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Population dynamics of Peromyscus maniculatus austerus and Microtus townsendii with supplementary food Taitt, Mary Joan 1978

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POPULATION DYNAMICS OF PEHOMYSCUS 1ANICULATDS AUS^ AND MICROTUSTOWSENDII WITH SUPPLEMENTARY FOOD by HART JOAN TAITT B. Sc.{Hons.), U n i v e r s i t y of London, England, 1967 H. .Sc., U n i v e r s i t y o f Durham, England, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) He accept t h i s t h e s i s as conforming to the r e g u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA August, 1978 /c) Mary J. T a i t t In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Co lumb ia , I ag ree that the 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 and s tudy . I f u r t h e r agree 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 purposes may be g r a n t e d by the Head o f my Department or by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d that 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 not be a l l o w e d w i thout my w r i t t e n p e r m i s s i o n . Department o f ~Z>OOl*.OCi Y The U n i v e r s i t y o f B r i t i s h Co lumbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 A b s t r a c t A number of f i e l d s t u d i e s suggest that some v e r t e b r a t e p o p u l a t i o n s a re l i m i t e d by spacing behaviour, , Small mammals of the genus P§roraj;scus and Microtus e x h i b i t s p a c i n g behaviour by possessing home ranges, but they have c o n t r a s t i n g p a t t e r n s of pop u l a t i o n f l u c t u a t i o n , Deermice (£tE21Y§£S§ §£•) f l u c t u a t e a n n u a l l y but maintain f a i r l y constant numbers from year t o year, whereas v o l e s {Microtus sja.) ' c y c l e ' , r e a c h i n g peak d e n s i t i e s every 2 to 5 years. One use of the home range i s f o r food-gathering, T h e r e f o r e , these experiments were designed t o i n v e s t i g a t e the i n f l u e n c e o f food a v a i l a b i l i t y on the home range and p o p u l a t i o n dynamics of l o c a l deermice and vo l e s ( P. maniculatus austerus and M. tow n s e n d i i ) . The a d d i t i o n of food i n l a t e winter r e s u l t e d i n a doubling of the number of deermice. Immigration was 2.5 times t h a t of an unfed c o n t r o l . T h i s c o u l d be e x p l a i n e d s i n c e r e s i d e n t deermice reduced the s i z e of t h e i r home ranges. Deermice p o p u l a t i o n s with e x t r a food i n c r e a s e d t h e i r r e p r o d u c t i v e output compared with c o n t r o l s : l a r g e r numbers o f mice bred, and f o r longer p e r i o d s , more young were r e c r u i t e d , they grew f a s t e r and reached s e x u a l maturity e a r l i e r . I t i s suggested t h a t the onset and c e s s a t i o n of breeding i n deermice are proximate responses to food a v a i l a b i l i t y , Deermouse dynamics may be c l o s e l y t i e d to the temporal and s p a t i a l a v a i l a b i l i t y of food through the s p a t i a l o r g a n i s a t i o n o f i n d i v i d u a l s . I t i s a l s o suggested that females, because of t h e i r energy demands f o r l a c t a t i o n , and t h e i r i n f l u e n c e on the s u r v i v a l o f young, may be more s e n s i t i v e to these food c o n d i t i o n s and hence e x e r t a strong i i i i n f l u e n c e on deermouse p o p u l a t i o n dynamics. Vole p o p u l a t i o n s with, low- and i n t e r m e d i a t e - l e v e l s o f food peaked a t twice the c o n t r o l d e n s i t y , and a populat i o n with a h i g h - l e v e l of food reached seven times c o n t r o l d e n s i t y . . Voles immigrated to e s t a b l i s h e d p o p u l a t i o n s , and c o l o n i z e d vacant areas i n p r o p o r t i o n to the food a v a i l a b l e . L i k e deermice, r e s i d e n t s reduced the s i z e of t h e i r ranges. Breeding was enhanced i n a l l fed v o l e p o p u l a t i o n s . Omnivorous deermice had l a r g e r ranges than d i d the herbivorous v o l e s , but both s p e c i e s responded to e x t r a food by red u c i n g t h e i r range s i z e , so the s m a l l e s t deermouse ranges were the s i z e of l a r g e v o l e ranges. The r e s u l t s i n d i c a t e that home range s i z e i n both s p e c i e s i s res p o n s i v e to food a v a i l a b i l i t y , and t h a t the c o n c e n t r a t i o n of food i n the ' t y p i c a l ' h a b i t a t s of these s m a l l mammals i s d i f f e r e n t . I f , as suggested, the hetero g e n e i t y of deermouse-food i n the f o r e s t r e s u l t s i n an annual c y c l e i n numbers, then the reduced heterogeneity of vo l e - f o o d i n g r a s s l a n d s may i n f l u e n c e v o l e dynamics. i v l i l L E OF CONTENTS Ab s t r a c t ................... i i TABLE OF CONTENTS ...«><>•> . . . . . . . . . . • » • . » » » « • » « . * < • , . • « » » . , X V LX ST OF T ABLES . .«.««•*«. .•*««...*»« ..*«.*«.«.*«« #•.»«»«••• vxx LIST OF FIGURES ....... i x ACKNOWLEDGEMENT x i GENERAL INTRODUCTION . . .-. . . ... 1 PEROBYSCUS MANICULATUS 7 1 # X n tx o citic t i o n *i •• » » • • > » » * » » * » • * * • » » * • < > * * 7 2. Methods And Experiments 11 Study Area And Trapping Technigues .................. 11 Experimental Design ................................. 14 3. Re s t i l t s . 21 Se c t i o n A: T r a p p a b i l i t y And R e l i a b i l i t y Of The Data ... 21 Se c t i o n B: Population Density 24 Male Dynamics . 24 Female Dynamics ..................................... 31 Summary .. «. ... ... *» . * ?*• • • * .... .» ...... ,.*». .««»*. ** 32 Se c t i o n C: Breeding Season ............................ 33 Food And The Onset And C e s s a t i o n Of Breeding . ,. . 33 Food And Winter Breeding 35 Weather And Breeding .......... 36 Summary 39 Se c t i o n D: Breeding Success 40 Summary »»«*-. •«.»»«.. ..«.«..«•««................ 45 Sec t i o n E: Immigration And S u r v i v a l ................... 46 Immigration *....•..............»•...,..**»..«...»..• 46 S ur v i v a l *» ..»»..».»»..»*...*......*«.*. ...»•-.• .. *. • * 50 V S u in in 9. L? y i t t i i i t i i v i i t t i i i i i i i i i i t t i • * • • * * # # * • » * • • • • • * ; • * 5 5 Section F: Weights, Sexual Maturity And Growth 56 Winter Weights Of Larger Mice 57 Winter Weights Of Smaller Mice ...................... 59 Overwintering Weight And Breeding C o n d i t i o n 62 Sexual M a t u r i t y ..................................... 65 Growth ..... . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Food Consumed ..........»..................<•••••••«> 70 Summary ...........«...,...,..,...*.».........*•*..*.. 72 Secti o n G: Home Sanges .......... ...................... 73 Summary 80 S e c t i o n H: M i c r o t u s oregoni: A P o s s i b l e Competitor? ... 81 Food Manipulation And Species Numbers 83 Removal Of M. oregoni 84 S u in in ci y ***#•« ••**•» « ***»•*•••*> «•#•»•••#»««§••••* 86 4. D i s c u s s i o n 88 Male And Female Numbers ............................. 90 Breeding Season 92 S p a t i a l O r g a n i s a t i o n 94 I m p l i c a t i o n s For P o p u l a t i o n R e g u l a t i o n I n Peromyscus 96 MICROTUS TOWNS ENDII ............ .A 101 1. I n t r o d u c t i o n •.»»»«...»«.....*•••••.«..*••»•••••*.•*•* 101 2. Methods And Experiments 105 G r i d s E, R, T And M : Long-term Food Supplementation 106 Grids 1, 2, and 3 : C o l o n i z a t i o n and Food Density ... 108 G r i d s I and G : Spring D e c l i n e and Food ............. 109 Food Supply •«,....*..»,.,..*•.,...*.**•.........*•*. 110 3. R e s u l t s •**».,. i . « » . . . . . . * . . . . , . . , . . . . • « . . . . , * » * : » . . . . ' • • 113 v i S e c t i o n I : L o n g - t e r m F o o d S u p p l e m e n t a t i o n .............113 *Xr 9. p p a. Ibi 1 i t y * * * • »*# * * * * » * * * * • • * * • # »•# * * • » • » * • * * » * # . 1 1 3 P o p u l a t i o n D e n s i t y < t * » * • • » • • • • • • • • * « • • • • • • 1 1 4 XHQ m i g r a t i o n * • « * * * * » * o « * * * • * * -* * » » » *-•*#• * • « * * • • * • » 121 B r e e d i n g S e a s o n »*«»*«»» »•»•#•• * * • • * * * » • * 1 2 5 B r e e d i n g S u c c e s s •.*••****•«•*•**•••**••*•**•••••«*•* 1 2 9 W e i g h t s And G r o w t h .«*.*»»«••«•*'»«»•••.•.«»•«•«•«••••.. 131 S u r v i v a l i « * « i * 4 i t i i i < i < i i i 4 4 i * » t * i i * i i i i i t * t i i i i i i i i i 1 3 5 Home Range S i z e • * • * • * * * • * * . * » * * * * ? • • * • * • « * * 1 3 6 S e c t i o n I I : C o l o n i z a t i o n a n d Foo d D e n s i t y ............ 139 P o p u l a t i o n D e n s i t y 140 X n s i n i g r s t i o n *«.*«»•:*«* A .--i *» * » • *.*#*•.<•* « * « • » * • * * • * 1 4 3 B r e e d i n g c o n d i t i o n And R e p r o d u c t i v e S u c c e s s ......... 145 S e c t i o n I I I : S p r i n g D e c l i n e And Food S u p p l e m e n t a t i o n .. 147 4» D i s c u s s i o n * * * 4 k s • - » * • * * « * * • • * • • * * • • • * * • • « » * • 1 5 0 G E J$ E R A X CONCXUSXONS * « » * * •* • «» * * • * • * • * # » * • «* *» * * * « * *#* • * * • • • 154 XITERATUR E CXT ED ******•<»» • « * * * * * • * * « * • * * ? * • • * « * • * * • • * * * * « • 1 6 0 A3?I?E^DXCES • * * * * « * § * .» « * • » * ? * • » • * » * • * * # » » a # » * * * * * « • * * « • * * • • * 1 6 5 v i i L I S T OF TABLES Table I. T r a p p a b i l i t y of T>± manjculatus, 23 Table I I , Mean Density of P^ . manieulatus On Each G r i d , . . 28 Table I I I . Sew Young Mice Caught on G r i d 1 and Gr i d 5 i n the Last Two Trapping P e r i o d s . 41 Table IV. J u v e n i l e s R e c r u i t e d i n t o Each P o p u l a t i o n i n Each Six-month Period, 43 Table V. Index of E a r l y J u v e n i l e S u r v i v a l . 45 Table VI, The Sex and Age of Immigrants to G r i d s 1 and 2 i n Each Three-month Period. , ., .. 49 Table VII. Mean Minimum 14-day S u r v i v a l of Sub-adult and J u v e n i l e Mice on G r i d s 1 and 2. .. . . 54 Table VIII, The Mean Weight of Oats Removed from Feeding S t c i t i ons »«*•»•»•» «<•***««•• *<* • *•»•«•*•»»• •»«»•**•**• "71 Table IX, The Mean Home Range S i z e {sg M) of Male and Female Mice on C o n t r o l and Food G r i d s . 74 Table x. The Mean Number of M_j. oregoni i n Each Ea. l^Si£Sl^ili§ P o p u l a t i o n . ............................. 82 Table XI. The Number of P.. j a n i c u l a t u s And M t oregoni During P a r t i a l Removal of the Vole from Grids 1 And 5, . 85 Table XII. Fa c t o r s Suggested as Necessary to Show That Spacing Behaviour L i m i t s P o p u l a t i o n S i z e . 103 Table XIII. T r a p p a b i l i t y of Vo l e s on G r i d s E, S, and T, . 114 Table XIV, E a r l y J u v e n i l e S u r v i v a l on G r i d s E, R, T, and M # *••**«*•-*****« # « * « »»*<*<•*<•*••*«* »•* •» • * »* »»•*.* »•••«***** 130 Table XV. S u r v i v a l of Voles on G r i d s E, R, T, and M, .... 136 Table XVI, Mean Home Range S i z e {sg m) of Voles on G r i d s v i i i E f j and *««.**••*•«• •**•* « •*••«•**•**•• 137 Table XVII, New Voles Captured on Grids 1, 2, and 3 i n Each C o l o n i z a t i o n Period, »....,-,,•.., , , . , . . . 1 4 4 T a b l e XVIII. Mean Number of Males S c r o t a l and Females L a c t a t i n g on Grids 1, 2, and 3. *. 146 Table XIX. P e r i o d s of Maximum and Minimum Numbers at E i e f e l from 1 972 to 1975, 148 i x LIST OF FIGURES F i g u r e 1. A General Model Of Spacing Behaviour. 3 F i g u r e 2. L o c a t i o n o f P.. maniculatus Grids 1 2 F i g u r e 3., The Arrangement of Traps and Food S t a t i o n s . ...... 1 5 F i g u r e 4. Summary of P. maniculatus Experimental Design. 1 8 F i g u r e 5 . Number of Male and Female Mice on G r i d 1 and G r i d F i g u r e 6. Number of Mice on G r i d 1 and Gri d 4 . . . . . . . . . . . . . 2 9 F i g u r e 7. Summary of Breeding Season Data f o r G r i d s 1 , 2 , 3 i ci n d 5» » •» • a * . • • * * * » » • * • « * • « » » * * » » » » » • • • a •»«»#>» 37 Figure 8. Immigration of Mice t o G r i d 1 and Grid 2 4 7 F i g u r e 9. Minimum 1 4 D a y - s u r v i v a l of Adu l t s on G r i d 1 and G ITXCl 2 f » t * * 1 V 1 * « • I 4 f t » 1 t t « • * 1 I * 1 . 4 4 t t * I * » * * t « • I 4 t 4 * • * * 4 51 F i g u r e 1 0 . Winter Weights of I n d i v i d u a l Large Mice on the C o n t r o l s and Food G r i d s . ................... , .. .. 5 8 Figure 1 1 . Winter Weights o f I n d i v i d u a l Small Mice on the C o n t r o l s and Food G r i d s . ............................... 6 0 F i g u r e 1 2 . Weight D i s t r i b u t i o n o f Males on G r i d 1 and G r i d 2 . * « . * » . . . . . . . . . . * . « . « . . . . . . • . . « . . . . . » . . . . . . . . . . . . . . . . . 6 3 Figure 1 3 , Median Body Weight a t Sexual Ma t u r i t y On Grids Ijr 2/ And 3* • **.»• * » • « * * * * » • • o * * .»• •****•*•«**•« * 6*7 Fi g u r e 1 4 . Frequency D i s t r i b u t i o n of Male Home Range S i z e s on the C o n t r o l ( g r i d 1 ) and Food Gr i d ( 2 ) . . .......... 7 6 F i g u r e 1 5 , Frequency D i s t r i b u t i o n of Female Home Range S i z e s on the C o n t r o l ( g r i d 1 ) and Food Grid ( 2 ) . 7 8 F i g u r e 1 6 . Summary of the E f f e c t o f Supplemental Food on PA. !§£iculatus P o p u l a t i o n s . .. ., ,, , .. 8 9 Figure 17, L o c a t i o n o f M„ to w n s e n d i i Grids at Ladner, B. * 'a % 9 * # •* * * * » * a * * * * a * ; » • : » * : * * ••» a # * * * <* J> * * * # * « * • a. * * * F i g u r e 18, Summary of M. townsendii Experimental Design. F i g u r e 19. Number of Voles on G r i d s E, fi, and T. ....... Fig u r e 20. Number of Hales and Females cn G r i d s E and M. Figu r e 21, New Voles Captured on Grids E, 8, T, and M. . Fi g u r e 22. Breeding A c t i v i t y on G r i d s E, R, T, and W. Fi g u r e 23. Mean Weights o f Voles on Gr i d s E, R, and T.,. F i g u r e 24. Number of Voles on Grids 1, 2, and 3........ ACKNOWLEDGEMENTS I wish to 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 encouragement and a s s i s t a n c e throughout t h i s study. I thank him and the members of my committee, Dr. D, C h i t t y , Dr. J . N. M. Smith, Dr. A. R. E. S i n c l a i r , and Dr. R. M, F. . S. S a d l e i r f o r t h e i r c r i t i c a l r eading of e a r l i e r d r a f t s of t h i s manuscript, and t h e i r suggestions f o r making i t more c o n c i s e . I t would not have been p o s s i b l e to c o l l e c t such an extensive body of data without f i e l d a s s i s t a n c e from Dr. R, Boonstra, Dennis Draper, Dr. R. H i l b o r n , J a n i c e LeDuc, Micheal Moult, Nick M o r r i s , J a r o s l a v Pieman, Dr. J . A , R e d f i e l d , Donna Stace-Smith, Tom S u l l i v a n , and Irene Wingate, I am very g r a t e f u l to a l l these people. F i n a l l y , I thank Dr. C. J . Krebs f o r g i v i n g me access t o h i s computer programs f o r the a n a l y s i s o f t h e demographic data, and Dr. D. B. Hawes f o r s e t t i n g up the home range program on the O.'.B, C, computer. I acknowledge the U n i v e r s i t y of B r i t i s h Columbia f o r t h e i r f i n a n c i a l support i n the form o f a Graduate Student F e l l o w s h i p and a Teaching A s s i s t a n t s h i p . 1 GENERAL INTRODUCTION A major f i e l d of i n t e r e s t i n p o p u l a t i o n ecology i s the apparent tendency of n a t u r a l p o p u l a t i o n s to maintain t h e i r d e n s i t y at a c e r t a i n l e v e l when they have the . c a p a c i t y t o i n c r e a s e e x p o n e n t i a l l y , A number of hypotheses have been advanced to e x p l a i n t h i s phenomenon. These hypotheses span a broad spectrum from those proposing t h a t c o n t r o l i s e x t r i n s i c to those suggesting that p o p u l a t i o n s are i n t r i n s i c a l l y self-regulated» One hypothesis t h a t i s c o n s i s t e n t with r e c e n t r e s e a r c h i n t o p o p u l a t i o n l i m i t a t i o n i n v e r t e b r a t e s was f i r s t suggested by Wynne-Edwards (1962,1965). He s t a t e d t h a t animals which are capable of s o c i a l behaviour can d i s p e r s e themselves i n space and time and so avoid o v e r e x p l o i t a t i o n of t h e i r environmental resources. The extensive review of vertebrate p o p u l a t i o n s t u d i e s by Watson and Moss (1970) i n d i c a t e s that i n t e r f e r e n c e phenomena i n the form of t e r r i t o r i a l and other types of dominance and spacing behaviour l i m i t some v e r t e b r a t e populations, A problem a r i s e s when one attempts t o look f o r the cause of these i n t e r f e r e n c e phenomena. For example, can one design a p e r t u r b a t i o n that w i l l a f f e c t the s p a c i n g behaviour of i n d i v i d u a l s and produce a p r e d i c t a b l e change i n t h e i r p o p u l a t i o n dynamics? Two r e c e n t reviews on the r o l e of behaviour i n the r e g u l a t i o n of animal p o p u l a t i o n s o f f e r l i t t l e encouragement, but at the same time they i l l u s t r a t e the problem. Brown (1976) warns t h a t , "The ' l i m i t a t i o n ' , ' c o n t r o l ' or ' r e g u l a t i o n ' o f p o p u l a t i o n numbers by behaviour i s o f t e n d i s c u s s e d as i f behaviour were the 2 only important fa c t o r , " King (1973) suggests that, "Perhaps behavioural explanations of phenomena, l i k e population regulation, are made when the explanations amenable to measurement and manipulation are unsatisfactory, almost any type of behaviour can provide an impenetrable refuge for an infirm theory." A reason for the current skepticism as well as the d i f f i c u l t y of designing an experimental approach to the problem, i s that the proximate causes of behaviour are multivariate i n nature (Crook,1965). E x t r i n s i c a l l y , the temporal and s p a t i a l pattern of physical resources and i n t e r s p e c i f i c interactions (for example, food, nest s i t e s , predators, and competitors) i n the environment determines how animals move around and use the resources they reguire. Also, the individuals themselves: their number, size, sex, age, phenotypic experience and genetic make-up may determine when and how they interact. Therefore, the s p a t i a l arrangement of a given population i s a re s u l t of e x t r i n s i c and i n t r i n s i c factors acting at s p e c i f i c and probably dif f e r e n t times (Brown and Orians,1970). Where the removal, of dominant indivi d u a l s results in the settlement and successful breeding of animals that previously were r e s t r i c t e d , I consider that the operative behaviour i s the sp a t i a l organisation of established members of the population. This s p a t i a l pattern of individuals could r e s u l t from an integration of both e x t r i n s i c and i n t r i n s i c features (Figure 1). The dynamics of such a population may be a conseguence of the s p a t i a l organisation of i n d i v i d u a l members. But i t may also be d i r e c t l y influenced by certain features of both the e x t r i n s i c and i n t r i n s i c environment. In addition we have to allow for 3 Figure 1, A general model of spacing behaviour. large arrows indicate small arrows indicate ? Indicates possible major interactions other possible interactions feedbacks 4 EXTRINSIC PHYSICAL RESOURCES : e-g-, NEST SITES, FOOD 1NTERSPECIFICALLY: e-g-, PREDATION COMPETITION INTRINSIC SEX, AGE, SIZE, BREEDING CONDITION, PHENOTYPIC EXPERIENCE GENETIC MAKEUP SPATIAL ORGANIZATION D Y N A M I C S NUMBER BREEDING REPRODUCTIVE OUTPUT IMMIGRATION SURVIVAL GROWTH RATE 9 5 feedbacks, as indicated in Figure 1, For example, an increase i n density, may feed back to s p a t i a l organisation. The ranges of indi v i d u a l s may become reduced in s i z e or overlap more, causinq animals to interact more frequently. Such an increase in interaction may lead to an i n t r i n s i c response, for example, some animals may leave or be excluded, while others may stay but cease reproduction temporarily. Further, the reduction or increasing overlap of ranges could r e s u l t i n a resource becoming over-utilized, But we can perturb such a system by increasing a resource such as food supply, then trace the e f f e c t of such a perturbation on the components, l a t e r , removal of the perturbation may give insight i n t o feedbacks that are operating. One of the categories of dominance behaviour l i s t e d by Watson and Moss i s 'a system of home ranges', k home range was defined by Burt (1943) as "that area traversed by the in d i v i d u a l i n i t s normal a c t i v i t i e s of food gathering, mating, and caring for young". The small mammals Peromyscus and Microtus both exhibit this form of s p a t i a l organisation. Experiments by Sadleir (1965) and Healey (1967) on P. maniculatus and by Krebs Hi* (1976) on M» townsendii, show that the removal of residents results i n colonization by previously-excluded individuals. The general trends in population numbers have been f a i r l y well established for these two groups of small mammals. The c y c l i c fluctuations of microtine rodents have been observed repeatedly: their numbers increase, peak and decline over a two-to five-year period (review by Krebs and Myers, 1974). Ferpmyscus, on the other hand, over most of i t s range appears not to fluctuate in such a c y c l i c manner, Instead, the 6 r e p e a t a b l e p a t t e r n i s an a n n u a l c y c l e i n numbers (McCabe a nd B l a n c h a r d , 1.950 and S a d l e i r , 1965). These two s p e c i e s , t h e r e f o r e , have c o n t r a s t i n g p o p u l a t i o n d y n a m i c p a t t e r n s , b u t i n d i v i d u a l s o f e a c h e x h i b i t t h e same i n t e r f e r e n c e phenomenon. In t h i s s t u d y , I have c o n c e n t r a t e d on one f e a t u r e o f t h e home r a n g e , n a mely i t s i m p o r t a n c e i n f o o d - g a t h e r i n g . T h e s e two s m a l l mammals d i f f e r i n t h e n a t u r e of t h e i r f o o d t y p e , E§£ojnyscus i s a n o m n i v o r o u s s e e d - e a t e r , e x p l o i t i n g a s p a t i a l l y and t e m p o r a l l y v a r i a b l e f o o d s u p p l y , M i c r o t i n e s , on t h e o t h e r h a n d , a r e g r a z e r s . T h e i r f o o d s u p p l y i n open g r a s s l a n d s a p p e a r s t o be f a r l e s s s p a t i a l l y a n d t e m p o r a l l y r e s t r i c t e d . McNab (1963) d e m o n s t r a t e d t h a t " c r o p p e r s " , f o r e x a m p l e g r a z e r s , had s m a l l e r home r a n g e s t h a n " h u n t e r s " s u c h as g r a n i v o r o u s s m a l l mammals. He a s c r i b e d t h i s t o t h e " c o n c e n t r a t i o n o f f o o d m a t e r i a l s " w i t h i n t h e h a b i t a t s u t i l i z e d by t h e s e g r o u p s . By s u p p l e m e n t a l f e e d i n g I have a t t e m p t e d t o m a n i p u l a t e t h e t e m p o r a l and s p a t i a l a v a i l a b i l i t y o f f o o d t o n a t u r a l p o p u l a t i o n s o f P g r o m y s c u s sanisalBiUS S J J s t e r u s • ( B a i r d ) a n d M i c r o t u s l o w n s e n d i i (Bachman) . I have measured t h e e f f e c t o f t h i s e x t r i n s i c m a n i p u l a t i o n o n t h e home r a n g e s and p o p u l a t i o n d y n a m i c s o f b o t h s p e c i e s . 7 PEROHYSCUS MANIC0LATUS 1. INTRODUCTION £e£2!y.§£!3§ shows an a n n u a l c y c l e i n numbers. The number o f m i c e r e a c h e s a peak a t t h e end o f t h e b r e e d i n g s e a s o n i n t h e f a l l o r e a r l y w i n t e r . When b r e e d i n g b e g i n s i n s p r i n g t h e r e w i l l be a s h a r p d e c l i n e i n numbers i f p o p u l a t i o n d e n s i t y i s s t i l l h i g h . T h i s w i l l n o t o c c u r i f p o p u l a t i o n d e n s i t y i s low a t t h e end o f w i n t e r ( P e t t i c r e w a n d S a d l e i r , 197 4) . The d e n s i t y o f m i c e t h e n r e m a i n s l o w t h r o u g h o u t t h e e n s u i n g summer b r e e d i n g p e r i o d . McCabe and B l a n c h a r d (1950) were t h e f i r s t t o s u g g e s t t h a t P e r o m y s c u s p o p u l a t i o n s m i g h t be c o n t r o l l e d by some f o r m o f i n t r i n s i c b e h a v i o u r a l m e chanism, S a d l e i r ( 1 9 6 5 ) , w o r k i n g on E* i s n i c u l a t u s , h y p o t h e s i z e d t h a t j u v e n i l e s s u r v i v e p o o r l y d u r i n g t h e summer b r e e d i n g s e a s o n b e c a u s e t h e y a r e c o m p e t i n g w i t h a g g r e s s i v e b r e e d i n g a d u l t s , He p r e d i c t e d t h a t t h e autumn r i s e i n p o p u l a t i o n i s a r e s u l t o f b e t t e r s u r v i v a l o f j u v e n i l e s , w h i c h may be c o r r e l a t e d w i t h a d e c l i n e i n a g g r e s s i v e n e s s o f a d u l t s . H e a l e y (1967) c o n f i r m e d t h i s c o r r e l a t i o n and d e m o n s t r a t e d , b o t h i n t h e l a b o r a t o r y and t h e f i e l d t h a t a d u l t m a l e a g g r e s s i o n r e d u c e s s u r v i v a l and g r o w t h of j u v e n i l e s . The c e n t r a l u n a n s w e r e d q u e s t i o n i s : what d e t e r m i n e s t h e number o f a n i m a l s t h a t e s t a b l i s h t h e s i z e o f t h e b r e e d i n g p o p u l a t i o n i n t h e s p r i n g ? Fordham (1971) was t h e f i r s t t o s u g g e s t t h a t t h e number o f male and f e m a l e P, m a n i c u l a t u s i n t h e b r e e d i n g p o p u l a t i o n may be 8 r e g u l a t e d d i f f e r e n t l y . He s u p p l i e d supplemental food t o £* maniculatus p o p u l a t i o n s from mid-February t o September, 1968, The number of females i n c r e a s e d , but the number of males remained the same, P e t t i c r e w and S a d l e i r (1974) i n c o r p o r a t e d t h i s r e s u l t i n t o t h e i r g eneral model of p o p u l a t i o n r e g u l a t i o n i n E» i§2iculatus > They propose that during the breading season the number of. males i s r e g u l a t e d i n an 'undetermined manner' by a g o n i s t i c behaviour. But they suggest t h a t the number of females i n c r e a s e s throughout the breeding season, In a recent paper, F a i r b a i r n (1977) suggested t h a t females t h a t bred e a r l y d i e d and so reduced the s p r i n g d e n s i t y of females. F i n a l l y , P e t t i c r e w and S a d l e i r (1974) suggest that a f t e r the i n i t i a l autumn i n c r e a s e i n d e n s i t y the numbers of both sexes of mice d e c l i n e s l o w l y f o r the l e n g t h of the non-breeding pe r i o d * A more ge n e r a l guestion i s : what determin es the onset and c e s s a t i o n of breeding i n the f i r s t placa ? T h i s c o u l d be a c o m p l i c a t i n g f ea t u r e because, as Petticrew an d S a d l e i r (1974) suggest, the length of the non- breeding seas on i t s e l f may have some i n f l u e n c e on the s i z e of the i p o p u l a t i o n that w i l l breed. The use of food items by Peromyscus v a r i e s t h e i r s e asonal a v a i l a b i l i t y (Jameson, 1952), A v a r i e t y of seads are taken i n the f a l l , winter and s p r i n g , then the d i e t i s expanded i n summer to i n c l u d e f r u i t s and animal m a t e r i a l , p a r t i c u l a r l y i n s e c t s , The o v e r a l l resource p a t t e r n i n the f o r e s t i s probably r e l a t i v e l y s t a b l e to Peromyscus p o p u l a t i o n s , But c e r t a i n food items, e s p e c i a l l y those t h a t p r o v i d e the energy for female l a c t a t i o n , are probably s p a t i a l l y and t e m p o r a l l y v a r i a b l e to i n d i v i d u a l mice. For example, the mast produced by t r e e s and the products 9 of f r u i t i n g shrubs and p l a n t s are s e a s o n a l l y r e s t r i c t e d and l o c a l l y abundant, S a d l e i r e t a l , (1973) suggest that r e p r o d u c t i o n ceases i n the f a l l because at normal winter temperatures so much energy i s r e g u i r e d f o r maintenance t h a t l a c t a t i o n energy c o s t s cannot be met, But a number of s t u d i e s (on Peromyscus - Jameson, 1955; and on Apodemus s y l y a t i c u s -Smyth, 1966 and Hansson, 1971) have shown that the breeding season has been extended i n t o the winter f o l l o w i n g e s p e c i a l l y abundant s u p p l i e s o f n a t u r a l food. The breeding season might, t h e r e f o r e , be c o n f i n e d to a p a r t i c u l a r p e r i o d of the year because of ambient temperature and food a v a i l a b i l i t y . E xperimental attempts t o e l u c i d a t e the r e l a t i o n s h i p between food supply and the onset and c e s s a t i o n of breeding have so f a r produced no r e a l l y c o n s i s t e n t p a t t e r n , B e n d e l l (1959) s u p p l i e d supplemental food t o a Peromyscus leucopus p o p u l a t i o n ; but found no e f f e c t on the l e n g t h of the breeding season, Fordham (1970) found that male P, maniculatus came i n t o breeding c o n d i t i o n a month e a r l i e r on an area s u p p l i e d with excess food than d i d c o n t r o l males. He a l s o had pregnant females on h i s food g r i d two months before they appeared on h i s c o n t r o l area. In a long-term study on A£odemus s y l v a t i c u s i n England, the s t a r t of the breeding season was advanced i n p o p u l a t i o n s with supplemental food by two to three weeks i n t h r e e out of f o u r y ears (Watts, 1970 and Flowerdew, 1972, 1973), However, the end of the breeding season i n these s t u d i e s was "hardly a f f e c t e d " by the a d d i t i o n of food* I designed my experiments to p r o v i d e a v a r i e t y of times a t which supplemental food was a v a i l a b l e to P, §anieulat us „ 1 0 S p e c i f i c a l l y , I was a t t e m p t i n g t o tsst t h e f o l l o w i n g h y p o t h e s e s : -H 1 : The b r e e d i n g d e n s i t y o f b o t h male and f e m a l e P. J L S a a i c u l a t u s w i l l i n c r e a s e i f f o o d i s added e a r l y i n t h e y e a r . H2: The t e m p o r a l and s p a t i a l a b u n d a n c e o f f o o d h a s a m a j o r e f f e c t on t h e o n s e t , i n t e n s i t y , and c e s s a t i o n o f b r e e d i n g i n P. m a n i c u l a t u s . 11 2, METHODS AND EXPERIMENTS St udy Area And T r a c i n g Technigngs A l l f i v e of my P, maniculatus p o p u l a t i o n s were l o c a t e d i n the U n i v e r s i t y Endowment Lands (Figure 2) . T h i s area o f f o r e s t i s i n the ' c o a s t a l douglas f i r ' zone ( K r a j i n a , 1969), Apart from the dominant Douglas f i r (Pseudotsuga m e n z i e s i i ) , t h e r e are some western red cedar (Thuja p l i c a t a ) and western hemlock (Tsuga h§tsrop_hylla) t r e e s . The undergrowth c o n s i s t s c h i e f l y of s a l a l ( G a u l t h e r i a s h a l l o n ) , with sword f e r n (Polystichum munitum) , salmonberry (Rubus s p e c t a b i l i s ) , and t r a i l i n g b l a c k b e r r y (Rubus ursinus) i n c e r t a i n areas. Each P, maniculatus p o p u l a t i o n was monitored i n a g r i d of 7X7 t r a p p i n g s t a t i o n s . The d i s t a n c e between t r a p s on each g r i d was 50 f t ( g r i d s 2,3 and 4) and 15 m ( g r i d s 1 and 5), so the area of each g r i d was approximately 0.84 hectares. G r i d s were trapped at two-weekly i n t e r v a l s with Long worth l i v e - t r a p s (grids 2, 3 and 4) and Sherman t r a p s ( g r i d s 1 and 5 up to November 1974) (see Appendix 1). Traps c o n t a i n i n g c o t t o n b a t t i n g and a l a r g e handful of whole oats were set l a t e i n the evening of the f i r s t day of each two-day t r a p p i n g p e r i o d . Mice were removed from the t r a p s e a r l y next morning and the t r a p s r e s e t f o r a second night. During the i n t e r i m non-trapping p e r i o d the t r a p doors were locked open, Each mouse captured f o r the f i r s t time was tagged with a numbered f i n g e r l i n g f i s h tag, On t h i s and a l l subsequent captures of the i n d i v i d u a l , i t s number, g r i d l o c a t i o n , and 12 F i q u r e 2. l o c a t i o n o f P_ raaniculatus g r i d s on the u n i v e r s i t y endowment l a n d s , Vancouver, B, C. Numbers c o r r e s p o n d t o those a s s i g n e d t o each g r i d , 14 weight to the nearest gram were recorded. The breeding condition was assessed as follows: males - testes position {abdominal or s c r o t a l ) , females - vaginal perforation and nipple size (small, medium or large) > A l l l i t t e r s in traps and obvious pregnancies were recorded, I^J9e£iSsnial Design The experimental treatment i s described here for each grid and summarized for a l l grids in Figure 4, Grid 1 This grid was run as a control throughout the period of my research. Fairbairn (1976) trapped the grid as her control during the f i r s t year from November, 1973 to October, 1974. She used a s l i g h t l y different trapping technigue than I did (see Appendix 1), I took the gr i d over in November, 1974 and trapped i t u n t i l the close of my study in August, 1975. Grid 2 I started trapping mice on this grid i n November, 1973, At the end of the second trapping period i n December, I put out twenty-five feeding stations, which were 80-fl oz opaque p l a s t i c containers containing a known weight of whole oats (see Figure 3) . At the end of each trapping period, that i s , once every two weeks, I measured how much food had been consumed from each station and added a weighed amount of fresh oats. This supplemental feeding was continued for one-and-a-half years. A l l 15 Fiqure 3, The arrangement of traps and food stations, X = trap s i t e 0 = trap s i t e • food station 16 A B c D E F G 1 X X X X X X X 2 X 0 0 0 0 Q X 3 X ' ' 0 Q 0 0 - 0 X A X 0 0 0 0 0 X 5 X 0 0 0 0 0 X 6 X 0 0 0 0 0 X 7 X X X X X X X 17 food s t a t i o n s were removed a t the end of June, 1975, The g r i d was trapped f o r a f u r t h e r month and c l o s e d i n J u l y 1975. G r i d 3 T h i s g r i d was a l s o set up i n November, 1973. I began supplemental f e e d i n g one month l a t e r than on G r i d 2. again t w e n t y - f i v e f e e d i n g s t a t i o n s were provided and whole oats was added to t h i s population. The same method of supplemental feeding was • c a r r i e d out on t h i s g r i d . Food s t a t i o n s were withdrawn a f t e r the t r a p p i n g period a t the end of August, 1974. The p o p u l a t i o n was monitored f o r a f u r t h e r f o u r t e e n weeks, then c l o s e d i n November 1974. G r i d 4 T h i s g r i d was e s t a b l i s h e d on June 26th, 1974. Supplemental food was addad i n t w e n t y - f i v e s t a t i o n s on August 9th, 1974. On October 18th, a f t e r ten weeks of supplemental f e e d i n g at t h i s l e v e l , the s t a t i o n s were reduced to nine f o r the next ten weeks ( s t a t i o n s were on rows: B, D, and F, at l o c a t i o n s : 2, 4, and 6-see Figure 3) . S t a t i o n s were f u r t h e r reduced to four on December 28th ( s t a t i o n s on rows: C and E, at l o c a t i o n s : 3 and 5), and r e t a i n e d at t h i s l e v e l f o r f o u r t e e n weeks. On A p r i l 18th, 1975 a l l four s t a t i o n s were removed, and the p o p u l a t i o n was monitored f o r a f u r t h e r s i x weeks. G r i d 5 Throughout the whole of the f i r s t year t h i s g r i d was run as 18 Figure 4, Summary of P_ maniculatus experimental design. Winter months are shaded, = food added ^ = food reduced = food removed 1973 1974 1975 N D J F M A M J J A S O N O J F M A M J J A grid 1* contro l 4 20 a c o n t r o l by F a i r b a i r n (1976), I added t w e n t y - f i v e f e e d i n q s t a t i o n s on November 15th, 1974. The g r i d r e c e i v e d supplemental food f o r the next f o u r months. I removed a l l s t a t i o n s on March 7th and monitored the p o p u l a t i o n f o r the next four months without a food supplement. On the 2 8th of June, 1975 I resumed feeding the p o p u l a t i o n from twenty-five s t a t i o n s and continued t h i s treatment u n t i l the c l o s e of my study on August 22nd, 1975. In Appendix 1, I have d e s c r i b e d events t h a t may have reduced my t r a p p i n g e f f i c i e n c y on each g r i d . 21 3, RESULTS Se c t i o n A: Tr_ap_p_abJ_lity_ And R e l i a b i l i t y . Qf The Data Krebs (1966) abandoned using c a p t u r e - r e c a p t u r e techniques f o r e s t i m a t i n g the p o p u l a t i o n s i z e of v o l e s because of the problems of non-random sampling. H i l b o r n at a l . , (1976) have shown that i n t e n s i v e t r a p p i n g can provide a c c u r a t e p o p u l a t i o n estimates provided the t r a p p a b i l i t y o f the p o p u l a t i o n i s f a i r l y high. T h e r e f o r e , the r e l i a b i l i t y of my population estimates as w e l l as the v a l i d i t y o f the demographic a n a l y s i s depends upon the t r a p p a b i l i t y of mice on each g r i d . In e s t i m a t i n g the t r a p p a b i l i t y of a p o p u l a t i o n , I f i r s t d i s c a r d e d any mouse with only one or two captures. The estimate f o r the remaining mice i s the number of times each i n d i v i d u a l was captured d i v i d e d by the number of times i t was p o t e n t i a l l y exposed to ca p t u r e during i t s l i f e on the g r i d , expressed as a percentage. The f i r s t and l a s t captures are always s u b t r a c t e d . For example, a mouse l i v i n g on the g r i d f o r e i g h t t r a p p i n q p e r i o d s was captured s i x times, i t s t r a p p a b i l i t y i s : -(6-2) X 100 = 61% (8-2) I have c a l c u l a t e d t h i s t r a p p a b i l i t y o f mice f o r each p o p u l a t i o n per month (see Appendix 2) . On both c o n t r o l g r i d s there was a c o n s i d e r a b l e drop i n t r a p p a b i l i t y (males and females on g r i d 1, females only on g r i d 5) i n January 1974. T h i s d i d not occur i n e i t h e r sex on the 22 supplemental food g r i d 2: but females d e c l i n e d on g r i d 3, which was not s u p p l i e d with supplemental food u n t i l the end of the month. P r a c t i c a l l y a l l other cases of low t r a p p a b i l i t y c o i n c i d e d e i t h e r with very c o l d weather or raccoon d i s t u r b a n c e (see Appendix 1), Cold weather, e s p e c i a l l y s n o w f a l l , may reduce the m o b i l i t y of mice. Also the Longworth t r a p mechanism may become fr o z e n during very c o l d n i g h t s , and hence reduce the t r a p p a b i l i t y of P, maniculatus, T r a p p a b i l i t y i n high d e n s i t y p o p u l a t i o n s tends to be lower than i n low d e n s i t y p o p u l a t i o n s (e.g. see grand t o t a l s f o r g r i d s 1 and 2 i n Table I ) . In 1974 males had a h i g h e r t r a p p a b i l i t y than females on f o u r out of f i v e g r i d s . But females were more tr a p p a b l e than males i n 1975 i n three out of f o u r g r i d s . T a b l e I. The t r a p p a b i l i t y o f ffianic u l a t us ir. each p o p u l a t i o n . Min inn?, number a l i v e i s q i v a n p a r e n t h e s e s . HL=males, FM-femaias. Season G r i d 1 G r i d 2 G r i d 3. G r i d 4 Grid- 5 Winter 1973-4 77 (26) 69 (20) 85 /29) 74 {35) 82(17) 37( 15) — -- 86(6) 67(3) S u i t i e : 1974 82{33) 75(31) 64( 53) 57(65; 85 (30) 9 1 (30) 6 9 ( 33) 63 (25) 8 1(24) 57(20) T o t a l 1974 80 (45) 72(36) 72 (64) 63(7«) 83 (38) 89 (35) 69 (33) 63 (25) 34 (27) 60 (25) Winter 1974-5 74<12) 68 (13) 65 (29) 77 (20) -- -- 64(32) 69( 19) 61 (22) 76 (2'i) 3uff.E3r 1975 73 ( 23) 79(23) 49(28) 65(31) -- -- 57 (20) 69 (23) 3 3 (29) 68(25) T o n a l 1975 . 73 (25) 75 (23) 55 (42) 70 (39) -- -- 62 (36) 69 (13)' 7 5 (41) 69 (32) Grand T o t a l 77 (65) 74 (53) 66 (97) 65 (I04) 33(38) 89 (35) 63( 52) 67(37) 73( 58) 67(49) 24 S e c t i o n B: P o p u l a t i o n Density H i l b o r n e t a l (1976) have shown that the r e l i a b i l i t y o f miniraium number a l i v e ( MNA ) as a p o p u l a t i o n measure drops o f f d r a m a t i c a l l y when t r a p p a b i l i t y f a l l s below 65%, At a t r a p p a b i l i t y of 65% and s u r v i v a l of 0.8, MNA g i v e s a 10% underestimate of p o p u l a t i o n s i z e . I have noted by an the p e r i o d s when the t r a p p a b i l i t y was egual to or lower than 65% on each P. maniculatus g r i d (see Appendix 3). T h i s means t h a t the e r r o r i n MNA i s -10% or worse i n each p o p u l a t i o n f o r those t r a p p i n g periods, Kale Dynamics Male numbers on the two c o n t r o l g r i d s are given i n Appendix 3, and p l o t t e d f o r g r i d 1 i n f i g u r e 5. A f t e r the autumn i n c r e a s e i n 1973, male d e n s i t y d e c l i n e d : on g r i d 1 from 25 to 9 and on g r i d 5 from 16 t o 7. When breeding began i n June 1974, t h e r e were 12 males on g r i d 1 and 7 on g r i d 5. Both c o n t r o l g r i d s showed a s l i g h t autumn i n c r e a s e , but numbers on g r i d 1 then continued to d e c l i n e The a d d i t i o n of food at the end of December 1973, c l e a r l y r e s u l t e d i n the male p o p u l a t i o n on g r i d 2 i n c r e a s i n g r a p i d l y from 19 to 33 by the end of A p r i l (Figure 5). Food had a s i m i l a r e f f e c t on g r i d 3, where i t was added a month l a t e r ; the male pop u l a t i o n here doubled i n a month (Appendix 3), Through May and i n t o June 1974, males d e c l i n e d on both g r i d s i n s p i t e ' of there being superabundant oats. The p o p u l a t i o n on g r i d 2 then remained f a i r l y s t a b l e through the f a l l and wintsr but at a higher 25 F i g u r e 5 Number o f male and f e m a l e m i c e on g r i d 1 and g r i d 2. S i n t e r months a r e s h a d e d . u p p e r g r a p h s = g r i d 1 ( c o n t r o l ) l o w e r g r a p h s = q r i d 2 ( l o n g - t e r m f o o d ) A - f o o d a d d e d t o g r i d 2 d o t t e d l i n e = m a l e s s o l i d l i n e = f e m a l e s MINIMUM NUMBER 27 d e n s i t y than that of the c o n t r o l s (see Table I I ) , In the l a s t t r a p p i n g i n October, male d e n s i t y dropped from 20 to 14: t h i s was probably the r e s u l t o f a raccoon i n v a s i o n (see Appendix 1), At the end of February the f o l l o w i n g year, t h e number of males on g r i d 2, which was s t i l l r e c e i v i n g supplemental food, was 22 compared with 12 at t h i s time on the c o n t r o l . A l s o on g r i d 5 with food t h i s year, there were 16 compared with 9 males in the p r e v i o u s s p r i n g without food (Appendix 3), Through March to June 1975, t r a p s i n the g r i d 2 p o p u l a t i o n were d i s t u r b e d by raccoons and cats, These predators and the i n f l u x of M. oregoni may have r e s u l t e d i n the reduced d e n s i t y of P. maniculatus on g r i d 2 t h i s summer. Gr i d 4 has an i n t e r e s t i n g p a t t e r n of male dynamics over the 1974 winter p e r i o d compared with that on the c o n t r o l (see Figure 6). When food was added in t w e n t y - f i v e s t a t i o n s a t the beginning of August, male numbers rose from 15 to 23 by mid-October, while males on the c o n t r o l i n c r e a s e d only from 9 to 12, With the r e d u c t i o n i n food s t a t i o n s at t h i s time t o nine, males went out of breeding c o n d i t i o n on g r i d 4, and numbers rose to a winter peak of 27 compared with 14 on the c o n t r o l . A f t e r food was r e s t r i c t e d to f o u r s t a t i o n s a t the end of December, males d e c l i n e d from 27 to 19 by the end of A p r i l , w h i l e the c o n t r o l i n c r e a s e d from 14 to 18, Then the food was completely removed, and males d e c l i n e d by n e a r l y 50% (from 19 to 11) over the f o l l o w i n g s i x weeks t h a t g r i d 4 was trapped. A s p r i n g i n f l u x of I " oregoni to a l l g r i d s may have reduced the number of P. maniculatus a l i t t l e (see S e c t i o n H) . But the e a r l y s t a r t and long d u r a t i o n of the d e c l i n e on g r i d 4 alone suggests t h a t once Table II. The seaa density of £=_ sanicu la tug on each q r i d over three month i n t e r v a l s i n 1974-75. ML= E a l e s , FM=feaales. M o n t h s G r i d J 2 Grid 3 SEiS. i Gri d- 5 I i i ML ilL FK HL FM Jlov- Dec 1973 19. g 13.8 17.2 17. 4 11.0 6.3 1 4. 2 20.0 Jan- Kar 1 974 18 . 0 16.2 26.0** 31.2** 12.3* 12.5* 9.0 12. 8 Apr-Jun ti 13. 6 15.7 26.0** 27.0** 10.4** 13.6** -- 7.4 11.43 Jul-Sep ti 10. 5 15. 2 19.1** 1 6.6** 3.4* 7.8* 16.0* 14.2* 7. 8 9. 0 Oct-Dec tt 10. 1 11.9 17.3** 16.7** 1 2. 3 8.3 22. 5* 18. 3* 1 2. 4* 12. 3* .Jan-Kar 1975 12. 8 13.2 18.5** 15.7** 22.5* 15.3* 15.8* 18. 0* kpr-Jun it 17. 4 15. 1 17. 1** 1 8. 0** -- 16.2 10.6 16.6 18. 0 Jul-Auq it 10. 3 13.3 14. 0 7. 5 — — — — 13.0** 14.3** f u l l complement cf food for the whole period * = p a r t i a l complement or fed for only part of the period OO 29 F i q u r i 6. Number o f m i c e on q r i d 1 and q r i d U, W i n t e r months a r e s h a d e d , u p p a r q r a p h - q r i d 1 ( c o n t r o l ) l o w e r q r a p h = q r i d H ( r e d u c e d - f o o d ) A 1 = f o o d i n 25 s t a t i o n s 2 = f o o d r e d u c e d t o 9 s t a t i o n s 4 3 - f o o d r e d u c e d t o 4 s t a t i o n s if'' U = f o o d r e m o v e d d o t t e d l i n e = m a l e s s o l i d l i n e = f e m a l e s o 31 the food was reduced to 4 s t a t i o n s , t h i s was i n s u f f i c i e n t t o maintain the high d e n s i t y p o p u l a t i o n through the s p r i n g , £§J§I§ SlH§lic§ Female mice on the c o n t r o l g r i d s reached winter p o p u l a t i o n peaks i n December 1974 (Appendix 3), On g r i d 1, females maintained f a i r l y s t a b l e numbers through the s p r i n g 1974 (Figure 5) , while females on g r i d 5 d e c l i n e d , On both g r i d s the lowest numbers were reached when the breeding season was f u l l y underway i n l a t e summer 1974, A f t e r food was added to g r i d 2, females i n c r e a s e d and reached peak numbers mere r a p i d l y than d i d males (Figure 5) , On g r i d 3 females doubled t h e i r numbers f i v e weeks a f t e r the food was added at the end of January 1974 (see Appendix 3). As i n males, female numbers d e c l i n e d over the summer on both food g r i d s . The mean winter p o p u l a t i o n of females on the c o n t r o l was 11.9 compared with 16. 7 on g r i d 2 with food (Table I I ) . T h i s was i n s p i t e of a r e d u c t i o n i n females from 22 t o 15 f o l l o w i n g the raccoon i n v a s i o n (Appendix 1). On t h i s long-term food g r i d s p r i n g numbers peaked at 22 at the end of March compared with 16 on the c o n t r o l , As with males, female s p r i n g d e n s i t i e s i n 1975 on g r i d 2, although higher than those on the c o n t r o l , are lower than those i n 1974. As suggested, t h i s may have r e s u l t e d from raccoon or c a t d i s t u r b a n c e and the immigration of JL oregoni., A l s o , 1975 was a more n e a r l y normal breeding year on the c o n t r o l (see S e c t i o n C ) , which suggests that n a t u r a l food c o n d i t i o n s may have been b e t t e r i n 1975. A l s o , immigration to the c o n t r o l was 3 2 higher and c l o s e r to the r a t e f o r g r i d 2 t h i s year (see S e c t i o n E) , On g r i d 5, which was s u p p l i e d with food i n twenty-five s t a t i o n s i n November 1974, females i n c r e a s e d to 21 by the end of March 1975 compared with 11 i n March 1974. By c o n t r a s t , females on g r i d 4 s t a r t e d to d e c l i n e when the food s t a t i o n s were reduced to nine on October 1974 and continued to d e c l i n e throughout the s p r i n g (see F i g u r e 6). Summary The a d d i t i o n of food e a r l y i n the s p r i n g r e s u l t e d i n the numbers of both sexes d o u b l i n g or nearly doubling on the experimental g r i d s 2 and 3. The mid-winter a d d i t i o n of food to g r i d 5 reversed the normal slow winter d e c l i n e i n numbers,. The response of both sexes t o the removal of food v a r i e d a c c ording to the time of year. Spring removal o f food ( g r i d s 4 and 5) tended to cause a r e d u c t i o n i n both sexes. The summer removal from g r i d 2 had a s t r o n g e f f e c t on females, which d e c l i n e d from 20 to 6 i n f o u r weeks. Late f a l l removal (grid 3) caused animals to go out of breeding c o n d i t i o n (see S e c t i o n C) and proceed i n t o a normal f a l l i n c r e a s e . There was a tendency f o r both sexes on a l l g r i d s to d e c l i n e i n the summer o f 1975 as M, oregoni immigrated (see S e c t i o n H),, 33 Sect ion C: Breeding Season D e n s i t i e s change by b i r t h s , deaths, and movements. In t h i s s e c t i o n I assess the e f f e c t s of supplemental fe e d i n g on the p o t e n t i a l reproductve e f f o r t of the populations, I have defined the breeding season based on the e x t e r n a l examination of mice captured i n the f i e l d a t each two-week t r a p p i n g p e r i o d . I considered a l l males and females weighing 17g or more as a d u l t s (see Appendix 4). A breeding a d u l t male has s c r o t a l t e s t e s and a breeding female has medium or l a r g e n i p p l e s . I d e f i n e d the breeding season as the p e r i o d when 50% or more of the a d u l t males or females are i n breeding c o n d i t i o n . In Appendix 5, I present a t a b l e of the percentage of a d u l t males and females breeding i n each t r a p p i n g p e r i o d i n a l l f i v e P. maniculatus p o p u l a t i o n s . Food And The Onset And C e s s a t i o n Of Breeding I was i n t e r e s t e d i n measuring the e f f e c t of supplemental food on the onset, i n t e n s i t y , and extent of the breeding season on food g r i d s 2 and 3 compared with events on the c o n t r o l g r i d 1, No males were breeding on any g r i d s i n November or December 1973. Food was added to g r i d 2 a f t e r the second t r a p p i n g p e r i o d i n December 1973.. One month l a t e r 5 males had s c r o t a l t e s t e s , and s i x weeks l a t e r over 50% of the males were breeding i n t h i s p o p u l a t i o n . Food was s u p p l i e d one month l a t e r to g r i d 3. In the f o l l o w i n g two-week tr a p p i n g p e r i o d over 50%' of the males had s c r o t a l t e s t e s . T h i s l e v e l of breeding i n t e n s i t y was not reached on the c o n t r o l u n t i l the beginning of June, four months l a t e r 34 than on the food g r i d s * Males i n the c o n t r o l p o p u l a t i o n ceased breeding completely at the end of August 1974, Two males on g r i d 2, and a l l four males on g r i d 3 were s t i l l breeding'. The food was removed from g r i d 3 d u r i n g t h i s t r a p p i n g p e r i o d , and no more males were recorded breeding here, I was unable to t r a p the experimental g r i d s i n the f i r s t week of September or feed the g r i d 2 p o p u l a t i o n . P r a c t i c a l l y a l l the food s t a t i o n s were empty when I came to r e f i l l them at the end of the month. Male r e p r o d u c t i o n had ceased completely a t t h i s time on g r i d 2, but s t a r t e d again the f o l l o w i n g p e r i o d and continued f o r the r e s t of the study. Apart from one male that was s c r o t a l on the c o n t r o l i n October, no animals resumed breeding on t h i s g r i d u n t i l December. The onset of the breeding season on the c o n t r o l was much e a r l i e r i n 1975, over 50% of the males were s c r o t a l by the end of February. In f a c t , a p a r t from a drop at the beginning of May, the pattern of r e p r o d u c t i o n on the c o n t r o l g r i d i n 1975 was very s i m i l a r to t h a t on the long-term food g r i d . The p a t t e r n of breeding i n females was very s i m i l a r to that i n males. One month a f t e r the a d d i t i o n of food to g r i d 2, females were l a c t a t i n g i n January (see Appendix 5). On g r i d 3 females were l a c t a t i n g s i x weeks a f t e r food a d d i t i o n . In both cases females had p e r f o r a t e vaginas i n the t r a p p i n g p e r i o d immediately f o l l o w i n g food a d d i t i o n . One female was found l a c t a t i n g i n March on the c o n t r o l , but no others were found u n t i l J u l y . Females on the c o n t r o l bred f o r only one month compared with f i v e months on g r i d 2. Breeding stopped i n females on a l l three g r i d s f o r the whole of October, No breeding was 35 recorded on g r i d 3 once the food was removed, but females on food g r i d 2 s t a r t e d breeding i n November, and more than 50% were l a c t a t i n g by the end of January, L i k e the males on g r i d 1, females bred much e a r l i e r on the c o n t r o l i n 197 5 than i n 1974, The i n t e n s i t y of breeding dropped below 50% i n flay on the c o n t r o l while females on g r i d 2 continued at 50% or more. There was no response i n e i t h e r males or females to the removal of food from g r i d 2 in June 1975, Food And Winter Breeding On g r i d 5, I was i n t e r e s t e d t o see i f a p o p u l a t i o n would s t a r t to breed i f food were provided i n the middle of winter. Secondly, I wanted to see i f the breeding c o n d i t i o n i n l a t e s p r i n g would be a f f e c t e d by removal and l a t e r r e - a d d i t i o n of the supplemental food, G r i d 5 was run as a c o n t r o l from November 1973 to September 1974, The p a t t e r n of r e p r o d u c t i o n was very s i m i l a r t o that on g r i d 1 (see Appendix 5), I added food to t h i s p opulation a t the beginning of November 1974, Males became s c r o t a l and females p e r f o r a t e by the next t r a p p i n g period. Some females were l a c t a t i n g at the end of December, I removed food frcm t h i s p o p u l a t i o n i n e a r l y March, Male r e p r o d u c t i o n d e c l i n e d s l i g h t l y , but the number of females l a c t a t i n g dropped from 8 to 1 two months a f t e r food s t a t i o n s - w e r e removed. Over t h i s period female l a c t a t i o n s on the c o n t r o l d e c l i n e d from 7 to 3 and d i d not change on the long-term food g r i d . I r e - i n t r o d u c e d food to the g r i d at the end of June but n e i t h e r sex responded n o t i c e a b l y . On g r i d 4, I was i n t e r e s t e d i n s e e i n g how many feeding 36 s t a t i o n s were needed to prevent a c e s s a t i o n of breeding i n winter, Male breeding had almost ceased when I added food i n twenty-five s t a t i o n s a t the beginning of August, then ceased when the food ran out i n September (see Appendix 5), Three males became s c r o t a l i n October, but ceased breeding a f t e r the number of food s t a t i o n s was reduced to nine at the end of the month. Female l a c t a t i o n i n c r e a s e d from 43% to 82^ l a c t a t i n g a f t e r the food was added i n t w e n t y - f i v e s t a t i o n s . But a t the end of October and through November only one female was pregnant and l a c t a t i n g ; she was trapped twice out of a p o s s i b l e three times at one of the nine food l o c a t i o n s on the g r i d * By the time the food s t a t i o n s were reduced t o f o u r at the end of December, no mice were breeding on g r i d 4, . I§iljjher I S i JE§§ili2.3 The breeding performance of males and females on the c o n t r o l g r i d d i f f e r e d i n t h e onset and i n t e n s i t y of breeding i n 1974 and 1975 (see F i g u r e 7), February 1974 had c l o s e to the average mean temperature, p r e c i p i t a t i o n , and hours of sunshine (see Appendix 5), But March had double the normal p r e c i p i t a t i o n f o r the month;"this continued through a c o o l , r a t h e r cloudy A p r i l . Some males d i d become r e p r o d u c t i v e on the c o n t r o l s , but only one female was recorded breeding a t the end of March, May 1974 had the lowest average temperature ever recorded f o r t h i s month i n Vancouver,, May a l s o had two-and-a-half times the normal r a i n f a l l and 41 hours below the normal sunshine f o r the month. Reproductive a c t i v i t y h a l t e d i n males, and no l a c t a t i n g females were caught on the c o n t r o l s by the end of May, The weather 37 F i g u r e 7, Summary of breeding season data f o r g r i d s 1, 2, 3, and 5, Winter months are shaded. = food added = food removed wide l i n e = middle l i n e = narrow l i n e = 50% or more a d u l t s breeding l e s s than 50% of a d u l t s breeding no breeding 1973 1974 1975 N D J F M A M J J A S O N D ' J F M A M J J A 0 0 39 Improved through June when breeding began, In c o n t r a s t , October se t a new r e c o r d f o r the hours of sunshine. S i m i l a r l y , December was much milder than normal. One male on g r i d 1 had s c r o t a l t e s t e s i n October, In December four males had s c r o t a l t e s t e s and three females had p e r f o r a t e vaginas i n the c o n t r o l population, The s p r i n g of 1975 was not much warmer than that of 1974, but the hours of sunshine.and the t o t a l p r e c i p i t a t i o n were much c l o s e r t o normal. Summary The a d d i t i o n of supplemental food to g r i d s 2 and 3, at the end of December 1973 and the end of January 1974 r e s p e c t i v e l y , enabled mice on these g r i d s t o breed e a r l i e r and f o r much longer i n s p i t e of the poor weather i n 1974 (see F i g u r e 7), Breeding stopped on the c o n t r o l g r i d a t the beginning o f August 1974. I t ceased completely on g r i d 3 when food s t a t i o n s were removed at the end of the month and te m p o r a r i l y on g r i d 2 when the food s t a t i o n s were emptied. Otherwise mice on g r i d 2 with food i n twe n t y - f i v e s t a t i o n s continued to breed throughout the winter. Both sexes on g r i d 5 were stim u l a t e d to breed immediately a f t e r food was added to t h i s g r i d i n mid-winter 1 9 7 4 T h e r e s u l t s f o r g r i d 4 with regard to a t h r e s h o l d l e v e l of food abundance f o r winter breeding i n P, maniculatus are not c l e a r , But when the food s t a t i o n s were reduced from twenty-five to nine, breeding ceased i n t h i s p o p u l a t i o n during the winter of 1974. 40 S e c t i o n D: Breeding Success Breeding on the food g r i d s began f o u r months e a r l i e r than on the c o n t r o l s i n 1974. The d u r a t i o n and i n t e n s i t y of breeding on these g r i d s was a l s o much gr e a t e r than on the c o n t r o l s {see S e c t i o n C) . Did t h i s g r e a t e r r e p r o d u c t i v e e f f o r t r e s u l t i n a higher p r o d u c t i o n of young mice? Es t i m a t i n g the recruitment of young mice to a p o p u l a t i o n , p a r t i c u l a r l y i n the breeding season, i s d i f f i c u l t . F i r s t , a c c o r d i n g to Sadleir»s hypothesis (1965), male P. maniculatus are a g g r e s s i v e to a l l j u v e n i l e s . Secondly, young may be present i n the p o p u l a t i o n but ' p a s s i v e l y ' prevented from e n t e r i n g the t r a p s because l a r g e r mice are caught f i r s t . I h e l d a l l mice caught on g r i d s 1 and 5 d u r i n g the f i r s t n i g h t i n the August 20-22 t r a p p i n g s e s s i o n . T h i s meant t h a t a l l 49 t r a p s were a v a i l a b l e to any mice remaining on these g r i d s . In Table I I I , I compare the number of new subadult and j u v e n i l e mice caught i n t h i s t r a p p i n g p e r i o d with those caught i n the preceeding period i n August 6 - 8th. On each g r i d the t o t a l number of new young mice was much higher i n the 'removal' period (August 20 - 22nd) compared with the previous 'normal* t r a p p i n g period (August 6 - 8th). There were the same number cf breeding males on g r i d 1 i n both t r a p p i n g periods. F u r t h e r , on g r i d 5 the number of breeding males i n c r e a s e d to 6 out of 6 compared with 4 out of 5 i n the August 6 - 8th p e r i o d . T h e r e f o r e , there was presumably no decrease i n r e p r o d u c t i o n - r e l a t e d a g g r e s s i v e behavior of a d u l t males between the two t r a p p i n g periods. In a d d i t i o n , 9 out of 16 ( g r i d 5), and 7 out of 14 ( g r i d 1) new young mice i n the August 20 - 22nd perio d were caught i n the f i r s t check. T h i s suggests t h a t the second f a c t o r , namely the 41 Table I I I . New young mice caught on g r i d 1 and g r i d 5 i n the l a s t two t r a p p i n g periods. August 6-8 August 20-22 Sub-ad. Juv. T o t a l Sub-ad, Juv, T o t a l G r i d 1 1 1 2 8 6 14 G r i d 5 2 4 6 10 6 16 42 • p a s s i v e 1 e x c l u s i o n from t r a p s , was not the reason f o r the low ca t c h of j u v e n i l e s i n August 6 - 8th, Two r e c e n t l a b o r a t o r y s t u d i e s on Pergmyscus suggest t h a t maternal a g g r e s s i o n may be important i n the d i s p e r s a l of young mice, Rowley and C h r i s t i a n (1976) found t h a t l a c t a t i n g female P. leucopus had high l e v e l s o f aggression, and t h a t they, not males, were a g g r e s s i v e towards t h e i r own young, Savidge (1974) working with P, §aniculatus b a i r d i , found that a female's aggression towards her young was manifest only i f the female was pregnant with a subseguent l i t t e r . I f these r e s u l t s apply to f i e l d p o p u l a t i o n s , then r e p r o d u c t i v e females may have i n f l u e n c e d the observed changes i n the recruitment of young mice between the two t r a p p i n g p e r i o d s i n August 19 75. On g r i d 1, f o u r females were l a c t a t i n g and two of these plus two other females were pregnant i n the August 6 - 8th p e r i o d , but i n the f o l l o w i n g p e r i o d no females appeared to be pregnant although f i v e were l a c t a t i n g , On g r i d 5 no females i n e i t h e r p e r i o d were pregnant, but f i v e were l a c t a t i n g i n August 6 - 8th compared with only two on August 20 - 22nd, An index of r e p r o d u c t i v e output i s the number of j u v e n i l e s ( a l l mice weighing 13 g or l e s s ) r e c r u i t e d to each g r i d . In 1974 61 j u v e n i l e s were captured on g r i d 2 with supplemental food compared with o n l y 17 on t h e c o n t r o l (Table IV). Food was removed from g r i d 3 i n August 1974, yet t h i s g r i d produced 43 j u v e n i l e r e c r u i t s . Reproductive output on the c o n t r o l i n the f i r s t h a l f of 1975 was double t h a t of 1974. But g r i d 5, which was r e c e i v i n g food f o r the f i r s t two months of 1975, had 4 times the number of j u v e n i l e s i t had i n the previous c o n t r o l year. T a b l e IV. J u v e n i l e s r e c r u i t e d i n t o each po p u l a t i o n i n each six-month p e r i o d . 1974 1975 Jan-Jun Jul-Dec Jan-Jun G r i d 1 8 9 18 G r i d 2 36** 25** 27** G r i d 3 22* 21* Grid 4 — 2 0* 16* G r i d 5 7 19* 29* ** = g r i d r e c e i v i n g food throughout p e r i o d * = g r i d r e c e i v i n g food f o r part o f p e r i o d 44 The o v e r a l l breeding success was higher on g r i d s with supplemental food, but a l a r g e r number of a d u l t s were breeding on these g r i d s (Appendix 5). Was j u v e n i l e s u r v i v a l i n c r e a s e d ? I estimated e a r l y j u v e n i l e s u r v i v a l by counting the number of j u v e n i l e s r e c r u i t e d at time t , then d i v i d i n g t h i s by the number of females l a c t a t i n g a t time t-4 weeks (Krebs, 1966) . This index assumes t h a t a l l young from a l i t t e r are r e c r u i t e d at a j u v e n i l e weight, so i t i g n o r e s those t h a t may be r e c r u i t e d l a t e r as subadults or a d u l t s , In the f i r s t h a l f of 1974 e a r l y j u v e n i l e s u r v i v a l was h i g h e s t on g r i d 2, which was r e c e i v i n g supplemental food over the whole p e r i o d (Table V) . The e a r l y s u r v i v a l o f j u v e n i l e s i n the f a l l - winter 1974 was higher on g r i d s 2, 3, 4, and 5 than on the c o n t r o l . A l l t h e s e g r i d s were r e c e i v i n g some l e v e l of supplemental food over t h i s period. Summary The o v e r a l l breeding success, i n terms of the number of j u v e n i l e s r e c r u i t e d , was i n c r e a s e d by the a d d i t i o n of food. During the breeding season, t h i s was c h i e f l y a r e s u l t of the much higher r e p r o d u c t i v e a c t i v i t y on the supplemental food g r i d s . E a r l y j u v e n i l e s u r v i v a l i n the f a l l and winter was higher i n p o p u l a t i o n s t h a t r e c e i v e d some l e v e l of supplemental food. But e a r l y j u v e n i l e s u r v i v a l d u r i n g the summer was low i n the high d e n s i t y p o p u l a t i o n s o f breeding a d u l t s . as Table V. Index of e a r l y j u v e n i l e s u r v i v a l . The number of l a c t a t i n g females i s given i n paretheses. 1974 1975 Jan-Jun Jul-Dec Jan-Jun G r i d 1 0.00 (1) 0, 22 (27) 0. 38(34) Gri d 2 0. 26 (87) ** 0.40(43)** 0. 20 (86)* G r i d 3 0. 0 1 (41) * 0.54(13)* G r i d 4 — 0. 38 (21) * 0.35(26) * G r i d 5 0. 00(4) 0,45 (20) * 0, 36 (53)* ** = g r i d r e c e i v i n g food throughout p e r i o d * = g r i d r e c e i v i n g food f o r part of p e r i o d S e c t i o n E: Immigration And S u r v i v a l The f i r s t j u v e n i l e s were r e c r u i t e d t o the supplemental food g r i d 2 at the end of March 1974. The p o p u l a t i o n of males on t h i s g r i d had a l r e a d y i n c r e a s e d from 19 to 31, and females from 23 to 37 i n response to the a d d i t i o n of food. T h e r e f o r e , the r a p i d i n c r e a s e i n numbers roust have r e s u l t e d from the immigration of mice onto g r i d 2. Immigration I measured immigration as the t o t a l number of new mice captured d u r i n g each two-week t r a p p i n g period. In the f i r s t h a l f of 1974 number of immigrants onto g r i d 2 was much high e r than onto the c o n t r o l (Figure 8). Females in p a r t i c u l a r showed a high l e v e l of immigration: by the end of June 1974, a t o t a l of 68 had entered the g r i d 2 p o p u l a t i o n compared with 15 on the c o n t r o l . Immigration to the c o n t r o l i n c r e a s e d d u r i n g J u l y - September 1974 and was o n l y s l i g h t l y below that to g r i d 2» But through the winter immigration to g r i d 2 r o s e to 54 mice compared with only 20 on the c o n t r o l . T o t a l immigration to g r i d 5 was only 22 i n the winter of 1974/5, But when food was present overwinter i n 1975/6, immigration r o s e to 69 mice. The p a t t e r n of immigration of the two sexes was d i f f e r e n t (Table ¥ 1 ) , On both g r i d s 1 and 2 the sex r a t i o of immigrants was p r a c t i c a l l y even i n the s p r i n g and l a t e summer. But female immigrants predominated i n e a r l y summer and males d u r i n g the winter. T h i s r e s u l t e d i n an even immigrant sex r a t i o ( g r i d 1 65:59, g r i d 2 154:162). The age d i s t r i b u t i o n of immigrants of HI Figure 8, Immigration of mice t o summed f o r each three-month dotted l i n e = g r i d s o l i d l i n e = g r i d g r i d 1 and g r i d 2, The data are pe r i o d of the study. 1 (control) 2 (long-term food) f em a l es males J F M A M J J A S O N D J F M A M J 1974 1975 49 Table VI. The sex and age of immigrants to g r i d s 1 and 2 i n each three-month p e r i o d . The r a t i o s are the number of males:females, Ad. Sub-ad, Juv, Ad. Sub-ad. , Juv, Jan-Mar 1974 1:1 4: 2 2:3 16:8 4: 9 3: 10 Apr-Jun " 3:1 2:6 1:2 10:18 10:11 11:12 Jul-Sep " 7:6 3:5 3:4 9:5 5: 9 7:9 Oct-Dec « 6:0 10:2 0:2 17:3 14:11 4:5 Jan-Mar 1975 5:3 3:3 4:6 3:6 11:6 6:9 Apr-Jun « 5:4 3:4 3:5 12:14 6:11 6:6 T o t a l 27:15 25:22 13:22 67:54 50:57 37:51 50 each sex i s a l s o given i n Table VI. There are s i g n i f i c a n t l y more females among j u v e n i l e r e c r u i t s (chi sguare, 4.30, p<0.05). The sex r a t i o i s even i n mice immigrating to the sub-adult age c l a s s . But males tend to predominate in the a d u l t c l a s s ( c h i sguare, 3.83, 0„ 05<p<0.1) . T h e r e f o r e , females tend to immigrate as younger mice and males to immigrate l a t e r as a d u l t s . In the winter p e r i o d , when the immigrant sex r a t i o i s skewed towards males, we see that the age s t r u c t u r e a l s o r e f l e c t s t h i s trend, a l l male immigrants on the c o n t r o l and 31 out of 35 on g r i d 2 are i n the a d u l t or sub-adult age c l a s s i n October-December. S u r v i v a l Immigration to the long-term food g r i d was higher than that to the c o n t r o l , but the de n s i t y of mice on t h i s g r i d d e c l i n e d . I f m o r t a l i t y or 'emigration was higher on the food g r i d s , the changes i n d e n s i t y might be e x p l a i n e d . at present, we cannot separate emigration from death i n mice that cease being trapped. A l l we can c a l c u l a t e i s the r e t u r n of marked mice each t r a p p i n g p e r i o d . The value o f t h i s s u r v i v a l estimate i s c l o s e l y dependent on the t r a p p a b i l i t y of mice, s i n c e only mice caught i n co n s e c u t i v e t r a p p i n g p e r i o d s can be used i n the c a l c u l a t i o n , I c a l c u l a t e d the B e a n minimum s u r v i v a l per 14 days f o r each three-month p e r i o d o f the study, and p l o t t e d the r e s u l t s f o r a d u l t males and females i n Figure 9. In the j u v e n i l e and sub-adult age c a t e g o r i e s , the s u r v i v a l of males and females was very s i m i l a r , so I have combined the sexes f o r these two age groups (Table V I I ) . The s u r v i v a l o f a d u l t males and females on both the c o n t r o l 51 F i g u r e 9„ Minimum 14 d a y - s u r v i v a l of a d u l t s on g r i d 1 and g r i d 2. The h o r i z o n t a l l i n e i s drawn at survival=0.75. dotted l i n e = g r i d 1 (control) s o l i d l i n e = g r i d 2 (long-term food) females males 0.4 L ' T — ' — 1 1 JFM AMJ JAS OND JFM AMJ 197 4 1 975 53 and the long-term food g r i d i s high i n January-March i n both years, adul t female s u r v i v a l d e c l i n e d i n summer then rose t o 100% i n October to November on the c o n t r o l . a d u l t female s u r v i v a l on the food g r i d was much lower than t h a t on the c o n t r o l i n summer, but t r a p p a b i l i t y was lower and more females were breeding on g r i d 2. The high winter s u r v i v a l of females on both g r i d s c o i n c i d e s with the lowest r a t e of female immigration (see F i g u r e 8). The lower s u r v i v a l of the g r i d 2 females may be a s s o c i a t e d with t h e i r g reater r e p r o d u c t i v e e f f o r t ; they may be dying, or e m i g r a t i n g , or both. Myers and Krebs (1971) found t h a t a high p r o p o r t i o n of a d u l t female Micrgtus JBasnsylyanicus that moved i n t o a removal area i n summer was i n breeding c o n d i t i o n . Adult male s u r v i v a l a l s o d e c l i n e d i n summer, but, i n c o n t r a s t to females, i t remained low on both g r i d s through October to November. T h i s low male s u r v i v a l i n e a r l y winter c o i n c i d e s with the period of maximum immigration of males onto the g r i d s . E x c l u d i n g the winter months, the s u r v i v a l of j u v e n i l e and sub-adult mice i s low i n summer when most a d u l t s are i n r e p r o d u c t i v e c o n d i t i o n (see Table VII) . I f the October-December samples are not i n c l u d e d , t h e r e i s a weak negative c o r r e l a t i o n (r--0,76) between the combined s u r v i v a l of j u v e n i l e s and sub-adults and the number o f l a c t a t i n g females on g r i d 1, A st r o n g e r r e l a t i o n s h i p (r=-0. 93) e x i s t s on the supplemental food g r i d , i f the October-December peri o d i s excluded,, In c o n t r a s t , t h e r e i s no r e l a t i o n s h i p (r=0.18) between the s u r v i v a l of young and the number of males with s c r o t a l t e s t e s on the c o n t r o l but a weak one (r=-0,71) on g r i d 2. In s p i t e o f the negative e f f e c t of 54 Table VII. Mean minimum 14-day s u r v i v a l o f sub-adult and j u v e n i l e mice on g r i d s 1 and 2. Sample s i z e i s i n parenthses. Lfm i s the mean number of l a c t a t i n g females, Sm i s the number of s c r o t a l males. Gr i d 1 ( c o n t r o l ) G r i d 2 (food) Sub-ad,, Juv, Lfra/Sm Sub-ad, Juv, Ifm/Sm Jan-Mar 1974 0.87 0. 92 (63) (12) 0.0/0.5 0.88 (56) 0. 74 (19) 4.0/10.? Apr-Jun 0. 93 C69) 0. 50 (6) 0,0/3,9 0.65 (34) 0. 48 (25) 10.9/15. 9 Jul-Sep 0,62 (21) 0. 50 (8) 5. 2/2, 5 0.80 (45) 0,90 (20) 4, 6/4, 6 Oct-Dec 0. 74 (42) 0.75 (4) 0.0/0.1 0.58 (74) 0. 70 (10) 1, 6/4. 3 Jan-Mar 1975 0,92 (13) 0.7 3 (11) 3, 4/4,2 0.82 (33) 0, 88 (16) 6.0/4.7 Apr-Jun 0. 92 (37) 0. 67 (15) 3.0/9.3 0,73 (30) 0, 42 (12) 7. 7/8. 3 55 r e p r o d u c t i v e a d u l t s , f o r a given number of l a c t a t i n g females, the s u r v i v a l of young deermice i s higher i n the presence of e x t r a food. Summarj The p r o v i s i o n of food i n c r e a s e d the number of immigrants t o a p o p u l a t i o n a t a l l times o f the year. T o t a l immigration to a p o p u l a t i o n which was fed f o r one-and-a-half years was 2.5 times g r e a t e r than t h a t to a c o n t r o l p o p u l a t i o n . But s u r v i v a l of a d u l t mice i n t h i s long-term food p o p u l a t i o n was lower than on the c o n t r o l . The lower s u r v i v a l of a d u l t s with food may be a s s o c i a t e d with the i n c r e a s e d r e p r o d u c t i v e a c t i v i t y of mice i n the presence of supplemental food. T h i s may have r e s u l t e d from these re p r o d u c t i v e l y a c t i v e mice emigrating or dying. The r e s u l t s of these experiments i n d i c a t e t h a t breeding females have more i n f l u e n c e on the s u r v i v a l of young than males. 56 S e c t i o n F: Weights, Sexual Maturity. And Growth A number of s t u d i e s have shown that the mean weight of i n d i v i d u a l s i n p o p u l a t i o n s of north temperate small mammals decreases i n the f a l l and winter (see Iverson and Turner, 1974 f o r a review), These authors analyse i n d e t a i l the winter weight dynamics o f Microtus p e n n s y l v a n i c u s i n Manitoba over two years, They conclude t h a t the f a l l and winter decrease i n mean weight i s due p a r t l y t o the recruitment of s m a l l mice that f a i l t o g a i n weight, and weight l o s s by l a r g e r i n d i v i d u a l s . They suggest that the lower winter weights are adaptive because by decreasing i t s energy requirements an i n d i v i d u a l minimizes the amount of time i t spends f o r a g i n g and hence reduces i t s exposure to predators and bad weather. However, they f e e l t h a t the lack, of food i s not the proximate cause of t h i s r e d u c t i o n i n weight, P. maniculatus p o p u l a t i o n s show t h i s decrease i n mean weight d u r i n g the winter ( S a d l e i r , 1965 and S t e b b i n s , 1978). I was i n t e r e s t e d to t e s t the hypothesis t h a t food was not a proximate f a c t o r i n determining the f a i l u r e of smal l n e w l y - r e c r u i t e d P..maniculatus to gain weight and l a r g e r i n d i v i d u a l s to maintain weight during the winter, A second i n t e r e s t i n g f e a t u r e of winter weights i n mice i s dis c u s s e d by Flowerdew (1973). He shows t h a t t h e r e i s a trend i n Apodemus s y l v a t i c u s towards an e a r l i e r s t a r t t o the breeding season f o r a given i n c r e a s e i n weight l a t e i n the winter, I f t h i s a p p l i e s to P. !§fii£2l§tus, the a d d i t i o n of supplemental food should cause the mice t o breed e a r l i e r because they can i n c r e a s e t h e i r o v e r w i n t e r i n g weight. 57 Winter Weights Of Larger Mice In Figure 10 I have p l o t t e d the weight records f o r three l o n g - l i v e d mice from the c o n t r o l s and three from food g r i d 2, 111 three a d u l t mice on the c o n t r o l s show the c h a r a c t e r i s t i c d e c l i n e i n weight i n winter 1973. The two l o n g - l i v e d females, 10 40 and 594, a l s o d e c l i n e d i n weight i n the f o l l o w i n g winter. The two males on g r i d 2 responded immedlately t o the a d d i t i o n of food a t the end of December 1973, Both i n c r e a s e d i n weight and became s c r o t a l by the end o f January and the beginning of February, while male 1005 on the c o n t r o l never became s c r o t a l , Female 2759 was pregnant i n A p r i l , w h i l e the two c o n t r o l females were not pregnant u n t i l June and J u l y 1974. C o n t r o l females 594 ( g r i d 5) and 1040 ( g r i d 1) showed an almost i d e n t i c a l d e c l i n e i n weight i n t o the f a l l 1974. But as soon as t h e food was added to g r i d 5 the weight of female 594 i n c r e a s e d and she became pregnant i n December. These immediate responses to the a d d i t i o n of food suggest that indeed the winter d e c l i n e i n weight of l a r g e r mice can be reversed i n what appears to be a proximate response to food, S i n t e r Weights Of Smaller Mice The maintenance of a low winter weight i n young r e c r u i t s i s c l e a r l y shown i n a l l f o u r mice i n F i g u r e 11. The two mice on g r i d 1, male 891 and female 888, continued at low sub-adult weights i n t o the s p r i n g of 1974. But male 2796 and female 2788, which showed the same p a t t e r n i n i t i a l l y as the c o n t r o l mice, i n c r e a s e d i n weight r a p i d l y a f t e r the a d d i t i o n cf food. In f a c t . 58 Figure 10, Winter weights of i n d i v i d u a l l a r g e mice on the c o n t r o l s and food g r i d s . dotted l i n e = c o n t r o l g r i d s s o l i d l i n e = food g r i d s A 1 = food added to g r i d 2 2 = food added to g r i d 5 60 F i g u r e 11, Winter weights of i n d i v i d u a l s m a l l mice on the c o n t r o l s and food g r i d s , dotted l i n e = c o n t r o l g r i d s s o l i d l i n e = food g r i d s = food added to g r i d 2 WEIGHT IN GRAMS 62 both mice reached a d u l t weights and were breeding by the end of February, These weight responses o f s m a l l mice again seem to have r e s u l t e d from a proximate response to the i n c r e a s e d a v a i l a b i l i t y of food. Overwintering Weight And Breeding c o n d i t i o n To a v o i d the problem of the i n c r e a s e i n weight of females a s s o c i a t e d with pregnancy and l a c t a t i o n , I have co n s i d e r e d the weight d i s t r i b u t i o n of males only i n t h i s s u b - s e c t i o n . The mean weights of males on the c o n t r o l g r i d 1 and food g r i d 2 were s i m i l a r i n November-December 1973 before food was added (Appendix 7), Follo w i n g the a d d i t i o n of food to g r i d 2 at the end of December, the weights of a l l age c l a s s e s of males s t a r t e d to i n c r e a s e (see Fig u r e 12). A month l a t e r the mean weights of males on the food g r i d were s i g n i f i c a n t l y h i g h e r than those on the c o n t r o l . The mean weight remained s i g n i f i c a n t l y higher f o r the next s i x weeks, A l l males were i n the a d u l t weight c l a s s , t h a t i s over 17 g, by the end of February. In the f o l l o w i n g t r a p p i n g p e r i o d the mean reached 21.2 g on g r i d 2, which was the highest recorded d u r i n g the study,. The mean weight was not s i g n i f i c a n t l y higher at the end of March when the f i r s t j u v e n i l e s were r e c r u i t e d , but rose again and was s i g n i f i c a n t l y higher f o r the whole of A p r i l . A s i m i l a r p a t t e r n of weight i n c r e a s e occurred on g r i d 3. One month a f t e r food was added at the end of January 1974, the mean weight of males was s i g n i f i c a n t l y higher than t h a t on the c o n t r o l (p<0.05). The mid-winter a d d i t i o n of food to g r i d 5 a l s o gave the same r e s u l t as these e a r l y s p r i n g a d d i t i o n s . Food was added t o g r i d 5 at the 63 Figure 12, Height d i s t r i b u t i o n of males on g r i d 1 and g r i d 2. open r e c t a n g l e s = g r i d 1 ( c o n t r o l ) c l o s e d r e c t a n g l e s = g r i d 2 (long-term food) A = food added to g r i d 2 x = mean weights s i g n i f i c a n t l y d i f f e r e n t (p=0.05) WEIGHT IN GRAMS t/9 65 beginning of November 1974. The mean weight of males was s i g n i f i c a n t l y higher one month a f t e r food a d d i t i o n compared with t h a t i n the same p e r i o d the previous c o n t r o l year. The mean was s i g n i f i c a n t l y higher than t h a t on the c o n t r o l a t the beginning of December 1974, s i x weeks a f t e r food a d d i t i o n . The i n c r e a s e i n weight i n response to the a d d i t i o n o f food had a d i r e c t i n f l u e n c e on the breeding c o n d i t i o n of males. The breeding season was underway on g r i d 2 by the f i r s t p e r i o d i n February 1974, which was two weeks a f t e r the mean weight of males reached over 18 g. On g r i d 1, 50^ of the males were breeding by the f i r s t week of June, which i s one month a f t e r the mean weight reached and remained at or above 18 g. On the lower-density food g r i d s 3 and 5, 50% of the males were s c r o t a l one month a f t e r the mean weights had reached 17 g i n s p r i n g 1974 and winter 1975 r e s p e c t i v e l y . The p a t t e r n s f o r the remaining onsets and c e s s a t i o n s of breeding are not so c l e a r . One problem i s t h a t the mean weight i s dependent on the age d i s t r i b u t i o n of mice i n the sample. Secondly, the age at which i n d i v i d u a l s reach s e x u a l maturity i n a po p u l a t i o n may change throughout the breeding season, Sexual M a t u r i t y I was i n t e r e s t e d to see i f t h e r e was a change i n the age of se x u a l maturity through the breeding season and i n response to food, U n f o r t u n a t e l y t h e r e i s no convenient method of aging mice i n the f i e l d a part from weight, L e s l i e et a l . (1945) used a g u a n t i t a t i v e technique t o estimate median body weight at sexual 66 maturity. In Figure 13 I have p l o t t e d t h i s median body weight of males at s e x u a l maturity f o r both years combined on the food and c o n t r o l g r i d s . S i n c e 50% or more males on g r i d 1 i n 1974 bred only from June to August, I have grouped the data so that these three months form a p e r i o d . There i s a trend on the c o n t r o l f o r a decrease i n the median body weight at s e x u a l maturity with the progress of the breeding season. Only the heaviest males were s e x u a l l y mature during the e a r l y breeding season, The a d d i t i o n of supplemental food s i g n i f i c a n t l y reduced the weight at maturity of ,these e a r l y breeding males (p<0.05). Over the whole period of food a d d i t i o n , s e x u a l l y mature males were s i g n i f i c a n t l y l i g h t e r on both g r i d s 2 and 3 compared with those on the c o n t r o l . The l a t e summer median was lower on g r i d 2 compared with that on the c o n t r o l , but on the lowest d e n s i t y food g r i d 3 (av. den. 8 males) i t was s t i l l lower than t h a t on e i t h e r g r i d 1 (av. den. 11) or 2 (av. den. 17), T h i s suggests that d e n s i t y as w e l l as food may have been important i n determining the age o f s e x u a l maturity, Gr o wt h Supplemental food i n c r e a s e d the r e p r o d u c t i v e p o t e n t i a l of mice because i n d i v i d u a l s were ab l e to gain weight and become r e p r o d u c t i v e l y a c t i v e e a r l i e r . Second, the age of sexual m a t u r i t y was lowered i n males. A t h i r d p o s s i b i l i t y i s t h a t s u r p l u s food may a l s o have i n c r e a s e d the growth r a t e of mice. Growth r a t e s of mice i n these p o p u l a t i o n s can be c a l c u l a t e d from weight changes of i n d i v i d u a l s between t r a p p i n g p e r i o d s , Pregnant females are excluded from t h i s a n a l y s i s , Within each 67 F i g u r e 13. Median body weight at s e x u a l maturity on g r i d s 1, 2, and 3. The 95% confidence l i m i t s are given, ^ = g r i d 1 ( c o n t r o l ) ^ = g r i d 2 (long-term food) • = g r i d 3 (short-term food) 24 CO •a: CD 20 C D 16 v —I 1 1 I DJF MAM J J A SON 1 3 7 5 69 four-week p e r i o d , a l i n e a r r e g r e s s i o n was c a l c u l a t e d f o r each sex on the change i n weight a g a i n s t the mean weight, 1 T h i s was then adjusted f o r an average j u v e n i l e {12 g) , an average sub-adult (15 g) , and an average a d u l t (18 g) mouse t o give an instantaneous r e l a t i v e growth r a t e . The growth r a t e data f o r each sex-age group are given i n Appendix 8, A f t e r food was added to g r i d 2 a t the end of December 1973, the growth r a t e of a l l sex-age groups of mice i n c r e a s e d . F i r s t , both sexes i n a l l age c l a s s e s had growth r a t e s l e s s than zero on only seven o c c a s i o n s a f t e r food was added to g r i d 2 compared with f o r t y o c c a s i o n s f o r a l l sex-age groups on the c o n t r o l , Second, adult males and females on the food g r i d had higher growth r a t e s (sign t e s t , males p=0,01, females p=0„02) than v o l e s on the c o n t r o l , T h i s was a l s o the case f o r each sex i n the sub-adult (sign t e s t , males and females p=0.02), and j u v e n i l e ( s i g n t e s t , males p=0,06, females p=,02) age c l a s s e s . T h i r d , a d u l t males and females, and sub-adult females on g r i d 2, a l l had lower growth r a t e s than mice on the c o n t r o l i n September 1974; t h i s i s the p e r i o d when the food s t a t i o n s were emptied. Fourth, mice on the c o n t r o l had fewer growth r a t e s below zero i n the s p r i n g of 1975 compared with the previous year, when the onset of breeding was so l a t e . F i n a l l y , the h i g h e s t growth r a t e s i n a d u l t males on the c o n t r o l were recorded s i x weeks before the 50% breeding l e v e l was reached. Adult females had high growth r a t e s a month l a t e r . But on the food g r i d both sexes and a l l age groups had high growth r a t e s at the same time. , 70 Food Consumed I t i s d i f f i c u l t t o o b t a i n a meaningful measure of food consumption. The weight of food removed from f e e d i n g s t a t i o n s was recorded every two weeks. But mice may s t o r e the oats, so t h a t the t o t a l measured need not represent the consumption of food over the two-week p e r i o d , Consumption per i n d i v i d u a l may be an even l e s s s a t i s f a c t o r y measure. F i r s t , because MM A was not always a good measure of p o p u l a t i o n s i z e . Secondly, because i t i s not known whether a l l mice i n the p o p u l a t i o n had access t o the food s t a t i o n s . For example, Dunford (1970) found t h a t s e v e r a l chipmunks i n i t i a l l y f e d from seed p i l e s t h a t he provided. But a f t e r a s h o r t time one or two animals would become dominant and chase away su b o r d i n a t e s so that the l a t t e r ceased t o use the seed p i l e s , F i n a l l y , not a l l mice r e g u i r e the same amount of food, so i n d i v i d u a l s probably took d i f f e r e n t q u a n t i t i e s of oats. The mean weight o f oats removed every two weeks from the g r i d 2 food s t a t i o n s , and the mean consumption per i n d i v i d u a l i n the three-month p e r i o d s of t h e study are given i n Table VIII, Least oat food was removed from s t a t i o n s i n l a t e summer. This may r e f l e c t the use of a l t e r n a t i v e food items which become a v a l a b l e i n the f o r e s t at t h i s time o f year. D i e t a r y analyses i n d i c a t e that P, maniculatus switches from seeds to other food items, p a r t i c u l a r y arthropods, i n the summer (Jameson 1952 and B a t z l i 1977). In both years the l a r g e s t q u a n t i t y of p a t s was removed from the f e e d i n g s t a t i o n s i n the f i r s t h a l f of the year. T h i s i s the p e r i o d of maximum weight qain, January to March i s the p e r i o d when males qain weiqht before becoming r e p r o d u c t i v e . 71 Table VIII. The mean weight of oats removed from fe e d i n g s t a t i o n s and mean consumption per mouse on g r i d 2. Three-month Kg of Oats Removed G per mouse Pe r i o d per two-weeks per two-weeks Jan-Mar 1974 1.8 31 Apr-Jun 1.8 35 Jul-Sep 1.3 35 Oct-Dec " 1.2 37 Jan-Mar 1975 1.8 52 Apr-Jun »» 2.1 60 72 and A p r i l to June c o i n c i d e s with the maximum number of l a c t a t i n g females. M i l l a r (1975) demonstrated t h a t l a c t a t i n g female £. leucopus r e g u i r e d 2.6 times the amount of food taken by non-breeding females. The mean consumption per i n d i v i d u a l i n d i c a t e s t h a t mice ate the same g u a n t i t y of oats throughout 1974, However, i t i s not p o s s i b l e to say whether t h i s i s a t r u e measure of consumption. The higher consumption i n 1975 may i n d i c a t e t hat the emigrating M, oregoni were removing some of the oats t h i s year. Summary The winter d e c l i n e i n weight of l a r g e r mice and the maintenance of a low winter weight by s m a l l e r mice can be reversed by p r o v i d i n g supplemental food. The response i s immediate, which suggests t h a t these winter weight changes i n E> S a n i c u l a t u s may be proximate responses t o the a v a i l a b i l i t y of food. The r e s u l t i n g i n c r e a s e i n o v e r w i n t e r i n g weight enabled mice to come i n t o breeding c o n d i t i o n much e a r l i e r than on the c o n t r o l , The consumption of supplemental food a l s o lowered the age of male se x u a l maturity so that s m a l l e r males came i n t o breeding c o n d i t i o n e a r l i e r than on the c o n t r o l . F i n a l l y , mice of a l l age-sex c l a s s e s had higher growth r a t e s i n the presence o f e x t r a food. 73 S e c t i o n G: Home Banges S t i c k e l (1968) i n d i c a t e d t h a t the home ranges of P. leucopus vary i n s i z e between h a b i t a t s , She sugqested t h a t home range s i z e may be l a r g e r i n areas with lower food s u p p l i e s , A recent short-term study by Metzqar (1973) suggests that feeding and e x p l o r a t o r y home ranges of P. maniculatus are not s p a t i a l l y d i s t i n c t a c t i v i t i e s . I f fo o d - g a t h e r i n g i s an important component of P. maniculatus movement, the a d d i t i o n of food should reduce home range s i z e . I f i n d i v i d u a l s i n the present study concentrated t h e i r movements around f e e d i n g s t a t i o n s , t h i s might leave room f o r immigrants to the food g r i d s . I estimated home range s i z e by Koeppl et a l . ' s technique (1975), T h i s model c a l c u l a t e s an e l l i p t i c a l home range from the d i s t r i b u t i o n o f g r i d capture p o i n t s of an i n d i v i d u a l animal. The model does not c a l c u l a t e ranges f o r i n d i v i d u a l s t h a t are captured at only one l o c a t i o n , or a t two or more l o c a t i o n s i n a s t r a i g h t l i n e , I c a l c u l a t e d a home range estimate f o r such i i n d i v i d u a l s (Appendix 9). U n f o r t u n a t e l y , the 0,8-hectare g r i d s used i n t h i s study are too s m a l l t o give good home range data. Over h a l f the mice i n a l l p o p u l a t i o n s had 50% or more of t h e i r capture p o i n t s i n the outer l i n e s of t r a p s on these g r i d s . Since the percentage with such captures was almost equal f o r a l l pop u l a t i o n s I have i n c l u d e d these mice i n the data a n a l y s i s . But mice with 100% of t h e i r captures i n the outer l i n e of t r a p s were excluded. Home range s i z e i s h i g h l y v a r i a b l e w i t h i n each p o p u l a t i o n (see Table IX). Males have l a r g e r home ranges than females, For example, d u r i n g the one-and-a-half years of food a d d i t i o n to 74 Table IX, The mean home range s i z e (sg ra) of male and female mice on c o n t r o l and food g r i d s , G r i d 1 (control) G r i d 2 (food) Males Females Males Females Nov-Dec 1973 1442 (10) 1073 (10) 1035(13) 903 (11) Jan-Mar 1974 2273 (14) 1352(10) 14 89( 18) 796 (15) Apr-Jun tt 1511 (13) 1548 (10) 1203 <14) 639 (14) Ju1-Sep II 278 4 (9) 2881 (10) 1338(9) 815 (8) Oct-Dec H 1516 (6) 1571(10) 1513(7) 867 (8) Jan-Mar 1975 2241 (6) 2008 (8) 2089( 1 1) 919 (11) ftpr-Jun it 2105(15) 1795 (14) 3365(7) 868 <11) Means (1974-75) 2009 (45) 1615 (37) 1473(45) 974 (66) Grid 5 (without food) G r i d 5 (with food) 1757(17) 1753 (14) 1584 (13) 1151 (18) The means are f o r home ranges c a l c u l a t e d f o r the whole data s et from January 1974 to June 1975, 75 g r i d 2, males on the c o n t r o l had home ranges 1.24 times l a r g e r than those of the females, and on g r i d 2 they were 1.51 times l a r g e r , On a seasonal b a s i s , home ranges of males on the c o n t r o l are l a r g e i n l a t e winter and s p r i n g (Jan,-Mar.), They decrease i n e a r l y summer, peak i n l a t e summer ( J u l , - S e p . ) , and f i n a l l y decrease again i n the f a l l and e a r l y winter. Females on the c o n t r o l had a sea s o n a l p a t t e r n i n home range s i z e s i m i l a r to that o f the males, except i n the s p r i n g of 1974, when they showed l i t t l e home range expansion. But i n 1975, when breeding was underway much e a r l i e r , female home ranges were a l s o high i n the l a t e w i n t e r - s p r i n g p e r i o d , S t i c k e l (1968) a l s o reported that home ranges were l a r g e at the onset of breedi n g . The d i s t r i b u t i o n of home range s i z e s c a l c u l a t e d f o r males and females during the whole p e r i o d o f food a d d i t i o n are given i n F i g u r es 14 and 15, The mean home ranges over the whole perio d are s i g n i f i c a n t l y s m a l l e r on g r i d 2 (males; t=2.41, p<0.02, and females: t=4.52, p<0„001). In each three-month period (see Table IX), except f o r males i n Apr.-Jun, 1975, horns ranges o f both sexes are sm a l l e r i n the long-term food p o p u l a t i o n ( g r i d 2).,But on the low d e n s i t y g r i d 3 p o p u l a t i o n which r e c e i v e d food one month a f t e r g r i d 2, males had l a r g e r home ranges (30 27sg m) even than those on the low d e n s i t y c o n t r o l g r i d 5 over t h i s period (2638sg m) , Females i n t h i s p o p u l a t i o n had s m a l l e r home ranges (1488sq m) but not s i g n i f i c a n t l y s m a l l e r than those on g r i d 5 (2076sg m) , C e r t a i n topographic f e a t u r e s of t h i s g r i d may have had some i n f l u e n c e (see Appendix 1), Over the whole p e r i o d that g r i d 5 r e c e i v e d food (Nov.-Feb. +• Jul.-Aug.), compared with the same p e r i o d the previous c o n t r o l year, the mean home ranges o f 76 F i g u r e 14. Frequency d i s t r i b u t i o n of male home range s i z e s on the c o n t r o l ( g r i d 1} and food g r i d (2). 77 grid 1 grid 2 1 j 1 I I 1 1 I 1 i i .j I I i i i 2 0 0 0 4-0 0 0 SQUARE METERS 78 Figure 15s Freguency d i s t r i b u t i o n of female home range s i z e s on the c o n t r o l ( g r i d 1} and food g r i d (2), grid 1 •+ grid 2 0 10 0 0 20 0 0 SQUARE METERS 3 0 0 0 80 males and females were s m a l l e r . Females again had s t a t i s t i c a l l y s m a l l e r home ranges when the po p u l a t i o n was s u p p l i e d with excess food (t=2, 13, p<0,05). Summary. Home ranges of P, maniculatus are f l e x i b l e . Home ranges on the c o n t r o l changed s e a s o n a l l y , with a tendency for there to be l a r g e r home ranges i n both sexes during the breeding season, Mice reduced the s i z e o f t h e i r home ranges i n response to the a d d i t i o n of food. T h i s response to food i n d i c a t e s t h a t food-gathering i s an important determinant o f the movement pa t t e r n e x h i b i t e d by P. maniculatus. The reduced movement of males and females i n response t o supplementary food c o u l d have made 'space' a v a i l a b l e f o r immigrants. In f a c t , the p e r i o d with the highest r a t e of female immigration c o i n c i d e s with the s m a l l e s t home range s i z e recorded i n the study, A l s o , home ranges of females i n p a r t i c u l a r tended t o be s m a l l e r during p e r i o d s o f highest food consumption (Table VIII). T h i s suggests a c a u s e - e f f e c t r e l a t i o n s h i p between food a v a i l a b i l i t y and movement pa t t e r n s . Males a l s o reduced the s i z e of t h e i r home ranges, yet i n s p i t e o f being l i g h t e r than l a c t a t i n g females and having lower energy requirements, they maintained l a r g e r home ranges than d i d such females. T h i s suggests t h a t males maintain l a r g e r home ranges than are e n e r g e t i c a l l y neccessary. But, s i n c e the o v e r a l l sex r a t i o f o r the f e d p o p u l a t i o n remained at 1:1, males with e x t r a food may t o l e r a t e more range o v e r l a p than females. 81 S e c t i o n H: Micro tus o r e g o n i : A P o s s i b l e Competitor? In previous s e c t i o n s I have a l l u d e d t o p o s s i b i l e c ompetition from M. oregoni t h a t immigrated i n t o a l l P. j a n i c u l a t u s p o p u l a t i o n s i n 1975. When I s t a r t e d t r a p p i n g the P. maniculatus p o p u l a t i o n s on g r i d s 2 and 3, I tagged any !• Pj:egoni that entered the t r a p s . Over the f i r s t summer on these g r i d s , M. o r egoni made only s p o r a d i c appearances or disappeared, so I ceased t a g g i n g them. U n f o r t u n a t e l y , when they reappeared I d i d not have the time t o tag them i n d i v i d u a l l y . The number of «. o r e g o n i given i n Table X i s t h e r e f o r e a minimum pop u l a t i o n count. I t r e p r e s e n t s the l a r g e s t number I recorded i n e i t h e r check each t r a p p i n g p e r i o d . However, i n the penultimate t r a p p i n g p e r i o d on g r i d 2, I caught 23 M. oregoni i n one check, Then, i n the l a s t t r a p p i n g period I removed a l l i n d i v i d u a l s of both s p e c i e s t h a t were caught i n the f i r s t check, so a l l 49 t r a p s were a v a i l a b l e to any remaining mice of e i t h e r s p e c i e s , The t o t a l number of M,.oregoni recorded i n t h i s t r a p p i n g p e r i o d was 25, only 2 more mice than my count f o r the previous normal t r a p p i n g period. T h e r e f o r e , the f i g u r e s given f o r the fl. oregoni p o p u l a t i o n may not be toe u n r e a l i s t i c . Table X shows the mean number of H. oregoni f o r each three-month p e r i o d on each g r i d . There are u n f o r t u n a t e l y no data f o r the two c o n t r o l s i n the f i r s t h a l f of the study (Appendix 1). There was an i n c r e a s i n g immigration of M. oregoni t o a l l g r i d s i n 1975. The number o f a. oregoni compared with the number of P. maniculatus (Table II) i n d i c a t e s that there i s a r e c i p r o c a l change i n numbers of the two species., P e t t i c r e w and S a d l e i r (1974) found a s i m i l a r r e c i p r o c a l change i n numbers of 82 Table X. The mean number o f M^.orecjoni i n each P t maniculatus p o p u l a t i o n . G r i d 1 G r i d 2 G r i d 3 G r i d 4 G r i d 5 1973 Nov-Dec — 1.40 1974 Jan-Mar — 1.00 Apr-Jun -- 0.29 Jul-Sep -- 2,50 Oct-Dec 0. 57 2.25 1 975 Jan-Mar 8,0 5.17 Apr-Jun 9.86 6.4 1.60 1.50 0.57 8,40 7.60 10,25 2.86 0.0 7.8 3,8 4.8 8,71 83 these two s p e c i e s i n one of t h e i r p o p u l a t i o n s i n the UBC Research F o r e s t a t Haney, They concluded that some type of competitive i n t e r a c t i o n may have occurred. R e d f i e l d e t a l . (1977) showed t h a t M. tpwnsendii a c t i v e l y excluded P» a a n i c u l a t u s from g r a s s l a n d areas i n the Fraser River Delta. In the present study I was i n t e r e s t e d t o see whether food manipulations had any a f f e c t on the p r o p o r t i o n s of the two s p e c i e s o c c u r r i n g i n the f o r e s t . Also, I wanted t o see i f the H« orecipni immigration was having any e f f e c t on the £• !§.fiiculatus p o p u l a t i o n s . Food M a n i p u l a t i o n And Species Numbers In the l a s t h a l f of 1974 there were few i f any M, oregoni* on g r i d s 1, 2 and 5, But on g r i d s 3 and 4 numbers were higher. An i n t e r e s t i n g change i n numbers of both s p e c i e s occurred i n August 1974, when food was removed from g r i d 3 and added to g r i d 4. On g r i d 3, i n the f i r s t t r a p p i n g p e r i o d i n August 1974, there were 9 M. oregoni and 16 P. maniculatus. The food s t a t i o n s were removed d u r i n g t h i s t r a p p i n g p e r i o d . Two weeks l a t e r t h e r e were 16 M. pregoni- and 15 P. maniculatus. The numbers of P. maniculatus and M. oregoni on g r i d 4 a t the beginning of August were 27 and 13 r e s p e c t i v e l y , A f t e r two weeks of food the number of P. maniculatus r o s e t o 36 and the number of M_. ojcggoni d e c l i n e d to 4. S i m i l a r changes i n numbers of the two s p e c i e s occurred with the removal of food s t a t i o n s from g r i d 2 i n June 1975. fi. oregoni i n c r e a s e d from 13 to 23, and P.,maniculatus d e c l i n e d from 35 to 19. F i n a l l y , on g r i d 5, when food was removed i n March 1975, M. oregoni i n c r e a s e d from 3 to 13 i n two 84 weeks, but P, maniculatus i n t h i s case i n c r e a s e d i n i t i a l l y , then d e c l i n e d . There i s some i n d i c a t i o n o f a decrease i n £• l a B i S M l a i S S and a n i n c r e a s e i n fl. oregoni when food s t a t i o n s are removed from a population, There i s a l s o an i n d i c a t i o n of a r e v e r s e p a t t e r n of change of numbers when food i s added to a po p u l a t i o n . l§ffioyal Of n, oregoni In the l a s t t r a p p i n g p e r i o d of the study, I removed some of the f§. oregoBi from c o n t r o l g r i d 1 and food g r i d 5. In both po p u l a t i o n s I removed a l l M, oregoni t h a t were caught i n t r a p s on the f i r s t f o u r rows of each g r i d (rows D to G, see Fig u r e 3), I d i d t h i s p a r t i a l removal o f M. oregoni f o r two t r a p p i n g p e r i o d s (at the end of J u l y and the beginning of August). Table XI presents the t o t a l number of t r a p s occupied by each s p e c i e s f o r these l a s t t h r e e t r a p p i n g p e r i o d s , I» oregoni on rows A to C almost c e r t a i n l y moved i n t o rows D to G to r e p l a c e v o l e s removed. For example, on g r i d 1 there were 10 v o l e s i n the A to C rows and 12 i n the D to G rows i n i t i a l l y . By August 20-22, the number of v o l e s was reduced by h a l f . From the t o t a l number of t r a p s occupied by each s p e c i e s , we see t h a t a f t e r 2 weeks of M. oregoni removal t h i s s p e c i e s was reduced on g r i d s 1 and 5, P, maniculatus, on the other hand, i n c r e a s e d on both g r i d s . The changes i n MNA of P. maniculatus a l s o r e f l e c t t h i s t r e n d : on g r i d 1 numbers rose from 20 to 32 and on g r i d 5 from 25 to 35* The presence of IL oregoni appears to have depressed the numbers of P, maniculatus i n both p o p u l a t i o n s i n 1975, But the r e d u c t i o n o f P. l a n i c u l a t u s i n the 85 Table XI, The number of masiculatus and Mv. oregoni-du.ri.nq p a r t i a l removal of the vol e from q r i d s 1 and 5, 5.0. = M. or e g o n i and P.M. = P. maniculatus. The numbers are the number of t r a p s occupied by each s p e c i e s . G r i d 1 (control) Rows A-C Rows D-G G r i d 5(food) Rows A-C Rows D-G H.O, P.M. M.O. P.M. H.O, P.M. H.O. P.M. J u l 23-25 10 5 12* 12 17 12 16* 14 Auq 6-8 12 12 14* 10 9 14 5* 13 Auq 20-22 5 13 6 22 6 16 5 21 * = voles removed 86 presence of e x t r a food on g r i d 5 was not as great as on the c o n t r o l * To see which age group was being a f f e c t e d by t h i s i n t e r s p e c i f i c i n t e r a c t i o n , I have counted the number of r e c r u i t s i n the month b e f o r e M, oregoni removal and i n the month during p a t i a l removal. I t was not p o s s i b l e t o compare recruitment on the two halves of the g r i d s d u r i n g the removal p e r i o d s because H« oregoni d i s t r i b u t e d themselves evenly a c r o s s the g r i d i n the i n t e r v e n i n g two weeks. On both g r i d s combined a d u l t recruitment d i d not change, but sub - a d u l t s and j u v e n i l e recruitment in c r e a s e d from 5 to 37, T h i s i n d i c a t e s t h a t s ub-adults and j u v e n i l e s were prevented from e n t e r i n g the £, maniculatus p o p u l a t i o n s with l a r g e numbers of M. oregoni. Summary I t i s not p o s s i b l e to say what mechanism produced the change i n numbers observed i n the two s p e c i e s i n response to the a d d i t i o n and removal o f food. But i t i s i n t e r e s t i n g to note t h a t £• maniculatus home ranges on both c o n t r o l and food g r i d s were l a r g e r i n 1975 i n the presence o f M. oregoni than i n 1974 i n t h e i r absence. This may i n d i c a t e that the two s p e c i e s were using some resource i n common, even i f only 'space'. I n d i v i d u a l P. maniculatus may then have been f o r c e d to compensate f o r t h i s by e n l a r g i n g t h e i r home ranges i n the presence of the vole. But the responses of both s p e c i e s t o the removal of food suqgests t h a t P. maniculatus can outcompete M. oregoni i f food i s provided. Such a response may r e s u l t i f P. maniculatus i s more 87 s u c c e s s f u l at defending a l o c a l i z e d food supply than the v o l e . The removal experiments suggest t h a t subadult and j u v e n i l e I - ffasiculatus recruitment was reduced by M. oregoni. 88 4, DISCUSSION The r e s u l t s o f i n c r e a s i n g the temporal and s p a t i a l a v a i l a b i l t y of food to P. maniculatus p o p u l a t i o n s are summarized i n F i g u r e 16. Supplemental f e e d i n g changed the s i z e of the breeding p o p u l a t i o n , the extent and i n t e n s i t y of breeding, and the s p a t i a l o r g a n i s a t i o n o f i n d i v i d u a l mice. i§.l§ Ind Female Numbers The l a t e winter (December 1973 and January 1974) a d d i t i o n of food r e s u l t e d i n a doubling or near-doubling of the number of both sexes of P, maniculatus, thus c o n f i r m i n g the f i r s t p r e d i c t i o n (see H1, Introduction) . S i m i l a r , though l e s s dramatic, numerical responses followed the l a t e summer (August 1974) and mid-winter (November 1974) a d d i t i o n s o f supplemental food. The l a r g e l a t e winter i n c r e a s e i n both sexes occurred before j u v e n i l e s were born and r e s u l t e d from an i n c r e a s e i n immigration t o the food g r i d s . The r e s u l t s c o n t r a s t with those of Fordham (1971), who added food to P. maniculatus p o p u l a t i o n s i n mid-February and observed an i n c r e a s e i n female d e n s i t y , but no change i n males. S i n c e i n a •normal* year, males can have s c r o t a l t e s t e s by February (present study and S a d l e i r 1974), i t i s p o s s i b l e t h a t Fordham added food a f t e r the p o t e n t i a l breeding males had expanded t h e i r home ranges and p o s s i b l y excluded any su r p l u s males. The e a r l y p r o v i s i o n of supplemental food i n the present study preceeded any weight gain and home range expansion i n males. 89 F i g u r e 16. Summary of the e f f e c t of supplemental food on R±. maniculatus p o p u l a t i o n s . 90 EXTRINSIC PERTURBATION INTRINSIC RESPONSE Increase food -Increased growth Incr. winter weight—>. ^ Deer, age of maturity^ SPATIAL ORGANIZATION Deer, home range size DYNAMICS Incr. immigration Incr. breeding popn. o & S T Incr. reproductive output 91 These numerical responses of p. maniculatus to the a d d i t i o n of food i n l a t e winter suggest t h a t food a v a i l a b i l i t y i n f l u e n c e s the s i z e of the 'breeding p o p u l a t i o n , T h i s response may vary with the stage of the breeding c y c l e , the amount of n a t u r a l food present, and the number o f f e e d i n g s t a t i o n s , The importance of the stage i n the breeding c y c l e i s i n d i c a t e d i n the smaller numerical responses of p o p u l a t i o n s to l a t e summer and mid-winter food a d d i t i o n and removal. The complete and temporary removals of food i n l a t e summer and f a l l caused males f i r s t t o go out of r e p r o d u c t i v e c o n d i t i o n and then t o i n c r e a s e i n den s i t y , T h i s numerical response i s probably a consequence of the d e c l i n e i n male a g o n i s t i c behaviour a s s o c i a t e d with the c e s s a t i o n o f repr o d u c t i o n (Pergmyscus: S a d l e i r 1965 and Healey 1968, Microtus: Turner and Ivers e n 1973), Food removal e a r l i e r i n the breeding season had l i t t l e e f f e c t on males but d i d a f f e c t females. For example, the l a t e summer removal of food was followed by a d r a s t i c d e c l i n e from 20 to 6 females over s i x weeks, but 70% or more a d u l t females continued to l a c t a t e . By c o n t r a s t , s p r i n g food removal produced no change i n d e n s i t y but reduced the percent of females l a c t a t i n g from 73% t o 20%. The other two f a c t o r s that modified the numerical response of P. maniculatus t o supplemental food i n v o l v e the food i t s e l f . The consumption of supplemental oats d e c l i n e d during l a t e summer, which c o i n c i d e s with the period when granivorous rodents are known to expand t h e i r d i e t t o i n c l u d e arthropods, leaves, f r u i t s and seeds of growing p l a n t s (Pergmyscus: Jameson 1952 and B a t z l i 1977: Apodemus: Hansson 1971). I f the P, maniculatus p o p u l a t i o n s s u p p l i e d with oats expanded t h e i r d i e t to i n c l u d e 92 such n a t u r a l food items, and re-ad -justed t h e i r home ranges to in c l u d e these items, then t h i s could e x p l a i n the summer d e c l i n e i n d e n s i t y on the food g r i d s . T h i s might a l s o e x p l a i n why other p o p u l a t i o n s of granivorous mice s u p p l i e d only with supplemental seed food tend t o d e c l i n e i n d e n s i t y over summer i n s p i t e of the extr a food (Etromyscus: Fordham 1971 and Smith 1971: AEodemus: Watts 1969 and Flowerdew 1972), F i n a l l y , the r e d u c t i o n i n the number of fe e d i n g s t a t i o n s over winter was f o l l o w e d by a steepening of the normally slow overwinter d e c l i n e i n d e n s i t y . Meanwhile, two p o p u l a t i o n s f e d at high l e v e l s were s t a b l e or incr e a s e d , T h i s suggests t h a t the number of food s t a t i o n s a l s o a f f e c t e d the p o p u l a t i o n s ' response t o food. Breeding Season £§£omyscus normally does not breed i n winter. But a number of s t u d i e s have shown that the onset, d u r a t i o n , and c e s s a t i o n of breeding can be h i g h l y v a r i a b l e i n one l o c a t i o n from year t o year (see S a d l e i r 1974 f o r a review). In the present study P. maniculatus i n a c o n t r o l p o p u l a t i o n s t a r t e d t o breed t h r e e months l a t e r i n 1974 than 1975. The d u r a t i o n of the breeding season i t s e l f (at l e a s t 50% of males s c r o t a l or females l a c t a t i n g ) , was only two and a h a l f months compared with over s i x months i n 1975. The average monthly temperatures were below normal i n both years, but May 1974 had the lowest mean temperature on record. Male r e p r o d u c t i o n h a l t e d and females ceased breeding oh both c o n t r o l s by the end of May 1974, whereas no breaks i n rep r o d u c t i o n o c c u r r e d i n e i t h e r sex i n 1975. T o t a l p r e c i p i t a t i o n and hours of sunshine were c l o s e r to normal i n 93 1975, In f a c t , the unusually e a r l y onset of breeding t h i s year was probably a d i r e c t response to p a r t i c u l a r l y mild weather i n December 1974, In s p i t e of the r a t h e r * wintry* weather i n 1974, mice i n two p o p u l a t i o n s with supplemental food s t a r t e d t o breed, T h i s advanced the onset of breeding i n 1974 by four months compared with t h a t on the c o n t r o l . In f a c t , breeding i n the long-term food p o p u l a t i o n continued f o r seventeen months apart from a two-week c e s s a t i o n i n the f a l l 1974, T h i s c e s s a t i o n was probably i n response to a temporary exhaustion of food s u p p l i e s at t h i s time. The mid-winter a d d i t i o n o f food t o another p o p u l a t i o n r e s u l t e d i n males becoming s c r o t a l and females p e r f o r a t e a f t e r two weeks. These r e s u l t s confirm the p r e d i c t i o n s of Watts (1970) and s a d l e i r et a l , (1973), t h a t abundant food s u p p l i e s can o v e r r i d e the normal c e s s a t i o n of reproduction e x h i b i t e d by granivorous s m a l l mammals i n temperate winter c l i m a t e s . T h i s r e l a t i o n s h i p between food supply and r e p r o d u c t i v e c o n d i t i o n i s a l s o i n d i c a t e d i n the winter weight responses of mice t o supplemental food. Mice on a l l g r i d s showed the winter weight d e c l i n e c h a r a c t e r i s t i c of north temperate small mammals (Iverson and Turner, 1973). But i n a l l t h r e e p o p u l a t i o n s t h a t r e c e i v e d food i n winter, mice i n c r e a s e d i n weight immediately a f t e r food was made a v a i l a b l e . T h i s r e s u l t e d i n some i n d i v i d u a l s becoming r e p r o d u c t i v e in two weeks a f t e r the e a r l y November a d d i t i o n and f o u r weeks a f t e r the l a t e December a d d i t i o n . The del a y i n the l a t t e r case i s a r e s u l t of mice i n t h i s p o p u l a t i o n having reached the minimum o v e r w i n t e r i n g weight by the time food was added. The presence of e x t r a food reduced the c o s t of energy 94 a q u i s i t i o n and enabled deermice to maintain p o s i t i v e growth r a t e s and reproduce i n s p i t e of winter weather c o n d i t i o n s . Flowerdew (1972) a l s o found t h a t the mean weiqhts of AEodemus s y l v a t i g u s s u p p l i e d with supplemental food were 20% higher than those of c o n t r o l s . These r e s u l t s suggest that weight changes and, i n t u r n , the onset and c e s s a t i o n of breeding are proximate responses to food a v a i l a b i l i t y i n granivorous rodents, The consumption of supplementary food a l s o enhanced r e p r o d u c t i v e output, Larger numbers o f male and female mice bred on food g r i d s than on c o n t r o l s . The median body weight (which i s equated with age) of s e x u a l l y mature males was high a t the beginning of the breeding season on the c o n t r o l and then d e c l i n e d . But the a d d i t i o n of food reduced the weight at sexual maturity, p a r t i c u l a r l y i n the e a r l y breeding season. Also mice of both sexes on food g r i d s had higher growth r a t e s than c o n t r o l s at a l l times of the year, Therefore, younger mice were a b l e not only to breed but a l s o t o reach breeding weiqhts e a r l i e r , F i n a l l y , the hiqhest qrowth r a t e s c o i n c i d e d with p e r i o d s when most oats were removed from food s t a t i o n s . These r e s u l t s i n d i c a t e t h a t the abundance of foo d , throuqh i t s i n f l u e n c e on the weight o f i n d i v i d u a l s , a f f e c t s the onset, c e s s a t i o n , and i n t e n s i t y o f breeding i n P. maniculatus. S p a t i a l O r g a n i s a t i o n In a p o p u l a t i o n s u p p l i e d with supplemental food f o r one-and-a-half years, male deermice had average home ranqes 0,73 times the s i z e of male home ranqes i n a c o n t r o l p o pulation. Females reduced the s i z e of t h e i r home ranqes even f u r t h e r t o 95 0,60 times that of c o n t r o l s . I f home ranges space i n d i v i d u a l s In a p o p u l a t i o n (Watson and Moss 1970 ) , then t h i s r e d u c t i o n i n response to food r e p r e s e n t s a r e d u c t i o n i n the use of •space* by the r e s i d e n t s , T h i s could e x p l a i n the high r a t e of immigration to food p o p u l a t i o n s . In f a c t , female immigration was highest during the three-month p e r i o d when the lowest mean home range was recorded. F u r t h e r , females i n p a r t i c u l a r tended t o have s m a l l e r home ranges during p e r i o d s of highest oat removal from feeding s t a t i o n s . These r e s u l t s suggest that f o o d - g a t h e r i n g i s an important determinant o f home range s i z e i n both male and female £, maniculatus. The response o f P. maniculatus to the immigration of J3» 2 £ § a°Bi i n the present study could a l s o be i n t e r p r e t e d i n terms of s p a t i a l o r g a n i s a t i o n , Deermice i n a l l p o p u l a t i o n s had l a r g e r home ranges i n the presence of M,.oregoni. T h i s i n t e r s p e c i f i c competitor may have decreased the a v a i l a b i l i t y to deermice of food or some other e x t r i n s i c f a c t o r . I n d i v i d u a l deermice may have been f o r c e d to occupy l a r g e r home ranges, and perhaps a l s o exclude subordinate c o n s p e c i f i c s . T h i s would e x p l a i n the d e c l i n e i n deermouse densit y and the e x c l u s i o n of j u v e n i l e and sub-adult mice i n the presence of M. oregoni •Density compensation' i n the presence of a c o e x i s t i n g competitor has been observed i n b i r d s p e c i e s (Cody, 1974) . The r e d u c t i o n i n M. oregoni numbers and the i n c r e a s e i n P. maniculatus when supplemental food was provided r e g u i r e s f u r t h e r e x p l a n a t i o n , however. Perhaps deermice are b e t t e r a t e x p l o i t i n g seed foods ( C a l d w e l l , 1 9 6 4 ) , e s p e c i a l l y the clumped d i s t r i b u t i o n i n food s t a t i o n s used i n the present study. 96 I m p l i c a t i o n s For P o p u l a t i o n R e g u l a t i o n In Peromjscus What b e a r i n g , i f any, do the r e s u l t s of the present study have on the general p o p u l a t i o n model for P, maniculatus proposed by P e t t i c r e w and S a d l e i r (1974)? Deermice share with other small mammals the f r e g u e n t l y observed s p r i n g d e c l i n e i n p o p u l a t i o n s i z e a t the onset o f breeding. The extent of t h i s d e c l i n e i s v a r i a b l e and depends on the l e n g t h of the previous non-breeding season i n granivorous mice ( F u l l e r 196 9, Watts 1969 and Pett i c r e w and S a d l e i r 1974), The end r e s u l t , whether a l a r g e d e c l i n e from, high d e n s i t i e s , or a s m a l l one or none at a l l f o l l o w i n g lower overwinter d e n s i t i e s , i s a low d e n s i t y breeding p o p u l a t i o n from year to year, The manner i n which t h i s i s achieved i s as yet undetermined (Petticrew and S a d l e i r 1974),, The present study suggests that the a v a i l a b i l i t y of food may determine both the onset and c e s s a t i o n of breeding and hence the length of the non-breeding season. Mice i n popul a t i o n s s u p p l i e d with e x t r a food i n mid- and l a t e - w i n t e r not only put on weight and came i n t o breeding c o n d i t i o n but a l s o i n c r e a s e d i n density. T h i s i n c r e a s e r e s u l t e d from immigration i n response to the r e d u c t i o n i n home range s i z e by r e s i d e n t s r e c e i v i n g extra food. L i k e the European wood mouse (Flowerdew 1973) , deermice i n t h i s study had to gain weight from low o v e r w i n t e r i n g weights before s t a r t i n g to breed. T h i s p r e - r e p r o d u c t i v e weight gain i n c o n t r o l p o p u l a t i o n s was a s s o c i a t e d with home range expansion. Since the food experiments i n d i c a t e t h a t the s i z e of home ranges i s a f f e c t e d by food a v a i l a b i l i t y a t t h i s time, I suggest that males expand t h e i r home ranges i n the s p r i n g i n order to o b t a i n the e x t r a food r e q u i r e d to gain weight and reproduce. This 97 i n c r e a s e i n male home ranges c o u l d exclude s u b o r d i n a t e s {observed i n arena s t u d i e s by Eisenb e r g 19 68) and hence help to e x p l a i n the normal s p r i n g d e c l i n e i n male de n s i t y . F u r t h e r , i f l a r g e r males are dominant {Turner and Iverson 1973) , t h i s could e x p l a i n why d i s p e r s e r s are l i g h t - w e i g h t males ( F a i r b a i r n 1977) and a l s o why only the heaviest males become r e p r o d u c t i v e i n the e a r l y breeding season (present study) , Petticrew and S a d l e i r (1974) suggest that male d e n s i t y i s r e g u l a t e d throughout the breeding season, whereas females can i n c r e a s e through the season. They a l s o suggest that j u v e n i l e numbers are l i m i t e d by the a g o n i s t i c behaviour of r e p r o d u c t i v e males, In the present study, no c o r r e l a t i o n was found between the number of breeding males and j u v e n i l e s u r v i v a l , I n s t e a d , t h e r e was a negative c o r r e l a t i o n between j u v e n i l e s u r v i v a l and the number of l a c t a t i n g females, R e d f i e l d e t a l , (1978) a l s o found that j u v e n i l e recruitment i n M, townsendii was r e l a t e d to female but not male d e n s i t y , Female P. maniculatus i n the present study appeared to maintain n e a r l y constant p o p u l a t i o n s i z e through the breeding season than d i d males on the c o n t r o l i n 1974 (Figure 5) , Over t h e l o n g breeding season i n 1975, both sexes i n c r e a s e d and then d e c l i n e d , p o s s i b l y i n response t o H* oregoni immigration (Appendix 3), Over the same two-week period (May 29 - June 12) i n the 1974 breeding season both sexes d e c l i n e d on the food g r i d (Figure 5) . T h i s l a r g e d e c l i n e may have c o i n c i d e d with the summer switch i n d i e t from seed t o non-seed food items, I f so, not only were the sexes behaving s i m i l a r l y , but they a l s o may have responded to the d i s t r i b u t i o n of t h e i r perhaps p r e f e r r e d n a t u r a l food supply. The number of 98 females on the food g r i d a f t e r t h i s d e c l i n e was almost i d e n t i c a l to that on the c o n t r o l , The r e s u l t s o f these experiments suggest t h a t female r e p r o d u c t i o n i s s e n s i t i v e t o environmental c o n d i t i o n s , and that females thereby have a s t r o n g i n f l u e n c e on po p u l a t i o n dynamics i n P, maniculatus. a f t e r the s p a t i a l r e - o r g a n i z a t i o n of males in the s p r i n g an i n c r e a s i n g number of males become s c r o t a l , with s u f f i c i e n t food and warmer weather some females may then become pregnant and l a c t a t e a f t e r two to s i x weeks. I f i n s u f f i c i e n t food i s a v a i l a b l e , or weather i s poor, females may not ccrae i n t o breeding c o n d i t i o n u n t i l l a t e r . The long delay i n the onset o f female l a c t a t i o n on the c o n t r o l i n the c o l d s p r i n g of 1974 c o n t r a s t s with there being no delay the same year on g r i d s with e x t r a food, P e t t i c r e w and S a d l e i r (1974) suggest that over the con-breeding p e r i o d both sexes of P. maniculatus are r e g u l a t e d by the length of such seasons. at lower temperatures energy demands i n c r e a s e , and on the c o n t r o l , mice l o s t weight, ceased r e p r o d u c t i o n , and d e c l i n e d i n number. T h i s response was r e v e r s e d immediately i n winter by the a d d i t i o n of food to experimental g r i d s . Mice on such g r i d s gained weight, reproduced, and maintained f a i r l y s t a b l e winter d e n s i t i e s . By c o n t r a s t , P. maniculatus s u p p l i e d with decreasing l e v e l s of food overwinter tended to respond to each food r e d u c t i o n by a steepening of t h e i r overwinter d e c l i n e (Figure 6 ) . . T h i s suggests t h a t not only i s the l e n g t h of the non-breeding season determined by a combination o f food a v a i l a b i l i t y and weather c o n d i t i o n s , but a l s o that the patte r n of d e c l i n e w i t h i n such p e r i o d s can be i n f l u e n c e d by food a v a i l a b i l i t y . 99 I suggest t h a t n a t u r a l populations of P, maniculatus, through t h e i r s p a t i a l o r g a n i s a t i o n i n home ranges, are h i g h l y r e s p o n s i v e to the e x t r i n s i c environment. By means of t h i s s p a t i a l arrangement, i n d i v i d u a l s attempt t o gain access t o r e g u i r e d resources, maintain high weights, and reproduce. The system i s probably t i g h t l y r e g u l a t e d i n P. maniculatus by the se a s o n a l r e s t r i c t i o n s of t h e i r food supply, which r e s u l t s i n t h e i r l o s i n g weight over winter and cea s i n g to breed. Because of t h e i r high energy demands f o r l a c t a t i o n , females are probably more s e n s i t i v e to these c o n d i t i o n s than males and may e x e r t more i n f l u e n c e on p o p u l a t i o n dynamics during the breeding season. Such a dynamic system could be maintained through the i n t e r a c t i o n o f i n d i v i d u a l s as a r e s u l t of t h e i r s p a t i a l o r g a n i s a t i o n . I n t e r a c t i o n could be through d i r e c t c o n t a c t , or through o l f a c t o r y s t i m u l a t i o n as a r e s u l t of scent-marking by i n d i v i d u a l s (Eisenberg 1968). For example, through i n c r e a s e d i n t e r a c t i o n , i n d i v i d u a l s could monitor a r e d u c t i o n i n resource q u a n t i t y or g u a l i t y , and then respond by emigrating to an area of lower i n t e r a c t i o n i f such e x i s t s , or reducing the s i z e of t h e i r home ranges and ceas i n g t o breed. There are records of low r a t e s of l a c t a t i o n i n dense p o p u l a t i o n s and complete r e p r o d u c t i v e c e s s a t i o n i n extremely dense p o p u l a t i o n s of granivorous mice (see Terman 1968 f o r review, and Canham 1969), I t i s i n t e r e s t i n g to s p e c u l a t e t h a t these females may have experienced such high r a t e s of i n t e r a c t i o n when attempting to gain access t o re s o u r c e s t h a t r e p r o d u c t i o n was no longer p o s s i b l e . In the very dense p o p u l a t i o n , the high r a t e s of i n t e r a c t i o n per se may have had a d e t r i m e n t a l e f f e c t , such as 100 pregnancy block (Bruce 1959, E l e f t h e r i o u et a l . 1962, and Clulow and Langford 1971). The end of the normal breeding season might r e s u l t from a combination of higher energy demands and i n c r e a s e d i n t e r a c t i o n over decreasing food a v a i l a b i l i t y i n the autumn. Reduced access t o food c o u l d r e s u l t i n l o s s of weight and c e s s a t i o n of r e p r o d u c t i o n , The a s s o c i a t e d d e c l i n e i n r e p r o d u c t i v e behaviour r e s u l t s i n the observed i n c r e a s e to winter d e n s i t i e s . Some of these suggestions are h i g h l y s p e c u l a t i v e , But the presence of such a dynamic system of s p a t i a l o r g a n i s a t i o n could e x p l a i n the observed impact of supplementary fe e d i n g i n the present study. A l s o such a system, combined with the omnivorous nature o f t h e i r d i e t , c o u l d account f o r the r a p i d and s u c c e s s f u l e x p l o i t a t i o n of new food resources i n g e n e r a l by Perpmyscus, e s p e c i a l l y human garbage (Courtney and Fenton 1976) . 10 1 MICROTUS TOWNSENDII 1, .INTRODUCTION In s p i t e of h a l f a century o f r e s e a r c h , the p o p u l a t i o n dynamics of v o l e s remains somewhat of an enigma. The general p o p u l a t i o n trend of a two- to f i v e - y e a r ' c y c l e ' i n numbers has been observed r e p e a t e d l y (see review by Krebs and Myers 1974) . But there i s no consensus on the u n d e r l y i n g mechanism. The hypotheses range from s i n g l e f a c t o r e x t r i n s i c (weather, food, predation) or i n t r i n s i c (behaviour, genetics) e x p l a n a t i o n s , to rece n t suggestions that i n v o l v e a combination of s e v e r a l f a c t o r s (Bunnell 1973, B a t z l i 1975, C o l l i e r e t a l . 1976, Gaines and Rose 1976), Krebs et a l . (196 9) put a fence around an otherwise n a t u r a l p o p u l a t i o n of Microtus fiennsxlvanicus and M. ochrogaster, thus preventing immigration and emigration. The fenced p o p u l a t i o n i n c r e a s e d to un u s u a l l y high d e n s i t i e s , which r e s u l t e d i n voles o v e r g r a z i n g t h e i r food supply and consequently some i n d i v i d u a l s s t a r v i n g . T h i s experiment has been repeated with s i m i l a r r e s u l t s o n H i s r p t u s townsendii (Boonstra and Krebs 1977), These experiments suggest t h a t n a t u r a l v o l e p o p u l a t i o n s are normally a b l e to prevent s e r i o u s o v e r g r a z i n g of t h e i r food supply, F u r t h e r , removal experiments on these s p e c i e s (Myers and Krebs 1971, Krebs et a l , 1976) i n d i c a t e that v o l e s c o u l d do t h i s by ind u c i n g d i s p e r s a l of some i n d i v i d u a l s and hence l i m i t p o p u l a t i o n s i z e i n a given area. F i n a l l y , Krebs et a l . 1976, 102 found that the r e c r u i t s to areas from which r e s i d e n t v o l e s had been removed, were breeding a t lower weights than voles on the c o n t r o l . T h i s shows that excluded v o l e s were capable of breeding, and t h a t s m a l l e r v o l e s on the c o n t r o l were being prevented from breeding i n the presence of l a r g e r animals. These m i c r o t i n e s thus f u l f i l l the f i r s t three c o n d i t i o n s that Watson and Moss (1970) suggest i n d i c a t e t h a t spacing behaviour can l i m i t p o p u l a t i o n s i z e at l e a s t i n the s h o r t term (Table X I I ) . I t i s not known whether t h i s i s the case over long periods o f time (c o n d i t i o n D) . There i s very l i t t l e evidence f o r or a g a i n s t c o n d i t i o n E (Table XII) i n m i c r o t i n e s . Krebs and Delong (1965) provided s u r p l u s oats to a low d e n s i t y v o l e p o p u l a t i o n f o r ten months. The p o p u l a t i o n i n c r e a s e d a t 3% per week over f i v e months (reaching 8 times i t s o r i g i n a l d e n s i t y ) , then d e c l i n e d over the summer to the low s t a r t i n g d e n s i t y i n c o n t r a s t to the c o n t r o l , which continued t o i n c r e a s e . But t h i s experiment has been c r i t i c i s e d because of the type of food used (Watson and Moss 1970, B a t z l i and P i t e l k a 1971) . A l s o , there were many d i f f e r e n c e s i n the v e g e t a t i o n between the c o n t r o l and the experimental area, which was 10 km away. The s t a t u s of the experimental area was u n c e r t a i n , but the c o n t r o l was c l e a r l y an • i n c r e a s i n g ' p o p u l a t i o n , I was i n t e r e s t e d i n t e s t i n g c o n d i t i o n E on Microtus i P S n s e n d i i , and s e e i n g i f the number of i n d i v i d u a l s that could be supported i n a given area over a long p e r i o d of time was r e l a t e d to food d i s t r i b u t i o n (Section I) . Second, I wanted to see i f immigration to a new area, and the s i z e of the c o l o n i z i n g 103 Table XII. F a c t o r s suggested as necessary t o show t h a t spacing behaviour l i m i t s p o p u l a t i o n s i z e . A. A s u b s t a n t i a l part of the p o p u l a t i o n does not breed, B. Such non-breeders are capable of breeding i f dominant or t e r r i t o r i a l animals are removed. C. Breeding animals are not completely using up some resource. D. Long-term l i m i t a t i o n by spacing behaviour, E. I f c o n d i t i o n s A to D are met, and numbers change f o l l o w i n g changes i n food, then food and behaviour are both l i m i t i n g p o p u l a t i o n s i z e , Taken from Watson and Moss (1970) p.. 170. 104 p o p u l a t i o n was a f f e c t e d by food d i s t r i b u t i o n ( S e c t i o n I I ) . F i n a l l y , v o l e s , l i k e E» fflasiculatus, o f t e n e x h i b i t a s p r i n g d e c l i n e i n d e n s i t y , and I wanted to see i f t h i s was i n f l u e n c e d by the a v a i l a b i l i t y o f food, or by dominant r e s i d e n t v o l e s ( S e c t i o n III) . 105 2, METHODS AND EXPERIMENTS Study p o p u l a t i o n s o f M. towns e n d i i were s i t u a t e d i n two g r a s s l a n d areas south of Vancouver i n the F r a s e r R i v e r D e l t a of B.C. The Ladner study area i s 4,8 km east of ladner i n the abandoned a i r b a s e b o r d e r i n g Boundary Bay. The R e i f e l g r a s s l a n d i s s i t u a t e d on Westhara I s l a n d , which i s 6.4 km west of Ladner. Both g r a s s l a n d s have been undisturbed f o r at l e a s t ten years. The R e i f e l g r a s s l a n d i s a r e l a t i v e l y c o n f i n e d area bounded by farmland, w a t e r - f i l l e d d i t c h e s and a dyke beside the sea. In c o n t r a s t , the Ladner g r a s s l a n d i s bounded on only one s i d e by the Boundary Bay dyke, Two s i z e s of g r i d were marked out i n these grasslands. Large g r i d s had 100 t r a p s t a t i o n s a t 7,62 m i n t e r v a l s , u s u a l l y i n e i t h e r a 10X10 (0,47 hectares) or a 5X20 (0.44 hectares) p a t t e r n . Small g r i d s had e i t h e r 50 t r a p s i n a 5X10 p a t t e r n (0,21 hectares) or 49 t r a p s i n a 7X7 p a t t e r n (0.21 h e c t a r e s ) , V a r i a t i o n s i n the shape of l a r g e r g r i d s were made to f i t h a b i t a t r e s t r i c t i o n s : these are d e s c r i b e d i n more d e t a i l i n the r e l e v a n t s e c t i o n s . The t r a p p i n g regime was the same as that used i n !!• !§.niculatus, except t h a t M. townsendii were checked during the i n t e r v e n i n g day as well as the two o v e r n i g h t p e r i o d s i n each winter t r a p p i n g s e s s i o n . In summer, the daytime t r a p p i n g p e r i o d was abandoned a l t o g e t h e r i n order to avoid m o r t a l i t y from overheating. The type of data recorded on each vole was i d e n t i c a l to t h a t recorded f o r P. maniculatus. 106 G r i d s E , fi, T And M : Long-term Food Supplementation These f o u r g r i d s were l o c a t e d i n the Ladner study area (see Figu r e 17) „ G r i d E was the c o n t r o l population trapped by Krebs §t a l . (1976) from May 1971 to September 1975. G r i d s B and T were set up i n J u l y 1972. I n i t i a l l y , the 100 t r a p s on g r i d R were placed i n a 4X25 p a t t e r n , and g r i d T had 7 rows of 13 t r a p s and one row of 9 traps, But the area between g r i d s R and T and the c o n t r o l ( g r i d E ) became a shallow l a k e i n l a t e f a l l , so I r e a l i g n e d both experimental g r i d s on October 29th t o avoid f l o o d i n g the t r a p s . Both g r i d s were made i n t o 5X20 g r i d s running c l o s e and p a r a l l e l to the runway. At the end o f March 1973, a high d e n s i t y of food s t a t i o n s (1 per 22 sq m) was i n t r o d u c e d to g r i d T and a low d e n s i t y t o g r i d R (1 per 73 sq m) . I had intended t o run these two q r i d s as long-term food g r i d s with high and low d e n s i t i e s of food a v a i l a b l e f o r the d u r a t i o n of the study. u n f o r t u n a t e l y both were destroyed by ploughing i n A p r i l 1974 a f t e r only one year of supplemental f e e d i n g . In May 1974 I se t up g r i d M and provided i t with food s t a t i o n s at an in t e r m e d i a t e d e n s i t y (1 per 45 sq m) at the end of Auqust 1974, Th i s q r i d was monitored every two weeks u n t i l the end of the study i n September 1975. Food s t a t i o n s were removed from q r i d M ten weeks b e f o r e the end of the study. G r i d s 1, 2, and 3 : C o l p n i z a t i o n and Food Density These t h r e e s m a l l q r i d s were set up on the area that was plouqhed i n A p r i l 1974 (see F i q u r e 17), By the end of November 07 i g u r e 17, Location of M, townsendii g r i d s at Ladner, B. C. Numbers and l e t t e r s correspond to those assigned to each g r i d . 108 109 1974, grass was re-growing and some mice were immigrating i n t o the area. I was i n t e r e s t e d i n monitoring immigration to high, low, and zero supplemental food l e v e l s i n the breeding and non-breeding season. G r i d s j and G : Spr i n g D e c l i n e and Food G r i d I was the c o n t r o l used by LeDuc and Krebs(1975). I t was trapped c o n t i n u o u s l y from J u l y 1971 to September 1975, G r i d G was a l s o used by these authors. The dynamics of vo l e s on the two g r i d s was p r a c t i c a l l y i d e n t i c a l . In p a r t i c u l a r they e x h i b i t e d a non-breeding season which terminated i n a s u b s t a n t i a l s p r i n g d e c l i n e , I decided to use the two g r i d s to t e s t the e f f e c t of food a v a i l a b i l i t y on the magnitude of the sp r i n g d e c l i n e . On December 1st 1974, food s t a t i o n s were put out at each t r a p l o c a t i o n (1 per 47 sg m) on g r i d G. A l l food s t a t i o n s were removed on June 27th 1975, e i g h t weeks before the g r i d was l a s t trapped, I looked at the data a v a i l a b l e from previous years on the vo l e s that remained on g r i d I a f t e r each s p r i n g d e c l i n e . There was some i n d i c a t i o n t h a t such voles were l a r g e and came i n t o breeding c o n d i t i o n e a r l y . At the low p o i n t of the d e c l i n e , a l l males tended to be s c r o t a l a d u l t s and h a l f the females were p e r f o r a t e and some were l a c t a t i n g , I t h e r e f o r e decided to t e s t the h y p o t h e s i s that i f such v o l e s were removed frcm the po p u l a t i o n the v o l e s t h a t normally disappeared d u r i n g the d e c l i n e would stay and reproduce. Two smal l g r i d s were s e t up a t the beginning of January 1975. A l l males weighing > 70 g and 110 females weighing > 5H g were removed from one g r i d (Alpha), The second g r i d (Beta) was the c o n t r o l , where the same number of v o l e s was removed but as a random sample ac r o s s a l l s i z e c l a s s e s . F i n a l l y t o determine what v o l e s d i s p e r s e d from e s t a b l i s h e d p o p u l a t i o n s at B e i f e l through the 1975 s p r i n g d e c l i n e , I removed v o l e s c o n t i n u o u s l y from a 10X10 q r i d and a narrow s t r i p g r i d o f 2X13 t r a p s . Food Supply M i c r o t i n e s are h e r b i v o r e s , and the most e f f e c t i v e method of manipulating t h e i r food supply would be to provide them with e x t r a green v e g e t a t i o n or change t h e i r grass food supply i n s i t u by f e r t i l i z a t i o n or d e s t r u c t i o n . But the l o g i s t i c s and expense of the former on a l a r g e s c a l e over a long p e r i o d of time, and the consequence of the l a t t e r on h a b i t a t s t r u c t u r e made them unacceptable. Further, I wanted to c o n t r o l the d i s t r i b u t i o n of food f a i r l y c l o s e l y . F o l l o w i n g the advice of Dr, G. 0, B a t z l i ( personal communication), I provided v o l e s on g r i d s B and T with a l a b o r a t o r y chow (Purina #5321) f o r the d u r a t i o n of t h i s experiment. The chow was r e a d i l y taken by M. townsendii both i n the l a b o r a t o r y and from food s t a t i o n s i n the f i e l d , But the p e l l e t s were very s e n s i t i v e to moisture; a f t e r a p e r i o d of r a i n they became mushy, and developed mildew a few days l a t e r . J?.» townsendii r e a d i l y took whole oats from l i v e - t r a p s , so i n a l l other food experiments I used oats with a l a r g e handful of chow. The experimental design i s summarized i n F i g u r e 18, 111 F i g u r e 18. Summary of M, townsendii experimental design. The winter months are shaded. A 1 = food added to g r i d s R and T A 2 = " " " g r i d H A 3 = » « " g r i d s 1 and 2 A 4 = " " " » 2 and 3 A 5 = " " " g r i d G tp1 = food removed 1972 1973 1974 1975 J S N J ; M M J S N J M M J S N J M M J S GRID E GRID R H GRID T *" GRID M A 1 GRID 1 GRID 2 GRID 3 GRID I GRID Q GRID a GRID j3 A • A 4 A 5 H V ro 113 3. RESULTS S e c t i o n I : Long-term Food Supplementation Ifappabilitjr The t r a p p a b i l i t y of v o l e s on the c o n t r o l g r i d (E) and the experimental g r i d s (R, T, M) was c a l c u l a t e d i n the same manner as d e s c r i b e d f o r P, maniculatus. The v o l e s are more trappable than the deermice at s i m i l a r d e n s i t i e s (see Table I and Table X I I I ) . But the t r a p p a b i l i t y of v o l e s d e c l i n e s p r o g r e s s i v e l y as the number of i n d i v i d u a l s i n c r e a s e s , Male and female v o l e s are e g u a l l y t r a p p a b l e . Population Density The d e n s i t y of v o l e s on the c o n t r o l g r i d (E) was very low when i t was f i r s t e s t a b l i s h e d i n June 1971 (see Krebs et a l . 1976), From a winter (November) high of 27 v o l e s the p o p u l a t i o n d e c l i n e d t o 12 i n March 1972. When g r i d s R and T were e s t a b l i s h e d i n J u l y 1972, the c o n t r o l was i n c r e a s i n g towards i t s second winter peak of 58 voles (Figure 19). Before the realignment of t r a p s on October 29 1972, the number of v o l e s on both g r i d s R and T were s i m i l a r at 63 and 68 v o l e s r e s p e c t i v e l y compared with 48 on the c o n t r o l . But a f t e r realignment, the d e n s i t y on these two g r i d s i n c r e a s e d r a p i d l y , I t h i n k part of T a b l e X I I I . The t r a p p a b i l i t y of v o l e s OP. q r i d s E, R, and T. Minimum number a l i v e i s g i v e n i n p a r e n t h e s e s . ML=uales, FM-£eaales. Season G r i d E Grid R G r i d T G r i d K ML FM ML FM ML FM ML F-M-Kir.ter 1972-3 9 1 (33) 09(36) 85 (46) 87(59) 31 (72) 81(65) Sumner 1 973 78 (68) 83 (57) 74(1 1 2) 76 (104) 54 ( 167) 65 ( 133) — Winter 1973-4 85(36) 82(23) 90(57) 79(61) 63(135) 60(130) --Susner 1974 89(80) 31(79) Winter 1974-5 .78 (93) 77 {89) -- -- -- -- 59( 120) 49 (153) Grand T o t a l (1) 85 (124) 85 (104) 73 (166) 79 ( 156) 68 (264) 63(259) --Grand T o t a l (2) 71 (209) 71 (188) 47 (226) 41 (290) E = C o n t r o l , 3 = Lou-food, T = H i g h - f o o d , K = I n t e r m e d i a t e - f o o d .(1) = T r a p p a b i l i t y Over The P e r i o d J u l y 1972 To March 1974 (2) = T r a p p a b i l i t y Over The P e r i o d May 1974 To S e p t e a b e r 1975 115 Figure 19. Number o f v o l e s on g r i d s E, R, and T. A - g r i d E ( c o n t r o l ) x = g r i d H (low-food) 4» = g r i d T (high-food) A = food added to g r i d s R and T 117 the reason f o r t h i s i n c r e a s e i s that mice from the •marsh' area t h a t became a winter • l a k e 1 emigrated t o the s l i g h t l y higher ground along the edge o f the runway where the two experimental g r i d s were p o s i t i o n e d (Figure 18). The lake remains u n t i l A p r i l or May depending on the p r e c i p i t a t i o n , then i t becomes a temporary breeding h a b i t a t f o r M..townsendii i n summer. I delayed adding food s t a t i o n s to g r i d s R and T i n the hope t h a t the d e n s i t y o f v o l e s would d e c l i n e t o the l e v e l of the c o n t r o l . But over the three months (January to March), the c o n t r o l d e c l i n e d a t an average of 1% per week, while g r i d R continued to i n c r e a s e a t 2% per week and g r i d T at 4% per week. The sex r a t i o on both experimental g r i d s was biased i n favour of females (R 37:50, T 59:75), while on the c o n t r o l i t was even (19:17). The. number of voles on g r i d s R and T continued to i n c r e a s e a f t e r food was added on March 18 1973 ( F i g u r e 19) . The number of v o l e s on the low-food g r i d (R) i n c r e a s e d at 6% per week over the-next three months to peak at n e a r l y 3 times the c o n t r o l density., But the number of voles then d e c l i n e d s t e a d i l y (2% per week) on the low food g r i d (R) through the l a t e summer and autumn (July to October), The c o n t r o l (E) p o p u l a t i o n d e c l i n e d at 3% per week during October and November, while the low-food p o p u l a t i o n i n c r e a s e d at 1% per week. The p o p u l a t i o n (T) of voles s u p p l i e d with a h i g h - l e v e l o f food, in c o n t r a s t to both the c o n t r o l and low-food p o p u l a t i o n , continued t o i n c r e a s e a t 2% per week throughout the whole p e r i o d (March to December 1973). I n i t i a l l y , females i n c r e a s e d more g u i c k l y than males on the high-food g r i d and became s i g n i f i c a n t l y ( c h i sguare, 6.30.3, p<0. 025) more abundant over the p e r i o d A p r i l to June 1973 (Appendix 10), But 118 by the end of the year the sex r a t i o was even. The o v e r a l l d e n s i t y o f v o l e s on the c o n t r o l and low-food g r i d d i d not change a p p r e c i a b l y over the f i r s t three months o f 1974. The sex r a t i a on the c o n t r o l became s l i g h t l y biased towards males, while on the low-food g r i d males d e c l i n e d i n numbers and females i n c r e a s e d . Over the same p e r i o d , the d e n s i t y of v o l e s i n the high-food p o p u l a t i o n d e c l i n e d because the number of males d e c l i n e d by 30%. T h e r e f o r e , both food g r i d s had p o p u l a t i o n s biased towards females. G r i d M was set up as a 10X10 g r i d of t r a p s at the end of Hay 1974. The d e n s i t y of v o l e s on the g r i d M was c l o s e to that of the c o n t r o l (Figure 20) at the end of August, when an i n t e r m e d i a t e d e n s i t y of food s t a t i o n s was added to g r i d M. The number of v o l e s on both g r i d s increased r a p i d l y ( c o n t r o l 7,3%, i n t e r m e d i a t e - f o o d 9.6% per week) over the next f o u r months, Again, the p o p u l a t i o n i n c r e a s e on the food g r i d was uneven between the sexes, females increased f a s t e r than males. The c o n t r o l p o p u l a t i o n remained f a i r l y s t a b l e over the e a r l y summer then i n c r e a s e d again towards the end of the study i n the f a l l of 1975, The i n t e r m e d i a t e - f o o d p o p u l a t i o n diverged from the c o n t r o l i n December and i n c r e a s e d to double I t s number by March. T h i s i n c r e a s e was c o n t r i b u t e d to e q u a l l y over t h i s time by both sexes. Over the next three months male d e n s i t y d e c l i n e d while females i n c r e a s e d s t i l l f u r t h e r with intermediate food.. 1 1 9 F i g u r e 20. Number of males and females on g r i d s E and M. unjoined p o i n t s = g r i d E ( c o n t r o l ) j o i n e d p o i n t s = g r i d M (intermediate food) x = males + = females A ~ food added to g r i d M MINIMUM NUMBER 121 Immigration The t o t a l number o f new v o l e s captured on a l l g r i d s i n each three-month period of the study i s shown i n F i g u r e 21, There i s a f a i r l y c o n s i s t e n t p a t t e r n of capture o f new v o l e s on the c o n t r o l over the f i r s t two years of t h e study. Over the summer months the number of new v o l e s i s higher than over the winter periods. But a f t e r September 1974 n e a r l y twice as many new v o l e s were captured over the 1974/5 winter as over the previous two winters combined, Twice as many new v o l e s were recorded over the f o l l o w i n g summer compared with the previous year. G r i d s B and T had more new v o l e s than the c o n t r o l i n the nine months before food was provided. During the summer 1974, f o l l o w i n g food a d d i t i o n , the number of new voles on the low food g r i d (B) rose to 1.9 times the c o n t r o l number and on the high-food g r i d (T) to 2,5 times the c o n t r o l number. The low-food p o p u l a t i o n had almost the same number of new v o l e s as the c o n t r o l over the 1973/4 winter, but the high food g r i d maintained 2,9 times the c o n t r o l number. The number of new voles captured on the i n t e r m e d i a t e - f o o d g r i d (M) was 1.7 times that on the c o n t r o l overwinter (1974/5) and twice the c o n t r o l numbers over the summer 1975, The age and sex composition of new v o l e s e n t e r i n g each p o p u l a t i o n i s given i n Appendix 11, In terms of the sex composition of each age c l a s s , immigrants to a l l p o p u l a t i o n s of tpwnsendii e x h i b i t the same g e n e r a l pattern as P, maniculatus , The sex r a t i o i n the sub-adult category i s c l o s e to even,, Females predominate i n the j u v e n i l e age c l a s s , while new a d u l t s 122 F i g u r e 21. New v o l e s captured on g r i d s E, R, T, and M. = g r i d E (control) • = g r i d B (low food) 0 = g r i d T (high food) ^ •= g r i d H (intermediate food) A 1 = food added to g r i d s R and T A 2 - food added to g r i d M 124 i n a l l p o p u l a t i o n s are predominately males. A f t e r food was added to g r i d s R and T both p o p u l a t i o n s i n c r e a s e d over A p r i l - June 1973 because of high immigration of v o l e s of a l l age c l a s s e s , over the next t h r e e months, a d u l t immigration dropped on both experimental g r i d s to the same l e v e l s as on the c o n t r o l . Sub-adults and j u v e n i l e s entered the low-food p o p u l a t i o n (R) at the same high rate as i n the previous three months but then d e c l i n e d over winter to c o n t r o l l e v e l s , Meanwhile, the h i g h - l e v e l food g r i d (T) had twice as many immigrants i n the sub-adult age group i n l a t e summer and continued with high r a t e s i n f o the winter i n c o n t r a s t to the c o n t r o l and low-food g r i d s . The f i r s t new v o l e s on the in t e r m e d i a t e - f o o d g r i d (M) were i n the ad u l t and sub-adult age c l a s s . But over the l a s t s i x months (Janua ry to June), twice as many v o l e s immigrated to g r i d M as t o the c o n t r o l . Over the year that food was added to g r i d s R and T, the sex r a t i o of new v o l e s to the c o n t r o l was biased towards males (1 06:70). Males a l s o predominated (181:152) amongst immigrants t o the c o n t r o l over the nine months of i n t e r m e d i a t e food a d d i t i o n to g r i d M. The number of males amongst new v o l e s on each food g r i d , i n order o f food l e v e l from low to high, was: 1.5 times, 1.8 times, and 2.2 times the number on the c o n t r o l . S i m i l a r l y , new females on each food g r i d i n order were: 1. 7 times, 2.3 times, and 3,1 times those on the c o n t r o l . T h e r e f o r e , both sexes entered food g r i d s i n p r o p o r t i o n to the food a v a i l a b l e . 125 Br eedin_g Season I have separated the breeding data f o r the sexes of M* townsendii. Although some female M-. townsendii were p e r f o r a t e i n every t r a p p i n g p e r i o d of the study except one, the breeding i n t e n s i t y , In terms of the percentage of females l a c t a t i n g , seldom rose above 50%. Males, on the other hand, s u s t a i n e d a high l e v e l of breeding i n each season. The data are summarized i n Figure 22, where the breeding a c t i v i t y o f a d u l t s (voles weighing >42 g) on each g r i d i s given f o r the d u r a t i o n of the study. In a few i n s t a n c e s sub-adults (voles weighing 30 t o 42 g) were c o n s i d e r e d i f the percentage breeding was higher than i n the a d u l t age c l a s s . Before food was added to g r i d s R and T, male v o l e s on a l l g r i d s were breeding. The i n t e n s i t y of breeding i n males i n the h i g h e s t - d e n s i t y p o p u l a t i o n on g r i d T was s l i g h t l y l e s s than i n the other two populations. The f o l l o w i n g winter, males on the c o n t r o l had stopped breeding by the end of December 1973, On g r i d R, which had a higher d e n s i t y of voles and a low l e v e l of food, males had gone out o f breeding c o n d i t i o n s i x weeks e a r l i e r . But with a high l e v e l of food, males on g r i d T continued to breed. In f a c t they stopped breeding f o r only s i x weeks on g r i d T compared with 12 weeks on g r i d R, and ten weeks on the c o n t r o l . The breeding a c t i v i t y o f females was more s p o r a d i c and l e s s i n t e n s e than t h a t of males. There was no a p p r e c i a b l e i n c r e a s e i n breeding i n t e n s i t y of females with e x t r a food, but the p e r i o d of non-breeding was s h o r t e r i n the presence of a high d e n s i t y o f food. On g r i d R with a low d e n s i t y of food but twice the d e n s i t y 126 F i g u r e 22, Breeding a c t i v i t y on g r i d s E, R, T, and M, wide l i n e = 50% or more a d u l t s breeding middle l i n e = l e s s than 50% of a d u l t s breeding narrow l i n e = no breeding g r i d E (control) g r i d R (low food) g r i d T (high food) g r i d M (intermediate food) A 1 = food added to g r i d s R and T A. 2 = food added to g r i d M = food removed 1972 J S "N GRID E GRID R GRID T GRID M 1974 1975 M J S N J M M J 9 mm-4 V 2 1 2 8 of v o l e s compared with the c o n t r o l , females ceased breeding f o r twenty-six weeks compared with eighteen weeks on the c o n t r o l . Females i n the h i g h - d e n s i t y p o p u l a t i o n with a high l e v e l of food<T) d i d not breed f o r twelve weeks, Apart from one t r a p p i n g period a t the end of August 1975, males and females bred c o n t i n u o u s l y on the c o n t r o l from s p r i n g 1974 u n t i l the end of the study, More than 50% of the males were s c r o t a l on the int e r m e d i a t e - f o o d g r i d i n February 1975, t h i s was s i x weeks e a r l i e r than the c o n t r o l males, Females l a c t a t e d at a c o n s i s t e n t l y higher i n t e n s i t y i n the presence of intermediate food compared with c o n t r o l females i n 1975. A f t e r the removal of food s t a t i o n s at the beginning of J u l y , male r e p r o d u c t i o n dropped below 50% two weeks l a t e r and female breeding ceased completely, while both sexes on the c o n t r o l continued at t h e i r summer i n t e n s i t y . In summary, breeding of v o l e s i n the c o n t r o l p o p u l a t i o n a t Ladner was n e a r l y continuous throughout the study. T h i s made i t d i f f i c u l t to observe the i n f l u e n c e o f a d d i t i o n a l food on vole breeding. But, a pop u l a t i o n with double the c o n t r o l d e n s i t y of v o l e s , and onl y a l o w - l e v e l o f extra food, appeared to have a s h o r t e r breeding season than the c o n t r o l . But the a d d i t i o n of a h i g h - l e v e l of food appeared t o shorten the non-breeding season of both sexes i n a high d e n s i t y vole p o p u l a t i o n . F i n a l l y , i n 1975, v o l e s with an i n t e r m e d i a t e l e v e l o f food bred more i n t e n s i v e l y than c o n t r o l animals. When food was removed from t h i s p o p u l a t i o n , r e p r o d u c t i o n i n both sexes was reduced. 129 Breeding Success Reproductive output, i n terms of the number of younq r e c r u i t e d , v a r i e d widely on the c o n t r o l over the du r a t i o n of the study a t Ladner. For example, i n the three-month p e r i o d October to December 34 young were r e c r u i t e d i n 1972, 23 i n 1973, and 88 i n 1974. Before food was added to g r i d s R and T, 76 young had been r e c r u i t e d t o the c o n t r o l g r i d . The number r e c r u i t e d to g r i d S was 1.4 times h i g h e r (113) and to g r i d T was 1.9 times higher (148) than the number to the c o n t r o l , Over the s i x months f o l l o w i n g food a d d i t i o n 59 young were r e c r u i t e d t o the c o n t r o l (E) , The low-food p o p u l a t i o n had 163 young (2.8 times g r i d E) , and the high-food g r i d had 215 young (3.6 times g r i d E) over t h i s period. So the number of young e n t e r i n g t h e food g r i d s was twice the pre-food l e v e l . Before food a d d i t i o n to g r i d M, the number of young v o l e s present was s i m i l a r t o the number on the c o n t r o l . But durin g the next nine months over twice as many young(337) were r e c r u i t e d to the i n t e r m e d i a t e - f o o d p o p u l a t i o n than t o the c o n t r o l ( 1 5 9 ) . . E a r l y j u v e n i l e s u r v i v a l i s shown i n Table XIV. Before food was added g r i d T had higher j u v e n i l e s u r v i v a l but a lower r a t e of female l a c t a t i o n than g r i d R. J u v e n i l e s u r v i v a l on g r i d R was lower than on the c o n t r o l . In the s i x months a f t e r food a d d i t i o n , the number of l a c t a t i n g females on both food g r i d s was egual. J u v e n i l e s u r v i v a l on the food g r i d s was lower i n the f i r s t h a l f of 1973 than on the c o n t r o l , But i n the second h a l f of the summer i t improved on the food g r i d s , and was hig h e s t on the high-food g r i d (T). Before food was added to g r i d M, 1 3 0 Table XIV. E a r l y j u v e n i l e s u r v i v a l on g r i d s E, R, T, and M, The number of l a c t a t i n g females i s gi v e n i n parentheses. Date Gri d E G r i d R Grid T G r i d M (c o n t r o l ) (low-food) (high-food) (i n t . - f o o d ) J u l-Sep 1972 Oct-Dec " Jan-Mar " Apr-Jun " Jul-Sep " Oct-Dec " Jan-Mar 1973 Jul-Sep »» Oct-Dec » 2.13 (15) 3. 78 (9) 0.77 (13) 0. 96 (28) 0. 91(35) 1. 64 (14) 13. 00 (1) 0, 87 (46) 2,84 (31) 0. 44 (10) 2. 50(18) 0. 56 (43) 0.77(105) 1 .26(65) 25. 00 (1) 3. 75(4) 4. 33(9) 4. 79(14) 1. 27(33) 0. 78{ 105) 2. 02 (66) 2, 77(26) 1. 90 ( 10) 2. 05(17) 1. 49(71) Jan-Mar 1974 Apr-Jun " 0, 90 (49) 0. 16 (168) 1. 40 (77) 0. 56 (220) 131 j u v e n i l e s u r v i v a l was higher than on the c o n t r o l , but the number of females was a l s o lower,. In the f i r s t s i x months of 1975 the number of l a c t a t i n g females was higher i n the presence of food, and j u v e n i l e s u r v i v a l was a l s o b e t t e r than on the c o n t r o l . Weights And Growth The mean weights of males on g r i d s E, 8, and T are p l o t t e d i n F igure 23, There i s a s t r o n g annual c y c l e i n mean weight on a l l g r i d s . The lowest mean weights are recorded i n e a r l y to mid-winter and the highest weights i n e a r l y summer, The a d d i t i o n of food t o g r i d s R and T i n 1973 r e s u l t e d i n the mean weight of these p o p u l a t i o n s becoming s i g n i f i c a n t l y higher than that on the c o n t r o l by the f o l l o w i n g t r a p p i n g p e r i o d . On g r i d T with the hi g h e s t l e v e l of food the mean was s i g n i f i c a n t l y higher than the means on both g r i d R and the c o n t r o l . Four weeks l a t e r g r i d T reached i t s peak of 57g while the c o n t r o l was s t i l l o nly at 53g. The lowest winter weights on the c o n t r o l were lower than those on the low-food g r i d , which i n t u r n were lower than those on the high-food g r i d . In f a c t , v o l es on g r i d T maintained higher mean weights than d i d the c o n t r o l s from the end of November 1973 t o the ploughing of the g r i d i n A p r i l 1974. Instantaneous r e l a t i v e growth r a t e s were c a l c u l a t e d f o r vo l e s as d e s c r i b e d f o r P, l a n i c u l a t u s . Before food was added t o g r i d s R and T, the growth r a t e s on these h i g h - d e n s i t y g r i d s was lower than on the low d e n s i t y c o n t r o l (E) {see Appendix 12), In the period immediately f o l l o w i n g food a d d i t i o n , the growth r a t e s 132 F i g u r e 23, Mean weights of v o l e s on g r i d s E, R, and T, top = g r i d E (control) middle = g r i d R (low food) bottom = g r i d T (high food) A = food added to g r i d s R and T 6 0 GRID F. 5 0 / 40 3 0 H i l l 6 0 5 0 !-0 JO / A GRID R J r •(ID I I J F M A M J J A S O N D J F M k 1 9 7 3 1 9 7 4 134 i n a l l age-sex groups were higher on the low-food g r i d (B), and highest on the high food g r i d (T) , A month l a t e r , a l l v o l e s on the c o n t r o l had extremely high growth r a t e s . A l l age-sex groups i n the low food p o p u l a t i o n (R), except j u v e n i l e females i n August, had lower growth r a t e s than c o n t r o l s through the summer (June to August)* Voles i n the high-food p o p u l a t i o n (T) had higher growth r a t e s than v o l e s on g r i d R, but apart from a l l males i n May, each age-sex group had lower growth r a t e s than v o l e s on the low-density c o n t r o l u n t i l the end of J u l y , T h e r e f o r e , v o l e s with access to dry, supplemental mouse chow i n mid-summer had lower growth r a t e s than v o l e s on a low d e n s i t y c o n t r o l g r i d . T h i s s i t u a t i o n was r e v e r s e d i n the winter months. Voles on g r i d R had higher growth r a t e s than voles on the c o n t r o l 19 out of 24 p o s s i b l e times (sign t e s t , p=0.003). On the high-food g r i d (T) v o l e s had higher growth r a t e s 24 out of 30 times i n the p e r i o d from August to December 1973. In l a t e winter, the growth r a t e o f v o l e s on the c o n t r o l i n c r e a s e d , and, as i n P. maniculatus, males i n c r e a s e d before females. At t h i s time on the food g r i d s , as i n P, maniculatus with food, males and females had higher growth r a t e s at the same time. Growth r a t e s of v o l e s on the c o n t r o l through the 1974/5 winter were much higher than i n both previous w i n t e r s , , I n f a c t , no negative growth r a t e s were recorded on the c o n t r o l i n t h i s mild winter. However, v o l e s on the i n t e r m e d i a t e food g r i d (M) had higher growth r a t e s than c o n t r o l v o l e s 35 out of 42 times. But again, t h e e x t r a food, t h i s time a d m i t t e d l y o a t s , d i d not i n c r e a s e growth r a t e s i n the summer 1975 on g r i d M, 135 S u r v i v a l The s u r v i v a l of v o l e s on a l l g r i d s f l u c t u a t e d between 0,80 and 0.90 from J u l y 1972 to March 1974 (Table XV). In the nine months before food was added, v o l e s u r v i v a l on g r i d s R and T was higher than on the c o n t r o l , The a d d i t i o n of supplemental food to both experimental g r i d s d i d not change the s u r v i v a l i n t h e i r v o l e populations. But s u r v i v a l on g r i d M, with an i n t e r m e d i a t e l e v e l of food, was lower i n both sexes compared with the c o n t r o l from October 1974 to June 1975, In p a r t i c u l a r , male s u r v i v a l was below. .80 i n 17 out of a p o s s i b l e 18 sample periods. In the two months f o l l o w i n g food removal, male s u r v i v a l d e c l i n e d to 0,43 and female s u r v i v a l to 0,52 compared with 0.77 and 0,78 r e s p e c t i v e l y on the c o n t r o l . Horce Range S i z e Home ranges o f v o l e s were c a l c u l a t e d by the same method as described f o r P, SL§JLiculaj:us» Voles with one capture p o i n t , and more than one capture i n a s t r a i g h t l i n e (but not on the p e r i p h e r a l rows of the grids) have been i n c l u d e d u s i n g the estimates g i v e n i n Appendix 9. The home ranges were c a l c u l a t e d f o r blocks of data, one f o r the nine months before food was added to g r i d s R and T (July 1972 to March 1973), one f o r the same time p e r i o d a year l a t e r , when g r i d s R and T were r e c e i v i n g supplemental food, and one the f o l l o w i n g year, when q r i d M was being fed (see T a b l e XVI). In the c o n t r o l year (1972/3) there were no s i g n i f i c a n t d i f f e r e n c e s i n home range s i z e of the sexes on the c o n t r o l from those on the experimental g r i d s . But during 136 Table XV, S u r v i v a l o f v o l e s on g r i d s E, R, T, and M. The number of v o l e s i s gi v e n i n parentheses, followed by the number of p e r i o d s that s u r v i v a l was >0,90 / and CO, 80. J u l y 1972 to March J973 G r i d Males Females E 0.83(332) 7/6 0.87(381) 11/3 R 0.88(445) 12/3 0.89 (615) 12/3 T 0. 88(603) 7/3 0,93 (757) 15/1 Jul y 1973 to March 1974 Males Females 0.86 (433)5/6 0.89 (378) 10/1 0. 87(780) 9/3 0. 90(726) 9/1 0.87 (1 104)8/2 0.90 (249) 13/1 October 1974 to June J.975 E 0, 83(725) 3/8 0. 85 (787) 5/4 M 0.69(792) 1/17 0.78 (919) 5/18 E = c o n t r o l , R = low-food, T = high-food, M = interm.-food 137 Table XVI, Mean home range s i z e {sg m) of v o l e s on g r i d s E, B, . and H. The p e r i o d s are J u l y t o March i n each year. 121211 1573/4 12Z!£/5 G r i d E Males Females 804(42) 546 (45) 113 4(51) 608(38) 670 (85) 391 (79) G r i d E Males Females 1050(50) 578 (65) 445 (79)* 331(78) * --G r i d T Males Females 795(74) 423(89) 328(102) *** 261 (114)*** G r i d M Males Females — 447 (87) ** 231 (92)** *** = high food intermediate food low food 138 the p e r i o d of food a d d i t i o n , males on g r i d R and g r i d T had s i g n i f i c a n t l y s m a l l e r home ranges than d i d males on the c o n t r o l ( g r i d R: t=3,12, p<0,01; g r i d T: t=3,66, p<0.001), S i m i l a r l y , females on the experimental g r i d s had s m a l l e r home ranges than d i d c o n t r o l s over t h i s p e r i o d ( g r i d R: t=3. 14, p<0,01; g r i d T; t=3.88, p<0,001), Although males and females on the c o n t r o l had smaller home ranges i n the 1974/5 p e r i o d than i n the pre v i o u s two p e r i o d s , those with an i n t e r m e d i a t e l e v e l o f food ( g r i d M) had s i g n i f i c a n t l y s m a l l e r home ranges than c o n t r o l v o l e s (males: t=2.63, p<0,01; females: t=3, 33, p<0,001), Females had sm a l l e r home ranges than males i n a l l p e r i o d s on a l l g r i d s (except g r i d T with high-food) . 139 S e c t i o n I I : C o l o n i z a t i o n a nd Food Densi t y The most s i g n i f i c a n t response t o food a d d i t i o n i n both £• maniculatus and M. tqwnsendii was a l a r g e i n c r e a s e i n immigration. In the next s e r i e s of short-term experiments (see Fi g u r e s 17 and 18) , I attempted to see i f immigrants to vacant areas were s e t t l i n g i n response to the s p a t i a l p a t t e r n of food. The experiment was planned f o r the winter non-breeding season (1974/75) and the summer breeding season (1975). At the o u t s e t of the experiment on November 18-20th 1974, I removed a l l voles from a l l g r i d s . On November 25, a high d e n s i t y of food s t a t i o n s (1 per 11 sq m) was placed on g r i d 1, a lower d e n s i t y (1 per 33 sg m) on g r i d 2, and none on g r i d 3. A f t e r t h r e e weeks of immigration to these three food l e v e l s , a l l voles were removed i n the mid-December t r a p p i n g period. I reduced the den s i t y of food s t a t i o n s on g r i d 2 to one per 68 sg m. Voles were then allowed to immigrate f r e e l y f o r the next f o u r months to a l l three g r i d s . I reduced the density of food s t a t i o n s on both g r i d s 1 and 2 by h a l f i n January 1975. Food cans were removed a t the end of A p r i l , and a l l voles were removed f o r the next t h r e e t r a p p i n g s e s s i o n s . The experimental treatments were then switched so that no s t a t i o n s were placed on g r i d 1, g r i d 2 had a high d e n s i t y of food (1 per 87 sg m), and g r i d 3 now had a low d e n s i t y ( 1 s t a t i o n per 136 sq m). 140 Pop u l a t i o n Density The number of vo l e s removed ( v e r t i c a l l i n e s ) and the minimum number a l i v e for the i n t e r v e n i n g c o l o n i z a t i o n p e r i o d s are shown i n Fig u r e 24, P r a c t i c a l l y the same number of vo l e s were removed from a l l t h r e e g r i d s before food was added, and a l s o a f t e r the f i r s t week of food a d d i t i o n . . B u t over the second and t h i r d week of food a d d i t i o n t o g r i d s 1 and 2, 37 new v o l e s entered the high-food p o p u l a t i o n (1), 28 entered t h e low-food p o p u l a t i o n (2) , and only 19 entered the c o n t r o l p o p u l a t i o n (3). Over the four-month c o l o n i z a t i o n p e r i o d , the po p u l a t i o n with a high l e v e l of food (1) i n c r e a s e d at an average of 50% per week compared with 30% and 35% r e s p e c t i v e l y on the low-food g r i d (2) and the c o n t r o l (3), A f t e r the th r e e removal periods i n A p r i l and May 1975, the t o t a l number of v o l e s (males:females) t h a t had c o l o n i z e d the high-food g r i d was 188 (97 :91), compared with 1 16 (62:76) on the low-food g r i d (2), and only 53( 16: 37) on the c o n t r o l . T h e r e f o r e , three times as many voles c o l o n i z e d an area with high-food compared with one with no food, and twice as many c o l o n i z e d an area with low-food compared with one with no food, The second c o l o n i z a t i o n experiment was not run f o r a long enough p e r i o d . However, voles again c o l o n i z e d the highest-food area (2) at a higher r a t e (22% per week) than they d i d the low-food area (10% per week) and the c o n t r o l (10% per week). , 141 F i g u r e 24, Number of v o l e s on g r i d s 1, 2, and 3. V e r t i c a l l i n e s are the number of v o l e s removed, 9 = g r i d 1 {high food then no food) O = g r i d 2 (low food then high food) = g r i d 3 (no food then low food) A 1 = high food t o g r i d 1 and low to g r i d 2 JJJs - food removed 2 = high food t o g r i d 2 and low to g r i d 3 143 Immigration A t o t a l of 241 new v o l e s was captured on the high-food area (1) over the f i r s t c o l o n i z a t i o n period (Table XVII), Only h a l f t h i s nuraber(136) of new voles was captured on the low-food area (2) and s l i g h t l y g r e a t e r than a q u a r t e r the number (77) on the c o n t r o l (3) » In the two-month c o l o n i z a t i o n period i n summer 1975, 153 new voles were captured on the high-food g r i d (2), 143 on the low-food area (3), and 106 on the c o n t r o l (1) . During the f i r s t c o l o n i z a t i o n p e r i o d , the number of new female v o l e s on the low food area (2) and the c o n t r o l (3) were s i m i l a r . But n e a r l y three times the c o n t r o l number of females entered the high-food area (1), New males, on the other hand, entered the p o p u l a t i o n s i n p r o p o r t i o n to the food a v a i l a b l e . Over the short summer p e r i o d , the trend i n the sexes was r e v e r s e d , namely, females appeared to be more responsive to the d i f f e r e n t food l e v e l s . During the f i r s t c o l o n i z a t i o n p e r i o d , the age-sex composition of immigrants r e f l e c t s p r evious t r e n d s found f o r both s p e c i e s i n t h i s study. Namely, the sex r a t i o of sub-adults e n t e r i n g a l l g r i d s combined was c l o s e to even. Adult immigrants were predominantly males (163:110), and j u v e n i l e s were predominantly females (24:44), A second f e a t u r e i s that an i n c r e a s i n g number of s m a l l e r v o l e s immigrated t o areas i n p r o p o r t i o n to the food a v a i l a b l e . During the f i r s t c o l o n i z a t i o n p e r i o d , only 27% of the immigrants to the c o n t r o l (3) were j u v e n i l e s and sub-adults compared with 35% and 47% of those to the low- and high-food areas r e s p e c t i v e l y . 144 Table XVII. New v o l e s captured on g r i d s 1, 2, and 3 i n each c o l o n i z a t i o n period, Ratio o f number of males:females Jan, to May 1975 Jun. t o J u l . 1975 G r i d 1 a d u l t s Sub-adults J u v e n i l e s 78:51** 34:35** 15:28** 36:26 16 :24 2:2 G r i d 2 A d u l t s Sub-adults J u v e n i l e s 56:32* 19:12* 8:9* 38:41** 20:30** 7:17** G r i d 3 a d u l t s Sub-adults J u v e n i l e s 29:27 7:6 1:7 46:33* 18:27* 9:7* ** = high food low food 145 Breeding C o n d i t i o n And Reproductive Success A l l v o l e s bred through the winter 19 74/5 at Ladner, so i t was not p o s s i b l e to d e t e c t any i n f l u e n c e of food on the breeding season on g r i d s 1, 2, and 3. The number of males and females i n r e p r o d u c t i v e c o n d i t i o n i s shown i n Table XVIII, Over the w i n t e r - s p r i n g c o l o n i z a t i o n p e r i o d , males and females bred at a higher i n t e n s i t y on g r i d s with e x t r a food. The number of females breeding i n the food p o p u l a t i o n s (1 and 2) was i n p r o p o r t i o n to the amount of food a v a i l a b l e . Over the s h o r t summer c o l o n i z a t i o n p e r i o d , males bred e q u a l l y i n t e n s i v e l y on g r i d s with and without food, but females again bred more i n t e n s i v e l y i n the presence o f e x t r a food. The number of young (<40g) r e c r u i t e d r e f l e c t e d the i n t e n s i t y of female breeding i n each population, Between January and May, 52 young v o l e s were r e c r u i t e d to the high-food g r i d (1), 28 t o the low-food (2) and only 10 to the c o n t r o l (3). Over the two month summer c o l o n i z a t i o n , 26 young were r e c r u i t e d to the high-food p o p u l a t i o n (2), 18 to the low-food (3) and only 9 to the c o n t r o l . 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 higher i n both c o l o n i z a t i o n p e r i o d s i n p r o p o r t i o n to the food made a v a i l a b l e to the p o p u l a t i o n , In the h i g h - f o o d p o p u l a t i o n s , 1,41 j u v e n i l e s were captured per l a c t a t i n g female i n the f i r s t p e r i o d , and 1.73 i n • the second. E a r l y j u v e n i l e s u r v i v a l i n the low-food p o p u l a t i o n s was 1,00, and 1,39, and on the c o n t r o l s i t was 0.50, and 0.82 i n each c o l o n i z a t i o n p e r i o d r e s p e c t i v e l y . 146 Table XVIII. Mean number of males s c r o t a l and females l a c t a t i n g on g r i d s 1, 2, and 3, Jan. to May 1975 Jun, t o J u l , 1975 G r i d 1 Bales 11.3** 12,8 Females 7.4** 3,0 G r i d 2 Males 12.7* 12.0** Females 5,0* 5. 3** G r i d 3 Males 4.1 15,3* Females 3.9 5.0* ** = high food * = low food 147 S e c t i o n I I I : S p r i n g D e c l i n e And Food Supple§entatign The f l u c t u a t i o n s i n the number o f mice on g r i d s I and G were very s i m i l a r t o one another from J u l y 1971 to J u l y 1973 (see leDuc and Krebs 1975). The pattern of change i n numbers i s a l s o remarkably c o n s i s t e n t between years over t h i s p e r i o d and again i n 1974. Table XIX l i s t s the t r a p p i n g p e r i o d s when the po p u l a t i o n on g r i d I reached maximum and minimum numbers each year. The p o p u l a t i o n was unusual i n 1971: i t was at a low-density when t r a p p i n g began, and r e p r o d u c t i o n continued throughout the f i r s t winter (see LeDuc and Krebs 1975) , In 1972 and 1973 the p o p u l a t i o n peaked i n December then d e c l i n e d over the s p r i n g to reach i t s lowest l e v e l s i n l a t e A p r i l and e a r l y May i n a l l t h r e e years. In 197 3 and 19 74 the number of voles at the end of t h i s s p r i n g d e c l i n e was almost egual at 46 and 41 r e s p e c t i v e l y , and 100% o f the adult males were s c r o t a l . T h i s p a t t e r n i s s u g g e s t i v e of the annual c y c l e i n numbers e x h i b i t e d by R' I j i f i i c u l a t u s . The p o p u l a t i o n d e c l i n e d i n the s p r i n g to a low-density breeding p o p u l a t i o n . Then, when breeding ceased i n the f a l l the p o p u l a t i o n i n c r e a s e d t o a winter peak i n d e n s i t y . Food s t a t i o n s were p l a c e d on g r i d G on December 1st 1974, Both g r i d G and the c o n t r o l g r i d I populations were w e l l i n t o t h e i r winter i n c r e a s e by t h i s time, But the d e n s i t y p a t t e r n over t h i s winter turned out to be completely d i f f e r e n t from that i n a l l other years. Males on g r i d I peaked e a r l y i n January at a much higher d e n s i t y than i n previous y e a r s , and females continued to i n c r e a s e i n numbers u n t i l March, Further, n e i t h e r sex e x h i b i t e d a s p r i n g d e c l i n e i n d e n s i t y ; by the end o f A p r i l 148 Table XIX, P e r i o d s of maximum and minimum numbers at R i e f e l from 1972 to 1975. Maximum numbers T o t a l Males Females 10 t o 12th January 1972 170 12 to 14th December 1972 231 22 to 24th December 1973 233 6 t o 8th January 1975 327 77 93 1 18 149 9 3 (*2) 138 115 174 (+10) Minimum numbers 1-3rd May 1972 30 A p r i l t o 2 May 1973 15 to 17th A p r i l 1974 28 to 30th A p r i l 1974 88 46 41 273 26 14 15 95 (+2) 62(-2) 32 26 (+2) 178 (- or + #) i n d i c a t e s the weeks before or a f t e r the date given t h a t numbers peaked f o r t h i s sex 149 1975, the number of males and females was s i x times that o f the previous two years (see T a b l e XIX). T h i s , combined with the very low t r a p p a b i l i t y of v o l e s i n the extremely dense popu l a t i o n on g r i d I throughout the summer of 1975, and the r e s u l t i n g o v e r g r a z i n g , made any attempt a t data comparison between the two g r i d s meaningless. S i m i l a r l y , the number of d i s p e r s i n g v o l e s was f a i r l y low on the removal areas which made the r e s u l t s of the s e l e c t i v e removal experiment almost meaningless. I t would be very i n t e r e s t i n g t o conduct these experiments again under more t i g h t l y c o n t r o l l e d c o n d i t i o n s . In order t o overcome the p o s s i b i l i t y of being swamped by v o l e s immigating to the e x t r a food, I would conduct t h e f e e d i n g experiment and s e l e c t i v e removals on fenced g r i d s with a d i s p e r s a l ' s i n k ' i n c l u d e d (of the type now being used by Beacham, Ph. D, i n progress) . 150 l | . DISCUSSION I t i s hard to i n t e r p r e t the the e f f e c t o f the three l e v e l s of long-term food supplementation on the dynamics of these !• i p w n s e n d i i p o p u l a t i o n s . T h i s i s because o f : -1. The c o n t r o l g r i d (E) e x h i b i t e d an unusual p a t t e r n of f l u c t u a t i o n over the f o u r years of the study. The d e n s i t y of v o l e s was f a i r l y s t a b l e f o r the d u r a t i o n of the low- and h i g h - l e v e l food experiments, But, d u r i n g the p e r i o d of i n t e r m e d i a t e food supplement to g r i d M, the number of v o l e s on the c o n t r o l proceeded to i n c r e a s e f a i r l y s t e a d i l y , 2. I t i s d i f f i c u l t t o decide what i n f l u e n c e the seasonal a v a i l a b l i t y o f breeding h a b i t a t had on experimental g r i d s R and T. In the summer the 'lake area* d r i e d out and was c o l o n i z e d by v o l e s , but i n the autumn and winter the area became a shallow l a ke. 3. The two long-term food g r i d s were destroyed a f t e r only one year of supplementary feeding, 4. Apart from g r i d s R and T, I was not able to feed these fl« a s u i t a b l e supplementary food. In f a c t , I doubt the e f f e c t i v e n e s s of the chow that was used on g r i d s R and T i n wet and very dry weather c o n d i t i o n s . Before food was added, the density of v o l e s on experimental g r i d s R and T i n c r e a s e d ; t h i s was probably i n response to f l o o d i n g of the 'marsh*. In r e t r o s p e c t , I should have delayed adding food u n t i l the l a k e had d r i e d out i n the summer of 1973, because some voles might have l e f t both areas (R and T) to c o l o n i z e the new h a b i t a t , and hence the d e n s i t i e s on these g r i d s 151 might have been c l o s e r to t h a t on the c o n t r o l (E). However, a f t e r one year o f l o w - l e v e l food supplement, the g r i d E p o p u l a t i o n s t i l l had twice the number of voles that were present on the c o n t r o l (E) , The g r i d T p o p u l a t i o n reached 4.2 times the c o n t r o l d e n s i t y a f t e r one year of high-food compared with i t s pre-food d e n s i t y of 2, 8 times the c o n t r o l d e n s i t y . A f t e r nine months of i n t e r m e d i a t e food, g r i d n had double the c o n t r o l d e n s i t y of v o l e s , having been at the same d e n s i t y p r i o r to food a d d i t i o n . T h e r e f o r e , over the periods that these experiments were conducted, areas with e x t r a food were able to maintain higher vole p o p u l a t i o n s than that on a c o n t r o l area with no food. Also, p o p u l a t i o n s with i n t e r m e d i a t e and high d e n s i t i e s of food s t a t i o n s doubled t h e i r pre-food numbers. The number of new v o l e s captured on an area was always higher i n p o p u l a t i o n s with e x t r a food. A l s o v o l e s immigrated i n t o the p o p u l a t i o n s i n p r o p o r t i o n to the e x t r a food a v a i l a b l e . T h i s may have been f a c i l i t a t e d by males and females reducing the s i z e of t h e i r home ranges, on areas with e x t r a food compared with those of v o l e s on the c o n t r o l areas. The c o l o n i z a t i o n of vacant areas provided with d i f f e r e n t l e v e l s of food confirmed the o b s e r v a t i o n s f o r the long-term food g r i d s . The l a r g e s t p o p u l a t i o n s were e s t a b l i s h e d on areas with the h i g h e s t l e v e l s of extra food. Also, such areas had higher numbers of immigrants i n p r o p o r t i o n to the food a v a i l a b l e . I t was d i f f i c u l t to detect any i n f l u e n c e of food a v a i l a b i l i t y on the breeding season i n ML .townsendii, p a r t l y because breeding was almost continuous on the c o n t r o l s . The non-breeding season was s h o r t e r i n a p o p u l a t i o n with a high 152 l e v e l of food compared with one with no e x t r a food. Also more male and female v o l e s bred on areas with e x t r a food. Larger numbers of young v o l e s were r e c r u i t e d t o food p o p u l a t i o n s than to the c o n t r o l s , and e a r l y j u v e n i l e s u r v i v a l was improved with e x t r a food. F i n a l l y , mean growth r a t e s of v o l e s were higher over winter on areas with a d d i t i o n a l food. Since the present study was i n i t i a t e d , two f e e d i n g experiments conducted on the European bank vole £2§thripnpmys g l a r e p l u s have been reported, Flowerdew (1973) i n England found no s i g n i f i c a n t change i n the breeding season of bank v o l e s s u p p l i e d with e x t r a wheat f o o l . But these voles d i d have higher o v e r w i n t e r i n g weights than d i d the c o n t r o l s . Andrzejewski (1975) i n Poland found t h a t 90% of female bank v o l e s had an open vagina i n e a r l y s p r i n g on a p l o t with h i g h - l e v e l oat food, compared with 53% on a low-food area and only 23% on a c o n t r o l p l o t . Both s t u d i e s reported l i t t l e change i n s u r v i v a l of v o l e s on t h e i r food p l o t s , Andrzejewski (1975) found a l a r g e i n c r e a s e In immigration to h i s food p l o t s , but Flowerdew (1972, 1973) found an i n c r e a s e one year but not the next. In a l a t e r short-terra experiment, Flowerdew (1976) concluded t h a t h i s e g u i v o c a l response r e s u l t e d from the f a c t t h a t the bank v o l e s d i d not s i g n i f i c a n t l y change t h e i r d i s t r i b u t i o n i n response to the e x t r a food. In another experiment i n Poland, Andrzejewski and Mazurkiewicz (1976) found t h a t an i s l a n d p opulation o f bank v o l e s with supplemental food had s m a l l e r home ranges than they had had i n previous non-food years. The e g u i v o c a l nature of some of the r e s u l t s of these food 153 experiments on voles may be a conseguence of the type of supplementary food that was provided, I f vo l e s p r e f e r green p l a n t food, as suggested by B a t z l i (1974), or r e q u i r e i t t o reproduce, as suggested by Negus et a l , (1977), then such r e s u l t s might be explained,. But i n s p i t e of the type of supplementary food used i n the present study, the r e s u l t s i n d i c a t e t h a t v o l e s are s e n s i t i v e to the a v a i l a b i l i t y of food resources i n t h e i r environment. 154 GENERAL CONCLUSIONS The s m a l l mammals P. maniculatus and M, townsendii e x h i b i t spacing behaviour by the p o s s e s s i o n of home ranges. T h i s behaviour may l i m i t the number o f breeding i n d i v i d u a l s i n a given area. Two r e c e n t experiments by F a i r b a i r n (1976) on 1* ffia£iculatus and Krebs et a l . (manuscript) on M. townsendii provide more thorough evidence f o r t h i s , By using a 'pulsed removal' design these authors showed t h a t d i s p e r s e r s i n t o an area from which r e s i d e n t s had been removed were able to c o l o n i z e and e s t a b l i s h v i a b l e p o p u l a t i o n s . Thus both P. maniculatus and fl. townsendii f u l f i l l c o n d i t i o n s A and B (Table XII) of the experimental design suggested by Watson and Moss.(1970) f o r showing t h a t spacing behaviour l i m i t s p o p u l a t i o n s i z e . P. mafiicsiaiMs i° the present study was able to breed i n winter when e x t r a food was provided. T h i s may i n d i c a t e t h a t food i t s e l f l i m i t s breeding i n P, maniculatus at t h i s time of the year, There i s no evidence, at p r e s e n t , as to whether food or any other e x t r i n s i c r e s o u r c e i s l i m i t i n g f o r these s m a l l mammal po p u l a t i o n s ( c o n d i t i o n C). F u r t h e r , i t i s a l s o not p o s s i b l e t o say whether sp a c i n g behaviour l i m i t s these p o p u l a t i o n s over long p e r i o d s of time ( c o n d i t i o n D) . The o v e r a l l purpose of the present study was to see i f s p a c i n g behaviour (measured as home ranges) of P. ma,ni£ula,tus and M. townsendii c o u l d be changed f o l l o w i n g an experimental change i n food a v a i l a b i l i t y ( t e s t i n g c o n d i t i o n E, see Table X I I ) , P. maniculatus with e x t r a food reached a maximum d e n s i t y 155 of 82 mice/hectare compared with 33 mice/hectare on the c o n t r o l , J9.» i2IS§&a§.ii with a high l e v e l of ex t r a food peaked at 734 v o l e s / h e c t a r e compared with 209 v o l e s / h e c t a r e i n a low food p o p u l a t i o n , and 98 v o l e s / h e c t a r e on the c o n t r o l , The f o l l o w i n g year, an i n t e r m e d i a t e food p o p u l a t i o n of M. townsendii peaked a t 504 v o l e s / h e c t a r e compared with the c o n t r o l , which t h i s year had 280 v o l e s / h e c t a r e . Females i n both s p e c i e s responded more g u i c k l y than males to the a d d i t i o n of food, Immigration to a l l p o p u l a t i o n s of both s p e c i e s with e x t r a food was always higher than to the c o n t r o l s . The age-sex d i s t r i b u t i o n of the immigrants to a l l p o p u l a t i o n s showed the same p a t t e r n : there were s i g n i f i c a n t l y more females i n t h e j u v e n i l e c l a s s , and s i g n i f i c a n t l y more males i n the a d u l t c l a s s . Residents i n po p u l a t i o n s of both s p e c i e s reduced t h e i r home range s i z e i n the presence of e x t r a food, and t h i s probably f a c i l i t a t e d the i n c r e a s e d immigration. T h e r e f o r e , the home ranges of these s m a l l mammals were re s p o n s i v e to changes i n food a v a i l a b i l i t y , and t h i s a f f e c t e d t h e i r p o p u l a t i o n dynamics, thus p r o v i d i n g evidence f o r c o n d i t i o n E (Table XII), But having done t h i s , I t h i n k i t remains t o be determined whether any other f a c t o r s (as suggested i n F igure 1) can a c t on sp a c i n g behaviour i n t h i s way. F l e x i b i l i t y of s p a t i a l o r g a n i s a t i o n i n response to food a v a i l a b i l i t y has r e c e n t l y been documented f o r other v e r t e b r a t e s . Mares et a l , (1976) showed that Eastern chipmunks reduced the s i z e of t h e i r home ranges i n response to t h r e e weeks of food supplementation. Iguanid l i z a r d s reduced the s i z e of t h e i r t e r r i t o r i e s i n response to only 14 days of food a d d i t i o n (Simon 1975). Both s p e c i e s expanded t h e i r r e s p e c t i v e home ranges 156 f o l l o w i n g withdrawal of the e x t r a food. These s t u d i e s , l i k e the present one, are short-term experimental p e r t u r b a t i o n s . But a study by Zach and F a l l s (1975) i n d i c a t e s that such responses can and do occur n a t u r a l l y . They observed t h a t ovenbirds responded to a spruce budworm outbreak by reducing t h e i r t e r r i t o r y s i z e and i n c r e a s i n g i n d e n s i t y . These s t u d i e s i n d i c a t e t h a t the s p a t i a l o r g a n i s a t i o n o f e s t a b l i s h e d i n d i v i d u a l s can change i n response t o p e r t u r b a t i o n s i n food a v a i l a b i l i t y , and that t h i s i n tu r n can change the number o f i n d i v i d u a l s t h a t occur i n a given area. P. maniculatus and M, townsendii have d i f f e r e n t food h a b i t s . The r e s u l t s of the present study c o n f i r m HcNab's (1963) o b s e r v a t i o n , t h a t the g r a n i v o r e s have l a r g e r home ranges than the h e r b i v o r e s . But a f t e r the p r o v i s i o n of ex t r a food, female E. S S U i c u l a t u s reduced t h e i r home ranges to the s i z e of those o f male M. townsendii. This supports McNab's (196 3) c o n c l u s i o n t h a t the d i f f e r e n c e i n home range s i z e i n the two groups i s a r e s u l t of the d i s t r i b u t i o n of food i n the h a b i t a t . F u r t h e r , the present study i n d i c a t e s t h a t both s p e c i e s can respond to d i f f e r e n t food d e n s i t i e s . T h e r e f o r e , i t i s i n t e r e s t i n g t o s p e c u l a t e that the s p a t i a l and temporal pattern of food i n the ' t y p i c a l ' h a b i t a t s that the two groups of s p e c i e s have been s t u d i e d i n p r e v i o u s l y may have i n f l u e n c e d the dynamics observed. I have suggested that P, maniculatus s u r v i v e s the temporal r e s t r i c t i o n of i t s food supply i n the f o r e s t overwinter by ceasing to breed, and hence e x h i b i t s an annual c y c l e i n numbers. By c o n t r a s t , the food supply i n a t y p i c a l homogeneous gras s l a n d h a b i t a t appears to be f a r l e s s r e s t r i c t e d f o r low d e n s i t y 157 i 5 i £ I ° i a s p o p u l a t i o n s . For example, a low d e n s i t y , i n c r e a s i n g p o p u l a t i o n of m i c r o t i n e s u s u a l l y breeds through i t s f i r s t winter, which may i n d i c a t e no temporal r e s t r i c t i o n of food and r e s u l t i n high p o p u l a t i o n d e n s i t i e s . On the other hand, the r e s u l t s of the present study i n d i c a t e t h a t P. maniculatus w i l l breed overwinter i n the f o r e s t i f ex t r a food i s made a v a i l a b l e , F u r t h e r , a p o p u l a t i o n o f M. pennsylvanicus l i v i n g i n woodland h a b i t a t (Grant, 1975) appeared t o cease r e p r o d u c t i o n i n winter, Perhaps a Micrptus p o p u l a t i o n would e x h i b i t an annual c y c l e i f i t s food supply c o u l d be temporally r e s t r i c t e d . Perhaps g r a s s l a n d p o p u l a t i o n s of M. tpwnsendii are a case i n point* Temporal and s p a t i a l h e t e r o g e n e i t y of whole p i e c e s of h a b i t a t may be imposed by the winter f l o o d i n g of these g r a s s l a n d s . In summer such areas are good breeding h a b i t a t , and the voles move out to use them._ But i n winter they have to l e a v e them f o r patches of higher ground, which themselves may not be high enough to allow deep burrows. T h i s i n c r e a s e d d e n s i t y may, through i n c r e a s e d i n t e r a c t i o n , as suggested f o r P. maniculatus, cause a d e c l i n e i n weight and h a l t r e p r o d u c t i o n , thus g i v i n g r i s e to an annual c y c l e i n numbers. F i n a l l y , a major d i f f e r e n c e between the two groups i s the f a c t that the food supply of v o l e s i s a l s o t h e i r cover p r o t e c t i o n . Under c o n d i t i o n s o f peak d e n s i t y , v o l e s have been observed t o reduce t h e i r major food p l a n t s by as much as 85% ( B a t z l i and P i t e l k a 1970). L o c a l l y , the s p a t i a l and temporal heterogeneity of v o l e h a b i t a t could serve to p r o t e c t patches o f grass from such o v e r e x p l o i t a t i o n by !!• t2HB§§B2ii' Then, f o r example, i f an impact of high d e n s i t i e s of v o l e s i s a r e d u c t i o n i n cover, t h i s w i l l occur i n patches. 158 But, as the water-table subsides i n the s p r i n q , the remaining v o l e s w i l l be able to invade the p r e v i o u s l y f l o o d e d patches, which now may be f a v o u r a b l e r e f u q i a , as w e l l as burrow deeper i n the o l d patches. a s i m i l a r patchwork of f a v o u r a b l e and temporally unfavourable h a b i t a t on a l a r q e r s c a l e may be important to m i c r o t i n e dynamics i n general. These s p e c u l a t i o n s are too parsimonious, but they should be r e a d i l y t e s t a b l e . I t would s t i l l be i n t e r e s t i n g t o determine the e f f e c t of food and removal of dominants on t h e s p r i n g d e c l i n e i n v o l e s , as o u t l i n e d i n S e c t i o n I I I above. Since voles responded to e x t r a food by i n c r e a s e d immigration, t h i s experiment should be conducted i n an e n c l o s u r e with a d i s p e r s a l sink (see Beacham Ph. D. i n progress) , a s u i t a b l e •herbivore-food-supplement* should a l s o be used. Second, i f f u t u r e feeding experiments are c a r r i e d out i n open p o p u l a t i o n s of e i t h e r s p e c i e s , i t would be i n t e r e s t i n g to p r o v i d e food t o the c e n t r a l part of a l a r g e trapped area. I t should then be p o s s i b l e t o determine whether the immigrants are l o c a l animals or not, T h i r d , i t would be i n t e r e s t i n g t o t e s t some of the i d e a s r a i s e d on the i n f l u e n c e o f h a b i t a t heterogeneity. S p e c i f i c a l l y , can P, maniculatus be induced to ' c y c l e * i f a c o n t i n u a l supply o f s u i t a b l e food i s provided? Can the winter weight syndrome be reversed and r e p r o d u c t i v e c o n d i t i o n manipulated i n both s p e c i e s by the p r o v i s i o n of e x t r a food i n the f a l l ? Can t h i s a l s o be done by reducing p o p u l a t i o n d e n s i t y i n the f a l l , hence a l l o w i n g the remaining animals unmolested access to resources? I f r e p r o d u c t i o n can be maintained i n t o the winter, what e f f e c t does t h i s have on f u t u r e p o p u l a t i o n dynamics, and i n v o l e s , on the 159 s t r u c t u r e and q u a l i t y o f t h e grasses, F i n a l l y , i f a patchwork, of good and poor h a b i t a t of d i f f e r e n t s i z e s can be c r e a t e d , what e f f e c t does t h i s have on t h e movement p a t t e r n s of i n d i v i d u a l v o l e s , t h e i r r e p r o d u c t i v e c o n d i t i o n , s u r v i v a l , and p o p u l a t i o n dy na mics? 160 L I T E R A T U R E C I T E D Andrzejewski, R, 1975, Supplementary food and the winter dynamics of bank vo l e p o p u l a t i o n s , Acta t h e r i o l . 20: 23-40 Andrzejewski, R,, and M, Mazurkiewicz, 1976. Abundance of food supply and s i z e o f the bank v o l e s home range. Acta t h e r i o l , 21:237-253. 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No. 2 amer, Soc, Mammal. S u l l i v a n , T. P. 1 9 7 6 , 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 the deermouse, Peromyscus §a£iculatus. M. Sc. T h e s i s , U n i v e r s i t y of B. C., Vancouver, B. C, Terman, C. R. 1 9 6 8 , P o p u l a t i o n dynamics, p. 4 1 2 - 4 5 0 In J , A, King (ed,) B i o l o g y of Peromyscus (Rodentia)..Spec. Publ, No. 2 Amer, Soc, . Mammal, Turner, B, N,, and S . , L . I v e r s o n . 1 9 7 3 . The annual c y c l e of a g g r e s s i o n i n male Mi c r o t u s £§nnsylyanicus, and i t s r e l a t i o n to p o p u l a t i o n parameters, Ecology 5 4 : 9 6 7 - 9 8 1 . Watson, A,, and P.. Moss. 1 9 7 0 . Dominance, spacing behavior, and a g g r e s s i o n i n r e l a t i o n t o p o p u l a t i o n l i m i t a t i o n i n v e r t e b r a t e s , p. 1 6 7 - 2 1 8 In Animal p o p u l a t i o n s i n r e l a t i o n to t h e i r food r e s o u r c e s . E d i t e d by A, Watson, B l a c k w e l l , Oxford. Watts, C H , S. 1 9 7 0 . E f f e c t of supplementary food on breeding i n woodland rodents. J . Mamm. 5 1 : 1 6 9 - 1 7 1 . Watts, C H. s. 1 9 6 9 . The r e g u l a t i o n of wood mouse (Apodemus s y l y a t i c u s ) numbers i n Wytham Woods, Ber k s h i r e . J . Anim, E c o l . 3 8 : 2 8 5 - 3 0 4 , Wynne-Edwards, V. C. 1 9 6 2 . . Animal d i s p e r s i o n i n r e l a t i o n t o s o c i a l behaviour, O l i v e r and Boyd, Edinburgh, Wynne-Edwards, V. C, 1 9 6 5 . , S e l f - r e g u l a t i n g systems i n p o p u l a t i o n s of animals. Science, 1 4 7 : 1 5 4 3 - 1 5 4 8 , Zach, R,, and J , B. F a l l s . 1 9 7 5 . Response of the ovenbird to an outbreak of spruce budworm. Can, J , Zool. 5 3 : 1 6 6 9 - 1 6 7 2 . 166 Appendix J P o s s i b l e Experimental Problems G r i d 1 -I was not s a t i s f i e d with the Sherman t r a p s which Daphne F a i r b a i r n had been using on t h i s g r i d . The nature of the tr a p p r o h i b i t s one from p u t t i n g what I c o n s i d e r i s s u f f i c i e n t c o t t o n b a t t i n g and food t o keep mice warm and fed over a c o l d night i n winter. The system with the Sherman t r a p was to use' a small piece o f a r t i f i c i a l c o t t o n and a piece of mouse chow attached to the s i d e of the tr a p by peanut butter, I used t h i s method f o r the two t r a p p i n g p e r i o d s i n November, 1974, During these t r a p p i n g p e r i o d s two mice d i e d from having i n s u f f i c i e n t food and i n s u l a t i o n . Two more mice were extremely weak; they had eaten the peanut b u t t e r but l i t t l e of the chow. Neither of the l a t t e r two were present on the g r i d again. A f i f t h mouse had attempted to chew on the g r i l l a t the r e a r of the t r a p , i t s t e e t h had become stuck i n the wire and i t was dead. In a d d i t i o n , I r e g u l a r l y caught Microtus oregoni i n my Longworth l i v e t r a p s , whereas Daphne F a i r b a i r n caught only dead ones i n her Sherman tr a p s , Since I had not l o s t any mice up to t h i s p o i n t i n my study, I abandoned the use o f Sherman t r a p s and used Longworths from December 1974 onwards. The g r i d was not trapped on January 8 - 10th, 1975 because of a warning o f heavy s n o w f a l l . There was an immigration of M, oregoni during the summer of 1975, 167 G r i d 2 T h i s g r i d was not trapped d u r i n g the f i r s t p e r i o d i n September, 1974, U n f o r t u n a t e l y , a l l the food s t a t i o n s were depleted over t h i s period. The t h i r d trapping period i n October, 1974 was d i s r u p t e d by an i n v a s i o n of a raccoon. The raccoon was seen on the g r i d , most of the t r a p s were d i s t u r b e d , and some had blood on them. The second check was not run on January 9th, 197 5 because of a snow warning. The c a t c h during the f i r s t t r a p p i n g p e r i o d i n February was reduced because many t r a p s f r o z e . Baccoons or p o s s i b l y a f e r a l c a t were a problem on March 5th, A p r i l 17th, May 1st, and June 27 i n 1975. T h i s , combined with the l a r g e i n f l u x of M. oregoni, caused me to abandon t h i s g r i d a month e a r l i e r than I had intended. G r i d 3 I set up t h i s g r i d at the same time as g r i d 2, i n t e n d i n g to feed i t a t a lower food l e v e l . It became apparent t h a t the p o p u l a t i o n s were not comparable: g r i d 3 had only h a l f the p o p u l a t i o n of G r i d 2, I t h i n k t h a t because of i t s l o c a t i o n t h i s g r i d was s i m i l a r t o S a d l e i r ' s G r i d B ( S a d l e i r , 1965). His g r i d was i n a s m a l l ' i s l a n d ' of f o r e s t , and the P. maniculatus po p u l a t i o n became e x t i n c t d u r i n g h i s study.,Apart from a l a r g e i n f l u x of M. oregoni a f t e r I removed the food s t a t i o n s from t h i s g r i d , nothing e l s e untoward happened. 1 6 8 G r i d 4 The second check on January 9th was abandoned because of snow warnings, and i n February many t r a p s were f r o z e n i n the f i r s t t r a p p i n g p e r i o d , which reduced the c a t c h . Raccoons or a cat d i s t u r b e d the t r a p s on March 20th and 21st and again on A p r i l 4 t o 5th and May 15th, M, oregoni immigrated onto t h i s g r i d i n l a t e winter and s p r i n g . G r i d 5 Again I was not s a t i s f i e d with the Sherman t r a p s on t h i s g r i d so I r e p l a c e d them with Longworths on December 11th, 1974. No t r a p p i n g was done because of the snow warning i n January 1975, A f t e r the f i r s t removal o f food from t h i s g r i d there was an i n f l u x of M„ oregoni, which maintained high numbers f o r the r e s t of the study. 169 Appendix 2 Egromjrscus: T r a p p a b i l i t y Of Males (ML) And Females (FM) Date G r i d •1 G r i d 2 G r i d 3 G r i d 4 G r i d 5 ML I i I i FM ML FM ML FM ML FM Nov. 1973 79 82 94 80 100 75 88 70 Dec, ti 88 84 89 82 84 79 97 71 Jan, 1974 51 45 85 71 61 77 79 42 Feb. ii 86 73 80 71 91 99 86 59 Mar. , ii 85 60 60 51 82 86 100 68 Apr. i? 100 81 68 71 67 91 77 84 May, II 89 84 71 61 54 80 94 94 Jun. it 94 69 89 64 78 100 100 91 J u l . II 92 70 67 64 100 71 88 85 100 69 Aug, ii 79 91 69 70 100 71 65 83 100 67 Sep, , ii 68 75 50 65 80 100 69 50 57 64 Oct. i i 57 67 45 48 92 93 67 59 82 60 Nov. it 33 67 81 76 75 64 58 62 Dec. •• 86 70 69 82 7 3 81 50 96 Jan. 1975 89 67 63 74 6 1 72 78 75 Feb, ti 74 67 59 78 55 64 63 79 Mar, n 72 82 53 73 60 72 76 78 Apr, n 73 79 60 68 54 62 79 73 May, ii 75 76 44 68 75 83 93 67 Jun. ii 89 89 41 43 85 63 J u l , i i 64 76 56 100 76 68 Aug, « 57 70 75 50 170 P e r o f ^ s c u s : Minimum Number A l i v e Males {ML) And Females (FM) Date G r i d 1 G r i d 2 G r i d 3- G r i d G r i d 5 ML FM ML FM ML FM ML FM ML FM Nov. 1973 12 12 11 11 14 07 1 2 20 21 13 17 15 12 07 14 19 Dec. II 25 14 19 18 11 06 16 22 21 14 20 20 09 07 15 21 20 16 19 23 09 07 14 18 Jan, 1974 21* 15* 21 23 09* 07 10 13* 21* 16* 23 30 09* 10 10 13* Feb, it 18 17 24 30 12 14 09 14* 16 16 29 3 7 18 15 09 13* Mar, II 16 17* 28* 33* 16 13 08 11 16 16* 31* 34* 13 16 0,8 13 Apr. it 13 16 31 30 11 20 08 12 14 14 3 3 36 10 19 07 10 Ma y, « 15 15 28 27* 09* 16 08 11 15 16 27 3 3* 10* 13 09 12 14 17 26 2 8* 15* 10 07 11 Jun. II 12 16 18 19* 09 08 07 11 12 16 20 16* 09 09 06 03 06 12 J u l , II 10 17 22 15* 08 09 15 09 09 11 11 17 21 17* 07 07 13 12 06 08 Aug. II 09 13 17 16 08 08 15* 12 08 09 09 14 18 17 07 08 19* 18 06 07 Sep, it 09 15 07* 08* 12 15 18* 18* 11 07 18 20* 1 1* 10* Oct. II 12* 13 19 20 13 11 2 1 19* 13 10* 12* 14 20 22 14 10 23 21* 14 12* 09* 12 14 15 11 07 22 21* 15 12* Nov, II 07 12 16 19 11 05 20 19 1 1* 13* 05 12 16 14 23 13 13* 12* Dec, II 12 10 16 13 26 17 09 13 14 10 20 14 27 18 12 14 Jan. 1975 -- 14* 11 2 3* 15 — 10 11 16* 1 1 24* 16 14 16 Feb, II 11 11 19* 15 24* 17* 15* 15 12 11 22* 17 2 3* 16* 16* 19 Mar. ti 15 15 20* 18 22* 17 15 19 16 18 20* 22 19* 14 19 21 Apr. ii 16 16 22* 20 19* 09* 16 18 18 14 19* 19 19* 12* 17 18 May. it 18 16 15* 1 1 16 14 19 19 18 17 18* 16 16 10 18 19 21 19 16* 20 1 1 08 17 18 Jun, II 17 14 15 16 17* 13* 20* 20* 15 14 18* 16* J u l , II 08* 08* 14 12 10* 18* 09 06 1 1 1 1 15 14 Aug. •t 09* 16* 11 16 10 20 13* 15* 171 Appendix 4 £§£21_yscus Age C l a s s e s Female weights are confounded by the i n c r e a s e i n weight a s s o c i a t e d with pregnancy, so I have co n s i d e r e d males only, Taking the data f o r both breeding seasons f o r the c o n t r o l g r i d , I c a l c u l a t e d the percentage o f males which had s c r o t a l t e s t e s i n each weight c l a s s : -I©iaht C l a s s (g) Number of Males % of Mai es S c r o t a l 10 3 0 11 8 0 12 6 0 13 17 0 14 18 11 15 30 13' 16 32 19 17 61 33 18 64 45 19 72 57 The percentage of mice that are s e x u a l l y mature changes a b r u p t l y from J\9% to 33% at 17 g. Also t h e a c t u a l number of s e x u a l l y mature males changes from 6 mice to 20 mice. T h e r e f o r e , I c o n s i d e r that mice o f 17g and heavier are a d u l t s . No mice weighing 13g or lower were ever found with s c r o t a l t e s t e s , I c o n s i d e r a l l these mice to be j u v e n i l e s . Subadults are mice weighing between 14g and 16g. Other workers on P. maniculatus have used the f o l l o w i n g c u t o f f s : -F a i r b a i r n (1976): " A l l mice weighing at l e a s t 15 g were considered to be a d u l t , " 172 S a d l e i r (1965) : Mice "weighing l e s s than 16 g were considered j u v e n i l e u n l e s s they had e i t h e r p e r f o r a t e vagina or descended t e s t e s . " S u l l i v a n (1977): a l l mice weighing 17 g or more were a d u l t s . Sub-adults weighed from 13 to 16 g and j u v e n i l e s weighed 12 g or l e s s . 1 7 3 r - C ! ! " 2 c 2 C ! l n n n n 3 : 0 ^ r m , S i n ^ ^ ' " r ' M ' " n u ! ^ ' ' 1 0 *~ ,_ *~ ^ K£> ^ *- ™> ^  ^  mi fni r- o o o o o o o o o o o o in r- a- o r» ro o o r- o o o o o o o o >o i o o ^ r- ro o O O O o -t co ro o V O O CN CO :rj- O O VO O O ^ I I O \C I O ^  r* :f I N r i m co i - m CO rr "Ol « - I- t - T- «-•HI Ul n i> ^ ^ m r - n j a « CM I N o .-f CM , c o i n j o o ;r o M c co i t ^ co .-j cn <-- r - io r*N r- »- cn i n vo 5 : I O O O O O O O O O ! " O O O o r o o O o r - O O i r i o O O C D O O C O r o o I O O =t in o c*i VO o •— Oi r - o r^ O o O n K l cn in ifl m in i n co a - m r i j c ^ i n i i / i o ^ r ^ r ^ r o i n r o c i c o ' T v o o o c o o r:i rJI D l C l •HI "Cl 0)1 a'l u l oil Mi .-il mi RI O-'l M •vi -II u i -HI ~\ o i Dl C71 III 4-'I Cl 0)1 Ol u l OJ| CM Ml il-l M •Cl -'I U l n --r co h c - in -r .-j r- ro oi -r *o :f oo m r' t - cc n o cc co i n ojricnrjrsr-r\ijr-r-o Lf. ro rn r-l 1 o o O cr rn o o o o o in o m ro r- in o o rn cr CO i 1 cr ro r~ - >- C N -o C N ro vO m CD _____ o o . ^_ t— r- rr. (IJ vO „ CN IT) T — r- L A m lO C A CO 00 o m o o O | 1 o o m o o o o o CN m r- CT o ro C O o ro cr cr rn 1 i m cr ro in vO r- CO r- Ol CO L O ' . O r- in < N C N C N C N * C N o o O 1 i O O O O o •Ul •H| C l i n a c o u j i n ^ i D n i N U I i n — . - — ^ . ^ , _ ^ . - . i O i n o o « ~ - — — ^ — .—. , ^ . ^ O ' - n j n n m m i / i r - ' - r - i - o i r ^ c c a i n j n i n j »— o ro C N C N ^ J I — — — — — — , w _ w K i o o o o o o o o o ^ o o o r o o c n c o i n o o r ^ o o I o o o o o l O i O i D i r i c o i o r - c O h i o o i o c o o I — ~~ — — . ^ r j r - . i f - ( \ | O r o r , n o „ ^ . ^  . — co . r- , ^ C N o o <o o „ s-, — Ci d '2. ZL. Ci i Ci ^ "~ r s l ™ Ci ^ d _ *~ ^ ^ °" ^ *~ ° ™ ~r *~ r o c 0 ^ ^ ^ '° °'' *~ '° , _ , — *~ m ^ ^ t . i D O o o o o i . ' i r i c i i n O M N i n - ' - o o i t M T r i I o o o o i n o o : r o - - c c r - ' — a - c - r o o o o i f c ^ o r o o n g r - i N j i n i n c o i n i o r ' v o M ^ c c c o i n I r - m i n t - i N i n ' O O i c D C O i o c V o i n i o i o r ' . o ^ C l m „ ~ O r t —. C M n r> CC HI T - : f CP rr CC. ^ r - in - ^ . « . 0 n „ . t— ^ ^ r - l ^ c c j ' co*r- ^ ^ ^ l o o o o o o i o r o c c c 7 i - - o c N o c o a i r - i n r o u i o i n i o o c T ^ i n c o o c r m ^ c i r - - c r ) r - o i c N v o i n r ' j c N o o r o a o c N r ^ r ^ c x ' c o o c r o c o c o r - . c o r ^ i n c o o i t ^ o i r ^ i n r ^ r o c N ^ c ^ j c N f ^ c o c r c x i c r i ^ c ^ c x i o o o o r ^ Ol Wl ell PI Ol CM ^ ccr i n ro co *- c i c:f ro i"N i n i/i ro i n o i r- r - co »— ai co f> u~i ro vo in m vo >n co r-- co co in r~ 't> co d r - r- T — aj r- o i r - | C i . | i n o o O O O O O O O O O O O O O O O C O r o O r o c n r i | 0 0 0 0 0 0 0 I 0 ' ? O C C ^ O f i f O r -~f o co in o ro vO _ _ | l ^ i n <N. r o c o o r o c N c N I c N m r - c o u o m . r o r - y c N c j - m i h r j - i n •H| . u l „ „ _ „ c l — .— —~ —- —. ^ ^ O O — . ^ O ' - ^ - - . ^ - . ^ ^ ^ . . — ~~ —. « . CN . . ^ .— . „ n c l -n t CN ^ ^ „ , | w ^ w ^ U n ^ t t ^ l L r " T " C l ' : C , r " ' " l 7 ' r O C i a : r V ^ ; J r l i n f , , r ' r 4 l N | L n ' - M 1 ' Q l C M C lO C M C - >~ r- ^  -O C O U l c O s ^ l o o o o o o o o T - o o c r j i n c r o c o r ^ c o r - r o r - o o o o o o o o o r o i ;r C N f i c o o r o r - o a i c o o o o r - c > o r- C N C N C N J C N C O c O C O c o v O i n UT n i r i N l O C - O C O i c i n i o C C O O O m i f u l o i 7 Ol z - _ _ = <DI 4--I CCI o i ^ X) l l u Q| O CU CD cu- o. 11 a 3 3 0) is a Cu re 10 z z r z z _ = -*~* •M > r j a Si M Ll 0 o •5 cj «J a r j C J 3 O « T 2^ •a: 3: 174 Ap_£en gix 6 l e a t h e r Summary For Nov. J973 To August 1974 2a£e Sean Temj), { F) P r e c i p i t . ( i n s . ) Sunshine(hrs,) Nov. 1973 39.6 [-3, 3) 7. 24 ( + 1.6 8) 54, 5 i [-15.1) Dec, « 41. 5 i 1 + 2,6) 9.22 (+2.71) 39. 1 [-5. 3) Jan. 1974 36.7 | [+0, 4) 7, 05 ( + 1.25) 83. 8 (+28.5) Feb. i i 40,3 ( + 0. 3) 4, 30 (-0. 29) 62.9 [-2 9. 6) Mar. i i 43, 0 [ + 0.5) 7,34 (+3.65) 117. 1 [-12, 4) Apr. n 48.2 (-0. 1) 3, 96 ( + 1, 57) 131.0 (-49.0) May. i i 51.0 (-3.3) 4. 64 ( + 2.77) 2 11.6 (-41,4) Jun, n 58, 2 (-1.4) 0.79 (-0,99) 260. 7 ( + 17, 2) J u l . ii 60.6 (-2. 8) 2. 97 (+1,8) 254. 8 [-49. 7) Aug. rt 63.2 ( + 0. 5) 0.13 (-1.33) 30 5. 1 ( + 50.1) Sep, n 6.0. 3 ( + 2. 7) 0,93 (-1,48) 252. 5 (+64. 6) Oct. » 50. 5 ( + 0.4) 2. 01 (-2.8) 179. 0 ( + 6 3, 2) Nov. ii 43, 6 (+0,7) 7.60 ( + 2.04) 63. 7 (-5.9) Dec, • i 41.6 ( + 2.7) 7.40 (+0.89) 49, 8 (+5.4) J an. 1975 36.3 (0. 0) 6. 80 (+1.0) 57, 5 ( + 2.3) Feb. ti 36. 7 (- 3. 2) 5,22 ( + 0,63) 69, 0 (-23. 5) Mar. II 41, 2 (-1.3) 3. 80 (+0. 1 1) 173. 2 (+43.7) Apr. ti 45. 0 (-3.1) 0. 93 (-1, 46) 177.7 (-2.3) May, ti 53. 1 (-1.3) 1, 73 (-0.14) 258, 4 (+5.4) Jun. ii 57.7 (-1.8) 0.97 (-0. 81) 218. 0 (-25.5) J u l . II 64. 4 ( + 1. 0) 0.47 (-0.7) 333. 5 (+29.0) Aug. ti 60. 4 (-2,0) 3. 98 (+2.52) 145. 7 (-109. 3) Che d e v i a t i o n from the •Normal Monthly Mean 1 i s given parenthses. These records are from the Vancouver I n t e r n a t i o n a l A i r p o r t Atmospheric S t a t i o n o f Environment Canada. 175 Appendix 7 Perpmyscus: Mean Weight Of Males And Females In Each Trapping Period, MI=males And FMffemales, G r i d 3 G r i d 4 G r i d 5 IJ3 ML FM ML FM 14,4 15.4 15,4 14,6 14. 2 15.4 15.2 13.8 14.4 16.7 15.4 14.6 15. 1 15.2 15,0 14, 1 15. 1 15.4 14. 9 13.9 17.0 15.0 15.2 14.5 16, 9 14.7 15, 5 13, 3 16. 4 15.4 15. 9 14.0 19.0 18.3 15.9 14.8 1 9 , 1 1 8 , 6 16.5 15,1 18.3 19.6 16.5 14.4 19.8 19.4 16.5 15,5 18.1 16.7 16,3 15.5 17, 4 18.8 16,0 15.6 15.0 17,4 17.2 16.5 16.6 17,9 17.6 16.7 16.8 18,6 15, 3 14. 8 17.0 15.7 17.8 17,0 16,5 15.7 17,3 18.4 18,3 17.2 17,7 19.3 16.0 19.6 18.2 18.8 15.7 19.8 16.5 17.1 17,1 18,7 16,8 18.7 17.0 18.0 17.1 17.8 17.2 16.2 12. 8 14, 5 Date G r i d 1 G r i d 2 ML FM ML FM Nov. 1973 16. 0 15.6 15, 2 13, 1 14. 9 15.7 14. 4 14. 0 Dec. it 15.7 15. 2 15. 1 14, 0 15, 9 15, 4 16. 1 13, 9 15, 5 14 ,5 15. 8 13, 4 Jan, 1974 16. 2 — 16, 6 14, 5 16, 8 14.8 18. 2 14.9 Feb. ti 16. 8 15,7 18, 2 16, 0 17. 3 14, 8 19. 8 1 7. 5 Mar, » 17. 2 15. 5 21. 2 19, 7 17. 2 15,6 18, 6 1 8. 0 Apr, ti 18, 3 16, 1 20. 1 20, 3 16. 8 15, 9 19.0 17. 9 May, n 17, 1 16,2 18. 7 18. 5 18, 1 16. 6 18, 9 1 8. 2 18, 1 17. 2 18, 9 19, 6 Jun. ii 17.7 14 , 8 19. 1 18. 1 17. 4 16. 8 18. 6 17. 3 J u l , ti 19, 9 18,3 19. 2 19.4 18. 5 19,1 16. 7 17. 6 Aug, it 17. 9 18.6 16.3 1 8. 3 16. 9 18,0 15. 9 15, 8 Sep. ii 17. 0 16, 1 16. 1 16. 6 15. 4 14. 1 Oct, ii 17, 7 17. 1 16. 0 14, 3 16, 0 15, 4 16. 3 15, 0 15. 9 16.8 17. 2 16.3 Nov. it 16.7 16.7 18, 1 17, 6 16.7 17. 1 17, 9 18, 1 Dec. ii 16. 3 16. 9 17. 9 17. 9 17. 8 18, 5 17, 5 17,7 Jan. 1975 — — 17, 6 20. 1 18. 4 21.1 17,4 19. 8 Feb. it 19.6 20, 6 17, 1 16, 8 19, 4 21 . 8 18, 0 18. 8 Har. I I 17,0 18.8 18. 2 18. 5 17. 1 16. 9 17. 6 17, 4 Apr. , n 17, 3 15. 9 17. 6 18. 8 17. 9 18,6 18. 5 18. 2 May. it 18. 1 16. 9 19, 2 21.0 18. 8 17, 0 18. 2 1 8, 5 18, 7 17. 2 18. 8 19, 1 Jun. it 18. 9 18.9 17.7 20, 1 19.4 19,0 18, 3 17, 6 J u l . ti 18, 5 19,7 17, 3 20. 2 18.3 20.7 15, 9 20. 0 Aug. it 18. 3 15. 1 20. 3 17. 1 14. 5 16.7 15.9 15.2 12,6 13.9 14,6 14.4 16,4 15.9 15,5 14,6 15.3 14.9 16.4 15.2 14,7 13.6 15,3 15.1 16.3 16,0 15.5 15.3 15. 3 15,6 16.8 15. 2 17.6 17, 1 16,9 15.8 17. 1 16, 8 17.1 15,6 18,0 17.6 17,5 15.9 18, 7 19, 9 18.9 17,0 — 19.4 16.5 16.6 18.0 19.5 16,6 16,7 17.8 20.0 17, 1 17. 8 16,9 20,4 16,6 17.9 18,5 19.2 17,5 17.7 18,4 20.4 19.3 15.2 18.0 19,0 17.2 16.1 16.4 19.3 17.7 16.5 15.4 19,3 18.2 16.6 16.6 19.3 16.6 17.4 17,3 17,6 17.9 20,0 19,6 19.6 17.5 17.7 19.3 17.5 18.5 14.6 15.2 November 1973 60 -134 127 203 193 539 129 -120 1 80 -12 231 96 - 135 -149 -15 -281 1 07 -412 1 64 -224 188 -126 213 -27 J a n u a r y 1974 396 -25 464 . 145 532 -66' 524 4 00 652 476 779 551 13 185 215 82 4 18 -21 4 16 753 451 655 505 553 March " - 10 -49 44 107 97 264 238 431 473 741 703 1002 -64 63 - 200 37 -316 12 501 525 10 20 9 40 1540 1352 May " 4 60 355 -52 235 -564 204 559 487 1035 85 2 16 12 1216 -63 -367 464 -653 9'J1 -9 48 204 536 4 6 9 742 734 948 2 14 1048 -4 13 1009 -1040 971 359 739 462 1434 565 2078 J u l y " -405 -7 -474 36 -543 79 -1 07 639 425 1363 95 2093 -229 -315 - 1 31 -256 -33 - 1 96 233 -223 543 356 797 941 September " 317 146 -63 337 -453 627 -1 47 -935 3 13 9 774 952 -441 -196 . -51 289 333 775 276 877 8 97 40 3 1518 -72 Koveaber " - 1 49 5 399 -236 947 -476 2'64 150 251 57 4 239 939 709 227 344 1 96 -21 165 286 216 533 -50 790 -317 Ja n u a r y 1975 1 15 404 -302 220 -720 36 235 497 951 940 1 667 1332 695 725 16 13 1 1 95 254 1 1665 66 3 365 1233 10 91 179 9 13 77 'March " 245 622 739 11 52 1232 16 82 275 552 719 883 1163 12 15 332 272 823 517 1314 763 566 779 1432 1198 2298 16 17 May " 98 24 -8 274 - 1 14 524 194 957 252 1627 310 2296 262 377 547 6 24 833 872 2 50 906 13 9 1080 127 1254 254 483 411 577 568 671. 267 149 353 2 27 449 305 u l t i p l y each number by 10-s (cj) Da t a a r e q i v e n f o r ma l e s and fe rcales ir . each age c l a s s : Ad = a d u l t s Sa = s u b - a d u l t s Ju = - j u v e n i l e s 177 Ap_p_endix 9 Peromyscus And M i c r o t u s : S t r a i g h t - l i n e Home Range C a l c u l a t i o n I c a l c u l a t e d home range s i z e s f o r each animal with a l l capture p o i n t s i n a s t r a i g h t l i n e by m u l t i p l y i n g the number o f d i f f e r e n t capture l o c a t i o n s (cap) i t v i s i t e d by the area of these c i r c l e s : -G r i d s 1,5 : r=7,50m ( h a l f the d i s t a n c e between stakes) G r i d s 2,3,4 : r=7.6 2m ( " " " '« ) A l l vole g r i d s : r=3,8 1m { »' »» " " ) S 2 I § . Range Area ( i n sguare meters) Gr i d s 1 cap 2 cap 3cap 1,5 176. 7 353.4 530.1 2,3, 4 182. 4 364. 8 547.2 A l l v o l e g r i d s 45,6 91. 2 136. 8 178 Append i x 1.0 Microtjjs: Minimum Number Aliy_e on Grids E, R, and T i n Each Three-month Period. P a t e J u l Sep 1972 Oct Dec » Jan Mar 1973 Apr Jun J u l Sep " Oct Dec " Jan Mar 1974 G r i d E ML FM 16,3 14.3 22, 2 25,0 22.0 23,3 26.3 23,0 29.4 26.9 23, 8 22.7 25,9 21,1 G r i d R H i I J 2 0, 0 24.9 27, 7 37,8 34,7 48.9 46.7 58,8 58. 3 54,1 46.8 46,5 42, 3 50.7 Grid T Ml FM 23.9 23,0 39. 5 50,5 52.6 71.4 54.7 84.3 77,4 91.3 109,8 109,8 86.9 109.1 ML = Males And FM = Females !li££2tus: Immigration l o L2B.S~i.eEIB. Food 5.Ei^§ G r i d E G r i d 3 G r i d T Grid M Ad Sa J u Ad Sa Ju Ad Sa Ju Ad Sa J u l Sep 1972 23:17 9:11 12:12 Oct Dec " Jan Mar " Apr Jun " " J u l Sep " Oct Dec " Jan Mar 1973 Get Dec " 7:1 4:3 12: 17 10:1 2:7 4:4 25: 10 5: 15 7:4 10:3 13:9 3:9 0: 0 6:2 8:3 13: 0 4: 4 2:6 27:9 29:19 23:27 Jan Mar 1974 • 39:14 8:17 7:16 23:15 12:12 4:26 7:5 7:6 17:18 12:5 6:9 4:8 24: 1 4 24: 1 6 20: 28 . 7:1 27:8 22:27 3:1 10:7 2:7 15: 3 7: 9 2:3 27:16 13:15 8:14 8:12 19:19 13:20 22:6 11:12 5:17 24:21 21:27 20:21 9:5 53:37 23:29 9: 5 32: 40 10:3 30:12 7:15 0:5 37:33 10:10 12:11 Apr Jun u 37: 41 7: 13 4:6 42:33 37:40 19:22 60:3 9 2 9:40 19:28 62:72 36:36 25:37 E = c o n t r o l , R = low-food, T = high-food, M = intermediate-food Ratio = aales :f eaal es, Ad = adults, Sa = sub-adults, Ju = "juveniles v£3 M i c r o t u s : i C s i i i i i t a n . s o u s " R e l a t i v e G r o w t h G r i d £ S a J u A d N o v . 1 9 7 2 4 9 8 / - h. 1 6 0 4 / 1 4 8 7 0 9 / 3 5 8 8 0 / 9 3 3 5 5 / 29 3 5 2 8 / 3 6 4 7 0 0 / 4 31 4 9 0 / - 7 5 J a n . 1 9 7 3 3 6 5 / 17 6 6 7 / 1 5 5 9 4 8 / 2 9 3 4 0 8 / 3 6 8 3 5 3 / 5 3 4 4 4 0 / 5 3 7 5 2 2 / 6 3 9 1 4 1 / 3 4 7 M a r . 1 9 7 3 1 2 4 / 10C 8 9 / - 3 4 5 3 / - 1 6 3 7 1 6 / 5 7 1 1 7 6 8 / 2 6 3 2 3 2 4 / 2 7 7 2 3 7 9 / 2 9 2 5 7 9 / 2 2 7 M a y 1 9 7 3 7 4 1 / 7 5 4 9 0 1 / 1 1 7 3 1 0 6 1 / 1 5 9 3 8 5 9 / 1 0 7 3 6 3 5 / 4 77 3 4 2 / 6 5 3 1 0 4 9 / 8 2 3 - 6 7 / - 1 3 3 J u l . 1 9 7 3 3 4 1 / 2 14 4 6 7 / 5 0 4 5 9 2 / 7 9 3 3 9 / - 2 0 6 6 3 / - 2 37 9 2 / - 4 3 1 2 1 / 2 01 - 2 5 S / - 5 5 7 - 9 1 / 2 9 1 - 6 4 / 2 5 1 - 3 7 / 2 12 - 9 5 / - 2 7 3 S e p . 1 9 7 3 2 / - 27 4 1 2 5 / - 7 9 2 4 3 / 1 0 5 7 2 / 1 3 7 - 2 1 5 / - 5 4 - 1 7 2 / 9 3 - 1 2 8 / 2 5 1 - 1 2 1 / - 3 5 N o v . 1 9 7 3 - 4 7 6 / - 3 4 0 - 3 2 0 / - 3 1 S - 1 6 4 / - 2 9 6 2 5 6 / - 5 2 - 9 6 / 1 2 5 2 3 1 / 1 4 9 5 5 7 / 1 7 3 5 5 7 / 4 3 9 J a n . 1 9 7 4 5 5 3 / 1 7 2 6 9 1 / 2 4 3 8 2 5 / 3 1 4 3 9 6 / 2 1 5 7 9 7 / 'J \J 9 3 1 / 7 0 5 1C 6 5 / 9 11 6 5 1 / 6 j 5 1 9 7 4 2 3 9 / 2 1 6 2 6 0 / 7 0 5 2 8 2 / 1 1 9 4 1 6 8 / 4 5 0 2 0 7 / 5 3 7 2 0 3 / 3 4 7 2 0 9 / 1 1 5 7 5 7 3 / 6 ^ 1 M u i t i p l y e a c h a u s = c o n t r o l , 2 = l o w - f D a t a a r e q i v . e n f o r % a 1 e s / f e :i: R a t a s of V o l f t s O n G r i d s s , R , A n d ?-. G r i d R G r - i d S a A- 5 a J-u-2 9 3 / 1 4 5 5 1 6 / 1 9 7 1 0 5 / - 1 8 3 2 6 2 / - 2 8 4 1 9 / 1 2 6 7 6 2 / 6 2 1 0 3 5 / 1 9 8 4 1 1 / - 5 2 5 6 5 / 2 1 7 1 3 / 1 0 4 5 7 5 / 5 3 5 7 4 2 / 7 0 2 6 3 4 / 5 9 7 7 7 5 / 6 2 2 9 1 7 / 5 4 7 1 2 9 / 2 4 1 1 1 6 / 1 3 5 2 4 6 / 1 3 5 3 6 1 / 3 7 7 4 7 5 / 5 6 9 8 6 6 / 7 6 0 10 1 6 / 9 4 3 1 1 7 3 / 6 1 3 1 4 1 5 / 8 84 1 6 5 2 / 1 1 5 1 7 1 3 / 2 5 1 3 4 7 / 2 7 5 3 3 2 / 7 39 4 0 5 / 1 0 9 9 4 7 9 / 1 4 6 0 1 0 6 6 / 1 5 2 6 1 2 7 4 / 1 9 7 4 1 0 5 1 / 6 12 1 2 9 3 / 8 9 1 1 5 3 5 / 1 1 7 0 - 2 1 / - 4 0 2 5 / 5 3 8 3 0 / 2 5 1 1 0 7 9 / 4 15 1 3 2 7 / 5 3 2 3 7 / - 5 9 1 3 5 / 6 9 14 1 / - 4 4 2 1 9 / 4 5 2 9 3 / 1 3 4 - 1 4 . 2 / - 4 3 2 - 2 7 / - 3 0 6 - 2 3 9 / • - 3 7 1 - 2 1 3 / - - 2 6 7 - 1 4 6 / - 1 6 4 - 3 3 / - 1 5 6 2 9 / - 4 0 1 6 3 / 2 3 4 2 5 5 / 2 3 8 3 4 7 / 3 4 1 7 0 / 71 6 9 / 6 1 5 6 / 1 2 0 15 6 / 1 4 5 1 5 6 / 1 7 0 - 4 4 / - 2 3 2 / 3 i 1 0 0 / - 2 7 3 3 / - 6 4 - 4 4 / - 1 0 2 2 3 5 / - 2 0 2 1 4 / 1 1 8 7 / 13 1 9 5 / 13 3 0 3 / 11 5 0 1 / 3 3 9 4 4 6 / 3 3 3 4 3 6 / - " 2 7 3 4 0 / - 2 0 2 4 4 / - 1 2 5 1 0 / 1 2 8 6 2 4 / 4 0 4 6 0 / 4 8 5 5 9 / 1 4 5 6 5 9 / 2 4 1 7 6 6 / 3 1 9 8 8 1 / 9 4 4 3 6 6 / 5 3 1 3 9 0 / 5 9 7 4 1 4 / 6 6 3 1 7 2 / 1 C 0 0 1 5 6 / 1 5 5 5 7 9 / 4 1 8 7 0 0 / 6 4 3 3 2 2 / 8 6 3 7 5 3 / 9 4 4 9 4 4 / 1 2 4 3 6 3 7 / 2 9 2 8 5 2 / 5 74 1 0 6 7 / 8 5 6 h e r b y 10 s ( q ) o o d , T = h i q h - f o o d a l e s i n e a c h a q e c l a s s : -u l t s J u = " j u v e n i l e s CO o 

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