d e n t p o p u l a t i o n s from f a l l 1977 t o s p r i n g 1978. .............................. ... 98 Table 3.5. P r o b a b i l i t y of an i n d i v i d u a l Mierotus townsendii being k i l l e d by a v i a n predators i n r e l a t i o n to week of f i r s t c apture i n 1977. A l l g r i d s and both sexes are pooled. ............................ 99 Table 4.1. Minimum s u r v i v a l r a t e s per 14-day p e r i o d f o r j u v e n i l e Mierotus townsendii i n combined l i v e - t r a p and p i t f a l l p o p u l a t i o n . ............................. ......... 113 Table 4.2. Minimum s u r v i v a l r a t e s per 14-day p e r i o d f o r Mierotus townsendii caught by Longworth l i v e - t r a p s . A l l p o p u l a t i o n s were pooled f o r each i n t e r v a l . Sample s i z e s are i n parentheses. 125 Table 4.3. Minimum s u r v i v a l r a t e per 14-day p e r i o d f o r Mierotus townsendii caught by p i t f a l l s . A l l p o p u l a t i o n s x i i are pooled f o r each i n t e r v a l . Sample s i z e s are i n parentheses. ............................................. 127 Table 5.1. Instantaneous r a t e s of d e c l i n e per week, number (and % of population) of males a t the s t a r t and end, p r o p o r t i o n of heavy-weight males, and v a r i a n c e o f body weights d u r i n g the breeding d e c l i n e s of 1977 and 1978. ... 157 Tabl e 5.2. Instantaneous r a t e s o f d e c l i n e per week, p r o p o r t i o n of heavy-weight females, and v a r i a n c e of body weights d u r i n g t h e breeding d e c l i n e s of 1977 and 1978. ... 158 Table 5.3. Average number of wounds per animal f o r males and females d u r i n g breeding d e c l i n e s . Sample s i z e i s i n parentheses. ......... ..................... 163 Table 6.1. Numbers o f Hi c r o t u s townsendii f i r s t caught i n p i t f a l l s and l a t e r i n l i v e - t r a p s , and those caught only i n p i t f a l l s . Data are pooled over the e n t i r e study. ......... 191 Tabl e 6.2. Numbers of H i c r o t u s townsendii f i r s t caught i n l i v e - t r a p s and l a t e r i n p i t f a l l s , and those caught only i n l i v e - t r a p s . . 192 Table 6.3. Mean weight i n grams a t f i r s t c a p ture (• 1 SE) of H i c r o t u s townsendii i n l i v e - t r a p s and p i t f a l l s f o r fo u r p o p u l a t i o n s . Data f o r each g r i d are pooled over the e n t i r e study. Sample s i z e s a r e i n parentheses. 194 Table 6.4. Numbers of Mi c r o t u s townsendii i n each s i z e and sex c l a s s a t f i r s t capture i n l i v e - t r a p s and p i t f a l l s . ... 195 Table 6.5. Percentage d i s t r i b u t i o n by s i z e c l a s s of i n t e r v a l between f i r s t capture i n p i t f a l l s and f i r s t capture i n l i v e - t r a p s of Mi c r o t u s t o w n s e n d i i . .. . . .... ,.............. 197 Table 6.6. Percentage d i s t r i b u t i o n by s i z e c l a s s of i n t e r v a l between f i r s t and l a s t p i t f a l l c a p t u r e s of M i c r o t u s townsendii caught only i n p i t f a l l s . 1976 and 1977 are combined. ....................... ......................... 198 Table 6.7. Number of d i s p e r s i n g M icrotus townsendii caught i n l i v e - t r a p s and p i t f a l l s . Data are pooled over the e n t i r e study. 200 Table 6.8. J u v e n i l e minimum s u r v i v a l r a t e s and index of j u v e n i l e s u r v i v a l o f Krebs and DeLong (1965) f o r t o t a l p o p u l a t i o n s and f o r l i v e - t r a p p o p u l a t i o n s of Microtus townsendii. ............................... ......... ...... 202 Tab l e 6.9. Demographic a t t r i b u t e s o f male Microtus townsendii caught dur i n g summer 1976 i n p i t f a l l s and l i v e - t r a p s . .... 204 Table 6. 10. Demographic a t t r i b u t e s of male Microtus townsendii caught dur i n g summer 1977 i n p i t f a l l s and l i v e - t r a p s . .............................................. 206 Table 6.11., Demographic a t t r i b u t e s of male H i c r o t u s townsendii f i r s t caught i n p i t f a l l s i n 1976 t h a t were caught i n l i v e - t r a p s and of those males t h a t were not. ... 208 Table 6.12. Demographic a t t r i b u t e s of male Microtus townsendii f i r s t caught i n p i t f a l l s i n 1977 t h a t were caught i n l i v e - t r a p s and of those males t h a t were not. ... 209 x i v LIST OF FIGURES F i g u r e 1.1. Experimental design used i n t h i s study. G r i d s A and D a r e unfenced c o n t r o l s and g r i d s B and C are e n c l o s e d experimentals. .......................... 7 Fi g u r e 1.2. P o p u l a t i o n d e n s i t y of Mi c r o t u s townsendii on c o n t r o l g r i d A and number of d i s p e r s e r s . ............... 16 F i g u r e 1.3- P o p u l a t i o n d e n s i t y o f Mi c r o t u s townsendii on c o n t r o l g r i d D and number of d i s p e r s e r s . ............... 18 Figure 1.4. P o p u l a t i o n d e n s i t y of Microtus townsendii on fenced g r i d B and number of d i s p e r s e r s . 20 F i g u r e 1-5- P o p u l a t i o n d e n s i t y o f Microtus townsendii on fenced g r i d C and number of d i s p e r s e r s . ................ 22 F i g u r e 1.6. S i z e c l a s s e s of d i s p e r s i n g Microtus townsendii from fenced g r i d B r e s i d e n t p o p u l a t i o n . ................ 25 Fi g u r e 1.7. S i z e c l a s s e s of d i s p e r s i n g Microtus townsendii from fenced g r i d C r e s i d e n t p o p u l a t i o n 27 Fi g u r e 1.8. Mean body weights of r e s i d e n t male M. townsendii i n fenced p o p u l a t i o n s B (BB) and C (CR), and those t h a t d i s p e r s e d (BD, CD). 48 Figure 1.9. Body weight d i s t r i b u t i o n s of r e s i d e n t (N=1255) and d i s p e r s i n g (N=92) male M. townsendii from fenced g r i d B d u r i n g summer 1977. ............................. 52 F i g u r e 3.1. P o p u l a t i o n d e n s i t y o f Mi c r o t u s townsendii determined by l i v e - t r a p s on c o n t r o l g r i d D d u r i n g 1976-78. Both sexes are combined. Non-breeding p e r i o d s are shaded. 90 F i g u r e 4.1. P o p u l a t i o n d e n s i t y determined from l i v e - t r a p s XV o f B i c r o t u s townsendii on fenced g r i d C during 1976-78. Both sexes are combined. Non-breeding p e r i o d s are shaded. ..................... ... 110 F i g u r e 4.2. S u r v i v o r s h i p curves f o r young Mierotus townsendii on c o n t r o l g r i d ft between b i r t h and re c r u i t m e n t . The number born i n each year was estimated from the number of l a c t a t i n g females; the number o f r e c r u i t s was known. 116 F i g u r e 4.3. S u r v i v o r s h i p curves f o r young Mierotus townsendii on fenced g r i d B between b i r t h and rec r u i t m e n t . The number born i n each year was estimated from the number of l a c t a t i n g females; the number of r e c r u i t s was known. .................................... 118 F i g u r e 4.4. S u r v i v o r s h i p curves f o r young Mierotus townsendii on fenced g r i d C between b i r t h and re c r u i t m e n t . The number born i n each year was estimated from the number of l a c t a t i n g females; the number of r e c r u i t s was known. 120 F i g u r e 4.5. S u r v i v o r s h i p curves f o r young Mierotus townsendii on c o n t r o l g r i d D between ' b i r t h and r e c r u i t m e n t . The number born i n each year was estimated from the number of l a c t a t i n g females; the number of Figu r e 5.1. P o p u l a t i o n d e n s i t y , instantaneous r a t e of i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r Mierotus townsendii on c o n t r o l g r i d & during 1976-1978. 142 F i g u r e 5.2. P o p u l a t i o n d e n s i t y , i n s t a n t a n e o u s r a t e of i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r x v i M i c r o t u s t o w n s e n d i i on c o n t r o l g r i d D during 1976-1978. Non-breeding p e r i o d s are shaded 144 Figure 5.3. P o p u l a t i o n d e n s i t y , i n s t a n t a n e o u s r a t e of i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r Microtus townsendii on fenced g r i d B during 1976-1978. Non-breeding p e r i o d s are shaded. ....................... 146 Figure 5.4. P o p u l a t i o n d e n s i t y , i n s t a n t a n e o u s r a t e of i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r M i c r o t u s townsendii on fenced g r i d C during 1976-1978. Non-breeding p e r i o d s are shaded. .......................149 F i g u r e 5.5. Histogram of body weight d i s t r i b u t i o n s of male M i c r o t u s townsendii on fenced g r i d C. .................. 151 Fig u r e 5.6. Histogram of body weight d i s t r i b u t i o n s of male M i c r o t u s townsendii on fenced g r i d B. ....... 153 F i g u r e 6.1. P o p u l a t i o n d e n s i t y of Microtus townsendii on c o n t r o l g r i d A. ...181 Figure 6.2. P o p u l a t i o n d e n s i t y of Microtus townsendii on fenced g r i d B. ......................................... 183 F i g u r e 6.3. P o p u l a t i o n d e n s i t y o f Microtus townsendii on fenced g r i d C. 185 F i g u r e 6.4. P o p u l a t i o n d e n s i t y of Microtus townsendii on ACKNOWLEDGMENTS I wish t o express my s i n c e r e thanks t o ray s u p e r v i s o r , Dr. C h a r l e s J . Krebs, f o r h i s c o n t i n u i n g a d v i c e , encouragement and a s s i s t a n c e throughout t h i s study. H i s c r i t i c i s m s , along with those of the other members of my committee. Dr. J . Myers, Dr. L. Gass, e s p e c i a l l y those of Dr. Dennis C h i t t y , whose help I g r e a t l y a p p r e c i a t e d , and a l s o those of Ms. B. W i t h l e r , have made t h i s t h e s i s more readable and c o n c i s e . , Thanks are due to L. Nordstrom and C. Fleming who a s s i s t e d me i n g a t h e r i n g some of the data presented i n t h i s t h e s i s . Dr. C h a r l e s Krebs gave me access t o s e v e r a l computer programs f o r the a n a l y s i s of demographic d a t a . The Canadian W i l d l i f e S e r v i c e k i n d l y allowed me to use p a r t o f t h e i r p roperty on S e i f e l I s l a n d as a study a r e a . I r e c e i v e d support both from OBC teaching a s s i s t a n t s h i p s and NBC postgraduate s c h o l a r s h i p s . 1 GENERAL INTRODUCTION Many m i c r o t i n e p o p u l a t i o n s f l u c t u a t e i n numbers, and the causes of these f l u c t u a t i o n s a re s t i l l p oorly understood. D i s p e r s a l i s an important mechanism of p o p u l a t i o n r e g u l a t i o n (Krebs e t a l . , 1969), and may a l t e r the genetic and b e h a v i o u r a l c h a r a c t e r i s t i c s o f the p o p u l a t i o n (Krebs et a l . , 1973). D i s p e r s a l i s most common i n i n c r e a s i n g p o p u l a t i o n s (Myers and Krebs, 1971; Krebs e t a l . , 1976), but has not been demonstrated t o occur i n d e c l i n e s . These r e s u l t s o r i g i n a t e from a design based on the removal g r i d t e chnique, whereby v o l e s c o l o n i z i n g a trapped-out area are de f i n e d as d i s p e r s e r s and removed. T h i s experimental design r e s t s on many tenuous assumptions (Krebs et a l . , 1976). I chose t o study d i s p e r s a l i n Mierotus townsendii by e n c l o s i n g n a t u r a l f i e l d p o p u l a t i o n s i n a vo l e - p r o o f fence and by p r o v i d i n g a v o l e - f r e e unfavourable h a b i t a t f o r p o t e n t i a l d i s p e r s e r s t o e n t e r . T h i s technique, u n l i k e the removal g r i d d e s ign, has the advantage of enumerating a l l d i s p e r s e r s because v o l e s must enter an unfavourable h a b i t a t ( v o l e - f r e e area) to d i s p e r s e . T h e r e f o r e , by p r o v i d i n g an unfavourable h a b i t a t f o r s e p a r a t i n g d i s p e r s e r s from r e s i d e n t s , I was abl e to c a t c h a l l d i s p e r s e r s from two p o p u l a t i o n s d u r i n g the course o f a two-year c y c l e . I examined whether or not d i s p e r s a l occurs during d e c l i n e p e r i o d s , and I give the r e s u l t s i n S e c t i o n 1. The d e t e r m i n a t i o n o f whether or not c e r t a i n c h a r a c t e r s have a h e r i t a b l e b a s i s i s o f t e n d i f f i c u l t t o a s c e r t a i n i n f i e l d p o p u l a t i o n s . H i l b o r n (1975) r e p o r t e d t h a t l i f e t i m e (time between 2 f i r s t and l a s t captures) was non-randomly d i s t r i b u t e d among l i t t e r s only i n unfenced, i n c r e a s i n g Microtus p o p u l a t i o n s , and thus l i f e t i m e was i n h e r i t e d . I trapped p a r t i a l and complete l i t t e r s of M. townsendii i n p i t f a l l t r a p s i n order to i n v e s t i g a t e h e r i t a b i l i t y of d i f f e r e n t c h a r a c t e r s . I was able to compare v a r i a b i l i t y i n d u r a t i o n of l i f e and d i s p e r s a l tendency w i t h i n and among l i t t e r s , and I e v a l u a t e the h e r i t a b i l i t y of l i f e t i m e and d i s p e r s a l tendency i n i n c r e a s i n g and peak M. townsendii p o p u l a t i o n s i n S e c t i o n 2. P r e d a t i o n i s not necessary f o r d e c l i n e s i n v o l e p o p u l a t i o n s (Krebs and Myers, 1974). However, i n d i v i d u a l s eaten by pr e d a t o r s may be the v u l n e r a b l e segment of the p o p u l a t i o n and be predisposed t o m o r t a l i t y f o r s o c i a l reasons ( E r r i n g t o n , 1956). Few s t u d i e s have i n t e n s i v e l y f o l l o w e d a marked p o p u l a t i o n of v o l e s to o b t a i n l i f e h i s t o r i e s of the animals and r e l a t e d them to subsequent p r e d a t i o n (Boonstra, 1977). In my study, avian p r e d a t o r s were present during l a t e f a l l and winter of 1977 at a time of d e c l i n i n g v o l e numbers. I examined t h e p r o p o r t i o n o f l o s s accounted f o r by p r e d a t i o n and determined whether or not pre d a t o r s were s e l e c t i v e f o r e i t h e r the s i z e or sex of the prey (Section 3). S u r v i v a l of j u v e n i l e v o l e s i s v a r i a b l e d u r i n g a p o p u l a t i o n f l u c t u a t i o n , with p a r t i c u l a r l y poor s u r v i v a l i n the d e c l i n e (Krebs and Myers, 1 9 7 4 ) . L i t t l e i s known about the s u r v i v a l of n e s t l i n g s because they are i n the nest and untrappable; i n d i r e c t t e c h n i q u e s must be used t o estimate t h e i r s u r v i v a l . Apparent s u r v i v a l r a t e s o f a group of v o l e s may be dependent upon the t r a p p i n g technique. In my study I measured j u v e n i l e s u r v i v a l 3 d i r e c t l y by enumeration with p i t f a l l s , and compared praweanling and j u v e n i l e s u r v i v a l r a t e s i n i n c r e a s i n g , peak, and d e c l i n i n g p o p u l a t i o n s . I compared s u r v i v a l r a t e s of each age and sex c l a s s i n l i v e - t r a p s and p i t f a l l s t o see i f estimated s u r v i v a l depended upon t r a p type ( S e c t i o n 4 ) . One of the major q u e s t i o n s concerning m i c r o t i n e f l u c t u a t i o n s i s the cause of m o r t a l i t y during p o p u l a t i o n d e c l i n e s . There has been a tendency to r e l a t e a l l d e c l i n e s to a s i n g l e necessary c o n d i t i o n (Krebs and Myers, 1974). Spacing behaviour i s p o s t u l a t e d as a necessary c o n d i t i o n f o r d e c l i n e s by Krebs and Boonstra (1978), because i t can cause e i t h e r death or d i s p e r s a l from the p o p u l a t i o n . In S e c t i o n 5 I provide a d e t a i l e d demographic a n a l y s i s o f two d e c l i n e s i n f o u r M. townsendii p o p u l a t i o n s and e v a l u a t e the c h a r a c t e r i s t i c s o f the d e c l i n e i n r e l a t i o n to the C h i t t y hypothesis, the Krebs and Boonstra (1978) h y p o t h e s i s , and the food h y p o t h e s i s ( P i t e l k a , 1958). The t o t a l p o p u l a t i o n enumeration technique used i n s e v e r a l s t u d i e s , i n c l u d i n g t h i s one, assumes t h a t most of the a d u l t p o p u l a t i o n i s ca p t u r e d . However, most s t u d i e s use o n l y l i v e - t r a p s to enumerate the p o p u l a t i o n and t h i s assumption i s r a r e l y t e s t e d . Do l i v e - t r a p s c a t c h most of the l a r g e r animals? I f some v o l e s are caught only i n p i t f a l l s , whereas others are caught both i n l i v e - t r a p s and p i t f a l l s , then are t h e r e any demographic d i f f e r e n c e s between these groups of vo l e s ? Do l i v e - t r a p s and p i t f a l l s sample the same segment of the pop u l a t i o n ? These q u e s t i o n s are answered i n S e c t i o n 6. Because the study can be d i v i d e d i n t o s i x major aspects t h a t are r e l a t e d but s e l f - c o n t a i n e d , I decided t o w r i t e the 4 s e c t i o n s as independent papers s u i t a b l e f o r p u b l i c a t i o n a f t e r s l i g h t r e v i s i o n . T herefore, t h e r e i s some r e p e t i t i o n among the s e c t i o n s with regard to experimental d e s i g n , t r a p p a b i l i t y , and l i t e r a t u r e c i t e d . L i t e r a t u r e c i t e d Boonstra, R. 1977. Pr e d a t i o n on H i c r o t u s townsendii p o p u l a t i o n s : Impact and v u l n e r a b i l i t y . Can. J . Z o o l . 55: 1631-1643. E r r i n g t o n , P. L.. 1956. F a c t o r s l i m i t i n g higher v e r t e b r a t e p o p u l a t i o n s . Science 124: 304-307. H i l b o r n , R. 1975. S i m i l a r i t i e s i n d i s p e r s a l tendency among s i b l i n g s i n f o u r s p e c i e s of v o l e s ( M i c r o t u s ) . Ecology, 56: 1221-1225. Krebs, C. J . , B. L. K e l l e r , and R. H. T a m a r i n . 1 9 6 9 . Microtus p o p u l a t i o n b i o l o g y : Demographic changes i n f l u c t u a t i n g p o p u l a t i o n s o f M. ochr o g a s t e r and M. £§flfisylyaniens i n southern Indiana. Ecology, 50: 587-607. Krebs, C. J . , M. S. Gaines, B. L. K e l l e r , J . H. Myers, and R. H. Tamarin. 1973. Pop u l a t i o n c y c l e s i n smal l r o d e n t s . S c i e n c e , 179: 35-41. Krebs, C. J . and J . H. Myers. 1974. P o p u l a t i o n c y c l e s i n s m a l l mammals, adv. E c o l . Res. 8: 267-399. Krebs, C. J . , I . Wingate, J . LeDuc, J . A. .R e d f i e l d , M. T a i t t , and R. H i l b o r n . 1976. H i c r o t u s p o p u l a t i o n b i o l o g y : D i s p e r s a l i n f l u c t u a t i n g 1 p o p u l a t i o n s of M. townsendii. Can. J . Zool. 54: 79-95. Krebs, C. J . and S. Boonstra. 1978. Demography of the s p r i n g 5 d e c l i n e i n p o p u l a t i o n s o f the vole Mierotus townsendii. J . Anim. E c o l . 47: 1007-1015. Myers, J . H., and C . J . Krebs 1971. Ge n e t i c , b e h a v i o r a l , and r e p r o d u c t i v e a t t r i b u t e s of d i s p e r s i n g f i e l d v o l e s Mierotus p e n n s y l v a n i c u s and Mierotus ochrpqaster. E c o l . Monogr. 44: 53-78. P i t e l k a , F. A. 1958. Some aspects of p o p u l a t i o n s t r u c t u r e i n the short-term c y c l e of the brown lemming i n northern A l a s k a . Cold Spring Harb. Symp. Quant. B i o l . 22: 237-251. 6 SECTION 1. DISPERSAL DURING POPULATION FLUCTUATIONS OF THE VOLE MICEOTUS TOWNSENDII I n t r o d u c t i o n Many raicrotine rodent p o p u l a t i o n s f l u c t u a t e p e r i o d i c a l l y i n s i z e , and d i s p e r s a l may be a necessary component of normal p o p u l a t i o n r e g u l a t i o n i n v o l e s (Krebs e t a l . 1973 ,. L i d i c k e r 1975). D i s p e r s a l r e g u l a t e s d e n s i t i e s below the l e v e l s e t by the food supply (Krebs e t a l . , 1969), and because d i s p e r s i n g v o l e s are not a b e h a v i o u r a l l y or g e n e t i c a l l y random subsample o f the p o p u l a t i o n , d i s p e r s a l has the p o t e n t i a l t o a l t e r p o p u l a t i o n c h a r a c t e r i s t i c s (Myers and Krebs 1971b, Krebs et a l . 1976). D i s p e r s a l i s rep o r t e d t o be most common i n i n c r e a s i n g p o p u l a t i o n s (Myers and Krebs 1971b, Krebs et a l . 1976), but has not been demonstrated as s u f f i c i e n t t o account f o r d e c l i n i n g p o p u l a t i o n s ( C h i t t y and Phipps 1966, Krebs 1966, Myers and Krebs 1971b). However, Pearson (1963) and H i l b o r n and Krebs (1976) invoked d i s p e r s a l as a p l a u s i b l e e x p l a n a t i o n of some d e c l i n e s . The i d e n t i f i c a t i o n of d i s p e r s i n g v o l e s i s d i f f i c u l t , and most s t u d i e s employ the method of d e s i g n a t i n g i n d i v i d u a l s captured on a trapped-out area as d i s p e r s i n g i n d i v i d u a l s (Myers and Krebs 1971b, H i l b o r n and Krebs 1976, Krebs e t a l . 1976, F a i r b a i r n 1978). However, t h i s e xperimental design i n v o l v e s s e v e r a l assumptions (Krebs et a l . , 1976) and does not enumerate d i s p e r s e r s t h a t do not enter l i v e - t r a p s set biweekly i n the removal area. I t may be necessary t o enumerate a l l d i s p e r s e r s 7 from a p o p u l a t i o n i n order to d e t e c t d i s p e r s a l d u r i n g p o p u l a t i o n d e c l i n e s . In t h i s study, I provided an area f o r v o l e s to d i s p e r s e i n t o from fenced p o p u l a t i o n s of Mierotus t o w n s e n d i i ; I hoped t o enumerate a l l d i s p e r s i n g v o l e s and t o determine i f d i s p e r s a l i s a necessary or s u f f i c i e n t c o n d i t i o n ( C h i t t y , 1960) f o r p o p u l a t i o n d e c l i n e s i n v o l e s . Methods The study was conducted on E e i f e l I s l a n d i n the F r a s e r B i v e r d e l t a near Vancouver, B r i t i s h Columbia. The study area was part of a Timothy hay f i e l d owned by the Canadian W i l d l i f e S e r v i c e . although Timothy (Phleura pratense) was the dominant s p e c i e s , ftgrostis a l b a , Lplium perenne, and Holeas l a n a t u s were a l s o p r e s e n t . The f i e l d was cut i n the f a l l of 1975 and grazed by waterfowl during the winter of 1975-1976, so t h a t the grass was s h o r t and cover s c a r c e when t r a p p i n g s t a r t e d i n the s p r i n g of 1976. In May 1976 I s e t up two e n c l o s u r e s ( g r i d s B and C) and two unfenced g r i d s ( g r i d s a and D) and trapped each Mierotus townsendii p o p u l a t i o n u n t i l June 1978. Three males and three females were added t o each e n c l o s u r e a f t e r the f i r s t two l i v e - t r a p s e s s i o n s produced l e s s than f i v e i n d i v i d u a l s . I mowed an area i n s i d e each e n c l o s u r e every two weeks d u r i n g the growing season i n order t o c r e a t e a h a b i t a t unfavourable f o r the v o l e s , and a c r o s s which d i s p e r s i n g v o l e s had t o pass. T h i s a r e a was 18.3 m (60 f t ) wide and 53.3 m (175 f t ) long ( F i g . 1.1). On the c o n t r o l g r i d s I mowed an area of equal s i z e during 1976, but dur i n g 1977 and 1978 I reduced i t s width t o 9.1 m (30 f t ) to 8 F i g u r e 1.1. Experimental design used i n t h i s study. G r i d s A and D are unfenced c o n t r o l s and g r i d s B and C are enclosed ex p e r i mentals. The study area was s i t u a t e d i n a Timothy (Phle_M! pratense) h a y f i e l d and bounded on one s i d e by a w a t e r - f i l l e d d i t c h . Ditch < > i > B Disperser Traps c Fence c * < D Mowed A rea Scale 100 ft 30 m 10 attempt to c a t c h more d i s p e r s i n g v o l e s from these unfenced g r i d s . I a l s o mowed an area 0.6 m wide i n s i d e and o u t s i d e the fence p e r i m e t e r to discourage v o l e s from burrowing near the fence. I c o n s t r u c t e d the e n c l o s u r e s by using 6.3 mm (1/4 in) mesh hardware c l o t h extending 0.5 m both below and above ground l e v e l where b o r d e r i n g mowed areas, and both 0.6 m below and above ground l e v e l where bordering unmowed areas. The experimental g r i d s were bounded on three s i d e s by the fence and on one s i d e by the mowed area, which was a l s o fenced i n . The c o n t r o l g r i d s were bordered on one s i d e by a w a t e r - f i l l e d d i t c h , on another by the mowed a r e a , and on the o t h e r two by the remainder of the hay f i e l d ( F i g . 1.1) . Each g r i d had 49 t r a p s i t e s 7.6 m (25 f t ) a p a r t , i n a 7 x 7 p a t t e r n . The Longworth l i v e - t r a p s were p r e b a i t e d f o r f o u r weeks before t r a p p i n g commenced, and oats were r e p l e n i s h e d as necessary. Each t r a p - s i t e had one Longworth l i v e - t r a p and one p i t f a l l t rap s i m i l a r to the type d e s c r i b e d by Boonstra and Krebs (1978). In October 1976, an a d d i t i o n a l Longworth l i v e - t r a p was placed at each t r a p - s i t e . The l i v e - t r a p s , b a i t e d with o a t s and s u p p l i e d with c o t t o n bedding, were s e t every second week on Monday a f t e r n o o n , checked Tuesday morning and afternoon, and checked and locked open on Heknesday morning. During summer, high daytime temperatures r e s t r i c t e d t r a p p i n g t o n i g h t s only. In the winter of 1977-78, one i n s t e a d of two checks were made on Tuesdays. P i t f a l l t r a p s were used from May through October 1976, A p r i l through September 1977, and A p r i l through June 1978. They were not used i n the i n t e r v e n i n g p e r i o d s because of winter 11 f l o o d i n g . They were set every week on Wednesday morning, checked Wednesday a f t e r n o o n and Thursday morning, and c l o s e d on Thursday a f t e r n o o n . When both types o f t r a p s were s e t i n one week, the p i t f a l l s were s e t a f t e r the Longworth l i v e - t r a p s had been l o c k e d open. Voles l e a v i n g a t r a p p i n g area to c r o s s the e n t i r e 18... 3-m width of the mowed area were s u b j e c t t o capture i n f i v e - g a l l o n p i t f a l l t r a p s i n s t a l l e d at 8.9-m (29 f t ) i n t e r v a l s along the fence (see F i g . 1.1). One-gallon cans f i t t e d with Longworth t u n n e l s were set along the same fence half-way between p i t f a l l s . There were two l i v e - t r a p s per s i t e i n the e n c l o s u r e s , t o t a l l i n g 12 per e n c l o s u r e , and one per s i t e on the c o n t r o l s . These d i s p e r s e r p i t f a l l s and l i v e - t r a p s , b a i t e d with o a t s and s u p p l i e d with c o t t o n , remained c o n t i n u a l l y s e t , except t h a t r a i n prevented the use of these l a r g e p i t f a l l s from November to February. A l l t r a p s were checked on each v i s i t . Each v o l e was ear-tagged on f i r s t c apture, and i t s weight, capture l o c a t i o n , sex, r e p r o d u c t i v e c o n d i t i o n , wounding on rump, and r e l a t i v e s i z e o f hip glands were recorded. A l l animals were r e l e a s e d immediately a f t e r b e i n g processed., Voles were c l a s s i f i e d as f o l l o w s : a d u l t >42 g ; subadult 30 - 42 g; j u v e n i l e <30 g. About 30,000 captures were reco r d e d on n e a r l y 5,000 i n d i v i d u a l s i n t h i s study. Voles c r o s s i n g the e n t i r e width of the mowed are a were d e f i n e d as d i s p e r s e r s . , I assumed t h a t untagged v o l e s caught i n d i s p e r s e r t r a p s i n s i d e an enclo s u r e had been born on the g r i d and were untagged d i s p e r s e r s . D i s p e r s i n g v o l e s were removed from the g r i d s . Untagged v o l e s caught i n d i s p e r s e r t r a p s on 12 c o n t r o l g r i d s were not c l a s s i f i e d as d i s p e r s e r s as t h e i r b i r t h place was unknown. Animals escaping from the e n c l o s u r e s were a l s o c l a s s i f i e d as d i s p e r s e r s ; there were 16 known escapes during 1976, 12 d u r i n g 1977, and 2 d u r i n g 1978. The t o t a l enumeration technique of Krebs (1966) was used i n determining p o p u l a t i o n s i z e . A l l of the demographic a n a l y s i s to f o l l o w assumes t h a t most of the i n d i v i d u a l s i n a p o p u l a t i o n are caught d u r i n g each sampling p e r i o d . The estimate of t r a p p a b i l i t y used was: Number of a c t u a l captures f o r an animal T r a p p a b i l i t y = Number of p o s s i b l e captures f o r that animal N where N i s the number of v o l e s caught more than twice and t r a p p a b i l i t y i s summed over a l l N animals. The f i r s t and l a s t times o f capture are excluded from the summation of a c t u a l and p o s s i b l e c a p t u r e s , because an animal must be caught a t these times. R e s u l t s T r a p p a b i l i t y The t r a p p a b i l i t y of the v o l e s i n Longworth l i v e - t r a p s on both c o n t r o l g r i d s i s presented i n Table 1.1, and the t r a p p a b i l i t y on both experimental g r i d s i s presented i n Table 1.2. T r a p p a b i l i t y was g e n e r a l l y above 60%, and approximately equal f o r males and females. A 60% t r a p p a b i l i t y l e v e l combined 1 3 Table 1.1. T r a p p a b i l i t y e s t i m a t e s f o r H i c r o t u s townsendii i n Longworth l i v e - t r a p s on c o n t r o l g r i d s A and D. T i s t r a p p a b i l i t y , N i s sample s i z e . G r i d A G r i d D P e r i o d Males Females Males Females T N T N T N T N Hay -Oct. 1976 0.36 14 0.53 27 0.66 32 0.64 39 Nov. -Feb. 1977 0.63 88 0.60 89 0.69 116 0.65 112 Mar.-Oct. 0.64 169 0.66 192 0.67 202 0.66 190 Nov.-Feb. 1978 0.59 97 0.58 129 0.64 111 0.65 96 Mar. -Jun. 1.00 14 0.73 36 1.00 8 0.81 25 T o t a l 0.65 382 0.63 473 0.67 469 0.66 462 14 Table 1.2. T r a p p a b i l i t y e s t i m a t e s f o r Mierotus townsendii. i n Longworth l i v e - t r a p s on experimental g r i d s B and C. T i s t r a p p a b i l i t y , N i s sample s i z e . G r i d a G r i d D Males Females Males Females P e r i o d T N T N T N T N May -Oct. 1976 0.60 40 0.65 70 0.61 28 0.61 54 Nov. -Feb. 1977 0.59 152 0.63 147 0.67 116 0. 57 107 Mar.-Oct. 0.64 272 0.63 235 0.69 220 0.62 212 Nov.-Feb. 1978 0.64 130 0.56 88 0.64 96 0.64 118 Mar. -Jun. 0.91 42 0.74 37 0.95 32 0.91 57 T o t a l 0. 64 636 0.68 577 0.69 492 0.64 548 15 with good s u r v i v a l i n d i c a t e s t h a t the minimum number a l i v e estimate i s q u i t e c l o s e to the a c t u a l p o p u l a t i o n s i z e ( H i l b o r n e t - a l . - . , • 1976) . . T r a p p a b i l i t y was r e l a t i v e l y constant except d u r i n g the s p r i n g of 1978, when t r a p p a b i l i t y was u s u a l l y over 80%. T h i s i n c r e a s e i n t r a p p a b i l i t y was con c u r r e n t with a p e r i o d of low v o l e d e n s i t y . P o p u l a t i o n Density Changes i n M. townsendii p o p u l a t i o n s were s i m i l a r on a l l f o u r g r i d s . The trappable p o p u l a t i o n s were low when t r a p p i n g began i n May 1976 and remained low throughout the summer ( F i g s . 1.2, 1.3, 1.4, 1.5). They i n c r e a s e d r a p i d l y a t approximately 9% per week d u r i n g the f a l l and winter, owing t o delayed capture i n l i v e - t r a p s , u n t i l the s t a r t of the 1977 breeding season i n March. A s p r i n g d e c l i n e occurred on a l l f o u r g r i d s c o v e r i n g the 12-week p e r i o d t o mid-June 1977. F o l l o w i n g t h e d e c l i n e , i n which males s u s t a i n e d g r e a t e r l o s s e s than females, the p o p u l a t i o n s i n c r e a s e d at approximately 5% per week u n t i l October 1977. T h i s summer recover y i n numbers i n c r e a s e d the p o p u l a t i o n s i z e s t o near or above the l e v e l s t h a t occurred before the s t a r t of the breeding season. A f t e r October 1977, the p o p u l a t i o n s d e c l i n e d a t 5% per week throughout the f a l l and win t e r . At the onset of the 1978 breeding season the r a t e of d e c l i n e i n c r e a s e d t o approximately 50% per week f o r males and 18% per week f o r females, and the pop u l a t i o n s d e c l i n e d to very low l e v e l s d u r i n g the s p r i n g and e a r l y summer of 1978. 16 F i g u r e 1.2. P o p u l a t i o n d e n s i t y of Mierotus townsendii on c o n t r o l g r i d A and number o f d i s p e r s e r s . Non-breeding p e r i o d s are shaded. Both sexes are combined i n the d i s p e r s a l p l o t . D i s p e r s e r s were d e f i n e d as v o l e s c r o s s i n g an unfavourable h a b i t a t and were removed as they were captured. Grid A Residents 18 F i g u r e 1.3. P o p u l a t i o n d e n s i t y o f Mierotus townsendii on c o n t r o l g r i d D and number of d i s p e r s e r s . Non-breeding p e r i o d s are shaded. Both sexes are combined i n the d i s p e r s a l p l o t . 200 Grid D Residents 1976 1977 1978 20 F i g u r e 1.4. P o p u l a t i o n d e n s i t y of H i c r o t u s townsendii on fenced g r i d B and number o f d i s p e r s e r s . Non-breeding p e r i o d s are shaded. V e r t i c a l l i n e s mark the l i m i t s of the seasons. Both sexes are combined i n the d i s p e r s a l p l o t . Grid B Residents 11 1 I 1~ i i I • i . * « * _ i 4 Grid B Dispersers M J S N J M M J S N J M M J 1976 1977 1978 22 F i g u r e 1.5. P o p u l a t i o n d e n s i t y o f Hicro'tus townsendii on fenced g r i d C and number o f d i s p e r s e r s . Non-breeding p e r i o d s are shaded. V e r t i c a l l i n e s mark the l i m i t s of the seasons. Both sexes are combined i n the d i s p e r s a l p l o t . 23 Grid C Residents Grid C Dispersers 1976 1977 1978 24 P o p u l a t i o n Density And D i s p e r s a l The d i s p e r s a l p a t t e r n s f o r the c o n t r o l p o p u l a t i o n s are presented i n F i g u r e s 1.2 and 1.3. The trends were s i m i l a r to those of the two fenced p o p u l a t i o n s ( F i g s . 1.4 and 1.5), except t h a t the number of d i s p e r s e r s recovered was lower on the c o n t r o l s , probably because d i s p e r s i n g animals could l e a v e not only a c r o s s the mowed area but a l s o from the s i d e s of the g r i d . Voles d i s p e r s i n g from the experimental g r i d s c o u l d l e a v e o n l y v i a the mowed area. D i s p e r s i n g v o l e s from the: c o n t r o l g r i d s c o u l d a v o i d the mowed are a , and o n l y an unknown f r a c t i o n of d i s p e r s e r s was enumerated. The numbers of d i s p e r s e r s recovered on the fenced g r i d s were -higher because d i s p e r s i n g v o l e s had no o p t i o n but t o c r o s s the mowed ar e a . On g r i d B the f i r s t capture l o c a t i o n s of d i s p e r s i n g v o l e s were randomly d i s t r i b u t e d (X 2=0.34, P>0.50), with as many d i s p e r s e r s o r i g i n a t i n g from the h a l f of the g r i d d i s t a l t o the mowed area as from the h a l f a d j a c e n t . However, on g r i d C, the f i r s t capture l o c a t i o n s of d i s p e r s i n g v o l e s were non-randomly d i s t r i b u t e d , with more v o l e s o r i g i n a t i n g from the h a l f of the g r i d bordering the mowed area (X 2=12.25, P<.001). P o t e n t i a l d i s p e r s e r s at the f a r end of g r i d C may not have l o c a t e d the d i s p e r s a l area and thus remained i n the p o p u l a t i o n . There were f o u r p e r i o d s of i n c r e a s e d d i s p e r s a l during the study p e r i o d . The f i r s t occurred i n the f a l l of 1976. Both a d u l t s and s u b a d u l t s d i s p e r s e d d u r i n g t h i s i n t e r v a l ( F i g s . 1.6 and 1.7). D i s p e r s a l was low d u r i n g the the 1976-77 winter but 25 F i g u r e 1 . 6 . S i z e c l a s s e s of d i s p e r s i n g Mierotus townsendii from fenced g r i d B r e s i d e n t p o p u l a t i o n . 27 F i g u r e 1.7. S i z e c l a s s e s of d i s p e r s i n g Mierotus towjisendii from fenced g r i d C r e s i d e n t p o p u l a t i o n . 29 i n c r e a s e d d r a m a t i c a l l y a f t e r the 1977 breeding season began. Adult d i s p e r s a l i n c r e a s e d s h a r p l y from mid-March to May, a f t e r which i t was low u n t i l October 1977. Subadult and j u v e n i l e d i s p e r s a l i n c r e a s e d r a p i d l y i n June and J u l y , d e c l i n e d through August and e a r l y September, and then i n c r e a s e d i n l a t e September and October. There was l i t t l e d i s p e r s a l d u r i n g the winter of 1977-1978 and e s s e n t i a l l y no d i s p e r s a l d u r i n g the p r e c i p i t o u s 1978 s p r i n g d e c l i n e . To summarize, a p e r i o d of d i s p e r s a l occurred i n the f a l l of 1976 and 1977, a d u l t d i s p e r s a l i n c r e a s e d r a p i d l y d u r i n g the 1977 s p r i n g d e c l i n e , and su b a d u l t d i s p e r s a l was high d u r i n g the 1977 summer. D i s p e r s a l r a t e i n t h i s study was measured i n two ways. The f i r s t was by cou n t i n g the average number of d i s p e r s e r s per two-week i n t e r v a l f o r d i f f e r e n t seasons, and the second was by d i v i d i n g the average number of d i s p e r s e r s per two weeks i n a season by the average d e n s i t y f o r t h a t season. Table 1.3 pr e s e n t s the d i s p e r s a l r a t e r e s u l t s f o r the experimental p o p u l a t i o n s . The number of d i s p e r s e r s c o l l e c t e d on the c o n t r o l areas was not high enough to provide a meaningful a n a l y s i s . The p o s s i b i l i t y of c o r r e l a t i o n between these two measures of d i s p e r s a l r a t e and the d e n s i t y of the r e s i d e n t p o p u l a t i o n s , r a t e o f p o p u l a t i o n change ( r ) , or the season was i n v e s t i g a t e d by grouping the data i n t o e i g h t p e r i o d s . I c o n s t r u c t e d a v a r i a b l e c a l l e d season and gave i t a value of spring=1, summer=2, fa.ll=3, and winter=4 i n order to examine s e a s o n a l d i s p e r s a l r a t e s . The only s t a t i s t i c a l l y s i g n i f i c a n t c o r r e l a t i o n was between the d e n s i t y o f the r e s i d e n t p o p u l a t i o n s and the number of d i s p e r s e r s (r=.52, n=16, P<.05). More v o l e s d i s p e r s e d from high d e n s i t y 30 T a M e 1.3. P o p u l a t i o n d e n s i t y , r a t e o f i n c r e a s e , and d i s p e r s a l r a t e s f o r p o p u l a t i o n s o f Microtus townsendii. . . a l l data are mean values f o r the p e r i o d . Rate of Average Populn. Av. No. D i s p e r s e r s P e r i o d Season Density Growth* D i s p e r . 2 D e n s i t y 3 G r i d B Summer 1976 2 9.7 +0.1413 0.7 0.072 F a l l 1976 3 71.8 +0.1590 10.3 0.143 Winter 1976-77 4 248. 1 +0.0390 1.9 0.008 Spring 1977 1 176.6 -0.0601 30.8 0. 174 Summer 1977 2 208. 1 +0.0417 25.7 0. 124 F a l l 1977 3 215.3 -0. 1170 22. 0 0. 102 Winter 1977-78 4 143.9 -0.0443 2.3 0.016 S p r i n g 1978 1 53. 5 -0.1720 1.8 0.034 G r i d C Summer 1976 2 11.5 +0.1129 0.6 0.052 F a l l 1976 3 60.8 +0.1438 4.0 0.066 Winter 1976-77 4 164.7 +0.0318 0.5 0.003 Spring 1977 1 168.0 -0.0224 19.5 0. 116 Summer 1977 2 179. 8 +0.0264 12.0 0.067 F a l l 197 7 3 201.5 -0.0121 10.5 0.052 Winter 1976-•77 4 153. 1 -0.0379 1.8 0.012 Spririg 1978 1 49.7 -0.2762 0.5 0.010 1 in s t a n t a n e o u s r a t e per week 2 per two weeks 3 d i s p e r s e r s per two weeks per r e s i d e n t 31 p o p u l a t i o n s than from low d e n s i t y p o p u l a t i o n s . The r a t e of p o p u l a t i o n i n c r e a s e was not c o r r e l a t e d with e i t h e r the average number o f d i s p e r s e r s per two weeks (r=.06, P>.80) or with d i s p e r s s a l r a t e measured as the number of d i s p e r s e r s per two weeks per r e s i d e n t (r=.23, P>.30). More d i s p e r s e r s d i d not l e a v e a f a s t - g r o w i n g p o p u l a t i o n than a slow-growing or d e c l i n i n g p o p u l a t i o n . The c o r r e l a t i o n between season and the two d i s p e r s a l measures approached s t a t i s t i c a l s i g n i f i c a n c e (P<.08). I f s p r i n g 1978 i s excluded from the a n a l y s i s , there i s a c o r r e l a t i o n between season and number of d i s p e r s e r s (r=-.64, n=14, P<.02) and season and number o f d i s p e r s e r s per r e s i d e n t (r=-.75, n= 14, P<.005). T h i s r e s u l t i n d i c a t e s t h a t from May 1976 t o February 1978, d i s p e r s a l was highest i n the s p r i n g and summer and lowest i n the f a l l and winter. Seasonal and d e n s i t y e f f e c t s on d i s p e r s a l r a t e were i n v e s t i g a t e d by a two-way a n a l y s i s of v a r i a n c e on the data i n Table 1.3- The d e n s i t y index was assigned a value of 1 when d e n s i t y was l e s s than 100 animals, 2 when i t was between 100 and 200 animals, and 3 when i t was g r e a t e r than 200 animals. There was a s t r o n g seasonal e f f e c t on d i s p e r s a l r a t e (P<-01), with winter having the lowest r a t e . There was no s t r o n g e f f e c t of d e n s i t y on d i s p e r s a l r a t e (P>.08), with a l l three d e n s i t y c l a s s e s having s t a t i s t i c a l l y equal d i s p e r s a l r a t e s ( d i s p e r s e r s per two weeks per r e s i d e n t ) . In summary, the number of d i s p e r s i n g v o l e s was c o r r e l a t e d with the season and the d e n s i t y of the r e s i d e n t p o p u l a t i o n s . D i s p e r s a l r a t e i n c r e a s e d r a p i d l y i n the s p r i n g i f the r e s i d e n t p o p u l a t i o n had a moderate d e c l i n e , decreased throughout the 32 summer and f a l l when the r e s i d e n t p o p u l a t i o n i n c r e a s e d , and was low d u r i n g the w i n t e r . D i s p e r s a l can occur when the r e s i d e n t p o p u l a t i o n i s i n c r e a s i n g or d e c l i n i n g and i t i s not dependent on the r a t e of p o p u l a t i o n i n c r e a s e . D i s p e r s a l does not occur when the r e s i d e n t p o p u l a t i o n s u f f e r s a severe s p r i n g d e c l i n e (Krebs and Boonstra, 1978) . S u r v i v a l And D i s p e r s a l To determine what p r o p o r t i o n of the l o s s of i n d i v i d u a l s from a p o p u l a t i o n can be accounted f o r by d i s p e r s a l and how t h i s p r o p o r t i o n v a r i e s with p o p u l a t i o n d e n s i t y changes and season, I examined those v o l e s tagged on the t r a p p i n g g r i d s and l a t e r captured i n d i s p e r s e r t r a p s . Only the r e s u l t s f o r the fenced g r i d s a r e l i s t e d as I assumed a l l d i s p e r s e r s from these g r i d s were enumerated. Host tagged d i s p e r s e r s t h a t had been captured at l e a s t once i n Longworth l i v e - t r a p s b e f o r e d i s p e r s a l were caught s h o r t l y a f t e r l e a v i n g the t r a p p i n g g r i d . The average i n t e r v a l s between l a s t capture i n l i v e - t r a p s on the g r i d and capture i n e i t h e r type of d i s p e r s e r t r a p was 3.2 weeks f o r 204 animals from g r i d B and 2.8 weeks f o r 101 animals from g r i d C. The i n t e r v a l was l e s s than 6 weeks f o r the m a j o r i t y of d i s p e r s e r s t h a t had p r e v i o u s l y entered Longworth l i v e - t r a p s . The average i n t e r v a l f o r d i s p e r s e r s t h a t had been captured a t l e a s t once i n p i t f a l l t r a p s p r i o r to d i s p e r s a l was 8.0 weeks f o r 348 animals from g r i d B and 7.9 weeks f o r 166 animals from g r i d C. About 25% o f these d i s p e r s e r s had a 10-week or g r e a t e r l a g between l a s t p i t f a l l 33 capture on the g r i d and subsequent capture i n d i s p e r s e r t r a p s . The h i g h e r mean l a g time occurred because the p i t f a l l s were i n o p e r a b l e d u r i n g the f a l l and winter r a i n y season, and some v o l e s d i s p e r s e d before the p i t f a l l s were operable i n the s p r i n g . D i s p e r s i n g animals c o u l d have been p r e v i o u s l y trapped i n l i v e - t r a p s , p i t f a l l s , both , o r n e i t h e r . Table 1.4 g i v e s the l o s s e s of v o l e s from the combined l i v e - t r a p and p i t f a l l p o p u l a t i o n s , the number of tagged d i s p e r s e r s , and the percentage l o s s e x p l a i n e d by d i s p e r s a l . J u v e n i l e l o s s e s and j u v e n i l e d i s p e r s e r s are excluded from the a n a l y s i s . The r e s u l t s i n Table T. 4 i n d i c a t e t h a t d u r i n g the 1977 s p r i n g d e c l i n e d i s p e r s a l accounted f o r over 50% of the l o s s from the t o t a l tagged p o p u l a t i o n when j u v e n i l e s were excluded, but during the 1978 s p r i n g , i t accounted f o r l e s s than 10%. D i s p e r s a l always accounted f o r a higher p r o p o r t i o n of the l o s s from g r i d B than from g r i d C. D i s p e r s a l r a t e s were higher i n the 1977 d e c l i n e than i n e i t h e r i n c r e a s e p e r i o d before or a f t e r i t . The percentage of l o s s e x p l a i n e d by d i s p e r s a l was not r e l a t e d t o the r a t e of i n c r e a s e of the r e s i d e n t p o p u l a t i o n s (r=.05, n=16, P>.80), nor to the d e n s i t y of the r e s i d e n t p o p u l a t i o n s (r=.12, n=16, P>.60). However, i f s p r i n g 1978 i s excluded, the percentage of l o s s decreased from s p r i n g t o winter (r=-.78, n=14, P<. 001). Provided t h a t t h e r e i s no severe s p r i n g d e c l i n e i n p o p u l a t i o n s i z e , d i s p e r s a l can account f o r over 50% of the l o s s d u r i n g the s p r i n g , l e s s of the l o s s d u r i n g the summer and f a l l , and s t i l l l e s s o f any winter l o s s . S u r v i v a l r a t e s of male Mierotus townsendii on the experimental g r i d s were c l o s e l y r e l a t e d t o the p o p u l a t i o n growth 34 Table 1.4. Number of tagged Mierotus townsendii d i s a p p e a r i n g from combined l i v e - t r a p and p i t f a l l p o p u l a t i o n s on both experimental g r i d s and number o f tagged d i s p e r s e r s caught. J u v e n i l e l o s s e s and d i s p e r s e r s are excluded. No. tagged v o l e s Number % l o s s l o s t from r e s i d e n t of tagged e x p l a i n e d by Peri o d p o p u l a t i o n d i s p e r s e r s d i s p e r s a l Males Females Males Females Males Females G r i d B Summer 1976 12 7 5 6 42 86 F a l l 1976 39 41 18 12 46 29 Winter 1976-77 74 47 22 15 30 32 Spri n g 1977 130 89 74 60 56 67 Summer 1977 153 166 49 67 32 40 F a l l 1977 58 36 22 11 38 31 Winter 1977-78 83 61 4 1 5 2 Spri n g 1978 62 40 5 1 8 3 T o t a l 611 487 199 173 33 36 G r i d C Summer 1976 19 11 6 2 32 18 F a l l 1976 29 46 1 4 3 9 Winter 1976-77 53 28 4 2 8 7 Spring 1977 86 53 46 31 52 58 Summer 1977 137 142 34 23 25 16 F a l l 1977 54 36 10 7 19 19 Winter 1977-78 65 73 1 1 2 1 Sp r i n g 1978 42 53 0 3 0 6 T o t a l 485 442 102 73 21 17 35 r a t e s of the r e s i d e n t p o p u l a t i o n s (r=.61, n=16, PC.02) . Female s u r v i v a l r a t e s sere moderately r e l a t e d t o the p o p u l a t i o n growth r a t e s (r=.43, n=16, P<.07). There was no c o r r e l a t i o n between percentage of l o s s e x p l a i n e d by d i s p e r s a l and s u r v i v a l r a t e s of males or females. The percentage of d i s p e r s a l l o s s was mainly c o r r e l a t e d with season. Since d i s p e r s a l accounted f o r the m a j o r i t y of the l o s s from the p o p u l a t i o n d u r i n g the 1977 s p r i n g d e c l i n e , I assumed t h a t new a d u l t animals caught on the c o n t r o l g r i d s were d i s p e r s e r s . I then determined what p r o p o r t i o n of the d i s p e r s e r s was r e c r u i t e d t o the p o p u l a t i o n and i f s u c c e s s f u l r e c r u i t s were he a v i e r than u n s u c c e s s f u l r e c r u i t s . Table 1.5 i n d i c a t e s t h a t females were more s u c c e s s f u l i n being r e c r u i t e d i n t o the p o p u l a t i o n s than males, and that heavier animals g e n e r a l l y were more s u c c e s s f u l r e c r u i t s , although the weight d i f f e r e n c e between r e c r u i t s and n o n - r e c r u i t s was not s t a t i s t i c a l l y s i g n i f i c a n t . Becruitment r a t e s were higher on g r i d A than on g r i d D, probably because the p o p u l a t i o n d e n s i t y was lower on g r i d A at t h i s time. D i s p e r s a l And Time Of B i r t h The time of d i s p e r s a l of young v o l e s i s dependent upon t h e i r season o f b i r t h ( K a l e l a 1957, Anderson 1970). T a b l e s 1.6 and 1.7 show the p r o b a b i l i t i e s of d i s p e r s a l as a f u n c t i o n of the month o f f i r s t c a p t u r e . Voles born i n the s p r i n g and summer d i s p e r s e b e f o r e the s t a r t of the next breeding season, whereas animals born i n the l a t e summer or e a r l y f a l l d i s p e r s e a f t e r i t . Thus, f a l l - b o r n d i s p e r s e r s tend to be o l d e r than s p r i n g - b o r n 36 Table 1.5. Mean weight {+ 1 SE) of s u c c e s s f u l and u n s u c c e s s f u l r e c r u i t s t o c o n t r o l g r i d s d u r i n g the 1977 s p r i n g d e c l i n e . G r i d & G r i d D Males Females Males Females No. l i v i n g l e s s than 2 weeks Weight a t f i r s t capture 11 57.3 (2. 4) 2 52.0 (2.0) 20 59.3 (2. 3) 7 58.4 (4. 7 ) No. l i v i n g 2 or more weeks Weight a t f i r s t capture 26 62.7 (2. 0) 15 55.6 (2.3) 32 64.0 (1. 6) 30 55. 1 (1.?) ^ s u c c e s s f u l r e c r u i t s 70 88 61 81 37 Table 1. 6. P r o b a b i l i t y of an i n d i v i d u a l f i r s t caught from s p r i n g to winter d i s p e r s i n g before the s t a r t of the next breeding season. G r i d s B anc C, 1976 and 1977, and both sexes are pooled. Month New animals Number d i s p e r s i n g P r o b a b i l i t y A p r i l 81 30 0.37 May 284 94 0.33 June 632 111 0. 18 J u l y 447 80 0. 18 Aug. 359 44 0. 12 Sept. , 286 18 0.06 Oct. 201 15 0.07 Nov- 72 4 0.06 Dec. 66 0 0.00 38 Table 1.7. P r o b a b i l i t y of an i n d i v i d u a l d i s p e r s i n g during 1977 given i t s month of f i r s t c apture i n 1976. Both sexes and g r i d s B and C are combined. New animals Number d i s p e r s i n g P r o b a b i l i t y of Month i n 1976 i n 1977 d i s p e r s a l May 11 0 0.00 June 59 2 0.03 J u l y 32 1 0.03 Aug. 250 20 0.08 Sept. 265 41 0. 15 Oct. 182 41 0.23 Nov. 70 17 0.24 Dec. 55 14 0.25 3 9 d i s p e r s e r s . Sex R a t i o s Of D i s p e r s i n g Voles I compared the sex r a t i o s of d i s p e r s i n g and r e s i d e n t v o l e s by t a l l y i n g each animal every time i t was known to be a l i v e and summing the data over s e a s o n a l p e r i o d s . Table 1.8 g i v e s the sex r a t i o s f o r the d i s p e r s i n g and r e s i d e n t p o p u l a t i o n s on a l l f o u r g r i d s . Sex r a t i o s f o r the r e s i d e n t p o p u l a t i o n were c a l c u l a t e d f o r the combined l i v e - t r a p - and p i t f a l l - e n u m e r a t e d p o p u l a t i o n . These data i n d i c a t e that more males d i s p e r s e d than females. The d i s p e r s e r s ' sex r a t i o was u s u a l l y higher than t h a t of the r e s i d e n t s f o r the same season on the same g r i d . Although there i s u s u a l l y a d e f i c i e n c y of males i n the r e s i d e n t p o p u l a t i o n (Myers and Krebs 1971a), th e r e i s an excess i n the d i s p e r s i n g p o p u l a t i o n . The sex r a t i o decreased on a l l g r i d s d u r i n g the s p r i n g d e c l i n e s but recovered d u r i n g the summer. D i s p e r s e r s vary i n age, and I i n q u i r e d i f an excess of males o c c u r r e d i n a l l age c l a s s e s . The age c l a s s e s used were de f i n e d by weight and were t h e r e f o r e a crude i n d i c a t i o n of age. The sex r a t i o s of the t h r e e age c a t e g o r i e s were compared by t a l l y i n g an animal each time i t was caught and summing the data f o r the whole study. There was an excess of female j u v e n i l e d i s p e r s e r s from g r i d B (X 2=3.98, P<.05) (Table 1.9), but no s i g n i f i c a n t d i f f e r e n c e i n j u v e n i l e sex r a t i o s on g r i d C (X 2=1.85, P>.15). D i s p e r s i n g s u b a d u l t s had s i m i l a r sex r a t i o s t o those of r e s i d e n t subadults on both g r i d s . D i s p e r s i n g a d u l t s on g r i d C had s i g n i f i c a n t l y h i g h e r sex r a t i o s than r e s i d e n t 4 0 Table 1.8. Sex r a t i o s ( p r o p o r t i o n of males) i n r e s i d e n t and d i s p e r s i n g p o p u l a t i o n s of Mierotus townsendii on both c o n t r o l and experimental g r i d s . Sample s i z e i s i n parentheses. G r i d Season A B C D Residents Summer 1976 0. 48 (298) 0. 40 (416) 0.31 (453) 0. 37 (645) F a l l 1976 0. 43 (606) 0. 44 (1482) 0.42 (1282) 0. 45 (867) winter 1976 0. 47 (1598) 0. 51 , (3341) 0.49 (2583) 0. 52 (2009) Spring 1977 0. 40 (1400) 0. 40 (2174) 0.46 (1985) 0. 40 (1871) Summer 1977 0. 43 (3943) 0. 55 (5658) 0.46 (5021) 0. 49 (4244) F a l l 197 7 0. 44 (907) 0.59 (1111) 0.47 (1052) 0. 53 (961) Winter 1977 0. 43 (1192) 0. 58 (1364) 0.44 (1218) 0. 55 (1189) Spr i n g 1978 0. 24 (263) 0. 51 (379) 0.32 (324) 0. 25 (182) T o t a l 0. 43 (10207) 0. 51 (15925) 0.45 (13918) 0. 48 (11968) D i s p e r s e r s Summer 1976 (0) 0. 83 (6) 0.80 (5) 0. 80 (10) F a l l 1976 oZ "50 (2) 0. 50 (38) 0.47 (15) 0. 33 (3) Winter 1976 (0) 0. 63 (30) 0.67 (6) 0. 00 (D S p r i n g 1977 ~0~. "80 (5) 0. 50 (173) 0.54 (116) 0. 57 (7) Summer 1977 0. 50 (8) 0. 49 (187) 0.56 (84) 0. 75 (8) F a l l 1977 0. 50 (8) 0. 50 (88) 0.62 (42) 1. 00 (3) Winter 1977 0. 33 (3) 0. ,76 (21) 0.64 (14) (0) Spring 1978 1. 00 (2) 0. 73 (11) 0.00 (3) — (0) T o t a l 0. 53 (28) 0. 52 (554) 0.56 (285) 0. 66 (32) 41 Table 1.9. P r o p o r t i o n of males a c c o r d i n g t o age group i n d i s p e r s i n g and r e s i d e n t p o p u l a t i o n s o f Mierotus townsendii. Sample s i z e i s i n parentheses. Data are summed from 1976 u n t i l 1978. Po p u l a t i o n Adult Subadult J u v e n i l e Resident G r i d B 0.55 (6480) 0.46 (1621) 0.51 (112) G r i d C 0.50 (4696) 0.37 (1884) 0.44 (1088) D i s p e r s e r s 0.33 (33) G r i d B 0.61 (290) 0.44 (231) G r i d C 0.71 (136) 0.41 (117) 0.56 (32) 4 2 a d u l t s (X 2=24.03, p<.0001),-as d i d d i s p e r s i n g a d u l t s on g r i d B (X 2=3.94, P<.05). The excess of male d i s p e r s e r s was c o n f i n e d to a d u l t s . Sexual M a t u r i t y and D i s p e r s a l The r e p r o d u c t i v e c o n d i t i o n of the vo l e s was assessed by e x t e r n a l c h a r a c t e r i s t i c s . M a t u r i t y i n males was judged by the presence of s c r o t a l t e s t e s , and i n females by the presence of a p e r f o r a t e v a g i n a , enlarged mammary glands, open pubic symphyses, or a l i t t e r i n a t r a p . J u v e n i l e r e s i d e n t s and d i s p e r s e r s were i n the same r e p r o d u c t i v e c o n d i t i o n d u r i n g the same season, as were a d u l t male r e s i d e n t s and d i s p e r s e r s . However, a d u l t female d i s p e r s e r s were more f r e q u e n t l y i n r e p r o d u c t i v e c o n d i t i o n than were r e s i d e n t a d u l t females on g r i d B i n the f a l l o f 1976 (X 2=8. 18, P<.005) and i n the f a l l of 1977 (X 2=9.88, P<.001). U n f o r t u n a t e l y , I d i d not capture enough a d u l t females d i s p e r s i n g from g r i d C duri n g these p e r i o d s to make a meaningful a n a l y s i s . I compared the r e p r o d u c t i v e m a t u r i t y of d i s p e r s i n g subadult v o l e s with t h a t of r e s i d e n t s u b a d u l t s f o r both experimental p o p u l a t i o n s {Table 1.10). The p r o p o r t i o n of d i s p e r s i n g s u b a d u l t males and females which were s e x u a l l y mature was s i g n i f i c a n t l y g r e a t e r than the p r o p o r t i o n i n the r e s i d e n t p o p u l a t i o n f o r both g r i d s (P<.001). The d i f f e r e n c e i n maturity between d i s p e r s i n g and r e s i d e n t subadults was s t a t i s t i c a l l y s i g n i f i c a n t (P<.01) during each season t h a t a s u f f i c i e n t number of d i s p e r s i n g s u b a d u l t s was captured. T h i s d i f f e r e n c e was most marked d u r i n g 43 Table 1.10. Percentage of su b a d u l t v o l e s i n breeding c o n d i t i o n i n r e s i d e n t and d i s p e r s i n g p o p u l a t i o n s . Sample s i z e i s i n parentheses. G r i d B G r i d C Resident D i s p e r s i n g Resident D i s p e r s i n g Male Female Male Female Male Female Male Female Summer 1976 33 48 100 100 32 56 100 (24) (48) (2) (D (19) (52) (D TO) F a l l 1976 12 26 33 80 11 23 100 57 (100) (112) (6) (10) (72) (94) (1) (7) Winter 1976 0 4 0 0 0 5 (80) (93) (3) (8) (108) (185) (0.) l 0 ) Spring 1977 38 44 57 94 20 45 53 78 (53) (45) (23) (32) (41) (47) (17) (27) Summer 1977 4 5 35 42 7 8 36 32 (467) (431) (55) (71) (381) (448) (25) ( 2 2 ) F a l l 1977 0 0 9 35 0 3 20 50 (15) (38) (11) (19) (34) (94) (5) (12) Winter 1977 0 0 0 0 0 0 (27) (62) (3) "(0) (40) (175) ~ 0 ) (D Spr i n g 1978 50 55 100 0 0 I M I I „ t (4) (22) " 0 ) (D (4| (90) ~ 0 ) (0) T o t a l minus winters 9 15 40 58 9 13 43 53 1 (663) (696) (97) (134) (551) (825) (49) (68) 44 the summer and f a l l o f 1977 when d e n s i t i e s were high ( F i g s . 1.4 and 1.5). Less than 10% of t h e r e s i d e n t s ubadults were s e x u a l l y mature whereas about 30% of the d i s p e r s i n g s u b a d u l t s were mature. I f the data f o r the non-winter seasons i s summed, about 30% more subadult males and females were i n breeding c o n d i t i o n i n the d i s p e r s i n g p o p u l a t i o n s than i n the r e s i d e n t p o p u l a t i o n s . I c a l c u l a t e d weight a t s e x u a l maturity t o determine i f d i s p e r s i n g v o l e s mature at the same weight as r e s i d e n t v o l e s . The weight a t s e x u a l maturity f o r l i v e - t r a p p e d v o l e s was c a l c u l a t e d by the technique of L e s l i e e t a l . (1945). I grouped data f o r 3-month p e r i o d s to determine percentage mature i n 4-g weight c l a s s e s , and then f i t t e d a p r o b i t l i n e t o the percentage mature data. The weight a t s e x u a l maturity f o r r e s i d e n t and d i s p e r s i n g v o l e s from g r i d B i s shown i n Table 1.11. There was an annual c y c l e i n the median weight a t sexual maturity i n the r e s i d e n t p o p u l a t i o n f o r both males and females. Only a few heavy v o l e s or none at a l l were s e x u a l l y mature d u r i n g the winters. Both d i s p e r s i n g males and females matured a t lower weights than i n r e s i d e n t p o p u l a t i o n s i n a l l seasons f o r which there was an adequate sample of d i s p e r s e r s . The d i s p e r s i n g males matured at s i g n i f i c a n t l y lower weights i n a l l seasons, but i n females t h e r e was some o v e r l a p of the c o n f i d e n c e i n t e r v a l s . The data from g r i d C confirmed these t r e n d s , with both male and female d i s p e r s e r s maturing a t lower weights than the r e s i d e n t s (Table 1-12) . Body Height And D i s p e r s a l Table 1.11. Median weight at s e x u a l maturity f o r g r i d B male and female r e s i d e n t s and d i s p e r s e r s . 95% c o n f i d e n c e l i m i t s are i n parentheses. P e r i o d R e s i d e n t s 1 D i s p e r s e r s Males June-Aug. 1976 42.0 (40.8, 43. 3) Sept..—Nov. 1976 64.1 (59.4, 69. 2) 49.8 (47.3, 52.3)* March-May 1977 48.6 (46.7, 50. 6) 40.7 (38. 2, 43.4) * June-Aug. 1977 56.5 (53.8, 59. 5) 38.7 (37.5, 40.0) * Sept.-Nov. 1977 >70 66.6 (53. 8, 82.5) March-May 1978 56. 1 (53.9, 58. 5) Females June-Aug. 1976 33.7 (29.4, 38. 7) Sept.-Nov 1976 49.9 (47.7, 52. 3) 38.4 (12.5, 118.2) March-May 1977 44. 1 (40. 1, 48. 5) 30.0 (21.5, 42.0) June-Aug 1977 54.0 (52.5, 55. 5) 38.4 (29. 8, 49.6) * Sept.-Nov. 1977 >70 59.2 (31.6, 111.2) March-May 1978 42.4 (33.0, 54. 8) 1 Median weight at s e x u a l maturity was above 70 g f o r r e s i d e n t males and females from Dec.-Feb. 1977 and Sept.-Feb. 1978. * P<.05 46 Table 1. 12. Median weight at s e x u a l maturity f o r g r i d C male and female r e s i d e n t s and d i s p e r s e r s . 95% co n f i d e n c e l i m i t s are i n parentheses. P e r i o d R e s i d e n t s 1 D i s p e r s e r s M a l es June-Aug. 1976 46.0 (43.7, 48.3) Sept.-Nov. 1976 60-6 (56.9, 64.4) March-May 1977 48. 1 (46.8, 49.4) 44. 1 (39.5, 49.2) June-Aug. 1977 51.2 (49.5, 52.9) 37.8 (36.5, 39. 1) * March-May 1978 52.3 (50.8, 55.0) Females June-Aug. 1976 34.3 (30.4, 38.6) Sept.-Nov. 1976 50.4 (46.7, 54.4) March-May 1977 39.6 (35.5, 44.2) 29.3 (17.8, 48.2) June-Aug. 1977 47.2 (45.7, 48.8) 21.2 (0.0, >200) March-May 1978 36.4 (29.5, 44.9) 1 Median weight at s e x u a l maturity was above 70 g f o r r e s i d e n t males and females from Dec.1976-Feb. 1977 and Sept. 1977 -Feb. 1978. * P<.05 47 Mean body weights and weight d i s t r i b u t i o n s o f d i s p e r s i n g and r e s i d e n t p o p u l a t i o n s on the two experimental g r i d s were compared f o r d i f f e r e n t seasons. The mean weights o f d i s p e r s i n g and r e s i d e n t males f o r g r i d s B and C are p l o t t e d i n F i g . 1.8. The data f o r r e s i d e n t p o p u l a t i o n s were c a l c u l a t e d by summing the capture weights of a l l i n d i v i d u a l s throughout each season. Thus, a s i n g l e i n d i v i d u a l might have been captured s e v e r a l times d u r i n g a season, and a l l i t s weight measurements are i n c l u d e d i n the determination of the mean. In 1976, d i s p e r s i n g males were h e a v i e r than r e s i d e n t males, whereas d u r i n g 1977 and 1978, d i s p e r s i n g males were l i g h t e r than r e s i d e n t males ( F i g . 1.8). However, during the same p e r i o d , d i s p e r s i n g females were l i g h t e r than r e s i d e n t females (Table 1.13). The 1977 summer and f a l l weights of both male and female d i s p e r s e r s i n d i c a t e d t h at a hig h e r p r o p o r t i o n of d i s p e r s e r s was i n the j u v e n i l e and subadult weight category compared with the r e s i d e n t p o p u l a t i o n (Table 1.9). For example, the d i f f e r e n c e f o r g r i d B males d u r i n g both summer 1977 (X 2=5.57, P<. 02) and f a l l 1977 (X 2=19.66, P<.0001) was s t a t i s t i c a l l y s i g n i f i c a n t . The g r i d B female comparisions f o r summer 1977 (X 2 = 17.28, P<.0001) and f a l l 1977 (X 2=6.30, P<.02) were a l s o s i g n i f i c a n t . F i g u r e 1.7 i l l u s t r a t e s t h i s r e s u l t with the sharp i n c r e a s e i n subadult and j u v e n i l e d i s p e r s a l i n the summer and f a l l of 1977. The t o t a l summation o f these data f o r both experimental g r i d s i n d i c a t e t h a t j u v e n i l e and subadult males c o n s t i t u t e d a higher p r o p o r t i o n o f d i s p e r s i n g males than expected ( g r i d B X2=1G.Q5, P<.001, g r i d C X 2=3.54, P<.07). The same r e s u l t o c c u r r e d i n females ( g r i d B X 2=70.12, P<. 0001, g r i d C X 2=27.61, P<.0001). & 48 F i g u r e 1.8. Hean body weights of r e s i d e n t male ti. townsendii i n fenced p o p u l a t i o n s B (BE) and C (CE), and those t h a t d i s p e r s e d (BD, CD). Data are grouped over the seasons i n d i c a t e d i n F i g s . 1.4 and 1.5. V e r t i c a l bars give 95% c o n f i d e n c e l i m i t s . E ro i_ O U ) I O CO c CD <U 2 6 5 r 6 0 55 5 0 4 5 4 0 35 30 CR-L CDi BD BR BD BR CD CR I BR I CR BD CD I BRj BD I CR CD I BRj BD IT CR CD I BR I i . CR B D CD I BR I CR 1 CD BD BR I CR BD SUMMER FALL WINTER SPRING SUMMER FALL WINTER SPRING !976 1977 1978 50 Table 1.13. P r o p o r t i o n of males and females t h a t are j u v e n i l e or subadult i n d i s p e r s i n g and r e s i d e n t p o p u l a t i o n s during d i f f e r e n t seasons. Sample s i z e i s i n parentheses. G r i d B G r i d C Resident D i s p e r s i n g Resident D i s p e r s i n g Male Female Male Female Male Female Male Female Summer 1976 0.54 0.55 0.40 1.00 0.37 0.4 8 0.25 1.00 (110) (171) (5) (D (94) (179) («»)- (4) F a l l 1976 0.58 0.45 0.32 0.68 0.69 0.44 0. 14 0.88 (395) (558) (19) (1:9) (277) (425) (7) C8) Winter 1976 0.08 0. 16 0.16 0. 64 0. 18 0.36 0.00 0.50 (915) (898) (19) (11) (645) (592) (4) (2) S p r i n g 1977 0.27 0.17 0.30 0.46 0.18 0.18 0.38 0.62 (658) (950) (87) (81) (732) (841) (63) (53) Summer 1977 0.57 0.56 0.70 0-78 0.61 0.57 0.69 0.78 (1255) (1149) (92) (95) (986) (1330) (48) (37) F a l l 1977 0.04 0.20 0.20 0.38 0. 10 0.38 0.22 0.69 (336) (190) W) (42) (266) (248) (27) (13) Winter 1977 0.06 0. 18 0.17 0.00 0. 12 0.59 0.00 1.00 (474) (282) (16) (5) (345) (424) (9) (D S p r i n g 1978 0.06 0. 18 0.00 1. 00 0.05 0.46 • • 0.00 (167) (146) (8) (D (98) (196) (0) (1) T o t a l 0. 30 0. 33 0.39 0.57 0.33 0.43 0.40 0.67 (4310) (4344) (290) (255) (3443) (4235) (162) (119) 51 higher p r o p o r t i o n of j u v e n i l e and subadult females d i s p e r s e d than d i d males. F i g u r e 1.9 shows that on g r i d B during the summer of 1977 about 5% o f the male d i s p e r s e r s were over 54 g, but over 1555 o f the r e s i d e n t male p o p u l a t i o n weighed more than 54 g. T h i s i n d i c a t e d t h a t d u r i n g the summer l a r g e male v o l e s tend not to d i s p e r s e , and the same r e s u l t was obtained f o r the f a l l data. There was no evidence of d i s p e r s a l o f heavy-weight males d u r i n g the d e c l i n e t h a t s t a r t e d i n the f a l l o f 1977. D i s c u s s i o n I undertook t h i s study of d i s p e r s a l t o determine i f d i s p e r s a l can account f o r p o p u l a t i o n d e c l i n e s and t o examine d i f f e r e n c e s between r e s i d e n t and d i s p e r s i n g v o l e s . In t h i s study, v o l e s were d e f i n e d as d i s p e r s e r s i f they c r o s s e d an unfavourable h a b i t a t . D i s p e r s a l i n s m a l l mammal p o p u l a t i o n s has been mainly s t u d i e d by means o f a removal g r i d s i t u a t e d near a c o n t r o l g r i d , with t r a p p i n g on the removal g r i d o c c u r r i n g f o r two days every two weeks (Myers and Krebs 1971b, Krebs e t a l . 1976, F a i r b a i r n 1978). T h i s design assumes t h a t d i s p e r s i n g v o l e s f i n d the removal g r i d , remain t h e r e long enough t o be trapped, and enter Longworth l i v e - t r a p s s e t biweekly. F u r t h e r assumptions of t h i s e xperimental design are d i s c u s s e d i n Krebs e t a l . (1976). The b a s i c assumption i n v o l v e d i n t h i s study i s t h a t areas mowed biweekly i n the growing season c o n s t i t u t e d unfavourable h a b i t a t , and that t h i s h a b i t a t b a r r i e r separated d i s p e r s i n g and r e s i d e n t animals. The b a r r i e r seemed to be e f f e c t i v e because 52 F i g u r e 1.9. Body weight d i s t r i b u t i o n s of r e s i d e n t (H=1255) and d i s p e r s i n g (N=92) male M. townsendii from fenced g r i d B d u r i n g summer 1977. Grid B Residents Summer 1977 Body Weight, Grams 5 4 the number of tagged d i s p e r s e r s o r i g i n a t i n g from the c o n t r o l p o p u l a t i o n s was about 15% of the number o r i g i n a t i n g from the experimental p o p u l a t i o n s , and d e n s i t i e s were s i m i l a r i n the p o p u l a t i o n s . T h i s r e s u l t i n d i c a t e s t h a t when g i v e n an a l t e r n a t i v e , v o l e s avoided the mowed areas and sought other avenues of d i s p e r s a l . I t appears probable t h a t v o l e s c r o s s i n g t h i s unfavourable h a b i t a t were attempting to l e a v e the p o p u l a t i o n and were not merely on a b r i e f f o r a y i n t o a d j o i n i n g areas. The i d e a t h a t d i s p e r s a l i n s m a l l mammals i s undertaken at the time of puberty i s g e n e r a l l y accepted (Howard 1949, B l a i r 1953, Smith 1968, C h r i s t i a n 1970). Stemming from t h i s i d e a i s the s u g g e s t i o n that d i s p e r s a l occurs mainly i n the s p r i n g and summer, owing to the attainment of puberty a t t h i s time ( K a l e l a 1957, Anderson 1970). Hurray (1967) and C h r i s t i a n (1970) suggest t h a t s o c i a l l y s u b ordinate i n d i v i d u a l s d i s p e r s e . The r e s u l t s o f the present study i n d i c a t e t h a t t h e r e i s i n c r e a s e d d i s p e r s a l by both young males and young females d u r i n g the summer and f a l l . . The i n c r e a s e d d i s p e r s a l of young animals dur i n g these p e r i o d s suggests t h a t they may be d r i v e n from the r e s i d e n t p o p u l a t i o n s by l a r g e r , more a g g r e s s i v e animals. T h i s r e s u l t agrees with the s u g g e s t i o n of S a d l e i r (1965) and Healey (1967) t h a t young Peromyscus males are d r i v e n from the r e s i d e n t p o p u l a t i o n by dominant male a g g r e s s i o n . Young voles of both sexes may be f o r c e d t o d i s p e r s e through a g g r e s s i v e i n t e r a c t i o n s with a d u l t females too., V o l e s born near the s t a r t of the s p r i n g breeding season have a higher p r o b a b i l i t y of d i s p e r s i n g before the s t a r t of next 55 year's b r e e d i n g season than do v o l e s born near the end of the breeding season. T h i s r e s u l t supports Anderson's (1970) i d e a t h a t s p r i n g - b o r n v o l e s are more l i k e l y t o d i s p e r s e w i t h i n the year of t h e i r b i r t h . Late summer- and f a l l - b o r n v o l e s have a higher p r o b a b i l i t y of d i s p e r s i n g a f t e r the s t a r t of the next year's breeding season than do t h e i r s p r i n g - b o r n c o u n t e r p a r t s . Since the average l i f e t i m e of v o l e s i s under s i x months, f a l l - b o r n v o l e s should have a higher p r o b a b i l i t y of d i s p e r s i n g the f o l l o w i n g s p r i n g than t h e i r s p r i n g - b o r n comparisons simply because more fa11-born than s p r i n g - b o r n v o l e s are a l i v e at t h i s time. T h i s r e s u l t a p p l i e s o n l y i f a moderate s p r i n g d e c l i n e occurs d u r i n g the f o l l o w i n g breeding season. L i t t l e or no d i s p e r s a l accompanies a severe s p r i n g d e c l i n e (Krebs and Boonstra, 1978). Bate o f d i s p e r s a l i s h i g h e r i n i n c r e a s i n g p o p u l a t i o n s (Hyers and Krebs 1971b, Krebs et a l . 1976) with f a s t e r growing p o p u l a t i o n s having r e l a t i v e l y more d i s p e r s a l . D i s p e r s a l has not been demonstrated t o i n c r e a s e d u r i n g d e c l i n e p e r i o d s with a removal g r i d d e s i g n . H i l b o r n and Krebs (1976) presumed t h a t d i s p e r s a l o c c u r r e d during a d e c l i n e because no dead r a d i o a c t i v e l y tagged v o l e s were found on the g r i d s , but the removal g r i d d e sign d i d not d e t e c t these presumed d i s p e r s e r s . These r e s u l t s l e a d t o a p o s i t i v e c o r r e l a t i o n beteen the r a t e of p o p u l a t i o n growth (r) and the number of d i s p e r s e r s l e a v i n g a p o p u l a t i o n . Thus, the removal g r i d e v a l u a t i o n of d i s p e r s a l r a t e s i n d i c a t e s t h a t , when c o n t r o l p o p u l a t i o n s are growing most r a p i d l y , they expel the g r e a t e s t number o f d i s p e r s e r s . In t h i s study, no c o r r e l a t i o n i s found between the r a t e of p o p u l a t i o n 56 growth and the number of d i s p e r s e r s . T h i s l a c k of a p o s i t i v e c o r r e l a t i o n was not dependent upon the grouping o f the time p e r i o d s . I f the data are grouped i n t o summer and winter p e r i o d s (Krebs e t a l . 1976) , the 1977 summer was c h a r a c t e r i z e d by the h i g h e s t d i s p e r s a l r a t e s i n the study (0.12 d i s p e r s e r s per 2 weeks per r e s i d e n t ) , but an average p o p u l a t i o n growth r a t e of -0.4% per week (Table 1.3). The 1976 f a l l p o p u l a t i o n s had the h i g h e s t growth r a t e s recorded i n the study (15% per week), but a d i s p e r s a l r a t e of 0.10 d i s p e r s e r s per two weeks per r e s i d e n t . D i s p e r s a l was not r e l a t e d to the r a t e of p o p u l a t i o n growth, but r a t h e r t o the season of the year. The l a c k of a p o s i t i v e c o r r e l a t i o n between p o p u l a t i o n growth r a t e and d i s p e r s a l r a t e i n t h i s study was because, u n l i k e r e s u l t s with the removal g r i d design, high d i s p e r s a l r a t e s were conc u r r e n t with a moderately d e c l i n i n g p o p u l a t i o n . Why, d u r i n g moderate s p r i n g d e c l i n e s , does the experimental design i n t h i s study r e v e a l d i s p e r s a l t h a t i s not detected by the r e m o v a l - g r i d design? High d i s p e r s e r m o r t a l i t y r a t e s , l a c k of a t t r a c t i o n to the removal g r i d or to l i v e - t r a p s on i t , or g r e a t e r e f f e c t i v e n e s s of p i t f a l l s than l i v e - t r a p s i n c a t c h i n g d i s p e r s e r s are p o s s i b l e e x p l a n a t i o n s . Male v o l e s u s u a l l y show p e r i o d s of low s p r i n g s u r v i v a l (Krebs e t a l . 1969, Krebs and Myers 1974). During a moderate s p r i n g d e c l i n e , an i n c r e a s e i n male d i s p e r s a l can account f o r t h i s lower observed s u r v i v a l . Female s u r v i v a l d i d not decrease as much as male s u r v i v a l d u r i n g the 1977 s p r i n g d e c l i n e , and t h i s r e s u l t may be p a r t i a l l y accounted f o r by there having been fewer female d i s p e r s e r s as compared with males. 57 L i d i c k e r (1962, 1975) suggests that vole p o p u l a t i o n s may e x h i b i t p r e s a t u r a t i o n d i s p e r s a l i n which i n d i v i d u a l s emigrate before the p o p u l a t i o n reaches maximum d e n s i t y . These i n d i v i d u a l s are not n e c e s s a r i l y s u b o r d i n a t e animals. F i g u r e 1.8 shows t h a t male d i s p e r s e r s d u r i n g the summer and f a l l o f 1976 tended to be l a r g e r than the average male r e s i d e n t v o l e . These animals d i s p e r s e d when d e n s i t i e s were low as compared with the 1976-77 f a l l d e n s i t y . Female d i s p e r s e r s , however, were s m a l l e r than female r e s i d e n t s d u r i n g the same time i n t e r v a l . Although the number of d i s p e r s e r s was s m a l l , the number of d i s p e r s e r s c o r r e c t e d f o r d e n s i t y i n 1976 was comparable with t h a t i n the summer and f a l l p e r i o d s of 1977. A f t e r the f a l l of 1976, d i s p e r s e r s tended to be l i g h t e r than r e s i d e n t s , which suggests t h a t these d i s p e r s e r s were sub o r d i n a t e animals. L i d i c k e r (1962) has suggested t h a t s m a l l mammal p o p u l a t i o n s may be r e g u l a t e d by an i n c r e a s e i n the p r o p o r t i o n emigrating as d e n s i t y i n c r e a s e s . one would expect a p o s i t i v e c o r r e l a t i o n between p r o p o r t i o n emigrating and p o p u l a t i o n d e n s i t y . Although more v o l e s do d i s p e r s e from dense p o p u l a t i o n s , t h i s study i n d i c a t e s t h a t t h e r e i s no c o r r e l a t i o n between the p r o p o r t i o n e m i g r a t i n g ( d i s p e r s e r s per two weeks per r e s i d e n t ) and p o p u l a t i o n d e n s i t y . An a n a l y s i s of v a r i a n c e i n d i c a t e d t h a t with seasonal e f f e c t s removed, th e r e was no e f f e c t of d e n s i t y on d i s p e r s a l r a t e . Thus a h i g h e r p r o p o r t i o n of v o l e s does not d i s p e r s e from a denser p o p u l a t i o n . The p r o p o r t i o n of d i s p e r s e r s was seasonally-dependent, not density-dependent., More males than females d i s p e r s e d , i n agreement with Hyers and Krebs (1971b) and Krebs et a l . (1976). T h i s l a r g e r 58 p r o p o r t i o n of male d i s p e r s e r s depressed the r e s i d e n t s ' sex r a t i o , e s p e c i a l l y d u r i n g the 1977 s p r i n g d e c l i n e . , However, about 35-40% of the d i s p e r s i n g v o l e s were females, and they c o n s t i t u t e d a s i z e a b l e p r o p o r t i o n of the d i s p e r s e r s . F a i r b a i r n (1977) found t h a t male Peromyscus d i s p e r s e d i n the s p r i n g but females a p p a r e n t l y d i e d . Tamarin (1977a) found t h a t i n Microtus p e n n s y l v a n i c u s there was an excess of male d i s p e r s e r s i n the winter and an excess of female d i s p e r s e r s i n the summer. The attainment of sexual maturity appears t o be l i n k e d with d i s p e r s a l tendency i n subadult v o l e s . During a l l seasons i n which some su b a d u l t s d i s p e r s e d , a higher p r o p o r t i o n of d i s p e r s i n g than r e s i d e n t subadult v o l e s was i n r e p r o d u c t i v e c o n d i t i o n . Sexual maturation of the s u b a d u l t s probably g i v e s r i s e to a g g r e s s i v e i n t e r a c t i o n s with dominant voles and the e x p u l s i o n of s u b a d u l t s . Myers and Krebs (1971b), Myers (1974), and Krebs e t a l . (1976) found an i n c r e a s e d p r o p o r t i o n of s e x u a l l y mature d i s p e r s i n g s u b a d u l t females but were unable to show c l e a r l y the same e f f e c t f o r subadult males. However, because they used a removal p l o t experimental design, they could not determine whether some , s u b a d u l t s became s e x u a l l y mature bef o r e d i s p e r s a l or whether the low d e n s i t y on the removal area g u i c k l y s t i m u l a t e d r a p i d r e p r o d u c t i v e maturation. My r e s u l t s show t h a t some s u b a d u l t s become r e p r o d u c t i v e l y mature before they d i s p e r s e . However, not a l l of the d i s p e r s i n g s u b a d u l t s were s e x u a l l y mature. High d e n s i t i e s d u r i n g 1977 i n c r e a s e d the median weight a t s e x u a l maturity, and t h i s may have been a c o n t r i b u t i n g f a c t o r t o lowering the p r o p o r t i o n of s e x u a l l y mature d i s p e r s i n g s u b a d u l t s . 59 Tamarin (1977) suggests t h a t the higher the c o r r e l a t i o n between d e n s i t y and d i s p e r s a l r a t e , the more l i k e l y i t i s to be s a t u r a t i o n d i s p e r s a l . Tamarin views s a t u r a t i o n d i s p e r s a l as a random n o n - s e l e c t i v e process, and p r e s a t u r a t i o n d i s p e r s e r s are presumed to be d i f f e r e n t i n q u a l i t y from the r e s i d e n t p o p u l a t i o n . The r e s u l t s of t h i s study i n d i c a t e t h a t d i s p e r s i n g v o l e s d i f f e r i n q u a l i t y from r e s i d e n t v o l e s d u r i n g a l l seasons and at d i f f e r i n g d e n s i t i e s . For example, the sex r a t i o s and age d i s t r i b u t i o n s of d i s p e r s e r s are not a random sample of the r e s i d e n t p o p u l a t i o n f o r most seasons, because d i s p e r s e r s are more o f t e n i n r e p r o d u c t i v e c o n d i t i o n than are r e s i d e n t s . C l e a r l y , the d i s p e r s e r s i n t h i s study cannot a l l be regarded as p r e s a t u r a t i o n d i s p e r s e r s ; t h e r e f o r e d i s p e r s a l o c c u r r i n g at peak d e n s i t i e s , which must by d e f i n i t i o n be s a t u r a t i o n d i s p e r s a l , can be a s e l e c t i v e process, because th e d i s p e r s e r s are not a random sample o f the p o p u l a t i o n . Tamarin (1978) a l s o suggests t h a t v o l e c y c l e s are the r e s u l t of d i s p e r s a l d u r i n g severe d e c l i n e s , and t h a t p r e d a t i o n r e s u l t s i n a high m o r t a l i t y among the d i s p e r s e r s . T h i s study does not support t h i s view f o r two reasons. There was e s s e n t i a l l y no d i s p e r s a l during the severe 1978 s p r i n g d e c l i n e . Thus t h e r e were no d i s p e r s i n g animals upon which pre d a t o r s c o u l d a c t . In a d d i t i o n , p r e d a t i o n was l i g h t during the l a t e winter and s p r i n g of 1978 (see p r e d a t i o n s e c t i o n ) and cannot account f o r the a c c e l e r a t e d l o s s d u r i n g the 1978 breeding season. C l e a r l y , n e i t h e r d i s p e r s a l , nor p r e d a t i o n , nor any of t h e i r i n t e r a c t i o n s are necessary c o n d i t i o n s f o r severe p o p u l a t i o n d e c l i n e s . 60 H i l b o r n and Krebs (1976), i n a study of Microtus to w n s e n d i i . found no d i s p e r s a l i n a moderate d e c l i n e and presumed t h a t d i s p e r s a l o c c u r r e d d u r i n g a severe d e c l i n e . D i s p e r s a l was measured by the presence or disappearance of r a d i o a c t i v e l y - t a g g e d v o l e s . I found e x a c t l y the opposite i n the fenced p o p u l a t i o n s . The d i s c r e p a n c y between the r e s u l t s of t h i s study and those of H i l b o r n and Krebs i s d i f f i c u l t t o e x p l a i n . Krebs and Boonstra (1978) suggest t h a t the r e s u l t s of H i l b o r n and Krebs (1976) may be an area e f f e c t not g e n e r a l l y v a l i d f o r Microtus t o w n s e n d i i . I have demonstrated t h a t d i s p e r s a l i s a s u f f i c i e n t c o n d i t i o n f o r a moderate p o p u l a t i o n d e c l i n e i n v o l e s , but i s not necessary f o r a severe d e c l i n e . However, d i s p e r s a l i s common i n the s p r i n g t o f a l l p e r i o d and may c o n s t i t u t e an important s e l e c t i v e f o r c e on the p o p u l a t i o n . Turner and Iverson (1973) found t h a t i n Microtus pennsylyanicus p o p u l a t i o n s . l a r g e males a r e dominant over s m a l l males. D i s p e r s i n g voles tend to be s m a l l e r than r e s i d e n t v o l e s and presumably are l e s s a g g r e s s i v e animals. The c o n s i s t e n t d i s p e r s a l of l e s s a g g r e s s i v e animals should i n c r e a s e the p r o p o r t i o n of a g g r e s s i v e animals which a f f e c t p o p u l a t i o n processes ( C h i t t y 1967, 1970). The d e c l i n e s from November 1977 u n t i l breeding began i n March 1978 were c h a r a c t e r i z e d by no wounding and no d i s p e r s a l , and the subsequent severe breeding season d e c l i n e s had l i t t l e wounding and almost no d i s p e r s a l . There was thus no evidence f o r i n c r e a s e d a g g r e s s i v e i n t e r a c t i o n s between i n d i v i d u a l s i n these d e c l i n e s as p o s t u l a t e d by C h i t t y (1967, 1970). D i s p e r s a l may stop vole p o p u l a t i o n s from i n c r e a s i n g i n d e f i n i t e l y , but i t 61 cannot account f o r the subsequent d e c l i n e . T h i s study suggests some areas that should be explo r e d f u r t h e r , because the f a c t o r s i n d u c i n g d i s p e r s a l should be examined. D i s p e r s a l and a g g r e s s i o n may be l i n k e d , and i f ag g r e s s i o n and hormone l e v e l s are l i n k e d (Beeman, 1947), a l t e r a t i o n o f hormone l e v e l s should i n f l u e n c e d i s p e r s a l r a t e s (Krebs e t a l . , 1977). Removal of l a r g e , dominant i n d i v i d u a l s d u r i n g a moderate s p r i n g d e c l i n e should decrease d i s p e r s a l r a t e s . Winter d i s p e r s a l r a t e s should i n c r e a s e i f breeding can be induced d u r i n g t h a t time. L i t e r a t u r e c i t e d Anderson, P. K. 1970. E c o l o g i c a l s t r u c t u r e and gene flow i n s m a l l mammals. Symp. Z o o l . Soc. Lond. 26: 299-325. Beeman, E. A. 1947. The e f f e c t of male hormone on a g g r e s s i v e behavior of mice. P h y s i o l . Zool. 20: 373-405. B l a i r , W. F. 1953. Population dynamics of rodents and other s m a l l mammals. Adv. Genet. 5: 1-41. Boonstra, R. And C. J . Krebs. 1978. P i t f a l l t r a p p i n g of Microtus townsendii. J . Mammal. 59: 136-148. C h i t t y , D. 1960. Po p u l a t i o n processes i n the vole and t h e i r r e l e v a n c e t o g e n e r a l theory. Can. J . Z o o l . 38: 99-113. , 19 67. The n a t u r a l s e l e c t i o n of s e l f - r e g u l a t o r y behaviour i n animal p o p u l a t i o n s . Proc. E c o l . . Soc. Aust. 2: 51-78. , 1970. V a r i a t i o n and p o p u l a t i o n d e n s i t y . Symp. Z o o l . 6 2 Soc- Lond. 2 6 : 3 2 7 - 3 3 3 . C h i t t y , D. And E. Phipps. 1 9 6 6 . Seasonal changes i n s u r v i v a l i n mixed p o p u l a t i o n s of two s p e c i e s of v o l e . J . Anim. E c o l . 3 5 : 3 1 3 r 3 3 1 . C h r i s t i a n , J . J . 1 9 7 0 . S o c i a l s u b o r d i n a t i o n , p o p u l a t i o n d e n s i t y , and mammalian e v o l u t i o n . Science, 1 4 6 : 1 5 5 0 - 1 5 6 0 . F a i r b a i r n , D. J . 1 9 7 7 . The s p r i n g d e c l i n e i n deer mice: death or d i s p e r s a l ? Can. J . Z o o l . 5 5 : 8 4 - 9 2 . , • , 1 9 7 8 . D i s p e r s a l of deer mice, Peroayscus manicnlatus. Proximal causes and e f f e c t s on f i t n e s s . O e c o l o g i a , 3 2 : 1 7 1 - 1 9 3 . Healey, M. C. 1 9 6 7 . Aggression and s e l f - r e g u l a t i o n of p o p u l a t i o n s i z e i n deermice. Ecology, 4 8 : 3 7 7 - 3 9 2 . H i l b o r n , R. and C. J . Krebs. 1 9 7 6 . Fates o f d i s a p p e a r i n g i n d i v i d u a l s i n f l u c t u a t i n g p o p u l a t i o n s of Mierotus townsendii. Can. J . Z o o l . 5 4 : 1 5 0 7 - 1 5 1 8 . H i l b o r n R., J . A. R e d f i e l d , and C. J . Krebs., 1 9 7 6 . On the r e l i a b i l i t y o f enumeration f o r mark and r e c a p t u r e census of v o l e s . Can. J . Z o o l . 5 4 : 1 0 1 9 - 1 0 2 4 . Howard, W . E. 1 9 4 9 . D i s p e r s a l , amount of i n b r e e d i n g , and l o n g e v i t y i n a l o c a l p o p u l a t i o n of p r a i r i e deermice on the George Reserve, Southern Michigan. Univ. 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Krebs, C. J . , I . Wingate, J . LeDuc, J . B e d f i e l d , M. T a i t t , and B. H i l b o r n . 1976. Mierotus p o p u l a t i o n b i o l o g y : D i s p e r s a l i n f l u c t u a t i n g p o p u l a t i o n s of M. townsendii. Can. J . Z o o l . 54: 79-95. Krebs, C. J . , Z. T. H a l p i n , and J . N. M. Smith. 1977. Ag g r e s s i o n , t e s t o s t e r o n e , and the s p r i n g d e c l i n e i n p o p u l a t i o n s of the v o l e Mierotus t o w n s e n d i i . Can. J . Z o o l . 55: 430-437. Krebs, C. J . and R. Boonstra. 1978. Demography of the s p r i n g d e c l i n e i n p o p u l a t i o n s of the vole Mierotus townsendii. J . Anim. E c o l . 47: 1007-1015. L e s l i e , P. H., J . S. Perry, and J . S. Watson. 1945. The det e r m i n a t i o n of the median body-weight a t which female r a t s reach maturity. Proc. Z o o l . Soc. Lond. 115: 473-488. L i d i c k e r , W. Z. 1962. E m i g r a t i o n as a p o s s i b l e mechanism p e r m i t t i n g the r e g u l a t i o n of p o p u l a t i o n d e n s i t y below c a r r y i n g c a p a c i t y . Amer. Nat: 29-33. 64 , 1975. The r o l e of d i s p e r s a l i n the demography of s m a l l mammals. Pp. 103-134. In F. B. G o l l e y , K. Petrusewicz, and L. Byszkowski (eds.) Small mammals: t h e i r p r o d u c t i v i t y and p o p u l a t i o n dynamics. I n t . B i o l . Progr., V o l . 5. Cambridge Univ. P r e s s , Great B r i t a i n . Murray, B. G. 1967. D i s p e r s a l i n v e r t e b r a t e s . Ecology 48: 975-978. Myers, J . H. 1974. Genetic and s o c i a l s t r u c t u r e of f e r a l house mouse p o p u l a t i o n s on G r i z z l y I s l a n d , C a l i f o r n i a . Ecology 55: 747-759. Myers, J . H. and C. J . Krebs. 1971a. Sex r a t i o s i n open and enclosed vole p o p u l a t i o n s : Demographic i m p l i c a t i o n s . Amer. Nat. 105: 325-344. Myers, J . B. and C. J . Krebs. 1971b. G e n e t i c , b e h a v i o r a l and r e p r o d u c t i v e a t t r i b u t e s of d i s p e r s i n g f i e l d v o l e s M i c r o t u s pennsylvanicus and Microtus pchroqaster. E c o l . Monogr. 44: 53-78. Pearson, 0. P. 1963. H i s t o r y o f two l o c a l outbreaks of f e r a l house mice. Ecology 44: 540-549. S a d l e i r , R. M. F. S. 1965. The r e l a t i o n s h i p between a g o n i s t i c behaviour and p o p u l a t i o n changes i n the deer mouse, Peromyscus maniculatus (Wagner). J. Anim. E c o l . 14: 331-352. Smith, M. H. 1968. D i s p e r s a l of the o l d - f i e l d mouse, Peromyscus p o l i o n o t u s . B u l l . Georgia Acad. S c i . 26: 45-51. Tamarin* R. H. 1977. D i s p e r s a l i n i s l a n d and mainland v o l e s . Ecology, 58: 1044-1054. 65 , 1978. D i s p e r s a l , p o p u l a t i o n r e g u l a t i o n , and K - s e l e c t i o n i n f i e l d mice. Amer. Nat. 112: 545-555. Turner, B. N a n d S. L. I v e r s o n . 1773. The annual c y c l e o f a g g r e s s i o n i n male Microtus pennsylyanicus. and i t s r e l a t i o n to p o p u l a t i o n parameters. Ecology, 54: 967-981, 66 SECTION 2. DISPERSAL TENDENCY AND DURATION OF LIFE OF LITTERMATES DURING POPULATION FLUCTUATIONS OF THE VOLE HICROTUS TOWNSENDII I n t r o d u c t i o n M i c r o t i n e rodent p o p u l a t i o n s f l u c t u a t e p e r i o d i c a l l y , and Krebs e t a l - (1973) have suggested t h a t d i s p e r s a l i s necessary f o r normal p o p u l a t i o n r e g u l a t i o n i n v o l e s . D i s p e r s i n g v o l e s are not a random subsample of the p o p u l a t i o n (Myers and Krebs, 1971; Krebs et a l . , 1976), and s e l e c t i o n through d i s p e r s a l may a l t e r the g e n e t i c composition, age and sex r a t i o s , and r e p r o d u c t i v e a t t r i b u t e s of a p o p u l a t i o n . Howard (1960) suggested t h a t there i s a polymorphic g e n e t i c b a s i s f o r d i s p e r s a l tendency w i t h i n a p o p u l a t i o n . I f tendency to d i s p e r s e and l i f e s p a n are under g e n e t i c c o n t r o l , then l i t t e r m a t e s should be more s i m i l a r i n these t r a i t s than i n d i v i d u a l s from d i f f e r e n t l i t t e r s . I f so, the v a r i a b i l i t y i n these t r a i t s should be g r e a t e r among l i t t e r s than w i t h i n l i t t e r s . In f a c t , H i l b o r n (1975) r e p o r t s t h a t d i s p e r s a l was non-random w i t h i n l i t t e r s f o r f o u r Microtus s p e c i e s d u r i n g p e r i o d s of p o p u l a t i o n i n c r e a s e . The present study examines whether or not l i f e s p a n and tendency t o d i s p e r s e are h e r i t a b l e c h a r a c t e r i s t i c s i n i n c r e a s i n g and peak M i c r o t u s townsendii p o p u l a t i o n s by comparing among and w i t h i n - l i t t e r v a r i a b i l i t y f o r each t r a i t . Methods 6 7 T h i s study was c a r r i e d out on R e i f e l I s l a n d i n the F r a s e r R i v e r d e l t a near Vancouver, B r i t i s h Columbia. The study area was part of a Timothy hay f i e l d owned by the Canadian W i l d l i f e S e r v i c e . The f i e l d was c u t i n the 1975 f a l l and grazed by waterfowl d u r i n g the f o l l o w i n g winter so t h a t both cover and M. townsendii were s c a r c e when t r a p p i n g began i n May 1976. I s e t up two e n c l o s u r e s and two unfenced g r i d s i n May 1976 and trapped them u n t i l June 1978. An area i n s i d e each e n c l o s u r e was mowed every two weeks d u r i n g the growing season i n order t o c r e a t e a h a b i t a t unfavourable f o r v o l e s . The mowed area was 18.3 m (60 f t ) wide and 53.3 m (175 f t ) long and was adjacent to each t r a p p i n g g r i d . Each g r i d had 49 t r a p - s i t e s s e t 7.6 m (25 f t ) a p a r t , i n a 7 x 7 p a t t e r n . At each t r a p - s i t e were two Longworth l i v e - t r a p s and one p i t f a l l t r a p s i m i l a r t o the type d e s c r i b e d by Boonstra and Krebs (1978). Longworth l i v e - t r a p s and p i t f a l l s were b a i t e d with o a t s , and provided with c o t t o n bedding. A l l g r i d s were trapped with Longworth l i v e - t r a p s f o r two days every two weeks, and with p i t f a l l s f o r two days every week. P i t f a l l s were used from May through October 1976, A p r i l through September 1977, and A p r i l through June 1978. During the i n t e r v a l s between these p e r i o d s , the p i t f a l l s c o u l d not be used because of winter f l o o d i n g . Each v o l e was ear-tagged on f i r s t c a p t u r e , and i t s weight, capture l o c a t i o n , sex, r e p r o d u c t i v e c o n d i t i o n , wounding on rump, and r e l a t i v e s i z e of h i p glands were recorded. A l l animals were r e l e a s e d immediately a f t e r being processed. On each g r i d I placed t r a p s t o c a t c h v o l e s t h a t c r o s s e d the 6 8 18.3 m width of the mowed area. F i v e - g a l l o n p i t f a l l t r a p s were i n s t a l l e d at 8.9 m (29ft) i n t e r v a l s along the fence b o r d e r i n g the mowed a r e a . One-gallon cans f i t t e d with Longworth tunne l s were s e t along the same fence half-way between p i t f a l l s . There were two l i v e - t r a p s per s i t e i n the e n c l o s u r e s , t o t a l l i n g 12 per e n c l o s u r e , and one per s i t e on the c o n t r o l s . The l a r g e p i t f a l l s and l i v e - t r a p s , b a i t e d with oats and s u p p l i e d with c o t t o n , remained c o n t i n u a l l y s e t , except when r a i n prevented the use of p i t f a l l s from November to February. A l l t r a p s were checked on each v i s i t . Voles c r o s s i n g the e n t i r e width of the mowed area and e n t e r i n g these t r a p s were c o n s i d e r e d to be d i s p e r s e r s and were removed from the p o p u l a t i o n s . L i t t e r m a t e s were d e f i n e d as new v o l e s under 40 g i n weight caught i n the same p i t f a l l on the t r a p p i n g g r i d s and i n the same t r a p p i n g check and having a maximum weight range of 4 g from the he a v i e s t to the l i g h t e s t . Pearson (1960) suggested t h a t runway systems were occupied by d i s c r e t e Microtus population u n i t s , with f a m i l i e s the most common u n i t . Thus new small Microtus caught i n the same p i t f a l l at the same time should be l i t t e r m a t e s . The l i f e t i m e of each vole i s d e f i n e d as the number of weeks between i t s f i r s t and l a s t c a p t u r e . Any vo l e a c c i d e n t a l l y k i l l e d was e l i m i n a t e d from the a n a l y s i s along with i t s l i t t e r m a t e s . I examined tendency t o d i s p e r s e by c o n s t r u c t i n g a dummy v a r i a b l e f o r each i n d i v i d u a l i n a l i t t e r and g i v i n g i t a value of two i f the animal d i s p e r s e d between the time of f i r s t c apture and the s t a r t o f next year's breeding season and one i f i t remained. I c o n s i d e r e d only l i t t e r s with two or more d i s p e r s i n g 6 9 s i b l i n g s to determine whether l i t t e r m a t e s d i s p e r s e d at the same time. Non-dispersing members of these l i t t e r s were excluded from the a n a l y s i s . For each i n d i v i d u a l , I recorded the g r i d , week, l i t t e r number with i n week, l i f e t i m e , f i r s t capture weight, r e l a t i v e growth r a t e , and d i s p e r s a l . Each p o p u l a t i o n phase was analyzed s e p a r a t e l y because, a c c o r d i n g t o the c l a s s i f i c a t i o n used by Krebs e t a l . (1973), s p r i n g 1976 t o s p r i n g 1977 was an i n c r e a s e p e r i o d (r>0.03/wk), s p r i n g t o f a l l o f 1977 a peak p e r i o d (-0.02<r<0. 03) , and f a l l 1977 t o summer 1978 a d e c l i n e p e r i o d (r<-0.02). Only one l i t t e r was caught d u r i n g the d e c l i n e so no r e s u l t s f o r the d e c l i n e are presented. I used an a n a l y s i s of v a r i a n c e t o separate v a r i a t i o n i n l i f e t i m e due t o area and week of c a p t u r e , from w i t h i n - and a m o n g - l i t t e r v a r i a b i l i t y . I used a p a r t i a l l y - n e s t e d two-way a n a l y s i s of v a r i a n c e to examine f o u r sources of v a r i a t i o n i n a d d i t i o n t o the w i t h i n - l i t t e r v a r i a b i l i t y . The g r i d source of v a r i a t i o n removed v a r i a b i l i t y r e s u l t i n g from l i t t e r s on one g r i d l i v i n g l o n g e r than l i t t e r s on another. The weeks source of v a r i a t i o n removed v a r i a b i l i t y r e s u l t i n g from d i f f e r e n c e s i n the time br e e d i n g b e g i n s . Thus, v o l e s born e a r l y i n an i n c r e a s i n g p o p u l a t i o n may l i v e l o n g e r than v o l e s born i n a peak or d e c l i n i n g p o p u l a t i o n . The i n t e r a c t i o n term removed v a r i a b i l i t y p e c u l i a r t o any p a r t i c u l a r combination of g r i d and week. The f o u r t h source o f v a r i a t i o n r e s u l t e d from a m o n g - l i t t e r d i f f e r e n c e s . The design allowed the p a r t i t i o n i n g of weeks sum of squares i n t o v a r i a t i o n r e s u l t i n g from d i f f e r e n t f i r s t - c a p t u r e weeks and v a r i a t i o n r e s u l t i n g from a grid-week i n t e r a c t i o n . 70 However, the design was unbalanced, as the weeks i n which l i t t e r s were f i r s t captured were not the same on each p a i r o f g r i d s . Sometimes t h i s unbalanced c o n d i t i o n caused the degrees of freedom f o r the grid-week i n t e r a c t i o n to be i n s u f f i c i e n t to giv e a r e l i a b l e estimate of the i n t e r a c t i o n mean square. In such cases I used a t o t a l l y nested a n a l y s i s of v a r i a n c e , which has the disadvantage, however, of i n c o r p o r a t i n g i n t e r a c t i o n v a r i a t i o n i n t o the weeks v a r i a t i o n . I f the average l i f e s p a n o f a l l l i t t e r s i s the same, then the mean square f o r l i t t e r s should equal the ' e r r o r ' or w i t h i n - l i t t e r mean square. A l a r g e r l i t t e r mean square i n d i c a t e s d i f f e r e n c e s i n average l i t t e r l i f e . G r i d s , weeks, and l i t t e r s a re c o n s i d e r e d random e f f e c t s i n the s t a t i s t i c a l models used. Thus, l i t t e r mean square d i v i d e d by e r r o r mean square t e s t s s i g n i f i c a n c e o f l i t t e r v a r i a t i o n , i n t e r a c t i o n mean square over l i t t e r mean square t e s t s the s i g n i f i c a n c e of i n t e r a c t i o n v a r i a t i o n , and the i n t e r a c t i o n mean square i s t h e d i v i s o r f o r t e s t s o f s i g n i f i c a n c e o f weeks and g r i d s v a r i a t i o n . R e s u l t s P o p u l a t i o n Density And D i s p e r s a l The Mierotus townsendii p o p u l a t i o n s were low when t r a p p i n g began i n May 1976 and remained low throughout the summer ( F i g . 1.4). They i n c r e a s e d at about 9% per week d u r i n g the f a l l and winter, owing to delayed r e c r u i t m e n t t o l i v e - t r a p s (see t r a p p i n g 71 s e c t i o n ) . A s p r i n g d e c l i n e s t a r t e d i n March 1977 with the onset of breeding, and oc c u r r e d i n a l l f o u r p o p u l a t i o n s , with males s u s t a i n i n g g r e a t e r l o s s e s than females. A summer recovery i n numbers i n c r e a s e d the p o p u l a t i o n s i z e s by October 1977 t o l e v e l s that occurred i n March. A f t e r October, both males and females d e c l i n e d at a r a t e of about 5% per week throughout the f a l l and winter. A f t e r breeding s t a r t e d i n March 1978, the r a t e o f d e c l i n e i n c r e a s e d t o about 50% per week f o r males and 18% per week f o r females, and the p o p u l a t i o n s d e c l i n e d to very low l e v e l s d u r i n g the s p r i n g and e a r l y summer of 1978. There were f o u r p e r i o d s o f d i s p e r s a l during the study. The f i r s t o c c u r r e d i n the 1976 f a l l with both s u b a d u l t s and a d u l t s d i s p e r s i n g ( F i g . 1.4). D i s p e r s a l was low d u r i n g the 1976 winter but i n c r e a s e d d r a m a t i c a l l y owing t o a d u l t d i s p e r s a l a f t e r the 1977 breeding season began. Subadult d i s p e r s a l i n c r e a s e d i n June and J u l y 1977, d e c l i n e d through August, and i n c r e a s e d i n l a t e September and October 1977, and a d u l t d i s p e r s a l a l s o i n c r e a s e d i n September and October. There was l i t t l e d i s p e r s a l during the 1977 winter and e s s e n t i a l l y no d i s p e r s a l d u r i n g the severe 1978 s p r i n g d e c l i n e . . L i t t e r m a t e L i f e s p a n The observed l i t t e r s i z e s i n each year f o r v o l e s i n the experimental and c o n t r o l p o p u l a t i o n s are l i s t e d i n Table 2.1. The average l i t t e r s i z e i s f i v e i n Microtus townsendii (Boonstra and Krebs, 1978), so t h a t most l i t t e r s of s i z e two or t h r e e are a subsample of the l i t t e r s born. The a n a l y s e s of v a r i a n c e 72 Table 2.1. L i t t e r s i z e s from the c o n t r o l and experimental p o p u l a t i o n s i n 1976 and 1977. L i t t e r m a t e s were d e f i n e d as new v o l e s under 40 g i n weight caught i n the same p i t f a l l i n the same t r a p p i n g check and having a maximum weight range of 4 g from the h e a v i e s t t o the l i g h t e s t . r — — T !— T T ., , | L i t t e r | Experimental g r i d s I C o n t r o l g r i d s I I s i z e | 1976 I t 1977 1976 r 1977 | I 2 i 72 135 | 28 | 77 | i 3 | 20 41 | 7 | 32 | | 4 | 10 | 13 j 1 | 10 | I 5 i 3 | 7 | 1 ) 4 I | 6 | 1 1 | 0 | 1 | | 7 | 1 1 0 1 I |Hean+1SE| 2.54 + 0. 091 2.49+0.08 I 2. 32+0.11 | 2 .58+0. 111 i i i . . — i — — L - _ , i 73 i n d i c a t e t h a t f o r both the c o n t r o l and experimental p o p u l a t i o n s i n both y e a r s , l i t t e r m a t e s tended t o have l i f e s p a n s more s i m i l a r than expected by chance (Table 2.2). The s i g n i f i c a n t F - r a t i o s f o r l i t t e r s were not a r e s u l t of l a r g e r animals at f i r s t capture l i v i n g l o n g e r , because t h e r e was no r e l a t i o n between weight at f i r s t c a p ture and l i f e t i m e . The weeks source of v a r i a t i o n f o r both the c o n t r o l and experimental g r i d s was s i g n i f i c a n t i n 1977 but not i n 1976. In 1976 the average l i t t e r m a t e l i f e t i m e was 21 weeks, whereas i n 1977 i t was 11 weeks. The grid-week i n t e r a c t i o n was not s i g n i f i c a n t f o r any g r i d i n any year. The g r i d source of v a r i a t i o n was always i n s i g n i f i c a n t , which i n d i c a t e d t h a t the average l i f e s p a n of l i t t e r s was the same on both p a i r s of g r i d s . H i l b o r n (1975) a t t r i b u t e d s i g n i f i c a n t F - r a t i o s i n the a m o n g - l i t t e r t e s t s to non-random d i s p e r s a l among l i t t e r s . I was a b l e t o examine i f my s i g n i f i c a n t r e s u l t s were e n t i r e l y a t t r i b u t a b l e to d i s p e r s a l . By removing known d i s p e r s e r s and l i t t e r s whose r e s u l t a n t s i z e was then one, I was able t o t e s t the h y p othesis t h a t , i n n o n - d i s p e r s i n g l i t t e r s , there was a random assortment of l i f e s p a n s among s i b l i n g s . I c o u l d t e s t t h i s h y p o t h e s i s on l i t t e r s born i n the experimental g r i d s o n l y by assuming that a l l d i s p e r s e r s were caught. T h i s was c l e a r l y not the case f o r the c o n t r o l g r i d s . The a n a l y s e s of v a r i a n c e i n d i c a t e t h a t l i f e s p a n was non-random w i t h i n l i t t e r s f o r both 1976 and 1977 on the fenced g r i d s (Table 2.3)- Some l i t t e r s s u r v i v e d b e t t e r than o t h e r s even when d i s p e r s a l l o s s e s were e l i m i n a t e d . Thus, d i s p e r s a l was not the only f a c t o r a c t i n g t o produce d i f f e r e n t i a l l i f e s p a n s 74 Table 2.2. A n a l y s i s of variance t a b l e s t o t e s t non-random l i f e t i m e s among Microtus townsendii l i t t e r s . L i f e t i m e was d e f i n e d as the time between f i r s t and l a s t c a p t u r e s . Sum of Mean Source df Sguares Square F P C o n t r o l g r i d s 1976 Gr i d s 1 45.3 45.3 0. 10 0.7587 Weeks 12 4498. 1 374.8 0.86 0.6364 GXW 3 1311.8 437.3 0.92 0.4527 L i t t e r s 20 9543.4 477.2 2.71 0.0024** E r r o r 49 8638.8 176.3 T o t a l 85 24037.4 C o n t r o l g r i d s 1977 G r i d s 1 136.9 136.9 0.60 0.4573 Weeks 18 8713.2 484.1 2. 11 0.0396* GXW 14 3198.4 228.5 1.04 0.4 266 L i t t e r s 91 20077.7 220.6 1.46 0.0154* E r r o r 198 30005.4 151.5 T o t a l 322 62131.6 Experimental g r i d s 1976 G r i d s 1 0.6 0.6 0.00 0.9 165 Weeks 15 10759.5 717.3 2. 55 0.0619 GXW 11 3100.3 281.9 0.54 0.8679 L i t t e r s 79 4 0946.9 518; 3 1.79 0.0009*** E r r o r 164 4 7563.6 288.2 T o t a l 271 102370.9 Experimental g r i d s 1977 G r i d s 1 43. 1 43.1 0.25 0.6314 Weeks 17 14084.8 828.5 4.74 0.0022** GXW 15 2622.8 174.8 0.68 0.7978 L i t t e r s 164 41887.5 255.4 1.83 0.0000*** E r r o r 295 41071.3 139.2 T o t a l 492 99709.5 * p<.05 ** p<.01 *** p<.001 7 5 Table 2.3. A n a l y s i s of v a r i a n c e t a b l e s t o t e s t non-random l i f e t i m e s w i t h i n l i t t e r s with d i s p e r s e r s removed. A l l r e s u l t a n t l i t t e r s of s i z e 1 .were .eliminated. These data are thus a subset of those i n Table 2. 2 Sum o f Mean Source df Squares Square F P Experimental g r i d s 1976 G r i d s 1 69.6 69.6 0. 24 0.6381 weeks 14 6764.4 483.2 1. 66 0.2010 GXW 11 3200.0 290.9 0. 54 0.8672 L i t t e r s 68 364 20.3 535.6 1. 71 0.0036** E r r o r 143 44591.7 311.8 T o t a l 237 91046.0 Experimental g r i d s 1977 G r i d s 1 323.3 323.3 1. 33 0.2694 Weeks 15 11400.2 760.0 3. 12 0.0 231* GXW 13 3161.2 243.2 0. 99 0.4672 L i t t e r s 113 27792.8 246.0 1. 35 0.0336* E r r o r 199 36289.9 182.4 T o t a l 341 78967.4 * P<.05 ** P<.01 76 among l i t t e r s . The weeks source of v a r i a t i o n was again s i g n i f i c a n t i n 1977 but not i n 1976. L i t t e r m a t e D i s p e r s a l Tendency I examined non-random d i s p e r s a l f o r l i t t e r s born i n the fenced p o p u l a t i o n s . The analyses of v a r i a n c e i n d i c a t e d t h a t d i s p e r s a l tendency was non-random among l i t t e r s i n 1977, but not i n 1976 (Table 2.4). The v a r i a t i o n among weeks was s i g n i f i c a n t i n both y e a r s . New animals f i r s t caught i n the s p r i n g and summer were more prone to d i s p e r s e before the next breeding season than were new animals caught i n the l a t e summer and f a l l (see d i s p e r s a l s e c t i o n ) . The g r i d s source o f v a r i a t i o n was s i g n i f i c a n t i n each yea r , which i n d i c a t e s t h a t more v o l e s d i s p e r s e d from g r i d B than g r i d C (see d i s p e r s a l s e c t i o n ) . One problem with t h i s a n a l y s i s i s that the d i s p e r s a l v a r i a b l e was not normally d i s t r i b u t e d as assumed by an a n a l y s i s of v a r i a n c e . However, non-normality would b i a s only the p r o b a b i l i t y l e v e l s . The l i t t e r mean square was s t i l l 1.68 times l a r g e r than the e r r o r mean square. Given the robustness of a n a l y s i s of v a r i a n c e t o non-normality and the unusually low p r o b a b i l i t y l e v e l s , the 1977 data suggest t h a t tendency to d i s p e r s e was non-randomly d i s t r i b u t e d among l i t t e r s . Given t h a t tendency t o d i s p e r s e was non-random among l i t t e r s , d i d s i b l i n g s d i s p e r s e at the same time? One way to answer t h i s q u e s t i o n was to examine the d u r a t i o n of l i f e of d i s p e r s i n g s i b l i n g s . I f they do d i s p e r s e at the same time, then a m o n g - l i t t e r v a r i a b i l i t y i n d u r a t i o n of l i f e should be l a r g e r 77 Table 2.4. analyses of v a r i a n c e t a b l e s f o r d i s p e r s a l tendency w i t h i n l i t t e r s born i n experimental p o p u l a t i o n s d u r i n g 1976 and 1977. D i s p e r s a l tendency i s measured by a v o l e t s w i l l i n g n e s s to c r o s s unfavourable h a b i t a t . Source df Sum of Squares Mean Square F P Experimental g r i d s 1976 G r i d s 1 0. 197 0. 197 7. 72 0.0173* Weeks 15 2.931 0. 195 7. 64 0.0010** GXW 11 0.281 0.003 0. 33 0.976 3 L i t t e r s 79 6. 129 0.078 1. 11 0.2838 E r r o r 165 11.517 0.070 T o t a l 271 21.055 Experimental g r i d s 1977 G r i d s 1 1.541 1.541 8. 68 0.0097** Weeks 17 14.715 0.866 4. 88 0.0019** GXW 15 2. 663 0. 178 0. 91 0.5587 L i t t e r s 164 32.123 0. 196 1. 68 0.0001*** E r r o r 295 34.414 0.117 T o t a l 492 85.456 * P<.05 ** P<.01 *** P<.001 78 than w i t h i n - l i t t e r v a r i a b i l i t y . I used a t r i p l y - n e s t e d a n a l y s i s of v a r i a n c e to i n v e s t i g a t e t h i s p o s s i b i l i t y because i n t e r a c t i o n degrees of freedom were i n s u f f i c i e n t t o give a r e l i a b l e e s timate f o r the i n t e r a c t i o n mean square. Among 1976 l i t t e r s few had two or more animals d i s p e r s i n g before the s t a r t of the 1977 breeding season. I was t h e r e f o r e unable t o make a meaningful a n a l y s i s f o r t hat year. Only the r e s u l t s f o r 1977 l i t t e r s are presented i n Table 2.5. These r e s u l t s i n d i c a t e t h a t when l i t t e r m a t e s d i s p e r s e d , they tended t o leav e a t the same time. The i n s i g n i f i c a n t weeks F - r a t i o i n d i c a t e s t h a t l i t t e r m a t e s d i s p e r s e d a t about the same age independent of t h e i r week of f i r s t c a p ture. Thus spri n g - b o r n and summer-born d i s p e r s e r s both l e f t the p o p u l a t i o n a t the same age. The average i n t e r v a l between f i r s t and l a s t c a p t u r e s f o r l i t t e r m a t e s born i n 1977 and d i s p e r s i n g before March 1978 was 2-5 weeks. I f f a l l - ^ b o r n animals d i s p e r s e d , they d i d so i n the f o l l o w i n g s p r i n g , and t h e i r observed l i f e t i m e was t h e r e f o r e g r e a t e r than s p r i n g - and summer-born v o l e s . The i n s i g n i f i c a n t g r i d s * F - r a t i o means t h a t the average age of d i s p e r s e r s was about the same on each g r i d . D i s c u s s i o n From these a n a l y s e s , we see t h a t , whether they d i d or d i d not d i s p e r s e , l i t t e r m a t e s tended to have s i m i l a r l i f e s p a n s i n both the i n c r e a s i n g and peak phases of p o p u l a t i o n growth; t h a t tendency to d i s p e r s e was non-random w i t h i n l i t t e r s i n the peak phase and l i k e l y non-random i n the i n c r e a s i n g phase; and th a t l i t t e r m a t e s d i s p e r s e d at about the same age. The average l i f e t i m e ( i n t e r v a l between f i r s t and l a s t 7 9 Table 2.5. A n a l y s i s of v a r i a n c e t a b l e t o t e s t whether d i s p e r s i n g M. townsendii s i b l i n g s have the same l i f e s p a n . So ur ce df Sum of Squares Mean Square F P Experimental g r i d s 1977 G r i d s 1 6.9 6.9 0. 55 0. 4764 Weeks 14 174.6 12.5 0. 53 0. 8709 L i t t e r s 11 260.4 23.7 13. 23 0. 0000*** E r r o r 41 73.3 1.8 T o t a l 67 515.2 *** P<.001 80 captures) of a l i t t e r decreased d u r i n g the p o p u l a t i o n c y c l e . There was no s i g n i f i c a n t weekly v a r i a t i o n i n the average l i f e t i m e of l i t t e r s born d u r i n g the 1976 i n c r e a s i n g phase breeding season. L i t t e r s born d u r i n g t h i s i n t e r v a l tended to have l o n g e r average l i f e t i m e s (21 weeks) than l i t t e r s born duri n g the 1977 peak phase breeding season (11 weeks). During t h i s peak phase there was a s i g n i f i c a n t weekly v a r i a t i o n i n average l i t t e r l i f e t i m e . L i t t e r s born near the beginning of the breeding season had a l o n g e r average l i f e t i m e (14 weeks) than l i t t e r s born near the end (7 weeks). The shortened l i f e t i m e of l a t e r born l i t t e r s i n 1977 can be accounted f o r by the t i m i n g of the d e c l i n e a f t e r the end of the breeding season. The d e c l i n e t h a t f o l l o w e d the 1976 breeding season di d not begin u n t i l the s p r i n g o f 1977, whereas the d e c l i n e f o l l o w i n g the 1977 breeding season began i n the same f a l l . Thus l i t t e r s born c l o s e to a time of i n c r e a s e d m o r t a l i t y have s h o r t e r l i f e s p a n s than l i t t e r s born e a r l i e r . D i s p e r s a l was only one of s e v e r a l f a c t o r s i n f l u e n c i n g the h e t e r o g e n e i t y i n average l i f e s p a n among l i t t e r s , as n o n - d i s p e r s i n g l i t t e r s s t i l l had d i f f e r e n t i a l l i f e t i m e s . H i l b o r n (1975) repo r t e d that l i f e t i m e was random among l i t t e r s i n unfenced i n c r e a s i n g p o p u l a t i o n s . H i l b o r n ' s method of l i t t e r c l a s s i f i c a t i o n was dependent upon s m a l l v o l e s being trapped i n Longworth l i v e - t r a p s . However, there i s evidence t h a t many Microtus townsendii a v o i d capture i n l i v e - t r a p s u n t i l adulthood (Boonstra and Krebs 1978, see t r a p p i n g s e c t i o n ) so that H i l b o r n may have analyzed p a r t i a l l i t t e r s . T h i s problem was present i n t h i s study, but by p i t f a l l t r a p p i n g I was a b l e to t r a p more 81 nearly-complete l i t t e r s at a younger age than i f only l i v e - t r a p s were used. M i s c l a s s i f i c a t i o n o f l i t t e r m a t e s would i n c r e a s e the v a r i a b i l i t y w i t h i n l i t t e r s ( e r r o r mean sq u a r e ) , which would decrease the chance of d e t e c t i n g d i f f e r e n c e s among l i t t e r s . Tendency t o d i s p e r s e was non-random among l i t t e r s i n 1977, but not i n 1976. The 1976 r e s u l t may be p a r t i a l l y e x p l a i n e d by low d e n s i t i e s d u r i n g the s p r i n g and e a r l y summer (Figure 1.4). I f v o l e s d i s p e r s e d from t h e i r b i r t h s i t e t o vacant areas on the same g r i d and remained there, they would have d i s p e r s e d but would not have been re c o g n i z e d as such by my methods. The 1976 d i s p e r s a l p a t t e r n a l s o l a c k e d the summer i n c r e a s e i n subadult and j u v e n i l e d i s p e r s a l observed i n 1977 (see d i s p e r s a l s e c t i o n ) . F a i l u r e to detect e a r l y d i s p e r s e r s i n 1976 may account f o r the random d i s t r i b u t i o n of d i s p e r s a l tendency among l i t t e r s i n 1976. The a n a l y s e s of va r i a n c e i n d i c a t e that d i s p e r s a l tendency was non-random w i t h i n l i t t e r s and d i s p e r s i n g l i t t e r m a t e s tended t o l e a v e the p o p u l a t i o n a t the same time. D i f f e r e n t i a t i o n i n l i t t e r d i s p e r s a l tendency supports Howard's (1960) hypothesis of a g e n e t i c b a s i s f o r d i s p e r s a l . Hyers and Krebs (1971) and Krebs e t a l . (1976) r e p o r t that d i s p e r s i n g v o l e s d i f f e r i n a l l e l i c f r e q u e n c i e s from r e s i d e n t v o l e s . T h i s r e s u l t i s c o n s i s t e n t with the idea t h a t t h e r e i s a g e n e t i c b a s i s f o r d i s p e r s a l tendency, but d i r e c t measurement of v a r i a b i l i t y i n l i t t e r d i s p e r s a l tendency p r o v i d e s a s t r o n g e r t e s t . The f a c t t h a t s p r i n g and summer-born d i s p e r s i n g l i t t e r s l e a v e a t the same age, independent of week of b i r t h , suggests t h a t d i s p e r s a l age may be g e n e t i c a l l y determined. A s i g n i f i c a n t weeks e f f e c t would mean that d i s p e r s i n g l i t t e r s from d i f f e r e n t 82 weeks d i s p e r s e at d i f f e r e n t ages, thus i m p l i c a t i n g environmental cues, as the environment changes d u r i n g the breeding season. For example, i t might be expected that summer-born d i s p e r s e r s remain i n the p o p u l a t i o n l o n g e r than spr i n g - b o r n d i s p e r s e r s i f a g g r e s s i v e behaviour d e c l i n e s d u r i n g the breeding season. The n o n s i g n i f i c a n t weeks r e s u l t does not r u l e out an environmental cue f o r d i s p e r s a l , but simply means t h a t any environmental cue a c t s on d i s p e r s i n g animals at the same age throughout the e n t i r e s p r i n g t o f a l l p e r i o d . I f young v o l e s a t t a i n s e x u a l m a t u r i t y at the same age throughout t h i s p e r i o d , a g g r e s s i v e i n t e r a c t i o n s between a d u l t s and these newly mature v o l e s may r e s u l t i n l i t t e r m a t e s d i s p e r s i n g a t the same age. Anderson (1975) found t h a t age at puberty was h i g h l y c o r r e l a t e d among l i t t e r m a t e s , as were growth r a t e s , so t h a t synchronous l i t t e r m a t e maturation c o u l d account f o r d i s p e r s a l at the same age. T h i s study i n d i c a t e s t h a t s p r i n g - and summer-born d i s p e r s e r s have an average l i f e t i m e of 2.5 weeks and t h e r e f o r e those i n d i v i d u a l s born i n the f a l l and d i s p e r s i n g the next s p r i n g must have g r e a t e r observed l i f e t i m e s . Howard (1949) and Anderson (1970) suggested t h a t s p r i n g - b o r n v o l e s d i s p e r s e d a t younger ages than autumn-born v o l e s , and the r e s u l t s of t h i s study support t h i s view. H i l b o r n (1975) found t h a t l i f e t i m e was non-random w i t h i n l i t t e r s d u r i n g p e r i o d s of p o p u l a t i o n i n c r e a s e , but random d u r i n g peak and d e c l i n e p e r i o d s . By assuming t h a t l i f e t i m e i s a measure of d i s p e r s a l tendency, he concluded that d i s p e r s a l was non-random w i t h i n l i t t e r s d uring i n c r e a s e p e r i o d s . The present study i n d i c a t e s a m o n g - l i t t e r v a r i a b i l i t y f o r both d i s p e r s a l 8 3 tendency and l i f e t i m e i n peak p o p u l a t i o n s . H i l b o r n l i n k e d l i f e t i m e and d i s p e r s a l tendency and suggested t h a t d i s p e r s a l tendency was h e r i t a b l e i n i n c r e a s i n g p o p u l a t i o n s but not i n peak or d e c l i n i n g ones. I t i s d i f f i c u l t t o see why h e r i t a b i l i t y o f l i f e t i m e s hould be c o n f i n e d to i n c r e a s i n g p o p u l a t i o n s because t h i s study shows t h a t l i f e t i m e i s a l s o h e r i t a b l e i n n o n - d i s p e r s i n g v o l e s . H i l b o r n suggested t h a t randomness of l i f e t i m e among l i t t e r s i n peak p o p u l a t i o n s was i n d i c a t i v e of l i t t l e d i s p e r s a l . The r e s u l t s of t h i s study suggest t h a t d i s p e r s a l i s an important component of p o p u l a t i o n s r e g u l a t i o n i n peak p o p u l a t i o n s . Fencing a peak p o p u l a t i o n should provide a t e s t of the r e g u l a t o r y a s p e c t s of d i s p e r s a l . I f d i s p e r s a l i s important i n p o p u l a t i o n r e g u l a t i o n a t peak d e n s i t i e s , then e n c l o s e d peak p o p u l a t i o n s should i n c r e a s e i n d e n s i t y f a r above n a t u r a l l e v e l s (Krebs e t a l . , 1969), and i f d i s p e r s a l i s unimportant, then no change i s expected i n numbers compared with those i n an unfenced p o p u l a t i o n . Boonstra (1977) enc l o s e d a peak p o p u l a t i o n of Microtus t o w n s e n d i i and found a fence e f f e c t . E i t h e r these r e s u l t s are unique to M. townsendii or H i l b o r n ' s r e s u l t s are not g e n e r a l l y v a l i d f o r Microtus s p e c i e s . One d i f f i c u l t y with using a n a l y s e s of variance to t e s t h e r i t a b i l i t y of l i f e t i m e i s t h a t i n p e r i o d s of high m o r t a l i t y , l i f e t i m e may be so short that even i f l i f e t i m e were s t r o n g l y h e r i t a b l e , t h e r e might not be enough d i f f e r e n c e s among l i t t e r s to d e t e c t h e r i t a b i l i t y . T h i s study i n d i c a t e s t h a t l i f e t i m e i s h e r i t a b l e even i f d i s p e r s e r s are e l i m i n a t e d . Very l a r g e samples of n e a r l y complete l i t t e r s may be necessary t o d e t e c t h e r i t a b i l i t y o f l i f e t i m e i n d e c l i n i n g p o p u l a t i o n s . 8U C h i t t y (1967) suggested t h a t animals s e l e c t e d f o r t h e i r t h r e a t behaviour dominate peak and d e c l i n i n g p o p u l a t i o n s because they have been s e l e c t e d f o r s u r v i v a l under c o n d i t i o n s of mutual i n t e r f e r e n c e , and that t h i s change i n the p r o p e r t i e s of the i n d i v i d u a l s a l s o reduces t h e i r f i t n e s s to endure environmental c o n d i t i o n s . C h i t t y suggested t h a t t h i s b e h a v i o u r a l change i s the d r i v i n g mechanism behind p o p u l a t i o n f l u c t u a t i o n s , and t h a t t h i s change has a g e n e t i c b a s i s . Krebs e t a l . (1973) hypothesized t h a t s e l e c t i v e d i s p e r s a l d u r i n g the i n c r e a s i n g phase of growth b r i n g s about a change w i t h i n the p o p u l a t i o n t h a t i n c r e a s e s the frequency of such animals. T h i s m o d i f i e d hypothesis assumes that as d e n s i t y i n c r e a s e s , d e n s i t y - i n t o l e r a n t , p a s s i v e v o l e s d i s p e r s e so p o p u l a t i o n s i n the l a t e peak and e a r l y d e c l i n e phase are c h a r a c t e r i z e d by a high frequency of a g g r e s s i v e , n o n - d i s p e r i n g v o l e s . The r e s u l t s of t h i s study support some as p e c t s o f t h i s h y p o t h e s i s . L i t t e r s i n peak I p o p u l a t i o n s , and l i k e l y i n i n c r e a s i n g ones, were d i f f e r e n t i a t e d with r e s p e c t to tendency to d i s p e r s e . I f tendency to d i s p e r s e and a g g r e s s i v e behaviour are both h e r i t a b l e and are i n v e r s e l y r e l a t e d , then d i s p e r s a l p r o v i d e s the mechanism by which the p r o p o r t i o n of a g g r e s s i v e animals should i n c r e a s e . However, t h e r e was no i n d i c a t i o n i n t h i s study of v o l e s i n d e c l i n i n g p o p u l a t i o n s being more a g g r e s s i v e . Krebs (1970) a l s o found t h a t v o l e s i n d e c l i n i n g p o p u l a t i o n s were l e s s a g g r e s s i v e than i n peak p o p u l a t i o n s . Anderson (1975) found that i n M. t o w n s e n d i i , the g e n e t i c i n f l u e n c e upon a g g r e s s i v e behaviour was low, so that g e n e t i c change a s s o c i a t e d with s e l e c t i o n upon a g g r e s s i v e behaviour was not a d r i v i n g mechanism of p o p u l a t i o n 85 r e g u l a t i o n . The present study suggests some areas t h a t should be explored f u r t h e r . Fencing peak m i c r o t i n e p o p u l a t i o n s w i l l e v a l u a t e the importance of d i s p e r s a l i n p o p u l a t i o n r e g u l a t i o n at high d e n s i t y . Techniques f o r i n utero marking would c o n c l u s i v e l y e s t a b l i s h l i t t e r m a t e s and enable d i r e c t measurements of h e r i t a b i l i t y of d i s p e r s a l tendency and l i f e t i m e . L i t e r a t u r e c i t e d Anderson, J . L. 1975. Phenotypic c o r r e l a t i o n s among r e l a t i v e s and v a r i a b i l i t y i n r e p r o d u c t i v e performance i n p o p u l a t i o n s o f the vole H i c r o t u s t o w n s e n d i i . Ph. D. Thesis Department of Zoology, U n i v e r s i t y of B r i t i s h Columbia, Vancouver, BC. Anderson, P. K. 1970. E c o l o g i c a l s t r u c t u r e and gene flow i n s m a l l mammals. Symp. Z o o l . Soc. Lond. 26: 299-325. Boonstra, R. 1977. A f e n c i n g experiment on a h i g h - d e n s i t y p o p u l a t i o n o f H i c r o t u s townsendii. Can. J . Z o o l . 55: 1166-1175. Boonstra, R. and C. J . Krebs. 1978. P i t f a l l t r a p p i n g of H i c r o t u s t o w n s e n d i i . J . Mammal. 59: 136-148. C h i t t y , D. 1967. . The n a t u r a l s e l e c t i o n of s e l f - r e g u l a t o r y behaviour i n animal p o p u l a t i o n s . Proc. E c o l . Soc. Aust. 2: 51-78. H i l b o r n , R. 1975. S i m i l a r i t i e s i n d i s p e r s a l tendency among s i b l i n g s i n f o u r s p e c i e s o f v o l e s ( H i c r o t u s ) . Ecology, 56: 1221-1225. 86 Howard, W. E. 1949- D i s p e r s a l , amount of i n b r e e d i n g , and l o n g e v i t y i n a l o c a l p o p u l a t i o n of p r a i r i e deermice on the George Reserve, Southern Michigan. Oniv. Mich. C o n t r i b . Lab. V e r t . B i o l . 43: 1-52. , 1960. Innate and environmental d i s p e r s a l of i n d i v i d u a l v e r t e b r a t e s . Am. M i d i . Nat. 63: 152-161. Krebs, C. J . 1970. Mierotus p o p u l a t i o n b i o l o g y : B e h a v i o r a l changes a s s o c i a t e d with the p o p u l a t i o n c y c l e i n M. ochrogaster and M. pennsylvanicus. Ecology, 51:34-52. Krebs, C. J . , B. L. K e l l e r , and B. H. Tamarin. 1969. Mierotus p o p u l a t i o n b i o l o g y : Demographic changes i n f l u c t u a t i n g p o p u l a t i o n s of M. ochrogaster and M. p e n n s y l v a n i c u s i n southern I n d i a n a . Ecology, 50: 587-607. Krebs, C. J . , M. S. Gaines, B. L. K e l l e r , J . H. Myers, and B. H. Tamarin. 1973. P o p u l a t i o n c y c l e s i n s m a l l rodents. S c i e n c e , 179: 35-41. Krebs, C. J . , I. „ wingate, J . LeDuc, J . A. R e d f i e l d , M. T a i t t , and B. H i l b o r n . 1976. M i e r o t u s p o p u l a t i o n b i o l o g y : D i s p e r s a l i n f l u c t u a t i n g p o p u l a t i o n s of M. townsendii. Can. J . Z o o l . 54: 79-95. Myers, J . H., and C. J . Krebs 1971. Genetic, b e h a v i o r a l , and r e p r o d u c t i v e a t t r i b u t e s of d i s p e r s i n g f i e l d v o l e s Mierotus p e n n s y l v a n i c u s and Mierotus ochrogaster. E c o l . Monogr. 44: 53-78. Pearson, O. P. 1960. Habits of Mierotus c a l i f o r n i c u s r e v e a l e d by automatic photographic r e c o r d e r s . E c o l . Monogr. 30: 231-249. 8 7 SECTION 3- SELECTIVITY OF AVIAN PREDATION IN DECLINING POPULATIONS OF THE VOLE MICROTUS TOWNSENDII I n t r o d u c t i o n The cause of c y c l i c a l f l u c t u a t i o n s i n v o l e abundance i s an area of c o n t i n u i n g r e s e a r c h , and there i s no consensus concerning the mechanisms u n d e r l y i n g p o p u l a t i o n d e c l i n e s (Krebs and Myers, 1974). Pearson (1964, 1966, 1971) suggested t h a t p r e d a t i o n d u r i n g d e c l i n e s i n f l u e n c e s both the amplitude and d u r a t i o n of the f l u c t u a t i o n . S e v e r a l a d d i t i o n a l s t u d i e s have i n d i c a t e d t h a t mammalian p r e d a t i o n may i n f l u e n c e some m i c r o t i n e p o p u l a t i o n d e c l i n e s (Maher 1967, MacLean e t a l . 1974, F i t z g e r a l d 1977). Other s t u d i e s suggest that avian p r e d a t o r s may account f o r some d e c l i n e s (Thompson 1955, Maher 1970). The f i r s t o b j e c t i v e o f t h i s study was to assess the impact o f avian p r e d a t o r s on a d e c l i n i n g H i c r o t u s townsendii p o p u l a t i o n . E r r i n g t o n (1956) suggested t h a t p r e d a t o r s are s e l e c t i v e f o r i n d i v i d u a l s whose p o s i t i o n i n the s o c i a l s t r u c t u r e i n the p o p u l a t i o n renders them v u l n e r a b l e to p r e d a t i o n . , A second o b j e c t i v e of the present study was t o determine i f avian p r e d a t o r s were s e l e c t i v e f o r s i z e and sex of prey d u r i n g p o p u l a t i o n d e c l i n e s i n v o l e s . Methods The study area was part of a Timothy hay f i e l d owned by the Canadian W i l d l i f e S e r v i c e on R e i f e l I s l a n d i n the Fraser River 88 d e l t a near Vancouver, B r i t i s h Columbia. Four Mierotus townsend i i p o p u l a t i o n s were l i v e - t r a p p e d with Longworth l i v e - t r a p s and p i t f a l l t r a p s from May 1976 u n t i l June 1978. There were two unfenced c o n t r o l g r i d s (A and D) and two enclosed experimental g r i d s (B and C). The t r a p p i n g technique c o n s i s t e d of biweekly sampling o f the p o p u l a t i o n with l i v e - t r a p s and weekly sampling from s p r i n g t o f a l l with p i t f a l l t r a p s . During the winter the p i t f a l l s c o u l d not be used because of f l o o d i n g . Each g r i d had 49 t r a p p i n g s t a t i o n s s e t i n a 7 x 7 p a t t e r n with s t a t i o n s 25 f e e t a p a r t . Two Longworth l i v e - t r a p s and one p i t f a l l t r a p were l o c a t e d a t each s t a t i o n . Voles were ear-tagged with aluminum ear tags, and the t r a p l o c a t i o n , sex, r e p r o d u c t i v e c o n d i t i o n , and weight o f each animal were recorded. A l l v o l e s were immediately r e l e a s e d a f t e r b e i n g processed. Avian p r e d a t o r s r e g u r g i t a t e some i n d i g e s t i b l e food components as p e l l e t s , and these p e l l e t s were c o l l e c t e d once per month from February 1978 to June 1978. P e l l e t s d e p o s i t e d i n the l a t e f a l l and e a r l y winter were assumed t o have remained i n t a c t , because l i t t l e decomposition of the p e l l e t s o ccurs d u r i n g the winter (Boonstra, 1977). Areas searched i n c l u d e d the study area, d i t c h e s , the f i e l d s adjacent to the study area, and nearby c o n i f e r o u s t r e e s and barns. I made no attempt t o d i s t i n g u i s h between p e l l e t s of d i f f e r e n t predatory s p e c i e s because t h e r e i s o f t e n c o n s i d e r a b l e s i z e o v e r l a p among them. The p e l l e t s were soaked i n 10% potassium hydroxide f o r 24 t o 72 hours. The remains were poured over a fine-mesh screen and washed with a stream o f water t o remove d i g e s t e d h a i r and f e a t h e r s . A l l ear tags were saved. 89 R e s u l t s Vole Abundance The v o l e d e n s i t i e s were low when t r a p p i n g began d u r i n g the summer of 1976 and remained low u n t i l the f a l l ( F i g . 3.1). The p o p u l a t i o n s i n c r e a s e d i n numbers d u r i n g the winter of 1976-1977, owing to delayed capture i n l i v e - t r a p s , and were a t a r e l a t i v e peak of abundance i n the s p r i n g of 1977. A moderate s p r i n g d e c l i n e i n p o p u l a t i o n s i z e o c c u r r e d i n the s p r i n g of 1977, but by f a l l the p o p u l a t i o n s had recovered to l e v e l s approximately e q u a l to or above those of the s p r i n g . D e c l i n e s s t a r t i n g i n October 1977 occurred i n a l l fou r p o p u l a t i o n s , and these d e c l i n e s l a s t e d u n t i l June 1978, when the study t e r m i n a t e d . The per i o d o f i n t e r e s t i n t h i s study was t h i s l a s t p o p u l a t i o n d e c l i n e spanning an eight-month p e r i o d from October 1977 to June 1978. Incidence Of P r e d a t o r s The major a v i a n p r e d a t o r s seen on the area were the gre a t blue heron (Ardea herodius) and marsh hawk ( C i r c u s cyaneus). The h i g h e s t d e n s i t y o f herons occ u r r e d i n December 1977, when seven herons were s i g h t e d on the t r a p p i n g g r i d s . Other avian p r e d a t o r s i n c l u d e d the barn owl (Tyto a l b a ) , s h o r t - e a r e d owl (ftsip flammeus), great-horned owl (Bubo v i r g i n i a n u s ) , snowy owl (Nyctea s c a n d i a c a ) , and r e d - t a i l e d hawk (Buteo l a g o g u s ) . 90 F i g u r e 3.1. P o p u l a t i o n d e n s i t y o f Mierotus townsendii determined by l i v e - t r a p s on c o n t r o l g r i d D d u r i n g 1976-78. Both sexes are combined. Non-breeding p e r i o d s are shaded. Minimum Number Alive v c_ ( D O O T 1—I I I I I I CA) O O T 1 CD 9 CD 2 0 8 2 92 P r e d a t o r s were i n h i g h e s t abundance i n the l a t e f a l l and e a r l y winter of 1977 and d e c l i n e d i n abundance through 1978. By the s p r i n g o f 1978 one heron was o c c a s i o n a l l y seen on the g r i d s and o t h e r d i u r n a l r a p t o r s were p e r i o d i c a l l y s i g h t e d . Predator Impact The p r o p o r t i o n of the l o s s of tagged v o l e s which was due to p r e d a t i o n i s shown i n Table 3.1. There are s e v e r a l reasons to b e l i e v e t hat these values underestimate the i n t e n s i t y of p r e d a t i o n on the tagged p o p u l a t i o n s . I missed c o l l e c t i n g some unknown p r o p o r t i o n o f the p e l l e t s because not a l l of the r o o s t i n g areas were known and some p e l l e t s may have been de p o s i t e d i n w a t e r - f i l l e d d i t c h e s near the study area. I may have overlooked p e l l e t s i n the areas t h a t I searched, and t r a n s i e n t p r e d a t o r s may have de p o s i t e d p e l l e t s f a r from the study area. However, I assume that the r e l a t i v e amount of p r e d a t i o n i s a c c u r a t e l y estimated i n Table 3.1 even i f the a b s o l u t e amount i s underestimated. Avian p r e d a t o r s had the g r e a t e s t impact i n e a r l y winter and were l e s s important i n s p r i n g (Table 3.1). Wore of the l o s s i n males was due t o p r e d a t i o n than i n females f o r each p e r i o d . The p r o p o r t i o n of l o s s accounted f o r by p r e d a t i o n tended to be between 10 to 15%, although a t l e a s t 25% of the l o s s i n a s i n g l e week was due t o p r e d a t i o n . The impact of the avian p r e d a t o r s was c o r r e l a t e d with the d e n s i t y o f the vole p o p u l a t i o n s (r=.99, P<.02) (Table 3.2). Avian p r e d a t o r s had the g r e a t e s t impact on the highest d e n s i t y 93 Table 3- 1. Number o f tagged v o l e s d i s a p p e a r i n g from four Mierotus townsendii p o p u l a t i o n s and percentage recovered from avian predator p e l l e t s . R e s u l t s from a l l g r i d s were pooled i n each p e r i o d . Males Females Number % Number % Time d i s a p p e a r i n g recovery d i s a p p e a r i n g r e c o v e r y Oct.-Nov. 1977 147 9.5 108 2.8 Dec.-Jan. 1978 169 15.4 170 10.6 Jan.-Feb. 1978 154 13.0 147 11.6 Mar.-Jun. 1978 172 4.7 164 4. 3 T o t a l 642 10.6 589 7.6 9 4 Table 3.2. Percentage of v o l e m o r t a l i t y accounted f o r by avian p r e d a t o r s and average M. townsendii d e n s i t y f o r 1977-1978. Data f o r p e r i o d s i n Table 3-1 are pooled f o r each g r i d . Both sexes are combined. % m o r t a l i t y Average v o l e G r i d due to p r e d a t i o n d e n s i t y D 6. 9 109 A 7.6 111 C 9.2 121 B 13. 3 134 95 vole p o p u l a t i o n and e x h i b i t e d density-dependent p r e d a t i o n . Sex R a t i o Of Voles Eaten By Predators To determine i f predators p r e f e r e n t i a l l y removed one sex from the p o p u l a t i o n s , I compared the sex r a t i o of the sample eaten by predators with the sex r a t i o of the r e s i d e n t p o p u l a t i o n s . The sex r a t i o of the r e s i d e n t p o p u l a t i o n s was determined by summing the minimum number of each sex known to be a l i v e i n each t r a p p i n g p e r i o d from the s p r i n g of 1977 u n t i l the end of the study. The sex r a t i o of the sample of v o l e s eaten by p r e d a t o r s was determined by i n c l u d i n g a l l those animals known to have been eaten d u r i n g the same p e r i o d . The t o t a l number of v o l e s known t o have been eaten was 162. For each g r i d , the sex r a t i o (males/females) was higher i n the sample eaten by predators than i n the r e s i d e n t p o p u l a t i o n (Table 3.3). When the data f o r a l l g r i d s were pooled, t h e r e was a s i g n i f i c a n t d i f f e r e n c e between the sex r a t i o s of the sample of v o l e s eaten by p r e d a t o r s and those of the r e s i d e n t p o p u l a t i o n s (X 2=10.28, P<.001). Hales were s e l e c t i v e l y removed by avian p r e d a t o r s . Body Weight D i s t r i b u t i o n s To see i f predators s e l e c t e d v o l e s a c c o r d i n g to body s i z e , I grouped t h e animals i n t o 'small* and ' l a r g e ' f o r each sex. Males g r e a t e r than 57 g and females g r e a t e r than 45 g were l a r g e ; o t h e r s were s m a l l . I used these weights so t h a t when the 9 6 Table 3.3. Sex r a t i o s (males:females) of eaten and r e s i d e n t Mierotus townsendii p o p u l a t i o n s . Sample s i z e s are i n parentheses. P e r i o d i s from s p r i n g 1977 t o s p r i n g 1978. G r i d Eaten by p r e d a t o r s Be s i d e n t Males:Females Males:Females k 1.33 (35) 0.72 (7857) B 2.40 (51) 1. 10 (10962) C 1.33 (42) 0.83 (9862) D 1.00 (34) 0.91 (8644) T o t a l 1.49 (162) 0.90 (37325) 97 sexes were combined, there were approximately equal numbers i n the •small" and »large' c a t e g o r i e s . For the v o l e s eaten by p r e d a t o r s , I used t h e i r l a s t known weight t o a s s i g n them t o a s i z e c a t egory. T a b l e 3.4 i n d i c a t e s t h a t s m a l l e r animals occurred more f r e q u e n t l y i n the. samples eaten by p r e d a t o r s than i n the r e s i d e n t p o p u l a t i o n s (X 2=4.45, P<-05). Smaller males tended to be the animals most l i k e l y s e l e c t e d by avian predators, whereas l a r g e r females were the l e a s t l i k e l y . The average d u r a t i o n of l i f e , measured as the time between f i r s t and l a s t c a p t u r e , was h i g h e r i n females eaten by p r e d a t o r s (21.2 weeks) than i n males (18.0 weeks). T h i s i s c o n s i s t e n t with predator s e l e c t i o n of s m a l l e r and presumably younger males. P r o b a b i l i t y Of P r e d a t i o n I examined whether or not the week of f i r s t capture of a vole was r e l a t e d t o i t s p r o b a b i l i t y of being s e l e c t e d by a v i a n p r e d a t o r s . Seek of f i r s t capture was assumed to be a measure o f age i n t h i s a n a l y s i s . For example, those v o l e s f i r s t caught i n October would be younger and s m a l l e r than those f i r s t caught i n august. Table 3.5 i n d i c a t e s t h a t the p r o b a b i l i t y of being eaten by p r e d a t o r s i n c r e a s e d from s p r i n g t o f a l l . T h i s r e s u l t suggests t h a t there was s e l e c t i o n f o r younger and thus s m a l l e r animals. However, because the l i f e s p a n o f an i n d i v i d u a l v o l e may be only a few months, v o l e s born i n the s p r i n g and e a r l y summer of 1977 w i l l n e c e s s a r i l y have a lower chance of being eaten by predators because many were a l r e a d y dead when the 98 Table 3 . 4 . Number of l a r g e and s m a l l male and female Mierotus townsendii i n eaten and r e s i d e n t p o p u l a t i o n s from f a l l 1 9 7 7 to s p r i n g 1 9 7 8 . Eaten by p r e d a t o r s Besident G r i d Males Females Males Females < 5 8 g > 5 8 g < 4 6 g > 4 6 g < 5 8 g > 5 8 g < 4 6 g > 4 6 g a 5 7 6 7 3 6 4 2 8 3 2 7 1 5 6 0 B 2 3 5 7 4 6 0 8 3 6 4 2 6 4 3 6 0 c 1 3 4 8 4 5 4 6 1 5 3 5 3 1 3 3 0 D 8 4 3 7 4 6 0 3 0 6 2 1 3 4 7 8 T o t a l 4 9 2 0 2 4 2 2 1 9 7 8 1 1 0 6 1 2 7 9 1 7 2 8 99 Table 3-5. P r o b a b i l i t y of an i n d i v i d u a l Mierotus townsendii b e i n g k i l l e d by avian p r e d a t o r s i n r e l a t i o n t o week of f i r s t c a pture i n 1977. A l l g r i d s and both sexes are pooled. New Number P r o b a b i l i t y Month animals eaten of p r e d a t i o n May 624 19 0.030 June 1022 58 0.057 J u l y 639 34 0.053 Aug. 189 14 0.074 Sept. 75 7 0.093 Oct. 46 7 0. 152 100 d e n s i t y of p r e d a t o r s i n c r e a s e d i n the f a l l . D i s c u s s i o n S e l e c t i v e p r e d a t i o n would a l t e r the age and sex s t r u c t u r e of the v o l e p o p u l a t i o n , e s p e c i a l l y d u r i n g t h e nonbreeding season., T h i s study i n d i c a t e s t h a t there was p r e f e r e n t i a l p r e d a t i o n of Mi c r o t u s towndendii males by a v i a n p r e d a t o r s . In a s i m i l a r study on M. townsendii, Boonstra (1977) r e p o r t e d t h a t a v i a n p r e d a t o r s e x h i b i t e d no pr e f e r e n c e f o r e i t h e r sex. However, i f the data presented i n Table 4 of Boonstra (1977) are pooled f o r the t h r e e p e r i o d s c o n s i d e r e d , then males occur more f r e q u e n t l y i n the tagged avian sample than i n the r e s i d e n t p o p u l a t i o n (X 2=7.32, P<.01). Thus i n Boonstra's study t h e r e a l s o may have been s e l e c t i o n of M. townsendii males by the avian p r e d a t o r s . Thompson (1955) r e p o r t e d t h a t snowy owls s e l e c t i v e l y preyed upon male brown lemmings (Lemmus s i b i r i e u s ) d u r i n g e a r l y , but not l a t e summer. S t e n d e l l ( c i t e d i n Boonstra 1977) found t h e r e was s e l e c t i o n f o r male Microtus c a l i f o r n i c u s by barn owls but not by s h o r t - e a r e d owls. Southern and Lowe (1968) r e p o r t e d t h a t there was no s e l e c t i o n f o r e i t h e r sex by owls f o r Apodemus or Clethrionomys. Maher (1970) repo r t e d on a study of Pomarine Jaeger ( S t e r e g r a r i u s pomarinus) pr e d a t i o n on brown lemmings i n Alaska. Assuming a 50:50 sex r a t i o i n the r e s i d e n t p o p u l a t i o n of lemmings, Maher showed t h a t p r e d a t o r s k i l l e d both sexes e q u a l l y i n 1956 (Maher's Table 7) (X 2=2.30, P<.25), but they p r e f e r r e d males i n 1960 (X 2=5.28, P<.05). U n f o r t u n a t e l y , Maher d i d not census the r e s i d e n t lemming p o p u l a t i o n s , and thus the a c t u a l sex r a t i o i s unknown. 101 G e n e r a l l y t h e r e tends t o be a d e f i c i e n c y of males i n r e s i d e n t m i c r o t i n e p o p u l a t i o n s (Myers and Krebs 1971), so t h a t d e s p i t e no apparent s e l e c t i o n (50% males) i n a sample eaten by p r e d a t o r s , i t i s d i f f i c u l t to draw any c o n c l u s i o n s with c e r t a i n t y without more knowledge of the prey p o p u l a t i o n . S e l e c t i o n of males by a v i a n p r e d a t o r s may be due to g r e a t e r movement by males and the r e s u l t a n t exposure to p r e d a t o r s (Thompson, 1955). I t may a l s o p a r t i a l l y account f o r the d e f i c i e n c y of males which o f t e n c h a r a c t e r i z e s r e s i d e n t vole p o p u l a t i o n s (Myers and Krebs, 1971). T h i s study i n d i c a t e s t h a t there was s e l e c t i v e p r e d a t i o n of s m a l l e r and presumably younger M. townsendii. Boonstra (1977) reported t h a t there was no d i f f e r e n c e i n body weights between r e s i d e n t M. townsendii and those tagged v o l e s known to have been eaten. However, the data i n Boonstra's Table 5 i n d i c a t e that the mean weights of the tagged sample eaten by p r e d a t o r s were c o n s i s t e n t l y lower than those o f the c o n t r o l p o p u l a t i o n (1 t i e , 5 below). T h i s i n d i c a t e s t h a t there was s e l e c t i o n f o r l i g h t animals i n the sample eaten by p r e d a t o r s ( o n e - t a i l e d s i g n t e s t , P=0.03). Lo c k i e (1955) found t h a t / i n the summer, sho r t - e a r e d owls took a higher p r o p o r t i o n of young than d i d h i s t r a p s . However, t h i s c o u l d be a r e s u l t of t r a p s s e l e c t i n g l a r g e r animals, p r e d a t o r s s e l e c t i n g s m a l l e r animals, or both. Si z e comparisons between v o l e s k i l l e d by predators versus those not k i l l e d are o n l y v a l i d i f both c l a s s e s of v o l e s have been sampled by the t r a p s . Southern and Lowe (1968) repo r t e d t h a t there was no s i z e s e l e c t i o n on Clethrlgngmy.s g l a r e o l u s or ftpodemus s y l v a t i c u s p o p u l a t i o n s by owls at any time of the year. 102 E r r i n g t o n (1956) suggested t h a t p r e d a t o r s do not randomly prey upon i n d i v i d u a l s from a p o p u l a t i o n but i n s t e a d remove those i n d i v i d u a l s whose s o c i a l p o s i t i o n renders them more v u l n e r a b l e to p r e d a t i o n . In v o l e p o p u l a t i o n s , s m a l l e r animals tend to be subordinate (Turner and I v e r s o n , 1973). S e l e c t i v e removal of s m a l l e r and presumably s u b o r d i n a t e i n d i v i d u a l s supports E r r i n g t o n ' s h y p o t h e s i s t h a t s u b o r d i n a t e i n d i v i d u a l s are the more v u l n e r a b l e t o p r e d a t i o n . fioberts and Wolfe (1974) repo r t e d t h a t a r e d - t a i l e d hawk s e l e c t i v e l y removed sub o r d i n a t e animals, whereas a c a t removed dominant i n d i v i d u a l s i n a c o t t o n r a t (Sigmodon h i s p i d u s ) p o p u l a t i o n . Pearson (1966) suggested t h a t mammalian pred a t o r s are able t o remove i n d i v i d u a l s r e g a r d l e s s of t h e i r s o c i a l p o s i t i o n . C h r i s t i a n (1975) found that domestic c a t s s e l e c t e d tagged Microtus pennsylvanicus at random from the p o p u l a t i o n . Mammalian predators may be r e s p o n s i b l e f o r the f l u c t u a t i o n and d u r a t i o n of low numbers i n a m i c r o t i n e c y c l e (Pearson 1966, 1971, MacLean et a l . 1974, F i t z g e r a l d 1977). Mammalian pre d a t o r s l a c k the m o b i l i t y of a v i a n p r e d a t o r s and are thus presumed t o e x e r t heavy p r e d a t i o n on d e c l i n i n g m i c r o t i n e p o p u l a t i o n s even when avian p r e d a t o r s have l e f t owing t o low vole d e n s i t i e s . Mammalian pr e d a t o r s are presumed t o cause i n v e r s e density-dependent m o r t a l i t y i n d e c l i n i n g v o l e p o p u l a t i o n s , and thereby account f o r extended p e r i o d s of low v o l e numbers, whereas the avian p r e d a t o r s i n t h i s study showed a p o s i t i v e density-dependent impact on the d e c l i n i n g v o l e p o p u l a t i o n s . There are no n a t u r a l mammalian pr e d a t o r s on S e i f e l I s l a n d , and a v i a n predators appeared to have g r e a t e s t e f f e c t at 103 high v o l e p o p u l a t i o n d e n s i t i e s , because the p r o p o r t i o n of l o s s accounted f o r by a v i a n p r e d a t i o n d e c l i n e d as v o l e p o p u l a t i o n d e n s i t y d e c l i n e d . The m o b i l i t y of these p r e d a t o r s allowed them to move to new areas when the prey p o p u l a t i o n d e n s i t i e s became low. High r a t e s of p o p u l a t i o n d e c l i n e can occur i n the absence of any s i g n i f i c a n t p r e d a t i o n (Boonstra, 1977), and thus p r e d a t i o n i s not a necessary component o f d e c l i n e s i n m i c r o t i n e p o p u l a t i o n s . Rather, i t may be a s u f f i c i e n t c o n d i t i o n , or simply one of s e v e r a l c o n t r i b u t i n g f a c t o r s , r e s u l t i n g i n some m i c r o t i n e d e c l i n e s . The major d i f f i c u l t y i n doing a d e s c r i p t i v e study on p r e d a t i o n of v o l e s i s that the t o t a l impact of p r e d a t i o n i s unknown unl e s s 100% of the p e l l e t s and s c a t s produced by the p r e d a t o r s are recovered. T h i s c o n d i t i o n was not s a t i s f i e d i n t h i s study, and there i s no independent method of determining the a b s o l u t e p r e d a t i o n r a t e . Unless n e a r l y 100% of the p e l l e t s and s c a t s can be recovered, d e s c r i p t i v e s t u d i e s should be abandoned i n f a v o u r of an experimental approach. To t e s t the h y p o t h e s i s t h a t p r e d a t i o n i s e i t h e r necessary or s u f f i c i e n t t o account f o r m i c r o t i n e d e c l i n e s , one c o u l d e n c l o s e a m i c r o t i n e p o p u l a t i o n with a p r e d a t o r - p r o o f f e n c e , w i t h i n which an area f o r the v o l e s to d i s p e r s e i n t o i s provided i n order t o prevent the p o p u l a t i o n s from r e a c h i n g abnormally high d e n s i t i e s . T h i s design c o u l d a l s o t e s t the h y p o t h e s i s o f Pearson (1966, 1971) t h a t mammalian p r e d a t i o n i n t e n s i f i e s i n d e c l i n i n g v o l e p o p u l a t i o n s , and accounts f o r extended p e r i o d s of low vole abundance a f t e r a d e c l i n e . 104 L i t e r a t u r e c i t e d Boonstra, R. 1977. Pr e d a t i o n on Mierotus townsendii p o p u l a t i o n s : impact and v u l n e r a b i l i t y . Can. J . Z o o l . 55: 1631-1643. C h r i s t i a n , D. P. 1975. V u l n e r a b i l i t y of meadow v o l e s , Mierotus pennsylvanicus, t o p r e d a t i o n by domestic c a t s . Amer. Mid. Hat. 93: 498-502. E r r i n g t o n , P. L. 1956. F a c t o r s l i m i t i n g h i g h e r v e r t e b r a t e p o p u l a t i o n s . , S c i e n c e , 124: 304-307. F i t z g e r a l d , B. M. 1977. Weasel p r e d a t i o n on a c y c l i c p o p u l a t i o n o f the montane vole (Mierotus montanus) i n C a l i f o r n i a . J . Anim., E c o l . 46: 367-397. Krebs, C. J . and J . H. Myers. 1974. P o p u l a t i o n c y c l e s i n s m a l l mammals. Adv. E c o l . fies. 8: 267-399. HacLean, S. F., J r . , B. M. F i t z g e r a l d , and F. A- P i t e l k a . 1974. P o p u l a t i o n c y c l e s i n A r c t i c lemmings: winter r e p r o d u c t i o n and p r e d a t i o n by weasels. A r c t . A lp. Res. 6: 1-12. Maher, W. J . 1967. Pr e d a t i o n by weasels on a winter p o p u l a t i o n o f lemmings. Banks I s l a n d , Northwest T e r r i t o r i e s . Can. F i e l d - N a t . 81: 248-250. .., 1970. The pomarine jaeger as a brown lemming predator i n n o r t h e r n A l a s k a . Wilson B u l l . 82: 130-157. Myers, J . H. and C. J . Krebs. 1971. Sex r a t i o s i n open and enclo s e d vole p o p u l a t i o n s : demographic i m p l i c a t i o n s . Amer. Nat. 105: 325-344. Pearson, 0. P. 1964. Carnivore-mouse p r e d a t i o n : an example of 105 i t s i n t e n s i t y and b i o e n e r g e t i c s . J . Mammal. 45: 177-188. 1966. The prey of c a r n i v o r e s d u r i n g one c y c l e of mouse abundance. J . Anim. E c o l . 35: 217-233. 1971. A d d i t i o n a l measurements of the impact of c a r n i v o r e s on C a l i f o r n i a v o l e s (Mierotus c a l i f o r n i c u s ) . J . Mammal. 52: 41-49. Roberts, M. W. and J . L. Wolfe. 1974. S o c i a l i n f l u e n c e s on s u s c e p t i b i l i t y t o p r e d a t i o n i n co t t o n r a t s . J . Mammal. 55: 869-872. Southern, H. N. and 7. P. W. Lose. 1968. The p a t t e r n of d i s t r i b u t i o n of prey and pr e d a t i o n i n tawny owl t e r r i t o r i e s . J . Anim. E c o l . 37: 75-97. Thompson, D. Q. 1955. Ecology of the lemmings. A r c t . , I n s t . North Am., F i n a l Rep. P r o j . ONR-133. 63p. Turner, B. N. and S. L. I v e r s o n . 1973. The annual c y c l e of a g g r e s s i o n i n male Mierotus pennsylvanicus, and i t s r e l a t i o n t o p o p u l a t i o n parameters. Ecology, 54: 967-981. 106 SECTION 4. SURVIVAL IN FLUCTUATING POPULATIONS OF THE VOLE MICROTUS TOWNSENDII I n t r o d u c t i o n M i c r o t i n e rodent p o p u l a t i o n s f l u c t u a t e i n abundance, and changes i n s u r v i v a l r a t e s i n f l u e n c e p o p u l a t i o n growth i n v o l e s (Krebs and Myers, 1974). The s u r v i v a l o f j u v e n i l e v o l e s i s low i n d e c l i n i n g p o p u l a t i o n s (Godfrey 1955; Krebs et a l . 1969), and j u v e n i l e s u r v i v a l i s a major determinant of the subsequent r a t e of p o p u l a t i o n change (Krebs and Myers, 1974). J u v e n i l e v o l e s are d i f f i c u l t t o t r a p i n l i v e - t r a p s , but an index o f s u r v i v a l can be used t o determine j u v e n i l e s u r v i v a l (Krebs and DeLong, 1965). However, t h i s index i s a f f e c t e d by j u v e n i l e t r a p p a b i l i t y , and may vary owing to changes i n t r a p p a b i l i t y of j u v e n i l e s (Boonstra and Krebs, 1978). The o b j e c t i v e o f t h i s study was to measure d i r e c t l y minimum s u r v i v a l of j u v e n i l e , subadult, and a d u l t v o l e s d u r i n g a p o p u l a t i o n c y c l e , and to determine whether or not t r a p p i n g technique i n f l u e n c e s observed minimum s u r v i v a l of a s i z e c l a s s of v o l e s . Methods The study area was p a r t o f a Timothy hay f i e l d owned by the Canadian W i l d l i f e S e r v i c e on R e i f e l I s l a n d i n the F r a s e r River d e l t a near Vancouver, B r i t i s h Columbia. Although Phleum pratense was the dominant s p e c i e s , A g r o s t i s a l b a , Lolium 107 perenne, and Holcus l a n a t u s were a l s o present. Four Mierotus townsendii p o p u l a t i o n s were l i v e - t r a p p e d from May 1976 u n t i l June 1978. P o p u l a t i o n s B and C were enc l o s e d by a v o l e - p r o o f f e n c e , and p o p u l a t i o n s A and D were unfenced c o n t r o l s . Each g r i d had 49 t r a p s t a t i o n s s e t 7.6 m (25 feet) a p a r t i n a 7 x 7 p a t t e r n . Each s t a t i o n c o n s i s t e d of two Longworth l i v e t r a p s and one p i t f a l l t r a p s i m i l a r t o the type d e s c r i b e d by Boonstra and Krebs (1978). The l i v e - s t r a p s , b a i t e d with o a t s and s u p p l i e d with c o t t o n , were set every second week on Monday a f t e r n o o n , checked Tuesday morning and a f t e r n o o n , and checked and l o c k e d open on Wednesday morning. P i t f a l l t r a p s were used from May to October 1976, A p r i l to September 1977, and A p r i l t o June 1978. In the i n t e r v a l s between these p e r i o d s , the p i t f a l l s c o u l d not be used owing to winter f l o o d i n g . They were s e t ev e r y week on Wednesday morning, checked Wednesday a f t e r n o o n and Thursday morning, and c l o s e d on Thursday a f t e r n o o n . When both types of t r a p s were s e t i n one week, the p i t f a l l s were s e t a f t e r the l i v e - t r a p s had been l o c k e d open. A l l v o l e s were ear-tagged, and t a g number, weight, capture l o c a t i o n , sex, r e p r o d u c t i v e c o n d i t i o n , and wounding were recorded on each c a p t u r e . Voles were r e l e a s e d immediately a f t e r p r o c e s s i n g and were c l a s s i f i e d as f o l l o w s : adult>42g; subadult 30-42g; j u v e n i l e <30g. The t o t a l enumeration technique of Krebs (1966) was used i n the d e t e r m i n a t i o n of demographic d a t a . The estimate of t r a p p a b i l i t y used was: 108 number of captures f o r an animal T r a p p a b i l i t y = number o f p o s s i b l e c a p t u r e s f o r t h a t animal N where N i s the number of v o l e s caught more than t w i c e , and t r a p p a b i l i t y i s summed over a l l N animals. The f i r s t and l a s t times o f capture are excluded i n the summation, because animals must be caught at these times. R e s u l t s / / T r a p p a b i l i t y The t r a p p a b i l i t y e s t i m a t e s o f l i v e - t r a p s and p i t f a l l s were c a l c u l a t e d independently f o r each type of t r a p . For example, v o l e s known t o be present from capture i n p i t f a l l t r a p s , but not i n l i v e - t r a p s , were not i n c l u d e d i n t r a p p a b i l i t y e stimates of those c a p t u r e d by l i v e - t r a p s . T r a p p a b i l i t i e s o f males and females i n l i v e - t r a p s were s i m i l a r and r e l a t i v e l y constant from summer 1976 u n t i l s p r i n g 1978. In the 1976 summer t r a p p a b i l i t y was 61% f o r 324 animals, and i n the 1977 summer i t was 65% f o r 1692 animals. Low d e n s i t i e s i n the s p r i n g of 1978 i n c r e a s e d male t r a p p a b i l i t y t o 95% f o r 96 animals and female t r a p p a b i l i t y to 81% f o r 155 animals. Hale and female t r a p p a b i l i t i e s i n p i t f a l l s were s i m i l a r . T r a p p a b i l i t y i n p i t f a l l s d u r i n g summer 1976 was 35% f o r 727 animals and i n the 1977 summer was 28% f o r 1726 animals. T r a p p a b i l i t y c ould not be c a l c u l a t e d f o r s p r i n g 1978 because no 109 animal was captured more than t w i c e . P i t f a l l s were only about one-half as e f f e c t i v e as l i v e - t r a p s i n r e c a p t u r i n g v o l e s . T h i s lower t r a p p a b i l i t y i n d i c a t e s t h a t v o l e s a v o i d repeated capture i n p i t f a l l s . P o p u l a t i o n Density The vole p o p u l a t i o n s were low when t r a p p i n g began i n May 1976, and the p o p u l a t i o n sampled by l i v e - t r a p s remained low throughout the summer ( F i g . 4.1). Pop u l a t i o n s i z e s i n c r e a s e d d u r i n g the f a l l and winter of 1976-1977, owing to delayed capture i n l i v e - t r a p s , and d e n s i t i e s were high i n the s p r i n g of 1977. Moderate s p r i n g d e c l i n e s , l a r g e l y a t t r i b u t a b l e to d i s p e r s a l , occurred (see d i s p e r s a l s e c t i o n ) , but by f a l l the vo l e p o p u l a t i o n s had recovered to l e v e l s e q u a l to or gr e a t e r than those of the s p r i n g . The summer of 1977 was c h a r a c t e r i z e d by peak d e n s i t i e s . D e c l i n e s i n a l l f o u r p o p u l a t i o n s began i n October 1977 and p e r s i s t e d u n t i l June 1978, when the study ended. The r a t e of d e c l i n e d u r i n g the 1978 breeding season was p a r t i c u l a r l y severe (see d e c l i n e s e c t i o n ) and provides an i d e a l o p p o r t u n i t y t o compare j u v e n i l e s u r v i v a l with t h a t i n i n c r e a s i n g and peak p o p u l a t i o n s . J u v e n i l e S u r v i v a l The estimate of j u v e n i l e s u r v i v a l i s d i r e c t because l a r g e numbers of j u v e n i l e s were enumerated i n e i t h e r l i v e - t r a p s or p i t f a l l s . S u r v i v a l r a t e s a re based on the combined l i v e - t r a p 110 F i g u r e 4.1. P o p u l a t i o n d e n s i t y determined from l i v e - t r a p s o f Mierotus townsendii on fenced g r i d C d u r i n g 1976-78. Both sexes are combined. Non-breeding p e r i o d s a r e shaded. 112 and p i t f a l l c a p t u r e s . Table 4.1 shows that j u v e n i l e s u r v i v a l decreased d u r i n g a p o p u l a t i o n c y c l e . J u v e n i l e s i n the low d e n s i t y , i n c r e a s i n g phase summer of 1976 s u r v i v e d b e t t e r , on average, than j u v e n i l e s i n the peak d e n s i t y summer of 1977 (X 2=6.87, P<.01), and j u v e n i l e s i n t h i s summer s u r v i v e d b e t t e r , on average, than j u v e n i l e s i n the severe 1978 s p r i n g d e c l i n e s (X 2=10.20, P<.01). J u v e n i l e s i n the enclosed p o p u l a t i o n s B and C may have s u r v i v e d b e t t e r i n the summer of 1976 than j u v e n i l e s i n the g r i d h and D p o p u l a t i o n s (X 2=2.81, P<. 10) . The e f f e c t of a d u l t s upon j u v e n i l e s u r v i v a l was i n v e s t i g a t e d because s i g n i f i c a n t i n v e r s e c o r r e l a t i o n s have been rep o r t e d between mean a d u l t female d e n s i t y and j u v e n i l e s u r v i v a l (Boonstra 1978; fiedfield et a l . 1978). The p r o p o r t i o n of a d u l t s i n the p o p u l a t i o n was obt a i n e d by summing the number of captures o f each s i z e c l a s s d u r i n g a breeding p e r i o d and d i v i d i n g t o t a l a d u l t captures by t o t a l c a p t u r e s f o r each sex. The average a d u l t d e n s i t y f o r each sex i n a p e r i o d was obt a i n e d by m u l t i p l y i n g the average d e n s i t y by the p r o p o r t i o n of a d u l t s . Data were summed d u r i n g the breeding p o r t i o n of the year. There was no c o r r e l a t i o n between a d u l t d e n s i t y and j u v e n i l e s u r v i v a l among ye a r s f o r males (r=.43, n=12, P>.10) or females (r=.48, P>.10). The comparison of a l l f o u r p o p u l a t i o n s w i t h i n one year rev e a l e d no r e l a t i o n s h i p between mean a d u l t d e n s i t y and j u v e n i l e s u r v i v a l i n males (1976 r=-.37, 1977 r=-.22, 1978 r=-.04) or females (1976 r=-.34, 1977 r=-.49, 1978 r=-.41). w i t h i n one year, p o p u l a t i o n s with higher d e n s i t i e s of e i t h e r a d u l t males or females d i d not have s i g n i f i c a n t l y lower j u v e n i l e s u r v i v a l . Bowever, whereas j u v e n i l e s u r v i v a l and a d u l t d e n s i t y were not 113 T a b l e 4.1. Minimum s u r v i v a l r a t e s per 14-day p e r i o d f o r j u v e n i l e Microtus townsendii i n combined l i v e - t r a p and p i t f a l l p o p u l a t i o n . A l l p o p u l a t i o n s and both sexes are pooled f o r each i n t e r v a l . Sample s i z e s are i n parentheses. G r i d A B C D T o t a l Summer 1976 0.58 0.70 0.66 0.44 0.63 (113) (380) (319) (142) (954) Summer 1977 0.58 0.56 0.63 0.50 0.58 (398) (668) (695) (425) (2186) Spring 1978 0.25 0.25 0. 00 0.25 0.20 (4) (6) (2) (8) (20) 114 r e l a t e d , t h e r e was a r e l a t i o n between p r o p o r t i o n of a d u l t s i n the p o p u l a t i o n and j u v e n i l e s u r v i v a l f o r a d u l t males (r=-.93, n=12, P<.0001), but not f o r females (r=-.46, P>.10). Po p u l a t i o n s with h i g h e r p r o p o r t i o n s of a d u l t males had s i g n i f i c a n t l y lower j u v e n i l e s u r v i v a l , but t h i s does not e s t a b l i s h a c a u s a l r e l a t i o n s h i p . Preweanling S u r v i v a l The s u r v i v a l r a t e of v o l e s between b i r t h and weaning has been d i f f i c u l t t o estimate because preweanlings are v i r t u a l l y untrappable, and few j u v e n i l e s are caught i n l i v e - t r a p s . However, because p i t f a l l t r a p s permitted the capture of l a r g e numbers of j u v e n i l e s i n t h i s study, I was a b l e to estimate preweanling s u r v i v a l i n the f o l l o w i n g manner. The minimum number o f l a c t a t i n g females d u r i n g each of the three breeding p e r i o d s was known. A female was assumed t o be l a c t a t i n g d i f f e r e n t l i t t e r s i f she was caught l a c t a t i n g two weeks or more apa r t . L i t t e r s i z e i s approximately f i v e (Boonstra and Krebs, 1978), and thus the t o t a l number of young born i n each p o p u l a t i o n i n each breeding season can be c a l c u l a t e d . The average weight a t f i r s t capture of both males and females l e s s than 4 0 g was 24 g. T h i s weight was converted to age by the use of growth r a t e eguations (Boonstra and Krebs, 1978), and the average animal was estimated to be 5 weeks o l d a t f i r s t c a p t u r e . Weaning was assumed to occur a t three weeks of age (Hamilton, 1941; Richmond and Conway, 1969). Preweanling s u r v i v a l c o u l d then be estimated. For example, on g r i d B d u r i n g summer 1976, 115 j u v e n i l e minimum s u r v i v a l was 0.70 per two weeks, and I captured 343 post-weanlings. The maximum number of vo l e s a l i v e at weaning can be estimated as 343/0.70 = 490. The t o t a l number of young produced by 141 l a c t a t i n g females can be estimated as 705. Preweanling s u r v i v a l per two weeks was then (490/705) 2 / 3 . Preweanling s u r v i v a l r a t e s r e p o r t e d i n t h i s study were s u b j e c t t o e r r o r f o r s e v e r a l reasons. Hot a l l o f the v o l e s born on a g r i d were caught i n l i v e - t r a p s or p i t f a l l s before they reached 40 g. Growth r a t e s of young v o l e s vary throughout the summer, so not a l l 24 g v o l e s were f i v e weeks o l d . , Some l a c t a t i n g females may have avoided c a p t u r e , so t h a t the number of young born was underestimated. However, although the abso l u t e estimates of preweanling s u r v i v a l may be i n e r r o r , I assume t h a t the r e l a t i v e e s t i m a t e s are ac c u r a t e . . The two-week minimum s u r v i v a l r a t e s of t r a p p a b l e j u v e n i l e s are shown i n Table 4.1. I assumed t h a t f o r each s p e c i f i c p o p u l a t i o n and breeding season combination, the minimum s u r v i v a l r a t e o f the t r a p p a b l e j u v e n i l e s was the minimum s u r v i v a l r a t e of a l l post-weanlings. F i g u r e s 4.2, 4.3, 4.4, and 4.5 show the s u r v i v o r s h i p curves f o r preweanlings and post-weanlings i n a l l fo u r p o p u l a t i o n s over t h r e e breeding seasons. S e v e r a l trends are apparent. Preweanling s u r v i v a l was hi g h e s t i n the 1976 i n c r e a s i n g p o p u l a t i o n s and lowest i n the 1977 peak p o p u l a t i o n s . In 1976, i t was a l s o higher i n the enclosed than i n the unenclosed p o p u l a t i o n s . Preweanling s u r v i v a l was g r e a t e r d u r i n g the 1978 d e c l i n e than i n the 1977 peak, but co u l d not be c a l c u l a t e d f o r the 1978 g r i d C p o p u l a t i o n because post-weanling s u r v i v a l was z e r o . Preweanling s u r v i v a l was higher than 116 F i g u r e 4.2. S u r v i v o r s h i p curves f o r young Mierotus townsendii on c o n t r o l g r i d A between b i r t h and r e c r u i t m e n t . The number born i n each year was estimated from the number of l a c t a t i n g females; the number of r e c r u i t s was known. 117 3 0 0 0 r 0.57 1000h 0.58 5 0 0 (D > CD £ 100 5 0 10 1 1977 0 .58 \ 0 . 2 5 \ \ \ 1976 \ Weaning i \ D 1978 ^ A v e r a g e Age 0 at Recru i tment Age, Weeks 118 F i g u r e 4.3. S u r v i v o r s h i p curves f o r young Mierotus townsendii on fenced g r i d B between b i r t h and r e c r u i t m e n t . The number born i n each year was estimated from the number of l a c t a t i n g females; the number of r e c r u i t s was known. 119 3000 — 0 . 6 8 <D £ Z 100 -at Recruitment Age; Weeks 120 Figu r e 4.4. S u r v i v o r s h i p curves f o r young Mierotus townsendii on fenced g r i d C between b i r t h and re c r u i t m e n t . The number born i n each, year was estimated from the number of l a c t a t i n g females; the number of r e c r u i t s was known. No r e c r u i t s were caught i n 1978. 30001 0.63 1000 - 0.63 500^ 100 5 0 0.92 0.66 1977 1976 10' Weaning Average Age at Recrui tment J L J i 0 1 Age, Weeks 1 2 2 F i g u r e 4.5. S u r v i v o r s h i p curves f o r young Mierotus townsendii on c o n t r o l g r i d D between b i r t h and r e c r u i t m e n t . The number born i n each year was estimated from the number of l a c t a t i n g females; the number o f r e c r u i t s was known. 123 3 0 0 0 1000 5 0 0 r > < <D _Q E 13 100 h 0.68 5 0 t> 1977 1976 \ \ 0.25 \ \ \ 10. Weaning \ 0 2 3 4 Age, Weeks 1978 - ' -•Average Age 5 at Recrui tment 124 post-weanling s u r v i v a l i n a l l t h r e e breeding seasons and was a l s o l e s s v a r i a b l e . S u r v i v a l o f the young v o l e s decreased a f t e r weaning, presumably at the time they l e f t the nest. Adult And Subadult S u r v i v a l Adult s u r v i v a l can be an important determinant of po p u l a t i o n growth (Krebs and Myers, 1974). Loss from a s p e c i f i c age group of animals i n c l u d e s a component both f o r death and e m i g r a t i o n . Values f o r s u r v i v a l r a t e s are a l s o i n f l u e n c e d by the t r a p p a b i l i t y o f animals. An age group with low t r a p p a b i l i t y w i l l tend t o have ap p a r e n t l y low s u r v i v a l . , Minimum s u r v i v a l r a t e s of d i f f e r e n t age groups caught by Longworth l i v e - t r a p s are shown i n Table 4.2. Spring was d e f i n e d as the i n t e r v a l between the s t a r t of breeding i n March and the end of the s p r i n g d e c l i n e , summer as the i n t e r v a l between s p r i n g and the non-breeding season, and winter as the non-breeding season. S u r v i v a l values were hi g h i n i n c r e a s i n g p o p u l a t i o n s , d e c l i n e d moderately i n the peak 1977 summer, and decreased d r a s t i c a l l y with the onset of breeding i n 1978. Females s u r v i v e d b e t t e r than males as a d u l t s (X 2=15.68, P<.001) and as su b a d u l t s (X 2=6.67, P<-01). J u v e n i l e males and females s u r v i v e d e q u a l l y w e l l . A d u l t s s u r v i v e d b e t t e r than s u b a d u l t s i n males (X 2=15.68, P<.001) and i n females (X 2=35.96, P<. 001). Subadults s u r v i v e d b e t t e r than j u v e n i l e s i n males (X 2=22.50, P<-001) and i n females (X 2=59.76, P<.001). These r e s u l t s i n d i c a t e t h a t l a r g e r animals s u r v i v e d b e t t e r than s m a l l e r ones and t h a t females s u r v i v e d b e t t e r than males. Subadult s u r v i v a l , compared 125 Table 4.2. Minimum s u r v i v a l r a t e s per 14-day p e r i o d f o r Microtus townsendii caught by Longworth l i v e - t r a p s . A l l p o p u l a t i o n s were pooled f o r each i n t e r v a l . Sample s i z e s are i n parentheses. Males Females Adults Subadults J u v e n i l e s A d u l t s Subadults J u v e n i l e s Summer 0.70 0.56 0.47 0.86 0. 67 0.47 1976 (228) (48) (34) (468) (115) (43) Winter 0.92 0.93 0.83 0.93 0. 93 0. 76 1976 (1727) (201) (12) (1590) (446) (25) S p r i n g 0.78 0.42 0.08 0.91 0. 85 0.22 1977 ( 1502) (19) (25) (1638) (157) (18) Summer 0.82 0.76 0.73 0.84 0.75 0.63 1977 (1129) (409) (108) (1637) (426) (142) Winter 0.82 0.70 0.82 0.81 : 1.00 1977 (2551) (142) loT (1868) (663) (1) S p r i n g 0.48 0.25 0.33 0.66 0.60 0.60 1978 (141) (4) (3) (339) (109) (5) T o t a l 0.82 0.76 0.59 0.86 0.81 0.59 (7278) (823) (182) (7540) (1916) (234) 126 with t h a t o f a d u l t s , dropped s h a r p l y i n the s p r i n g of 1977. The di m i n i s h e d s u r v i v a l i n the subadult group was a r e s u l t o f d i s p e r s a l of s m a l l e r animals a t t h i s time (see d i s p e r s a l s e c t i o n ) . Hinimum s u r v i v a l of males was higher during the 1976 winter than d u r i n g the 1977 winter i n a d u l t s (X 2=85.79, -P<..0.001) and su b a d u l t s (X 2=43.84, P<.0001) (Table 4.2). Female minimum s u r v i v a l was a l s o h i g h e r during the 1976 winter than d u r i n g the 1977 winter i n a d u l t s (X 2=92.50, P<.0001) and suba d u l t s (X 2=30.54, P<.0001). Male and female minimum s u r v i v a l were the same d u r i n g the w i n t e r s , but female minimum s u r v i v a l d u r i n g the s p r i n g breeding season was hig h e r than male s u r v i v a l i n 1977 (X 2=108.60, P<.001) and i n 1978 (X 2=14.18, P<.001). Minimum s u r v i v a l r a t e s o f the t h r e e age groups caught by p i t f a l l s are shown i n Table 4.3. Rates f o r a d u l t s and suba d u l t s caught i n p i t f a l l s were much lower than f o r those caught i n l i v e - t r a p s . The p i t f a l l data a l s o i n d i c a t e t h a t females s u r v i v e d b e t t e r than males as a d u l t s (X 2=88.41, P<.001) and as su b a d u l t s (X 2=4.03, P<-05), whereas j u v e n i l e s o f e i t h e r sex s u r v i v e d e q u a l l y w e l l . However, a d u l t and subadult males s u r v i v e d e q u a l l y p o o r l y , while j u v e n i l e s males s u r v i v e d b e t t e r than s u b a d u l t s (X 2=34.96, P<.001). Adult females s u r v i v e d b e t t e r than s u b a d u l t s , but not b e t t e r than j u v e n i l e s , and j u v e n i l e females s u r v i v e d b e t t e r than s u b a d u l t s (X 2=24.92, P<.01). These r e s u l t s again i n d i c a t e t h a t females s u r v i v e d b e t t e r than males, but t h a t s m a l l e r animals tended to s u r v i v e b e t t e r than l a r g e r animals. Because of d i f f e r e n t trends i n the two types of t r a p s . 127 Table 4.3. Minimum s u r v i v a l r a t e per 1,4-day p e r i o d f o r Mierotus townsendii caught by p i t f a l l s . A l l p o p u l a t i o n s a re pooled f o r each i n t e r v a l . Sample s i z e s a r e i n parentheses. Males Females Adults Subadults J u v e n i l e s A d u l t s Subadults J u v e n i Summer 0.38 0.36 0.41 0.56 0.52 0.57 1976 (549) (274) (402) (744) (332) (410) Spring 0.39 0*16' 0.42 0.62 0. 16 0.36 1977 (596) (156) (332) (961) (112) (329) Summer 0.20 0.31 0.46 0.26 0.31 0. 44 1977 (693) (928) (633) (642) (975) (678) S p r i n g 0.00 0.00 0.02 0.09 0.06 0.04 1978 (5) (4) (7) (37) (4) (5) T o t a l 0.30 0.30 0.43 0.48 0.34 0.45 (1843) (1362) (1374) (2384) (1423) (1422) 128 minimum s u r v i v a l r a t e s of v a r i o u s age and sex c l a s s e s caught i n l i v e - t r a p s and p i t f a l l s were not s t r o n g l y c o r r e l a t e d . Minimum s u r v i v a l r a t e s of subadult males, as determined from l i v e - t r a p and p i t f a l l c a p t u r e s , were moderately c o r r e l a t e d (r=.88, n=4, P<.15), as were a d u l t female minimum s u r v i v a l r a t e s (r=.88, n=4, P<.15). However, with i n a given type of t r a p , the c o r r e l a t i o n of male and female minimum s u r v i v a l r a t e s w i t h i n r e s p e c t i v e age groups was good. As determined from p i t f a l l c a p t u r e s , male and female minimum s u r v i v a l r a t e s were c o r r e l a t e d i n a d u l t s (r=.98, n=4, P<.02), s u b a d u l t s (r=.93, P<.07), and j u v e n i l e s (r=.91, P<.10). From l i v e - t r a p s , the minimum s u r v i v a l r a t e s were c o r r e l a t e d i n a d u l t s <r=.87» n=6, P<.03) and i n j u v e n i l e s (r=.88, n=5, P<.05), but not i n s u b a d u l t s (r=.69). Minimum s u r v i v a l r a t e s of animals enumerated by l i v e - t r a p s and p i t f a l l s may not be comparable un l e s s the age group being c o n s i d e r e d has s i m i l a r t r a p p a b i l i t y i n both types of t r a p s . D i s c u s s i o n S u r v i v a l i n d i f f e r e n t age and sex groups can have a s t r o n g e f f e c t on the r a t e of p o p u l a t i o n growth, but the b a s i s of v a r i a b i l i t y i n s u r v i v a l r a t e s i s s t i l l c o n t r o v e r s i a l . The e l u c i d a t i o n of changes i n the s u r v i v a l r a t e s of d i f f e r e n t age groups d u r i n g the course of a p o p u l a t i o n c y c l e may be b e n e f i c i a l i n e v a l u a t i n g a proposed mechanism of p o p u l a t i o n r e g u l a t i o n . R e s u l t s of t h i s study i n d i c a t e t h a t preweanling s u r v i v a l i s l e s s v a r i a b l e than post-weanling s u r v i v a l . Preweanling s u r v i v a l d i d not become markedly worse i n the d e c l i n e phase, but i n s t e a d improved over peak phase l e v e l s . The depression of preweanling 129 s u r v i v a l i n peak p o p u l a t i o n s may be due i n p a r t to d i s p e r s a l of young animals b e f o r e they have been enumerated by t r a p s . Over 30% o f the v o l e s t h a t d i s p e r s e d from the e n c l o s e d p o p u l a t i o n s were untagged (see t r a p p i n g s e c t i o n ) , and d i s p e r s a l of s m a l l v o l e s was h i g h e s t i n the peak 1977 p o p u l a t i o n s (see d i s p e r s a l s e c t i o n ) . T h i s decreases the number of post-weanlings enumerated and thereby reduces the c a l c u l a t e d number of i n d i v i d u a l s a l i v e a t weaning. C a l c u l a t e d preweanling s u r v i v a l i s thus reduced. Hoffmann (1958) estimated preweanling s u r v i v a l by comparing the number of a c t i v e mammae of females with the number of p l a c e n t a l s c a r s . He found t h a t n e s t l i n g s u r v i v a l i n Microtus montanus i n c r e a s e d d u r i n g a p o p u l a t i o n d e c l i n e . Hoffmann r e p o r t e d an average preweanling s u r v i v a l r a t e of 77% per two weeks i n M. c a l i f o r n i c u s and 73% i n M. montanus. assuming weaning occurs at three weeks of age. Boonstra and Krebs (1978) estimated preweanling s u r v i v a l i n a peak M. townsendii p o p u l a t i o n t o be 86% per two weeks. Krebs (1964) estimated lemmimg preweanling s u r v i v a l a t 63% per two weeks i n a peak p o p u l a t i o n and 28% i n a d e c l i n e . The r e s u l t s of t h i s study i n d i c a t e a preweanling s u r v i v a l of 78% per r two weeks i n i n c r e a s i n g M. townsendii p o p u l a t i o n s , 63% i n peak p o p u l a t i o n s , and 70% i n d e c l i n i n g p o p u l a t i o n s . Preweanling m o r t a l i t y i s not a s i g n i f i c a n t component of d e c l i n i n g vole p o p u l a t i o n s . In 1976, both preweanling and post-weanling s u r v i v a l were higher i n the enclosed p o p u l a t i o n s than i n p o p u l a t i o n s i n which immigration was p o s s i b l e . Two p o s s i b l e e x p l a n a t i o n s are that e i t h e r there was i n c r e a s e d d i s p e r s a l o f post-weanlings from the open p o p u l a t i o n s or that immigrants reduced both preweanling and 130 post-weanling s u r v i v a l . There was no evidence of i n c r e a s e d post-weanling d i s p e r s a l i n open p o p u l a t i o n s i n 1976 (see d i s p e r s a l s e c t i o n ) . I t appears t h a t immigrants may have had d e l e t e r i o u s e f f e c t s on s u r v i v a l of s m a l l animals i n the summer of 1976 but not i n any other year. J u v e n i l e s u r v i v a l decreased during the p o p u l a t i o n f l u c t u a t i o n and was p a r t i c u l a r l y low i n the d e c l i n e . Poor s u r v i v a l i n j u v e n i l e s i s c h a r a c t e r i s t i c o f d e c l i n i n g v o l e p o p u l a t i o n s (Godfrey 1955; Krebs e t a l . 1969; Krebs and Myers 1974). However, low j u v e n i l e s u r v i v a l i s not r e s t r i c t e d t o d e c l i n i n g p o p u l a t i o n s . Getz (1960) estimated t h a t 90% o f v o l e s born i n a low p o p u l a t i o n died i n t h e i r f i r s t month of l i f e . J u v e n i l e s u r v i v a l can be a s i g n i f i c a n t determinant of p o p u l a t i o n growth r a t e (Krebs and Myers, 1974). Boonstra (1978) r e p o r t e d an i n v e r s e c o r r e l a t i o n between a d u l t female d e n s i t y and s u r v i v a l of young Microtus t o w n s e n d i i . As female d e n s i t y i n c r e a s e d , s u r v i v a l of i n t r o d u c e d young decreased. R e d f i e l d e t a l . (1978) found t h a t i n a M. townsendii p o p u l a t i o n a r t i f i c i a l l y d e p l e t e d of males, the number of young v o l e s r e c r u i t e d per l a c t a t i n g female was i n v e r s e l y c o r r e l a t e d with female d e n s i t y . Boonstra (1978) suggested t h a t b e h a v i o u r a l responses of a d u l t females toward young may be important i n p o p u l a t i o n dynamics o f M i c r o t u s . These r e s u l t s o r i g i n a t e from p o p u l a t i o n s i n which the p r o p o r t i o n of males was reduced f a r below n a t u r a l l e v e l s . I f these types of experiments are to be i n t e r p r e t e d i n r e l a t i o n to the key g u e s t i o n of c a u s a t i o n o f m i c r o t i n e d e c l i n e s , then the hypothesis generated must be that p o p u l a t i o n s with h i g h e r p r o p o r t i o n s or higher 131 d e n s i t i e s of females should experience l o s e r j u v e n i l e s u r v i v a l i n any c y c l i c phase. Krebs e t a l . (1969) found t h a t j u v e n i l e s u r v i v a l was not dependent on female d e n s i t y . T h i s study showed that . there was no r e l a t i o n between j u v e n i l e s u r v i v a l and ad u l t female d e n s i t y o r between j u v e n i l e s u r v i v a l and p r o p o r t i o n s of females t h a t were a d u l t s . However; th e r e was a r e l a t i o n between p r o p o r t i o n of males t h a t were a d u l t and j u v e n i l e s u r v i v a l , which suggests t h a t a d u l t males i n f l u e n c e d j u v e n i l e s u r v i v a l , S a d l e i r (1965) and Healey (1967) found t h a t the presence o f a d u l t males i n the breeding season reduced j u v e n i l e s u r v i v a l i n Peromyscus maniculatus, and C h i t t y and Phipps (1966) repo r t e d s i m i l a r f i n d i n g s f o r Mierotus a g r e s t i s and Clethriongmys g l a r e o l u s . J u v e n i l e s u r v i v a l may be reduced i n p o p u l a t i o n s i n which the sex r a t i o i s r a d i c a l l y a l t e r e d t o f a v o u r females, but there i s no evidence t o i l l u s t r a t e the same e f f e c t i n n a t u r a l p o p u l a t i o n s . Why do j u v e n i l e s e x h i b i t much lower s u r v i v a l r a t e s i n d e c l i n e p e r i o d s than i n e i t h e r i n c r e a s e o r peak periods? C h i t t y and Phipps (1966) suggested that a g g r e s s i v e behaviour by a d u l t v o l e s , e s p e c i a l l y males, leads t o e i t h e r j u v e n i l e death or e m i g r a t i o n and low s u r v i v a l r a t e s . Adult males have been reported to reduce j u v e n i l e s u r v i v a l i n Peromyscus ( S a d l e i r , 1965; Healey, 1967) and i n Apodemus (Flowerdew, 1974). These s p e c i e s f o l l o w an annual p o p u l a t i o n c y l e and j u v e n i l e s u r v i v a l i s reduced when the a d u l t s are br e e d i n g . In the present study, i n c r e a s e d d i s p e r s a l by j u v e n i l e s was not apparent d u r i n g the d e c l i n e phase (see d i s p e r s a l s e c t i o n ) , and j u v e n i l e s presumably died on the g r i d s . E i t h e r a d u l t a g g r e s s i v e behaviour was not connected with the disappearance o f j u v e n i l e s i n t h i s d e c l i n e . 132 or the k i n d o f b e h a v i o u r a l i n t e r a c t i o n s between a d u l t s and j u v e n i l e s changed so t h a t young v o l e s d i e d without d i s p e r s i n g . S u r v i v a l r a t e s o f the d i f f e r e n t age c l a s s e s i n l i v e - t r a p s and p i t f a l l s were not s t r o n g l y c o r r e l a t e d . However, because t r a p p a b i l i t y was higher i n l i v e - t r a p s than i n p i t f a l l s , minimum s u r v i v a l r a t e s c a l c u l a t e d from l i v e - t r a p enumerations are more r e l i a b l e . A c c o r d i n g l y , the d i s c u s s i o n of a d u l t and subadult s u r v i v a l w i l l be based on l i v e - t r a p data. S u r v i v a l o f a d u l t s was h i g h e r i n winter than summer. Lower summer s u r v i v a l r a t e s of a d u l t s can be accounted f o r by d i s p e r s a l of s m a l l a d u l t s and i n f e s t a t i o n s of p a r a s i t i c b o t f l i e s (Cuterebra sp.) and the grey f l e s h f l y ( W o h l f a h r t i a v i g i l ) (Boonstra, 1977a). Subadult s u r v i v a l i n the s p r i n g of 1977 was lower than a d u l t s u r v i v a l , a r e s u l t that i s c o n s i s t e n t with i n c r e a s e d d i s p e r s a l of s m a l l e r animals during t h i s p e r i o d . A d u l t s u r v i v a l was lower d u r i n g the 1977 w i n t e r , a post-peak phase time, than d u r i n g the 1976 winter, a time o f i n c r e a s i n g p o p u l a t i o n s i z e , and two e x p l a n a t i o n s are p o s s i b l e . Voles i n the post-peak winter have been s e l e c t e d f o r s u r v i v a l under c o n d i t i o n s o f mutual i n t e r f e r e n c e , and t h i s change i n p r o p e r t i e s of the i n d i v i d u a l s a l s o reduces t h e i r f i t n e s s to withstand o t h e r environmental c o n d i t i o n s ( C h i t t y , 1965). In a d d i t i o n , s i n c e t h e r e i s now a high p r o p o r t i o n of animals s e l e c t e d f o r ' t h r e a t ' , the d i r e c t l o s s e s through d i s p e r s i o n w i l l a l s o be more pronounced. The a l t e r n a t i v e view i s t h a t the h a b i t a t changed i n q u a l i t y , and t h a t food and cover were i n s h o r t e r supply d u r i n g the 1977 winter, and predators more abundant, than d u r i n g the 1976 w i n t e r . The hay was s p a r s e r and 133 s h o r t e r d u r i n g the 1977 summer than d u r i n g the 1976 summer, and t h i s p h y s i c a l d e t e r i o r a t i o n i n the q u a l i t y of the h a b i t a t may account f o r the lowered s u r v i v a l d u r i n g the 1977 winter. Both e x p l a n a t i o n s may have a p p l i e d t o the p o p u l a t i o n s d u r i n g t h i s p e r i o d . minimum s u r v i v a l r a t e s o f a d u l t s and s u b a d u l t s captured by p i t f a l l s were l e s s than the r a t e s of a p p a r e n t l y comparable animals captured by l i v e - t r a p s , and s e v e r a l e x p l a n a t i o n s are p o s s i b l e . Older v o l e s could a v o i d c a p t u r e i n p i t f a l l s , and t h e i r observed minimum s u r v i v a l r a t e s simply r e f l e c t a lower t r a p p a b i l i t y i n p i t f a l l s . T r a p p a b i l i t y in p i t f a l l s was lower than i n l i v e - t r a p s , and t h i s e x p l a n a t i o n seems most p l a u s i b l e . A l t e r n a t i v e l y , l i v e - t r a p s and p i t f a l l s may c a t c h d i f f e r e n t s o c i a l c l a s s e s , and p i t f a l l s c a t c h a c l a s s of subordinate v o l e s t h a t s u r v i v e p o o r l y . T h i s e x p l a n a t i o n may acccount f o r some of the d i f f e r e n c e i n observed minimum s u r v i v a l r a t e s , because some v o l e s caught on l y i n p i t f a l l s d i s p e r s e d before e n t e r i n g l i v e - t r a p s , thus lowering the apparent s u r v i v a l of the p i t f a l l - c a u g h t v o l e s . However, I suggest t h a t the m a j o r i t y of the d i f f e r e n c e i n apparent s u r v i v a l between v o l e s caught i n l i v e - t r a p s and p i t f a l l s i s a t t r i b u t a b l e t o d i f f e r e n t t r a p p a b i l i t i e s i n the two types of t r a p s . S u r v i v a l during the s p r i n g of 1978 was poor i n a l l age groups. What caused t h i s low s u r v i v a l ? C h i t t y (1967) i m p l i e d t h a t b e h a v i o u r a l i n t e r a c t i o n s are necessary f o r a p o p u l a t i o n d e c l i n e . In peak season d e c l i n e s the i n t e r a c t i o n s are supposed to be between 'avoidance 1 ( d o c i l e ) and ' t h r e a t ' (aggressive) types; i n d e c l i n e season d e c l i n e s the i n t e r a c t i o n s are supposed 1 3 4 to be mostly between t h r e a t types. Boonstra (1977b) hypothesized that during l a t e d e c l i n e s and periods of low numbers, v o l e s become so a g g r e s s i v e as to be unable to mate s u c c e s s f u l l y , o r i f they do mate, t h a t they w i l l destroy young i f mating i s s u c c e s s f u l . However, Krebs and Myers (1974) concluded t h a t t h e r e was no evidence to i n d i c a t e that pregnancy r a t e v a r i e d i n r e l a t i o n to the p o p u l a t i o n c y c l e . Moreover, Hoffmann (1958) and I found t h a t preweanling s u r v i v a l i n c r e a s e d i n d e c l i n i n g p o p u l a t i o n s compared with peak ones. I f b e h a v i o u r a l i n t e r a c t i o n s become more i n t e n s e i n severe d e c l i n e s and the p r o p o r t i o n o f a g g r e s s i v e animals i n c r e a s e s as C h i t t y (1967) suggested, then adverse b e h a v i o u r a l i n t e r a c t i o n s were necessary f o r the low s u r v i v a l d u r i n g the severe d e c l i n e of s p r i n g 1978. However, because d i s p e r s a l and wounding r a t e s were lower i n the 1978 breeding d e c l i n e than i n 1977 (see d e c l i n e s e c t i o n ) , these hypothesized b e h a v i o u r a l i n t e r a c t i o n s , i f they are p r e s e n t , must r e s u l t i n death without heavy wounding or d i s p e r s a l . Boonstra (1977b) proposed a model of vole behaviour i n which females compete f o r space i n order to r a i s e young, whereas males compete f o r females with which to mate. While t h i s model of i n c r e a s e d a g g r e s s i v e i n t e r a c t i o n s between i n d i v i d u a l s may e x p l a i n p e r i o d s of reduced s u r v i v a l accompanied by an i n c r e a s e i n d i s p e r s a l , i t i s inadequate to account f o r d e c l i n e s i n non-breeding seasons, f o r d i f f e r e n c e s i n s u r v i v a l between non-breeding seasons, and f o r r a p i d changes i n s u r v i v a l when breeding begins d u r i n g a d e c l i n e i n which there i s no d i s p e r s a l and l i t t l e wounding. 135 fire i n d i v i d u a l s i n d e c l i n i n g v o l e p o p u l a t i o n s the most a g g r e s s i v e of any phase, as suggested by C h i t t y (1967)? Krebs (1970) found t h a t male Mierotus i n peak d e n s i t i e s were the most a g g r e s s i v e i n d i v i d u a l s , whereas d e c l i n e males were i n t e r m e d i a t e i n a g g r e s s i v e behaviour between i n c r e a s e and peak p o p u l a t i o n males. Thus a g g r e s s i v e behaviour i n t h i s case was r e l a t e d more to d e n s i t y than to c y c l i c a l phase. Turner and Iverson (1973) found t h a t i n M. pennsylyanicus, l e v e l s of aggression d i d not vary c o n s i s t e n t l y with p o p u l a t i o n d e n s i t y . Males were more a g g r e s s i v e i n the breeding season than i n the winter non-breeding season. There i s no evidence t h a t a g g r e s s i v e n e s s , d i r e c t l y measured by b e h a v i o u r a l o b s e r v a t i o n s i n the l a b o r a t o r y , i s h i g h e s t i n the d e c l i n e phase o f the m i c r o t i n e c y c l e ; i n f a c t i t seems to have lessened by that time. Thus a g g r e s s i v e behaviour seems t o be a poor e x p l a n a t i o n f o r low s u r v i v a l during the d e c l i n e ; but e x t r i n s i c f a c t o r s (food, p r e d a t o r s , etc.) are a poor e x p l a n a t i o n f o r r a p i d changes i n s u r v i v a l with the onset of the 1978 breeding season. The present study suggests s e v e r a l a r e a s t h a t should be explored f u r t h e r . The cause o f low j u v e n i l e s u r v i v a l d u r i n g the severe s p r i n g d e c l i n e of 1978 i s of prime i n t e r e s t . Removal of a d u l t male or a d u l t females before an a n t i c i p a t e d severe d e c l i n e should improve j u v e n i l e s u r v i v a l i f b e h a v i o u r a l i n t e r a c t i o n s are the cause of low j u v e n i l e s u r v i v a l i n severe d e c l i n e s . I f C h i t t y (1967) and Krebs and Boonstra (1978) are c o r r e c t i n s u g g e s t i n g t h a t s p a c i n g behaviour i s more severe i n d e c l i n i n g p o p u l a t i o n s , and presumably can account f o r the low s u r v i v a l , one c o u l d a d m i n i s t e r drugs t o reduce a g g r e s s i o n (Vessey, 19 67) 136 or i n c r e a s e aggression (Krebs e t a l . • , 1977) i n a v o l e p o p u l a t i o n about t o undergo a severe d e c l i n e and observe i t s demographic consequences. L i t e r a t u r e c i t e d Boonstra, R. 1977a. E f f e c t of t h e p a r a s i t e W o h l f a h r t i a v i g i l on M i c r o t u s townsendii p o p u l a t i o n s . Can. J . Z o o l . 55: 1057-1060. — , 1977b. E f f e c t of c o n s p e c i f i c s on s u r v i v a l d u r i n g p o p u l a t i o n d e c l i n e s i n Microtus townsendii. J . Anim. E c o l . . 46: 835-851. , 1978. E f f e c t of a d u l t Townsend v o l e s ( M i c r o t u s townsendii ) on s u r v i v a l of young. Ecology, 59: 242-248. Boonstra, H. and C. J . Krebs. 1978. P i t f a l l t r a p p i n g of Microtus townsendii. J. Mammal. 59: 136-148. C h i t t y , D. 1965. P r e d i c t i n g q u a l i t a t i v e changes i n i n s e c t p o p u l a t i o n s . Proc. 12th I n t . Congr. Entomol. Lond. 384-386. , 1967. The n a t u r a l s e l e c t i o n of s e l f - r e g u l a t o r y behaviour i n animal p o p u l a t i o n s . Proc. E c o l . Soc. Aust. 2: 51-78. C h i t t y , D. and E. Phipps. 1966. Seasonal changes i n s u r v i v a l i n mixed p o p u l a t i o n s of two v o l e s p e c i e s . J . Anim. E c o l . 35: 313-331. Flowerdew, J . R. 1974. F i e l d and l a b o r a t o r y experiments on the s o c i a l behaviour and p o p u l a t i o n dynamics of the wood mouse (Apodemus §xlvaticus). J . Anim. E c o l . 43: 137 499-511. Getz, L. L. 1960. A pop u l a t i o n study o f the v o l e , Mierotus p e n n s y lv a n i c u s . Amer. M i d i . Nat. 64: 392-05. Godfrey, G- K. 1955. Observations on the nature o f the d e c l i n e i n numbers of two Mierotus p o p u l a t i o n s . J . Mammal. 36: 209-214. Hamilton* H. J . , J r . 1941. Reproduction o f the f i e l d v o l e Mierotus pennsylvanicus (Ord) . C o r n e l l Univ. A g r i c . Exper. Sta., Memoir, 237: 1-23. Healey, M. C. 1967. Aggression and s e l f - r e g u l a t i o n of p o p u l a t i o n s i z e i n deermice. Ecology, 48: 377-392. Hoffmann, R. S. 1958. The r o l e of r e p r o d u c t i o n and m o r t a l i t y i n p o p u l a t i o n f l u c t u a t i o n s of voles (Mierotus). E c o l . Monogr. 28: 79-109. Krebs, C. J . 1964. The lemming c y c l e at Baker Lake, Northwest T e r r i t o r i e s , d u r i n g 1959-62. A r c t i c I n s t . N. Amer. Tech. Pap. No. 15. pp 1-104. Krebs, C. J . 1966. Demographic changes i n f l u c t u a t i n g p o p u l a t i o n s o f Mierotus c a l i f o r n i c u s . E c o l . Monogr. 36: 239-273. , 1970. Mierotus p o p u l a t i o n b i o l o g y : b e h a v i o r a l changes a s s o c i a t e d with the p o p u l a t i o n c y c l e i n M. ochrogaster and M. pennsylvanicus. Ecology, 51: 34-52. Krebs, C. J . and K. T. DeLong. 1965. A Mierotus p o p u l a t i o n with supplemental food. J . Mammal. 46: 566-573. Krebs, C. J . , B. L. K e l l e r , and R. H. Tamarin 1969. Mierotus p o p u l a t i o n b i o l o g y : demographic changes i n f l u c t u a t i n g p o p u l a t i o n s of M. ochrogaster and M. 138 pennsylvanicus i n southern I n d i a n a . Ecology, 50: 587-60 7. Krebs, C. J . and J . H. Myers. 1974. P o p u l a t i o n c y c l e s i n s m a l l mammals. Adv. E c o l . Hes. 8: 267-399. Krebs, C. J . , Z. T. H a l p i n , and J . N. M. Smith. 1977. Aggr e s s i o n , t e s t o s t e r o n e , and the s p r i n g d e c l i n e i n p o p u l a t i o n s of the vole Microtus townsendii. Can. J . Z o o l . 55: 430-437. Krebs, C. J . and B. , Boonstra. 1978. Demography of the s p r i n g d e c l i n e i n p o p u l a t i o n s of the v o l e H i c r o t u s t o w n s e n d i i . J . Anim. E c o l . 47: 1007-1015. R e d f i e l d , J . A., M. J . T a i t t , and C. J . Krebs. 1978. Experimental a l t e r a t i o n of sex r a t i o s i n p o p u l a t i o n s of H i c r o t u s t o w n s e n d i i . a f i e l d v o l e . Can. J . ; Z o o l . 56: 17-27. Richmond, H. and c. H. Conway. Management, breeding and r e p r o d u c t i v e pervormance of the v o l e H i c r o t u s qchroqaster, i n a l a b o r a t o r y colony. Lab. Anim. Care, 19: 80-87. S a d l e i r , B. H. F. S. 1965. The r e l a t i o n s h i p between a g o n o s t i c behaviour and p o p u l a t i o n changes i n the deer mouse, Peromyscus maniculatus (Wagner). J . Anim. E c o l . 14: 331-352. Turner, B. N. and S. L. I v e r s o n . 1973. The annual c y c l e of a g g r e s s i o n i n male H i c r o t u s pennsylvanicus, and i t s r e l a t i o n t o p o p u l a t i o n parameters. Ecology, 54: 967-981. Vessey, S. 1967. E f f e c t s of chlorpromazine on a g g r e s s i o n i n l a b o r a t o r y p o p u l a t i o n s of w i l d house mice. Ecology, 48: 367-376. 139 SECTION 5. DEMOGRAPHY OF DECLINING POPULATIONS OF THE VOLE MICROTUS TOWNSENDII I n t r o d u c t i o n M i c r o t i n e p o p u l a t i o n s undergo p e r i o d i c changes i n abundance, the cause o f which i s s t i l l c o n t r o v e r s i a l (Krebs and Myers, 1974). Sudden d e c l i n e s u s u a l l y occur i n the s p r i n g at t h e s t a r t of the annual breeding season ( C h i t t y and Phipps 1966; Flowerdew 1972; F a i r b a i r n 1977), and can be v a r i a b l e i n magnitude, r a t e , and d u r a t i o n (Krebs and Boonstra, 1978). Krebs and Boonstra (1978) suggest t h a t s p r i n g d e c l i n e s i n v o l e s are a r e s u l t o f s p a c i n g behaviour by breeding animals, which causes e i t h e r death or d i s p e r s a l from the p o p u l a t i o n . In t h i s study, I provide a d e t a i l e d demographic a n a l y s i s of two d e c l i n e s i n f o u r p o p u l a t i o n s of Mierotus t o w n s e n d i i , and i n t e r p r e t the r e s u l t s i n r e l a t i o n to the C h i t t y h y p othesis ( C h i t t y , 1967), the Krebs and Boonstra (1978) h y p o t h e s i s , and the food h y p o t h e s i s ( P i t e l k a 1958, S c h u l t z 1964, 1969). Methods The study area was part o f a Timothy hay f i e l d owned by the Canadian W i l d l i f e S e r v i c e on R e i f e l I s l a n d i n the F r a s e r River d e l t a near Vancouver, B r i t i s h Columbia. Pale am- pratense was the dominant s p e c i e s : A g r o s t i s a l b a , Lolium perenne. and Holeus l a n a t u s were a l s o present. Four Mierotus townsendii p o p u l a t i o n s were l i v e - t r a p p e d from 140 May 1976 u n t i l June 1978. P o p u l a t i o n s B and C were enc l o s e d by a v o l e - p r o o f fence, p o p u l a t i o n s A and D were unfenced c o n t r o l s . Each g r i d had 49 t r a p s t a t i o n s s e t 7.6 m (25 f e e t ) apart i n a 7 x 7 p a t t e r n . Each s t a t i o n had two Longworth l i v e - t r a p s and one p i t f a l l t r a p s i m i l a r to the type d e s c r i b e d by Boonstra and Krebs (1978). The l i v e - t r a p s , b a i t e d with oats and s u p p l i e d with c o t t o n , were s e t every second week on Monday a f t e r n o o n , checked Tuesday morning and af t e r n o o n , and checked and loc k e d open on Wednesday morning. P i t f a l l t r a p s were used from May through October 1976, A p r i l through September 1977, and A p r i l through June, 1978, but could not be used at other times on account of winter f l o o d i n g . They were s e t every week on Wednesday morning, checked Wednesday a f t e r n o o n , Thursday morning, and c l o s e d on Thursday a f t e r n o o n . When both t r a p types were set i n one week, the p i t f a l l s were set a f t e r the l i v e - t r a p s had been locked open. A l l v o l e s were ear-tagged, and on each c a p t u r e , tag number, weight, capture l o c a t i o n ^ sex, r e p r o d u c t i v e c o n d i t i o n , and wounding were recorded. A l l v o l e s were r e l e a s e d immediately a f t e r b e i n g processed. B e s u l t s P o p u l a t i o n Density The p o p u l a t i o n changes i n Microtus townsendii were s i m i l a r but not i d e n t i c a l on a l l f o u r g r i d s ; hence I w i l l d i s c u s s each 141 s e p a r a t e l y by using one as a model, and then by d e s c r i b i n g r e l e v a n t d i f f e r e n c e s i n the ot h e r s . Figure 5.1 shows d e n s i t y changes on c o n t r o l g r i d A. In May 1976, when t r a p p i n g began, the l i v e - t r a p p o p u l a t i o n d e n s i t y was low and remained low during the summer, and i n c r e a s e d a t 8% per week d u r i n g the f a l l and winter, owing to delayed capture i n l i v e - t r a p s . The p o p u l a t i o n d e c l i n e d with the onset of the bree d i n g season i n March 1977. T h i s d e c l i n e was s u s t a i n e d almost e n t i r e l y by the males, which d e c l i n e d at 8% per week compared with 2% per week f o r females. The p o p u l a t i o n i n c r e a s e d d u r i n g the summer and e a r l y f a l l a t an average r a t e of ,3% per week, reached a peak i n October 1977, and then d e c l i n e d a t 355 per week. The r a t e of d e c l i n e a c c e l e r a t e d r a p i d l y a f t e r the s t a r t of the 1978 breeding season f o r both males and females ( F i g . 5.1). In 1978, the i n i t i a l breeding d e n s i t y of males was 46% of t h a t i n 1977, and of females was 62%. However, the breeding season r a t e of d e c l i n e of both males and females was f i v e times f a s t e r i n 1978 than i n 1977. F i g u r e 5.2 shows d e n s i t y changes on c o n t r o l g r i d D. A f t e r breeding s t a r t e d i n March 1977 the po p u l a t i o n underwent a s p r i n g d e c l i n e i n males but not i n females. The r a t e o f d e c l i n e i n c r e a s e d i n both sexes a f t e r the 1978 breeding season began, with male d e c l i n e r a t e s s i x times f a s t e r and female d e c l i n e r a t e s 24 times f a s t e r than those of the 1977 breeding d e c l i n e ( F i g . 5.2). I n i t i a l 1978 br e e d i n g p o p u l a t i o n s of both sexes were about 25% as l a r g e as those of 1977. Density changes on experimental g r i d B are shown i n F i g u r e 5.3. The p o p u l a t i o n d e c l i n e d a f t e r breeding began i n March 1977, with the r a t e about f o u r times f a s t e r i n males than i n 142 F i g u r e 5.1. P o p u l a t i o n d e n s i t y , i n s t a n t a n e o u s r a t e of i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r Mierotus townsendii on c o n t r o l g r i d A d u r i n g 1976-1978. Non-breeding periods are shaded. Instantaneous r a t e of i n c r e a s e curve smoothed by 3-point running average. The h o r i z o n t a l l i n e on the s u r v i v a l p l o t i s a s u r v i v a l r a t e of 0.707, below which h a l f the p o p u l a t i o n disappears every f o u r weeks. Density 1976 1977 1978 144 Figu r e 5.2. P o p u l a t i o n d e n s i t y , i n s t a n t a n e o u s r a t e of i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r Microtus townsendii on c o n t r o l g r i d D d u r i n g 1976-1978. Non-breeding p e r i o d s are shaded. 145 146 F i g u r e 5.3. P o p u l a t i o n d e n s i t y , i n s t a n t a n e o u s r a t e of i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r Microtus townsendii on fenced g r i d B d u r i n g 1976-1978. Non-breeding p e r i o d s are shaded. Density m Rate of Increase 1976 1977 1978 148 females. a f t e r breeding began i n the s p r i n g of 1978, the d e c l i n e a c c e l e r a t e d to r a t e s about t h r e e times g r e a t e r f o r males and f i v e times g r e a t e r f o r females than i n the 1977 s p r i n g d e c l i n e . The male i n i t i a l breeding p o p u l a t i o n was 38%, and the female p o p u l a t i o n 30%, of the e q u i v a l e n t 1977 p o p u l a t i o n s . F i g u r e 5.4 shows d e n s i t y changes on experimental g r i d C. The p o p u l a t i o n d e c l i n e d a f t e r the onset of the 1977 breeding season; males d e c l i n e d about t h r e e times f a s t e r than females. The r a t e of d e c l i n e a c c e l e r a t e d a f t e r the 1978 breeding began, with male d e c l i n e r a t e s 10 times g r e a t e r and female r a t e s over 20 times g r e a t e r than those of the 1977 s p r i n g d e c l i n e . The males d e c l i n e d t o e x t i n c t i o n i n May at an average r a t e of 88% l o s s per week a f t e r breeding s t a r t e d , whereas females d e c l i n e d at 28% per week. I n i t i a l 1978 breeding d e n s i t i e s were o n e - h a l f i n i t i a l 1977 breeding d e n s i t i e s i n both sexes. Body Weights Body weight d i s t r i b u t i o n s of g r i d C males are shown i n F i g u r e 5-5 and of g r i d B males i n F i g u r e 5.6. There was much seasonal and y e a r l y v a r i a t i o n i n body weights. Voles on a l l g r i d s were h e a v i e s t i n the s p r i n g of 1977, a time of peak d e n s i t y . average male weight on g r i d C a t the s t a r t of breeding was 63.5 g i n March 1977 and 54.6 g i n March 1978, and female weights were 50.9 g and 44.1 g r e s p e c t i v e l y . Males averaged 16% l a r g e r and females 15% l a r g e r i n s p r i n g 1977 than s p r i n g 1978. The same p a t t e r n was found on the other g r i d s . G r i d A males were 2% l a r g e r , g r i d B males 8% l a r g e r , and g r i d D males 8% 149 Figure 5.4. P o p u l a t i o n d e n s i t y , instantaneous r a t e of i n c r e a s e , and minimum two-week s u r v i v a l r a t e f o r H i c r o t u s townsendii on fenced g r i d C duri n g 1976-1978. Non-breeding p e r i o d s are shaded. Densi ty Ra te of Increase Surv iva l M J S N J M M J S 1976 1977 N J M M J 1978 151 F i g u r e 5.5. Histogram of body weight d i s t r i b u t i o n s of male Mierotus townsendii on fenced g r i d C. 153 Fi g u r e 5.6. Histogram o f body weight d i s t r i b u t i o n s of male H i e r o t u s t o w n s e n d i i on fenced g r i d B. 155 l a r g e r i n s p r i n g 1977 compared with s p r i n g 1978. G r i d A females were 3% l a r g e r , g r i d B females 5% l a r g e r , and g r i d D females 2% l a r g e r i n s p r i n g 1977 compared with those i n s p r i n g 1978. F i g u r e s 5.5 and 5.6 show t h a t during the l a t e winter and s p r i n g o f 1978 the body weight of males on g r i d C was l e s s v a r i a b l e than t h a t of the males on g r i d B. Smaller males were present on g r i d C from l a t e winter u n t i l the s t a r t o f the breeding season i n March 1978, whereas on g r i d B, s m a l l e r males disappeared d u r i n g t h i s i n t e r v a l . The f i g u r e s a l s o show th a t d u r i n g t h i s i n t e r v a l t h e r e was an absence of l a r g e r - s i z e d males on g r i d C. Rates Of D e c l i n e The average r a t e of d e c l i n e per week was used as a measure of i n t e n s i t y of the s p r i n g d e c l i n e . The o v e r a l l percentage l o s s o f . i n d i v i d u a l s was r e l a t e d to the r a t e of d e c l i n e both i n males (r=.94, n=8, P<-001) and i n females (r=-92, n=8, P<.001) . The breeding d e c l i n e s l a s t e d about as long on a l l g r i d s w i t h i n the same year. Thus, the magnitude of the s p r i n g d e c l i n e was r e l a t e d t o the r a t e , not the d u r a t i o n , of d e c l i n e . The s p r i n g of 1978 was c h a r a c t e r i z e d by high r a t e s of d e c l i n e , so t o determine whether or not the r a t e of d e c l i n e was c o r r e l a t e d with the p r o p o r t i o n of heavy-weight v o l e s i n a p o p u l a t i o n , I examined vari a n c e of body weights w i t h i n a po p u l a t i o n i n r e l a t i o n t o the r a t e of d e c l i n e before and a f t e r breeding began. I c a l c u l a t e d the r a t e of d e c l i n e before breeding as the average of t h e f o u r preceding sampling p e r i o d s . 156 c o v e r i n g an eight-week p e r i o d p r i o r t o breeding. The v a r i a n c e of body weights was a weighted average of the v a r i a n c e s of the p r e v i o u s f o u r p e r i o d s . Variance i n body weights was c o r r e l a t e d with r a t e s o f d e c l i n e a f t e r b r e e d i n g began i n males (r=-.92, n=4, P<. 10) (Table 5.1) and i n females (r=-.88, n=4, P<. 13) (Table 5.2). T h i s c o r r e l a t i o n i n d i c a t e s t h a t the h i g h e r the v a r i a n c e i n body weights b e f o r e breeding began i n 1978, the slower the r a t e of d e c l i n e a f t e r breeding began. The r a t e of d e c l i n e b e f o r e breeding began i n 1978 was i n v e r s e l y r e l a t e d to the r a t e a f t e r breeding began (r=-.95, n-U, P<.05), which i n d i c a t e s t h a t g r i d s t h a t experienced a higher r a t e o f d e c l i n e i n the eight-week i n t e r v a l b e f o r e breeding tended t o have a lower d e c l i n e r a t e once breeding began. I examined the r a t e of d e c l i n e both i n 1977 and i n 1978 to determine i f d e c l i n e r a t e s were c o r r e l a t e d with the p r o p o r t i o n of heavy-weight animals i n the p o p u l a t i o n s . In males, the p r o p o r t i o n of the p o p u l a t i o n above 70 g was c a l c u l a t e d f o r an eight-week p e r i o d a f t e r breeding began i n 1977 and 1978. In females, the p r o p o r t i o n above 58 g was c a l c u l a t e d f o r 1977 and above 50 g f o r 1978. I lowered the c u t - o f f weight i n 1978 because t h e r e were few females above 58 g, and the r e l a t i v e rankings of the g r i d s d i d not change. In 1977, p o p u l a t i o n s with higher p r o p o r t i o n s of heavy animals d e c l i n e d f a s t e r (males r=.92, females r = . 9 0 , P<.10). A l s o , p o p u l a t i o n s with higher p r o p o r t i o n s of heavy animals and higher body weight v a r i a n c e had more d i s p e r s a l ( d i s p e r s a l s e c t i o n ) , which accounted f o r the higher r a t e s of d e c l i n e i n these p o p u l a t i o n s . In c o n t r a s t , p o p u l a t i o n s i n 1978 with a 157 Table 5.1. Instantaneous r a t e s of d e c l i n e per week, number (and % of population) of males a t the s t a r t and end, p r o p o r t i o n of heavy-weight males, and v a r i a n c e of body weights d u r i n g the breeding d e c l i n e s of 1977 and 1978. Body weight v a r i a n c e i s c a l c u l a t e d f o r an eight-week p e r i o d b e f o r e breeding begins. G r i d Rate of D e c l i n e Number (%) at s t a r t Number (%) Prop.> at end % l o s s 70g. Body Ht. Variance 1977 breeding d e c l i n e s A 0.0754 84 (51) 35 (35) 58.3 0.232 84.3 B 0.1038 132 (50) 38 (29) 71.2 0.371 82.2 C 0.0586 105 (51) 39 (33) 62.1 0.208 99.4 D 0.0809 103 (52) 39 (31) 62.1 0.231 86.5 1978 breeding d e c l i n e s A 0.3806 42 (46) 2 (11) 95.2 0.235 71.0 B 0.2792 56 (59) 6 (35) 89.3 0.241 : 73.5 C 0.8815 34 (37) 0 (0) 100.0 0.000 36.0 D 0.4580 39 (51) 1 (8) 97.4 0. 108 50.8 158 Table 5.2. Instantaneous r a t e s of d e c l i n e per week, p r o p o r t i o n of heavy-weight females, and v a r i a n c e of body weights d u r i n g the breeding d e c l i n e s of 1977 and 1978. Body weight v a r i a n c e i s c a l c u l a t e d f o r an eight-week p e r i o d before breeding begins. Bate of Number Number Prop. 1 > Body Wtw G r i d D e c l i n e at s t a r t at end % l o s s 58 g. Variance 1977 breeding d e c l i n e s k 0.0206 82 65 20.7 0.089 52.7 B 0.0304 131 94 28.2 0.20 6 54.0 C 0.0203 102 81 20.5 0.146 50.8 D 0.0055 94 88 6.4 0.068 46.9 1978 breeding d e c l i n e s k 0.1399 49 16 67.3 0.534 29.8 B 0.1582 39 11 71.8 0.481 33.0 C 0.2835 58 6 89.7 0.086 22.9 D 0.1308 37 12 67.6 0.543 41.0 1 1978 p r o p o r t i o n > 50 g. 159 higher p r o p o r t i o n of heavy animals had slower r a t e s of d e c l i n e (males r=-.93, females r=-.99, P<.10). The f a s t e r the d e c l i n e i n the breeding season, the l e s s was the d i s p e r s a l of e i t h e r sex (males r=-.82, n=4, P<.20; females r=-.90, P<.10)« Thus d i s p e r s a l can account f o r more of the l o s s i n the modest 1977 d e c l i n e than i n the severe 1978 breeding d e c l i n e . In summary, the i n i t i a l d e n s i t y of the breeding p o p u l a t i o n of males and females was not c o r r e l a t e d with subsequent r a t e s of p o p u l a t i o n d e c l i n e i n the s p r i n g . When d e c l i n e s were concurrent with p e r i o d s of i n c r e a s e d d i s p e r s a l , d e n s i t y and r a t e of d e c l i n e were s l i g h t l y c o r r e l a t e d , which suggests that d i s p e r s a l d e c l i n e s c o u l d be density-dependent w i t h i n one season. However, when v o l e s d i s p e r s e d , there was a s t r o n g e r c o r r e l a t i o n between r a t e of d e c l i n e and p r o p o r t i o n of heavy-weight animals, with the higher the p r o p o r t i o n of heavy-weight v o l e s , the f a s t e r the s p r i n g d e c l i n e . There was a l s o a c o r r e l a t i o n between d e n s i t y and p r o p o r t i o n of heavy-weight males (r=.83# n=8, P<.01) and 1977 females (r=.91, n=4, P<.10), but not 1978 females (r=-.79, n=4, P<.25). These data suggest t h a t the p r o p o r t i o n of heavy-weight animals, not d e n s i t y , i s the r e l e v a n t v a r i a b l e to measure to p r e d i c t the r a t e of s p r i n g d e c l i n e . S i z e - s e l e c t i v i t y Of S u r v i v a l I examined minimum s u r v i v a l r a t e s d u r i n g the 1977 breeding d e c l i n e to determine i f s u r v i v a l was s i z e - s e l e c t i v e . Males above 59 g and females above 49 g were ' l a r g e ' ; o t h e r s were 160 • s m a l l 1 . The s u r v i v a l of l a r g e animals was scored ••' when i t was g r e a t e r than the s u r v i v a l o f s m a l l animals during a week, and when i t was lower, a s i g n t e s t a n a l y s i s was conducted on the minimum s u r v i v a l r a t e s estimated biweekly of l a r g e and s m a l l animals of both sexes on each g r i d d u r i n g the d e c l i n e . T h i s a n a l y s i s i n d i c a t e d t h a t l a r g e r males s u r v i v e d b e t t e r than s m a l l e r males ( 2 3 + , 9 - , P < . 0 5 ) ; s i m i l a r l y with females ( 2 4 + , 8 - , P < . 0 1 ) . D i s p e r s a l o f l i g h t e r animals was common d u r i n g t h i s d e c l i n e ( d i s p e r s a l s e c t i o n ) , and the lower s u r v i v a l r a t e s o f l i g h t e r males and females may be a t t r i b u t e d t o d i s p e r s a l . I i n v e s t i g a t e d whether or not male s u r v i v a l was s i z e - s e l e c t i v e b efore and d u r i n g the 1 9 7 8 breeding season by c a l c u l a t i n g minimum s u r v i v a l r a t e s f o r the h e a v i e r 8 0 % and l i g h t e r 2 0 % p r o p o r t i o n s of the male p o p u l a t i o n f o r each sampling p e r i o d over four p e r i o d s ( 8 weeks) before breeding s t a r t e d and from then u n t i l the end of the d e c l i n e . I c a l c u l a t e d male minimum s u r v i v a l r a t e s of a f i x e d percentage r a t h e r than a f i x e d weight because I wanted t o see whether or not s u r v i v a l of a group of v o l e s was dependent upon t h e i r being the l i g h t e s t i n d i v i d u a l s present i n the p o p u l a t i o n a t a p a r t i c u l a r week. G r i d C males were e l i m i n a t e d from t h i s a n a l y s i s because they were at r e l a t i v e l y uniform body s i z e and t h e r e f o r e p r o v i d e d i n s u f f i c i e n t phenotypic range f o r s e l e c t i o n t o a c t upon ( F i g . 5 . 5 ) . A s i g n t e s t a n a l y s i s f o r the remaining three g r i d s i n d i c a t e d t h a t the heavy-weight males s u r v i v e d s i g n i f i c a n t l y b e t t e r than the l i g h t - w e i g h t males ( 1 6 + , 5 - , P < - 0 4 ) . Heavier males were at a s e l e c t i v e advantage over l i g h t e r males before and d u r i n g the 1 9 7 8 breeding d e c l i n e . 161 Female s u r v i v a l r a t e s were a l s o compared before and d u r i n g the breeding season i n a s i m i l a r manner, but the s i g n t e s t a n a l y s i s showed no s i g n i f i c a n t d i f f e r e n c e s i n s u r v i v a l between l a r g e r and s m a l l e r females i n 1978. Growth Bates Instantaneous growth r a t e s were examined f o r s i z e c l a s s e s of v o l e s from mid-December u n t i l l a t e March i n 1977 and 1978. S i z e c l a s s e s were: under 40 g, 40-49 g, 50-59 g, and g r e a t e r than 59 g. I d i d a four-way a n a l y s i s of c o v a r i a n c e with growth ra t e as the dependent v a r i a b l e , weight as a c o v a r i a t e , and sex, g r i d , year, and weight c l a s s as the f o u r i n d i c e s . The a n a l y s i s i n d i c a t e d t h a t d u r i n g t h i s time of year, there were no s i g n i f i c a n t d i f f e r e n c e s i n growth r a t e s between sexes, among g r i d s , or between y e a r s ; but t h e r e was a d i f f e r e n c e between weight c l a s s e s (F=5.4Q, df=3,4602, P<-01), and the i n t e r a c t i o n between year and s i z e c l a s s was s i g n i f i c a n t (F=9. 14, P<.001). The f o l l o w i n g comparisons of winter growth r a t e s are based upon mean growth r a t e s c o r r e c t e d f o r s i z e e f f e c t s by c o v a r i a n c e a n a l y s i s . Voles under 40 g l o s t weight over winter, an average l o s s of 0.20% per day. They l o s t 0.10% per day i n 1977 and 0.27% per day i n 1978. Voles 40-49 g gained 0.12% per day i n 1977, but l o s t 0.03% per day i n 1978. Voles over 49 g gained weight i n both y e a r s , about 0.09% per day i n 1977, and 0.22% per day i n 1978. Small v o l e s l o s t weight more r a p i d l y i n 1978 than i n 1977, but l a r g e r voles gained weight more r a p i d l y d u r i n g the same winter. 162 Hounding Rates Tab l e 5.3 shows t h a t wounding was more severe i n both sexes du r i n g the 1977 s p r i n g d e c l i n e than i n the 1978 s p r i n g d e c l i n e . Wounding i n c r e a s e s a f t e r breeding begins but r e q u i r e s about s i x weeks to reach maximum i n t e n s i t y (Krebs and Boonstra, 1978). However, the breeding d e c l i n e was s h o r t e r i n 1978 than i n 1977, and i n 1978 the v o l e s may simply not have had enough time to accumulate wounds. To i n v e s t i g a t e t h i s p o s s i b i l i t y , I compared the average number of wounds per i n d i v i d u a l f o r the f i r s t f o u r sampling p e r i o d s (6 weeks) of t h e two breeding d e c l i n e s . When wounding was higher i n a g i v e n week i n 1977 than i n 1978, the comparison was s c o r e d 1 ; when i t was lower i t was scored T h i s s i g n t e s t a n a l y s i s showed t h a t wounding l e v e l s were hig h e r i n the 1977 breeding d e c l i n e i n females (13+, 1-, 2 t i e s , P<.01), but not s i g n i f i c a n t l y i n males (11*, 5-, P>.10). The 1978 breeding d e c l i n e was not accompanied by unusually h i g h wounding l e v e l s . There was a r e l a t i o n between p r o p o r t i o n of heavy animals and average number of wounds i n c u r r e d d u r i n g the breeding season i n males (r=.73, n=8, P<-05) and i n females (r=.68, n=8, P<.10). P o p u l a t i o n s with higher p r o p o r t i o n s of heavy animals tended to have more wounding and a l s o t o have lower s u r v i v a l of l i g h t - w e i g h t animals during t h e d e c l i n e s . I compared the wounding and breeding c o n d i t i o n i n v o l e s t h a t s u r v i v e d and those that disappeared during each d e c l i n e . Wounding was more severe i n s u r v i v o r s of the 1978 d e c l i n e than i n those l o s t . There was no s i g n i f i c a n t d i f f e r e n c e i n wounding 1 6 3 Table 5.3. Average number of wounds per animal f o r males and females d u r i n g breeding d e c l i n e s . Sample s i z e i s i n parentheses. G r i d A B C D Male Female Male Female Male Female Male Female 1977 1. 10 0.06 1. 15 0.04 1.05 0.07 1.05 0.09 (4 61) (619) (299) (424) (503) (547) (425) (536) 1978 0.40 0.04 0.85 0.02 0.28 0.03 0.62 0.02 (59) (139) (157) (133) (95) (187) (42) (94) 164 between s u r v i v o r s and n o n - s u r v i v o r s of the 1977 d e c l i n e , which l a s t e d l o n g e r than the 1978 breeding d e c l i n e . During both d e c l i n e s , s u r v i v i n g v o l e s were more o f t e n i n breeding c o n d i t i o n than those t h a t disappeared. The 1977 r e s u l t can be accounted f o r by d i s p e r s a l o f l i g h t - w e i g h t v o l e s t h a t were l e s s o f t e n r e p r o d u c t i v e l y mature than were l a r g e r v o l e s d u r i n g the 1977 d e c l i n e . The 1978 d i f f e r e n c e r e f l e c t s the f a c t t h a t v o l e s s u r v i v i n g the f i r s t f o u r weeks of the 1978 breeding d e c l i n e had a l o n g e r p e r i o d to become r e p r o d u c t i v e t han those t h a t disappeared soon a f t e r breeding s t a r t e d . D i s c u s s i o n P o p u l a t i o n d e c l i n e s are h i g h l y v a r i a b l e i n s m a l l mammals. C h i t t y (1955) recognized three types of m i c r o t i n e d e c l i n e s based on the r a t e of p o p u l a t i o n decrease and the d u r a t i o n and t i m i n g of the d e c l i n e . Type M d e c l i n e s are very r a p i d p o p u l a t i o n decreases i n which numbers f a l l t o a low d u r i n g the winter or e a r l y s p r i n g a f t e r a peak ye a r . Type G d e c l i n e s are of a maximum d u r a t i o n of one year and have no recovery i n numbers dur i n g t h e breeding season. Type H d e c l i n e s i n v o l v e a gradual decrease i n numbers over one or more years with some recovery d u r i n g the breeding season. Spr i n g d e c l i n e s i n p o p u l a t i o n s i z e are a c h a r a c t e r i s t i c of many s m a l l mammal p o p u l a t i o n s ( C h i t t y and Phipps 1966, Krebs 1966, Myers and Krebs 1971). The r a t e of d e c l i n e i n c r e a s e s a f t e r the breeding season begins and marked decreases i n s u r v i v a l i n one or both sexes occur. These d e c l i n e s can vary both i n r a t e and i n d u r a t i o n (Krebs and Boonstra, 1978). 165 V a r i a b i l i t y i n the r a t e of some s p r i n g d e c l i n e s may depend on the p r o p e r t i e s of the i n d i v i d u a l s i n the p o p u l a t i o n at the time of the d e c l i n e . Krebs and Boonstra (1978), a f t e r f a i l i n g to demonstrate a c o r r e l a t i o n between the p r o p o r t i o n of males over 70 g i n March and the r a t e of the breeding d e c l i n e , showed t h a t t h e r e was a c o r r e l a t i o n between the p r o p o r t i o n of heavy-weight males and the r a t e of s p r i n g d e c l i n e i n the f o l l o w i n g year. Peak p o p u l a t i o n s c o n t a i n l a r g e r v o l e s than i n c r e a s i n g or d e c l i n i n g ones, and r a t e s of d e c l i n e i n the breeding season are higher i n d e c l i n i n g p o p u l a t i o n s than i n peak ones. T h e r e f o r e , because males i n the i n c r e a s i n g phase are s m a l l e r than those i n the peak phase, and the r a t e of d e c l i n e i n the peak phase breeding season i s l e s s than i n the d e c l i n e phase breeding season, t h e r e must n e c e s s a r i l y be a c o r r e l a t i o n between body s i z e and r a t e of d e c l i n e i f a one-year time l a g i s i n t r o d u c e d . Thus the Krebs and Boonstra s u g g e s t i o n t h a t P o p u l a t i o n s with l a r g e - s i z e d males thus are programmed to drop s h a r p l y i n numbers the f o l l o w i n g s p r i n g ' * simply shows that a d e c l i n e phase f o l l o w s a peak. C o r r e l a t i o n s between d e c l i n e r a t e s and body s i z e w i t h i n a year may have g r e a t e r b i o l o g i c a l r e l e v a n c e , and t h i s study shows t h a t when d i s p e r s a l was common, as at a peak, p o p u l a t i o n s with h i g h e r p r o p o r t i o n s o f l a r g e - s i z e d v o l e s had f a s t e r r a t e s o f d e c l i n e . They a l s o had more d i s p e r s a l (see d i s p e r s a l s e c t i o n ) , which accounts f o r the f a s t e r d e c l i n e r a t e s . In d e c l i n e phase p o p u l a t i o n s , d i s p e r s a l was low d u r i n g the b r e e d i n g season, and p o p u l a t i o n s with higher p r o p o r t i o n s of l a r g e - s i z e d v o l e s had slower r a t e s o f d e c l i n e than p o p u l a t i o n s with few l a r g e v o l e s . 166 Krebs and Boonstra (1978) suggest that i n H i c r o t u s t o w n s e n d i i , p o p u l a t i o n s with higher p r o p o r t i o n s of males above 70 g i n March have more i n t e n s i v e s p r i n g d e c l i n e s the next year. However, t h i s study showed the o p p o s i t e r e s u l t , and whatever the cause of disappearance d u r i n g the 1978 b r e e d i n g d e c l i n e , heavy-weight v o l e s seemed b e t t e r s u i t e d to environmental c o n d i t i o n s . D e c l i n e s i n v o l e p o p u l a t i o n s have been e x p l a i n e d with e i t h e r e x t r i n s i c (food, p r e d a t o r s , etc.) or i n t r i n s i c (behaviour, g e n e t i c s ) mechanisms of d e c l i n e . The r e s u l t s of t h i s study p a r t i a l l y support both; viewpoints. Two d e c l i n e s i n p o p u l a t i o n s i z e were recorded i n t h i s study; one from March to June 1977 t h a t was concurrent with wounding and d i s p e r s a l , and one from October 1977 u n t i l June 1978 t h a t was c h a r a c t e r i z e d by the absence o f wounding and d i s p e r s a l i n the non-breeding season and, d u r i n g the breeding season, by a marginal i n c r e a s e i n wounding, but not i n d i s p e r s a l . According t o the C h i t t y h y p o t h e s i s ( C h i t t y , 1967), only one e x p l a n a t i o n i s r e q u i r e d to account f o r these d e c l i n e s ; v o l e s a t the beginning of the two breeding d e c l i n e s have been subjected t o d i f f e r e n t s e l e c t i o n p r e s s u r e s , and d i f f e r e n c e s between the two d e c l i n e s are due to changes i n the p r o p e r t i e s of the i n d i v i d u a l s . , The main p r o p e r t y assumed to change i n frequency i s behaviour, and the p r e d i c t i o n from the C h i t t y hypothesis i s t h a t , on average, i n d i v i d u a l s i n d e c l i n i n g p o p u l a t i o n s show more t h r e a t behaviour than do i n d i v i d u a l s i n i n c r e a s i n g or peak ones. The fundamental assumption i s that s e l e c t i o n f o r a b e h a v i o u r a l change i s at the expense of the t r a i t s t h a t are s e l e c t i v e l y advantageous d u r i n g the i n c r e a s e . Voles i n l a t e peak and d e c l i n i n g p o p u l a t i o n s have 167 been s e l e c t e d f o r s u r v i v a l under c o n d i t i o n s o f mutual i n t e r f e r e n c e , and t h i s change i n p r o p e r t i e s of the i n d i v i d u a l s a l s o reduces t h e i r f i t n e s s to endure environmental c o n d i t i o n s ( C h i t t y , 1965). Krebs and Boonstra (1978) proposed t h a t v a r i a b i l i t y i n s p a c i n g behaviour causes v a r i a b i l i t y i n the r a t e of d e c l i n e d u r i n g breeding seasons. Bore severe breeding d e c l i n e s are a r e s u l t of more vigorous spacing behaviour by the i n d i v i d u a l s present i n t h a t d e c l i n e . The Krebs and Boonstra hypothesis adequately e x p l a i n s the 1977 breeding d e c l i n e . The d e c l i n e s t a r t e d with the onset of b r e e d i n g , wounding i n c r e a s e d , and d i s p e r s a l of s m a l l e r - s i z e d v o l e s i n c r e a s e d . The r a t e of d e c l i n e was h i g h e s t i n those p o p u l a t i o n s t h a t had the h i g h e s t v a r i a b i l i t y i n body weight, the highest p r o p o r t i o n of heavy-weight v o l e s , and the h i g h e s t wounding l e v e l s . These o b s e r v a t i o n s support the h y p o t h e s i s t h a t a g g r e s s i v e i n t e r a c t i o n s between v o l e s i n breeding c o n d i t i o n l e d t o d i s p e r s a l o f the s m a l l e r , and presumably s u b o r d i n a t e , i n d i v i d u a l s . The second d e c l i n e i n t h i s study had both a non-breeding and breeding component, and 60 to 70% of the p o p u l a t i o n disappeared i n the non-breeding component; with minimum s u r v i v a l r a t e s above 0.80 per two weeks i n t h i s p e r i o d , but w e l l below the r a t e of 0.93 per two weeks observed i n the 1976 winter (see s u r v i v a l s e c t i o n ) . Krebs and Boonstra (1978) d i d not d i s c u s s non-breeding season l o s s e s , and C h i t t y and Phipps (1966) suggested t h a t when non-breeding l o s s e s are r e l a t i v e l y s l i g h t , they may be accounted f o r by e x t r i n s i c f a c t o r s . Instead, they argued t h a t what needs t o be e x p l a i n e d are d r a s t i c changes i n 168 s u r v i v a l , f o r example, those that occur between the non-breeding and breeding seasons, and any w i t h i n the breeding season. C h i t t y (1967) and Krebs and Boonstra (1978) would p r e d i c t t h a t s p a c i n g behaviour d u r i n g the breeding p o r t i o n s of both d e c l i n e s was d i f f e r e n t , i . e. t h a t i t was more i n t e n s e i n the 1978 d e c l i n e . The l o g i c a l assumption i s t h a t , on average, v o l e s were more a g g r e s s i v e i n the 1978 breeding d e c l i n e than d u r i n g the 1977 d e c l i n e . I f s e v e r i t y o f s p r i n g d e c l i n e s i s simply the r e s u l t o f more vigorous s p a c i n g behaviour, then i t seems reasonable to expect t h a t , d u r i n g the severe 1978 breeding d e c l i n e , d i s p e r s a l of l i g h t - w e i g h t voles should have been higher than i n 1977, as should wounding l e v e l s . A l s o , p o p u l a t i o n s with h i g h e r p r o p o r t i o n s o f heavy-weight animals should have had f a s t e r r a t e s of d e c l i n e . P r e c i s e l y the o p p o s i t e r e s u l t s were observed i n t h i s study, and one must e i t h e r r e j e c t t h i s h y p o t h e s i s , or assume t h a t t h e b e h a v i o u r a l i n t e r a c t i o n s caused death i n some manner without heavy wounding or d i s p e r s a l . . Barnett (1958) re p o r t e d t h a t a g g r e s s i v e i n t e r a c t i o n s between wild r a t s (Battus norvegicus) i n the l a b o r a t o r y sometimes r e s u l t e d i n weight l o s s and death of s u b o r d i n a t e s with l i t t l e o r no wounding. Wounding r a t e data may or may not be r e l e v a n t t o the C h i t t y h y p o t h e s i s . C h i t t y (1967) made no p r e d i c t i o n s of wounding l e v e l s i n the d i f f e r e n t phases of the m i c r o t i n e c y c l e , but i f v o l e s i n d e c l i n i n g p o p u l a t i o n s are suggested t o be the most a g g r e s s i v e of any c y c l i c a l phase, and i f the frequency of wounding i s a f u n c t i o n of the number of a g g r e s s i v e animals, then presumably wounding l e v e l s should be highest i n d e c l i n i n g 169 p o p u l a t i o n s . However, the C h i t t y hypothesis a p p l i e s without d i s t i n c t i o n t o both sexes. Females i n d e c l i n i n g p o p u l a t i o n s can have high death r a t e s , but l i t t l e wounding, which suggests, i f the C h i t t y h y p othesis i s c o r r e c t , t h a t wounding may not be a r e l e v a n t index of the s e v e r i t y of behaviour that may be i n v o l v e d i n these d e c l i n e s . I f wounding i s a measure of some forms of a g g r e s s i o n , and i f f i g h t i n g i s necessary f o r a p o p u l a t i o n d e c l i n e , then animals i n d e c l i n i n g p o p u l a t i o n s should show marked i n c r e a s e s i n wounding. In M i c r o t u s t o w n s e n d i i . wounding d u r i n g the winter i s very low (Krebs and Boonstra, 1978) and i n c r e a s e s with the onset of breeding i n the s p r i n g i n a s i m i l a r way to t h a t i n M. p e n n s y l v a n i c u s ( C h r i s t i a n 1971, Turner and Iverson 1973), and i n winter b r e e d i n g i n M. c a l i f o r n i c u s ( L i d i c k e r , 1973). Severe d e c l i n e s i n p o p u l a t i o n s i z e may occur i n M. townsendii during the non-breeding season i n the absence o f any wounding (Krebs and Boonstra, 1978), and over 70-80% of the p o p u l a t i o n may disappear d u r i n g t h i s p e r i o d . There i s no evidence t h a t a g g r e s s i v e behaviour i s i n v o l v e d i n these non-breeding season d e c l i n e s . Rose and Gaines (1976) found t h a t wounding was high i n breeding winter p o p u l a t i o n s of M. o c h r o g a s t e r , and males had comparable wounding i n peak and d e c l i n e p e r i o d s . In a study o f H* pennsylvanicus p o p u l a t i o n s . Rose (1978) d e f i n e d a peak phase as a 3- t o 4-month p e r i o d i n which a p o p u l a t i o n decreased by 62%, an e a r l y d e c l i n e as comprising 7 months which had a 42% l o s s , and a l a t e d e c l i n e o f 12 months i n which p o p u l a t i o n s i z e had no net l o s s . C l e a r l y these d e f i n i t i o n s are q u e s t i o n a b l e , as i s h i s c o n c l u s i o n of more severe wounding i n a l a t e d e c l i n e 170 p o p u l a t i o n . His data show t h a t severe wounding i s not a necessary c o n d i t i o n f o r a p o p u l a t i o n d e c l i n e . The r e s u l t s of t h i s study and those r e p o r t e d i n the d i s p e r s a l s e c t i o n p a r t i a l l y support the C h i t t y h y p o t h e s i s . D i s p e r s a l was common i n the i n c r e a s i n g and peak p o p u l a t i o n s , and d i s p e r s e r s are a non-random sample of the p o p u l a t i o n (Myers and Krebs 1971, Krebs et a l . 1973). D i s p e r s a l can be a mechanism by which n a t u r a l s e l e c t i o n a c t s upon the p o p u l a t i o n . I f there i s a polymorphism f o r tendency to d i s p e r s e , then those prone to d i s p e r s e l e a v e the p o p u l a t i o n i n the i n c r e a s e and peak phase, so t h a t i n d i v i d u a l s present i n the l a t e peak and e a r l y d e c l i n e have been s e l e c t e d f o r s t a y i n g put, which may mean t h a t they are more a g g r e s s i v e . During the non-breeding season e x t r i n s i c f a c t o r s d e p l e t e the p o p u l a t i o n , but s u r v i v a l remains good. The minimum s u r v i v a l r a t e s of v o l e s during the 1977 post-peak phase winter were s i g n i f i c a n t l y l e s s than d u r i n g the 1976 i n c r e a s i n g phase winter, which may i n d i c a t e t h a t the f i t n e s s of the p o p u l a t i o n had been reduced, which, a c c o r d i n g t o C h i t t y (1965), may have occurred because v o l e s i n l a t e peak and d e c l i n i n g p o p u l a t i o n s had been s e l e c t e d f o r s u r v i v a l under c o n d i t i o n s of mutual i n t e r f e r e n c e . However, when breeeding begins, the animals i n t e r a c t more, but because most i n d i v i d u a l s prone to d i s p e r s e have a l r e a d y l e f t i n the i n c r e a s e and peak phase, the remaining animals tend to remain behind. These i n d i v i d u a l s d i e by an unknown mechanism t h a t l e a v e s few wounds. One problem with t h i s view i s e x p l a i n i n g how s p a c i n g behaviour r e s u l t s i n death. I t seems i m p l a u s i b l e that b e h a v i o u r a l i n t e r a c t i o n s become so severe as to cause death with 171 no d i s p e r s a l . Indeed, on g r i d C, where both males and females v a r i e d l e a s t i n weight, the males d e c l i n e d t o e x t i n c t i o n and only s i x females s u r v i v e d . I t seems d i f f i c u l t t o accept a b e h a v i o u r a l mechanism t h a t causes v i r t u a l e x t i n c t i o n of the p o p u l a t i o n , because as d e n s i t y approaches z e r o , the r a t e of b e h a v i o u r a l i n t e r a c t i o n s must n e c e s s a r i l y decrease. E x t r i n s i c f a c t o r s were i n v o l v e d t o v a r y i n g degrees i n the second d e c l i n e . Avian predators accounted f o r some of the l o s s (see p r e d a t i o n s e c t i o n ) , and c i r c u m s t a n t i a l evidence i n d i c a t e d t h a t food may have been l i m i t i n g . For example, d u r i n g the summer of 1976 the Timothy g r a s s grew to heights of 1.5 to 2 m, so presumably food and cover were p l e n t i f u l d u r i n g the winter of 1976- 77. These good c o n d i t i o n s may have helped the animals to s u r v i v e w e l l and be abundant; because s u r v i v a l was e x c e l l e n t , and i n i t i a l breeding d e n s i t i e s were high a t the s t a r t o f the 1977 breeding season. A cover of dead grass r e t a r d e d growth durin g the 1977 summer, so t h a t the grass was s p a r s e r and s h o r t e r (0.5 t o 1m). S u r v i v a l was lower i n the winter o f 1977- 78 than i n the previous winter, and l e s s dead g r a s s was present d u r i n g t h i s time, so presumably food was i n s h o r t e r supply. S u r v i v a l o f s m a l l e r animals was reduced, s m a l l v o l e s l o s t weight, and p o p u l a t i o n s with higher v a r i a b i l i t y i n body weights had f a s t e r pre-breeding season d e c l i n e s . Breeding season d e c l i n e s were f a s t e r i n those p o p u l a t i o n s that had lower v a r i a b i l i t y i n body weights. F i g u r e 5.6 shows t h a t only l a r g e males were l e f t at the end of the d e c l i n e . Presumably these animals would have been the b e t t e r competitors i f reso u r c e s were l i m i t i n g . The r a t e s of d e c l i n e i n c r e a s e d s h a r p l y a f t e r breeding 172 began, which supports the h y p o t h e s i s of an i n c r e a s e d demand f o r r e s o u r c e s i n a l r e a d y s h o r t supply. I f food abundance i s i n v o l v e d i n d e c l i n e s , then the a p p l i c a t i o n o f f o o d t o a d e c l i n i n g p o p u l a t i o n should have some measurable e f f e c t . Krebs and DeLong (1965) f e d a Mierotus c a l i f o r n i c u s p o p u l a t i o n and found that although the d e c l i n e was not prevented, i t may have been delayed and slower i n the food-enhanced p o p u l a t i o n than i n the c o n t r o l . Fordham (1971) s u p p l i e d excess food to a p o p u l a t i o n of Peromyscus maniculatus and found t h a t while females i n c r e a s e d i n d e n s i t y , males remained about the same as the c o n t r o l p o p u l a t i o n . T a i t t (1978) s u p p l i e d excess food to p o p u l a t i o n s of P. maniculatus and M. t o w n s e n d i i and found t h a t p o p u l a t i o n d e n s i t i e s and winter mean weights were hig h e r on the food-enhanced g r i d s than on c o n t r o l g r i d s . Flowerdew (1972) and Hansson (1971) both r e p o r t e d higher i n i t i a l s p r i n g breeding p o p u l a t i o n s of Apodemus on food-enhanced g r i d s than on c o n t r o l g r i d s . However, Cole and B a t z l i (1978) f e d a Mierotus ochrogaster p o p u l a t i o n and found that although the food-enhanced p o p u l a t i o n showed higher breeding r a t e s , higher a d u l t s u r v i v a l , and higher growth r a t e s than the c o n t r o l p o p u l a t i o n , supplementary f e e d i n g d i d not prevent or d e l a y the d e c l i n i n g d e n s i t i e s t h a t occurred i n l a t e summer and f a l l i n both p o p u l a t i o n s . D e c l i n e r a t e s a c c e l e r a t e d r a p i d l y when breeding began i n 1978, and the major problem with the food hypothesis i s t h a t i t seems unable t o e x p l a i n the sudden drop i n s u r v i v a l with the onset of breeding. I t seems u n l i k e l y t h a t food was exhausted at p r e c i s e l y t h i s time. However, the r o l e of food can be 173 i n v e s t i g a t e d by s u p p l y i n g food t o a p o p u l a t i o n i n a v o l e - p r o o f e n c l o s u r e , i n c l u d i n g an area f o r the v o l e s to d i s p e r s e i n t o . I f the amount of d i s p e r s a l i s dependent upon i n i t i a l breeding d e n s i t y and not i n d i v i d u a l p r o p e r t i e s , then presumably a food-enhanced p o p u l a t i o n should have higher winter s u r v i v a l , h igher i n i t i a l breeding d e n s i t i e s , and a breeding season d e c l i n e that might be due to d i s p e r s a l , whereas the c o n t r o l p o p u l a t i o n would have a severe d e c l i n e with no d i s p e r s a l . The b e h a v i o u r a l p r e d i c t i o n s from the Krebs and Boonstra (1978) and the C h i t t y hypothesis can be t e s t e d by behaviour-modifying drugs, such as t r a n q u i l i z e r s . I f a g g r e s s i v e b e h a v i o u r a l i n t e r a c t i o n s are necessary t o account f o r severe breeding d e c l i n e s i n which there i s no d i s p e r s a l , one could a d m i n i s t e r drugs to reduce a g g r e s s i o n (Vessey^ 1967) or i n c r e a s e a g g r e s s i o n (Krebs et a l . , 1977) i n a v o l e p o p u l a t i o n about to undergo a severe d e c l i n e and observe i t s demographic consequences. Experimental manipulations are e s s e n t i a l to determine the cause of disappearance of v o l e s i n severe d e c l i n e s . L i t e r a t u r e c i t e d B a r n e t t , S. A. 1958. P h y s i o l o g i c a l e f f e c t s of • s o c i a l s t r e s s ' i n w i l d r a t s - I . The a d r e n a l c o r t e x . J. Psychosomatic Res. 3: 1-11. Boonstra, B. and C. J . Krebs. 1978. P i t f a l l t r a p p i n g of H i c r o t u s townsendii. J . Mammal. 59: 136-148. C h i t t y , D. 1955. Adverse e f f e c t s of p o p u l a t i o n d e n s i t y upon the v i a b i l i t y of l a t e r g e n e r a t i o n s . I n The Numbers of Man r 174 and Animals. (eds. J . B. Cragg and N. W. P i r i e ) pp. 57-67. O l i v e r and Boyd, London. . 1965. P r e d i c t i n g q u a l i t a t i v e changes i n i n s e c t p o p u l a t i o n s . Proc. 12th I n t . Congr. Entomol. Lond. 384-386. 1967. The n a t u r a l s e l e c t i o n of s e l f - r e g u l a t o r y behaviour i n animal p o p u l a t i o n s . Proc. E c o l . Soc. Aust. 2: 51-78. C h i t t y , D. and E. Phipps. 1966. Seasonal changes i n s u r v i v a l i n mixed p o p u l a t i o n s of two vole s p e c i e s - J . Anim. E c o l . 35: 313-331. C h r i s t i a n , J . J . 1971. F i g h t i n g , m a t u r i t y , and p o p u l a t i o n d e n s i t y i n Microtus p e n n s y l v a n i c u s . J . Mammal. 52: 556-567. Co l e , F. R. and G. 0. B a t z l i . 1978. I n f l u e n c e of supplemental f e e d i n g on a v o l e p o p u l a t i o n . J . Mammal. 59: 809-819. F a i r b a i r n , D. J . 1977. The s p r i n g d e c l i n e i n deer mice: Death or d i s p e r s a l ? Can. J . Z o o l . 55: 84-92. Flowerdew, J . B. 1972. The e f f e c t of supplementary food on a p o p u l a t i o n of wood mice (Apodemus s y l v a t i c u s ) . J . Anim. E c o l . 41: 553-566. Fordham, B. H. 1971. F i e l d p o p u l a t i o n s of deerraice with supplemental food. Ecology, 52: 138-146. Hansson, L. 1971. Small rodent food, f e e d i n g and p o p u l a t i o n dynamics. Oikos, 22: 183-198. Krebs, C. J . and K. T. DeLong. 1965. A Mic r o t u s p o p u l a t i o n with supplemental food. J . Mammal. 46: 566-573. 175 Krebs, C. J . , M. S. Gaines, B. L. K e l l e r , J . fi. Myers , and R. H. Tamarin. 1973. Population c y c l e s i n s m a l l r o d e n t s . S c i e n c e , 179: 35-41. Krebs, C. J . and J . H. Myers. 1974. Po p u l a t i o n c y c l e s i n s m a l l mammals. Adv. E c o l . Res. 8: 267-399. Krebs, C. J . , Z. T. H a l p i n , and J . N. M. Smith. 1977. Aggression, t e s t o s t e r o n e , and the s p r i n g d e c l i n e i n p o p u l a t i o n s o f the vole Mierotus townsendii. Can. J . Z o o l . 55: 430-437. Krebs, C. J . and E. Boonstra. 1978. Demography of the s p r i n g d e c l i n e i n p o p u l a t i o n s of the vole Mierotus t o w n s e n d i i . J . Anim. E c o l . 47: 1007-1015. L i d i c k e r , W. Z. 1973. Reg u l a t i o n of numbers i n an i s l a n d p o p u l a t i o n o f the C a l i f o r n i a v o l e , a problem i n community dynamics. E c o l . Monogr. 43: 271-302. Myers, J . H. and C. J . Krebs. 1971. G e n e t i c , b e h a v i o r a l , and r e p r o d u c t i v e a t r r i b u t e s of d i s p e r s i n g f i e l d v o l e s M i e r o t u s pennsylvanicus and Mierotus ochrogaster. E c o l . Monogr. 41: 53-78. P i t e l k a , F. A. . 19 58. Some as p e c t s of po p u l a t i o n s t r u c t u r e i n the short-term c y c l e of the brown lemming i n northern A l a s k a . Cold S p r i n g Harb. Symp. Quant. B i o l . 22: 237-251. Rose, R. K. 1978. Le v e l s o f wounding i n the meadow v o l e , Mierotus pennsylvanicus. J . Mammal. In p r e s s . Rose, R. K. and M. S. Gaines. 1976. L e v e l s of ag g r e s s i o n i n f l u c t u a t i n g p o p u l a t i o n s of the p r a i r i e v o l e , Mierotus o c h r o g a s t e r , i n eas t e r n Kansas. J . Mammal. 57: 43-57. 176 S c h u l t z , A. H. 1964. The n u t r i e n t recovery hypothesis f o r A r c t i c m i c r o t i n e c y c l e s . I I . Ecosystem v a r i a b l e s i n r e l a t i o n t o A r c t i c m i c r o t i n e c y c l e s . In G r a z i n g i n t e r r e s t r i a l and marine environments (D. J . C r i s p , ed.), Pp. 57-58, B l a c k w e l l , Oxford. , 1969. A study of an ecosystem: the a r c t i c tundra. Pp. 77-93, i n The ecosystem concept i n n a t u r a l r e s o u r c e management (G. H. Van Dyne, e d . ) . Academic P r e s s , New York. 383p. T a i t t , M. J . 1978. P o p u l a t i o n dynamics of Peromyscus maniculatus austerus and Microtus townsendii with supplementary food. Ph. D. t h e s i s . Department of Zoology, U n i v e r s i t y of B r i t i s h Columbia, Vancouver, BC. Vessey, S. 1967. E f f e c t s of chlorpromazine on aggression i n l a b o r a t o r y p o p u l a t i o n s of w i l d house mice. Ecology, 48: 367-376. 1 7 7 SECTION 6. PITFALL VERSUS LIVE-TRAP ENUMERATION OF FLUCTUATING POPULATIONS OF THE VOLE MICROTUS TOWNSENDII I n t r o d u c t i o n Two main techniques are used to estimate the s i z e of s m a l l mammal p o p u l a t i o n s . The s t o c h a s t i c model of J o l l y (1965) assumes that a l l i n d i v i d u a l s have an equal p r o b a b i l i t y of capture, an assumption that i s i n v a l i d f o r many m i c r o t i n e rodent p o p u l a t i o n s ( L e s l i e et a l . , 1953; Krebs, 1966). The second method, of d i r e c t enumeration, assumes that v i r t u a l l y a l l o f the i n d i v i d u a l s are caught (Krebs, 1966; H i l b o r n e t a l . , 1976). However, i n most s t u d i e s , t h i s assumption i s u n t e s t a b l e because on l y Longworth l i v e - t r a p s are used. The f i r s t purpose of t h i s study was to determine i f l i v e - t r a p s enumerate a l l the i n d i v i d u a l s known t o be present i n the p o p u l a t i o n a t both high and low d e n s i t i e s . Longworth l i v e - t r a p s and p i t f a l l t r a p s sample d i f f e r e n t segments of a m i c r o t i n e p o p u l a t i o n (Boonstra and Krebs, 1978). P i t f a l l s tend to c a t c h younger animals, whereas l i v e - t r a p s c a t c h o l d e r i n d i v i d u a l s . Many i n d i v i d u a l s caught by p i t f a l l s f a i l t o e n t e r l i v e - t r a p s , whereas o t h e r s may take s e v e r a l weeks to be r e c r u i t e d t o l i v e - t r a p s (Boonstra and Krebs, 1978). The second purpose of t h i s study was to examine the c h a r a c t e r i s t i c s of animals caught by both types of t r a p s i n order t o determine why some i n d i v i d u a l s f a i l t o be r e c r u i t e d t o l i v e - t r a p s and to determine i f the two t r a p types sample d i f f e r e n t s o c i a l segments 178 of the a d u l t p o p u l a t i o n . Methods The study area was p a r t of a Timothy hay f i e l d owned by the Canadian W i l d l i f e S e r v i c e on R e i f e l I s l a n d i n the F r a s e r R i v e r d e l t a near Vancouver, B r i t i s h Columbia. Phleum pratense was the dominant s p e c i e s ; A q r o s t i s a l b a , Lolium perenne, and Holeus  l a n a t u s were a l s o present. Four M i c r o t u s townsendii p o p u l a t i o n s were l i v e - t r a p p e d from May 1976 u n t i l June 1978. P o p u l a t i o n s B and C were enc l o s e d by a v o l e - p r o o f f e n c e , and p o p u l a t i o n s A and D were unfenced c o n t r o l s . Each g r i d had 49 t r a p s t a t i o n s set 7.6 m (25 feet) a p a r t i n a 7 x 7 p a t t e r n . Each s t a t i o n had two Longworth l i v e - t r a p s and one p i t f a l l t r a p s i m i l a r t o the t y p e d e s c r i b e d by Boonstra and Krebs (1978). The l i v e - t r a p s , b a i t e d with oats and s u p p l i e d with c o t t o n , were s e t every second week on Monday a f t e r n o o n , checked Tuesday morning and a f t e r n o o n , and checked and l o c k e d open on Wednesday morning. P i t f a l l t r a p s were used from May through October 1976, A p r i l through September 1977, and A p r i l through June 1978. During the i n t e r v a l s between these p e r i o d s , the p i t f a l l s c o u l d not be used because of winter f l o o d i n g . , They were s e t every week on Wednesday morning, checked Wednesday aft e r n o o n , Thursday morning, and c l o s e d on Thursday a f t e r n o o n . When both trap types were set i n one week, the p i t f a l l s were s e t a f t e r the l i v e - t r a p s had been l o c k e d open. A l l v o l e s were ear-tagged, and on each c a p t u r e , tag number, weight, capture l o c a t i o n , sex, r e p r o d u c t i v e c o n d i t i o n , and 179 wounding were recorded. a l l v o l e s were r e l e a s e d immediately a f t e r being processed. Voles were c l a s s i f i e d as f o l l o w s : adult> 42g; subadult 30-42g, j u v e n i l e <30g. Vol e s c r o s s i n g a s t r i p 60 f t wide (18.3 m) of mowed grass i n s i d e each enclosure were d e f i n e d as d i s p e r s e r s . I t was assumed t h a t untagged v o l e s caught i n d i s p e r s e r t r a p s i n s i d e an en c l o s u r e had been born on the g r i d and were t h e r e f o r e untagged d i s p e r s e r s . D i s p e r s i n g voles were removed from the g r i d s . The t o t a l enumeration technique of Krebs (1966) was used i n the d e t e r m i n a t i o n of demographic d a t a . The estimate o f t r a p p a b i l i t y used was: number of captu r e s f o r an animal T r a p p a b i l i t y = number of p o s s i b l e c a p t u r e s f o r t h a t animal N where N i s the number of vo l e s caught more than t w i c e , and t r a p p a b i l i t y i s summed over a l l N animals. The f i r s t and l a s t times o f capture are excluded i n the summation, because an animal must be caught at these times. R e s u l t s T r a p p a b i l i t y The t r a p p a b i l i t y e stimates c a l c u l a t e d independently f o r i n d i v i d u a l s known to be present but not i n l i v e - t r a p s were of l i v e - t r a p s and p i t f a l l s were each t r a p type. For example, from capture i n p i t f a l l t r a p s not i n c l u d e d i n t r a p p a b i l i t y 180 e s t i m a t e s of those captured i n l i v e - t r a p s . T r a p p a b i l i t i e s o f males and females i n l i v e - t r a p s were s i m i l a r and r e l a t i v e l y constant from summer 1976 u n t i l s p r i n g 1978. In the 1976 summer t r a p p a b i l i t y was 61% f o r 324 animals and i n the 1977 summer i t was 65% f o r 1692 animals. Low d e n s i t i e s i n the s p r i n g of 1978 i n c r e a s e d male t r a p p a b i l i t y t o 95% f o r 96 animals and female t r a p p a b i l i t y t o 81% f o r 155 animals. Males and females had s i m i l a r t r a p p a b i l i t y i n p i t f a l l s , and during summer 1976 i t was 35% f o r 727 animals, and i n the 1977 summer i t was 28% f o r 1726 animals. T r a p p a b i l i t y c o u l d not be c a l c u l a t e d f o r s p r i n g 1978 because no animal was captured more than twice. P i t f a l l s were only about one-half as e f f e c t i v e as l i v e - t r a p s i n repeated captures of v o l e s . Many voles avoided repeated capture i n p i t f a l l t r a p s . P o p u l a t i o n Density The d e n s i t y of the vole p o p u l a t i o n s was low when t r a p p i n g began i n May 1976 and remained low throughout the summer. During t h i s p e r i o d the p i t f a l l p o p u l a t i o n counts were s i m i l a r to the combined l i v e - t r a p and p i t f a l l counts, which i n d i c a t e s t h a t the p i t f a l l s were c a p t u r i n g p r a c t i c a l l y a l l of the v o l e s known to be a l i v e ( F i g s . 6.1, 6.2, 6.3, 6.4). The l i v e - t r a p counts were approximately one-half the magnitude of e i t h e r the p i t f a l l or combined counts. L i v e - t r a p p o p u l a t i o n counts s t e a d i l y i n c r e a s e d throughout the 1976 winter a t a time when the p o p u l a t i o n s were not breeding. Thus t h e r e was a c o n s i d e r a b l e l a g i n re c r u i t m e n t of young v o l e s to the l i v e - t r a p s during t h i s 181 F i g u r e 6.1. P o p u l a t i o n d e n s i t y o f Microtus townsendii on c o n t r o l g r i d A. Minimum number a l i v e (MNA) i s given f o r the p o p u l a t i o n known to have entered l i v e - t r a p s , f o r those known t o have entered p i t f a l l s , and f o r those known to have entered one or both types of t r a p s . Non-breeding p e r i o d s are shaded. 400 r 1976 1977 1978 183 F i g u r e 6.2. P o p u l a t i o n d e n s i t y of M i c r o t u s townsendii on fenced g r i d B. Minimum number a l i v e (MN&) i s given f o r the p o p u l a t i o n known t o have entered l i v e - t r a p s , f o r those known to have entered p i t f a l l s , and f o r those known to have entered one or both types of t r a p s . Non-breeding p e r i o d s are shaded. Minimum Number Al ive 78 T 185 F i g u r e 6.3. P o p u l a t i o n d e n s i t y o f M i c r o t u s townsendii on fenced g r i d C. Minimum number a l i v e (MNA) i s given f o r the p o p u l a t i o n known to have entered l i v e - t r a p s , f o r those known to have entered p i t f a l l s , and f o r those known to have entered one or both types of t r a p s . Non-breeding p e r i o d s are shaded. Minimum Number Alive 9 8 T 187 Fi g u r e 6.4. P o p u l a t i o n d e n s i t y o f Mierotus townsendii on c o n t r o l g r i d D. Minimum number a l i v e (NN&f i s given f o r the p o p u l a t i o n known to have entered l i v e - t r a p s , f o r those known to have entered p i t f a l l s , and f o r those known t o have entered one or both types of t r a p s . Non-breeding p e r i o d s are shaded. Minimum Number Al ive 88T 189 time. F i g u r e s 6.1, 6.2, 6.3, and 6.4 i n d i c a t e t h a t a l l p o p u l a t i o n s d e c l i n e d i n the s p r i n g o f 1977. By t h i s time the l i v e - t r a p p o p u l a t i o n estimates were s i m i l a r ; to the combined l i v e - t r a p and p i t f a l l e s t i m a t e s , which i n d i c a t e s t h a t by s p r i n g 1977 l i v e - t r a p s were enumerating most of the v o l e s known t o be a l i v e . The p i t f a l l p o p u l a t i o n e s t i m a t e s i n c r e a s e d r a p i d l y d u r i n g e a r l y summer 1977 a t a r a t e o f about 15% per week compared with a 4% per week i n c r e a s e i n the l i v e - t r a p p o p u l a t i o n s . The p a t t e r n of 1976 repeated i t s e l f i n the e a r l y summer o f 1977. The p i t f a l l p o p u l a t i o n estimates were again s i m i l a r to the combined l i v e - t r a p and p i t f a l l e s t i m a t e s , whereas l i v e - t r a p s enumerated approximately o n e - h a l f of the v o l e s known to be a l i v e . During the l a t e summer of 1977 with the t a p e r i n g o f f of r e p r o d u c t i o n , the p i t f a l l p o p u l a t i o n d e c l i n e d at about 20% per week while the l i v e - t r a p p o p u l a t i o n was r e l a t i v e l y s t a b l e . There was again a c o n s i d e r a b l e time lag i n recru i t m e n t to l i v e - t r a p s , but by s p r i n g 1978 the l i v e - t r a p s were enumerating most of the animals known to be a l i v e . The v o l e p o p u l a t i o n s d e c l i n e d s h a r p l y i n s p r i n g 1978 before i t was p o s s i b l e t o t r a p with p i t f a l l s . Thus, few animals were caught i n p i t f a l l s a t t h i s time. During t h i s p e r i o d l i v e - t r a p s were enumerating more v o l e s than the p i t f a l l s were. F i r s t Capture Of Voles I n L i v e - t r a p s And P i t f a l l s I compared the d i s t r i b u t i o n s o f i n i t i a l v o l e c a p t u r e s i n l i v e - t r a p s and p i t f a l l s . T h i s comparison i n c l u d e d only the time 190 when both t r a p types were used c o n c u r r e n t l y . I n i t i a l c aptures i n l i v e - t r a p s d u r i n g the winter when p i t f a l l s were not o p e r a t i n g were excluded from t h i s a n a l y s i s . An i n d i v i d u a l f i r s t caught i n p i t f a l l s c o u l d be r e c r u i t e d t o l i v e - t r a p s at any time d u r i n g the study. Approximately 3700 v o l e s were f i r s t caught i n p i t f a l l s and about 450 were f i r s t caught i n l i v e - t r a p s . Table 6.1 shows t h a t 45% o f the v o l e s f i r s t caught i n p i t f a l l s d i d not en t e r l i v e - t r a p s . In the unfenced c o n t r o l p o p u l a t i o n s , i n t o which immigration was p o s s i b l e , 50% of the a d u l t males f i r s t caught i n p i t f a l l s were not caught i n l i v e - t r a p s , whereas 43% of a d u l t females were not caught i n l i v e - t r a p s . In the fenced p o p u l a t i o n s 46% of the a d u l t males and 26% of the a d u l t females were not caught i n l i v e - t r a p s . When the data are pooled over a l l s i z e and sex c l a s s e s , a g r e a t e r p r o p o r t i o n o f animals i n the unfenced c o n t r o l p o p u l a t i o n s f a i l e d to enter l i v e - t r a p s than i n the fenced experimental p o p u l a t i o n s (Xz=12.?8, P<.01). Table 6.2 shows t h a t 40% of the 447 animals f i r s t caught i n l i v e - t r a p s d u r i n g the s p r i n g to f a l l p e r i o d i n each year were not trapped i n p i t f a l l s . However, p i t f a l l s have a higher p r o b a b i l i t y than l i v e - t r a p s of c a p t u r i n g a v o l e f o r the f i r s t time. Of the 435 f i r s t - c a u g h t a d u l t males, p i t f a l l s accounted f o r 80%, and of the 289 f i r s t - c a u g h t a d u l t females, they accounted f o r 79%. The r e s u l t s a r e even more s t r i k i n g f o r the j u v e n i l e c l a s s , i n which 94% of 2213 f i r s t - c a u g h t i n d i v i d u a l s were captured i n p i t f a l l s . A f t e r v o l e s have entered the t r a p p a b l e p o p u l a t i o n , p i t f a l l s a re a b l e to d e t e c t them e a r l i e r than l i v e - t r a p s . 1 9 1 Table 6.1. Numbers of Microtus townseadii f i r s t caught i n p i t f a l l s and l a t e r i n l i v e - t r a p s , and~those caught only i n p i t f a l l s . Data are pooled over the e n t i r e study. Males Females Group J u v e n i l e Subadult Ad u l t J u v e n i l e Subadult Adult T o t a l C o n t r o l p o p u l a t i o n s F i r s t caught i n p i t f a l l s 387 233 224 374 231 147 1596 L a t e r caught i n l i v e - t r a p s 188 114 111 200 132 84 829 Caught o n l y i n p i t f a l l s 199 119 113 174 99 63 767 Experimental p o p u l a t i o n s F i r s t caught i n p i t f a l l s 638 305 122 675 260 8 1 2081 L a t e r caught i n l i v e - t r a p s 371 168 66 388 15 1 60 1204 Caught on l y i n p i t f a l l s 267 137 56 287 109 21 877 A l l p o p u l a t i o n s F i r s t caught i n p i t f a l l s 1025 538 346 1049 491 228 3677 L a t e r caught i n l i v e - t r a p s 559 282 177 588 283 144 2033 Caught o n l y i n p i t f a l l s 466 256 169 461 208 84 1644 192 Table 6.2. Numbers o f Mierotus townsendii f i r s t caught i n l i v e - t r a p s and l a t e r i n p i t f a l l s , and those caught only i n l i v e - t r a p s . F i r s t captures of voles i n l i v e - t r a p s when p i t f a l l s were not c o n c u r r e n t l y trapped were excluded. Males Females Group J u v e n i l e Subadult A d u l t J u v e n i l e Subadult Adult T o t a l C o n t r o l p o p u l a t i o n s F i r s t caught i n l i v e - t r a p s 33 40 66 33 38 50 260 L a t e r caught i n p i t f a l l s 20 24 33 23 21 33 154 Caught o n l y 10 6 i n l i v e - t r a p s 13 16 33 10 17 17 Experimental p o p u l a t i o n s F i r s t caught i n l i v e - t r a p s 29 39 23 44 41 11 187 L a t e r caught i n p i t f a l l s 23 21 13 30 20 7 114 Caught o n l y 73 i n l i v e - t r a p s 6 18 10 14 21 4 A l l p o p u l a t i o n s F i r s t caught i n l i v e - t r a p s 62 79 89 77 79 61 447 L a t e r caught i n p i t f a l l s 43 45 46 53 41 40 268 Caught on l y 21 179 i n l i v e - t r a p s 19 34 43 24 38 193 Body Height D i f f e r e n c e s At F i r s t Capture In L i v e - t r a p s And P i t f a l l s T a b l e 6.3 shows the mean body weights of male and female M i c r o t u s townsendii a t f i r s t c apture i n l i v e - t r a p s and p i t f a l l s . These means i n c l u d e weights of a l l i n d i v i d u a l s a t f i r s t c apture i n one type of t r a p , r e g a r d l e s s of whether or not they had a l r e a d y been caught i n the other type of t r a p . For example, a l l i n d i v i d u a l s i n i t i a l l y trapped i n p i t f a l l s and which l a t e r entered l i v e - t r a p s were i n c l u d e d i n the c a l c u l a t i o n of mean weight a t f i r s t capture i n l i v e - t r a p s . The mean body weight of males at f i r s t capture i n l i v e - t r a p s was about 15 g more-than i n p i t f a l l s {Table 6.3). Females were about 12 g h e a v i e r a t f i r s t capture i n l i v e - t r a p s than i n p i t f a l l s . Males and females i n the unfenced p o p u l a t i o n s A and D were s i g n i f i c a n t l y h e a v i e r at f i r s t c apture i n p i t f a l l s compared with males and females i n the e n c l o s e d p o p u l a t i o n s {Table 6.3). Immigration o f heavier animals i n t o the unfenced p o p u l a t i o n s may account f o r t h i s d i f f e r e n c e . Mean weight a t f i r s t capture i n l i v e - t r a p s was higher than i n p i t f a l l s . What i s the b a s i s of t h i s d i f f e r e n c e ? T a b l e 6.4 shows the d i s t r i b u t i o n of f i r s t c a p t u r e s i n l i v e - t r a p s and p i t f a l l s of a l l v o l e s tagged or untagged a c c o r d i n g to sex and s i z e c l a s s e s . P i t f a l l s were more l i k e l y t o capture j u v e n i l e v o l e s , whereas l i v e - t r a p s were more l i k e l y t o capture a d u l t s . A higher p r o p o r t i o n of females entered l i v e - t r a p s as j u v e n i l e s or s u b a d u l t s than d i d males i n both the c o n t r o l (X 2=45.57, P<.0001) and fenced p o p u l a t i o n s (X 2=60.46, p<.0001) (Table 6.4). Young 194 Table 6.3. Mean weight i n grams at f i r s t capture (+ 1 SE) of Mierotus townsendii i n l i v e - t r a p s and p i t f a l l s f o r f o u r p o p u l a t i o n s . Data f o r each g r i d are pooled over the e n t i r e study. Sample s i z e s are i n parentheses. G r i d L i v e - t r a p s P i t f a l l s Males Females Males Females A ; 4 8.8+0.5 42.6+0.4 35. 1+0.8 32.4+0.7 (378) (386) (426) (424) B 46.7+0.4 42.8+0.3 29. 1+0.3 29.1+0.4 (480) (414) (661) (593) C 4 5.2+0.5 39.8+0.3 29.1+0.5 25.8+0.4 (391) (402) (530) (545) D 49.0+0.4 44.9+0.4 36. 1 + 0.5 36.9 + 0.7 (459) (396) (568) (516) T o t a l 47.4+0.5 42-5+0.3 32. 1+0.6 30.8 + 0. 5 (1708) (1598) (2185) (2078) 195 Table 6.4. Numbers of M i c r o t u s townsendii i n each s i z e and sex c l a s s at f i r s t c apture i n l i v e - t r a p s and p i t f a l l s . Data are pooled over the e n t i r e study. L i v e - t r a p s P i t f a l l s age c l a s s Males Females T o t a l Males Females T o t a l C o n t r o l p o p u l a t i o n s J u v e n i l e 67 80 147 403 395 798 Subadult 195 293 488 264 256 520 a d u l t 575 409 984 327 289 616 Experimental p o p u l a t i o n s J u v e n i l e 99 137 236 674 702 1376 Subadult 241 336 577 333 292 625 a d u l t 531 343 874 184 144 328 196 males are l e s s l i k e l y t o e n t e r l i v e - t r a p s than young females. These r e s u l t s suggest t h a t there i s a time l a g between f i r s t capture i n p i t f a l l s and f i r s t capture i n l i v e - t r a p s . How i s t h i s time l a g d i s t r i b u t e d over the age and sex c l a s s e s ? Table 6.5 shows that a d u l t males f i r s t caught i n p i t f a l l s were not caught i n l i v e - t r a p s f o r about 7 weeks, whereas a d u l t females were not caught i n l i v e - t r a p s f o r 5 weeks. J u v e n i l e males took 2 weeks and j u v e n i l e females 4 weeks lo n g e r than a d u l t s t o e n t e r l i v e - t r a p s . Over 50% of those j u v e n i l e males f i r s t caught i n p i t f a l l s which d i d en t e r l i v e - t r a p s were a d u l t s by t h a t time (Table 6.5). J u v e n i l e females f i r s t caught i n p i t f a l l s were more l i k e l y t o en t e r l i v e - t r a p s f o r the f i r s t time as s u b a d u l t s . The h e a v i e r an i n d i v i d u a l at f i r s t c apture i n p i t f a l l s , the sooner i t tends t o enter the l i v e - t r a p p o p u l a t i o n . The maximum i n t e r v a l recorded i n t h i s study between f i r s t c a p tures i n p i t f a l l s and l i v e - t r a p s was 54 weeks f o r a subadult female. Table 6.1 shows t h a t 1644 v o l e s were caught o n l y i n p i t f a l l s . Are these i n d i v i d u a l s present i n the p o p u l a t i o n f o r a long time? Table 6.6 i n d i c a t e s t h a t over 50% of the i n d i v i d u a l s caught only i n p i t f a l l s were trapped only once. The average time between f i r s t and l a s t c a p t u r e s o f these : i n d i v i d u a l s was l e s s than 2 weeks. These animals comprise a s u b s t a n t i a l p o r t i o n of the p o p u l a t i o n about which nothing would be known i f enumeration had been by l i v e - t r a p s o n l y . The maximum d u r a t i o n of l i f e recorded f o r an i n d i v i d u a l caught only i n p i t f a l l s was 38 weeks f o r a j u v e n i l e female. 197 Table 6.5. Percentage d i s t r i b u t i o n by s i z e c l a s s of i n t e r v a l between f i r s t c apture i n p i t f a l l s and f i r s t c a p ture i n l i v e - t r a p s of Mierotus townsendii. C l a s s i f i c a t i o n used i s s i z e at f i r s t c a pture i n p i t f a l l s . A l l g r i d s and years are combined. Duration (weeks) Males Females J u v e n i l e Subadult Adult J u v e n i l e Subadult Adui' 1-5 43.3 47.5 54.8 44. 5 48.1 71. 3 6-10 22.7 20.9 25.4 24. 6 23.7 18. 2 11-15 15.0 16.3 10.2 1 1 . 6 13.4 7.0 16-20 9.1 6.0 3.4 7.0 5.3 2. 1 >20 9.9 9.3 6.2 12.3 9.5 1. 4 Average . 1+1. in t e r v a l + 2 S E 9.2+1.3 8.6+1. 1 7. 1 + 1.2 9.3+1.7 8 .8+3.7 5 Sample s i z e 559 282 177 588 283 144 C e n t e r i n g l i v e - t r a p s as: J u v e n i l e s 15.0 17.6 Subadults 31.3 28.1 43. 4 39.6 A d u l t s 53.7 7 1 . 9 100.0 39.0 60.4 100. 0 198 Table 6.6. Percentage d i s t r i b u t i o n by s i z e c l a s s of i n t e r v a l between f i r s t and l a s t p i t f a l l captures o f Mierotus townsendii caught only i n p i t f a l l s . 1976 and 1977 are combined. Duration (weeks) Males Females J u v e n i l e Subadult a d u l t J u v e n i l e Subadult AduH C o n t r o l p o p u l a t i o n s 0 62.4 67.2 69.0 54. 0 66.7 66. 7 1-5 29.6 31.1 27.4 35. 1 25. 3 30. 1 6-10 7.5 1.7 2.7 9.8 8.0 1.6 >10 0.5 0.0 0.9 1. 1 0. 0 1.6 Sample s i z e 199 119 113 174 99 63 Experimental p o p u l a t i o n s 0 47.6 55.5 62.4 56. 1 68.8 71.4 1-5 43. 1 34.3 30.4 33. 8 23.9 23.8 6-10 7.8 7.3 5.4 8.4 7. 3 0.0 >10 1.5 2.9 1.8 1.7 0.0 4.8 Sample s i z e 267 137 56 287 109 21 199 D i s p e r s a l The e x p e r i m e n t al design used i n t h i s study i d e n t i f i e d d i s p e r s i n g v o l e s . A d i s p e r s i n g v o l e c o u l d p r e v i o u s l y have been caught i n l i v e - t r a p s , p i t f a l l s , both types of t r a p s , or n e i t h e r . Table 6.7 shows t h a t l i v e - t r a p s enumerated about 30% of a l l d i s p e r s e r s and 60% of tagged d i s p e r s e r s . P i t f a l l s , o p e r a t i n g only p a r t o f the year, caught 60% of the t o t a l d i s p e r s e r s and 90% of the tagged d i s p e r s e r s . P i t f a l l s were more e f f e c t i v e than l i v e - t r a p s i n c a p t u r i n g d i s p e r s e r s before they l e f t the p o p u l a t i o n . Table 6.1 i n d i c a t e s t h a t , e x c l u d i n g j u v e n i l e s , 193 males and 130 females i n the fenced p o p u l a t i o n s were caught only i n p i t f a l l s . What p r o p o r t i o n of these i n d i v i d u a l s emigrated be f o r e they entered l i v e - t r a p s ? Table 6.7 i n d i c a t e s t h a t t h e r e were 127 d i s p e r s i n g tagged males not enumerated by l i v e - t r a p s . Of these 127 i n d i v i d u a l s , 120 were e i t h e r s u b a d u l t s or a d u l t s . T h e r e f o r e , a maximum of 62% o f the subadult and a d u l t males caught only i n p i t f a l l s d i s p e r s e d . There were 122 known subadult and a d u l t female d i s p e r s e r s t h a t had been trapped i n p i t f a l l s o n ly. These d i s p e r s e r s accounted f o r a maximum o f 94% of the 130 subadult and a d u l t females i n the experimental p o p u l a t i o n s t h a t f a i l e d t o enter l i v e - t r a p s . J u v e n i l e S u r v i v a l J u v e n i l e s u r v i v a l i s u s u a l l y measured i n d i r e c t l y by means of an index (Krebs and DeLdng, 1965), because d i r e c t e s t i m a t i o n 200 Table 6.7. Number of d i s p e r s i n g M i c r o t u s townsendii caught i n l i v e - t r a p s and p i t f a l l s . Data are pooled over the e n t i r e study. G r i d B G r i d C Males Females Males Females Tagged d i s p e r s e r s caught p r e v i o u s l y by l i v e - t r a p s 120 84 61 40 Tagged d i s p e r s e r s not caught by l i v e - t r a p s 8 3 92 44 35 Tagged d i s p e r s e r s caught p r e v i o u s l y by p i t f a l l s 179 169 96 70 Tagged d i s p e r s e r s not caught by p i t f a l l s 24 7 9 5 T o t a l tagged d i s p e r s e r s 203 176 105 75 % tagged d i s p e r s e r s caught by l i v e - t r a p s 59 48 58 53 % tagged d i s p e r s e r s caught by p i t f a l l s 88 96 91 93 Untagged d i s p e r s e r s 85 91 58 47 T o t a l d i s p e r s e r s 288 267 163 122 % t o t a l d i s p e r s e r s caught by l i v e - t r a p s 42 31 27 33 % t o t a l d i s p e r s e r s caught by p i t f a l l s 62 63 59 57 201 of s u r v i v a l i s dependent upon c a p t u r i n g l a r g e numbers of j u v e n i l e s . T h i s c o n d i t i o n was s a t i s f i e d i n t h i s study, and a comparison between observed j u v e n i l e s u r v i v a l and the index of Krebs and DeLong (1965) can be made. J u v e n i l e minimum s u r v i v a l r a t e s per two weeks were hi g h e s t i n the 1976 i n c r e a s i n g p o p u l a t i o n s and lowest i n the 1978 d e c l i n i n g ones, as was the Krebs and DeLong index f o r the combined l i v e - t r a p and p i t f a l l - c a u g h t p o p u l a t i o n (Table 6.8), and both measures o f j u v e n i l e s u r v i v a l were c o r r e l a t e d (r=.69, P<.02). However, the data d e r i v e d s o l e l y from l i v e - t r a p p i n g show t h a t the index i s lowest i n the 1977 peak p o p u l a t i o n s , and i s not c o r r e l a t e d with observed minimum s u r v i v a l r a t e s (r=.22). In these high d e n s i t y Jl- t ownsendii p o p u l a t i o n s , the Krebs and DeLong index i s too low. A Comparison Of a d u l t Hales Caught In L i v e - t r a p s and P i t f a l l s I t has a l r e a d y been demonstrated t h a t p i t f a l l s tend to capture j u v e n i l e v o l e s , whereas l i v e - t r a p s capture a d u l t s . However, a d u l t v o l e s can be caught i n both types of t r a p s . Are there any demographic d i f f e r e n c e s between a d u l t v o l e s caught i n p i t f a l l s and those caught i n l i v e - t r a p s ? Assuming that s u b o r d i n a t e v o l e s h a v e more wounds, I enguired i f l i v e - t r a p s and p i t f a l l s sampled d i f f e r e n t s o c i a l segments of the p o p u l a t i o n . In a d d i t i o n t o r e p r o d u c t i v e maturity and wounding l e v e l s , I a l s o examined degree of p a r a s i t i c i n f e c t i o n i n a d u l t male v o l e s . Females were excluded from the a n a l y s i s because they were seldom wounded and t h e i r r e p r o d u c t i v e m a t u r i t y was d i f f i c u l t to a s s e s s . 2 0 2 Table 6.8- J u v e n i l e minimum s u r v i v a l r a t e s and index of j u v e n i l e s u r v i v a l of Krebs and DeLong (1965) f o r t o t a l p o p u l a t i o n s and f o r l i v e - t r a p p o p u l a t i o n s o f Mi c r o t u s townsendii. Data are pooled over the e n t i r e breeding season i n eacn year. 1978 i n c l u d e s March t o June o n l y . The t o t a l p o p u l a t i o n i n c l u d e s combined l i v e - t r a p and p i t f a l l p o p u l a t i o n s . Minimum s u r v i v a l Index Index Year G r i d per 2 weeks t o t a l p o p u l a t i o n l i v e - t r a p s 1976 D 0.44 1.06 0.38 A 0.58 1.16 0.46 B 0.70 2. 54 0.82 C 0.66 2.55 0.77 1977 D 0.50 0.83 0.13 A 0.58 1.07 0.21 B 0.56 1.27 0.16 C 0.63 0.60 0.13 1978 D 0.25 0.55 0.50 A 0.25 0.40 0.40 B 0.25 0.60 0.43 C 0.00 0.50 0.00 20 3 because females i n the e a r l y stage of pregnancy are not e x t e r n a l l y d i s t i n g u i s h a b l e from nonreproductive females. Males were o r g a n i z e d i n t o f i v e 10-g weight c l a s s e s from 40-90 g. The number of wounds, number of p a r a s i t i c b o t f l i e s (Cuterebra sp.|, and r e p r o d u c t i v e s t a t e were recorded f o r males captured i n l i v e - t r a p s or p i t f a l l s . Male r e p r o d u c t i v e maturity was e x t e r n a l l y judged by t e s t e s s i z e and p o s i t i o n . These data were recorded f o r each capture i n each t r a p type, and the data were summed over the i n t e r v a l s i n which both t r a p t y p es were operated c o n c u r r e n t l y . Data on b o t f l y p a r a s i t i s m were summed only i n the p e r i o d s of i n f e s t a t i o n (from J u l y to September i n each y e a r ) . The average number of wounds per male and per r e p r o d u c t i v e male were c a l c u l a t e d f o r each weight c l a s s i n each t r a p - t y p e p o p u l a t i o n . ft t o t a l of 20 comparisons were made i n each year. When males caught i n p i t f a l l s had more wounds than males caught i n l i v e - t r a p s , the comparison was scored as a • + *, when the p i t f a l l males had fewer wounds, the comparison was scored as a When no males were caught i n one or both c l a s s e s , the comparison was 'NC. A s i g n t e s t a n a l y s i s was used t o analyze the data. Table 6.9 i n d i c a t e s t h a t i n the summer of 1976, males of 40-59 g caught i n p i t f a l l s were more o f t e n i n r e p r o d u c t i v e c o n d i t i o n than were s i m i l a r - s i z e d males caught i n l i v e - t r a p s (40-49 g X*=3.89, P<.05; 50-59 g X*=15.93, P<-01). , The males i n these weight c l a s s e s caught i n p i t f a l l s a t t a i n e d r e p r o d u c t i v e maturity a t l i g h t e r weights than d i d s i m i l a r - s i z e d males caught i n l i v e - t r a p s . In 1976, r e p r o d u c t i v e males caught i n p i t f a l l s had more wounds (12+, 3-, 5NC, P<.05) as d i d a l l males (13+, 3-, 204 Table 6.9- Demographic a t t r i b u t e s of male B i c r o t u s townsendii caught d u r i n g summer 1976 i n p i t f a l l s and l i v e - t r a p s . A l l g r i d s are combined. The time i n t e r v a l i s the e n t i r e summer except f o r the b o t f l y r e s u l t s when males i n the b o t f l y season (July through September) were scored. Sample s i z e s are i n parentheses. Weight % s c r o t a l Wounds/male Wounds/scrotal male Bots/male P i t f a l l c a p t u r e s of males 40-49 23.5 (259) 0. 53 (259) 0.98 (61) 0.38 (213) 50-59 62-3 (199) 1.28 (199) 1.34 (124) 0.33 (153) 60-6 9 76.0 (146) 0. 82 (146) 0.96 (111) 0.68 (120) 70-79 92.8 (56) 0.68 (56) 0.65 (52) 0.63 (49) >80 91.7 (12) 0.50 (12) 0.45 (11) 0. 18 (11) T o t a l 53.4 (672) 0. 83 (672) 1.04 (359) 0. 45 (546) Longworth l i v e - t r a p c a p t u r e s of males 40-49 13.6 (88) 0.41 (88) 1.08 (12) 0.28 (72) 50-59 38.7 (111) 0.70 (111) 0.67 (43) 0.39 (92) 60-69 73.8 (107) 0.61 (107) 0.74 (79) 0.44 (86) 70-79 93.5 (46) 0.56 (46) 0.56 (43) 0.24 (29) >80 100.0 (6) 0.17 (6) 0.17 (6) 0.00 (2) T o t a l 51. 1 (358) 0.58 (358) 0.69 (183) 0.37 (281) 205 4NC, P<.05). Males caught i n p i t f a l l s a l s o had higher l e v e l s of b o t f l y i n f e s t a t i o n (12+, 3-, 1 t i e , 4NC, P<.05). Tabl e 6.10 shows that males 40-49 g caught i n p i t f a l l s d u ring 1977 were more o f t e n i n r e p r o d u c t i v e c o n d i t i o n than were s i m i l a r males caught i n l i v e - t r a p s (X 2=21.80, P<.001),. At t h i s peak d e n s i t y males caught i n p i t f a l l s had more wounds than d i d males caught i n l i v e - t r a p s (17+, 3-, P<.01) as d i d r e p r o d u c t i v e males (18+, 2-, P<-01). Hales caught i n p i t f a l l s had fewer b o t f l i e s (7+, 10-, 3 t i e s ) , but not s i g n i f i c a n t l y fewer. In 1978 males caught i n p i t f a l l s , whether or not they were r e p r o d u c t i v e , had more wounds (6+, 1-, 12NC), but there were not enough comparisons t o i n d i c a t e s i g n i f i c a n c e by the s i g n t e s t . No b o t f l y data are a v a i l a b l e i n 1978 because the study ended before the b o t f l y season began. In summary, males caught i n p i t f a l l s have higher l e v e l s of wounding and tend t o have g r e a t e r r a t e s of b o t f l y i n f e s t a t i o n . Males 40-49 g caught i n p i t f a l l s were more o f t e n i n r e p r o d u c t i v e c o n d i t i o n than were males of the same s i z e caught i n l i v e - t r a p s . Bates of male r e p r o d u c t i v e maturity were the same i n the h e a v i e r weight c l a s s e s . Males 40-49 g and g r e a t e r than 79 g tended to have lower wounding r a t e s than d i d medium-sized a d u l t males. Very l a r g e males had lower r a t e s of wounding. Wounding r a t e s were higher and b o t f l y i n f e s t a t i o n lower i n the peak 1977 p o p u l a t i o n s than i n the i n c r e a s i n g 1976 p o p u l a t i o n s . Why Are Some Males Not Caught In L i v e - t r a p s ? Some males f i r s t caught i n p i t f a l l s l a t e r entered 206 Table 6.10. Demographic a t t r i b u t e s of t a l e J i c r o t u s townsendii caught d u r i n g summer 1977 i n p i t f a l l s and l i v e - t r a p s . A l l g r i d s are combined. The i n t e r v a l i s the e n t i r e summer except f o r the b o t f l y r e s u l t s , when males i n the b o t f l y season only were scored. Sample s i z e i s i n parentheses. Height % s c r o t a l Hounds/male Hounds/scrota1 male Bots/male P i t f a l l c a p t u r e s of males 40-4 9 18.8 (768) 0.04 (768) 0.20 (14 5) 0. 16 (681) 50-59 53.7 (296) 0.96 (296) 1.78 (159) 0.22 (197) 60-69 93.4 (274) 1.76 (274) 1.85 (256) 0.34 (82) 70-79 98. 1 (267) 1.70 (267) 1.72 (262) 0.40 (42) >80 99.0 (101) 1.21 (101) 1.19 (100) 0.16 (19) T o t a l 54.0 (1706) 0.81 (1706) 1.48 (922) 0.20 (1021) Longworth l i v e - t r a p c a p t u r e s of males 40-4 9 9.2 (489) 0.06 (489) 0.49 (45) 0. 10 (459) 50-59 52.3 (386) 0.70 (386) 1.27 (202) 0.28 (265) 60-69 90.4 (468) 1.09 (468) 1. 17 (423) 0.49 (172) 70-79 95.6 (434) 1.00 (434) 0.93 (415) 0.24 (127) >80 100.0 (105) 1. 15 (105) 1.15 (105) 0.08 (26) T o t a l 63.2 (1882) 0.72 (1882) 1.08 (1190) 0.26 (1049) 207 l i v e - t r a p s ; some d i d not. Are t h e r e any d i f f e r e n c e s between these males? An a n a l y s i s s i m i l a r to the one p r e v i o u s l y o u t l i n e d was conducted on the s e two groups of i n d i v i d u a l s . , Hales caught onl y i n p i t f a l l s a re d e f i n e d as PO males while those males who are e v e n t u a l l y caught i n l i v e - t r a p s are PL males. Table 6.11 i n d i c a t e s t h a t i n 1976 males 40-59 g caught i n p i t f a l l s only were more o f t e n i n r e p r o d u c t i v e c o n d i t i o n than were males e v e n t u a l l y caught i n l i v e - t r a p s , but the d i f f e r e n c e was not s i g n i f i c a n t . PO males had more wounds than t h e i r PL c o u n t e r p a r t s (10 + , 6-, 4NC) , as d i d PO r e p r o d u c t i v e males (8+, 6-, 2 t i e s , 4NC), but the d i f f e r e n c e s were not s i g n i f i c a n t . PO males had higher r a t e s of b o t f l y i n f e s t a t i o n s than PL males (10*, 4-, 6NC), but again t h i s d i f f e r e n c e was not s i g n i f i c a n t . In 1977, males 40-49 g caught only i n p i t f a l l s were more of t e n i n r e p r o d u c t i v e c o n d i t i o n than were 40-49 g males e n t e r i n g l i v e - t r a p s {X*=22.55, P<.001) (Table 6.12). How PL males had more wounds (4+, 13-, 4NC, P<.05) as d i d PL r e p r o d u c t i v e males (3+, 14-, 3NC, P<.05) than t h e i r PO c o u n t e r p a r t s . PL males a l s o had more b o t f l i e s (3-, 14+, 3NC, P<.05). Under high d e n s i t y c o n d i t i o n s , those males t h a t e v e n t u a l l y were r e c r u i t e d t o l i v e - t r a p s had more wounds and more b o t f l i e s than those males caught only i n p i t f a l l s . Hales 40-49 g and g r e a t e r than 79 g agai n tended to have lower wounding r a t e s than d i d medium-sized a d u l t males. They a l s o had lower r a t e s of p a r a s i t i c b o t f l y i n f e c t i o n . PL males had h i g h e r wounding r a t e s i n the peak 1977 p o p u l a t i o n than d i d PL males i n the 1976 i n c r e a s i n g p o p u l a t i o n s . , Presumably t h e r e i s heightened c o m p e t i t i o n to enter the r e s i d e n t l i v e - t r a p 208 T a b l e 6.11. Demographic a t t r i b u t e s of male H i c r o t u s townsendii f i r s t caught i n p i t f a l l s i n 1976 t h a t were caught i n l i v e - t r a p s and of those males th a t were not. A l l g r i d s are combined. The time i n t e r v a l i s the e n t i r e summer except f o r the b o t f l y r e s u l t s , when males i n the b o t f l y season only were s c o r e d . Sample s i z e i s i n parentheses. Height % s c r o t a l Wounds/male Hounds/scrotal male Bots/male Males f i r s t caught i n p i t f a l l s but not i n l i v e - t r a p s 40-49 28.8 (90) 0.70 (90) 0.92 (64) 0.38 (76) 50-59 67. 1 (64) 1.30 (64) 1. 12 (43) 0.50 (48) 60-69 85.7 (28) 1.21 (28) 0.91 (24) 1.00 (15) 70-79 75.0 (8) 1.75 (8) 2-00 (6) 1.00 (7) >80 100.0 (D 0.00 (D 0.00 (D —:— • (0) T o t a l 52.4 (191) 1.01 (191) 1.06 (138) 0.51 (146) Hales f i r s t caught i n p i t f a l l s and l a t e r i n l i v e - t r a p s 40-49 20.7 (169) 0.44 (169) 0.94 (35) 0.38 (137) 50-59 60.0 (135) 1.27 (135) 1. 30 (81)- 0.25 (105) 60-6 9 73.0 (119) 0.72 (119) 0.89 (87) 0.63 (105) 70-79 95.8 (48) 0.50 (48) 0.52 (46) 0.57 (42) >80 90.9 (11) 0.54 (11) 0.50 (10) 0. 18 (11) T o t a l 53.7 (482) 0.75 (482) 0.94 (259) 0.43 (400) 20 9 T able 6.12. Demographic a t t r i b u t e s of male Mi c r o t u s townsendii f i r s t caught i n p i t f a l l s i n 1977 t h a t were caught i n l i v e - t r a p s and of those males t h a t were not. A l l g r i d s are combined. The time i n t e r v a l i s the e n t i r e summer except f o r the b o t f l y r e s u l t s , when males i n the b o t f l y season only were scored. Sample s i z e i s i n parentheses. Weight % s c r o t a l Wounds/male Wounds/scrota1 male Bots/male Males f i r s t caught i n p i t f a l l s but not i n l i v e - t r a p s 40-49 13.8 (550) 0.05 (550) 0.34 (76) 0.16 (518) 50-59 52.5 (236) 1.02 (236) 1.92 (124) 0.22 (152) 60-69 93. 1 (262) 1.80 (262) 1.89 (244) 0. 35 (74) 70-79 98.6 (284) 1.61 (284) 1.62 (280) 0.4 3 (40) >80 99.0 (95) 1.20 (95) 1.21 (94) 0.16 (19) T o t a l 57. 3 (1427) 0.92 (1427) 1.58 (818) 0.20 (803) Hales f i r s t caught i n p i t f a l l s and l a t e r i n l i v e - t r a p s 40-4 9 28. 1 (229) 0.08 (229) 0. 15 (65) 0.18 (163) 50-59 58.2 (67) 0.54 (67) 0.92 (39) 0.20 (45) 60-69 100.0 (15) 0.66 (15) 0.66 (15) 0.25 (8) 70-7 9 100.0 <*)• 1.00 (4) 1.00 (4) 0.00 (2) >80 100.0 (4) 0. 00 <<*> 0.00 (4) ____ (0) T o t a l 39.5 (319) 0.21 :, (319) 0.47 (127) 0. 19 (218) .210 p o p u l a t i o n i n peak p o p u l a t i o n s and g r e a t e r wounding l e v e l s are i n d i c a t i v e o f i n c r e a s e d i n t e r a c t i o n s among i n d i v i d u a l s . PO males i n 1977 had lower wounding r a t e s than pO males i n 1976. This low l e v e l of wounding i n 1977 may be p a r t i a l l y a t t r i b u t a b l e to the d i s p e r s a l of PO males b e f o r e they accumulated many wounds. Growth Rates Of Voles Caught In P i t f a l l s and L i v e - t r a p s and of Those Caught Only In P i t f a l l s The a n a l y s i s so f a r i n d i c a t e s t h a t male v o l e s f a i l i n g t o ent e r l i v e - t r a p s a t t a i n e d r e p r o d u c t i v e maturity a t l i g h t e r weights than d i d males e v e n t u a l l y e n t e r i n g l i v e - t r a p s . Were growth r a t e s i n these two groups of vo l e s d i f f e r e n t ? To i n v e s t i g a t e t h i s q u e s t i o n , I examined in s t a n t a n e o u s growth r a t e s (Brody, 1945) as determined from p i t f a l l - o n l y captures of these two groups of v o l e s . Growth r a t e data from both sexes were i n c l u d e d . I performed a 5-way a n a l y s i s of c o v a r i a n c e with growth r a t e as a dependent v a r i a b l e , weight as a c o v a r i a t e , and sex, g r i d , group, weight c l a s s , and year as the f i v e i n d i c e s . The a n a l y s i s i n d i c a t e d t h a t t h e r e was a s i g n i f i c a n t d i f f e r e n c e i n growth r a t e s between sexes <F=47.29, df=1, 3426, P<.0001), among g r i d s <F=2.87, P<.05) , between groups (F=8.60, P<.01), among weight c l a s s e s (F=132.46, P<.0001), and between years (F=13.47, P<.001). The s i g n i f i c a n c e of the c l a s s index shows t h a t there i s a d d i t i o n a l v a r i a b i l i t y i n growth r a t e s a s s o c i a t e d with body s i z e t h a t i s not removed by the l i n e a r r e g r e s s i o n of growth r a t e s on body weight.. 211 The f o l l o w i n g comparisons of growth r a t e s a r e based upon mean growth r a t e s c o r r e c t e d f o r s i z e e f f e c t s by c o v a r i a n c e a n a l y s i s . Males grew about 24% f a s t e r per day than females. Voles on the unfenced c o n t r o l g r i d s grew 8% f a s t e r per day than v o l e s on the enclosed g r i d s . PO v o l e s grew 11% f a s t e r than d i d PL v o l e s , whereas i n d i v i d u a l s i n summer 1976 grew 8% f a s t e r than i n summer 1977. The i n t e r a c t i o n between weight c l a s s and group was not s i g n i f i c a n t , which i n d i c a t e s t hat t h e r e was about the same d i f f e r e n c e between groups w i t h i n each weight c l a s s . The mean growth r a t e s f o r each weight c l a s s of the PO v o l e s were always higher than f o r the same weight c l a s s f o r the PL v o l e s . T h i s a n a l y s i s c l e a r l y i n d i c a t e s t h a t those v o l e s f i r s t caught i n p i t f a l l s but never caught i n l i v e - t r a p s had h i g h e r i n s t a n t a n e o u s growth r a t e s than d i d those v o l e s f i r s t caught i n p i t f a l l s and l a t e r i n l i v e - t r a p s . D i s c u s s i o n Small mammal p o p u l a t i o n s i z e can be estimated by s t o c h a s t i c s t a t i s t i c a l models ( J o l l y , 1965) , d e t e r m i n i s t i c models ( L e s l i e e t a l . , 1953), d i r e c t enumeration ( C h i t t y and Phipps, 1966; Krebs, 1966) or other methods (Smith e t a l . , 1975) . Small rodent p o p u l a t i o n s v i o l a t e the major assumption of the s t a t i s t i c a l models that a l l marked and unmarked animals are e q u a l l y c a t c h a b l e (Krebs, 1966). T h e r e f o r e , Krebs (1966, l a t e r papers) used d i r e c t enumeration as an a l t e r n a t i v e method of e s t i m a t i n g p o p u l a t i o n s i z e . The major assumption o f t h i s method i s t h a t l i v e - t r a p s c a t c h a l l the a d u l t animals i n the p o p u l a t i o n . I t i s known t h a t j u v e n i l e v o l e s do not r e a d i l y 212 e n t e r l i v e - t r a p s , and Krebs and DeLong (1965) measured j u v e n i l e s u r v i v a l i n d i r e c t l y by means of an index. F a i l u r e of l i v e - t r a p s to c a t c h a l l j u v e n i l e s , but not a l l a d u l t s , i s expected. Krebs (1966) suggested that he was able to enumerate 80 to 90% of M i c r o t u s c a l i f o r n i c u s i n d e n s i t i e s up t o 150 per a c r e . There was no method of c o n f i r m i n g t h i s e s t i m a t e . Above t h i s d e n s i t y Krebs f e l t 60 to 808 of the p o p u l a t i o n was enumerated. Krebs e t a l . (1969) b e l i e v e d t h a t they enumerated 90% of M. ochrogaster and 75% (50% i n summer) of M. pennsylvanicus p o p u l a t i o n s . A trap-out of e n c l o s u r e s suggested t h a t l i v e - t r a p p i n g enumerated most of the i n d i v i d u a l s (Krebs e t a l . , 1969). I t was assumed t h a t v o l e s did not a v o i d snap-traps used i n the t r a p - o u t . Boonstra and Krebs (1978) found t h a t of 213 a d u l t M. townsendii f i r s t caught i n p i t f a l l s , only 62% were e v e n t u a l l y caught i n l i v e - t r a p s . In the present study, of 574 a d u l t s f i r s t caught i n p i t f a l l s , 56% were e v e n t u a l l y caught i n l i v e - t r a p s . In M. townsendii p o p u l a t i o n s , the assumption that l i v e - t r a p s enumerate a l l a d u l t s i s c l e a r l y v i o l a t e d . L i v e - t r a p s do not succeed i n completely enumerating any age c l a s s of M. townsendii. S t u d i e s t y p i c a l l y use only l i v e - t r a p s , and the d e t e r m i n a t i o n of demographic c h a r a c t e r i s t i c s i s based upon data from enumerated v o l e s o n l y . What e r r o r s are i n t r o d u c e d when onl y l i v e - t r a p data are used t o estimate demographic parameters? T h i s study demonstrated t h a t v o l e s never caught i n l i v e - t r a p s had f a s t e r growth r a t e s than those e n t e r i n g l i v e - t r a p s . Thus, growth r a t e s d e r i v e d s o l e l y from l i v e - t r a p data underestimate the average growth r a t e . Smaller a d u l t males caught only i n p i t f a l l s a re more o f t e n i n 213 r e p r o d u c t i v e c o n d i t i o n than those which are r e c r u i t e d to l i v e - t r a p s . I t appears t h a t males which a t t a i n sexual m a t u r i t y e a r l i e r tend t o d i s p e r s e without being caught i n l i v e - t r a p s . Median weights at s e x u a l maturity d e r i v e d s o l e l y from l i v e - t r a p data o v e r e s t i m a t e the a c t u a l v a l u e s f o r the p o p u l a t i o n . The l o s s through d i s p e r s a l o f v o l e s a t t a i n i n g s e x u a l maturity before e n t e r i n g l i v e - t r a p s reduces the apparent p r o p o r t i o n of r e p r o d u c t i v e i n d i v i d u a l s i n the p o p u l a t i o n . The l o s s of these i n d i v i d u a l s a l s o a f f e c t s minimum f i n i t e s u r v i v a l r a t e s . The assumption i s made t h a t s u r v i v a l o f a p a r t i c u l a r age and sex c l a s s o b t a i n e d by l i v e - t r a p enumeration i s a r e p r e s e n t a t i v e estimate of the minimum s u r v i v a l o f t h a t c l a s s . Disappearance has both a death and d i s p e r s a l component. The a c t u a l minimum s u r v i v a l of an age group w i l l be lower than t h a t estimated by l i v e - t r a p s because they f a i l t o c a t c h a l l e a r l y maturing, d i s p e r s i n g i n d i v i d u a l s . Males caught i n p i t f a l l s d u r i n g the same time t h a t l i v e - t r a p s are o p e r a t i n g have more wounds and more p a r a s i t i c b o t f l i e s . Demographic data d e r i v e d from l i v e - t r a p p i n g may underestimate the a c t u a l wounding l e v e l s and p a r a s i t e l o a d s present i n the p o p u l a t i o n . The present study i n d i c a t e s t h a t l i v e - t r a p s d i d not t o t a l l y enumerate any segment of the M i e r o t u s townsendii p o p u l a t i o n s , and may produce bi a s e d demographic data f o r both i n c r e a s i n g and peak p o p u l a t i o n s , and l i k e l y f o r d e c l i n i n g p o p u l a t i o n s as w e l l . However, although the absolute estimates of many parameters may be i n e r r o r , I suggest that the r e l a t i v e magnitude of the parameters i s a c c u r a t e l y determined. For example, although median weight at s e x u a l maturity w i l l be b i a s e d i f only l i v e - t r a p data are used i n i t s 2 1 4 d e t e r m i n a t i o n , these data w i l l s t i l l demonstrate t h a t i n c r e a s i n g p o p u l a t i o n s have the lowest median weight at s e x u a l maturity and peak ones the h i g h e s t . T h i s study demonstrates t h a t l i v e - t r a p s d i d not a c c u r a t e l y enumerate Mierotus townsendii p o p u l a t i o n s . One e x p l a n a t i o n c o u l d be t h a t there were too few l i v e - t r a p s a v a i l a b l e to c a t c h a l l i n d i v i d u a l s l i k e l y t o e n t e r them. In t h i s study the d e n s i t y of l i v e - t r a p s used was g r e a t e r than or equal t o d e n s i t i e s used by o t h e r workers (Krebs, 1966; Krebs e t a l . 1969; Tamarin 1977). However, because t r a p p a b i l i t i e s were about 65% i n i n c r e a s i n g and peak p o p u l a t i o n s and over 80% i n the low d e n s i t y 1978 s p r i n g p o p u l a t i o n s , t h e r e were i n d i c a t i o n s of c o m p e t i t i o n among v o l e s f o r the l i v e - t r a p s . Van Vleck (1968) found a p o s i t i v e r e l a t i o n between p o p u l a t i o n d e n s i t y and the number of t r a p s necessary t o capture a ; constant p r o p o r t i o n of the p o p u l a t i o n . I n d i v i d u a l behaviour i s important i n determining which v o l e s a r e caught i n l i v e - t r a p s . There are i n d i c a t i o n s t h a t l i v e - t r a p s sample mainly the l a r g e r , dominant i n d i v i d u a l s . Kikkawa (1964) observed encounters of Clethrionomys g l a r e o l u s a t l i v e - t r a p s and noted that l a r g e r , dominant i n d i v i d u a l s chased away s m a l l e r , subordinate v o l e s . Summerlin and Wolfe (1973) a l s o found t h a t h e a v i e r , dominant Sigmodon h i s p i d u s were more l i k e l y t o be trapped than were s m a l l e r , subordinate r a t s . G l i w i c z (1970) r e p o r t e d t h a t young v o l e s have low t r a p p a b i l i t y i n l i v e - t r a p s , and suggested that i n d i v i d u a l s low i n the s o c i a l h e i r a r c h y , such as j u v e n i l e s , have low t r a p p a b i l i t y . A p o s i t i v e r e l a t i o n s h i p between s o c i a l dominance and t r a p p a b i l i t y has been 215 demonstrated i n s e v e r a l s p e c i e s (Davis and Emlen, 1956; Andrzejewski and Rajska, 1972; J o u l e and Cameron, 1974). I t appears t h a t some form of behaviour d e t e r s subordinate v o l e s from e n t e r i n g l i v e - t r a p s . T h i s study i n d i c a t e s t h a t t h e r e i s a c o n s i d e r a b l e time l a g between f i r s t capture in< p i t f a l l s and f i r s t c apture i n l i v e - t r a p s i n i n c r e a s i n g , peak, and d e c l i n i n g p o p u l a t i o n s . When d e n s i t i e s were low i n summer 1976 p i t f a l l s enumerated p r a c t i c a l l y a l l of the i n d i v i d u a l s known to be present i n the p o p u l a t i o n s , whereas l i v e - t r a p s took u n t i l s p r i n g 1977 to enumerate most o f the v o l e s known to be a l i v e . Even at low d e n s i t i e s M i crotus townsendii appears t o be r e l u c t a n t t o enter l i v e - t r a p s . In the s p r i n g and e a r l y summer of 1977 p i t f a l l s a g a i n enumerated most of the animals known to be a l i v e , whereas l i v e - t r a p s enumerated about 50% of the animals. At peak p o p u l a t i o n d e n s i t i e s a c o n s i d e r a b l e time l a g between f i r s t c apture i n p i t f a l l s and l i v e - t r a p s was maintained. L i v e - t r a p s adeguately enumerate the v o l e p o p u l a t i o n s only i n l a t e winter and e a r l y s p r i n g . Small v o l e s seem i n h i b i t e d i n e n t e r i n g l i v e - t r a p s , and t h i s i n h i b i t i o n accounts f o r the time l a g between f i r s t c a p t u r e s i n p i t f a l l s and l i v e - t r a p s . Boonstra and Krebs (1978) r e p o r t e d s i m i l a r time l a g s f o r M. townsendii between f i r s t c a p t u r e s i n p i t f a l l s and l i v e - t r a p s , and the study by Watts (1970) supports the h y p o t h e s i s t h a t young v o l e s a v o i d l i v e - t r a p s . He found t h a t on areas from which he removed l a r g e male Clethrionomys gap.peri, j u v e n i l e s entered l i v e - t r a p s at a younger age than on areas where a d u l t s were present. In unmanipulated p o p u l a t i o n s , the 216 young presumably avoided l i v e - t r a p s . An index of j u v e n i l e s u r v i v a l proposed by Krebs and DeLong (1965) i s i n f l u e n c e d by t r a p p a b i l i t y of j u v e n i l e s . Changes in t h i s index are a f f e c t e d by changes i n j u v e n i l e t r a p p a b i l i t y i n l i v e - t r a p s . The index d e r i v e d from l i v e - t r a p p i n g was lowest i n the 1977 peak d e n s i t y p o p u l a t i o n s , when few v o l e s f i r s t entered l i v e - t r a p s as j u v e n i l e s . D i r e c t enumeration of j u v e n i l e s , such as by p i t f a l l t r a p p i n g used i n t h i s study, may provide a more r e l i a b l e e s t i m a t i o n o f j u v e n i l e demography. Boonstra and Krebs (1978) found t h a t a d u l t fl. townsendii f i r s t caught i n p i t f a l l s took an average of s i x weeks to enter l i v e - t r a p s . They suggested t h a t t h i s time l a g may be p a r t l y due to the presence of immigrants low i n the s o c i a l h e i r a r c h y . Time would be r e q u i r e d f o r these animals t o move up i n the s o c i a l system and g a i n access t o l i v e - t r a p s . While t h i s e x p l a n a t i o n i s p o s s i b l y c o r r e c t , i n the fenced p o p u l a t i o n s some a d u l t s , which are d e f i n i t e l y not immigrants, s t i l l r e q u i r e a number of weeks to e n t e r l i v e - t r a p s . In the fenced p o p u l a t i o n s , of 66 a d u l t males f i r s t caught i n p i t f a l l s and e n t e r i n g l i v e - t r a p s , 56% r e q u i r e d more than 5 weeks t o enter them. However, only 33% of 60 females took more than 5 weeks. I t appears t h a t f a c t o r s o ther than immigration i n f l u e n c e an a d u l t * s a b i l i t y to enter l i v e - t r a p s , with males being more a f f e c t e d than females. Immigration r a t e s are d i f f i c u l t t o measure. New v o l e s caught i n unfenced open p o p u l a t i o n s may be immigrants or n a t i v e to the study area. Some workers have assumed t h a t new s m a l l v o l e s are born on the a r e a , bute t h a t new a d u l t v o l e s are immigrants ( H i l b o r n and Krebs, 1976; B e d f i e l d e t a l . , , 1978). 217 In the fenced p o p u l a t i o n s , 52% of the voles were i n the a d u l t weight c l a s s at f i r s t capture i n l i v e - t r a p s , although none were immigrants. The assumption that new a d u l t animals i n unfenced p o p u l a t i o n s are a l l immigrants i s thus wrong. H i l b o r n e t a l . (1976) developed a s i m u l a t i o n model to show t h a t when t r a p p a b i l i t y i s above 65%, the minimum number a l i v e estimate d e r i v e d from enumeration underestimated the a c t u a l p o p u l a t i o n s i z e by about 10%. T h e r e f o r e , i n t h i s study, when t r a p p a b i l i t y i n l i v e - t r a p s d u r i n g summer was about 65%, the model p r e d i c t s an underestimate of a c t u a l p o p u l a t i o n s i z e by 10%. In October 1976, l i v e - t r a p s underestimated the a c t u a l p o p u l a t i o n by 55 t o 70%; and i n J u l y 1977 the underestimate was between 50 and 60%. T h i s d i s c r e p a n c y i s a d i r e c t r e s u l t o f the absence o f a l a r g e c l a s s of v o l e s from l i v e - t r a p s . The Mierotus townsendii p o p u l a t i o n s were i n a low d e n s i t y i n c r e a s i n g phase i n the 1976 summer. , In mid-October 1976 p i t f a l l s were enumerating 69 t o 75 % of the t o t a l number of v o l e s known to be a l i v e while l i v e - t r a p s enumerated between 30 and 44%. The peak enumerated p o p u l a t i o n occurred i n J u l y 1977, when p i t f a l l s enumerated from 63 t o 72% and l i v e - t r a p s 42 to 49% of the t o t a l v o l e p o p u l a t i o n known to be a l i v e at t h a t time. The r e s u l t s of t h i s study i n d i c a t e that > l i v e - t r a p s may a c c u r a t e l y e s t i m a t e v o l e p o p u l a t i o n s i n the l a t e winter and e a r l y s p r i n g provided t h e r e has been no winter breeding. The c o n s i d e r a b l e delays between f i r s t capture i n p i t f a l l s and l i v e - t r a p s suggests t h a t l i v e - t r a p s do not a c c u r a t e l y enumerate the p o p u l a t i o n d u r i n g the b r e e d i n g season, nor f o r about f o u r months a f t e r breeding s t o p s . a l t e r n a t i v e methods of 218 enumeration, such as p i t f a l l t r a p p i n g , are necessary to enable one to enumerate more f u l l y the vole p o p u l a t i o n d u r i n g t h i s p e r i o d . These H. towns e n d i i p o p u l a t i o n s were i n a peak phase du r i n g the 1977 summer. The maximum d e n s i t i e s recorded v a r i e d between 401 and 530 voles per a c r e , among the h i g h e s t ever enumerated f o r m i c r o t i n e p o p u l a t i o n s . Only the r e s u l t s of Boonstra and Krebs (1978) are s i m i l a r , because both p i t f a l l s and l i v e - t r a p s were used c o n c u r r e n t l y i n t h e i r study. They recorded a maximum of 519 v o l e s per a c r e f o r a M. townsendii p o p u l a t i o n . Other s t u d i e s have used o n l y l i v e - t r a p s to enumerate vo l e p o p u l a t i o n s . C h i t t y (1952) recorded a d e n s i t y of 300 v o l e s per acre i n a H. a q r e s t i s p o p u l a t i o n . Krebs (1966) r e p o r t e d a maximum d e n s i t y of 324 M. c a l i f o r n i c u s per ac r e . Krebs e t a l . (1969) estimated maximum d e n s i t i e s of M. pennsylvanicus a t 60 v o l e s per a c r e and M. ochrqqaster a t 40 v o l e s per ac r e . M. breweri and M. pennsylvanicus d e n s i t i e s were each recorded at 50 v o l e s per acre by Tamarin (1977). The a c t u a l p o p u l a t i o n s i z e s i n the l a t t e r s t u d i e s may have been much hi g h e r than recorded as a r e s u l t of the f a i l u r e o f l i v e - t r a p s to enumerate s u b s t a n t i a l p o r t i o n s of the p o p u l a t i o n s . P i t f a l l s and l i v e - t r a p s , when operated c o n c u r r e n t l y , sample d i f f e r e n t segments of the p o p u l a t i o n . Adult males caught i n p i t f a l l s have more wounds and higher r a t e s of p a r a s i t i c i n f e s t a t i o n s than do s i m i l a r - s i z e d males caught i n l i v e - t r a p s . Although some of the wounds accumulated by males caught i n p i t f a l l s may be a r e s u l t of confinement with o t h e r v o l e s i n the p i t f a l l , I suggest t h a t these wounds c o n s t i t u t e a minor p o r t i o n 219 of the t o t a l wounding l o a d . I f one assumes t h a t subordinate v o l e s have more wounds, then i t appears t h a t p i t f a l l s sample the s u b o r d i n a t e i n d i v i d u a l s i n the p o p u l a t i o n . Andrzejewski and Rajska (1972), by t r a p p i n g a v o l e p o p u l a t i o n c o n c u r r e n t l y with l i v e - t r a p s and p i t f a l l s , found t h a t as i n d i v i d u a l s became o l d e r , they were more t r a p p a b l e i n l i v e - t r a p s and l e s s so i n p i t f a l l s . Newly i n t r o d u c e d animals, i r r e s p e c t i v e o f age, had s i m i l a r t r a p p a b i l i t i e s to those of r e s i d e n t j u v e n i l e s . Thus i t appears t h a t as i n d i v i d u a l s ascend i n the s o c i a l h e i r a r c h y , they are more l i k e l y to e n t e r l i v e - t r a p s . More s m a l l e r females enter l i v e - t r a p s than do males, which suggests t h a t s o c i a l a g gression exerted by a d u l t s may be g r e a t e r towards young males than females. An a n a l y s i s of c o v a r i a n c e i n d i c a t e d that growth r a t e s of males were f a s t e r than those of females, and t h a t growth r a t e s of v o l e s caught i n p i t f a l l s were f a s t e r than growth r a t e s o f v o l e s caught i n both types of t r a p s . Higher growth r a t e s are a s s o c i a t e d with f a s t e r s e x u a l maturation, which l e a d s to b e h a v i o u r a l i n t e r a c t i o n s with o t h e r r e p r o d u c t i v e l y mature v o l e s . Summer growth r a t e s were f a s t e r i n the low d e n s i t y 1976 p o p u l a t i o n s compared with those i n the peak d e n s i t y 1977 p o p u l a t i o n s and f a s t e r i n the lower d e n s i t y c o n t r o l p o p u l a t i o n s than i n the e x p e r i m e n t a l s . Vole d e n s i t y may i n f l u e n c e growth r a t e s by i n c r e a s i n g frequency of b e h a v i o u r a l i n t e r a c t i o n s between i n d i v i d u a l s i n high d e n s i t y p o p u l a t i o n s and thereby e i t h e r d i r e c t l y or i n d i r e c t l y r e t a r d growth. The p r e s e n t study demonstrates t h a t l i v e - t r a p s f a i l to capture a l a r g e p o r t i o n of the vole p o p u l a t i o n s , and that 220 t r a p p i n g with p i t f a l l s i s a n e c e s s i t y i f one i s i n t e r e s t e d i n o b t a i n i n g a comprehensive demographic a n a l y s i s . T h i s study a l s o suggests some areas t h a t s h o u l d be explo r e d f u r t h e r . The s o c i a l f a c t o r s t h a t prevent s m a l l e r , subordinate v o l e s from e n t e r i n g l i v e - t r a p s c o u l d be i n v e s t i g a t e d by examining the r o l e of agge s s s i o n , odour, or other f a c t o r s . L i t e r a t u r e c i t e d Andrze je w s k i , R. and E. Rajska. 1972. T r a p p a b i l i t y of bank v o l e s i n p i t f a l l s and l i v e t r a p s . Acta T h e r i o l . 17:41-56. Boonstra, R. and C. J . Krebs. 1978. P i t f a l l t r a p p i n g of Microtus t o w n s e n d i i . J . Mammal. 59: 136-148. Brody, S. 1945. B i o e n e r g e t i c s and growth. Reinhold Publ. Corp., New York. 1023p. C h i t t y , D. 1952. M o r t a l i t y among v o l e CMicrotus a g r e s t i s ) a t Lake Vyrnwy, Montgomeryshire i n 1936-9. P h i l . Trans. Roy. Soc. Lond., Ser. A. 236: 505-552. C h i t t y , D. and E. Phipps., 1966. Seasonal changes i n s u r v i v a l i n mixed p o p u l a t i o n s of two vol e s p e c i e s . J . Anim. E c o l . 35: 313-331. Davis, D. E. , and J . T. Emlen. 1956./ D i f f f e r e n t i a l t r a p p a b i l i t y of r a t s a c c o r d i n g t o s i z e and age. J . W i l d l . Mgmt. 20: 326-327. G l i w i c z , J . 1970. R e l a t i o n between t r a p p a b i l i t y and age of i n d i v i d u a l s i n a p o p u l a t i o n o f the bank v o l e . Acta T h e r i o l . 15: 15-23. H i l b o r n R., J . A. R e d f i e l d , and C. J . Krebs., 1976. On the 2 2 1 r e l i a b i l i t y o f enumeration f o r mark and re c a p t u r e census of v o l e s . Can. J . Z o o l . 54: 1019-1024. J o l l y , G. H. 1965. E x p l i c i t e s t i m a t e s from c a p t u r e - r e c a p t u r e data with both death and i m m i g r a t i o n - s t o c h a s t i c model. B i o m e t r i k a , 52: 225-247. J o u l e , J . and G. N. Cameron. 1974. F i e l d e s t i m a t i o n o f demographic parameters: i n f l u e n c e of Sigmodon h i s p i d u s p o p u l a t i o n s t r u c t u r e . J . Mammal. 55: 309-318. Kikkawa, J . 1964. Movements, a c t i v i t y , and d i s t r i b u t i o n of the s m a l l rodents Clethrionomy.s g l a r e o l u s and Apgdemus s y l v a t i c u s i n woodland. J . Anim. E c o l . 33: 259-299. Krebs, C. J . 1966. Demographic changes i n f l u c t u a t i n g p o p u l a t i o n s of Mierotus c a l i f o r n i c u s . E c o l . Monogr. 36: 239-273. Krebs, C. J . and K. T- DeLong. 1965. A Mierotus p o p u l a t i o n with supplemental food. J . Mammal. 46: 566-573. Krebs, C. J . , B. L. K e l l e r , and 8. H. Tamarin 1969. Mierotus p o p u l a t i o n b i o l o g y : Demographic changes i n f l u c t u a t i n g p o p u l a t i o n s of M. ochrogaster and M. pen n s y l v a n i c u s i n southern Indiana. Ecology 50: 587-607. L e s l i e , P. H., D. C h i t t y , and H. C h i t t y . 1953. The e s t i m a t i o n of p o p u l a t i o n parameters from data o b t a i n e d by means of the c a p t u r e - r e c a p t u r e method. I I I . An example of the p r a c t i c a l a p p l i c a t i o n s of the method. B i o m e t r i k a , 40: 137-169. R e d f i e l d , J . A., M. J . T a i t t , and C. J . Krebs. 1978. Experimental a l t e r a t i o n of sex r a t i o s i n p o p u l a t i o n s of Mierotus t o w n s e n d i i . a f i e l d v o l e . Can. J . Z o o l . 56: 222 17-27. Smith, M. H., E. H. Gardner, J . B. Gentry, D. W. Kaufman, and H. H. O ' F a r r e l l . 1975. Density e s t i m a t i o n s of s m a l l mammal p o p u l a t i o n s . Pp. 25-54, In Small mammals: p r o d u c t i v i t y and dynamics of p o p u l a t i o n s (F. B. G o l l e y , K. Petrusewicz, and L. Ryszkowski, eds.) Cambridge Univ. P r e s s , London. 451p. Summerlin, C. T. and J . L. Wolfe. 1973. S o c i a l i n f l u e n c e s on t r a p response o f the c o t t o n r a t . Sigmodon h i s p i d u s . Ecology, 54: 1156-1159. Tamarin, B. H. 1977. D i s p e r s a l i n i s l a n d and mainland v o l e s . Ecology 58: 1044-1054. Van V l e c k , D. B. 1968. , Movements of H i c r o t u s pennsylvanicus i n r e l a t i o n t o depopulated areas. J . Hammal. 49: 92-103. Watts, C. H. S. 1970. & f i e l d experiment on i n t r a s p e c i f i c i n t e r a c t i o n s i n the red-backed v o l e , Clethrionomys g a p p e r i . J . Mammal. 51: 341-347. 223 GENERAL SUMMARY I designed t h i s study t o i n v e s t i g a t e the r o l e of d i s p e r s a l i n p o p u l a t i o n r e g u l a t i o n of Mierotus townsendii. I was s p e c i f i c a l l y i n t e r e s t e d i n determining i f v o l e s d i s p e r s e d from d e c l i n i n g p o p u l a t i o n s . T h i s study i n d i c a t e d t h a t t h e r e are two types of p o p u l a t i o n d e c l i n e s i n v o l e s . The f i r s t i s t y p i c a l l y r e f e r r e d t o as a s p r i n g d e c l i n e because i t s t a r t s s h o r t l y a f t e r s p r i n g b r e e d i n g commences. During t h i s d e c l i n e wounding i n c r e a s e s and there i s a preponderance of d i s p e r s i n g s m a l l e r a d u l t s compared with the r e s i d e n t p o p u l a t i o n . These d e c l i n e s l a s t f o r t h r e e or f o u r months, a f t e r which the p o p u l a t i o n recovers by t h e f a l l to d e n s i t i e s about the same as those i n the s p r i n g . More males d i s p e r s e than females d u r i n g t h i s s p r i n g d e c l i n e . I suggest that t h i s d e c l i n e i s e x p l a i n e d by a g g r e s s i v e b e h a v i o u r a l i n t e r a c t i o n s between v o l e s coming i n t o breeding c o n d i t i o n . T h i s r e s u l t s i n i n c r e a s e d s p a c i n g behaviour and s m a l l e r , s u b o r d i n a t e i n d i v i d u a l s d i s p e r s e from the p o p u l a t i o n . The second type of d e c l i n e was t y p i f i e d i n t h i s study by the eight-month d e c l i n e from October 1977 u n t i l June 1978. Here the d e c l i n e s t a r t e d i n the f a l l a f t e r breeding stopped, continued through the winter, and the r a t e of d e c l i n e a c c e l e r a t e d when breeding began i n the s p r i n g . Most of the p o p u l a t i o n was l o s t d u r i n g the non-breeding season, and the d e c l i n e was c h a r a c t e r i z e d by n e g l i g i b l e wounding and e s s e n t i a l l y no d i s p e r s a l . The l a c k of wounding and d i s p e r s a l suggested t h a t b e h a v i o u r a l i n t e r a c t i o n s were not the d r i v i n g mechanism behind t h i s d e c l i n e . Rather, those animals that disappeared and 2 2 4 t h e r e f o r e d i e d were the s m a l l e r i n d i v i d u a l s i n the p o p u l a t i o n s . These i n d i v i d u a l s were c h a r a c t e r i z e d by negative growth r a t e s during the l a s t h a l f of the d e c l i n e , while l a r g e r i n d i v i d u a l s had p o s i t i v e growth r a t e s . I i n t e r p r e t these r e s u l t s to i n d i c a t e t h at food was i n s h o r t supply d u r i n g t h i s d e c l i n e and t h a t i n t e r f e r e n c e c o m p e t i t i t o n prevented the s m a l l e r members of the p o p u l a t i o n s from o b t a i n i n g an adeguate share o f the r e s o u r c e . I suggest t h a t food was a l i m i t i n g f a c t o r d u r i n g t h i s p e r i o d . D i s p e r s a l i s common i n peak H i c r o t u s townsendii p o p u l a t i o n s . In the peak summer, d i s p e r s a l of s u b a d u l t s i n c r e a s e d , and these subadults were more o f t e n i n r e p r o d u c t i v e c o n d i t i o n than were r e s i d e n t s u b a d u l t s . D i s p e r s e r s d u r i n g t h i s p e r i o d were l i g h t e r than r e s i d e n t v o l e s . I suggest that b e h a v i o u r a l i n t e r a c t i o n s between l a r g e r e s i d e n t s and s m a l l e r v o l e s becoming r e p r o d u c t i v e l y mature l e d to the i n c r e a s e d d i s p e r a l of s u b a d u l t s d u r i n g the summer. D i s p e r s a l tendency was non-randomly d i s t r i b u t e d among members of the p o p u l a t i o n s . L i t t e r m a t e s resembled each other i n terms of l i f e t i m e ( d u r a t i o n o f l i f e between f i r s t and l a s t capture) and d i s p e r s a l tendency. I f l i t t e r m a t e s d i s p e r s e d , they tended to l e a v e at the same age. Even among the n o n - d i s p e r s i n g animals l i f e t i m e was s t i l l non-randomly d i s t r i b u t e d . L i t t e r s i n both i n c r e a s i n g and peak p o p u l a t i o n s were d i f f e r e n t i a t e d with r e s p e c t to l i f e t i m e and d i s p e r s a l tendency. I conclude t h a t l i f e t i m e and d i s p e r s a l tendency had a h e r i t a b l e b a s i s i n M. t o w n s e n d i i . but whether or not the h e r i t a b i l i t y i s g e n e t i c a l l y or m a t e r n a l l y based i s unknown. 225 Avian p r e d a t o r s apparently had no s i g n i f i c a n t impact on the vole p o p u l a t i o n s - Although they were s e l e c t i v e f o r the s i z e and sex of t h e prey, they c o u l d not account f o r the ma j o r i t y o f the l o s s e s i n d e c l i n i n g p o p u l a t i o n s , even though t h i s study underestimated the t o t a l impact o f the p r e d a t o r s . T h i s study provided estimates of preweanling and j u v e n i l e s u r v i v a l i n i n c r e a s i n g , peak, and d e c l i n i n g v o l e p o p u l a t i o n s . Large numbers of j u v e n i l e s were caught i n p i t f a l l s , and d i r e c t enumeration showed t h a t j u v e n i l e s u r v i v a l was h i g h e s t i n i n c r e a s i n g p o p u l a t i o n s and lowest i n d e c l i n i n g ones. However, estimated preweanling s u r v i v a l was lowest i n peak p o p u l a t i o n s and a c t u a l l y i n c r e a s e d i n d e c l i n i n g p o p u l a t i o n s . Higher preweanling m o r t a l i t y d i d not occur i n the d e c l i n e . The c o n c u r r e n t use of l i v e - t r a p s and p i t f a l l s showed t h a t l i v e - t r a p s f a i l e d to enumerate a l l members of any s i z e c l a s s of vo l e s . I t a l s o showed that e s t i m a t e s of demographic parameters may be i n a c c u r a t e i f o n l y l i v e - t r a p s a re used, because some i n d i v i d u a l s d i s p e r s e d before they entered l i v e - t r a p s . Many new immigrants i n t o a p o p u l a t i o n s f a i l e d t o e n t e r l i v e - t r a p s , presumably because they were low i n the s o c i a l h e i r a r c h y . Minimum s u r v i v a l o f a s i z e c l a s s o f v o l e s was dependent upon the type of t r a p used i n enumeration, presumably because t r a p p a b i l i t y of a s i z e c l a s s was d i f f e r e n t i n l i v e - t r a p s and p i t f a l l s . Voles caught only i n p i t f a l l s had f a s t e r growth r a t e s than v o l e s f i r s t caught i n p i t f a l l s and l a t e r i n l i v e - t r a p s . I suggest t h a t these f a s t e r - g r o w i n g v o l e s reached s e x u a l maturity e a r l i e r than slower-growing v o l e s and t h e r e f o r e d i s p e r s e d e a r l i e r before they were s o c i a l l y dominant and c o u l d gain access 2 2 6 to l i v e - t r a p s . 

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