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Demography and dispersal in island and mainland populations of the deer mouse, Peromyscus maniculatus Sullivan, Thomas Priestlay 1976

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DEMOGRAPHY AND DISPERSAL IH ISLAND AND MAINLAND POPULATIONS OF THE DEER HOUSE, PeromYgcus maniculatus by THOMAS PRIESTLAY SULLIVAN B.Sc. (Hons.), University of B.C., 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Zoology He accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA JUNE, 1976 (c) Thomas P r i e s t l a y S u l l i v a n In 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 H e a d 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 The 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 Wesbrook P l a c e Vancouver, Canada V6T 1W5 i A b s t r a c t I f d i s p e r s a l i s reduced on i s l a n d s , then the demography of i s l a n d p o p u l a t i o n s of deer mice should be d i f f e r e n t from that of mainland p o p u l a t i o n s . , Areas of 1.1 ha were p e r i o d i c a l l y c l e a r e d of mice on Samuel I s l a n d (206 ha) and Saturna I s l a n d (3102 ha) i n the Gulf I s l a n d s of southwestern B r i t i s h Columbia., A s i m i l a r experiment was conducted on the mainland a t Maple Ridge, B.C., The average d e n s i t y of mice per hectare on Saturna (43.5) was twice t h a t on Samuel I s l a n d (22.0) and n e a r l y two and one-half times higher than t h a t on the mainland (18.7). The r e p r o d u c t i v e r a t e , as measured by l e n g t h of breeding season, number of s u c c e s s f u l pregnancies, p r o p o r t i o n o f breeding animals, and number of r e c r u i t s s u r v i v i n g t o breed, was much higher on Samuel I s l a n d than on e i t h e r Saturna or the mainland. S u r v i v a l was lowest on Samuel I s l a n d , with l i t t l e d i f f e r e n c e between the mainland and Saturna I s l a n d . , Mice on Samuel I s l a n d grew more than f i v e times f a s t e r than mainland animals, and Saturna growth r a t e s were double those on the mainland. I s l a n d a d u l t males showed very few a g g r e s s i v e t e n d e n c i e s i n l a b o r a t o r y behaviour t e s t s compared with the seasonal changes i n aggression r e p o r t e d i n the l i t e r a t u r e f o r mainland deer mice. D i s p e r s a l (or c o l o n i z a t i o n ) r a t e was reduced on the two i s l a n d s compared with t h a t on the mainland. Eecruitment of young animals occurred throughout the breeding season on the i s l a n d s but was delayed u n t i l the end of breeding on the mainland. There was l i t t l e d i f f e r e n c e i n the demographic a t t r i b u t e s of c o n t r o l p o p u l a t i o n s when compared with those of c o l o n i s t p o p u l a t i o n s on e i t h e r the i i mainland or the two i s l a n d s . These r e s u l t s i n d i c a t e t h a t seasonal changes i n aggressiveness of the a d u l t p o p u l a t i o n may be s u f f i c i e n t but not necessary to determine breeding d e n s i t y and seasonal changes i n s u r v i v a l of j u v e n i l e deer mice., A more i n t e n s i v e study i s r e g u i r e d , but r e g u l a t o r y processes i n p o p u l a t i o n s of Peromy_scus o a & i c u l a t u s may be d i f f e r e n t on i s l a n d s and perhaps should not be g e n e r a l i z e d over d i f f e r e n t geographic areas. i i i TABLE OF CONTENTS A i s s t i T c i c t • • * ' • « * » • *'• •« • • • • • » • • • * • • » * • •'* • * • * • • # • » • • • <« #, • • * • • • i . TABLE OF CONTENTS ., i i i LIST OF FIGURES ........................... , ............ . , i v LIST OF TABLES , V ACKNOWLEDGEMENTS .... ........... , v i i INTRODUCTION , 1 DESCRIPTION OF STUDY AREAS . 3 MATERIALS AND METHODS 5 BJBSUXITS «•#«*•'••**«••**••*.••• •»• •-••••<*- ••••••*•*••*,•*•• • • • • • • # • .. 8 Trappability ..... ...................................... 8 Population Density and Recruitment ....................,„ 9 Population Density and Dispersal ....................... 11 Reproduction 16 Reproduction and Dispersal ............................. ., 17 Mortality .. .. ..-.. . 19 Survival and Dispersal .................................. 23 Growth *..'.'«.«'.*'....'......*.«..'....'. .....«.•#..........•'., 23 Growth Rates and Dispersal ............................,, 25 Sex Ratios and Dispersal ...........-,.................., 27 Introduction Experiments ............... ................ 28 Behaviour 29 DISCUSSION ........... .• ... ... ............................ . , 30 SUMMARY . . . . . . 41 FIGURES' .................................. ................ . 42 TABLES , , 64 LITERATURE CITED . . . 7 9 i v LIST OF FIGURES F i g u r e 1. L o c a t i o n of study areas. . ..., 42 F i g u r e 2. A e r i a l photograph of study a r e a s . 44 F i g u r e 3, P o p u l a t i o n d e n s i t y on mainland c o n t r o l and experimental g r i d s . , .......................... , 46 F i g u r e 4. P o p u l a t i o n d e n s i t y on Samuel I s l a n d c o n t r o l and experimental g r i d s . , ........................... 48 F i g u r e 5. , P o p u l a t i o n d e n s i t y on Saturna I s l a n d c o n t r o l and experimental g r i d s . s; ................... ........ 50 F i g u r e 6. S u r v i v a l i n the t h r e e c o n t r o l p o p u l a t i o n s . ... 52 F i g u r e 7, Percentage of animals s e x u a l l y mature i n v a r i o u s weight classes.,, ....................... 54 F i g u r e 8. Body weight d i s t r i b u t i o n s f o r c o n t r o l p o p u l a t i o n s . ................................. 56 F i g u r e 9. Growth r a t e s f o r c o n t r o l and experimental p o p u l a t i o n s . . .................... 58 F i g u r e 10. , Median body weight at s e x u a l maturity f o r c o n t r o l and experimental p o p u l a t i o n s . ........ 60 F i g u r e 11. Schematic r e p r e s e n t a t i o n of seasonal changes i n p o p u l a t i o n s of deer mice., ......... 62 / LIST OF TABLES Tabl e 1. Table 2. T r a p p a b i l i t y estimates f o r c o n t r o l p o p u l a t i o n s . „ Adult and j u v e n i l e r e c r u i t m e n t i n the breeding 64 season. .,• . . ........ .. .«y,,, ,.,. 65 Tabl e 3, C o n t r o l p o p u l a t i o n d e n s i t y and c o l o n i z a t i o n data, 66 Table 4. Table 5. Tabl e 6. Ta b l e 7. ,, Table 8. Tabl e 9. T a b l e 10. Table 11, Tabl e 12. Tabl e 13. Table 14. Tabl e 15. Comparison of observed range length i n c o n t r o l and experimental p o p u l a t i o n s . . , . . . . . . . . . . . . . 67 P r o p o r t i o n of mice i n breeding c o n d i t i o n i n c o n t r o l p o p u l a t i o n s * , ...........*• . •.. *. ............. ... .. 68 P r o p o r t i o n o f mice i n breeding c o n d i t i o n on c o n t r o l and experimental areas d u r i n g removal weeks. . 69 Comparison of p r o p o r t i o n of mice i n breeding c o n d i t i o n f o r c o n t r o l and p u l s e p o p u l a t i o n s . ........... 70 S u r v i v a l r a t e s f o r c o n t r o l p o p u l a t i o n s . 71 I n d i c e s of e a r l y j u v e n i l e s u r v i v a l and r e p r o d u c t i v e data f o r c o n t r o l p o p u l a t i o n s . 72 Comparison of s u r v i v a l r a t e s f o r c o n t r o l and experimental p o p u l a t i o n s . .................... 73 Age c l a s s e s of animals. ...................,.. 74 Growth r a t e s f o r animals i n c o n t r o l p o p u l a t i o n s . Sex r a t i o s i n c o n t r o l and experimental p o p u l a t i o n s , ......................,,. Re s u l t s of i n t r o d u c t i o n experiments. ......... 76 R e s u l t s of behaviour t e s t s f o r i s l a n d mice. 77 • • » * 74 75 v i Table 16., Summary of demography and d i s p e r s a l i n i s l a n d and mainland p o p u l a t i o n s of deer mice. ........... 78 v i i ACKNOWLEDGEMENTS I wish t o thank my 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 i n v a l u a b l e guidance and encouragement, and Dr. James A. E e d f i e l d f o r p r o v i d i n g the i n i t i a l i n s p i r a t i o n f o r t h i s p r o j e c t . The help of Stan J a r v i s on fiayne I s l a n d and Mr. and Mrs. H. Olmstead on Samuel I s l a n d made the study p o s s i b l e . . Thanks go to Norton Clapp who k i n d l y permitted me the use of Samuel I s l a n d and the E c o l o g i c a l Reserves Committee, through Dr. V. J . Krajina,, f o r the use of the r e s e r v e on Saturna I s l a n d . Mr. J . Walters and h i s s t a f f at the U.B.C. Research F o r e s t were h e l p f u l and c o o p e r a t i v e throughout the study. Walter S r . , Walter J r . , and Rand a l l K a i s e r a s s i s t e d i n s e t t i n g up the i s l a n d g r i d s and t r a p p i n g the two s m a l l i s l a n d s . My mother k i n d l y and p a t i e n t l y provided food s u p p l i e s f o r the many e x c u r s i o n s to the i s l a n d s . Rudy Boonstra provided i n s p i r a t i o n d u r i n g my a n a l y s i s of the data. Dr. Z . I . H a l p i n d i d the behaviour t e s t s . The help of these people i s g r e a t l y a p p r e c i a t e d ! F i n a n c i a l support was provided by N.R.C. funds awarded t o C.J. Krebs and by a Gul f O i l Graduate F e l l o w s h i p f o r which I am most g r a t e f u l . 1 INTRODUCTION 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 has r e c e i v e d much a t t e n t i o n i n r e c e n t years. The r e l a t i o n of d i s p e r s a l to p o p u l a t i o n r e g u l a t i o n has been e x p e r i m e n t a l l y s t u d i e d i n Microtus spp, by Myers and Krebs (1971) and Krebs e t a l (1976) and i n Per.omy.scus spp, by Garten and Smith (1974) and F a i r b a i r n (1976)• I s l a n d s provide n a t u r a l l a b o r a t o r i e s where hypotheses concerning the p o s s i b l e r o l e of d i s p e r s a l i n r e g u l a t i n g numbers may be e x p e r i m e n t a l l y t e s t e d . The G u l f I s l a n d s o f f the c o a s t of southern B r i t i s h Columbia and the adjacent mainland provide areas f o r s t u d y i n g d i s p e r s a l i n p o p u l a t i o n s of deer mice, Peromvscus maniculatus. P o p u l a t i o n s of deer mice i n southern B.C. f l u c t u a t e s e a s o n a l l y with low s p r i n g breeding d e n s i t i e s and high d e n s i t i e s through the f a l l and winter. During the breeding season, recruitment i s low, with few j u v e n i l e s s u r v i v i n g to t r a p p a b l e age. I t i s c u r r e n t l y thought t h a t seasonal changes i n the s u r v i v a l of j u v e n i l e mice are determined by seasonal changes i n the a g g r e s s i v e n e s s of the a d u l t p o p u l a t i o n ( S a d l e i r 1965; Healey 1967; Fordham 1971; P e t t i c r e w and S a d l e i r 1974; and F a i r b a i r n 1976)., Previous work i n v o l v i n g Peromvscus maniculatus on the G u l f I s l a n d s has suggested t h a t i s l a n d mice are poorer d i s p e r s e r s and have a lower v a r i a n c e i n p o p u l a t i o n s i z e r e l a t i v e t o mainland mice ( R e d f i e l d 1976). I f d i s p e r s a l i s reduced on the i s l a n d s , how does the demography of i s l a n d p o p u l a t i o n s of deer mice compare with t h a t of mainland, p o p u l a t i o n s ? One technique f o r 2 determining these e f f e c t s i s t o produce a vacant area i n good deer mouse h a b i t a t and then to measure the r a t e of c o l o n i z a t i o n of the vacant h a b i t a t . The trapped-out area has been adopted as a technique f o r s t u d y i n g d i s p e r s a l . Resident deer mice are p e r i o d i c a l l y removed from an area, which d i s p e r s i n g animals then c o l o n i z e . T h i s study was designed t o : 1) compare demographic a t t r i b u t e s and s e a s o n a l changes i n i s l a n d and mainland p o p u l a t i o n s of deer mice, 2) t e s t the hypothesis t h a t r a t e s of d i s p e r s a l are lower on i s l a n d s than on the mainland, 3) compare demographic a t t r i b u t e s of d i s p e r s i n g animals with those of r e s i d e n t s i n c o n t r o l p o p u l a t i o n s . y 3 DESCRIPTION OF S T U D Y AREAS T h i s r e s e a r c h was done on Samuel I s l a n d and Saturna I s l a n d i n the G u l f I s l a n d s and on the mainland i n the U.B.C. Research F o r e s t at Maple Ridge, B.C. (Figure 1). Samuel I s l a n d i s 206 ha (510 acres) i n area with a f a i r l y dense cover of s u c c e s s i o n a l f o r e s t composed of Douglas f i r , western hemlock, western red cedar, and grand f i r . S a l a l dominates the understory. Saturna I s l a n d i s 3102 ha (7680 acres) i n area and e x h i b i t s v a r i o u s s u c c e s s i o n a l stages f o l l o w i n g l o g g i n g . , The area used i n t h i s study was v i r g i n Douglas f i r f o r e s t with western hemlock and western red cedar o f secondary importance. S a l a l was the most p r e v a l e n t s p e c i e s i n the understory. Two very s m a l l i s l a n d s were used as areas f o r i n t r o d u c t i o n s of i s l a n d and mainland p o p u l a t i o n s of mice.. King I s l e t i s 0.16 ha (0.4 acre) i n area with a v e g e t a t i o n cover of shrubs and herbs, and i s l o c a t e d between Samuel I s l a n d and Saturna I s l a n d . Reef I s l a n d i s 1.6 ha (4.Q acres) i n area, and has predominantly shrub and herb cover with some t r e e s (Douglas f i r and Madrone). I t i s l o c a t e d e a s t of Samuel I s l a n d . An a e r i a l photograph of these four i s l a n d study areas i s given i n F i g u r e 2. A l l these i s l a n d s are l o c a t e d i n the C o a s t a l Douglas f i r zone of K r a j i n a ' s (1965) b i o g e o c l i m a t i c c l a s s i f i c a t i o n . The c l i m a t e i s mild with l i t t l e temperature f l u c t u a t i o n and a f a i r l y low amount of p r e c i p i t a t i o n , most of which occurs i n the winter season. The mainland study area i s covered by second growth f o r e s t dominated by western hemlock and western r e d cedar with some 4 Douglas f i r . Ground cover ve g e t a t i o n i s s p a r s e . The Research Fores t i s l o c a t e d i n the C o a s t a l Western Hemlock zone ( K r a j i n a 1965) with g r e a t e r temperature f l u c t u a t i o n s and amounts of p r e c i p i t a t i o n than on the i s l a n d s . 5 MATERIALS AND METHODS From March 1974 to May 1975, t h r e e c o n t r o l and t h r e e pulse removal g r i d s were l i v e - t r a p p e d every two weeks with Longworth l i v e - t r a p s . A l l g r i d s were 1.1 h e c t a r e s i n area. One c o n t r o l g r i d and one pulse removal g r i d were l o c a t e d on each of the i s l a n d s as w e l l as the mainland. Large i s l a n d (Saturna) and mainland g r i d s had 4 9 t r a p s t a t i o n s (7x7), and s m a l l i s l a n d (Samuel) g r i d s had 50 t r a p s t a t i o n s (5x10). A l l t r a p s t a t i o n s , were l o c a t e d at 15.2-m i n t e r v a l s marked by f l a g g i n g tape and s t r i n g or stakes. One l i v e - t r a p was p l a c e d w i t h i n a 2-m r a d i u s of each s t a t i o n . , Traps were b a i t e d with peanut b u t t e r and Purina l a b chow, and T e r y l e n e b a t t i n g was s u p p l i e d as bedding. Traps were s e t on day 1, checked on days 2 and 3, and then locked open between t r a p p i n g p e r i o d s . . During November 1974 to March 1975, t r a p s were se t f o r one n i g h t only and checked the f o l l o w i n g day. Snow i n t e r r u p t e d t r a p p i n g from Dec.^ 28 to Jan. 24 on the i s l a n d s and from Dec. 28 to Mar. 12 on the mainland. A l l deer mice captured were weighed on P e s o l a s p r i n g balances, sexed and ear-tagged with s e r i a l l y numbered f i n g e r l i n g f i s h t a g s . The d u r a t i o n of the breeding season was noted by p a l p a t i o n of male t e s t e s and the c o n d i t i o n of v a g i n a l openings and mammaries of the females,„ Mice 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 on the c o n t r o l g r i d s . The pulse removal g r i d s were trapped on a 12-week c y c l e of 2 t r a p p i n g p e r i o d s of complete removal f o l l o w e d by 4 t r a p p i n g p e r i o d s of mark . and r e l e a s e t r a p p i n g . T h i s allowed mice to c o l o n i z e the removal area and e s t a b l i s h a r e s i d e n t p o p u l a t i o n between removal 6 p e r i o d s . D i s t a n c e s between c o n t r o l and p u l s e removal g r i d s were as f o l l o w s : 600 m on mainland, 1500 m on Saturna i s l a n d , and 600 m on Samuel i s l a n d . P o p u l a t i o n parameters were determined by enumeration technigues t o avoid the s t a t i s t i c a l assumptions of random sampling. H i l b o r n et a l . (1976) have demonstrated by a s i m u l a t i o n model t h a t enumeration technigues p r o v i d e s u f f i c i e n t l y a c c u r a t e estimates f o r a t r a p p i n g design i n which 80% or more of the animals are caught each sampling time. Age c l a s s e s of animals were determined by body weight. For the i n t r o d u c t i o n experiments, approximately egual numbers of i s l a n d and mainland mice were i n t r o d u c e d onto King I s l e t and Reef I s l a n d on May 10, 1974, , These i s l a n d s were not p r e v i o u s l y i n h a b i t e d by mice ( R e d f i e l d 1976), The areas were trapped J u l y 29-31 f o r two n i g h t s with sma l l Sherman l i v e - t r a p s , and the number of s u r v i v i n g animals as well as new o f f s p r i n g were recorded., A l l mice were removed from King I s l e t a t t h i s time, and the experiment was repeated with much hig h e r d e n s i t i e s of i s l a n d and mainland mice., T h i s i s l a n d was trapped again on Oct. 18-20 f o r two n i g h t s , and the number of s u r v i v i n g animals and o f f s p r i n g was recorded. For these experiments, mainland mice were c o l l e c t e d from the removal g r i d a t U.B.C. Research F o r e s t and from t r a p - l i n e s s e t on the U.B.C. Endowment Lands., I s l a n d mice were from the Samuel and Saturna I s l a n d pulse removal g r i d s . , Behaviour t e s t s i n the l a b were conducted on males i n r e p r o d u c t i v e c o n d i t i o n from Samuel and Saturna I s l a n d c o n t r o l p o p u l a t i o n s . No t e s t s were done on mainland animals., Fourteen 7 animals were c o l l e c t e d from each i s l a n d i n J u l y , 1975 and t e s t e d a f t e r a 3- to 5-day l a b adjustment p e r i o d . Tests were conducted i n a n e u t r a l arena i n the dark under a 100-watt red bulb. Each of seven p a i r s of animals was allowed a 5-minute h a b i t u a t i o n p e r i o d with behaviours (aggressive and cohesive) recorded a t 5-second i n t e r v a l s over an i n t e r a c t i o n p e r i o d of ten minutes, ; A b r i e f d e s c r i p t i o n of these behaviours i s given i n Table 15. 8 RESULTS In the a p p r o p r i a t e f i g u r e s and t a b l e s , each of the g r i d s i s designated by a l e t t e r i n the f o l l o w i n g manner: mainland c o n t r o l - A and pulse removal - B; Samuel I s l a n d c o n t r o l - C and pulse removal - D; Saturna I s l a n d c o n t r o l - E and pulse removal - F. Two age c l a s s e s of deer mice have been used by S a d l e i r (1965), B r i t t o n (1966), Healey (1967), and Fordham (1971): j u v e n i l e s and a d u l t s . , I r e f e r to these two age c l a s s e s of a d u l t s and j u v e n i l e s (animals i n j u v e n i l e and subadult age c l a s s e s pooled together) throughout the r e s u l t s and d i s c u s s i o n . J u v e n i l e s are c o n s i d e r e d to be young animals r e c r u i t e d d u r i n g the breeding season. The data a n a l y s i s i n t h i s study i s complicated by the same animals being captured i n s e v e r a l sampling p e r i o d s . Consequently, c h i - s q u a r e analyses have been u t i l i z e d f o r data i n which the samples are not completely independent. Examples are the p r o p o r t i o n o f animals i n breeding c o n d i t i o n , s u r v i v a l r a t e s , and sex r a t i o s . For t h i s reason, the t e s t s may not be s t a t i s t i c a l l y v a l i d but are used as an i n d i c a t i o n of the degree of d i f f e r e n c e between s e t s of data. Chi-sguare a n a l y s e s have been used to t e s t f o r d i f f e r e n c e s i n v a r i o u s demographic a t t r i b u t e s between c o n t r o l p o p u l a t i o n s , between c o n t r o l and removal animals, and between c o n t r o l and pulse p o p u l a t i o n s . T r a p p a b i l i t y . The demographic a n a l y s i s o f these i s l a n d and mainland p o p u l a t i o n s i s based on the assumption t h a t most of the i n d i v i d u a l s i n a given p o p u l a t i o n are captured. Maximum 9 t r a p p a b i l i t y has been d e f i n e d by Krebs et al. ( 1 976 ) as f o l l o w s : t r a p p a b i l i t y = Ho. a c t u a l l y caught at time i / No. known to be prese n t at time i Minimum unweighted t r a p p a b i l i t y i s a l e s s biased estimate s i n c e i t e l i m i n a t e s f i r s t and l a s t c a p t u r e s , and provides only one value f o r each i n d i v i d u a l r e g a r d l e s s of how l o n g i t l i v e s . Both of these estimates f o r the three c o n t r o l p o p u l a t i o n s are given i n Table 1. As expected, maximum t r a p p a b i l i t y i s higher and i s always above 79S. Minimum unweighted t r a p p a b i l i t y i s above 70S except f o r males on Saturna I s l a n d , There i s no d i f f e r e n c e i n t r a p p a b i l i t y of males and females on Samuel I s l a n d . ; Mainland females and Saturna males have the lowest t r a p p a b i l i t i e s , which i n the l a t t e r case i s probably a s s o c i a t e d with the higher d e n s i t y on t h i s area. £o£Sii§£i2a £LS.d Recruitment The p o p u l a t i o n changes f o r the t h r e e study areas were d i f f e r e n t and w i l l be d i s c u s s e d s e p a r a t e l y . The d e n s i t y of the c o n t r o l p o p u l a t i o n on the mainland i s shown i n F i g u r e 3, D e n s i t y was low b e f o r e breeding commenced and subsequently i n c r e a s e d owing t o an i n f l u x of new a d u l t animals 3-4 weeks a f t e r the breeding season had begun. Numbers averaged about seventeen through the breeding season with very l i t t l e r e c r u i t m e n t . The number of j u v e n i l e r e c r u i t s averaged 1.0 male and no females per t r a p p i n g week of breeding season. In September, breeding ceased and th e r e was the t y p i c a l i n c r e a s e i n 10 j u v e n i l e r ecruitment i n t o the p o p u l a t i o n . , More males were r e c r u i t e d i n t o the p o p u l a t i o n than females a t t h i s time. Numbers i n c r e a s e d through October t o a peak of 32 animals and then s t a b i l i z e d f o r the d u r a t i o n of the winter. In e a r l y s p r i n g , d e n s i t y again decreased at the s t a r t of the next breeding season. P o p u l a t i o n changes on the Samuel I s l a n d c o n t r o l g r i d are i l l u s t r a t e d i n F i g u r e 4., Breeding began i n e a r l y A p r i l and numbers d e c l i n e d u n t i l l a t e June, when recruitment of j u v e n i l e s i n c r e a s e d the d e n s i t y . Recruitment was continuous through the breeding season with peak d e n s i t y reached i n mid-November. J u v e n i l e r e c r u i t s averaged 2.9 males and 2,2 females per t r a p p i n g week of breeding season. More males were r e c r u i t e d than females d u r i n g the l a s t few weeks of the breeding season, otherwise t h e r e was very l i t t l e d i f f e r e n c e i n males and females throughout the study. Numbers were s t a b l e through the winter with l i t t l e r e c r u i t m e n t compared with t h a t i n the breeding season. There was approximately one and one-half times the number o f animals at the s t a r t of the 1975 breeding season compared with the previous year. The non-breeding season was f o u r months long on t h i s s m a l l i s l a n d compared with nine months du r a t i o n on the mainland. The c o n t r o l p o p u l a t i o n f o r Saturna I s l a n d i s shown i n F i g u r e 5., Breeding began i n e a r l y May with a s l i g h t d e c l i n e i n d e n s i t y u n t i l r e cruitment o f j u v e n i l e s s t a r t e d i n J u l y . J u v e n i l e r e c r u i t s averaged 3.1 males and 1.8 females per t r a p p i n g week of breeding season. Numbers peaked at the end of breeding i n September and remained high through the winter.. The 11 non-breeding season was approximately s i x months d u r a t i o n on t h i s l a r g e i s l a n d . The 1975 breeding season began i n l a t e March with j u v e n i l e s r e c r u i t e d i n t o the p o p u l a t i o n during A p r i l and May, Numbers d e c l i n e d at the s t a r t of both breeding seasons u n t i l r e c r u i t m e n t of young animals i n c r e a s e d the d e n s i t y . The average d e n s i t y of mice per h e c t a r e on Saturna (43,5) throughout the study i s twice t h a t on Samuel I s l a n d (22,0) and n e a r l y two and one^half times hi g h e r than t h a t on the mainland (18.7). £22Slition Density, and D i s p e r s a l R e s u l t s f o r the p u l s e removal g r i d on the mainland are shown i n F i g u r e 3. There are f o u r t o t a l removal p e r i o d s of one month d u r a t i o n each. The f i r s t , second, and l a s t removals are f o l l o w e d by a two-month i n t e r v a l . The t h i r d removal i s f o l l o w e d by a four-month i n t e r v a l when snow i n t e r r u p t e d t r a p p i n g f o r s e v e r a l weeks. C o l o n i z a t i o n and then establishment of a r e s i d e n t p o p u l a t i o n comparable to the pre-removal d e n s i t y as w e l l as that of the c o n t r o l g r i d occurred w i t h i n two weeks i n the f i r s t three pulse p e r i o d s . The response a f t e r the f o u r t h p u l s e removal i s somewhat s i m i l a r , but was hampered by skunk d i s t u r b a n c e on the t r a p p i n g area d u r i n g the second removal week and the f i r s t week of c o l o n i z a t i o n . .. A l s o , t h i s pulse removal occurred a t the same time as numbers on the c o n t r o l g r i d were d e c l i n i n g . A t o t a l of 95 animals was removed from t h i s g r i d d uring the study, R e s u l t s f o r the Samuel I s l a n d pulse removal g r i d are shown i n F i g u r e 4. The d i s p e r s a l ( c o l o n i z a t i o n ) r a t e o f animals onto t h i s g r i d was slow f o l l o w i n g the f i r s t and second removal weeks of the f i r s t pulse. Numbers d i d not reach the pre-removal or 12 c o n t r o l d e n s i t i e s u n t i l e i g h t weeks l a t e r . The ot h e r pulse p e r i o d s showed more immediate responses f o l l o w i n g each removal. Density due t o c o l o n i z a t i o n s t a b i l i z e d a t pre-removal and c o n t r o l l e v e l s 10 weeks a f t e r the second removal, s i x weeks a f t e r the t h i r d removal, and e i g h t weeks f o l l o w i n g the f o u r t h removal. The f i n a l pulse p e r i o d showed a higher d i s p e r s a l { c o l o n i z a t i o n ) r a t e compared with the f i r s t pulse but again recovery to a s t a b l e d e n s i t y was not completed. The f i r s t and l a s t pulse removals occurred approximately one year a p a r t . C o l o n i z a t i o n during the f i r s t two weeks of the f i n a l p ulse was g r e a t e r than t h a t of the f i r s t . , T h i s probably r e f l e c t s the higher c o n t r o l d e n s i t y (one and one-half times) and l a t e r s t a r t of the breeding season i n 1975 compared with d e c l i n i n g numbers a f t e r the s t a r t of the breeding season i n 1974. A t o t a l of 168 mice was removed from t h i s g r i d during the study. Data from the Saturna I s l a n d p u l s e removal g r i d are shown i n F i g u r e 5. D i s p e r s a l ( c o l o n i z a t i o n ) r a t e was slow d u r i n g the recovery f o l l o w i n g the f i r s t removal p e r i o d . Density d i d not s t a b i l i z e to the l e v e l of pre-removal or c o n t r o l p o p u l a t i o n s u n t i l e i g h t weeks a f t e r the removal. The second pulse had a much more r a p i d r a t e of d i s p e r s a l w i t h i n the f i r s t two weeks of c o l o n i z a t i o n . , Numbers s t a b i l i z e d at the pre-removal l e v e l about f o u r weeks i n t o the pulse and reached a d e n s i t y comparable to t h a t of the c o n t r o l a f t e r ten weeks. The t h i r d pulse showed a s i m i l a r i n i t i a l r a t e of c o l o n i z a t i o n and a f t e r f o u r weeks s t a b i l i z e d below pre-removal and c o n t r o l d e n s i t i e s . The f o u r t h pulse removal was i n t e r r u p t e d by snow, with the r e s u l t t h a t not a l l animals were removed. Subsequent c o l o n i z a t i o n near t o the 13 pre-removal l e v e l o ccurred w i t h i n two weeks, and a f t e r ten weeks was s t i l l below that of the c o n t r o l . The f i n a l p ulse removal took p l a c e i n the s p r i n g of 1975 approximately one year a f t e r the f i r s t . The removal p e r i o d s and i n i t i a l c o l o n i z a t i o n were very s i m i l a r . The b r e e d i n g season was i n progress each year and c o n t r o l and pre-removal d e n s i t i e s were comparable. Thus, t h i s f i n a l p ulse can be c o n s i d e r e d a s e q u e n t i a l r e p l i c a t e of the f i r s t p ulse p e r i o d . 1 t o t a l of 262 animals was removed from t h i s g r i d d u r i n g the study. A comparison of r e c r u i t m e n t d u r i n g removal weeks i n the breeding season f o r c o n t r o l and p u l s e removal g r i d s i s shown i n T a b l e 2. On the mainland, very few a d u l t s or j u v e n i l e s entered the c o n t r o l p o p u l a t i o n compared with the numbers t h a t were r e c r u i t e d onto the removal g r i d . However, on Samuel I s l a n d , during the removal weeks of the second and t h i r d p u l s e s , s i m i l a r numbers of male j u v e n i l e s but not females entered the c o n t r o l and experimental p o p u l a t i o n s . For t o t a l numbers, t h i s i s s i g n i f i c a n t l y d i f f e r e n t from that on the mainland (p<.05). In both p u l s e s , more a d u l t s c o l o n i z e d the removal area than were r e c r u i t e d i n t o the c o n t r o l p o p u l a t i o n and these animals were predominantly males. Saturna I s l a n d showed a g r e a t e r number of j u v e n i l e s c o l o n i z i n g the removal area compared with r e c r u i t s i n t o the c o n t r o l p o p u l a t i o n . When compared with the mainland, t h i s d i f f e r e n c e i s 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 (p=.06), but i s of b i o l o g i c a l s i g n i f i c a n c e because of i t s s i m i l a r i t y to Samuel I s l a n d . There i s l i t t l e d i f f e r e n c e between g r i d s with r e s p e c t to a d u l t s . I n t h i s study, d i s p e r s a l (or c o l o n i z a t i o n ) r a t e can be 14 measured i n t h r e e ways: number of new mice c o l o n i z i n g pulse removal g r i d during the two weeks f o l l o w i n g each t o t a l removal, recovery r a t i o and recruitment index (Krebs et al,... 1976). Observed range l e n g t h i s used as a p o s s i b l e check on these three techniques. Recovery r a t i o = No., mice c o l o n i z i n g removal g r i d a t time i / P o p u l a t i o n s i z e on c o n t r o l g r i d or p u l s e removal g r i d at time i R e l a t i v e recruitment index= No. mice c o l o n i z i n g removal g r i d a t time i / No. new r e c r u i t s tagged on c o n t r o l g r i d at i The f i r s t t h r e e measures f o r the experimental p o p u l a t i o n s are presented i n Table 3., The r e c o v e r y r a t i o may be used as a measure of the r e s i l i e n c y of the c o n t r o l p o p u l a t i o n as w e l l as the experimental p o p u l a t i o n which has c o l o n i z e d and r e s i d e d on the pulse removal g r i d . The average recovery r a t i o , expressed as a percentage, f o r the c o n t r o l p o p u l a t i o n s i s h i g h e s t on the mainland (pulses 1,2, and 3) at 95.8%. Both i s l a n d s have a much lower r e s i l i e n c y : Samuel I s l a n d with 57.3% and Saturna (pulses 1,2,3, and 5) a t 36.9%. There i s no o v e r l a p i n 95% c o n f i d e n c e l i m i t s between data f o r the mainland and Saturna I s l a n d which suggests a s i g n i f i c a n t d i f f e r e n c e i n t h i s measure of d i s p e r s a l r a t e . The average recovery r a t i o f o r the pulse p o p u l a t i o n s s i m i l a r l y i s h i g h e s t on the mainland a t 97.4%, with Samuel I s l a n d (59.9%) and Saturna (50.7%) being s u b s t a n t i a l l y lower,. 15 There i s v i r t u a l l y no overlap i n confidence l i m i t s between data for the mainland and Samuel Island and very l i t t l e overlap between the mainland and Saturna., The r e l a t i v e recruitment index averaged 8.25 on the mainland. This means for every new mouse captured on the control area during these removal weeks, approximately 8 animals colonized the pulse removal grid. The islands both had much lower average indices of recruitment, with Samuel Island at 3.44 and Saturna at 3.30. , Since islands have r e s t r i c t e d land area, observed range length should be a f a i r l y good index of the movement of mice and, consequently, the a b i l i t y to disperse. Table 4 summarizes observed range lengths for males and females on the control and pulse removal grids of each study area. Only mice with more than one capture were used for these calculations. Movement of males on the mainland control i s not s i g n i f i c a n t l y d i f f e r e n t from that on either i s l a n d control., However, Saturna males have a s i g n i f i c a n t l y smaller range length than do those on Samuel. Control and experimental males show no difference on either i s l a n d . Sales from the experimental population on the mainland show a s i g n i f i c a n t l y larger average range length than do the i r control counterparts. This i s b i o l o g i c a l l y s i g n i f i c a n t , since i t i s evident that those animals colonizing the pulse removal grid on the mainland have a higher rate of dis p e r s a l than s i m i l a r mice on either island. , Movement of females i s not s i g n i f i c a n t l y d i f f e r e n t f o r control and experimental populations between islands (Table 4). Mainland females have larger range lengths than females on the isl a n d s , and t h i s r e s u l t again i s 16 s t a t i s t i c a l l y and b i o l o g i c a l l y s i g n i f i c a n t . There i s no d i f f e r e n c e i n female movement between mainland c o n t r o l and experimental g r i d s . In summary, the number o f mice moving i n t o depopulated areas as measured by two recovery r a t i o s and recr u i t m e n t index i s much g r e a t e r than on e i t h e r i s l a n d . T h i s i s f u r t h e r supported by movement of animals, as measured by observed range l e n g t h , which i s s i g n i f i c a n t l y g r e a t e r on the mainland than on the i s l a n d s . , SMfiroduction The best v a r i a b l e to use f o r determining the breeding a c t i v i t y of a p o p u l a t i o n i s probably the percentage of l a c t a t i n g females (Krebs et a l . 1969). T h i s parameter underestimates the s t a r t of a c t i v e breeding by the l e n g t h of t h e g e s t a t i o n p e r i o d , which i s about three weeks. Percentage of females with medium to l a r g e n i p p l e s and percentage of males with s c r o t a l t e s t e s have been used t o determine the l e n g t h of breeding season f o r my th r e e c o n t r o l p o p u l a t i o n s . On the mainland, the 1974 breeding season extended from mid-June t o e a r l y September (ca., 12 weeks). On Samuel I s l a n d the d u r a t i o n of breeding was from m i d - A p r i l t o mid-December {ca. 34 weeks) with some animals i n r e p r o d u c t i v e c o n d i t i o n up to l a t e January 1975. The 1975 breeding season began i n e a r l y May a t both of these study areas. Breeding commenced i n mid-May 1974 on Saturna I s l a n d and extended to mid-September (ca. , 18 weeks).. There was s p o r a d i c breeding i n e a r l y October., The 1975 breeding season began i n mid-March and continued through t o the end of the study i n l a t e May. 17 Table 5 g i v e s t h r e e measures of breeding performance f o r the c o n t r o l p o p u l a t i o n s during June to August of the 1974 breeding season. A s i g n i f i c a n t l y h i g h e r percentage of a d u l t males was i n breeding c o n d i t i o n on Samuel I s l a n d compared with Saturna., Although 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 , the mainland p o p u l a t i o n a l s o had a higher percentage of males with s c r o t a l t e s t e s compared with Saturna, There was no d i f f e r e n c e i n percentage of a d u l t females with a p e r f o r a t e vagina. I s l a n d comparisons f o r j u v e n i l e s f o r these two measures of breeding i n t e n s i t y were not s i g n i f i c a n t l y different.„ Mainland animals i n t h i s age c l a s s c o u l d not be compared with those on the i s l a n d s because of low sample s i z e . , Samuel I s l a n d had a s i g n i f i c a n t l y higher percentage of a d u l t females l a c t a t i n g compared with Saturna and a 10% increment over the mainland. An estimate of l i t t e r s i z e f o r Saturna I s l a n d mice was obtained from l a b o r a t o r y animals and averaged 3.38 (N=32, range 2 to 5 ) . , T h i s f i g u r e may be compared with . mainland l i t t e r s i z e s determined by other workers (Sheppe 1963; S a d l e i r 1974). I S p r o d u c t i o n and Disp_er s a l A comparison of breeding i n t e n s i t y f o r a l l removal weeks f o r c o n t r o l and pulse removal g r i d s i s g i v e n i n Table 6. There was no d i f f e r e n c e i n breeding i n t e n s i t y between c o n t r o l and d i s p e r s i n g males of e i t h e r age c l a s s on any of the study a r e a s . A d u l t and j u v e n i l e females showed no d i f f e r e n c e i n breeding i n t e n s i t y as measured by l a c t a t i o n . Of the a d u l t females c o l o n i z i n g the removal g r i d on Samuel I s l a n d , the percentage with a p e r f o r a t e vagina was s i g n i f i c a n t l y higher compared with the c o n t r o l p o p u l a t i o n (p<.05). S i m i l a r l y , a d u l t female 18 c o l o n i z e r s on Saturna I s l a n d a l s o had a h i g h e r percentage (p=.06) f o r t h i s measure of breeding i n t e n s i t y . However, t h i s may not be b i o l o g i c a l l y s i g n i f i c a n t because c o n d i t i o n of the v a g i n a l opening shows much v a r i a b i l i t y ( S a d l e i r 1974) , and both i s l a n d experimental g r i d s have low sample s i z e r e l a t i v e t o those f o r c o n t r o l p o p u l a t i o n s . Percentage of l a c t a t i n g females i s probably more r e l i a b l e f o r determining the breeding a c t i v i t y of a p o p u l a t i o n but i t i s impossible t o get a r a p i d response i n the l a c t a t i o n measure to a sudden removal s i t u a t i o n , , The r e p r o d u c t i v e performance of the pulse p o p u l a t i o n s a f t e r c o l o n i z a t i o n i s given i n Table 7. There was a s i g n i f i c a n t l y higher percentage of a d u l t males with s c r o t a l t e s t e s on the Saturna experimental g r i d d u r i n g the f i r s t and second pulses as w e l l as the t o t a l . The second p u l s e p o p u l a t i o n on Samuel I s l a n d a l s o had more r e p r o d u c t i v e a d u l t males than d i d the c o n t r o l . Mainland a d u l t males showed no d i f f e r e n c e i n r e p r o d u c t i v e performance between c o n t r o l and pulse p o p u l a t i o n s . S i m i l a r l y , where sample s i z e allowed s t a t i s t i c a l comparison, j u v e n i l e s d i d not vary s i g n i f i c a n t l y i n c o n t r o l and pulse p o p u l a t i o n s f o r e i t h e r s c r o t a l t e s t e s or p e r f o r a t e vagina. Percentage of a d u l t females with p e r f o r a t e vagina a l s o f o l l o w s t h i s p a t t e r n . There i s no s i g n i f i c a n t v a r i a t i o n i n percentage of l a c t a t i n g females on the mainland or Samuel I s l a n d , but the t r e n d from the t o t a l s i s a lower number of females with medium t o l a r g e n i p p l e s among the p u l s e p o p u l a t i o n s compared with the c o n t r o l . The Saturna I s l a n d a d u l t females show a higher percentage of l a c t a t i o n i n the f i r s t p u l s e p o p u l a t i o n , a d i f f e r e n c e which may be b i o l o g i c a l l y s i g n i f i c a n t . , 19 To summarize, males and females colonizing the experimental grids following each removal week did not d i f f e r s i g n i f i c a n t l y i n reproductive condition from control animals for any of the study areas., However, in the subsequent pulse populations, breeding i n t e n s i t y was s i g n i f i c a n t l y higher in adult males on Saturna Island and during the second pulse on Samuel Island., It was s i g n i f i c a n t l y lower for l a c t a t i n g adult females during the second pulse on Saturna. Mainland control and experimental populations showed s i m i l a r reproductive attributes.. Mortality Mortality i n t h i s study i s represented by disappearance from the trappable population, and so includes emigration. Temporal changes i n survival rates are measured by d i r e c t enumeration, and include survival i n the trappable population and early juvenile s u r v i v a l . , Minimum s u r v i v a l rates for a l l males and females i n the three control populations i n t h i s study are presented i n Figure 6. The average s u r v i v a l rates are given for breeding (summer) and non-breeding (winter) seasons i n Table 8. These average rates are summed over a season with an i n d i v i d u a l mouse being t a l l i e d each time i t i s trapped., On the mainland, male s u r v i v a l showed a large drop at the sta r t of the breeding season, and was lowest i n July 1974 and again i n A p r i l 1975. Females survived poorly before the breeding season started, and then remained f a i r l y stable up to May of the following year, when survival slipped below 50%. Survival of males was lower in the summer compared with winter (p=.07) and female s u r v i v a l was the same i n each season. Male 20 s u r v i v a l was lower than t h a t of females i u summer (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 ) with no d i f f e r e n c e i n the , winter., Both males and females showed a steady i n c r e a s e i n s u r v i v a l towards the end of and a f t e r the breeding season., On Samuel I s l a n d , both sexes ( p a r t i c u l a r l y males) had very poor s u r v i v a l during most of the breeding season. I t improved toward the end of breeding and remained high through the winter. Average s u r v i v a l was s i g n i f i c a n t l y h igher (p<.01) i n winter f o r both males and females. Male s u r v i v a l was lower than that of females i n both seasons but the d i f f e r e n c e i s 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 . Saturna I s l a n d males had lowest s u r v i v a l d u r i n g summer 1974, i n c r e a s e d towards the end of breeding and remained high through the winter ( d i f f e r e n c e s i g n i f i c a n t a t p<.01). Female s u r v i v a l was high throughout the study, with l i t t l e d i f f e r e n c e between seasons. I t may be b i o l o g i c a l l y s i g n i f i c a n t t h a t s u r v i v a l of females was g r e a t e r than t h a t of males d u r i n g the breeding season. Table 8 p r e s e n t s comparisons of minimum s u r v i v a l r a t e s f o r d i f f e r e n t seasons and age c l a s s e s f o r males and females i n the three c o n t r o l p o p u l a t i o n s . Low sample s i z e f o r winter 1974 p o p u l a t i o n s at the s t a r t of the study d i d not permit s t a t i s t i c a l comparisons. During summer 1974, a d u l t males and j u v e n i l e females on Saturna I s l a n d had s i g n i f i c a n t l y b e t t e r s u r v i v a l than on Samuel I s l a n d . Mainland a d u l t s and Saturna j u v e n i l e males a l s o had higher s u r v i v a l than those of Samuel, but t h i s i s not s i g n i f i c a n t . . T o t a l s f o r the two age c l a s s e s show s i g n i f i c a n t l y higher s u r v i v a l on Saturna and the mainland f o r both males and females. T h i s t r e n d continued through winter 1974-75 f o r males, 21 but t h e r e are no d i f f e r e n c e s i n t o t a l female s u r v i v a l . Age c l a s s d i s t i n c t i o n s f o r deer mice can be determined only d u r i n g the breeding season. T o t a l s u r v i v a l through the study f o r a l l males and females was s i g n i f i c a n t l y lower on Samuel I s l a n d compared with Saturna. T h i s was a l s o true f o r t o t a l mainland males but not f o r females. T o t a l s u r v i v a l on the mainland and Saturna was very s i m i l a r f o r both sexes. E a r l y j u v e n i l e s u r v i v a l may be measured by an index r e l a t i n g r e c r u i t m e n t of young i n t o the t r a p p a b l e p o p u l a t i o n to the number of p o s s i b l y l a c t a t i n g females (Krebs 1966): index= No. s m a l l mice i n week t / No.„ females with medium to l a r g e n i p p l e s caught i n week t-4 Small mice were d e f i n e d as those l e s s than 17 g on the mainland, 19 g on Samuel I s l a n d , and 21 g on Saturna I s l a n d . .. D i f f e r e n t gram weights were used i n accordance with the d i s c u s s i o n of v a r i a t i o n i n body weights and growth r a t e s . Table 9 g i v e s the mean i n d i c e s f o r the three c o n t r o l p o p u l a t i o n s d u r i n g the breeding season and up to the end of recruitment of young. In the breeding season, e a r l y j u v e n i l e s u r v i v a l on Samuel I s l a n d was ten times h i g h e r than t h a t of the mainland and one and one-h a l f times higher than t h a t of Saturna I s l a n d . E a r l y j u v e n i l e s u r v i v a l was a l s o much b e t t e r on Saturna than on the mainland. With the a d d i t i o n of recruitment a f t e r the breeding season, the two i s l a n d i n d i c e s changed very l i t t l e . However, the mainland i n c r e a s e d to 0.66 and t h i s l a t e r e c r u i t m e n t was r e f l e c t e d i n the 22 i n c r e a s e i n t o t a l d e n s i t y i n the f a l l f o r the c o n t r o l p o p u l a t i o n (Figure 3) . The number of s u c c e s s f u l pregnancies, l i t t e r s i z e , and expected and observed number of j u v e n i l e s which were r e c r u i t e d i n t o each c o n t r o l p o p u l a t i o n d u r i n g the breeding season are l i s t e d i n Ta b l e 9. Samuel I s l a n d had the highest number of s u c c e s s f u l pregnancies as w e l l as observed number of j u v e n i l e s e n t e r i n g the p o p u l a t i o n . Saturna I s l a n d had s l i g h t l y more s u c c e s s f u l pregnancies than the mainland and a much higher r e c r u i t m e n t of j u v e n i l e s . Of animals which were r e c r u i t e d i n t o c o n t r o l p o p u l a t i o n s up to one month before the end of br e e d i n g , 46% males and 50% females s u r v i v e d a t l e a s t two weeks and 29% males and 47% females s u r v i v e d and bred on Samuel I s l a n d (Table 9). On Saturna, 60% males and 75% females s u r v i v e d , but no males and only 3 of 8 females s u r v i v e d and bred. The mainland had 2 of 3 males s u r v i v i n g a f t e r being caught two weeks before the end of breeding. In summary, t o t a l male s u r v i v a l f o r a l l c o n t r o l p o p u l a t i o n s and t o t a l female s u r v i v a l on Samuel I s l a n d were s i g n i f i c a n t l y lower i n summer compared with winter. Samuel a d u l t males and j u v e n i l e males and females s u r v i v e d very poorly r e l a t i v e to Saturna I s l a n d . A d u l t females showed l i t t l e d i f f e r e n c e . Winter s u r v i v a l r a t e s were poorest on Samuel I s l a n d , and showed very l i t t l e v a r i a t i o n between Saturna and the mainland. T o t a l s u r v i v a l through t h e study was lowest on Samuel, with l i t t l e d i f f e r e n c e between the mainland and Saturna I s l a n d . , E a r l y j u v e n i l e s u r v i v a l was highest on Samuel I s l a n d , f o l l o w e d by Saturna and then the mainland. 23 Survival and Dispersal To determine i f dispersing (colonizing) animals survived better than those i n control populations, survival rates should be compared between each two-month pulse period and a similar time period on the control. Table 10 presents t h i s comparison for males and females on the three study areas. There was no difference i n survival between residents and colonizing mice f o r either sex on the mainland. The poor s u r v i v a l of animals i n pulse no., 3 for both males and females was probably due to low sample size on the experimental g r i d . Total s u r v i v a l was si m i l a r for males and somewhat lower f o r pulse females compared with the control. Saturna Island likewise showed no difference between control and experimental animals. Survival was s l i g h t l y higher i n male populations of the f i r s t and l a s t pulses and lower i n pulse no,, 1 for females, but t o t a l s u r v i v a l was si m i l a r for both sexes. On Samuel Island, males survived s i g n i f i c a n t l y better on the experimental grid during the f i r s t and t h i r d pulses as well as o v e r a l l . Female survival was s i g n i f i c a n t l y higher for the f i r s t three pulse populations compared with control s u r v i v a l . , The fourth pulse showed a lower but s t i l l high s u r v i v a l rate r e l a t i v e to control. To summarize, there was l i t t l e variation i n su r v i v a l between control and col o n i s t populations on Saturna Island or the mainland. However, both male and female deer mice on Samuel Island survived better i n most pulse populations r e l a t i v e to control animals. Growth Growth rates may be used as a further index of conditions 24 wi t h i n p o p u l a t i o n s of Peromjjscus maniculatus. The a s p e c t s of growth t o be c o n s i d e r e d are s e x u a l maturity and body weight, growth r a t e r e g r e s s e d on body weight, and body weight d i s t r i b u t i o n s . Age a t s e x u a l m a t u r i t y i s a u s e f u l demographic v a r i a b l e f o r determining the age c l a s s e s of deer mice. Owing to the l a c k of a b e t t e r c r i t e r i o n , body weight must be used as an index of age. The percentage of s e x u a l l y mature animals i n a s e r i e s of weight c l a s s e s may be used to determine the weight l i m i t a t i o n s f o r j u v e n i l e s , s u b a d u l t s , and a d u l t s . F i g u r e 7 p r e s e n t s t h i s comparison f o r males and females i n the c o n t r o l and pulse p o p u l a t i o n s d u r i n g the breeding season. , Males and females and c o n t r o l and pulse p o p u l a t i o n s d i d not d i f f e r s i g n i f i c a n t l y with r e s p e c t t o these histograms. My age c l a s s e s assume j u v e n i l e s are seldom, i f ever, s e x u a l l y mature; i n d i v i d u a l s of which l e s s than 50% are mature i n the upper weight c l a s s are c a l l e d s u b a d u l t s ; and a d u l t s must have at l e a s t 50% of mice s e x u a l l y mature i n the lowest weight c l a s s . Osing these c r i t e r i a , I estimated the weight l i m i t s f o r age c l a s s e s from F i g u r e 7 and these l i m i t s are l i s t e d i n Table 11. To show t h a t the d i f f e r i n g weights of age c l a s s e s are e q u i v a l e n t among study areas (e. g. i s a 17-g a d u l t mouse on the mainland e q u i v a l e n t i n age to a 21-g a d u l t mouse on Saturna I s l a n d ? ) , t h e r e must be v a r i a t i o n i n growth r a t e s . Since growth r a t e i s dependent on body weight, r e g r e s s i o n s f o r these v a r i a b l e s may be compared between study a r e a s . Table 12 summarizes a n a l y s i s o f c o v a r i a n c e of growth r a t e regressed on body weight of animals l e s s than 20 g from June to October. 25 Males and females had s i m i l a r growth r a t e s and have been combined f o r the t h r e e c o n t r o l p o p u l a t i o n s . Samuel I s l a n d mice have a growth r a t e 5.6 times that of mainland mice and 2.8 times t h a t of Saturna mice, which i n t u r n grow twice as f a s t as mainland animals. Thus, the i s l a n d animals do grow much f a s t e r than t h e i r mainland c o u n t e r p a r t s , and so the age c l a s s e s used throughout t h i s paper are reasonably a c c u r a t e . Body weight d i s t r i b u t i o n s f o r breeding (summer) and non-breeding (winter) seasons f o r males i n c o n t r o l p o p u l a t i o n s are shown i n F i g u r e 8. Body weights were higher i n the summer compared with winter f o r the mainland and Samuel I s l a n d . There i s l i t t l e d i f f e r e n c e on Saturna, which may r e f l e c t the lower i n t e n s i t y of breeding on the l a r g e r i s l a n d . Both i s l a n d s have animals weighing up to 32 g., During breeding, there are very few mainland a d u l t s above 24 g i n weight and none above 26 g. Samuel I s l a n d has the h i g h e s t number of mice i n j u v e n i l e and subadult weight c l a s s e s . During the winter, most mainland males are i n one weight category (16-17 g ) , and on Samuel the m a j o r i t y weigh 20-23 g. Saturna males are evenly spread over 5 weight c l a s s e s (20-30 g) , Growth Bates and D i s p e r s a l The growth r a t e s of i n d i v i d u a l s comprising the pulse p o p u l a t i o n s on removal areas should be egual to or b e t t e r than those on c o n t r o l g r i d s . . Mean growth r a t e s (adjusted to 13-g animals on the mainland and Samuel I s l a n d and 15 g on Saturna Island) and 95% c o n f i d e n c e l i m i t s of males f o r c o n t r o l and pulse p o p u l a t i o n s are shown i n F i g u r e 9. None of the i n d i v i d u a l means i s s i g n i f i c a n t l y d i f f e r e n t because of the broad c o n f i d e n c e 26 i n t e r v a l s . Growth was s l i g h t l y b e t t e r on the mainland pulse removal g r i d d u r i n g the f i r s t , t h i r d , and f o u r t h p u l s e s . , On Samuel and Saturna I s l a n d s , growth r a t e s i n pulse p o p u l a t i o n s were l e s s (except pulse no. 3 on Saturna) than f o r c o n t r o l animals. There i s more v a r i a t i o n i n the female data, which are a l s o presented i n F i g u r e 9. Experimental females on the mainland had s l i g h t l y b e t t e r growth dur i n g the f i r s t and t h i r d p u l s e s , but a lower growth r a t e f o r the second p u l s e . There were i n s u f f i c i e n t data to p l o t an average value f o r the f o u r t h pulse p o p u l a t i o n . Saturna females showed s l i g h t l y b e t t e r growth i n the experimental p o p u l a t i o n r e l a t i v e to c o n t r o l f o r the f i r s t t h r e e p u l s e s and then decreased c o n s i d e r a b l y i n the f i n a l p u l s e . Females were growing b e t t e r i n the f i r s t , second, and f o u r t h p u l s e s , but much poorer d u r i n g the t h i r d pulse on Samuel I s l a n d . In g e n e r a l , growth of mainland animals remained f a i r l y s t a b l e through time whereas the growth r a t e s of Samuel I s l a n d animals decreased d u r i n g the study, and those of Saturna decreased and then improved i n the l a s t p u l s e . , Age at s e x u a l maturity i s a l s o a u s e f u l v a r i a b l e f o r determining i f c o l o n i z i n g mice are maturing at the same age as c o n t r o l animals. The weight a t sexual maturity f o r l i v e - t r a p p e d mice was estimated i n the same manner as d e s c r i b e d f o r v o l e s by Krebs e t a l . (1976). F i g u r e 10 presents these data f o r males and females d u r i n g the p e r i o d June t o October 1974. The body weight of male deer mice at s e x u a l maturity was the same f o r c o n t r o l and p u l s e p o p u l a t i o n s on the mainland.. However, i s l a n d e xperimental males tended t o mature at l i g h t e r weights than c o n t r o l animals, a d i f f e r e n c e that i s s i g n i f i c a n t on Saturna 27 I s l a n d but not on Samuel. Females on a l l study areas matured a t s l i g h t l y lower weights i n p u l s e compared with c o n t r o l p o p u l a t i o n s . From a comparison of male and female c o n t r o l animals i t i s apparent t h a t mainland females matured a t weights ne a r l y 3 g heavier than males, but i s l a n d mice showed very l i t t l e d i f f e r e n c e . In summary, animals on Samuel I s l a n d grew more than f i v e times f a s t e r than mainland mice, and Saturna growth r a t e s were double those on the mainland. T h i s v a r i a t i o n i n growth s u b s t a n t i a t e s age c l a s s d i s t i n c t i o n s showing Saturna a d u l t s 4 g h e a v i e r and Samuel a d u l t s 2 g h e a v i e r than t h e i r mainland c o u n t e r p a r t s . , Mainland experimental males tended t o have b e t t e r growth than c o n t r o l animals, whereas i s l a n d e x p e r i m e n t a l males had s l i g h t l y lower growth r a t e s . In g e n e r a l , female growth on experimental areas was b e t t e r than on c o n t r o l s , but t h e r e was some v a r i a t i o n f o r s p e c i f i c p ulse p e r i o d s . Both sexes of i s l a n d experimental animals and females on the mainland tended to mature a t l i g h t e r weights compared with c o n t r o l p o p u l a t i o n s . Sex R a t i o s and Disp_ersal The sex r a t i o of c o n t r o l areas has been estimated by t a l l y i n g each animal every time i t i s captured and summing these data f o r the e n t i r e study and the f o u r removal periods.„ T h i s technique p r o v i d e s a weighted average sex r a t i o f o r c o n t r o l and c o l o n i z i n g p o p u l a t i o n s . , Both the mainland and Saturna c o n t r o l p o p u l a t i o n s had a s i g n i f i c a n t l y h igher (p<.05) p r o p o r t i o n of males (0.58) compared with Samuel I s l a n d (0.50) . Table 13 g i v e s the sex r a t i o s f o r c o n t r o l and c o l o n i z i n g p o p u l a t i o n s o f the t h r e e study areas. More males c o l o n i z e d the removal area than 28 females on Samuel I s l a n d , and t h i s i s s i g n i f i c a n t f o r the t o t a l . The p r o p o r t i o n of mainland males v a r i e d from one pulse to the next, but there was no d i f f e r e n c e i n the t o t a l r a t i o . On Saturna, the p r o p o r t i o n of males was l e s s than the c o n t r o l i n the f i r s t removal p e r i o d , but there was a t r a n s i t i o n through the study to a higher p r o p o r t i o n i n the f i n a l removal. Since age c l a s s d i s t i n c t i o n s i n P. maniculatus can only be made d u r i n g the breeding season, there are i n s u f f i c i e n t data to determine a d i f f e r e n c e i n sex r a t i o between j u v e n i l e s from experimental and c o n t r o l areas and t h a t of a d u l t s . , Comparison of sex r a t i o s of mice c o l o n i z i n g removal areas with those of new r e c r u i t s on c o n t r o l areas showed no d i f f e r e n c e . iSJtEoduction Exp.erim.ents To t e s t f o r p o s s i b l e d i f f e r e n c e s between i s l a n d and mainland deer mice with r e s p e c t to c o l o n i z i n g a b i l i t y , behaviour, and s u r v i v a l , approximately egual numbers of animals were i n t r o d u c e d t o two small i s l a n d s . The r e s u l t s of three i n t r o d u c t i o n experiments are l i s t e d i n T a b l e 14. 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 (chi-sguare analyses) i n r e l a t i v e s uccess or s u r v i v a l o f e i t h e r i s l a n d or mainland mice. I t was not p o s s i b l e t o i d e n t i f y untagged o f f s p r i n g as of e i t h e r i s l a n d or mainland o r i g i n . The t o t a l number of mice i n t r o d u c e d onto Eeef I s l a n d averaged 19.75 animals per acre and the number i n t r o d u c e d i n the second King I s l e t experiment e q u a l l e d 167 per acre. At the end of these experiments the d e n s i t y on Eeef I s l a n d i n c r e a s e d t o 21.5 per acre and King I s l e t was 120 animals per a c r e . I t was hoped t h a t these abnormally high d e n s i t i e s might r e s u l t i n e i t h e r mainland or i s l a n d mice dominating owing 29 to a higher l e v e l of agonistic behaviour. However, no v a l i d conclusions may be drawn from these experiments except that deer mice can survive for a short period of time, and at le a s t some ind i v i d u a l s reproduce at these very high densities.. Behaviour This investigation into behaviour of islan d and mainland mice i s preliminary and represents the behaviour of animals under laboratory conditions. Results of behaviour tests f o r Samuel and Saturna Islands are l i s t e d i n Table 15.,, There appear to be very few aggressive tendencies i n islan d mice. Samuel Island males had a much shorter latency period before f i r s t recorded behaviour and twice the number of interaction i n t e r v a l s compared with those of Saturna. This may r e f l e c t a lower l e v e l of general a c t i v i t y among the large island animals compared with those from Samuel. These results are not consistent with those of Sadleir (1965), Healey (1967), and Fairbairn (1976) who showed seasonal changes i n aggressive behaviour of male adult deer mice. 30 DISCUSSION In the experimental part of t h i s study, deer mice were removed from areas f o r one month, f o l l o w e d by establishment of a r e s i d e n t p o p u l a t i o n d u r i n g the next two months,, Consequently, i t i s p o s s i b l e to measure the r a t e of c o l o n i z a t i o n onto the vacant area and monitor the performance of animals which become e s t a b l i s h e d . I n d i v i d u a l s which appear on the vacant area were c l a s s e d as ' d i s p e r s e r s , ' and are probably s u r p l u s animals d r i v e n out of adjacent p o p u l a t i o n s . T h i s experiment was designed t o study the c o l o n i z i n g a b i l i t y o f d i s p e r s e r s a t 3-month i n t e r v a l s throughout the year.. I t i s u n l i k e l y t h a t a l l the mice which c o l o n i z e d the pulse removal g r i d s were animals which would have been d i s p e r s i n g out of undisturbed p o p u l a t i o n s . ; However, I t h i n k i t i s safe t o assume t h a t t h i s sample would c o n t a i n a higher p r o p o r t i o n of d i s p e r s i n g i n d i v i d u a l s than would r e s i d e n t samples from c o n t r o l areas. The work of S t i c k e l (1946) and F a i r b a i r n (1976) with deer mice from continuous removal g r i d s tends to support t h i s assumption.. S e v e r a l other assumptions f o r experimental designs i n v o l v i n g removal of animals from an area have been d i s c u s s e d by Krebs e t a l . (1976). C o n t r o l and removal g r i d s were separated by a d i s t a n c e beyond which t h e r e c o u l d be any e f f e c t on p o p u l a t i o n processes i n the c o n t r o l area due to removal of mice from the experimental area. T h i s precluded c a t c h i n g tagged i n d i v i d u a l s which d i s p e r s e d from the c o n t r o l , but was necessary owing to the wide ranging a c t i v i t y o f deer mice ( S t i c k e l 1968). T h e r e f o r e , I am assuming my d i s t a n t c o n t r o l p o p u l a t i o n s g i v e an adequate 3 1 r e p r e s e n t a t i o n of events o c c u r r i n g around the removal area. A second assumption i s t h a t most r e s i d e n t and c o l o n i z i n g mice are removed during the two removal weeks between pulse p e r i o d s . V±. ffiasigulatus i s very t r a p p a b l e and r e a d i l y e n t e r s u n f a m i l i a r t r a p s . Thus, the wave of c o l o n i z i n g mice f o l l o w i n g a month removal are assumed to be s u r p l u s animals d i s p e r s i n g onto a vacant h a b i t a t . ; I t i s a l s o p o s s i b l e t h a t c o l o n i z i n g mice are j u s t wandering i n d i v i d u a l s which s h i f t homesites every few weeks or months. I do not have the b e h a v i o u r a l data to determine whether c o l o n i z i n g mice are s o c i a l l y s ubordinate animals or are wanderers moving through the p o p u l a t i o n . A pulse removal was used i n t h i s study i n s t e a d of a continuous removal because i t a l l o w s capture . of animals c o l o n i z i n g a vacant h a b i t a t and then monitors the demographic performance i n the f o l l o w i n g two months. I t should a l s o be noted that a continuous removal on Samuel I s l a n d may have had adverse e f f e c t s on the c o n t r o l p o p u l a t i o n by a c t i n g as a s i n k f o r t h i s s m a l l i s l a n d (510 a c r e s ) . P o p u l a t i o n s of deer mice have been e x p e r i m e n t a l l y s t u d i e d on the mainland of southwestern B r i t i s h Columbia by S a d l e i r (1965), Healey (1967) , Fordham (1971), P e t t i c r e w and S a d l e i r (1974), and F a i r b a i r n (1976)., Seasonal changes i n these p o p u l a t i o n s of P.. fflaniculatus are summarized i n F i g u r e 11. The r e s u l t s from my study a t the U.B.C Research F o r e s t at Haple Sidge tend to agree with most aspects of the demographic changes a s s o c i a t e d with t h i s annual c y c l e of numbers i n deer mice. There i s an i n c r e a s e r a t h e r than a d e c l i n e a t the onset of breeding. Overwinter m o r t a l i t y may have reduced the p o p u l a t i o n 32 to the p o i n t t h a t too few animals are present to e x p l o i t the h a b i t a t e f f i c i e n t l y and immigrants move i n from adjacent areas. As d i s c u s s e d by Healey (1967) and F a i r b a i r n (1976), the s p r i n g r e o r g a n i z a t i o n may not n e c e s s a r i l y be a d e c l i n e i n numbers. I t should i n v o l v e i n c r e a s e d immigration and e m i g r a t i o n , and d e n s i t y may decrease, i n c r e a s e , or show no net change. F a i r b a i r n (1976) a l s o suggests t h a t a decrease i n d e n s i t y of males a t t h i s time i s a response t o m o r t a l i t y o f e a r l y breeding females., However, the f i r s t l a c t a t i n g females on my mainland c o n t r o l g r i d d i d not s u f f e r heavy m o r t a l i t y . T h i s may be due to delayed s t a r t i n breeding (mid-June) i n 1974 such t h a t energy requirements (food and weather c o n d i t i o n s ) were f a v o u r a b l e f o r s u c c e s s f u l r e p r o d u c t i o n . P e t t i c r e w and S a d l e i r (1974) working i n t h i s same f o r e s t found the s t a r t of breeding t o be much e a r l i e r i n 1969 (February) and i n 1970 ( A p r i l ) . The p o p u l a t i o n s on Samuel I s l a n d and Saturna I s l a n d showed some d i f f e r e n c e s from the model p o p u l a t i o n presented i n F i g u r e 11. There are two b a s i c d i f f e r e n c e s i n demography, a p a r t from d e n s i t y , between the i s l a n d s and mainland. F i r s t l y , t h e r e i s a lower r a t e of d i s p e r s a l on the i s l a n d s as measured by observed range l e n g t h and by three i n d i c e s d e r i v e d from the number of new mice c o l o n i z i n g the pulse removal g r i d . T h i s i s most pronounced duri n g the s p r i n g r e o r g a n i z a t i o n , when male d e n s i t y on Saturna (1974 and 1975) and d e n s i t y of males and females on Samuel I s l a n d i s d e c l i n i n g . There does not appear t o be a s u r p l u s of animals on e i t h e r i s l a n d a t t h i s time. T h i s may be an i n d i c a t i o n t h a t animals are not being d r i v e n out of the r e s i d e n t p o p u l a t i o n by s o c i a l pressure, but are dying on 33 the g r i d . , T h i s i s f u r t h e r supported by the c o l o n i z a t i o n of experimental areas being more r a p i d i n removal weeks d u r i n g the r e s t of the year when a s u r p l u s or s p i l l - o v e r of animals must be a v a i l a b l e from the breeding season., Although r a p i d c o l o n i z a t i o n of my mainland pulse removal g r i d occurred before breeding, F a i r b a i r n (1976) has shown by two pulse removal experiments on U.B.C., Endowment Lands t h a t animals moving around i n u n d i s t u r b e d areas during the s p r i n g r e o r g a n i z a t i o n would r a p i d l y c o l o n i z e a depopulated area., Her c o n t r o l p o p u l a t i o n s were d e c l i n i n g at t h i s time, which i s s i m i l a r to what was happening on my i s l a n d c o n t r o l areas. Thus, these d i s p e r s i n g mainland mice r e p r e s e n t s u r p l u s animals which were presumably d r i v e n out of the r e s i d e n t p o p u l a t i o n . Breeding was continuous a f t e r s t a r t i n g i n A p r i l on Samuel I s l a n d and i n May on Saturna I s l a n d (1974). There was no pulse of e a r l y breeding b e f o r e the main breeding season began on the i s l a n d s compared with t h a t recorded by F a i r b a i r n (1976) on the U.B.C. Endowment Lands. An e x c e p t i o n may be the 1975 breeding season on Saturna I s l a n d which began i n March and appeared to taper o f f i n mid-May when the study ended. There was heavy m o r t a l i t y of e a r l y breeding females on Samuel I s l a n d , and s u r v i v a l of males was a l s o very poor at t h i s time. These r e s u l t s are c o n s i s t e n t with the poor s u r v i v a l of e a r l y breeding females and subsequent d e c l i n e i n d e n s i t y of males recorded by F a i r b a i r n (1976). However, on Saturna I s l a n d , the f i r s t l a c t a t i n g females s u r v i v e d very w e l l i n both 1974 and 1975., Secondly, r e c r u i t m e n t of j u v e n i l e s occurs throughout the breeding season with peak d e n s i t i e s reached before the end of 34 breeding., T h i s recruitment i n t o c o n t r o l p o p u l a t i o n s on the i s l a n d s was v e r i f i e d by the experimental areas., The e g u i v a l e n t number of j u v e n i l e s c o l o n i z i n g the pulse removal g r i d and e n t e r i n g the c o n t r o l p o p u l a t i o n d u r i n g removal weeks provided t h i s support. On the mainland, as expected, animals c o l o n i z i n g the pulse removal dur i n g removal weeks and the f i r s t two weeks of the second pulse p e r i o d were p r i m a r i l y j u v e n i l e s , , There was v i r t u a l l y no r e c r u i t m e n t i n t o the c o n t r o l p o p u l a t i o n a t t h i s time. T h i s r e s u l t c o n t r a d i c t s t h a t of F a i r b a i r n (1976), who had poor d i s p e r s a l o f j u v e n i l e s , but high d i s p e r s a l of a d u l t s onto removal areas. I t i s p o s s i b l e my experiment of p e r i o d i c a l l y removing animals d u r i n g breeding may not be comparable to a continuous removal experiment. However, the number of j u v e n i l e s versus a d u l t s d i s p e r s i n g onto a removal g r i d i s a f u n c t i o n of age c l a s s e s based on body weight.^ F a i r b a i r n (1976) used a j u v e n i l e weight c l a s s o f under 15 g ;whereas I used 17 g. Since j u v e n i l e animals may be growing r a p i d l y a t t h i s time of year, the high number of a d u l t s c o l o n i z i n g a depopulated area may i n c l u d e a f a i r p r o p o r t i o n of mice which are a c t u a l l y young r e c r u i t s . I f t h i s i s the case, then, animals not a b l e t o enter a r e s i d e n t p o p u l a t i o n d i s p e r s e i n an attempt to f i n d an open space t o s e t t l e in.,. T h i s was suggested by Healey (1967) as a mechanism f o r g e t t i n g r i d of the excess of r e c r u i t s during years of low m o r t a l i t y . P r e l i m i n a r y r e s u l t s from behaviour t e s t s i n d i c a t e t h a t i s l a n d a d u l t males i n breeding c o n d i t i o n show v i r t u a l l y no a g g r e s s i v e behaviour. T h i s i s f u r t h e r supported by H a l p i n ' s work (pers, comm. ) which showed no evidence of a g g r e s s i v e 35 behaviour between i s l a n d a d u l t s and j u v e n i l e s i n the n e u t r a l arena. On the mainland, S a d l e i r (1965), Healey (1967), and F a i r b a i r n (1976) have found an i n c r e a s e i n a d u l t male agg r e s s i o n d u r i n g the breeding season., S a d l e i r (1965) and Healey (1967) have a l s o e s t a b l i s h e d t h a t a d u l t males i n mainland p o p u l a t i o n s i n h i b i t growth and s u r v i v a l of j u v e n i l e s at t h i s time. A b e h a v i o u r a l d i f f e r e n c e between i s l a n d and mainland mice may e x p l a i n reduced d i s p e r s a l on the i s l a n d s , p a r t i c u l a r l y d uring the s p r i n g r e o r g a n i z a t i o n . I f th e r e i s l i t t l e a g g r e s s i v e i n t e r a c t i o n or s o c i a l pressure f o r c i n g animals out of the p o p u l a t i o n on the i s l a n d s i n order t o e s t a b l i s h a breeding d e n s i t y , then c o l o n i z a t i o n of depopulated areas w i l l be slow. The a b i l i t y of j u v e n i l e s to reach t r a p p a b l e age and even e n t e r the p o p u l a t i o n may a l s o be a r e s u l t of t h i s lack of a g g r e s s i v e behaviour. The r e p r o d u c t i v e r a t e as measured by length of breeding season, number of s u c c e s s f u l pregnancies, percentage of breeding animals, and number of r e c r u i t s s u r v i v i n g to breed i s much higher on Samuel I s l a n d than on e i t h e r Saturna or the mainland. T h i s high r e p r o d u c t i v e r a t e i s r e s p o n s i b l e f o r the high r e c r u i t m e n t and i s a s s o c i a t e d with subseguent poor s u r v i v a l during breeding on Samuel, That the f l o o d of r e c r u i t s on Samuel i s not j u s t a f u n c t i o n of the high r a t e of r e p r o d u c t i o n i s shown by the recr u i t m e n t i n t o the Saturna c o n t r o l p o p u l a t i o n which had a lower r e p r o d u c t i v e r a t e but higher s u r v i v a l . I f the Samuel p o p u l a t i o n can produce almost double the number of l i t t e r s , why i s the d e n s i t y of deer mice on t h i s i s l a n d not comparable to th a t on Saturna? 36 The higher d e n s i t y on Saturna probably r e f l e c t s a g r e a t e r abundance of food i n the v i r g i n Douglas f i r f o r e s t compared with t h a t i n the d r i e r s u c c e s s i o n a l f o r e s t h a b i t a t on Samuel I s l a n d . T h i s assumes t h a t animals i n both i s l a n d p o p u l a t i o n s are e q u a l l y non-aggressive, but i n , no way i m p l i c a t e s food as a f a c t o r l i m i t i n g the number of mice on the s m a l l e r island.,. Young animals on Samuel have growth r a t e s over two and one-half times those on Saturna, and t h i s allows r e c r u i t e d animals to breed as soon as p o s s i b l e before the end of the breeding season., T h i s i s c e r t a i n l y r e f l e c t e d i n the high number of r e c r u i t s s u r v i v i n g to breed i n the Samuel I s l a n d c o n t r o l p o p u l a t i o n . F u r t h e r evidence t h a t food l i m i t a t i o n i s u n l i k e l y , i s the success of i n t r o d u c e d deer mice onto King I s l e t and Eeef I s l a n d . Both these i s l a n d s were dry and rocky with few t r e e s and shrubs, but there o b v i o u s l y was enough e d i b l e m a t e r i a l t o support such high d e n s i t i e s of mice. R e d f i e l d (pers, comm.) i n t r o d u c e d c o h o r t s of 50 mice to each of s e v e r a l p r e v i o u s l y u n i n h a b i t e d i s l a n d s i n the G u l f I s l a n d s , and a l l p o p u l a t i o n s were t h r i v i n g one year l a t e r . The weather on the Gulf I s l a n d s i s much l e s s s easonal than on the mainland. Summers can be very dry, and so water c o u l d be a l i m i t i n g f a c t o r on s m a l l rocky i s l a n d s ; but again the success of my i n t r o d u c t i o n s would appear t o r e f u t e t h i s hypothesis., P r e d a t i o n appears as an u n l i k e l y source of m o r t a l i t y . No a v i a n or t e r r e s t r i a l predators were observed on Samuel I s l a n d . Raccoons and herons were o c c a s i o n a l l y observed to forage i n i n t e r t i d a l r e g i o n s and mink are known to i n h a b i t some of the s m a l l e r Gulf I s l a n d s , but were not seen on Samuel. 37 Thus, what i s regulating the numbers of deer mice during the spring reorganization and breeding season on these islands? Aggressive behaviour by dominant animals cannot be ruled out. Comparison of demographic attributes i n control and pulse populations showed no consistent differences throughout the study on the mainland and Saturna Island. Fairbairn ( 1 9 7 6 ) ' found a s i m i l a r r e s u l t on the U.B .C* Endowment Lands, and concluded that the p l a s t i c i t y of these animals, i n colonizing a vacant area and establishing a resident population, make deer mice very r e s i l i e n t to l o c a l extinctions. The s i t u a t i o n on Samuel Island was somewhat d i f f e r e n t , as more males colonized the pulse removal grids; but the sex r a t i o s t a b i l i z e d at 1 : 1 i n the succeeding pulse populations. Survival of males and females was s i g n i f i c a n t l y higher i n nearly every pulse population compared with the control. This suggests that perhaps some animals, p a r t i c u l a r l y males, may have been driven out of the resident population, and these possibly subordinate mice survived better on the depopulated areas. One might also expect growth rates to improve. However, males grew less and females s l i g h t l y better than their control counterparts. There were no other consistent differences i n demographic parameters between control and pulse populations on Samuel Island. There seems to be l i t t l e difference i n the p l a s t i c i t y of i s l a n d and mainland mice colonizing vacant habitat., I o r i g i n a l l y thought that i f there were reduced dispersal on the islands, i t would be a r e s u l t of the 'island* or 'fence effect'(Krebs et a l . 1969)., Hith t h i s lower rate of dispersal and movement, there might also be a reduction i n aggressive 38 tendencies such that a population could survive successfully on a small, f i n i t e area of land. However, i t seems unlikel y that Saturna Island at 3102 ha {7680 acres) could act as an 'island* to a population of deer mice; and yet reduced dispersal on t h i s i s l a n d was comparable to that of Samuel with an area of 206 ha (510 acres), If dispersal i s a function of island s i z e , then i t should be more r e s t r i c t e d oh islands smaller than Samuel, and more pronounced on larger areas such as Vancouver Island. HcCabe and Cowan {1945) reported lower densities of isla n d E§£°Siccus spp. compared with nearby mainland areas further north i n B r i t i s h Columbia. Redfield (1976) reported higher densities of deer mice on some of the Gulf Islands compared with the mainland of southwestern B.C. The 'fence e f f e c t ' (Krebs et a l . , 1969) may be operating to reduce dispersal resulting i n higher d e n s i t i e s . This i s i n accordance with the condition of 'frustrated d i s p e r s a l ' suggested by Lidxcker (1975). Populations of small mammals i n experimental enclosures and on natural islands would cope with frustrated dispersal by increased usage of marginal habitat or a combination of decreased b i r t h rate and increased death rate (Lidicker 1975).. Redfield (1976) has suggested that deer mice on the Gulf Islands are K-selected because of t h e i r larger body size and higher densities, which r e f l e c t different demographic parameters and reduced d i s p e r s a l compared with mainland populations. Studies i n Europe appear to support the contention that i s l a n d populations of rodents tend towards K-selection. an i s l a n d population of the Skomer vole jC 1 e t hrionomy.s gla.regl .3s skgmerensisJL exhibited larger body s i z e , higher d e n s i t i e s , very 39 s h o r t breeding season, and a slower maturation r a t e compared with p o p u l a t i o n s on the B r i t i s h mainland ( F u l l a g a r et a l . , 1963; J e w e l l 1966), P o p u l a t i o n s of the l o n g - t a i l e d f i e l d mouse ll£&demus svJLvaticus}. on some B r i t i s h i s l a n d s have l a r g e r body s i z e than mainland animals but t h e r e i s much v a r i a t i o n among i s l a n d s (Delany 1970). An i s l a n d p o p u l a t i o n of the bank vole i C l e t h r i o n o m x s S[la£solus)_ i n Poland showed delayed maturation of young born l a t e i n the breeding season (Bujalska and G l i w i c z 1968; Petrusewicz et a l . 1971). My i s l a n d p o p u l a t i o n s of Peromyseus maniculatu.s e x h i b i t l a r g e r body s i z e , higher d e n s i t i e s , and reduced d i s p e r s a l but they do not conform t o the c o r r e l a t e s of K - s e l e c t i o n l i s t e d by Pianka (1970). In p a r t i c u l a r , mice on Samuel I s l a n d have r a p i d growth r a t e s , a high r e p r o d u c t i v e r a t e , and poor s u r v i v a l which are a l l demographic c o r r e l a t e s of r - s e l e c t i o n . ^ Deer mice on Saturna I s l a n d have hig h e r growth r a t e s , a s l i g h t l y higher r e p r o d u c t i v e r a t e , and s i m i l a r s u r v i v a l compared with mainland animals, Thus, compared with Samuel I s l a n d , mice on Saturna appear to be s h i f t e d towards the r-endpoint of the r-K continuum. Mainland p o p u l a t i o n s of deer mice would probably a l s o be l o c a t e d towards the K-endpoint compared with animals on Samuel I s l a n d . Taxonomic s t u d i e s o f t h i s s p e c i e s on the northern G u l f I s l a n d s (McCabe and Cowan 1945) and the Queen C h a r l o t t e I s l a n d s ( F o s t e r 1965) have i n d i c a t e d t h a t t h e r e are s e v e r a l subspecies of deer mice on i s l a n d s along the west coast of B.C. T h i s i n f o r m a t i o n and the d i f f e r e n c e i n body s i z e and other e x t e r n a l c h a r a c t e r i s t i c s , such as pelage c o l o u r , between my i s l a n d and 40 mainland p o p u l a t i o n s , as w e l l as the g e n e t i c v a r i a t i o n shown by E e d f i e l d (1976), i n d i c a t e t h a t deer mice have probably been i s o l a t e d on the i s l a n d s f o r many thousands o f years. Thus, e v o l u t i o n a r y processes such as genetic d r i f t c o u l d be r e s p o n s i b l e f o r the d i s p e r s a l and recruitment p a t t e r n observed i n these i s l a n d mice. The e f f e c t o f r e s t r i c t e d l a n d area may have i n t e r a c t e d with the e v o l u t i o n o f the p o p u l a t i o n s . Regardless of the e v o l u t i o n a r y o r i g i n of t h i s d i s p e r s a l and re c r u i t m e n t p a t t e r n , i t may be concluded t h a t on these i s l a n d s seasonal changes i n aggressiveness of the a d u l t p o p u l a t i o n may be s u f f i c i e n t but not necessary to determine the breeding d e n s i t y and seasonal changes i n s u r v i v a l o f j u v e n i l e deer mice. A more i n t e n s i v e study of seasonal changes i n behaviour and the v a r i a t i o n i n demographic and d i s p e r s a l parameters over s e v e r a l years f o r i s l a n d p o p u l a t i o n s o f deer mice would r i g o r o u s l y t e s t the v a l i d i t y of t h i s c o n c l u s i o n , . I t perhaps can be s t a t e d with some c r e d i b i l i t y t h a t r e g u l a t o r y processes i n p o p u l a t i o n s of Peromyscus l a n i c u l a t u s could be d i f f e r e n t i n i s l a n d p o p u l a t i o n s and hence should not be g e n e r a l i z e d over d i f f e r e n t geographic areas. 41 SdHBAfiY The demographic and d i s p e r s a l r e s u l t s f o r i s l a n d and mainland p o p u l a t i o n s of deer mice i n t h i s study are summarized i n Table 16., 42 F i g u r e 1. L o c a t i o n of study areas: tT.B.C. Research F o r e s t at Maple Ridge, and Samuel I s l a n d , Saturna I s l a n d , Reef I s l a n d , and King I s l e t i n the G u l f I s l a n d s . Scale 1 cm = 1 » 2 8 K m 44 F i g u r e 2. A e r i a l photograph of i s l a n d study a r e a s : Samuel I s l a n d , Saturna I s l a n d , Reef I s l a n d , and King I s l e t . 46 F i g u r e 3. P o p u l a t i o n d e n s i t y on mainland c o n t r o l and experimental g r i d s , Peromxscus s a n i S U l a J t u s , . Non-breeding season i s shaded. V e r t i c a l dotted bars r e p r e s e n t s t a r t and end o f breeding a c t i v i t y . S o l i d v e r t i c a l bars e n c l o s e d e f i n e d b r e e d i n g season. Histograms i n d i c a t e number of new males (shaded) and females (unshaded) r e c r u i t e d i n t o c o n t r o l p o p u l a t i o n . 47 MAJJOND ODNTRDL 3UD P- MnNIOJLATLB May July Sept Nov Jan Mar May 1974 1975 M A I N _ J M V O R J L 5 E F O V C V A L G R I O P . M A N I T L L A T L E May July Sept Nov Jan Mar May 48 F i g u r e 4. P o p u l a t i o n d e n s i t y on Samuel I s l a n d c o n t r o l and experimental g r i d s , Peromy,scus maniculatus,^ Non-breeding season i s shaded. V e r t i c a l dotted bars r e p r e s e n t s t a r t and end of breeding a c t i v i t y . S o l i d v e r t i c a l bars e n c l o s e d e f i n e d b r e e d i n g season.. Histograms i n d i c a t e number of new males (shaded) and females (unshaded) r e c r u i t e d i n t o c o n t r o l p o p u l a t i o n . 50 F i g u r e 5., P o p u l a t i o n d e n s i t y on Saturna I s l a n d c o n t r o l and e xperimental g r i d s , Peromxscus l a n i c u l a t u s , . Non-breeding season i s shaded. V e r t i c a l dotted bars r e p r e s e n t s t a r t and end of breeding a c t i v i t y . S o l i d v e r t i c a l bars e n c l o s e d e f i n e d b r e e d i n g season. Histograms i n d i c a t e number of new males (shaded) and females (unshaded) r e c r u i t e d i n t o c o n t r o l p o p u l a t i o n . . 51 SATUF3NA JSLAtO UNTFCL GRID P« MWiaJLATUS Mar May July Sept Nov Jan Mar May 1974 1975 SATLRslA I5LAISD PULSE REMOVAL GRID P- MANIQJ-ATLB F i g u r e 6. Minimum s u r v i v a l r a t e s per 14 days f o r Peromvscus aMi.£ala£!i§ o n 'the mainland, Samuel I s l a n d , and Saturna I s l a n d c o n t r o l g r i d s . Non-breeding season i s shaded. V e r t i c a l dotted bars r e p r e s e n t s t a r t and end of breeding a c t i v i t y . S o l i d v e r t i c a l bars e n c l o s e d e f i n e d breeding season. Males and females are shown s e p a r a t e l y . „ 53 MAIiSLAND r X N T R T J L GRID P- (vWilOJLATLB 1974 1975 54 F i g u r e 7., O v e r l a p p i n g histograms of percentage of animals s e x u a l l y mature i n v a r i o u s body weight c l a s s e s f o r the three study areas., Males and females are combined. 55 i 1 ) 56 F i g u r e 8. Body weight d i s t r i b u t i o n s f o r breeding (summer - June to Aug.) and non-breeding (winter - Dec. To Feb.) seasons f o r male £§romy_scus maniculatus i n the three c o n t r o l p o p u l a t i o n s . 57 MAINLAND CONTROL GRID 3MJER MAINLAND CONTROL GRID WINTER BO'-r SO-•40. 30. SO* 4 0 . 1/0. IS. 14. IB- IB- 20- 55- 24. 56. 26' 30- 32- 34. S A M J E L E L A N D Q3NTR3L GRID S J * € F . v 40-. 30.1 10. G O -S O -40-1 30-ao-1 to. 10. 12- 14. 16. i a - EO. 2S> 24. 2G- EB- 30- 32- 34. S A M J E L ISLAND CONTROL GRID WINTER 40-1 •30.1 20-1 10.1 iQ. IH. 14. IS- IB- 20- 22- 24- 26- 2B- 30. 3H« 34. . 10. 12. 14. 16' IS" 20> 22- 24. EE- SB- 30- 3E> 34. 30. 20. SATURNA ISLAND CONTROL GRID S L * * € R 40-1 30. 20-10-SATURNA I S L A N D CONTROL GRID WINTER lO. 12- 14. 16. IB. 20- 22. 24. 26. 2B- 30. 32. 34 10. IE. 14. IB. IB' 20- 22. 24- 26. 2B. 30- 32- 34. WEIGHT CLASS g WEIGHT CLASS g 58 F i g u r e 9. Mean growth r a t e s with 95% confidence l i m i t s f o r c o n t r o l and experimental p o p u l a t i o n s of Peromy.scus maniculatus f o r the f o u r pulse p e r i o d s . , C o n t r o l r e p r e s e n t e d by c l o s e d c i r c l e and experimental by open c i r c l e . Sample s i z e above upper confidence l i m i t s . 59 GROWTH RATES CONTROL AND EXPERIMENTAL GRIDS MALES P- MANICLLATU5 0.05. 0*04 0-03I o-oal o-oii 0 . 0 0 -0.01L -o-oa i s S3 23 AS 23 • B S3 35 M i n B E F D 1 5 f III 1 2 23 S 3 T 33 June - July Aug i s -Oct 15 TIKE (MONTHS) Nov i s - J a n (i»ion<t») -Feb (Mainland) Mar - Apr (isunds) Apr - May (Mainland) GROWTH RATES CONTROL AND EXPERIMENTAL GRIDS FEMALES P- MANICULATLS 0-05, 7 1 0>o4 0 -031 0 - O S l O-Oli O-OOl -O-Oll 12 1 5 1 4 9 E F" ± C 1 3 3 3 4 l i t 5 7 IB 6 12 I » HI 40 T 17 I S £ 0 ss C ± ! June - July Aug 15 - Oct 15 Nov 15 - Jan (inlands) Mar - Apr (uumis) TIME QvONTHS) Feb (Mninund) Apr-May (Mainland) 60 F i g u r e 10. Hedian body weight at s e x u a l maturity and 95% confidence l i m i t s f o r males and females from each study area d u r i n g the p e r i o d June t o Aug. 1974. C o n t r o l p o p u l a t i o n s represented by c l o s e d c i r c l e and experimentals by open c i r c l e . MEDIAN BODY WEIGHT AT SEWJAL MATURITY FOR MALES AND FEMALES P- MANTM 1 ATLE SB' B 4 . I 224 BO-1 ta-l e • ! 14.1 1H< E T * T 6 0 B MALES JUNE TO AUGUST 1374 FEMALES Grid Designations: A - mainland control B - mainland pulse removal C - Samuel Island control D - Samuel Island pulse removal E - Saturna Island control j F - Saturna Island pulse removal 62 F i g u r e 11. Schematic r e p r e s e n t a t i o n of seasonal changes i n p o p u l a t i o n s of Peromjrscus maniculatus.. Hodel based on r e s e a r c h of S a d l e i r (1965), Healey (1967) , and F a i r b a i r n (1976). 63 64 Table 1. Trappability estimates for.Peronyscus maniculatus.on the three control areas for this study. Sample size in parentheses. Maximum trappability is the proportion, of those known to be alive that are actually caught in a trapping session. minimum unweighted trappability eliminates f i r s t and last captures and provides only one value for each individual regardless of.how long he lives. Mainland Samuel Island Saturna Island. Males Females Males Females Males. Females Maximum Trappability 0.91 (44) 0.79 (34) 0.96 (95) 0.94 (73) 0.80 (103) 0.84 (75) Minimum Unweighted 0.37 (29) 0.72 (23) 0.36 (34) 0.85 (39) 0.69 (66) 0.77 (46) Trappability 65 Table 2 . Comparison of a d u l t and j u v e n i l e recruitment during removal weeks i n the breeding season. Males and females are shown f o r c o n t r o l and pulse removal g r i d s of each study area. Adults J u v e n i l e s Males Females T o t a l Males Females T o t a l Mainland Removal No. 2 (Aug. 14-16;Aug. 28-30 C o n t r o l 5 ) 0 2 0 7 0 u 3 8 0 22 3 Samuel I s l a n d Removal No. 2 (Aug. 12-14.;Aug. 26 -23 C o n t r o l Removal No. 3 (Ilov. 1 -2; Nov. 15-16) C o n t r o l 10 ) 1 5 1 1 1 1 1 11 2 6 2 9 10 14 13 10 5 10 4 19 15 24 17 Saturna I s l a n d Removal No. 2 (Aug. 1 2 - U;Aug. 26 -28 C o n t r o l 6 ) 3 2 2 8 ' 5 19 10 22 9 41 19 Table 3 . Population density-for control populations and colonization data f or - the removal weeks i n each removal period, Peromyscus maniculatus. Control population Experimental population Mainland Density No. immigrants Recovery r a t i o (control) Recovery r a t i o '^re-removal Relative recruitment index week Removal No. 1 . I sj 7 9 10 12 142.8 133.3 . 100.0 120.0' 7 . 3 3 1st Removal No. 2. 17 16 17 12 100.0 7 5 . 0 130.8 9 2 . 3 9.67 Removal. No. 3 . ^ 25 25 16 15 64.0 60.0 72.7 68.2 7.75 , M / 1 s t Removal- Ho. 4« 21 17 0 4 0.0 23.5 0.0 . 4 4 . 4 -Mean value (Removal Nos. 1,2,+3} 95.8 97 . 4 8.25 95% confidence limits 58.4-133.2 7 1 . 1 - 1 2 3 . 6 Samuel Island week 1st Removal No.. 1. 2 n (j 10 8 8 3 80.0 37.5 50 .0 -18.8 3.'67 1st Removal No. 2. 22 19 16 1 4 72.7 73.7 6 9 . 6 6 0 . 9 1.76 Removal No. 3. 2T 3 5 17 13 81 .0 37.1 70.8 5 4 . 2 1.58 Removal No. 4« 30 29 13 13 4 3 . 3 • 44.8 72.2 72.2 3.71 Removal No. 5. 2rd 27 2 4 13 13 43.2 54.2 6 5 . 0 65 .0 6.50 Mean value 5 7 . 3 5 9 . 9 3 . 4 4 95% confidence lim i t s 4 4 . 6-70.0 48.2-71.5 Saturna Island week Removal No. 1. ^ 2nd 3 6 41 . .20 ' 6 55 .6 14 .6 58.8 . 17 .6 1.63; Removal No. 2. 39 51 27 22 6 9 . 2 43.1 103.3 84 .6 r 2.04 • Removal No. 3. • 48 49 13 •14 27.1 28 .6 28.9 31.1 6.75 1 st Removal No. 4. 2nd 4 4 4 5 4 .12 9.1 2 6 . 7 • 15 .4 4 6 . 2 16.00 1 st Removal No. 5. 2nd 4 7 . 38 19 6 4 0 . 4 15.8 6 1.3 19 .4 2.78 Mean value(Removal Nos. 1,2,3,+5) 3 6 . 8 . 5 0 . 7 3.30 9 5 / J confidence limits 21.0-52.7 24.1-77.3 67 Table 4. Results of Analysis of Covariance of observed range length regressed on number of times captured for males and females. Means compared by Duncan's Multiple Range Tests(D. M. R. T.). Males Source F value Probability Area 5.39 0.0001 D. M. R. T.(p=.05) Any means underscored by the same line are not significantly different from one another. Average Observed Range Length (m) 34. Q F 37.8 A 4 0 . 9 C 4 5 . 0 D 49.1 B 54.9 N=number of animals Females 81 84 35 59 57 Source F value Probability Area 5.36 0.0001 34 D. M. R. T. (p=.05) Any means underscored by the same line are not significantly different from one another. Average Observed Range Length (m) F 32.5 C 35.6 F 36.4 D 3 6 . 8 B 4 5 . 6 A 51.3 N=nuraber of animals 72 55 60 44 28 27 Grid Designations: A - mainland control B - mainland pulse removal C - Samuel control D - Samuel pulse removal E - Saturna control F - Saturna pulse removal 68 Table 5. Proportion of Peromyscus maniculatus in breeding condition on control areas during a comparable period of the breeding season. Sample size in parentheses. Season and Group Mainland Samuel Island Saturna Island Comparisons (A) (c) •(E) . A-C A-E" C-E June - August 1974 Testes Scrotal Adults .30 (46) .91 (23) .67 (75) C-E** Juveniles .14- (7) + .12 (26) .02 (43) Vagina Perforate Adults .15 (4-8) .22 (32) .15 (78) Juveniles .25 (4-)+ .17 (18) .13 ( 2 3 ) Nipples Med.-Large Adults .71 (48) .81 (32) .62 (78) C-E* % % - ' significant difference by Chi-square + inadequate sample size to test for difference 69 Table 6. Proportion of Peromyscus maniculatus in breeding condition on control and experimental areas for week of colonization following each removal during the breeding season.- Totals for adults and juveniles are shown for each area. Sample size in parentheses. Mainland Samuel Island Saturna Island Group Control Removal Control Removal Control Removal Testes Scrotal Adults 1.00 (10) 1.00 (5) 0.83 (24) 0.79 (29) 0.70 (89) 0.74 (31) Juveniles 0.00 (5) 0.21 (14) 0.00 (30) 0.05 (21) 0.09 (44) 0.11 (28) Vagina Perforate Adults 0.00 (15) • 0.00 (5) 0.16 (38) 0.36 (14) 0.20 (54) 0.38 (8) Juveniles 0.00 (0) 0.00 (S) 0.14 (14) 0.06 (16) 0.10 (39) 0 . 16 (43) Nipples Hed.-Larg Adults 0.80 (15) 1.00 (5) 0.68 (38) 0.71 (14) 0.28 (54) 0.13 (8) 70 Table 7. Comparison of proportion of animals i n breeding condition for control and experimental populations during the f i r s t and second Pulse Periods. Saaple size i n parentheses. Period and Mainland Samuel Island Saturna Island Group Control Removal Control Removal Control PLemoval. Pulse No. 1. Testes scrotal Adults 0.75 ( 3 6 ) 0.77 (30) 0.94 (17) 0.93 (30) 0.73 (52)* 0.92 (36) Juveniles 0.50 (-2) + 0.20 (5) + 0.25 (12) 4 0.00 ( 6 ) + 0.05 (20) 0.13 ( 3 ) + Vagina perforate Adults - ' 0.21 (33) 0.18 (22) 0.20 (20) 0.15 (20) 0.18 (56) 0.17 (22) Juveniles 0.25 (4) + 0.67 ( 3 ) + 0.27 (11) 0.50 (6) + 0.27 (11) 0.23 (13) Nipples med.-large Adults 0.67 (33) 0.41 (22) 0.90 (20) 0.75 (20) 0.63 (56) 0.86 (22) Pulse No. 2 . Testes scrotal Adults 0.26 (19) 0.20 (15) 0.65 (20)* 0.91 (33) 0.30 (56)* * 0.64 (22) Juveniles 0.00 (4-9) 0.03 (34) 0.13 (30) 0.25 (S) + 0.02 ( 6 0 ) 0.02' (53) Vagina perforate Adults 0.03 (29) 0 . 0 0 (15) 0 . 2 3 (35) 0.20 (35) 0.11 (38) 0.18 (11) Juveniles 0.00 (15) 0 . 0 0 (27) 0.00 ( 2 3 ) 0.22 (9) + 0 . 0 0 (54) 0.02 (50) Hippies med.-large Adults 0.45 (29) 0.53 (15) 0.71 (35) 0.57 .(35) 0.42 (38)* 0.09 (11) Total Testes scrotal Adults 0.53 (55) 0.53 (45) 0.78 (37) 1 0 . 9 2 (63) 0.51 (108) * 0.81 (58) Juveniles 0.02 (51) 0.05 (39) 0.17 ( 4 2 ) 0.14 (14) 0.03 (80) 0.03 ( 6 1 ) Vagina perforate Adults 0.13 (62) 0.11 (37) 0 . 2 2 (55) 0.18 (55) 0.15 ( 9 4 ) 0.15 (33) Juveniles 0.05 (19) 0.07 (30) 0.09 (34) 0.33 (15) 4 0.05 (65) 0.06 (63) Nipples med.-large Adults 0 . 5 6 ( 6 2 ) 0.46 (37) 0.78 (55) 0.64 (55) 0.57 (94) 0.61 (33) significant difference by Chi-square between control and removal samples + inadequate sample size to test for difference Table 8. Minimum survival rates per 14 days for the three control populations. Sample size in parentheses. 71 Season and Group Mainland . Samuel Island Saturna Island Comparisons (A) (c) (E) A-C A-E C-S Males Winter 1974-' Total ' 0.89 (18)+ 0.86 (7) + 0.84 ( 68 ) + Summer 1974 Total Adults . Juveniles 0.76 (45) 0.76 (41) 0.75 (4) + 0.64 (160) 0.69 (88) . 0.58 (72) 0.79 (153) 0.84 (100) 0.70 (53) C—s** C-3* Winter 1974-75 Total 0.87 (146) 0.80 (120) 0.90 (210) C-E-"-* Summer 1975 Total wm 0.89 (120) Total 0.85 (209) 0.71 (287) 0.86 (551) A-C** C-2** Females Winter 1974 Total 0.56 (9) + 1.00- (7) + 0.81 (48) • Summer 1974 Total Adults Juveniles 0.87 (45) 0.88 (41) 0.75 (4) + 0.73 (160) 0.82.(104) 0.57 (56) 0.87 (125), 0.85 (87) 0.92 (38) C-B*« C-E#* Winter 1974-75 • Total 0.87 (97) 0.87 (122) 0.88 (150) Summer 1975 Total _ _ 0.84 (69) Total 0.85 (151) 0.80 (289) 0.86 (392) C-E* * p<.05 significant difference by Chi-square ** p<.01 + inadequate sample size to test for difference 72 Table 9. Indices of early juvenile survival and reproductive data for the three control populations. Ranges for indices of survival based on samples including 2 or more females. Sample size in parentheses for. observed proportion of juveniles surviving and surviving to breed. N= number of trapping weeks. Mainland . Samuel Island Saturna Island Mean N . Range Mean N Range Mean N Range Breeding season 1974 O . U 6 0-0.29 1 . 46 18 0-2.75 0.98 9 0.25-4.5 Breeding Season + to end of recruitment 0 . 66 11 0-2.00 1 .49 20 0-2.75 1 . 00 1 4 0-4.5 Litter . size * 4.52 57 3.36-5.68 3.38 + 32 2-5 3 . 3 8 + 32 2-5 Number of successful pregnancies Total ij Ranrje Total M Ranre Total II Rqnrre 13 7 1-4 33 15 1-12 . 18 13 1 - 3 Expected number of juveniles 58.8 111.5 6 0 . 3 Observed number of juveniles 3 92 44, Males Females Males Females Males Fenales Proportion surviving1 0.67 (3) 0 0.46 (35) 0.50 (30) 0.60 (20) 0.75 (S) Proportion surviving and breeding^ 0.00 0 0 . 29 (35) 0.47 ( 3 0 ) 0.00 (20) 0.38 (8) * Sadleir 1974 +. laboratory litters from Saturna Island 1 animals caught up to one month before end of breeding and survive at least two vreeks 2 animals in reproductive condition before end of breeding . • Table TO. Comparison of minimum survival rates per 1 4 days for control and experimental populations during Pulse Periods. Sample size i n parentheses. 7 3 Mainland Samuel Island Saturna Island r c i x o a anu Group Control Removal. Control Removal Control Removal Males Pulse No. 1 . Total 0 .74 ( 3 1 ) 0 . 3 5 ( 2 7 ) 0 . 5 0 ( 2 4 ) * *0 .88 ( 2 6 ) 0 . 6 7 ( 5 8 ) 0 . 7 3 ( 3 3 ) Pulse No. 2 . _ Total 0 . 9 6 ( 43 ) 0 . 9 3 ( 4 0 ) 0 . 6 2 ( 4 2 ) 0 . 7 2 ( 3 2 ) 0 . 9 0 ( 9 3 ) 0 . 9 3 ( 5 8 ) Pulse No. 3 . Total 0 . 8 3 ( 3 0 ) ' 0 .54 ( 1 3 ) + 0 . 6 4 ( 5 5 ) * 0 . 9 0 ( 2 9 ) . 0 . 9 5 ( 6 6 ) 0 . 9 4 ( 3 1 ) Pulse No. 4 . Total 0 . 8 9 ( 1 9 ) 1.00 ( 1 3 ) 0 . 8 4 ( 4 9 ) 0 . 8 5 ( 4 1 ) 0 . 8 9 ( 6 6 ) 0 . 9 6 ( 2 5 ) Total 0 . 8 7 ( 1 2 8 ) 0 . 8 6 ( 9 3 ) 0 . 6 7 ( 1 7 0 ) " * 0 . 8 4 ( 1 2 3 ) 0 . S 6 ( 2 3 8 ) 0 . 8 9 ( 1 4 7 ) Females Pulse No. 1 . Total 0 . 9 2 ( 2 6 ) 0 . 8 6 ( 2 2 ) 0 . 5 9 ( 2 2 ) * 0 . 9 5 ( 1 7 ) 0 . 7 9 ( 5 6 ) 0 . 6 9 ( 2 6 ) -Pulse No. 2 . Total 0 . 8 8 ( 33 ) 0 . 8 4 ( 3 2 ) 0 . 7 0 ( 4 6 ) * * 1 . 0 0 ( 2 9 ) 0 . S 9 ( 7 3 ) 0 . 9 3 ( 4 1 ) Pulse No. 3 . Total 0 . 9 6 ( 2 3 ) . * * 0 . 5 4 ( 1 3 ) + 0 . 6 4 ( 3 9 ) * '"1.00 ( 1 4 ) 0 . 91 ( 4 4 ) 0 . 9 6 ( 2 5 ) Pulse No. 4 . Total 0 . 8 0 ( 1 5 ) 1 .00 ( 2 ) + 0 . 9 5 ( 5 3 ) 0 . 8 6 ( 2 8 ) 0 . 9 2 ( 3 8 ) 0 . 9 5 (21) . Total 0 . 9 0 ( 9 7 ) 0 . 8 0 ( 6 9 ) 0 . 7 6 (165)""" : " 0 . 94 ( 8 8 ) 0 . 3 7 ( 2 1 1 ) 0.33 ( 1 1 3 ) ** p<!oi significant difference by Chi-square between control and removal samples + inadequate sample size to test for difference Table 11. Age classes.of animals, based on body weight. 74 Study Area !/eight( grams) Juveniles Subadults Adults Mainland 0 - I2g 13 - I6g >1Vg Samuel Island 0 - 12g 13 - Ug >19g Saturna Island 0 - Ug 15 - 20g >21g Table 12. Analysis of Covariance for growth rates regressed on body weight of anirnals < 20 g for the period June to Oct. 1974.. Males and females are combined. Mean growth rates are given with sample size in parentheses. ? values and probabilities for comparisons of three studj areas are also shown. Mainland (A) Samuel Island (0) Jaturna Island Growth Rate 0.0020 (124) 0.0111 (47) •0.0040 (10S) Comparisons A - C A - E C - E F value 59.9 4.9 25.3 Probability 0.00 0.03 0.00 75 Table 13. Sex ratios(proportion of males) in control and experimental populations of Peromyscus maniculatus. Sample size in parentheses. Mainland Samuel Island Saturna Island Period Control Removal Control Removal Control Removal Removal No. 1. 0.69 (16; 0.59 (22) 0.41 (17) 0.73 (11) 0.57 (65) 0.42 (26) Removal No. 2. 0.50 (30) 0.61 (31) 0.51 (39) 0.63 (30) 0.58 (80) 0.51 (49) Removal No. 3. 0.67 (46) 0.61 (31) 0.54 (50) O.63. (30) 0.52 (61) 0.52 (27) Removal No. 4. 0.65 (26) 1.00 (4) 0.50 (52) 0.62 (26) 0.63 (49) 0.69 (16) Removal No. 5. - - 0.53 (45) 0.58 (26) 0.62 (81) 0.68 (25) Total 0.63 (118 ) 0.63 (88) 0.51 (203; * 0.63 (123, 0.58 (336) 0.55 (143) * p<.05 significant difference by Chi-square betv.-een control and removal samnles 76 Table 14. Results of introductions of island and mainland mice onto King Islet(0.4 acre) and Reef Isiand(4.0 acres). King Islet May 10 July 29-31 (100 trap nights) Males Females Total Males Females Total Ma inland Mice 2 2 4 2 1 3 Island Mice 2 2 4 2 1 . 3 (17 untagged ofi'sr ring; King Islet July 31 Oct. 18-20 (155 trap nights) Males Females Total Males Females Total Mainland Mice 26 7 33 12 1 13 Island Mice 20 14 34 12 . 8 20 (15 untagged offspring) Reef Island May 10 July 29-31 (200 trap nights) Males Females Total Males Females Total Mainland Mice 22 18 40 20 12 32 Island Mice 21 18 39 14 13 27 (27 untagged offspring) 77 Table 15. R e s u l t s of behaviour t e s t s f o r breeding males, Peromyscus maniculatus, from Samuel I s l a n d and Saturna I s l a n d . Mean number and range of 5 second intervals(maximum of 120) of each behaviour f o r I! =7 encounters are given f o r each i s l a n d . Samuel I s l a n d Saturna I s l a n d Behaviour Mean Range Mean Range Latency period/10 min. 2.8 1-6 8.7 4-10 Aggressive Behaviour -Upright 2.7 0-13 1.7 0-5 Box 0.0 - 0.3 0-2 Avoid 0.7 0-5 0.0 -Avoid leap 0.U 0-1 0.0 -Threat 0.0 - 0.0 -T i g h t 0.0 - 0.14 0-1 Cohesive Behaviour Approach 12.1 0-22 7.0 1-12 Nose to nose + mutual s n i f f 6.1 0-16 3.4 0-8 S n i f f 16.1 0-38 6.4 1-15 Huddle-contact 9.3 . 0-32 5.1 0-30 T o t a l (Number of 5 second i n t e r v a l s i n which animals i n t e r a c t e d ) 35.9 0-63 17.4 1-53 Table 1 6 . Summary of demography and dispersal in island and mainland populations of deer mice. 78 Mainland S amuel Island Saturna Island Average density (per hectare) Reproductive rate (1974) - length of breeding season (weeks) - number of successful pregnancies - proportion ox . ^(males) breeding animals ^(females) - proportion-of recruits surviving to breed Survival (adults males females .nd juveniles) (summer) (winter) (summer) (winter) Relative growth rates Body size (adults) Dispersal (colonization rate) - mean recovery ratio(to control population) - mean" recovery ratio(to pulse| • population) - mean relative recruitment index - observed range length (m) (males) (females) No. recruits per trapping week of breeding season (males) (females) Behaviour (breeding season) .18.7 12 13 0.80 0.71 0 0.76 0.87 0.87 0.87 1.0 >17 95.8 97.4 7.3 40.9 51.3 1.0 0 more aggressive Sadleir 1965) Healey 1967) Fairbairn 1976) 22.0 34 33 0.91 0.81 0.37 0.64 0.80 0.73 0.87 5.6 >19 57.3 59.9 3.6 45.0 35.6 2.9 2.2 less a(?:.rressive 43.5 0.67 0.62 0.11 0.79 0.90 0.87 0.88 2.0 >21 g 36.8 50.7 3.5 34.0 36.4 3.1 1.8 less 79 LITERATURE CITED B r i t t o n , H.fl. 1966. Reproductive success and s u r v i v a l of the young i n Pergmyjscus^ ft. Sc. T h e s i s , U n i v e r s i t y of B r i t i s h Columbia, Vancouver, B r i t i s h Columbia. B u j a l s k a , G. and J . G l i w i c z . 1968. P r o d u c t i v i t y i n v e s t i g a t i o n s of an i s l a n d p o p u l a t i o n of £l§ t h x i 2 S 2 I X I i g l a r e o l u s (Schreber 1780). I I I . I n d i v i d u a l growth c u r v e s . Acta7 T h e r i o l . 13: 427-433. Delany, H.J. 1970. V a r i a t i o n and ecology of i s l a n d p o p u l a t i o n s of the i o n g - t a i l e d f i e l d mouse ( AfiSlemus sy_lvaticus }. Symp., Z o o l . Soc. . Lond.,; So. 26, 283-295. F a i r b a i r n , D.J. 1976. P o p u l a t i o n processes i n Peromxscus: An experimental approach. Ph.D. T h e s i s , U n i v e r s i t y of B r i t i s h Columbia, Vancouver, B r i t i s h Columbia. Fordham, B.A, 1971. F i e l d p o p u l a t i o n s of deer mice with supplemental food. Ecology 52: 138-146. F o s t e r , J.B. 1965. E v o l u t i o n of mammals of the Queen C h a r l o t t e I s l a n d s , B.C., O c c a s i o n a l Papers, B r i t i s h Columbia P r o v i n c i a l Museum, no. 14. pp. 1-130. F u l l a g a r , P.J., P. A., J e w e l l , H.ti. L o c k l e y , and I . W. Rowlands. 1963. The Skomer vo l e ( Clettoionomvs a l a r g o l u s skomerensis ) and l o n g - t a i l e d f i e l d mouse ( A&odemus §2i2§.tieus ) on Skomer I s l a n d , Pembrokeshire i n 1960., Proc. Z o o l . Soc. Lond. 140: 295-314. Garten, C T . and M. H. Smith. 1974. Movement by o l d f i e l d mice and p o p u l a t i o n r e g u l a t i o n . . Acta. T h e r i o l . , 19; 513-514. Healey, H.iJ. 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 deer mice. Ecology 48: 377-392.. H i l b o r n , S., J.A. S e d f i e l d , and C.J. Krebs. 1976., 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 . „ In press. , J e s e l l , P.A. 1966. Breeding season and recruitment i n some B r i t i s h mammals c o n f i n e d on s m a l l i s l a n d s . I n : Comparative b i o l o g y of r e p r o d u c t i o n i n mammals. E d i t e d by I.H. Rowlands, pp., 89-116. Zoology S o c i e t y Symposium No. 15. K r a j i n a , V.J. 1965. B i o g e o c l i m a t i c zones and c l a s s i f i c a t i o n of B.C. E c o l . Western N.A. 1: 1-17. Krebs, C.J., B.L. K e l l e r , and R.B. Tamarin. 1969. H i c j o t u s 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 Mj. ochrogaster. and M t fiennsYlvanicus i n southern Indiana. Ecology 50: 587-607. Krebs, C.J., I. U n g a t e , J . LeDuc, J . A . R e d f i e l d , E . T a i t t , 80 and E. , H i l b o r n . 1976,^ Microtus 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 H.. townsendii . Can. J . Z o o l . 54: 79-95. L i d i c k e r , W.Z. 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. 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 . E d i t e d by K. Petrusewicz, F.B. G o l l e y , and L. Ryszkowski. Cambridge Univ. P r e s s , New York and London., Chap. 5, McCabe, T.T. And I. McT. Cowan. 1945., E§£21iscus maniculatus macroninus and the problem o f i n s u l a r i t y . Trans. l o y a l Can., I n s t . , 1: 172-215. Myers, J . H. and C.J. Krebs. 1971.., G e n e t i c , b e h a v i o u r a l , and re 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 voles Micjrotus E§£S§lllsnicus_s_ E c o l . Monogr. , 41: 53-78. Petrusewicz, K., G. B u j a l s k a , R, Andrejewski, and J . , G l i w i c z . 1971.„ P r o d u c t i v i t y processes i n an i s l a n d p o p u l a t i o n of £itlli£i2S2fflIs 2 l a r e o l u s t Ann. Zool . Fenn. 8: 127-32, P e t t i c r e w , B.G. and R.M.F.S. S a d l e i r . 1974. The ecology of the deer mouse Peromv_scus j a n i c u l a tus i n a c o a s t a l c o n i f e r o u s f o r e s t . I . Po p u l a t i o n dynamics., Can. J. Z o o l . ,. 52: 107-118. Pianka, E.B. 1970. On r and K s e l e c t i o n . Amer., Nat. 104: 592-597. R e d f i e l d , J.A., 1976. D i s t r i b u t i o n , abundance, s i z e , and g e n e t i c v a r i a t i o n of £eromjrscus fflaniculatus on the G u l f I s l a n d s of B r i t i s h Columbia?, CanT ~j7~~Zool. , 54: 463-474. 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 . 34: 331-3527 S a d l e i r , R.M.F.S. 1974. The ecology of the deer mouse i n a c o a s t a l c o n i f e r o u s f o r e s t . , I I . Reproduction. ,• Can. J. Z o o l . 52: 119-131. Sheppe, W. 1963. P o p u l a t i o n s t r u c t u r e of the deer mouse, Peromyscus, i n the P a c i f i c Northwest. J . Mamm. 44: 180-185." S t i c k e l , L.F. 1946. The source of animals moving i n t o a depopulated area.. J . , Mammal, 27: 301-307. S t i c k e l , L., 1968. Home range and t r a v e l s . In: B i o l o g y of PeromY.scus!. E d i t e d by J.A. King. S p e c i a l p u b l i c a t i o n number 2, Amer. S o c , Mammal, pp. 373-411.-

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