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Experimental studies of the population processes in the vole Microtus townsendii Boonstra, Rudy 1976

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EXPERIMENTAL STUDIES OF THE POPULATION PROCESS IN THE VOLE MICROTUS TO^NSJNDII by RUDY BOCNSTRA B . S c , U n i v e r s i t y of Cal g a r y , 1972 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Zoology We accept t h i s t h e s i s as conforming to the. r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA NOVEMBER, 1976 (§j Rudy Boonstra, 1976 In presenting this thesis in partia l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of ^ — O 0 (Q The University of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1WS Date i A b s t r a c t A number of f i e l d s t u d i e s on s m a l l mammals have suggested t h a t a g g r e s s i v e behavior may l i m i t breeding d e n s i t y . To i n v e s t i g a t e how the presence of one i n d i v i d u a l a f f e c t s a n o t h e r 1 s chances f o r s u r v i v a l , r e p r o d u c t i o n , and growth, I c a r r i e d out a s e r i e s of experimental s t u d i e s on Microtus townsendii near Vancouver, Canada. In the f i r s t experiments, p o p u l a t i o n d e n s i t y was reduced by removing voles before or during the s p r i n g d e c l i n e . D r a s t i c a r t i f i c a l r e d u c t i o n during a s p r i n g d e c l i n e improved female s u r v i v a l but not male s u r v i v a l ; s i m i l a r r e d u c t i o n i n the f a l l p r i o r to a d e c l i n e improved both male and female s u r v i v a l . S u r v i v a l was not density-dependent i n males, but was i n females duri n g one d e c l i n e . To e x p l a i n these r e s u l t s , a .behavioral model i s proposed i n which females compete f o r nest s i t e s and males compete f o r females. In a second experiment, a p o p u l a t i o n p r e d i c t e d t o experience a d e c l i n e was enclosed. I wanted to see i f preventing e m i g r a t i o n would stop the d e c l i n e . U n f o r t u n a t e l y the c o n t r o l area remained at peak d e n s i t i e s and n e i t h e r area s u f f e r e d a s p r i n g d e c l i n e . The enclosed p o p u l a t i o n had higher r a t e s of i n c r e a s e , reached higher d e n s i t i e s , and had higher s u r v i v a l r a t e s than those on the c o n t r o l area. T h i s r e s u l t e d i n severe o v e r g r a z i n g and a sharp p o p u l a t i o n d e c l i n e w i t h i n the e n c l o s u r e . These r e s u l t s i n d i c a t e that movements p l a y a necessary r o l e i n po p u l a t i o n r e g u l a t i o n when vo l e s are at peak, d e n s i t i e s . In a t h i r d experiment, the r o l e of predation i n causing p o p u l a t i o n changes was measured. P r e d a t i o n was not necessary to i n i t i a t e a d e c l i n e or to maintain i t . Of the tagged v o l e s known to have been eaten, there was no c o n s i s t e n t s e l e c t i o n by predators f o r e i t h e r c f the saxes, f o r any weight c l a s s , or f o r v o l e s with any other c h a r a c t e r i s t i c t h a t c o u l d be measured by l i v e - t r a p p i n g . Of the t o t a l number of v o l e s known t o have been eaten, avian p r e d a t o r s c o n s i s t e n t l y chose animals t h a t were s m a l l e r than those i n the tagged p o p u l a t i o n . T h i s i n d i c a t e s t h a t e i t h e r the l i v e - t r a p s s e l e c t i v e l y caught l a r g e animals, or a v i a n p r e d a t o r s s e l e c t i v e l y caught sma l l animals, or that both b i a s e s were pres e n t . In the f o u r t h experiment, the r o l e of r e s i d e n t a d u l t s i n determining j u v e n i l e s u r v i v a l was t e s t e d by removal of a d u l t s and by adding j u v e n i l e s to experimental p o p u l a t i o n s . S u r v i v a l of young was improved i n the absence of a l l a d u l t s , but not i n e i t h e r i n t a c t p o p u l a t i o n s , i n reduced p o p u l a t i o n s , o r i n female p o p u l a t i o n s . S u r v i v a l of young was b e t t e r i n periods of reduced a d u l t breeding. Height a t sexual maturity tended to be higher i n the presence of a d u l t s of the same sex. Growth i n young males tended to be reduced i n the presence of a d u l t males. These r e s u l t s i n d i c a t e t h a t females reduce s u r v i v a l of young, and suggest t h a t emphasis be placed on the study of female behavior as a f a c t o r a f f e c t i n g m i c r o t i n e numbers. In the f i n a l experiment, the s u r v i v a l and d i s p e r s a l r a t e s of very young v o l e s were examined to determine where the enormous l o s s between b i r t h and recruitment o c c u r r e d . A high d e n s i t y vole p o p u l a t i o n was trapped c o n c u r r e n t l y with l i v e - t r a p s and p i t f a l l t r a p s . Capture of a l a r g e number of young i n i i i p i t f a l l s i n d i c a t e d t h a t m o r t a l i t y was higher among post-weanlings than among any other age group. The p i t f a l l t r a p s enumerated up to twice as many animals as the l i v e - t r a p s , and over h a l f of the 1100 animals caught f i r s t i n p i t f a l l s were never caught i n l i v e - t r a p s . Trapping s o l e l y with l i v e - t r a p s may s e v e r e l y underestimate numbers (at l e a s t when d e n s i t i e s are high) and gi v e i n a c c u r a t e p o p u l a t i o n s t a t i s t i c s . P o p u l a t i o n s of J . townsendii f l u c t u a t e i n s i z e and C h i t t y ' s behavior h y p o t h e s i s p r e d i c t s t h a t spacing behavior u n d e r l i e s these d e n s i t y changes. My experimental r e s u l t s are c o n s i s t e n t with t h i s g e n e r a l view and add s p e c i f i c d e t a i l s to the mechanism by which d e n s i t y i s r e g u l a t e d . My experiments p i n p o i n t post weaning s u r v i v a l as a c r i t i c a l phase determining p o p u l a t i o n changes i n t h i s v o l e and focus a t t e n t i o n on two c r i t i c a l a r e a s : a d u l t female - postweanling i n t e r a c t i o n s and a d u l t female -a d u l t female i n t e r a c t i o n s . i v Table Of Contents a b s t r a c t ......................................... i TAELE OF CONTENTS i v LIST OF FIGURES • v i i LIST OF TABLES i x ACKNOWLEDGEMENTS . .. x i GENEBAL INTRODUCTION 1 SECTION 1. EFFECTS OF CONSPECIFICS ON SURVIVAL CUBING POPULATION DECLINES 6 I n t r o d u c t i o n 6 Methods .. 8 Vegetation 13 Re s u l t s 14 Po p u l a t i o n Density 14 S u r v i v a l 21 Movements 34 D i s c u s s i o n 37 Summary 49 SECTION 2. DISPERSAL IN DECLINE PERIODS . .. 54 I n t r o d u c t i o n .". 54 Methods 55 Re s u l t s 57 T r a p p a b i l i t y 57 Pop u l a t i o n Density 61 S u r v i v a l 67 Reproduction 73 V Sexual M a t u r i t y , 78 D i s c u s s i o n 81 Summary , 87 SECTION 3. IMPACT AND SELECTIVITY OF PEEDATION ....... . 88 I n t r o d u c t i o n 88 Methods 89 B e s u l t s 92 Incidence Of Predators 92 Amount Of P r e d a t i o n 95 Vole P o p u l a t i o n Density And Predator Impact ......... 97 S e l e c t i v i t y Of Predation 108 Sex Ratio 108 Body Weight D i s t r i b u t i o n s 111 Pr e d a t i o n On Tagged V c l e s ........................ 118 D i s c u s s i o n 120 Summary 127 SECTION 4. EFFECT OE ADULT ANIMALS ON YOUNG 128 I n t r o d u c t i o n 128 The 1974 Experiment , 129 Methods 129 Res u l t s 134 Po p u l a t i o n Density 134 S u r v i v a l In The Trappable P o p u l a t i o n ............. 138 E a r l y J u v e n i l e S u r v i v a l 141 D i s p e r s a l 142 Sexual M a t u r i t y 144 Growth 146 The 1975 Experiment 148 v i Bet hods ......................................... 148 Resul t s 151 Po p u l a t i o n Density 151 S u r v i v a l In The Trappable P o p u l a t i o n ............. 154 Reproduction And E a r l y J u v e n i l e S u r v i v a l ......... 156 I n t r o d u c t i o n Of Young 159 Sexual M a t u r i t y 168 Growth 170 D i s c u s s i o n 173 Summary 179 SECTION 5. THE RELIABILITY OF ENUMERATION TRAPPING AND SURVIVAL OF YOUNG 181 I n t r o d u c t i o n 181 Methods 183 Res u l t s ..... 187 T r a p p a b i l i t y 187 Po p u l a t i o n Density 189 S u r v i v a l 193 Body Weight D i f f e r e n c e s 203 P i t f a l l - t r a p p e d Voles 209 Pr o p o r t i o n Of The Sexes 216 D i s p e r s a l .". 219 D i s c u s s i o n 223 Summary 230 LITERATURE CITED 232 v i i LIST OF FIGURES Fi g u r e 1.1 A) Numerical Changes On G r i d s I , X, And Z. ..... 15 F i g u r e 1.1 B) Numerical Changes On G r i d s I, And L. ........ 17 F i g u r e 1.2 Standardized S u r v i v a l Rates On G r i d s L, X, And Z. 24 F i g u r e 1.3 R e l a t i o n s h i p Between Density And S u r v i v a l . ...... 30 F i g u r e 1.4 S u r v i v o r s h i p Curves For Young From Grids I And F i g u r e 1.5 Schematic Diagram Of Changes In S u r v i v a l . ...... 44 Appendix 1 S u r v i v a l Rates On G r i d s I , L, X, And Z. ........ 51 F i g u r e 2.1 Numerical Changes On The C o n t r o l And E n c l o s u r e G r i d s . 62 F i g u r e 2.2 Photograph Of Enclosure Boundary. .............. 65 F i g u r e 2.3 S u r v i v a l Hates On The C o n t r o l And E n c l o s u r e G r i d s . 68 F i g u r e 2.4 Percentage Of Adult Females L a c t a t i n g . ......... 75 Figure 2.5 Weight At Sexual maturity On The Two Areas. .... 79 F i g u r e 3.1 Numerical Changes On G r i d I, 1971-1974. ........ 98 F i g u r e 3.2 S u r v i v a l Rates And Numbers Disappearing By P r e d a t i o n . 104 F i g u r e 3.3 D i s t r i b u t i o n Of Body Weights Of Animals On The C o n t r o l Area And Of Animals Eaten By B i r d s Of Prey. ....116 Figu r e 4.1 Map Of The Study Area. 132 F i g u r e 4.2 Numerical Changes In 1974 On G r i d s I, L, X, And Z. . 135 F i g u r e 4.3 Numerical Changes In 1975 On The C o n t r o l , Female, I n t a c t , And Removal G r i d s . ....................... 152 v i i i F i g ure 4.4 S u r v i v o r s h i p Curves Of Introduced Young. .......162 F i g u r e 4.5 R e l a t i o n s h i p Between Female Density And D u r a t i o n Of L i f e Of Young ..... .165 Fi g u r e 5.1 Diagram Of The P i t f a l l Trap. 184 F i g u r e 5.2 Numerical Changes On G r i d I With Two Trapping Methods. 191 Fi g u r e 5.3 S u r v i v a l Rates Of Animals Caught By Two T r a p p i n g Methods 195 F i g u r e 5.4 S u r v i v o r s h i p Curves For Young Between B i r t h And Recruitment. .........................201 F i g u r e 5.5 D i s t r i b u t i o n Of Body Weights In Two Trap Types. 204 F i g u r e 5.6 R e l a t i o n s h i p Between Time Of Capture In Two Trap Types 207 F i g u r e 5.7 S u r v i v o r s h i p Curves Of Animals Caught Only In P i t f a l l s 212 i x U S T 0 1 TABLES Table 1.1 Summary Of Methods Dsed On Experimental Areas. .. 11 Table 1.2 V e g e t a t i o n On Trapping Areas. ................... 14 Table 1.3 Average D e n s i t i e s A f t e r Removal On Experimental Areas. ........ 20 Table 1.4 S u r v i v a l Rates On G r i d s I, L, X, And Z. ......... 22 T a b l e 1.5 Movements Between Traps On G r i d s I , L, X, And 2. 36 Table 2.1 T r a p p a b i l i t y On The C o n t r o l And Enclosure G r i d s . 59 Table 2.2 S u r v i v a l Rates On The C o n t r o l And E n c l o s u r e G r i d s 70 Table 2.3 P r o p o r t i o n Of Animals Breeding. 74 Table 3.1 Estimated Number Of B i r d s Of Prey Using Study Area. 94 Table 3.2 Number Of Prey Items C o l l e c t e d On Study Area. ... 96 Table 3.3 Number Of Voles Disappearing And Percent Recovered From P r e d a t o r s . ..............................102 Table 3.4 Comparison Cf Avian P r e d a t i o n On T o t a l And Tagged Vole P o p u l a t i o n . .107 Table 3.5 Sex R a t i o Of Voles On C o n t r o l And Of Voles Eaten By Predators. .......................................... 111 Table 3.6 Mean Body Height Of V c l e s On C o n t r o l And Of Voles Eaten By P r e d a t o r s . .....114 Table 3.7 Movements Of Voles On C o n t r o l G r i d And Of Voles Eaten By P r e d a t o r s . 119 T a b l e 4.1 S u r v i v a l Rates On Grids "I, G, L, X, And Z. ......139 Table 4.2 Number Of Young Per l a c t a t i n g Female I n 1974. ...142 Table 4.3 Percent Loss Owing To D i s p e r s a l To Other G r i d s . .143 Table 4.4 Body Weights At Sexual Maturity In 1974. 146 Table 4.5 S u r v i v a l Sates On The C o n t r o l , Female, And I n t a c t G r i d s . 155 Table 4.6 P r o p o r t i o n Of Males And Females In Breeding C o n d i t i o n . .. 157 Table 4.7 Number Of Young Per l a c t a t i n g Female I n 1975. ...159 Table 4.8 Summary Of Young Introduced Onto E x p e r i m e n t a l Areas .. 161 Table 4.9 Movement Of Young To Other Trapping Areas. ...... 167 T a b l e 4.10 Body Weights At Sexual M a t u r i t y In 1975. ....... 170 Table 4.11 R e l a t i v e Growth Rates Of Males On Experimental Areas ...........172 Table 5.1 T r a p p a b i l i t y In L i v e - t r a p s And P i t f a l l s . ........189 Tabl e 5.2 S u r v i v a l Rates Of Animals Caught In The Two Trap Types. 197 Table 5.3 Body Weight At F i r s t Capture In The Two Trap Types .206 Table 5.4 Number Of Animals Caught Only In P i t f a l l s And Caught In Both Trap Types. ......210 Table 5.5 Percent Of Voles Caught Only In P i t f a l l s . .......215 Table 5.6 Number Of R e c r u i t s Caught In L i v e - t r a p s And Percent P r e v i o u s l y Caught In P i t f a l l s . .................216 Table 5.7 P r o p o r t i o n Of Males In The Two Trap Types. 218 Table 5.8 D i s p e r s a l Frcm G r i d I To Other Areas .220 Table 5.9 Movements Of Animals Caught In The Two Trap Types. .......222 x i fiCKNOSLEDG EMENTS I wish to express by s i n c e r e thanks to Dr. Dennis C h i t t y , my s u p e r v i s o r , ana to Dr. Ch a r l e s Krebs f o r a d v i c e , encouragement, and a s s i s t a n c e throughout t h i s study. T h e i r c r i t i c i s m s and comments on t h i s manuscript have made i t more readable and c o n c i s e . The comments of the other members of my committee, Dr. L. Gass, Dr. N. L i l e y , and Dr. J . Myers, are a p p r e c i a t e d . Thanks are due to Dennis Draper, J a n i c e LeDuc, Michael Moult, Nick Morris, J a r o s l a v Pieman, Donna Stace-Smith, Tom S u l l i v a n , and Mary T a i t t , who a l l a s s i s t e d me i n g a t h e r i n g some of the data presented i n t h i s t h e s i s . Dr. Dennis C h i t t y , Dr. C h a r l e s Krebs, and J a n i c e LeDuc provided me with some of t h e i r data. , N e i l Dawe, C r a i g Runyan, and Brain Davies provided i n f o r m a t i o n on predators from the R e i f e l Haterfowl Refuge. Dr. C h a r l e s Krebs and Steve Borden a s s i s t e d with s t a t i s t i c a l a n a l y s i s and computing problems. Tom S u l l i v a n helped me throughout the study with u s e f u l d i s c u s s i o n s . I r e c e i v e d support from a t e a c h i n g a s s i s t a n t s h i p and from NRC and K i l l a m P r e - d o c t o r a l s c h o l a r s h i p s . F i n a l l y , I am s i n c e r e l y g r a t e f u l to my wife, B e t t y , whose concern, support, and encouragement have s u s t a i n e d me throughout these years of graduate study. 1 GENERAL INTRODUCTION Rates of i n c r e a s e among animal popu l a t i o n s over the long terra are c l o s e t o zero. T h i s s t a b i l i t y poses a problem, given t h a t the p o t e n t i a l r a t e of i n c r e a s e of p o p u l a t i o n s i s o f t e n very g r e a t . I t i s p o s s i b l e to study the r e g u l a t i o n of an animal p o p u l a t i o n which i s r e l a t i v e l y constant i n numbers by p e r t u r b i n g i t and observing the processes t h a t r e t u r n i t to the previous d e n s i t y (Murdoch 1970), or by s t u d y i n g p o p u l a t i o n s which show l a r g e changes i n abundance and observing the processes which act during the p e r i o d s of i n c r e a s e and decrease ( C h i t t y 1960). I have chosen to work on a m i c r o t i n e s p e c i e s , flic r o t a s t o w n s e n d i i , which undergoes the l a t t e r type of ' f l u c t u a t i o n s . Many Microtus p o p u l a t i o n s undergo r e g u l a r 3-4 year c y c l e s i n p o p u l a t i o n d e n s i t y . P o p u l a t i o n c y c l e s i n m i c r o t i n e s are accompanied by a syndrome of changes i n both b i r t h and death r a t e s (Krebs and Myers 1974), though the mechanisms r e s p o n s i b l e f o r these f l u c t u a t i o n s are as yet poorly . understood. Ideas concerning e x t r i n s i c agents of m o r t a l i t y such as s t a r v a t i o n , p r e d a t i o n , or d i s e a s e have been unable to e x p l a i n the c h a r a c t e r i s t i c changes that accompany c y c l e s , Ideas concerning i n t r i n s i c agents, i n which the e f f e c t s of one i n d i v i d u a l on another are b e l i e v e d to u n d e r l i e the causes of c y c l e s , have gained support i n the l a s t decade. In p a r t i c u l a r , the g e n e t i c - b e h a v i o r a l hypothesis of C h i t t y (1967) has been strengthened and s l i g h t l y m o d ified by the work of Krebs and h i s s t u d e n t s (Krebs et a l . 1973, Krebs e t a l . 1976). I wished to e x p l o r e some b e h a v i o r a l aspects of t h i s hypothesis, i n which 2 spacing benavior (or h o s t i l i t y ) i s b e l i e v e d r e s p o n s i b l e f o r d r i v i n g the c y c l e . The b a s i c g u e s t i o n I was i n t e r e s t e d i n was whether s o c i a l l y - i n c r e a s e d m o r t a l i t y or s o c i a l l y - d e c r e a s e d r e c r u i t m e n t cause changes i n the abundance o f vo l e s . I performed a number of short-term experiments designed to c l a r i f y how behavior might operate i n the f i e l d . One of the major g u e s t i o n s about m i c r o t i n e f l u c t u a t i o n s concerns the nature of the m o r t a l i t y f a c t o r a c t i n g during the p o p u l a t i o n d e c l i n e . Adult m o r t a l i t y i s g e n e r a l l y low durin g the i n c r e a s e and peak phases, but high during the d e c l i n e phase (Krebs and Myers 1914). I reasoned t h a t i f a g g r e s s i v e behavior between c o n s p e c i f i c s i s a function' of the r a t e of i n t e r a c t i o n , which i s presumably r e l a t e d t o d e n s i t y i n some p o s i t i v e way, a d r a s t i c r e d u c t i o n i n p o p u l a t i o n s i z e would decrease the r a t e of i n t e r a c t i o n , and i n c r e a s e the s u r v i v a l r a t e of the remaining a d n l t s . The r a t e o f i n t e r a c t i o n i s d i f f i c u l t t o measure i n the f i e l d . I t i s probably i n f l u e n c e d by a number of f a c t o r s which can be p i c t u r e d as f e l l o w s : H a b i t a t S t r u c t u r e A g g r e s s i v e Behavior Bate of I n t e r a c t i o n P o p u l a t i o n Density E x t r i n s i c Agents (Predators) S u r v i v a l Rate ( l o s s e s can be due to d i s p e r s a l c r m o r t a l i t y ) Of the f a c t o r s i n f l u e n c i n g the r a t e of i n t e r a c t i o n , d e n s i t y appeared e a s i e s t to s h i f t . The experiments d e s c r i b e d i n Se c t i o n 1 t e s t t h i s i d e a by means of a r t ' i f i c a l r e d u c t i o n s of vole p o p u l a t i o n s during and before d e c l i n e p e r i o d s . The f a t e of animals d u r i n g a d e c l i n e i s unknown, and 3 remains one of the puzzles of microtine demography. There i s no evidence that aggressive interactions, as evidenced by wounding, are s u f f i c i e n t to cause death during the decline (Krebs and Myers 1974). L i t t l e dispersal was found i n the decline; most took place i n the increase and peak periods (Myers and Krebs 1971, Krebs et a l . 1976). Preventing dispersal i n the increase and peak periods prevented population regulation (Krebs et a l . 1969); but we do not know what part dispersal plays in the decline. I reasoned that i f dispersal were prevented prior to a decline, yet a decline took place, then the l o s s of animals could be attributed to i n s i t u mortality. The experiment described i n Section 2 was done to test t h i s idea. One of the possible fates of animals during the decline (and at other times as well) i s to be eaten by a predator. The present evidence suggests that predation i s not necessary to cause declines (Krebs and Myers 1974). However, changes i n the amount or kind of predation may be used to indicate processes associated with changes i n s u r v i v a l in a vole population. Are the i n d i v i d u a l s eaten by predators the vulnerable segment of the population and predisposed to mortality because of s o c i a l reasons (Errington 1956, 1967), or are they a cross-section of a l l i n d i v i d u a l s i n the vole population? To examine these questions, I t r i e d to c o l l e c t a l l evidence of predation during two winters and one f a l l , and related t h i s to the vole populations that were being monitored by live-trapping (Section 3). In addition, the impact of predation on vole populations during declines was also examined. Changes i n s u r v i v a l of young appear to one of the c r i t i c a l 4 f a c t o r s a s s o c i a t e d with m i c r c t i n e c y c l e s . The death r a t e of young appears to i n c r e a s e d r a m a t i c a l l y i n peak and d e c l i n e p o p u l a t i o n s (Krebs and Myers 1974), but the cause i s unknown. To determine whether a d u l t presence i s one of the f a c t o r s a f f e c t i n g s u r v i v a l of young, I performed the experiments d e s c r i b e d i n S e c t i o n 4. They i n v o l v e d removing a l l a d u l t r e s i d e n t s , removing the m a j o r i t y of the a d u l t r e s i d e n t s , and removing a l l males. I t i s d i f f i c u l t to determine by the use of present l i v e - t r a p p i n g t e c h n i q u e s whether young v o l e s are s u r v i v i n g p o o r l y before o r a f t e r weaning, and whether they are d i s p e r s i n g before capture i n l i v e - t r a p s . To examine the f a t e of young, I trapped a v o l e p o p u l a t i o n c o n c u r r e n t l y with l i v e - t r a p s and p i t f a l l t r a p s (which c a t c h young at an e a r l y age). I n a d d i t i o n , the p i t f a l l t r a p p i n g allowed me to examine how e f f i c i e n t the t o t a l p o p u l a t i o n enumeration techniques are i n t h i s s p e c i e s ( C h i t t y and Phipps 1966, Krebs 1S66). T h i s technique has been used i n a l l subsequent s t u d i e s by Krebs and h i s s t u d e n t s , but the assumption t h a t t r a p s capture the major i t y of animals at r i s k has seldom been t e s t e d . Since the major i t y of demographic s t a t i s t i c s I c a l c u l a t e are made oh t h i s assumption, a t e s t of t h i s technique was d e s i r a b l e (Section 5). In summary, the gen e r a l o b j e c t i v e of t h i s study was to f i n d out i f the presence of c o n s p e c i f i c s decrease the s u r v i v a l , growth, and r e p r o d u c t i o n of i n d i v i d u a l v o l e s , and at what stages i n the l i f e c y c l e t h i s may occur. Because the study can be d i v i d e d i n t o f i v e major aspects t h a t a re r e l a t e d but s e l f - c o n t a i n e d , I decided to w r i t e the s e c t i o n s as independent papers s u i t a b l e f o r p u b l i c a t i o n a f t e r s l i g h t r e v i s i o n . There i s 5 t h e r e f o r e some r e p e t i t i o n between the s e c t i o n s , e s p e c i a l l y with regard to general trapping methods and methods of determining such things as t r a p p a b i l i t y estimates, s u r v i v a l r a t e s , and weights at sexual maturity. These areas of r e p e t i t i o n are b r i e f . 6 SECTION .1. EFFECTS OF CONSJEGIFICS ON SURVIVAL DURING POPULATION DECLINES I n t r o d u c t i o n I n t r a s p e c i f i c b e h a v i o r a l i n t e r a c t i o n s are b e l i e v e d to be c r i t i c a l i n the r e g u l a t i o n of many animal p o p u l a t i o n s (Watson and Moss 1970), Despite over 40 years of r e s e a r c h , the methods by which m i c r o t i n e p o p u l a t i o n s are r e g u l a t e d i s s t i l l unknown (f o r a review, see Krebs and Myers 1974), The e x t r i n s i c hypotheses of d i s e a s e ( F i n d l a y and Middleton 1934, C h i t t y 1954), food and n u t r i e n t s ( P i t e l k a 1964, S c h u l t z 1969), p r e d a t i o n (Pearson 1964, 1966, 1971), and weather ( F u l l e r 1969) have a l l been u n s u c c e s s f u l i n p r o v i d i n g necessary causes of c y c l e s . The most promising e x p l a n a t i o n to account f o r the c h a r a c t e r i s t i c changes that occur i n r e p r o d u c t i o n , m o r t a l i t y , d i s p e r s a l , and growth i s t h a t b e h a v i o r a l i n t e r a c t i o n s between animals are c a u s i n g the f l u c t u a t i o n s (Krebs and Myers 1974), C h r i s t i a n and Davis (1964) b e l i e v e d that as p o p u l a t i o n d e n s i t y i n c r e a s e d so d i d the frequency of i n t e r a c t i o n s , causing a p h y s i o l o g i c a l d e t e r i o r a t i o n of the animals mediated through endocrine pathways. This d e t e r i o r a t i o n r e s u l t e d i n depressed r e p r o d u c t i o n , depressed inflammatory responses, and i n c r e a s e d i n c i d e n c e of other p h y s i o l o g i c a l derangements. T h e i r emphasis was on reduced r e p r o d u c t i o n r a t h e r than on i n c r e a s e d m o r t a l i t y . T h i s h y p o t h e s i s c o u l d not e x p l a i n the periods of very severe and sudden m o r t a l i t y found i n many m i c r o t i n e c y c l e s (Godfrey 1955, 7 C h i t t y and Phipps 1966, and Krebs et a l . 1969), nor could i t account f o r the maintenance of high population l e v e l s a t t a i n e d by animals i n e n c l o s u r e s (Clarke 1955, Van aijngaarden 1960, Houlihan 1963, Krebs et a l . 1969). C h i t t y (1967) a l s o b e l i e v e d t h a t behavior was c r i t i c a l i n the r e g u l a t i o n of m i c r o t i n e s . However, he b e l i e v e d t h a t m i c r o t i n e f l u c t u a t i o n s were the r e s u l t of g e n e t i c , not purely phenotypic change. The hypothesis proposes t h a t i n a s s o c i a t i o n with the a l t e r n a t i n g phases of i n c r e a s e and decrease i n the c y c l e , there was a l t e r n a t i n g s e l e c t i o n pressure f o r non-aggressive and a g g r e s s i v e genotypes. The a g g r e s s i v e genotypes were thought to have an advantage by a d v e r s e l y i n f l u e n c i n g both r e p r o d u c t i o n and s u r v i v a l of t h e i r c o n s p e c i f i c s . During a p o p u l a t i o n d e c l i n e , C h i t t y f e l t t h a t s o c i a l i n t o l e r a n c e s of the animals themselves l e d t o m o r t a l i t y . T h i s was c o r r o b o r a t e d from work by Myers and Krebs (1971) which i n d i c a t e d t h a t l i t t l e d i s p e r s a l occurred i n the d e c l i n e . In t h i s study, I have attempted t o t e s t part of C h i t t y ' s hypothesis t h a t b e h a v i o r a l i n t e r a c t i o n s are r e s p o n s i b l e f o r the d e c l i n e . The s e c r e t i v e behavior of v o l e s demands t h a t i n d i r e c t methods be used t o t e s t the a s s o c i a t i o n or b e h a v i o r with p o p u l a t i o n change i n f i e l d experiments. My experiments are based on two c r i t i c a l assumptions: f i r s t , that the s i g n i f i c a n t a g o n i s t i c i n t e r a c t i o n s are a f u n c t i o n of the number of c o n t a c t s between i n d i v i d u a l s i n a f i e l d p o p u l a t i o n ; and second, t h a t the number of c o n t a c t s between i n d i v i d u a l s i s r e l a t e d to p o p u l a t i o n d e n s i t y i n some p o s i t i v e manner. Given these two assumptions, I p r e d i c t e d t h a t a d r a s t i c r e d u c t i o n i n population s i z e d u r i n g the 8 d e c l i n e should decrease the r a t e of c o n t a c t between i n d i v i d u a l s , and hence i n c r e a s e the s u r v i v a l r a t e of the remaining a d u l t s and t h e i r o f f s p r i n g . The f i e l d experiments i n v o l v e d c r e a t i n g low d e n s i t y p o p u l a t i o n s by c r o p p i n g d u r i n g and b e f o r e expected d e c l i n e s . Methods The study was c a r r i e d out on Westham I s l a n d i n the d e l t a of the F r a s e r R i v e r , near Vancouver, B r i t i s h Columbia, on a 37-acre (15 ha) p a r c e l of pastureland owned by the Department of N a t i o n a l Defence. G r i d I, which was used as the c o n t r o l i n the experiment of LeDuc and Krebs (1975) and had bean trapped s i n c e J u l y 1971, was used f o r the same purposes i n the present study. Two expe r i m e n t a l g r i d s were s e t up i n 1:973. G r i d X was set up on 2 February and f i r s t trapped one week l a t e r . However, few animals entered the t r a p s u n t i l the beginning of March. At t h a t time a l l but 20 of the animals trapped (11 males and 9 females) were removed; new r e c r u i t s caught i n subseguent weeks were a l s o removed. The animals chosen to remain i n the p o p u l a t i o n ware caught i n t r a p s t h a t were spaced as f a r apart as p o s s i b l e from one another. T h i s cohort was trapped u n t i l the end of A p r i l , when young of the year s t a r t e d to enter the p o p u l a t i o n . A l l new r e c r u i t s from then on were allowed to enter the p o p u l a t i o n u n t i l September. Because G r i d L had been s u b j e c t to previous manipulations i n the f a l l and winter of 1972-73 , the vole p o p u l a t i o n there may have been d i f f e r e n t from t h a t on the c o n t r o l g r i d . An 9 attempt was made to remove a l l r e s i d e n t s from t h i s g r i d by l i v e - t r a p p i n g and snap-trapping during February. In March, 24 animals (12 males and 12 females) removed from G r i d X were t r a n s f e r r e d to t h i s g r i d and r e l e a s e d at egual d i s t a n c e s about i t . A • s i m i l a r t r a p p i n g scheme to t h a t on G r i d X was then c a r r i e d out. The e x p e r i m e n t al s a t up used f o r g r i d X i n s p r i n g 1973 was r e p l i c a t e d on g r i d Z i n s p r i n g 1974. The c o n t r o l p o p u l a t i o n was again undergoing a severe s p r i n g d e c l i n e . G r i d Z was s e t up i n mid-February and f i r s t trapped on 4 March. A l l animals but a cohort of 20 (10 males and 10 females) were removed and a t r a p p i n g scheme s i m i l a r t o t h a t above was f o l l o w e d . Although most of the subseguent r e c r u i t s were removed, a s m a l l number of males was allowed to remain because of the extremely poor s u r v i v a l r a t e s o f the males i n the o r i g i n a l c o h o r t . Although t h i s g r i d was f o l l o w e d through the r e s t of the summer (see S e c t i o n 4), the present study examines the r e s u l t s only to the end of the d e c l i n e p e r i o d . In an e f f o r t to c r e a t e a low d e n s i t y g r i d throughout the winter of 1973-74, I cropped g r i d X of a l l a d u l t animals i n September, l e a v i n g only those animals to remain - which had not yet reached s e x u a l m a t u r i t y . A l l new r e c r u i t s (> 40 g) subseguent to t h i s were a l s o removed; thus on l y young remained i n the p o p u l a t i o n . Numbers continued to i n c r e a s e , so t h a t by the end of December, g r i d X had a p o p u l a t i o n d e n s i t y t h a t was h a l f t h a t of the c o n t r o l . I cropped the g r i d even f u r t h e r , t a k i n g o f f a l l r e c e n t r e c r u i t s and l e a v i n g only a cohort of 25-30 animals o r i g i n a t i n g from the l a t e summer and f a l l . At the onset of the 10 breeding season i n 1974, only young animals (<40 g at f i r s t capture) judged t o be o f f s p i n g of t h i s cohort were allowed to enter the p o p u l a t i o n . During l a t e September 1973, I removed the e n t i r e p o p u l a t i o n from g r i d L except f o r a few pregnant females, and stocked i t with pregnant females removed from g r i d X. These females were removed a f t e r they had weaned t h e i r young. Only young (< 40 g) were allowed to e n t e r the p o p u l a t i o n . T h i s m a n i p u l a t i o n was made to see how animals born i n the l a t e f a l l responded t o low d e n s i t i e s . T h i s p o p u l a t i o n was trapped i n a s i m i l a r manner to g r i d X. Table 1.1 summarizes these manipulations on the experimental g r i d s . Each t r a p p i n g g r i d except g r i d 1 had 100 t r a p p i n g p o i n t s arranged i n a 10 X 10 p a t t e r n . These areas were approximately 1.7 acre {.7 ha) i n s i z e . G r i d I was an i r r e g u l a r t r a p p i n g area with 68 t r a p p i n g p o i n t s and an area of .9 acre (.4 ha) bordered on one s i d e by a drainage d i t c h and on the other by a d i r t road. On g r i d s I, X, and Z each t r a p p i n g p o i n t was 7.6 m (25 f t ) from the next. On g r i d L the. t r a p p i n g i n t e r v a l was i n i t i a l l y 9.1 m (30 f t ) but was l a t e r changed to 7.6 m. Longworth l i v e - t r a p s were placed a t every s t a t i o n and b a i t e d with o a t s ; c o t t o n bedding was provided. A d d i t i o n a l t r a p s (up t o 50) were added to the c o n t r o l g r i d as d e n s i t y i n c r e a s e d . On a l l other g r i d s o n l y one t r a p per s t a t i o n was used. Traps were g e n e r a l l y s e t every second week i n the a f t e r n o o n , checked the f o l l o w i n g morning, the next a f t e r n o o n , and again on the second morning, when they were lock e d open and l e f t i n place ( o c c a s i o n a l l y a one-week t r a p p i n g i n t e r v a l and once a three-week t r a p p i n g i n t e r v a l ware used). 11 Table 1.1. Summary o f methods used on the experimental g r i d s . G r i d March -A p r i l 73 Summer 73 F a l l 73 Winter 73-74 March 74 removed a l l but 20 animals; removed a l l new r e c r u i t s a l l animals allowed to r e c r u i t removed a l l o l d and new a d ul t s; young allowed t o r e c r u i t removed a l l but 25-30 animals removed a l l animals; i n t r o d u c e d 24 animals from X same as X removed a l l animals except pregnant females and seme int r o d u c e d pregnant females from X; only young from these females allowed t o r e c r u i t removed a l l but 20 animals as on X 1973 12 During the summer, t r a p s were se t only i n evenings to avo i d m o r t a l i t y i n t r a p s during the heat of the day. A l l v o l e s were ear-tagged, and on each c a p t u r e the f o l l o w i n g data were recorded: t r a p l o c a t i o n ; sex; s e x u a l c o n d i t i o n ( f o r males p o s i t i o n of the t e s t e s and f o r females vagina p e r f o r a t e or not; n i p p l e s s m a l l , medium, or l a r g e ; pubic symphysis c l o s e d , s l i g h t l y open, or open; pregnant or n o t ) ; and weight. Animals were c l a s s i f i e d as a d u l t (> 4 3 g) or young. Few j u v e n i l e s were caught so t h a t no d i s t i n c t i o n was made between su b a d u l t s and j u v e n i l e s . I have attempted to enumerate the p o p u l a t i o n i n t h i s study, s i n c e i t i s not p o s s i b l e to sample Microtus p o p u l a t i o n s randomly (Krebs 1966). T h i s method of demographic a n a l y s i s r e q u i r e s that a l a r g e p r o p o r t i o n of the i n d i v i d u a l s of t r a p p a b l e age be captured every time. The t r a p p a b i l i t y of the p o p u l a t i o n was c a l c u l a t e d u s i n g the method of H i l b o r n et a l . (1976). During the f a l l , winter, and s p r i n g month's' t r a p p a b i l i t y was always hi g h , ranging from 82-91% on the c o n t r o l g r i d . There were no d i f f e r e n c e s i n t r a p p a b i l i t y between the saxes. Experimental g r i d s a l s o had high t r a p p a b i l i t i e s ranging from 74-90$. In the summer, t r a p p a b i l i t y on the c o n t r o l remained above 82$ while t h a t on g r i d s L and X was approximately 10-15% lower. T h i s was probably r e l a t e d to g r e a t e r competition f o r t r a p s on g r i d s L and X than on the c o n t r o l g r i d where more t r a p s were used. I b e l i e v e , t h e r e f o r e , t h a t the demographic s t a t i s t i c s obtained from these data are accurate f o r a d u l t s . Young v o l e s appear to be l e s s t r a p p a b l e than a d u l t s . 13 Vegetation The study area was a r e l a t i v e l y f l a t g r a s s l a n d , which has had grass on i t s i n c e 1946 and has not been grazed s i n c e 1963. An a e r i a l photograph of the area can be found i n LeDuc and Krebs (1975), and a s k e t c h map of the l o c a t i o n of the g r i d s can be found i n S e c t i o n 4. The dominant p l a n t s on the f o u r g r i d s are shown i n Table 1.2. Percent cover was determined by using 1 m2 sguare p l o t s at each t r a p p o i n t . There was a g r e a t d e a l of v a r i a b i l i t y between g r i d s , with Agropyron dominating g r i d s L and Z and Acjrostis a l b a dominating g r i d s I and X. However, grass cover dominated the v e g e t a t i o n on a l l g r i d s , always accounting f o r more than 903? of the cover. Few herbaceous s p e c i e s occurred; c r e e p i n g t h i s t l e (Cirsium arvgnse) was the most common. R e s u l t s P°£iii:^i2Il D e n s i t y The p o p u l a t i o n on the c o n t r o l g r i d decreased at an average r a t e of 8% per week from December 1972 to May 1973 ( F i g . 1.1), i n c r e a s e d by 4% per week from May to September, and by 6% per week from September to December. During e a r l y 1974, the p o p u l a t i o n again d e c l i n e d , f a l l i n g a t an average r a t e of 111 per week to reach a low i n A p r i l . The r a t e of l o s s of males was g e n e r a l l y more r a p i d than t h a t of females (2-3% more per week) 14 Table 1.2. Percent cover of four t r a p p i n g g r i d s on Mestham I s l a n d , 1974. G r i d G r i d ' G r i d G r i d I • 1 X 2 repens 27 52 20 42 M i o s t i s a l b a 58 22 55 28 Holeus l a n a t u s 2 4 10 15 JEM^SS • £XJ t e n s e 1 ' 1 1 Poa ££atense 9 7 11 11 C i r s i u j arvense 2 7 2 2 15 F i g u r e 1,1 A) Minimum number a l i v e on the c o n t r o l g r i d and on two experimental g r i d s (X and Z) i n 1973-74. Non-breeding p e r i o d s on the c o n t r o l are shaded. M i n i m u m Number A l i v e 9T 1 7 Figure 1 . 1 B) Minimum number a l i v e on the control grid and on grid L. (Grid L was 4 / 7 the area of the control grid.) 19 bat the r a t e o f g a i n was a l s o more r a p i d by the same amount. Shan s e t t i n g up g r i d s X and Z, I had hoped that the p o p u l a t i o n s on them were comparable t o those on the c o n t r o l . Before the f i r s t r e d u c t i o n , 73 animals were caught on g r i d X compared with 92 on the c o n t r o l g r i d . In 1974, 78 animals were caught on g r i d Z i n the f i r s t t r a p p i n g s e s s i o n compared with 110 animals on the c o n t r o l i n the same week, Because some animals may r e q u i r e more time to get used t o t r a p s , d e n s i t i e s on these two g r i d s were probably reasonably s i m i l a r to those on the c o n t r o l . T h e r e f o r e , these g r i d s were good experimental areas. The cropping procedures i n s p r i n g 1973 and s p r i n g 1974 were e f f e c t i v e i n m a i n t a i n i n g d e n s i t i e s c o n s i d e r a b l y below those on the c o n t r o l g r i d (Table 1.3 and F i g . 1.1a). In 1973, a f t e r the c r o p p i n g stopped, the p o p u l a t i o n on g r i d X q u i c k l y b u i l t up (averaging 12% gain per week) to reach 103 animals i n J u l y . i t then stayed at approximately t h i s number f o r the r e s t of the summer. From September to December, a t o t a l of 163 a d u l t s r e s i d e n t s and r e c r u i t s were removed from g r i d X, but the d e n s i t i e s of young continued to i n c r e a s e . In the next three and a h a l f months an a d d i t i o n a l 210 animals were removed from g r i d X." From l a t e March onward, numbers on X s t a r t e d i n c r e a s i n g because of r e c r u i t m e n t of young of the year. Throughout the f a l l , winter, and s p r i n g of 1973-74, g r i d X had d e n s i t i e s t hat were 1/4 - 2/5 that on the c o n t r o l . On g r i d L ( F i g . 1.1b), a f t e r cropping ceased i n 1973, d e n s i t y g u i c k l y i n c r e a s e d to become comparable t o t h a t on the c o n t r o l . However, t h i s g r i d showed a mid-summer drop i n numbers t h a t was not e v i d e n t on g r i d X and only occurred i n one t r a p p i n g 20 Table 1.3. Average d e n s i t i e s per 1.7 ac g r i d a f t e r removal on the experimental g r i d s . Density on g r i d L was c o r r e c t e d f o r a r ea. The percent that these made up of c o n t r o l d e n s i t i e s i s g i v e n i n parentheses. G r i d Spring Summer Sept. - Jan. - March -73 73 Dec. 7 3 March 74 A p r i l 74 I 26 {32%) 93 (12 1%) 51 (30%) 55 (40%) X 30 (4U) 68 (88%) 66 (39%) 32 (24%) Z 22 (32%) 21 s e s s i o n on the c o n t r o l g r i d . During the f a l l , a t o t a l of e i g h t pregnant females was i n t r o d u c e d on to g r i d L from g r i d X. These i n d i v i d u a l s plus the young from approximately ei g h t pregnancies from female r e s i d e n t s gave r i s e a l a t e f a l l c o h o r t of young. Throughout the f a l l , winter, and s p r i n g , g r i d L had d e n s i t i e s t h a t were 1/3 - 2/5 that on the c o n t r o l . S u r v i v a l The c a l c u l a t i o n of minimum s u r v i v a l r a t e s from one t r a p p i n g p e r i o d to the next i n c l u d e s two components, death and e m i g r a t i o n , which cannot be separated by the kind of t r a p p i n g data c o l l e c t e d i n t h i s study. Since I am p r i m a r i l y i n t e r e s t e d i n the e f f e c t of the experimental treatments on s u r v i v a l , I w i l l make comparisons between the c o n t r o l and the experimental g r i d s o n l y . The minimum s u r v i v a l rate's presented i n Table 1.4 are mean r a t e s f o r each p e r i o d , and an i n d i v i d u a l i s t a l l i e d each time i t i s captured w i t h i n a period. To compare these r a t e s , I have used ch i - s g u a r e a n a l y s i s on samples which are not completely independent. Though these comparisons are not v a l i d , they give an i n d i c a t i o n of the degree of d i f f e r e n c e between samples. F i v e p o i n t s are shown by Table 1.4. 1. On g r i d s L and X during the s p r i n g d e c l i n e of 1973 and on g r i d Z during the s p r i n g d e c l i n e of 1974, the males s u r v i v e d no b e t t e r than those on the c o n t r o l g r i d . 2. Females on g r i d X s u r v i v e d s l i g h t l y b e t t e r than those on the c o n t r o l g r i d (see below) but not on g r i d L. Females on g r i d Z, however, had a t o t a l s u r v i v a l r a t e t h a t was 20$ h i g h e r than t h a t 22 fable 1.4. Minimum s u r v i v a l rates per 14 days f o r M. townsendii. Sample sizes i n parentheses. "~ ~ ""* Males Females Grid adult Young Total adult Young Total Spring 1973 I .78 {110} .70 (10) .78 (120) . 83 (93) .85 (16 5) .85 (258) 1 .75 (29) .44 (6) .70 (35) .93 (15) .77 (3D .83 (46) X .70 (49) . 89 (9) .75 (58) .88 (23) .90 (57) .89 (80) Summer 1973 I .77 (222) .49 (39) .73 (261) . 84 (326) .69 (6 5) . 82 (391) L .73 (9 3) . 59 (29) .70 (125) .81 (105) • 78 (49) . 80 (154) X .77 (262) .61 (57) .75 (319) .81 (314) .85 (81) . 81 (395) F a l l - Winter 1973 I .86 (337) .74 ** (S7) .83 (4 34) . 87 (365) .85 (149) . 86 (514) L .89 (16) . 93 * (5 3) .92 (73) . 92 (38) .85 (50) .88 (88) X .91 (127) . 89 (92) .90 (219) . 87 (71) .95 (112) .92 (183) Winter - Spring 1974 I .71 (363) * . 79 (108) * .72 (471) .76 (197) ** .7 6 (310) ** . 76 (507) ** L .85 (73) ** .96 (25) .88 (9 8) ** . 98 (43) .94 * (67) .95 (110) ** X .94 (117) .71 (31) .89 (148) .88 (75) .8 9 (54) . 88 (129) Spring 1974 I .58 (89) .00 (2) .57 (91) .74 (86) .62 (32) .71 (118) z .55 (22) .67 (3) . 56 (25) . ?0 (20) .92 (12) . 91 (32) * P<.05 for n u l l hypothesis of no difference with control ** P<. 01 23 of the females on the c o n t r o l g r i d . 3. During the summer of 1973, when d e n s i t i e s were s i m i l a r on the c o n t r o l g r i d , g r i d 1, and g r i d X, s u r v i v a l i n a i l c l a s s e s i n both sexes 1was a l s o s i m i l a r . 4. During the f a l l i n c r e a s e p e r i o d , there was a g e n e r a l trend f o r higher s u r v i v a l r a t e s on g r i d s L and X ( s i g n i f i c a n t only i n young males on 1 and X, i n the t o t a l male sample on X, and i n the young females on X). 5. During the d e c l i n e p e r i o d i n the winter and e a r l y s p r i n g 1974, s u r v i v a l was c o n s i s t e n t l y 12-23$ higher on g r i d s L and X except among the young males on g r i d X. Fi g u r e s 1.2 a and b show the s t a n d a r d i z e d minimum s u r v i v a l r a t e s f o r males and females r e s p e c t i v e l y . The appendix to t h i s s e c t i o n g i v e s the a c t u a l changes i n minimum s u r v i v a l r a t e s per 14 days during 1973-74. Poor s u r v i v a l i s a r b i t a r i l y d e f i n e d to be any r a t e below .707 per two weeks (below which h a l f the p o p u l a t i o n d i s a p p e a r s every 4 weeks). In the s p r i n g of 1973, the s u r v i v a l r a t e s of males on g r i d s L and X f l u c t u a t e d g r e a t l y when compared with t h a t on the c o n t r o l . T h i s was caused i n part by the s m a l l sample s i z e . For example, by the beginning of A p r i l , there were only 3 males l e f t on X and 4 on L. S u r v i v a l was n e v e r t h e l e s s very poor i n c e r t a i n weeks - 5 out of 9 a d u l t males present on G r i d X i n mid-March disappeared within the next two weeks. During the summer, males on a l l g r i d s g e n e r a l l y showed s i m i l a r s u r v i v a l r a t e s , with t h a t of males on g r i d 1 f l u c t u a t i n g much more than those on the other two g r i d s . These poor s u r v i v a l r a t e s c o i n c i d e d with a drop i n d e n s i t y on g r i d L ( F i g . 1.1b). During the f a l l and winter of 1973-74, when d e n s i t i e s were 24 Figure 1.2 Differences in minimum sur v i v a l rates per 14 days between the experimental grids and the c o n t r o l g r i d . Minimum s u r v i v a l rates on the control are standardized to a base l i n e of zero. A) males B) females Standard ized M in imum Surv iva l Rate 30 2 0 10 0 -10 •20 •30 30 20 10 0 -10 20 30 20 10-0 26 F E M A L E S T i I i 1 — i 1 — i 1 1 — i r G r i d Z T i i i i i — i — i 1 — i — i — i — r ^ n f M M J 5 N i J M 1 9 7 3 1974 27 c o n s i s t e n t l y lower on g r i d s L and X, male s u r v i v a l was almost always b e t t e r than t h a t on the c o n t r o l . From mid-winter to s p r i n g , s u r v i v a l o f animals on the c o n t r o l g r i d f e l l d r a s t i c a l l y and t h i s was a s s o c i a t e d with a sharp d e c l i n e i n d e n s i t i e s ( F i g . 1 . 1 ) . • S u r v i v a l of animals on g r i d s L and X a l s o f e l l , although i t c o n tinued to remain h i g h e r than t h a t on the c o n t r o l . However, d i f f e r e n t age groups were a f f e c t e d on g r i d X. Breeding began approximately one month e a r l i e r on g r i d X than on the c o n t r o l g r i d , so t h a t young c f the year were already e n t e r i n g the p o p u l a t i o n w e l l b e f o r e the end of the d e c l i n e p e r i o d . I t was these young t h a t were e x p e r i e n c i n g the poor s u r v i v a l , not the overwintered a d u l t s as on the c o n t r o l . For example, at the beginning of A p r i l , the 12 young on g r i d X had a minimum s u r v i v a l r a t e of .50 per 14 days compared with a r a t e of .87 f o r 15 a d u l t males on g r i d X and of .42 f o r 19 a d u l t males on the c o n t r o l g r i d . On g r i d L a d u l t s were r e s p o n s i b l e f o r t h e 10% drop i n s u r v i v a l r a t e . On g r i d Z i f was a l s o the a d u l t s t h a t were s u r v i v i n g poorly. In females ( F i g . 1.2b), the d i f f e r e n c e s i n minimum s u r v i v a l r a t a between g r i d s were l e s s marked. During the 1973 s p r i n g d e c l i n e , only 26 animals (14 males and 12 females) were taken from g r i d X at the f i r s t removal, l e a v i n g a moderate d e n s i t y on t h i s g r i d . S u r v i v a l from t h i s t r a p p i n g session to the next was not improved over that on the c o n t r o l g r i d . However, a f t e r the next t r a p p i n g , when a l l but 20 of the trappable animals were removed and only 9 females remained, not one female disappeared d u r i n g the next 1 1/2 months. T h i s suggests t h a t severe r e d u c t i o n d i d have a pronounced i n f l u e n c e on female s u r v i v a l . 28 Females on g r i d Z responded i n a s i m i l a r manner d a r i n g the s p r i n g d e c l i n e of 1974, u n l i k e the females i n t r o d u c e d to g r i d L i n 1973. T h i s may have been r e l a t e d to the f a c t that the l a t t e r females were not r e s i d e n t s on the area as were the females on g r i d s X and Z. During the p e r i o d from May t o September 1973, when d e n s i t i e s were s i m i l a r on a l l area's, s u r v i v a l of females was g e n e r a l l y good but v a r i e d s p o r a d i c a l l y . Two p e r i o d s of poor s u r v i v a l occurred - one on g r i d L i n l a t e June and e a r l y J u l y which was p a r a l l e l e d by lower s u r v i v a l on the c o n t r o l g r i d and g r i d X; and one i n August on g r i d X - which was a l s o p a r a l l e l e d by lower s u r v i v a l on the other two g r i d s . The cause f o r the f i r s t p e r i o d of poor s u r v i v a l i s unknown, but i n t h e second, p a r a s i t i s m by b o t f l i e s (Cutsrebra sp.) and f l e s h f l i e s ( W o l hf ahrtia v i g i l ) o ccurred. During the f a l l and winter of 1973-74, while d e n s i t i e s were kept lower on g r i d s 1 and X, females on these g r i d s g e n e r a l l y s u r v i v e d b e t t e r than those on the c o n t r o l g r i d , although not as c o n s i s t e n t l y w e l l as the males. A drop i n female s u r v i v a l t h a t occurred on g r i d X i n e a r l y A p r i l , d i d not occur on g r i d L, but i t corresponded to a s i m i l a r drop on the c o n t r o l . On both the c o n t r o l and g r i d X, t h i s poor s u r v i v a l was found i n a d u l t s , but may have been r e l a t e d to the e a r l i e r onset of breeding i n g r i d X females than i n e i t h e r of the female p o p u l a t i o n s on the c o n t r o l g r i d or g r i d L. There i s no r e l a t i o n s h i p between d e n s i t y and s u r v i v a l d u r i n g the s p r i n g d e c l i n e of 1974 i n e i t h e r males or females ( F i g . 1.3). During the p e r i o d i n c l u d e d i n the a n a l y s i s (February 29 t o m i d - A p r i l ) , males cn a l l areas were comparable i n breeding c o n d i t i o n . However, i n females, l a c t a t i n g animals were caught 1 1/2 months e a r l i e r on g r i d X than on the c o n t r o l g r i d . T h i s may have obscured any r e l a t i o n s h i p between d e n s i t y and s u r v i v a l there may have been i n females. There was a l s o no c o r r e l a t i o n between d e n s i t y and s u r v i v a l i n the s p r i n g of 1973 i n males on g r i d s I and X (r=.04, n=12), but i n females the c o r r e l a t i o n was good (r=-.64, n=12, P<.05). Data from g r i d L were not i n c l u d e d i n t h i s l a t t e r a n a l y s i s because the g r i d L p o p u l a t i o n was made up of intr o d u c e d animals, not r e s i d e n t s as on g r i d s I and X. I conclude t h a t f a c t o r s other than d e n s i t y are i n f l u e n c i n g male s u r v i v a l d u r i n g the s p r i n g d e c l i n e . In females a negative r e l a t i o n s h i p between d e n s i t y and s u r v i v a l i s suggested. In c o n c l u s i o n , cropping an unmanipulated p o p u l a t i o n while a d e c l i n e was already i n progress had no e f f e c t on male s u r v i v a l but d i d improve female s u r v i v a l . T r a n s f e r r i n g i n d i v i d u a l s from a d e c l i n i n g area to an evacuated h a b i t a t improved n e i t h e r male nor female s u r v i v a l . Producing low d e n s i t y o v e r w i n t e r i n g p o p u l a t i o n s p r i o r to a d e c l i n e r e s u l t e d i n improved s u r v i v a l of both males and females both i n the f a l l and winter. The higher s u r v i v a l r a t e s of animals on g r i d X than those on the c o n t r o l g r i d during the f a l l and winter of 1973-74 could be a s s o c i a t e d with d i f f e r e n c e s i n age s t r u c t u r e . In the f a l l , a l l a d u l t s had been removed from the g r i d X p o p u l a t i o n while they were s t i l l p resent i n the c o n t r o l p o p u l a t i o n . To see i f the d i f f e r e n c e s in. s u r v i v a l were r e a l , I grouped the s u r v i v o r s h i p data f o r a l l young caught on the c o n t r o l g r i d and on g r i d X during September and October 1973 at a common o r i g i n and a l i f e 30 are 1,3 Relationship between density and minimum s u r v i v a l rate par 14 days on four areas i n t h e spring decline of 1974. (control g r i d •; grid L 0; grid X • ; grid Z A ) (males r=.03, n=16; females r=-.18, n=16) M I N I M U M S U R V I V A L R A T E O-O O" rv) CO-CO o co-rn z 00 — I J \ -<o~ (Jl O" 0) O" o -0) o 00 o o o a > > a m o O O m > m o 0) o 00 o o o o • m > m TE 32 t a b l e c a l c u l a t i o n f o r e x p e c t a t i o n of f u r t h e r l i f e given by L e s l i e e t a l . (1955) was used. F i g u r e 1.4 shows that the c a l c u l a t i o n f o r e x p e c t a t i o n of l i f e of young males on g r i d X was 7 weeks longer than f o r young males on c o n t r o l g r i d I while i n females i t was 4 weeks longer. During the f i r s t 12 weeks, males on both areas s u r v i v e d w e l l (minimum s u r v i v a l per 2 weeks was .93 on the c o n t r o l and .92 on X), whereas i n the next 12 weeks males on X continued to s u r v i v e w e l l (.98) while those on the c o n t r o l s u r v i v e d poorly (.67). T h i s c o i n c i d e d with the onset of breeding on the c o n t r o l . Females on the two g r i d s showed the same t r e n d , although the d i f f e r e n c e s were l e s s marked (.93 per 2 weeks on the c o n t r o l and .92 on g r i d X i n the f i r s t 12 weeks; .81 on the c o n t r o l and .92 on g r i d X i n the next 16 weeks). S i m i l a r c a l c u l a t i o n s were not made f o r g r i d L because few young had r e c r u i t e d by October, though e x p e c t a t i o n s of l i f e c a l c u l a t e d f o r a l l animals a l i v e i n December on the c o n t r o l g r i d and g r i d L show s i m i l a r high e x p e c t a t i o n s of l i f e f o r animals on g r i d L as compared with t h a t on the c o n t r o l . In c o n c l u s i o n , the i n c r e a s e d s u r v i v a l on g r i d X compared with t h a t on the c o n t r o l during the f a l l and winter of 1973-74 was a r e a l phenomenon, not a consequence of i t s age s t r u c t u r e . Movements The low male s u r v i v a l r a t e observed on the experimental g r i d s a f t e r c r opping d u r i n g the d e c l i n e , may s t i l l be r e l a t e d to b e h a v i o r a l i n t e r a c t i o n s . I t i s p o s s i b l e that a f t e r the ma j o r i t y of males were removed from g r i d X i n 1973 and from g r i d Z i n 33 Figure 1.4 S u r v i v o r s h i p curves f o r animals f i r s t trapped as young i n September and October. The v e r t i c a l l i n e s on the curves represent e i t h e r removal or a c c i d e n t a l death. Ex i s the expectation of l i f e <± 1 SE) . 34 35 1974, the remaining males moved over a greater a r e a , thereby keeping the i n t e r a c t i o n r a t e constant. An index of movement i s the change i n p o s i t i o n of capture from one t r a p p i n g p e r i o d to the next ( f i r s t capture p o i n t i n one p e r i o d to the f i r s t capture p o i n t i n the n e x t ) . Only movements of a d u l t s on the c o n t r o l g r i d , g r i d X, and g r i d Z were ccmpared because the i r r e g u l a r shape of g r i d L c o n s t r a i n e d movement. Although movements of both sexes d i d i n c r e a s e on g r i d X i n the s p r i n g o f 1973 and on g r i d Z i n the s p r i n g of 1974 compared with those on the c o n t r o l g r i d (Table 1.5), the d i f f e r e n c e s were not s i g i f i c a n t . The only s i g n i f i c a n t d i f f e r e n c e s were i n the summer when females moved l e s s on g r i d X than on the c o n t r o l , and i n the f a l l and winter, when both males and females on g r i d X moved more. Thus i n these l a t t e r p e r i o d s , a decrease i n d e n s i t y i n c r e a s e d movement, but d i d not lower s u r v i v a l (Table 1.4). Movements with i n a s i n g l e t r a p p i n g peri o d were a l s o examined. An animal c o u l d , on f i r s t capture, be caught a t or near the same l o c a t i o n every t r a p p i n g p e r i o d but be moving about a g r e a t deal w i t h i n the t r a p p i n g s e s s i o n . Caution i s needed i n i n t e r p r e t a t i o n of these data because they may be g r e a t l y a f f e c t e d by p o p u l a t i o n d e n s i t y i n r e l a t i o n to t r a p number and t r a p response of i n d i v i d u a l s . S i m i l a r r e s u l t s to t h a t obtained above f o r movements between t r a p p i n g periods were o b t a i n e d , with both sexes moving about more on g r i d X during the f a l l and winter of 1972 than on the c o n t r o l . However, i n 1973-74, there were s i g n i f i c a n t d i f f e r e n c e s between the c o n t r o l g r i d and g r i d 2 i n both sexes. Males on the c o n t r o l moved 31.4 f t compared with 59.5 f e e t on g r i d Z, and females on the c o n t r o l moved 26.7 f e e t 36 Table 1.5,. aean d i s t a n c e between s u c c e s s i v e p o s i t i o n s of capture i n f e e t f o r a d u l t s , along with 95% c o n f i d e n c e l i m i t s . D i s t a n c e s were measured from f i r s t capture p o i n t of tr a p p i n g p e r i o d t to f i r s t capture p o i n t c f t r a p p i n g p e r i o d t+1. Males Females H 95% c . l . x M 95% c . l . Spring 1973 C o n t r o l 30.7 111 24.6-36.8 30.2 87 22.1-38.3 G r i d X 39. 8 38 26.0-53. 5 39.9 21 24. 2-55.5 Summer 1973 C o n t r o l 35.2 170 30.6-39.7 28.7 270 24.8-32.6 G r i d X 35. 1 199 30. 5-39.6 23. 9 233 19. 9-27.9 F a l l 197 3 C o n t r o l 20.5 279 18. 0-23.0 19.8 310 17. 4-22.1 G r i d X 34.6 114 28.1-41.1 31,7 64 23.5-39,9 winter 1974 C o n t r o l 30. 7 258 26.6-34.8 25.6 150 21.6-29.5 G r i d X 42.3 110 36.0-48.6 47.1 66 38.3-55.9 Spring 1974 C o n t r o l 47. 4 52 35. 1-59. 6 32. 9 64 27. 1-38.7 G r i d Z 68.9 12 43.8-94.0 43.7 17 29.4-58.1 37 compared with 51.6 f e e t . T h i s suggests that at l e a s t i n 1974 on g r i d Z, i n c r e a s e d movement was a s s o c i a t e d with a d r a s t i c r e d u c t i o n i n p o p u l a t i o n s i z e . D i s c u s s i o n In t h i s study d r a s t i c r e d u c t i o n i n numbers of an a l r e a d y d e c l i n i n g p o p u l a t i o n of fl. townsendii d i d not improve the s u r v i v a l of males, but d i d improve that of females. These r e s u l t s suggest t h a t at l e a s t i n males, the h o s t i l i t y p o s t u l a t e d by C h i t t y (1967) may not be o c c u r r i n g . Mere the c o n d i t i o n s of these experiments s i m i l a r t o those f o r which I had made the p r e d i c t i o n of i n c r e a s e d s u r v i v a l of an a r t i f i c a l l y reduced p o p u l a t i o n i n a d e c l i n e ? Were the two d e c l i n e p e r i o d s observed ( s p r i n g 1973 and s p r i n g 1974) on the c o n t r o l g r i d s i m i l a r to ' t y p i c a l * m i c r o t i n e d e c l i n e s and, i f so, were the causes of these d e c l i n e s the same? The l i t e r a t u r e i n d i c a t e s t h a t the d e c l i n e phase i s very v a r i a b l e . C h i t t y (195 5) recognized three types of d e c l i n e : i n type M d e c l i n e s , the p o p u l a t i o n f a i l s t o overwinter, dropping i n numbers a f t e r a peak summer; i n type G d e c l i n e s , the p o p u l a t i o n o v e r w i n t e r s , but s t e a d i l y decreases thoughout the breeding season; and i n type H d e c l i n e s , the p o p u l a t i o n d e c l i n e s moderately over the winter a f t e r a peak phase, r e c o v e r s d u r i n g the f o l l o w i n g breeding season, and reachs i t s g r e a t e s t s c a r c i t y the f o l l o w i n g year. Krebs and Myers (1974) , i n t h e i r review of p o p u l a t i o n c y c l e s , found types H and G about e q u a l l y common i n the l i t e r a t u r e , and type M d e c l i n e s more r a r e . The fundamental 38 assumption by C h i t t y (1967) was t h a t the c a u s a l mechanism f o r these d e c l i n e s were the same. The two d e c l i n e p e r i o d s I observed suggested a type H d e c l i n e . LeDuc and Krebs (1975) re p o r t e d that the g r i d I c o n t r o l p o p u l a t i o n appeared to be going through a more or l e s s t y p i c a l m i c r o t i n e c y c l e , with the p o p u l a t i o n i n c r e a s i n g r a p i d l y i n the f a l l of 1971, winter breeding o c c u r r i n g d u r i n g t h e winter of 1971-72, high numbers being maintained i n the summer of 1972 together with h e a v i e r body weights, and a s p r i n g d e c l i n e i n 1973. The p o p u l a t i o n then recovered r a p i d l y and d e c l i n e d again i n s p r i n g 1974. However, u n l i k e the type H d e c l i n e , where lower d e n s i t y i s reached i n the second year than i n tae f i r s t , numbers f e l l no lower than i n 1973. T h i s suggests that there may be some demographic d i f f e r e n c e s between t h i s s p e c i e s and those r e p o r t e d by Krebs and Myers (1974). In f a c t , i n one of the townsendii p o p u l a t i o n s t h a t has been s t u d i e d f o r f i v e years (Krebs et a l . 1976 and Krebs pers. comm.) no major d e c l i n e s were e v i d e n t . The two d e c l i n e s observed i n t h i s study had r a t e s of l o s s that were as high as or higher than those i n most of the d e c l i n e s r e p o r t e d by Krebs and Myers (1974). These data, together with those c o l l e c t e d by LeDuc and Krebs (1975) suggest that, these d e c l i n e s are a l o c a l m o d i f i c a t i o n of the phenomenon we want t o e x p l a i n . In many s p e c i e s of m i c r o t i n e s , s p r i n g d e c l i n e s t h a t are s i m i l a r to the t y p i c a l c y c l i c d e c l i n e occur, though they are of a r e l a t i v e l y s h o r t d u r a t i o n . These s p r i n g d e c l i n e s o f t e n precede a peak year (Thompson 1955a, C h i t t y and C h i t t y 1952, Krebs 1964, Krebs et a l . 1969, Gaines and Krebs 1971); though i n M. townsendii s i m i l a r d e c l i n e s do hot appear to be r e l a t e d to 39 the peak (Krebs et a l . 1976). Spring d e c l i n e s are found i n other rodents ( S a d l e i r 1965, Flowardew 1974, F a i r b a i r n 1976). These d e c l i n e s are thought to be a s s o c i a t e d with an i n c r e a s e i n ag g r e s s i v e behavior due to the onset of breeding a c t i v i t y . The t y p i c a l m i c r o t i n e d e c l i n e may j u s t be a more severe v e r s i o n of the s p r i n g d e c l i n e . I assume t h a t t h i s i s the case, and t h a t the d e c l i n e s I observed should be s u i t a b l e p e r i o d s f o r t e s t i n g a h y p o t h e s i s about behavior. We may next c o n s i d e r i f the po p u l a t i o n s processes going on d u r i n g the two d e c l i n e s were s i m i l a r , so t h a t the g r i d X experiment i n the s p r i n g of 1973 and the g r i d Z experiment i n the s p r i n g of 19 74 were r e p l i c a t e s . There i s some evidence to suggest t h a t these p o p u l a t i o n s may have been d i f f e r e n t i n the two years. F i r s t , the r a t e s of d e c l i n e i n the two years were d i f f e r e n t . Second, the a l l e l i c frequency of LAP-F was d i f f e r e n t d u r i n g the two d e c l i n e s . In the study by LeDuc and Krebs (1975), the LAP-F freguency on the c o n t r o l g r i d f l u c t u a t e d around 30-40%, with the freguency i n the d e c l i n e of 1973 f a l l i n g from about 39% at the s t a r t to 27% a t the end. In the d e c l i n e of 1974, the freguency i n c r e a s e d from about 36% at the s t a r t to 58% at the end (Krebs pers. comm.). I f s e l e c t i o n p r e s s u r e was r e s p o n s i b l e f o r these changes, i t appeared to be o p e r a t i n g i n o p p o s i t e d i r e c t i o n s i n the two d e c l i n e s . However, LeDuc and Krebs (1975) were unable t o f i n d any c a u s a l l i n k between the g e n e t i c and demographic changes. Voles i n d e c l i n i n g p o p u l a t i o n s do not seem to be e x p e r i e n c i n g any s e r i o u s p a t h o l o g i c a l c o n d i t i o n . Shen members of d e c l i n i n g p o p u l a t i o n s cf M. • a g r e s t i s and W. c a l i f o r n i c u s were 40 brought i n t o the l a b , s u r v i v a l was normal (Newson and C h i t t y 1962, Krebs 1966). T h i s i s what suggested t o Newson and C h i t t y (1962) that the poor s u r v i v a l i n d e c l i n e s was a s s o c i a t e d with b e h a v i o r a l i n t e r a c t i o n s . However, s t u d i e s by C h i t t y and Phipps (1966), Myers and Krebs (1971) and Krebs e t a l . (1976) have found t h a t d e c l i n e p e r i o d s are not a s s o c i a t e d with h i g h r a t e s of d i s p e r s a l i n t o vacant h a b i t a t s . In the present study, no i n d i v i d u a l s were known to d i s p e r s e d u r i n g the 1973 d e c l i n e . G r i d G, a g r i d separated from g r i d L by only a 3 m road, experienced the same d e c l i n e as the co.nt.rcl (LeDuc and Krebs 1975) y e t no animals were caught moving from g r i d G t o L, a low d e n s i t y area. In the 1974 d e c l i n e , only 3 animals out of 359 animals d i s a p p e a r i n g from a l l g r i d s were known to have d i s p e r s e d from one g r i d to another. T h e r e f o r e , b e h a v i o r a l i n t e r a c t i o n s , i f they are o c c u r r i n g , must be severe enough to cause i n s i t u m o r t a l i t y . In o n l y one of the s t u d i e s r e p o r t e d by Krebs and Myers (1974) was wounding judged to be severe enough to cause death. None of the subsequent s t u d i e s which have looked at wounding have i n d i c a t e d t h a t wounding was ever mortal ( L i d i c k e r 1973, Rose and Gaines 1976 and Rose pers. comm.). We are l e f t with only two p o s s i b i l i t i e s . E i t h e r the animals are making long-range d i s p e r s a l movements such t h a t they are not a t t r a c t e d to vacant h a b i t a t nearby, or some f a c t o r which may be r e l a t e d t o a type of behavior not r e s u l t i n g i n i n t e n s i v e wounding i s causing i n s i t u death. H i l b o r n and Krebs (1976), using a r a d i o a c t i v e tagging technique, found a l a r g e amount of i n s i t u death i n one d e c l i n e and l i t t l e i n another, suggesting t h a t long-range d i s p e r s a l was a p o s s i b i l i t y . These r e s u l t s are ambiguous and thus the f a t e of 41 animals during a d e c l i n e remains unexplained. During the s p r i n g of 1973, females introduced onto q r i d L s u r v i v e d at r a t e s s i m i l a r to those on the c o n t r o l , whereas those on g r i d X s u r v i v e d much b e t t e r , as d i d those on g r i d Z i n the f o l l o w i n g year (Table 1.2 and F i g . 1.2b). However, i n the l a t t e r two cases these females ware r e s i d e n t on the areas on which they had overwintered, whereas i n the f i r s t case t h i s was not so. T h i s may account f o r t h e i r poor s u r v i v a l . This suggests t h a t f a m i l i a r i t y with a s i t e may co n f e r s u r v i v a l advantages to females at low d e n s i t i e s . I was a l s o not completely s u c c e s s f u l i n c r e a t i n g a vacant h a b i t a t on g r i d L before i n t r o d u c t i o n , f o r 18 untagged animals were caught one week a f t e r the i n t r o d u c t i o n . T h e r e f o r e , g r i d L was not a r e p l i c a t e of g r i d s X and Z. The d i f f e r e n c e s i n s u r v i v a l between the two sexes i n both years on g r i d s X and Z s t i l l remains p u z z l i n g . Because I was working with r e l a t i v e l y s m a l l numbers of animals i n both years, an e f f i c i e n t predator such as a f e r a l c a t might have caught enough of these i n d i v i d u a l s t o have s e r i o u s l y reduced the s u r v i v a l r a t e . However, the f a c t t h a t the poor s u r v i v a l was s e x - s p e c i f i c argues a g a i n s t t h i s l i n e of re a s o n i n g . Perhaps t h e r e was some p h y s i o l o g i c a l d e t e r i o r a t i o n i n males t h a t d i d not occur i n females and t h e r e f o r e predisposed the males to d i e r e g a r d l e s s of what happened to the p o p u l a t i o n i n which they were l i v i n g . I t i s hard to conceive of a p h y s i o l o g i c a l s t r e s s that would continue t o a f f e c t males i n reduced p o p u l a t i o n s but not females, whereas i n unmanipulated p o p u l a t i o n s , both sexes do p o o r l y . The s o c i a l o r g a n i z a t i o n of m i c r o t i n e rodents i s poorly 42 understood. There i s l i t t l e evidence f o r the type of t e r r i t o r i a l behavior seen i n b i r d s (Watson and Jenkins 1968, Krebs 1971, J e n k i n s et a l . 1975). l o attempt to e x p l a i n the r e s u l t s obtained i n the present study, I propose 'the f o l l o w i n g g e n e r a l model of vole behavior. A d u l t males and a d u l t females compete f o r d i f f e r e n t o b j e c t s . During the breeding season, females are p r i m a r i l y concerned with s e c u r i n g nest s i t e s and an area i n which t o r a i s e young. T h e i r main competitors are t h e r e f o r e other females. Females may a l s o be aggressive to males and young during the p e r i o d they are pregnant or l a c t a t i n g {Rowley and C h r i s t i a n 1976). Males, on the other hand, are p r i m a r i l y i n t e r e s t e d i n o b t a i n i n g females with which to mate. T h e i r main competitors are t h e r e f o r e other males i n breading c o n d i t i o n . They may be f o r c e d to l o c a l i z e t h e i r wandering t o a given area on which there are females because of the a g g r e s s i v e behavior of adjacent males. However, i f no females occur on t h e i r home range, they w i l l seek other females, thereby c o n t a c t i n g other males and g e t t i n g i n v o l v e d i n a g g r e s s i v e i n t e r a c t i o n s . The h i g h e r the d e n s i t y on an a r e a , the more f r e g u e n t these i n t e r a c t i o n s may become. The conseguences of these i n t e r a c t i o n s i s dependent not so much on d e n s i t y as on the phase of the c y c l e or perhaps i n our l o c a l s i t u a t i o n , the time of the year. During the d e c l i n e , when d e n s i t y i s r e l a t i v e l y low, these i n t e r a c t i o n s r e s u l t i n disappearance and presumably death, whereas i n the i n c r e a s e and peak phases disappearance- may more commonly be a s s o c i a t e d with d i s p e r s a l (Myers and Krebs 1971). During the non-breeding season, s u r v i v a l should be g e n e r a l l y h i g h f o r both sexes. Most of the m o r t a l i t y o c c u r r i n g at that time s h o u l d . 43 be due to such agents as predation, disease, and environmental stresses, so that both sexes would be affected i n a similar fashion and s u r v i v a l in good habitats should be high. If females are competing through s o c i a l i n t e r a c t i o n s for nest s i t e s during the breeding season, removal of the majority of adjacent females should r e s u l t i n increased s u r v i v a l ; t h i s did happen i n t h i s study. Removing males from unmanipulated populations at the onset of breeding had no e f f e c t on s u r v i v a l , whereas keeping numbers down throughout the winter did improve su r v i v a l (Fig. 1.5). I t i s l i k e l y that the overwintering low density populations had set up some sort of r e l a t i v e l y stable s o c i a l structure so that the onset of the breeding season did not r e s u l t i n serious mortality or extensive movements. On the other hand, removing animals from an unmanipuiated population might have resulted i n a serious disruption of s o c i a l structure, so that some males might be l e f t on home ranges without females and have to move either from the grid or within i t to look for them. Presumably males from both the low density overwintering populations and from populations cropped during the decline engaged i n aggressive interactions at the onset of breeding. However, the fact that only males from the l a t t e r populations suffered high loss rates suggest that either t h e i r i n t e r a c t i o n s were more severe, or their s u s c e p t i b i l i t y to these i n t e r a c t i o n s were d i f f e r e n t . Krebs et a l . (1973) made the prediction that the prevention of dispersal after a vole population had reached peak densities would r e s u l t i n a normal decline. This was based on the assumption that dispersal and selection during the increase aa F i g u r e 1.5. Schematic diagram of the changes i n s u r v i v a l r e s u l t i n g from r e d u c t i o n of a M. townsendii p o p u l a t i o n before and dur i n g a d e c l i n e . 45 cropp ing ! i i y t i. r p o o r male surv iva l I improved female surv iva l improved male surv iva l improved female su rv i va l F A L L W I N T E R S P R I N G 46 were s u f f i c i e n t to ensure a d e c l i n e 1 or 2 years l a t e r . I f d i s p e r s a l i s not important i n the d e c l i n e , my manipulation should not have i n f l u e n c e d the course of events. In males the d e c l i n e does appear to be i n e v i t a b l e i f the p o p u l a t i o n i s cropped too l a t e , but not i f the p o p u l a t i o n i s cropped before the d e c l i n e . There are a t l e a s t two p o s s i b l e e x p l a n a t i o n s f o r these r e s u l t s . 1. S e l e c t i o n i s necessary f o r a d e c l i n e to occur i n the manner t h a t C h i t t y (1967) proposed and e i t h e r the p o p u l a t i o n s on g r i d X and the c o n t r o l i n e a r l y f a l l 1973 were g e n e t i c a l l y d i f f e r e n t because of the p e r t u r b a t i o n on g r i d X i n the s p r i n g of 1973, or the two p o p u l a t i o n s were g e n e t i c a l l y the same i n e a r l y f a l l but the c r i t i c a l s e l e c t i o n o c c u r r e d l a t e r i n the year. 2. s e l e c t i o n i s not necessary f o r a d e c l i n e i n !• townsendii and the reason the males s u r v i v e d p o o r l y when cr o p p i n g occurred i n the d e c l i n e was because of some unknown f a c t o r a s s o c i a t e d with exposure to high d e n s i t y d u r i n g the p r e c e d i n g winter. However, high d e n s i t y i s not a p r e r e g u i s i t e f o r a d e c l i n e (see S e c t i o n 2). Movements of a d u l t males have been found to be l a r g e r than those of females only d u r i n g the breeding season (Krebs 1966) and home ranges of males are c o n s i s t e n t l y l a r g e r (Getz 196Tb, Ambrose 1969, Van Vleck 1969, Brooks and Banks 1971). Through the use of r a d i o - t r a c k i n g , Brooks and Banks (1971) were ab l e to f o l l o w movements of breeding c o l l a r e d lemmings. They found that a d u l t males had home ranges averaging 2.03 ha compared with only 0.16 ha i n females. The males moved about t h e i r home ranges c o n t i n u a l l y , v i s i t i n g each female w i t h i n i t every 1 or 2 days. S i m i l a r high r a t e s of a c t i v i t y were found i n 47 male brown lemmings (Eanks e t a l . 1975), and they p o s t u l a t e t h a t the reason f o r the movement of males i s to ensure t h a t they are present when the females are i n e s t r u s , thus i n c r e a s i n g t h e i r mating o p p o r t u n i t i e s . T h i s may indeed be a g e n e r a l phenomenon i n a l l m i c r o t i n e s , with males seeking females and i n the process encountering other males. Few s t u d i e s have examined behavior i n m i c r o t i n e s , and none has done so i n the f i e l d . Turner and Iverson (1973) found t h a t overwintered a d u l t males were more ag g r e s s i v e , had l a r g e r home ranges, and s u r v i v e d longer than young of the year. T h i s suggests t h a t becoming s e x u a l l y mature i n an e s t a b l i s h e d p o p u l a t i o n may be extremely hazardous u n t i l the overwintered animals have d i e d or been f o r c e d out. At high d e n s i t i e s , movements and home range of both sexes are l e s s (Getz 1961b, Krebs 1966), which i n d i c a t e t h a t r e s t r i c t e d movements may be o c c u r r i n g . Krebs (1970) found t h a t a g g r e s s i v e behavior was higher i n peak p o p u l a t i o n s than i n i n c r e a s i n g p o p u l a t i o n s , which suggests why movements are r e s t r i c t e d . There are s t i l l a number of problems with t h i s model. There i s no evidence f o r d i s p e r s a l during d e c l i n e p e r i o d s such as I suggest may occur. Therefore s o c i a l i n t o l e r a n c e s i n the d e c l i n e must e i t h e r l e a d t o m o r t a l i t y without movement or movement i s of a long-ranga nature. A second problem i s t o e x p l a i n severe d e c l i n e s from which there i s l i t t l e or no immediate recovery. Lagerspetz (1969) found that i n house mice a g g r e s s i o n towards another male s i g n i f i c a n t l y decreases the sexual behavior of an i n e x p e r i e n c e d male mouse towards a female i n heat. T h i s decrease i n sexual response was p a r t i a l l y due to a g g r e s s i v e behavior 48 towards the female. I f there i s a change i n l e v e l s of a g g r e s s i o n over the c y c l e r e s u l t i n g from e i t h e r a phenotypic or genotypic change i n the animals, perhaps the animals become so a g g r e s s i v e as to be unable t o mate s u c c e s s f u l l y , or i f mating i s s u c c e s s f u l , the young are destroyed. Aggressive behavior i n females, which has r e c e i v e d very l i t t l e a t t e n t i o n (Conley 1976) , may be c r i t i c a l f o r s u r v i v a l of the young. The present study suggests s e v e r a l areas that should be e x p l o r e d f u r t h e r . A g r e a t e r understanding of s o c i a l s t r u c t u r e and of the i n t e r a c t i o n s t hat go on i n a m i c r o t i n e p o p u l a t i o n i s needed. D i r e c t o b s e r v a t i o n of s m a l l enclosed p o p u l a t i o n s should be c a r r i e d out to observe i n t e r a c t i o n s among a d u l t males, a d u l t females, and young, and how these change as a f u n c t i o n of r e p r o d u c t i v e c o n d i t i o n and phase of the c y c l e . I n a f i e l d p o p u l a t i o n , a combination of i n t e n s i v e l i v e - t r a p p i n g and of monitoring a f a i r l y l a r g e number o f i n d i v i d u a l s by r a d i o - t r a c k i n g would allow one to form a much more dynamic p i c t u r e of the i n t e r a c t i o n s between i n d i v i d u a l s , the cause of these i n t e r a c t i o n s , and t h e i r r e s u l t s . The use of an e l e c t r i c a l t r a p p i n g system s i m i l a r to that designed by H a r l i n g (1971) would permit d e t e c t i o n of an animal as soon as i t was c a p t u r e d and thus i t need not be removed from the p o p u l a t i o n f o r as great a l e n g t h of time as i s now the case. R a d i o - t r a c k i n g of s m a l l mammals i s c u r r e n t l y r e c e i v i n g i n c r e a s i n g a t t e n t i o n (Brooks and Banks 1971, Chute et a l . 1974, Banks e t a l . 1975) and appears to be a f e a s i b l e method f o r s t u d y i n g m i c r o t i n e s i n the f i e l d . A p o p u l a t i o n expected to experience a d e c l i n e should be enclosed to see i f d i s p e r s a l i s important at t h i s time (see S e c t i o n 2). 49 F i n a l l y , to t e s t the i d e a that a d u l t breeding males are more i n t e r e s t e d i n o b t a i n i n g females t o mate with than on remaining on a home range, one could remove most of the females from an a c t i v e l y breeding p o p u l a t i o n . The p r e d i c t i o n i s that the number of a d u l t males remaining cn the g r i d would drop almost at once. An i n c r e a s e i n i n t e r a c t i o n between the remaining males might a l s o occur so t h a t wounding r a t e s might i n c r e a s e . Summary 1. To t e s t part of C h i t t y 1 s hypothesis (1967) that b e h a v i o r a l i n t e r a c t i o n s are r e s p o n s i b l e f o r d e c l i n e p e r i o d s , I p r e d i c t e d t h a t a d r a s t i c r e d u c t i o n i n p o p u l a t i o n s i z e d u r i n g a d e c l i n e should i n c r e a s e s u r v i v a l . 2. One c o n t r o l g r i d was monitored throughout t h i s study and the p o p u l a t i o n s on i t experienced severe s p r i n g d e c l i n e s i n 1973 and 1974. 3. Three experimental areas were s e t up; two i n e a r l y 1973 and one i n e a r l y 1974. The production of low d e n s i t i e s on two of the e x p e r i m e n t al areas dur i n g the s p r i n g d e c l i n e had no e f f e c t on male s u r v i v a l but d i d improve female s u r v i v a l . The t r a n s f e r o f i n d i v i d u a l s from a d e c l i n i n g p o p u l a t i o n to an evacuated area improved n e i t h e r male nor female s u r v i v a l . The p r o d u c t i o n of low d e n s i t y o v e r w i n t e r i n g p o p u l a t i o n s p r i o r to a d e c l i n e improved both male and female s u r v i v a l compared with t h a t on the c o n t r o l g r i d . 4. There was no r e l a t i o n s h i p between d e n s i t y and s u r v i v a l i n males i n d e c l i n e p e r i o d s ; i n females, there was a negative 50 rel a t i o n s h i p between density and survival i n one decline, although in the second decline t h i s relationship was not s i g n i f i c a n t . 5. Expectation of l i f e for animals caught as young in the f a l l prior to a spring decline was higher on a grid maintained at low density than on the control g r i d . 6. During the declines, movements of both males and females on the low density areas increased, but only on the g r i d reduced i n the f a l l was t h i s s i g n i f i c a n t . 7 . A behavioral model i s proposed in which females are believed to compete for nest s i t e s while males compete for females. 51 Appendix 1 Minimum s u r v i v a l r a t e per 14 days ou the c o n t r o l g r i d and t h r e e experimental g r i d s i n 1973-74. A h o r i z o n t a l l i n e i s drawn at a s u r v i v a l r a t e o f .707, below which h a l f the p o p u l a t i o n disappaears every 4 weeks ( c o n t r o l g r i d • t ; g r i d 1 • ; g r i d X 0 ; g r i d Z A )• Non-breeding p e r i o d s on the c o n t r o l are shaded. A) Males B) Females 52 31V& nVAIAdf lS lAlniAjIN I (AJ 53 q oo cp ^ 31VcJ IVAIAdns lAjniAllNI lAl 5a SECTION 2. DISPEESAL IN DECLINE PERIODS I n t r o d u c t i o n D i s p e r s a l i s an important demographic parameter i n rodent p o p u l a t i o n s (Howard 1960, Anderson 1970, Krebs and Myers 1974, L i d i c k e r 1975), and may be c r i t i c a l to m i c r o t i n e c y c l e s . D i s p e r s a l reduces d e n s i t i e s below the l e v e l set by the food supply (Krebs e t a l . 1969) and changes the g u a l i t y of the r e s i d e n t s , because the d i s p e r s e r s ' can d i f f e r from the r e s i d e n t s both b e h a v i o r a l l y and g e n e t i c a l l y (Myers and Krebs 1971). The amount of d i s p e r s a l appears to b e ' d i r e c t l y p r o p o r t i o n a l to the r a t e of p o p u l a t i o n growth (Myers and Krebs 1971; Krebs et a l . 1976), with animals l e a v i n g the p o p u l a t i o n during i n c r e a s e and peak p e r i o d s . However, d i s p e r s a l has not been i m p l i c a t e d as a cause f o r d e c l i n e periods ( C h i t t y and Phipps 1966, Myers and Krebs 1971, H i l b o r n and Krebs 1976). The v o l e s i n the d e c l i n e must, t h e r e f o r e , be e i t h e r dying i n s i t u or d i s p e r s i n g without s e t t l i n g i n nearby vacant h a b i t a t . T h i s study was designed to t e s t the h y p o t h e s i s t h a t d i s p e r s a l i s not necessary f o r p o p u l a t i o n d e c l i n e s i n Migrotus townsendii. S p r i n g d e c l i n e p e r i o d s have been observed i n a number of s m a l l mammal p o p u l a t i o n s (Krebs 1964, S a d l e i r 1965, C h i t t y and Phipps 1966, Krebs e t a l . 1969, F a i r b a i r n 1976) and a r e thought to be a s s o c i a t e d with s o c i a l s t r i f e at the beginning of the breeding season. The t y p i c a l c y c l i c d e c l i n e p e r i o d may be j u s t an accentuated s p r i n g d e c l i n e . S p r i n g d e c l i n e s have been found 55 i n other p o p u l a t i o n s of M. tgwnsendii (Krebs et a l . 1976). In t h i s study, a p o p u l a t i o n t h a t was expected to e x p e r i e n c e a s p r i n g d e c l i n e i n 197 5 was e n c l o s e d and i t s demographic h i s t o r y f o l l o w e d . I f d i s p e r s a l i s not a necessary component of a d e c l i n e , e n c l o s i n g a p o p u l a t i o n a b o u t t o d e c l i n e s h o u l d not i n f l u e n c e i t s course. I f the d e c l i n e c o n t i n u e s , i n s i t u m o r t a l i t y must be o c c u r r i n g . I f the c o n t r o l p o p u l a t i o n d e c l i n e s but the enclosed p o p u l a t i o n doss not, d i s p e r s a l must s t i l l play a r o l e . Methods T h i s study was c a r r i e d out on iestham I s l a n d i n the d e l t a of the F r a s e r River near Vancouver, B r i t i s h Columbia. The study area i s a p a s t u r e l a n d used by the Department of N a t i o n a l Defence f o r communication towers. The area has had a grass cover s i n c e 1946, and has not been grazed s i n c e 1963. I t i s g e n e r a l l y f l a t , with some low l y i n g areas which are f l o o d e d f o r p e r i o d s i n the winter. The l i v e - t r a p p i n g g r i d used as the c o n t r o l was G r i d I, which served the same purpose i n the experiment of LeDuc and Krebs (1975). T h i s g r i d has been trapped c o n t i n u o u s l y s i n c e J u l y 1971. I set up an e n c l o s u r e on January 16, 1975 about 60 m from g r i d I and trapped i t u n t i l November 12, 1975. I used 6.3 mm (1/4 in) mesh hardware c l o t h extending at l e a s t 0.3 m i n t o the ground and 0.6 m above the ground, and e n c l o s i n g an area of 0.30 h e c t a r e . The e n c l o s u r e was bordered on one side by a removal g r i d , on a second s i d e by a grass s t r i p and a w a t e r - f i l l e d 56 d i t c h , on the t h i r d s i d e by a p a r t i a l removal g r i d , and on a f o u r t h s i d e by a grass and willow s t r i p . One of the removal g r i d s was trapped every second week; the other was trapped i n a s i m i l a r f a s h i o n i n the f i r s t p a r t o f the summer, but with a six-week t r a p p i n g i n t e r v a l i n the l a t t e r p a r t of the summer and f a l l . Because of t h i s arrangement, I b e l i e v e I was a b l e to pi c k up most of the animals escaping from the enclosure. T h i s g r i d was ap p a r e n t l y escape-proof u n t i l June 11, when the f i r s t tagged animal was captured on a removal g r i d . The voles then s t a r t e d t u n n e l i n g underneath the f e n c e , s i n c e the' water t a b l e had dropped enough t o allow deep burrowing. I t r i e d to e l i m i n a t e as much of t h i s as I c o u l d by tamping g r a v e l along the fence edges, but was not completely s u c c e s s f u l . No v o l e s were known to have climbed over the top of the fe n c e . A f t e r June 11, 41 animals are known to have escaped from the encl o s u r e out of the 629 animals caught i n s i d e the area. The c o n t r o l g r i d was covered by a checkerboard of t r a p p o i n t s spaced 7.6 m (25 f t ) a p a r t , i n a 10 x 10 p a t t e r n . The en c l o s u r e was approximately h a l f the s i z e of the c o n t r o l , with t r a p s arranged i n a 7 x 7 p a t t e r n . Because of the cost i n b u i l d i n g the e n c l o s u r e and the l i m i t e d area a v a i l a b l e to put a g r i d of t h i s nature, I decided t h a t a g r i d h a l f the s i z e of a normal g r i d would be s u i t a b l e . Throughout t h i s study the c o n t r o l was trapped with 150 Longworth l i v e - t r a p s , with two t r a p s at every other t r a p p o i n t . The e n c l o s u r e was trapped with 74 t r a p s i n a s i m i l a r manner t i l l the end of June, when I added 24 more t r a p s , so t h a t each t r a p p o i n t had two tr a p s . The t r a p number was i n c r e a s e d at t h i s time because I f e l t the d e n s i t y was higher 57 in the enclosure than on the control, and I t r i e d as much as possible to provide an excess of traps. Traps were baited with oats; cotton s t u f f i n g was provided. The traps were set every second week on Monday afternoon, checked luesday morning, Tuesday afternoon, and Wednesday morning, when they were locked open and l e f t in place. During most of the summer, high temperatures r e s t r i c t e d trapping to nights only. A small number of p i t f a l l traps, similar to those described in section 5, were put out i n the enclosure to provide an index of how many animals were not being captured by the Longworth l i v e - t r a p s . On June 21, I put out 8 p i t f a l l s , one at each trap point near one corner of the g r i d , and two weeks l a t e r , added 7 more. These were set at the same time as the l i v e - t r a p s . A l l voles were ear-tagged, and the trap l o c a t i o n , sex, sexual condition, and weight of each animal was recorded. Animals escaping from the enclosure were not replaced . In t h i s paper animals are c l a s s i f i e d as adult (> 43 g ) , subadult (30 g to 42 g) or juvenile {< 30 g). Results 2ll££lbilit.y Since i t i s not possible to sample Microtus populations randomly (Krebs 1966), I resorted to the complete enumeration of the trappable population by intensive l i v e - t r a p p i n g . This technique has been also been used i n other studies of small 58 mammal p o p u l a t i o n s (Newson and C h i t t y 1963; C h i t t y and Phipps 1966; Krebs 1966 and subseguent papers). I t assumes t h a t most of the t r a p p a b l e p o p u l a t i o n i s caught each s e s s i o n . The measure of t r a p p a b i l i t y used i s the f o l l o w i n g f o r a l l N i n d i v i d u a l s : where N i s the number of voles known to be present f o r more than two c o n s e c u t i v e t r a p p i n g p e r i o d s . In the number of a c t u a l and p o s s i b l e c a p t u r e s , the f i r s t and l a s t times of capture are excluded from the c a l c u l a t i o n , s i n c e an animal i s n e c e s s a r i l y caught these times. The data could be separated i n t o three d i s t i n c t breeding p e r i o d s . The f i r s t p e r i o d c o n s i s t e d of the main breeding season, which l a s t e d from the beginning of the study to the and of June. The the second p e r i o d , i n which l i t t l e or no breeding o c c u r r e d , l a s t e d from the beginning of J u l y t o mid-September. The l a s t p e r i o d , i n which seme voles resumed breeding a c t i v i t y , l a s t e d from mid-September to mid-November. Table 2.1 shows t h a t t r a p p a b i l i t y of animals was moderately low f o r most of the study. In the s p r i n g period t r a p p a b i l i t y was r e l a t i v e l y high on both g r i d s , except i n females on the c o n t r o l . In the summer, t r a p p a b i l i t y dropped d r a s t i c a l l y on both g r i d s , but more so on the c o n t r o l . In the f a l l , the animals i n the e n c l o s u r e became more t r a p p a b l e , while only the males on the c o n t r o l showed a s l i g h t i n c r e a s e . On the whole, the animals i n the e n c l o s u r e were more tr a p p a b l e than those on the c o n t r o l . In N number of a c t u a l captures f o r a given animal i = i Nnumber of p o s s i b l e c a p t u r e s f o r that animal T r a p p a b i l i t y = 59 Table 2.1. .Percent t r a p p a b i l i t y estimated f o r M. t o w n s e n d i i on a c o n t r o l and a fenced area. Sample s i z e s i n parentheses. P e r i o d Males C o n t r o l Enclosure Females C o n t r o l E n c l o s u r e February - June 74 (220) 73 (106) 62 (280) 73 (129) J u l y - 50 Sept. 15 (217) 61 (110) 51 (263) 58 (146) Sept. 29 6 2 - November (135) 89 (49) 52 (135) 75 (65) 60 p a r t , t h i s may have been due to the greater numbers of mice per a v a i l a b l e t r a p on the c o n t r o l than i n the e n c l o s u r e , even though d e n s i t i e s may not have been g r e a t l y d i f f e r e n t . In the e n c l o s u r e , t h e r e was no edge a f f e c t because the t r a p p o i n t s occurred w i t h i n 3 m of the f e n c e , whereas on the c o n t r o l the outer edges of the g r i d were contiguous with g r a s s l a n d c o n t a i n i n g voles whose home ranges may have overlapped part of the t r a p p i n g area. The g r e a t e r t r a p p a b i l i t y of the volas i n the e n c l o s u r e i n the l a s t two p e r i o d s could a l s o be r a l a t e d to the e x t r a t r a p s put out on t h i s g r i d at the end of June. For the two g r i d s , I compared the t r a p p a b i l i t y of v o l e s i n the two t r a p p i n g s e s s i o n s p r i o r t o the i n t r o d u c t i o n of the e x t r a t r a p s onto the e n c l o s u r e with the t r a p p a b i l i t y of v o l e s i n the two t r a p p i n g s e s s i o n s a f t e r the e x t r a t r a p s were put out. No c o n s i s t e n t d i f f e r e n c e was found. T r a p p a b i l i t y f o r both males and females on the c o n t r o l dropped s i g h t l y d u r i n g the p e r i o d when e x t r a t r a p s were put out (1-4%). T r a p p a b i l i t y a l s o dropped 4% i n males i n the e n c l o s u r e , but remained constant f o r females. Throughout these f o u r t r a p p i n g s e s s i o n s t r a p p a b i l i t y remained 3-1% h i g h e r i n the e n c l o s u r e . I conclude that the a d d i t i o n of the e x t r a 25 t r a p s had no s i g n i f i c a n t e f f e c t on i n c r e a s i n g t r a p p a b i l i t y i n the e n c l o s e d p o p u l a t i o n s . The t h i r d reason f o r the g r e a t e r t r a p p a b i l i t y of v o l e s on the e n c l o s u r e , at l e a s t i n the l a s t p e r i o d , c o u l d be due t o a d e c l i n e the e n c l o s e d p o p u l a t i o n s experienced a t the end of the summer. 61 ?2£]iiSiion Density The t r a p p a b l e p o p u l a t i o n on the c o n t r o l d e c l i n e d only s l i g h t l y i n the s p r i n g o f 1975 ( F i g . 2.1), averaging 1.6% per week from 17 February t o 28 Hay. The c o n t r o l maintained a high d e n s i t y of over 250 animals f o r ' t h e e n t i r e study. The s p r i n g d e c l i n e was s u s t a i n e d almost e n t i r e l y by the male p o p u l a t i o n , which d e c l i n e d a t a r a t e o f 4.1% per week compared with a r a t e of d e c l i n e o f females of 0.04% per week. The p o p u l a t i o n then i n c r e a s e d throughout the summer (2.2% per week) to re a c h a peak i n mid-September of 366 animals. The male p o p u l a t i o n a l s o accounted f o r most o f t h i s i n c r e a s e , which was 4.9% per week compared with 0.5% per week f o r females. From mid-September to the end of October the p o p u l a t i o n again d e c l i n e d s l i g h t l y (3.6% per week), owing t o a d e c l i n e i n both male and female numbers, with males c o n t r i b u t i n g more to the d e c l i n e , The c o r r e l a t i o n between changes i n numbers i n the t o t a l p o p u l a t i o n with the change i n male numbers was much' higher (r=.86, n=17, P<»001) than with the change i n female numbers (r=.65, n=17, P < .01), i n d i c a t i n g t h a t males were the main f a c t o r i n o v e r a l l d e n s i t y changes. To compare the changes t h a t took plac e i n numbers i n the e n c l o s u r e with those on the c o n t r o l , I have c o r r e c t e d f o r the d i f f e r e n c e i n s i z e between the two areas. To i n c l u d e those animals l i v i n g on the perip h e r y of the c o n t r o l , I assumed i t s area i n c l u d e d an outer edge equal to the d i s t a n c e between two adjacent t r a p p i n g p o i n t s { 7.6 m ) on the three edges contiguous with g r a s s l a n d and 3.8 m on the edge a b u t t i n g a g a i n s t the d i t c h . 62 F i g u r e 2 . 1 Minimum number a l i v e on t h e c o n t r o l g r i d a n d on t h e e n c l o s u r e . Number on t h e e n c l o s u r e was c o r r e c t e d f o r a r e a . Summer n o n - b r e e d i n g p e r i o d i s s h a d e d . 1975 64 Most of the o v e r w i n t e r i n g animals i n the e n c l o s u r e had entered the t r a p p a b l e p o p u l a t i o n by A p r i l . F i g u r e 2.1 i n d i c a t e s t h a t the d e n s i t y at t h i s time was higher than t h a t of the c o n t r o l by about 60 - 80 animals. P a r t of t h i s d i f f e r e n c e may be r e l a t e d to o v e r e s t i m a t i o n o f the c o r r e c t i o n f a c t o r , but i t probably p r e s e n t s a r e a l t rend because of the h i g h e r s u r v i v a l r a t e s o f the v o l e s i n the e n c l o s u r e (see below). From the end of June t o the beginning o f August, the po p u l a t i o n ;'\>n the e n c l o s u r e underwent a period of r a p i d i n c r e a s e averaging 5.3% per week compared with 3.5% per week on the c o n t r o l . I t reached a d e n s i t y 1.4 times t h a t of the c o n t r o l . T h i s very high d e n s i t y o f v o l e s s e v e r e l y overgrazed the grass i n the en c l o s u r e ( F i g . 2.2) so t h a t about 3/4 of the area was completely denuded of a l l green g r a s s , only a shallow l a y e r of d r i e d grass stems being l e f t . On the c o n t r o l , patches of grass a l s o showed heavy g r a z i n g , although nothing comparable to t h a t i n the e n c l o s u r e . From August to mid-October, the p o p u l a t i o n i n the e n c l o s u r e d e c l i n e d at an average r a t e of 6.4$ per week. In the l a t t e r p a r t of August, with the onset of the autumn r a i n s , g r a s s s t a r t e d growing again. The p o p u l a t i o n continued to d e c l i n e at a l e s s e r r a t e . In mid-October the p o p u l a t i o n s t a r t e d i n c r e a s i n g a g a i n , a f t e r breeding had been progress f o r about one month. The •fence e f f e c t 1 (Krebs e t a l . 1969) was thus e v i d e n t i n t h i s peak p o p u l a t i o n , and re s u l t e d ' i n severe o v e r g r a z i n g w i t h i n a s i n g l e season a f t e r the e r e c t i o n o f the e n c l o s u r e . The c o r r e l a t i o n between the changes i n t o t a l numbers i n the p o p u l a t i o n with changes i n male numbers (r=.8l, n-17, P<.001) was onl y s l i g h t l y lower than with changes i n female numbers 65 Figure 2,2 Photograph of the boundary between the enclosure on the l e f t and a removal grid on the righ t , July 1975, 99 6 7 (r=.87, ii=17, P<.001), i n d i c a t i n g t h at changes i n both were r e s p o n s i b l e f o r o v e r a l l p o p u l a t i o n changes. S u r v i v a l Very few j u v e n i l e s were caught i n the l i v e - t r a p s d u r i n g t h i s study; most animals on both g r i d s entered the p o p u l a t i o n s as e i t h e r s u b a d u l t s or a d u l t s , and I w i l l t h e r e f o r e be r e f e r r i n g mainly to these c l a s s e s . The s u r v i v a l r a t e s i n Table 2.2 i n d i c a t e only that animals have disappeared from the t r a p p a b l e p o p u l a t i o n . Disappearance on the c o n t r o l can have been e i t h e r through death or e m i g r a t i o n , whereas disappearance i n the e n c l o s u r e can have been through death only, except f o r some animals escaping a f t e r June 11. S u r v i v a l r a t e s o f animals i n the en c l o s u r e were c o r r e c t e d to account f o r known escapees. To compare the mean s u r v i v a l r a t e s between the e n c l o s u r e and the c o n t r o l , I have used chi-sguare a n a l y s i s on samples which are not completely independent. Though the r e s u l t s of these t e s t s are not v a l i d , they give some i n d i c a t i o n of the degree of d i f f e r e n c e between d i f f e r e n t s e t s of data. During the s p r i n g , animals on the c o n t r o l s u r v i v e d very w e l l , i n c o n t r a s t to previous years, i n which p e r i o d s of poor s u r v i v a l were recorded (LeDuc and Krebs 1975; see S e c t i o n 1). Ad u l t females s u r v i v e d s i g n i f i c a n t l y b e t t e r than both a d u l t males and subadult females ( F i g . 2.3, Table 2.2). During the summer both males and females d i d p o o r l y , so that over h a l f the p o p u l a t i o n was d i s a p p e a r i n g every 28 days. These poor s u r v i v a l r a t e s are c h a r a c t e r i s t i c of H. townsendii every summer, and 68 F i g u r e 2.3 Mean minimum s u r v i v a l r a t e s per 1^ days. The h o r i z o n t a l l i n e i s drawn at .707, below which h a l f the p o p u l a t i o n d i s a p p e a r s every 4 weeks. 69 70 Table 2.2. Minimum s u r v i v a l r a t e per 14 days f o r M. town s e n d i i on the two areas. Sample s i z e s i n parentheses. Pe r i o d and Males Females C l a s s C o n t r o l Enclosure C o n t r o l E n c l o s u r e Feb. - June Adult .81 (861) .87* (474) . 90 (1181) .92 (609) Subadult .89 (19) .60 (10) .83 (111) .81 (52) J u v e n i l e .60 (5) .83 (6) .78 (9) .80 (5) J u l y - S e p t . 15 Adult .68 (452) .77* (269) .69 (53 3) .75 (323) Subadult .65 (120) .77 (70) .73 (186) .70 (153) J u v e n i l e . 50 (6) .00 (3) .67 (6) .82 (11) Sept, 29-Nov. Adult .70 (315) .77 (133) .73 (250) .89** (112) Subadult .78 (23) .43 (7) .69 (98) .84 (32) J u v e n i l e - - - - 1.00 (D .00 (D T o t a l Adult .75 (1628) . 82** (876) .82 (1964) . 86* (1044 Subadult .70 (162) .72 (87) .75 (39 5) .74 (237) J u v e n i l e .54 (11) . 56 (9) .75 (16) .76 (17) * P < .01 f o r n u l l hypothesis of no d i f f a r e n c e ~ w i t h c o n t r o l . ** P < .001 71 appear t o be a s s o c i a t e d with the presence of b o t f l i e s (Cuterebra sp.) and grey f l e s h f l i e s ( J o l h f a h r t i a v i g i l ' W a l k e r ) . S u r v i v a l i n c r e a s e d s l i g h t l y i n the f a l l , but" d i d not a t t a i n the s p r i n g l e v e l s . Over the e n t i r e study a d u l t females s u r v i v e d s i g n i f i c a n t l y b e t t e r than a d u l t males and subadult females (by 1% i n both c a s e s ) . Animals i n the en c l o s u r e g e n e r a l l y s u r v i v e d b a t t e r than d i d animals on the c o n t r o l . T h i s d i f f e r e n c e was most pronounced i n a d u l t males, with a d u l t s i n the en c l o s u r e s u r v i v i n g 1% b e t t e r than a d u l t s on the c o n t r o l . During the sp r i n g and the summer, a d u l t females s u r v i v e d only s l i g h t l y b a t t e r , whereas i n the f a l l t h e i r s u r v i v a l r a t e s were s i g n i f i c a n t l y higher (16X) than those on the c o n t r o l . In subadult males and femalas t h e r e were no c o n s i s t e n t d i f f e r e n c e s i n s u r v i v a l r a t e s . C o r r e l a t i o n between male and female s u r v i v a l r a t e s i n the en c l o s u r e was f a i r (r=.57, n=19, p<.01). C o r r e l a t i o n between the s u r v i v a l r a t a s of males i n the e n c l o s u r e and males on the c o n t r o l was good (r=.87, n=19, P<.001), while c o r r e l a t i o n between the s u r v i v a l r a t e s of females i n the en c l o s u r e and cn the c o n t r o l was f a i r (r=.60, n=19, P<.01). These c o r r e l a t i o n s i n d i c a t e that the o v e r a l l p a t t e r n of l o s s were s i m i l a r , even though e m i g r a t i o n was not p o s s i b l e ( f o r the most part) from the en c l o s u r e . T h i s suggests t h a t the other agents of l o s s causing i n s i t u m o r t a l i t y ware a c t i n g at s i m i l a r i n t e n s i t i e s i n both p o p u l a t i o n s . Although • animals g e n e r a l l y s u r v i v e d b e t t e r i n the en c l o s u r e than on the c o n t r o l , there was a p e r i o d of about one month from mid-August to mid-September when s u r v i v a l in the enclo s u r e dropped d r a s t i c a l l y ( F i g . 2.3). Females i n the e n c l o s u r e had a 7 2 mean s u r v i v a l r a t e of only .53 compared with .69 f o r females on the c o n t r o l (x 2=10.2, P<.005). Males on the two areas showed no s i g n i f i c a n t d i f f e r e n c e s i n s u r v i v a l r a t e s a t t h i s time (enclosuxe=.65; control^.62) , whereas the s u r v i v a l of males i n the e n c l o s u r e had been c o n s i s t e n t l y h i g h e r . P a r t of the poor s u r v i v a l i n the enclosure was probably due t o s t a r v a t i o n and d e s i c c a t i o n , s i n c e most of the e n c l o s u r e had no green g r a s s or cover. Part was due to escape from the e n c l o s u r e . The c o r r e l a t i o n between the number of animals d i s a p p e a r i n g from the e n c l o s u r e once the f i r s t escapee had been detected i n mid-June and the number subsequently caught on the adjacent removal areas was good (r=.72, n=11, P<.05). However, of the 302 animals d i s a p p e a r i n g from the enclosure from mid-June onwards, only 41 were captured o u t s i d e the e n c l o s u r e . The second i n d i c a t i o n t h a t disappearance was mostly due to i n s i t u m o r t a l i t y was t h a t a d i s p r o p o r t i o n a t e number of the escaping animals came from a corner of the g r i d where e x t e n s i v e t u n n e l i n g underneath the fence was evident (x 2=22.1, P<.001). The p o s i t i o n of the e n c l o s u r e between the two removal g r i d s and a w a t e r - f i l l e d d i t c h was such t h a t the m a j o r i t y of the e s c a p i n g voles should have been detected. The assumption I make i s t h a t escapees would have s e t t l e d on the removal area. I conclude that the e n c l o s u r e had a higher s u r v i v a l r a t e than the c o n t r o l during most of the study because of the absence of l o s s owing to d i s p e r s a l from the e n c l o s u r e . 73 J£§££°3uction To compare d i f f e r e n c e s i n r e p r o d u c t i o n between the two areas, I used the p o s i t i o n of the t e s t e s and the s i z e of n i p p l e s . No s u b a d u l t s ever e x h i b i t e d s i g n s of r e p r o d u c t i o n , so t h a t t h i s d i s c u s s i o n i s l i m i t e d to a d u l t s . Breeding occ u r r e d i n the s p r i n g and the f a l l of 1975, with very l i t t l e i n mid-summer (Table 2.3). The majority of a d u l t males (80%) had s c r o t a l t e s t e s by the beginning of March, but a l a r g e p r o p o r t i o n of females was not l a c t a t i n g u n t i l m i d - A p r i l . At the end of the s p r i n g breeding pe r i o d , the number breeding d e c l i n e d more a b r u p t l y i n females than i n males. In the summer, almost no females were l a c t a t i n g , while some males (5-20%) remained s c r o t a l . Breeding i n the f a l l was much l a s s i n t e n s e than i n the s p r i n g , with at most 20% of the females l a c t a t i n g and 75% of the males having s c r o t a l t e s t e s . Although t r a p p i n g d i d not c o n t i n u e u n t i l breeding stopped, the percent of r e p r o d u c t i v e a d u l t s was t a p e r i n g o f f more r a p i d l y than i t had i n the s p r i n g . B eproductive r a t e s i n the e n c l o s u r e were very s i m i l a r to those on the c o n t r o l ( F i g . 2.4). The c o r r e l a t i o n between the r e p r o d u c t i v e r a t e s of each of the sexes i n the e n c l o s u r e with those on the c o n t r o l was high (males r=.90, n=21; and females r=.80, n=21). Chi-sguare analyses were performed to t e s t f o r d i f f e r e n c e s between the two g r i d s . An animal was counted each time i t was captured. There were only two periods i n which s i g n i f i c a n t d i f f e r e n c e s between the two g r i d s o c c u r r e d . In the s p r i n g , a g r e a t e r number of a d u l t males were s c r o t a l i n the 74 Table 2.3. Mean r e p r o d u c t i v e r a t e s f o r a d u l t s on the c o n t r o l and i n the e n c l o s u r e . Data are the p r o p o r t i o n of i n d i v i d u a l s f a l l i n g i n t o each category summed over the e n t i r e p e r i o d . Sample s i z e s i n parentheses. P e r i o d Males S c r o t a l C o n t r o l Enclosure Females L a c t a t i n g C o n t r o l E n c l o s u r e February .79 (928) .85* (511) - June .35 (1298) .32 (670) J u l y - .18 (509) Sept. 15 . 15 (298) .02 (482) .01 (301) Sept. 29 .56 (410) - November .82** (174) 18 (426) .42** (225) * P < .01 f o r n u l l hypothesis of no d i f f e r e n c e with c o n t r o l . ** P < .001 7 5 ure 2.4 Changes i n the percentage of l a c t a t i n g females on the c o n t r o l g r i d and on the e n c l o s u r e . The summer non-breeding p e r i o d i s shaded. Percent Lac ta t ing 9L 77 enclosure than on the control, This appeared to be due to a s l i g h t l y e a r l i e r onset of breeding in the animals in the enclosure. In the f a l l , both males and females had s i g n i f i c a n t l y higher rates of reproduction in the enclosure than on the control. This was associated with lower densities i n the enclosure aft e r the decline i n the l a t e summer, and with a greater guantity of green grass v i s i b l e in the enclosure. In summary, voles in the enclosure had similar rates of reproduction to those on the control, except i n the f a l l when the rates were much higher i n the enclosure. To obtain a measure of the production per pregnancy, I calculated a index of survival of the young which has been used in other studies on Microtias (Krebs 1966, and l a t e r papers). This index was the number of new young trapped (< 40 g) per la c t a t i n g female over the entire study excluding the f a i l breeding period. On the control t h i s index was ,39 young per l a c t a t i n g female (N=457 females) compared with .49 in the enclosure (N=220). These indices were si m i l a r and very low. Many young were known to be avoiding traps u n t i l they reached adult weight (see Section 5). L i t t l e or no immigration into the enclosure probably occurred because of i t s position position between two removal grids and a wat e r - f i l l e d d i t c h . For t h i s reason i t i s possible to get an index of t o t a l production, because a l l new r e c r u i t s had to be born on the area. Over 50% of the new re c r u i t s i n the enclosure entered traps as adults. If we assume that immigration and emigration are equal on the control, a comparable figure for t o t a l production can be obtained. On the control t h i s index was 1.77 r e c r u i t s per l a c t a t i n g female 7 8 compared with 1.65 in the enclosure. The s l i g h t l y higher index on the control may have been due to immigration. Nevertheless, these estimates are roughly eguivalent, and indicate that there were no differences in production between the two areas. Sexual Maturity The differences in density and reproduction between the two areas can be influenced by differences i n the age at sexual maturity. Since the age of voles i s not known, weight i s used as an index of' age. The weight at sexual maturity for live-trapped voles was calculated by the use of the technique of L e s l i e et a l . (1945). Maturity i n males was judged by the presence of s c r o t a l testes. Maturity i n females was judged by the presence of a perforate vaginal o r i f i c e , or of medium to large size nipples, or of an open pubic symphsis, or of a l i t t e r i n the trap. Figure 2.5 shows that i n males, only large animals were mature i n the summer, and i n females, the high weights at sexual maturity indicate that almost none were becoming mature. In the spring and f a l l , weights at sexual maturity were 10-40 g less in both sexes for both areas. There are no differences i n weight at sexual maturity between the control and the enclosure in the spring breeding period. In the summer, females i n the enclosure attained sexual maturity at a s i g n i f i c a n t l y higher body weight than did females on the cont r o l . However, there were no differences i n the percentages of l a c t a t i n g females at t h i s time (Table 2.3), so that the difference i n weight at sexual maturity 79 ure 2.5 Median body weight from the control ( t ), and with 95% confidence l i m i t s . at sexual maturity i n animals the enclosure ( 0 ) , along LO 2 CQ CO C 3 3 i Med ian Weight at Sexual Ma tu r i t y , G. CO Ol vj co o o o o i 1 ~i Z > CO "O 2 D CQ LO c 3 3 o rn CO LO 01 ^ CD o o o o T I I -n m Z > r m LO 08 81 represented no r e a l difference i n reproductive output. In the f a l l , both males and females matured at s i g n i f i c a n t l y lower body weights i n the enclosure than on the control. This difference was reflected i n a much larger percentage of l a c t a t i n g females i n the enclosure (Fig. 2.2). The s i g n i f i c a n t differences between the control and the enclosure coincided with a lower density and more grass i n the enclosure. Discussion The a b i l i t y to predict the pattern of population change i n voles over a short time period (6 months) presents a good opportunity to test hypotheses of population processes. Unfortunately, we do not yet have t h i s a b i l i t y with microtines. The object of the present experiment was to prevent d i s p e r s a l to fi n d out i f i t s absence would prevent the decline expected i n the spring of 1975. The control population did not decline; i n f a c t , 1975 was a peak year. There were two obvious differences between the winter of 1974-75 and previous winters. One was the almost complete absence of the seasonal migration of a e r i a l predators. Very few short-eared owls (Asio flammeus), rough-legged hawks (Buteo iil.90_P.us) > marsh hawks (Circus cyaneus) , and snowy owls (Wyetea scandiaca) were present. However, great blue herons (Ardea herodus) were s t i l l observed hunting in the f i e l d s and barn owls (Hi2 alba) were known to inhabit the l o c a l farm buildings. Even the presence of the seasonal migratory predators in other years, however, f a i l e d to account for more than a small portion of the 82 t o t a l number of v o l e s t h a t disappeared (see S e c t i o n 3 ) . The second d i f f e r e n c e was that the animals were unusually heavy i n the winter and s p r i n g of 1975. For example, d u r i n g the f i r s t t r a p p i n g week i n February, 1975, males on the c o n t r o l g r i d had an average body weight of 63.7 ± 11.1 g (n=109) compared with 53.3 ± 9.4 g (n=63) d u r i n g the same month i n 19 74. Females showed a s i m i l a r 10 g d i f f e r e n c e feejtween the two y e a r s . High body weights are c h a r a c t e r i s t i c of peak p o p u l a t i o n s i n m i c r o t i n e s (Krebs and Myers 1974) and are a s s o c i a t e d with higher growth r a t e s i n i n c r e a s i n g and peak p o p u l a t i o n s (Krebs et a l . 1S69). The high body weights i n 1975 suggest t h a t t h e r e was something fundamentally d i f f e r e n t between the p o p u l a t i o n i n that year and those i n p r e v i o u s years. The second reason f o r t h i s study was to see i f M. townsendii responded t o a fence i n a manner s i m i l a r to s p e c i e s showing s t r o n g l y c y c l i c c h a r a c t e r i s t i c s (Krebs et a l . 1969) . Some of the jM. townsendii p o p u l a t i o n s s t u d i e d near Vancouver have-shown a number of the c h a r a c t e r i s t i c s a s s o c i a t e d with m i c r o t i n e c y c l e s such as winter breeding i n the i n c r e a s e p e r i o d , a shortened summer breeding season i n the year of high d e n s i t y , and no breeding i n the winter a f t e r a peak year (LeDuc and Krebs 1975). However, other p o p u l a t i o n s have shown a gradual i n c r e a s e i n numbers over a f i v e year p e r i o d with no marked d e c l i n e (Krebs e t a l , 1976; Krebs pers. comm.). In the d e c l i n e s observed i n t h i s s p e c i e s , numbers have never dropped below 44 animals per hectare before i n c r e a s i n g again. T h e r e f o r e , t h i s s p e c i e s g i v e s some i n d i c a t i o n t h a t i t does not f u n c t i o n i n a t y p i c a l m i c r o t i n e f a s h i o n i n t h i s area. 83 Of those who have examined the affect of enclosing a f i e l d population of microtines, only Krebs at a l . (1969) have given a detailed enough account to which to compare my study. They found the following c h a r a c t e r i s t i c s i n M. pennsylvanicus and !• pchrggastar populations when these were enclosed i n a fence: (1) higher rates of population increase i n the increase and peak periods on the fenced areas than on the unfanced areas; (2) much higher densities on :the fenced areas; (3) higher s u r v i v a l rates i n the fenced populations; (4) severe overgrazing of the vegetation, r e s u l t i n g i n a decline through starvation. The enclosed population i n my study exhibited a l l of these c h a r a c t e r i s t i c s . Since the population in t h i s study was at peak numbers when i t was enclosed, severe overgrazing occurred within 6 1/2 months of the erection of the fence. In the two species studied by Krebs et a l . (1969), the populations started at low densities and took 13-17 months before overgrazing the habitat. The evidence presented i n Section 5, where l i v e - t r a p s and p i t f a l l traps were used, indicates that the actual number of voles on tha control during the summer of 1975 was more than twice tha number enumerated by Longworth l i v e - t r a p s . A small number of p i t f a l l traps put out near the end of the spring breeding period i n the enclosure also indicated that a large segment of the population temporarily avoided capture i n l i v e - t r a p s . Of the 104 animals f i r s t caught in p i t f a l l s , 3 6 f a i l e d to enter l i v e - t r a p s subsequently. This evidence together with that of severs overgrazing and higher s u r v i v a l rates within 84 the e n c l o s u r e , suggests t h a t there were over 519 animals per acre (1281 par hectare) i n the e n c l o s u r e , which was the maximum number enumerated on the c o n t r o l . T h i s compares with a maximum of 200 animals per acre i n M. ochrogaster (4 94 per hectare) i n an e n c l o s u r e i n Indiana where only l i v e - t r a p s were used (Krebs et a l , 1969) . T h i s study again i n d i c a t e s the importance o f d i s p e r s a l i n r e g u l a t i n g Microtus d e n s i t i e s below the l i m i t set by the food supply. D i s p e r s a l from peak p o p u l a t i o n s of M. £ennsylvanicus was found to account f o r 33% of the l o s s e s of males from c o n t r o l areas and 25% of the l o s s e s of females (Myers and Krebs 1971). These workers found that d i s p e r s a l was r e l a t e d t o the onset of breeding c o n d i t i o n , and that more males di s p e r s e d than females. T h i s higher d i s p e r s a l r a t e i n males corresponded to the lower apparent s u r v i v a l r a t e o f t h i s sex. An estimate of the f r a c t i o n of the l o s s e s t h a t were p o s s i b l y due to d i s p e r s a l can be obtained i n the present study i f one assumes that the d i f f e r e n c e i n s u r v i v a l between the enclosed and c o n t r o l p o p u l a t i o n s i n each t r a p p i n g s e s s i o n was due to emi g r a t i o n from the c o n t r o l ; the r e s t was due t o i n s i t u m o r t a l i t y on both areas. Only the p e r i o d from February to J u l y was examined, because a f t e r t h i s time the number of animals escaping from the enclosure i n c r e a s e d s h a r p l y and s u r v i v a l f e l l d r a s t i c a l l y because of o v e r g r a z i n g . T h i s estimate i n d i c a t e s that d i s p e r s a l accounted f o r 37% of the l o s s e s i n males and 33$ of the l o s s e s i n females. These f i g u r e s are comparable t o those found by Myers and Krebs (1971). Since very few a d u l t v o l e s d i s a p p e a r i n g from the c o n t r o l g r i d were ever caught again on other t r a p p i n g areas (see S e c t i o n 5), the 85 animals that d i d d i s p e r s e must e i t h e r have moved o n l y s h o r t d i s t a n c e s before s u f f e r i n g high m o r t a l i t y r a t e s , o r must have moved long d i s t a n c e s before s e t t l i n g down. The cause of t h i s d i s p e r s a l i s unknown, but i s presumed to i n v o l v e some form of a g g r e s s i v e behavior. This a g g r e s s i v e behavior i s not s u f f i c i e n t to cause death i f d i s p e r s a l i s e l i m i n a t e d as evidenced by the i n c r e a s e d s u r v i v a l i n the e n c l o s u r e . ^hat a c t u a l l y k i l l s d i s p e r s e r s i s unknown, but may i n v o l v e s e l e c t i v e p r e d a t i o n on an animal u n f a m i l i a r with an area through whxch i t i s moving (Metzgar 1967) or f a i l u r e to f i n d a nest s i t e . The high r e p r o d u c t i v e r a t e s e x h i b i t e d i n t h e enclosed p o p u l a t i o n i n the f a l l (Table 2.3) may have been the r e s u l t of two f a c t o r s . F i r s t , the p o p u l a t i o n i n the e n c l o s u r e had j u s t s u f f e r e d a severe d e c l i n e and was at lower d e n s i t y compared to the c o n t r o l . C h a r a c t e r i s t i c a l l y , m i c r o t i n e s i n the peak phase have a shortened breeding season t h a t may be i n v e r s e l y r e l a t e d to d e n s i t y ( K a l e l a 1957; Zedja 1967). S i m i l a r shortened breeding seasons have been found at low d e n s i t i e s such as i n d e c l i n e years (Krebs 1964). This suggests that shortened breeding seasons may not be r e l a t e d t o d e n s i t y per se but to t h e phase of the m i c r o t i n e c y c l e . Secondly, t h e r e was a l a r g e f l u s h of green grass t h a t was more evident i n the e n c l o s u r e during t h i s p e r i o d . Negus and P i n t e r (1966) suggest that v e g e t a t i o n i n the e a r l y growth stages may s t i m u l a t e r e p r o d u c t i v e processes through e s t r o g e n i c - l i k e substances i n the p l a n t s . However, the onset of the breeding season may be delayed even i n the presence of growing veg e t a t i o n or continued i n t o the dry season ( Krebs 1966). Therefore the exact cause of the d i f f e r e n c e s i n breeding 86 r a t e s i s unknown. T h i s experiment f a i l e d to r e s o l v e the f a t e of animals during a p o p u l a t i o n d e c l i n e . H i l b o r n and Krebs (1976) attempted to determine the f a t e of d i s a p p e a r i n g M. townsendii by means of a r a d i o a c t i v e tagging technigue. His r e s u l t s were ambiguous, with many voles dying i n s i t u i n one d e c l i n e , and with many v o l e s d i s a p p e a r i n g and presumed t o have d i s p e r s e d d u r i n g another d e c l i n e . Krebs e t a l . (1973) p o s t u l a t e d t h a t d i f f e r e n t i a l d i s p e r s a l of c e r t a i n g e n e t i c and b e h a v i o r a l groups of animals observed during the phase of i n c r e a s e caused the q u a l i t y of the v o l e s remaining at peak d e n s i t i e s i n wild p o p u l a t i o n s to be d i f f e r e n t from the q u a l i t y of v o l e s i n e n c l o s u r e s , where no s e l e c t i o n through emigration c o u l d occur. E x a c t l y what causes the d e c l i n e remains a puzzle. An experiment, s i m i l a r to the one I performed here, on a p o p u l a t i o n e x p e r i e n c i n g a d e c l i n e would help to p i n - p o i n t how animals were d i s a p p e a r i n g and p o s s i b l y might l e a d to f u r t h e r c l u e s of why. An a l t e r n a t i v e experiment would be to provide a d i s p e r s a l s i n k (a vacant h a b i t a t from which animals are removed) ( L i d i c k e r 1975) f o r a fenced p o p u l a t i o n and f o l l o w t h i s p o p u l a t i o n through a c y c l e . T h i s would allow d i s p e r s e r s t o be i d e n t i f i e d and t o determine at which times d i s p e r s a l was o c c u r r i n g . 87 Summary. 1. The object of t h i s study was to test the hypothesis that dispersal i s not necessary for population declines i n !• l2W£sendii. A vole population predicted to decline i n the winter and spring of 1975 was enclosed and both this population and one on a control area were live-trapped untxl November. 2. Neither population declined i n the spring; the population on the control remained at peak densities. 3. The enclosed population showed a higher rate of increase once the breeding season had started, reached higher densities, and survived better than the control population. By mid-summer the enclosed population had severely overgrazed the vegetation and went into a sharp decline. 4. This study thus f a i l e d to resolve the fate of animals during decline periods. It did show, however, that t h i s vole species responded to a fence i n a manner si m i l a r other species that have been studied and indicates the importance of dispersal at peak densities. 88 SECTION 3. IMPACT AND SELECTIVITY OF PRgDATIO]} Igtrpdaction The impact of predators on prey continues to be a subject of t h e o r e t i c a l and experimental i n t e r e s t (see Huffaker 1971 and Murdoch and Oaten 1975 for reviews). In the study of microtine cycles, there are diverse opinions on the importance of predation, ranging from the be l i e f that predation truncates population peaks and causes decline periods (MacLean et a l . 1974), to the b e l i e f that predation acts only after some other agent has started the decline and then serves to depress the population i n low periods (Pearson 1966, 1971), to the b e l i e f that predation i s not necessary f o r the decline (Chitty 1960). The f i r s t reason f o r doing t h i s study was to assess the impact of both mammalian and avian predators on Microtus townsendii populations, e s p e c i a l l y during population declines. Almost every other study on t h i s topic has dealt only with either mammalian predators or avian predators but not both, so that the t o t a l e f f e c t of predation on a particular population was not known. A second reason for doing t h i s study was to examine i f predation was se l e c t i v e on certain members of the vole population. Errington (1956) believed that some segments of the population were more vulnerable because of their p o sition i n the s o c i a l hierarchy. These individuals were doomed to death by one agent or another, with predation just happening to be one of them. In contrast to t h i s , Pearson (1966) believed that 89 c a r n i v o r e s preying on Microtus p o p u l a t i o n s are capable of c a t c h i n g not only s i c k , homeless, or maladjusted animals, but a l s o h e althy r e s i d e n t animals. No study has i n t e n s i v e l y s t u d i e d a marked p o p u l a t i o n of voles i n order to get l i f e h i s t o r i e s of the animals and then r e l a t e d t h i s t o subsequent p r e d a t i o n . In t h i s study, marked v o l e p o p u l a t i o n s were trapped every two weeks, and during the f a l l and winter, a l l evidence o f predation ( p e l l e t s and scats) was c o l l e c t e d . Methods The study area was on Sestham I s l a n d i n the F r a s e r Hiver d e l t a , near Vancouver, B r i t i s h Columbia, on a 37-acre (15 ha) p a r c e l of g r a s s l a n d owned by the Department of N a t i o n a l Defence as a t r a n s m i t t e r - r e c e i v e r s i t e . T h i s area i s r e l a t i v e l y f l a t , has a number of drainage d i t c h e s running through i t , and i s e n c l o s e d by fences. The area i s surrounded by c u l t i v a t e d f i e l d s to the east and south, a s a l t marsh and the sea to the west, and a waterfowl sanctuary to the n o r t h . The only nearby areas where v o l e s can l i v e are a permanent pasture (approximately 6 ha i n s i z e 170 m to the south) which, because of i n t e n s i v e g r a z i n g by c a t t l e i n the summer, probably c o n t a i n e d few v o l e s , and s m a l l grass margins o c c a s i o n a l l y o c c u r r i n g along the dykes. The vole p o p u l a t i o n on the study area i s thus r e l a t i v e l y i s o l a t e d (see LeDec and Krebs (197 5) f o r an a e r i a l photograph of the a r e a ) . L i v e - t r a p p i n g of the M. townsendii p o p u l a t i o n has been c a r r i e d out here s i n c e J u l y 1971. The g e n e r a l technique i n v o l v e d l i v e - t r a p p i n q v o l e s on 1.7 90 acre {.7 ha) g r i d s . Each of 100 t r a p p i n g p o i n t s was l o c a t e d 25 f t (7.6 m) from the next. A d d i t i o n a l t r a p s (up to 100) were added as vo l e d e n s i t y i n c r e a s e d . Longworth l i v e - t r a p s were b a i t e d with o a t s ; c o t t o n s t u f f i n g was provided f o r warmth. The t r a p s were set every second week i n the afternoon, checked the next morning, the next a f t e r n o o n , and again on the second morning, when they were l o c k e d open and l e f t i n p l a c e . During the summer, t r a p s were set only i n the evenings t o avoid m o r t a l i t y i n t r a p s during the heat of the day. A l l v o l e s were ear-tagged (Aluminum t a g s ) , and ' the tr a p l o c a t i o n , sax , r e p r o d u c t i v e c o n d i t i o n , and weight of each animal was recorded. In t h i s paper, u n l e s s otherwise i n d i c a t e d , animals are c l a s s i f i e d as a d u l t {> 43 g ) , subadult (30 g t o 42 g ) , or j u v e n i l e (< 30 g). Population s t a t i s t i c s were determined by enumeration techniques to a v o i d s t a t i s t i c a l assumptions of random sampling. These s t a t i s t i c s are b e l i e v e d to be accurate because of high t r a p p a b i l i t i e s (> 80%) i n winter, when most of the d e t e c t a b l e p r e d a t i o n occurred ( H i l b o r n e t a l . 1976). B i r d p e l l e t s and c a r n i v o r e s c a t s were c o l l e c t e d on the study area from February 1973 to May 1974. The area was searched thoroughly about once per month by walking along a l l fence l i n e s , paths, d i t c h e s , roadways, underneath antenna guy l i n e s , and a r b i t r a r i l y over the f i e l d s , as well as through t r a p p i n g g r i d s . Information was a l s o o b t a i n from n a t u r a l i s t s from the waterfowl sanctuary who c o l l e c t e d p e l l e t s every 2-3 months from r o o s t i n g s i t e s i n the sanctuary and from l o c a l farm b u i l d i n g s . I searched fence l i n e s , d i t c h margins, and dykes on the pasture south of the main study area every 2-3 months, as we l l as 91 neighboring farm fence l i n e s . I am reasonably confident that most of the p e l l e t s and scats were collected during the winter, when heavy rains flattened the standing grass, making them easy to see. I found no evidence that scat was buried. A few pe l l e t s and scats were probably missed each time I collected. Therefore the data were grouped into broad periods of time. With the growth of grass in A p r i l , my a b i l i t y to find p e l l e t s and scat decreased markedly so that the data from l a t e spring and summer gives no true i n d i c a t i o n of the actual extent of predation. I did not expect any p e l l e t s or scats to survive from one winter to the next because of high rates of decay i n summer. However I assumed that p e l l e t s and scats deposited in late f a l l and winter would remain r e l a t i v e l y i n t a c t u n t i l spring. Of the pellets c o l l e c t e d - i n February 1973, very few showed signs of weathering. Therefore, the majority of the predation which I was able to detect occurred during the f a l l , winter, and early spring. Every f a l l there i s an i n f l u x of raptors from northern Canada and Alaska, some of whom presumably spend the winter i n t h i s area. The major avian predators seen on the area were short-eared owls (Asio flammeus Pontoppidan), great-horned owls (Bubo yira.inia.nus Gme 1 in) , snowy owls (Nyctea scandiaca L.) , barn owls (Tytp alba Scopolil)-, marsh hawks (Circus cyaneus L.) , rough-legged hawks (Butep lagopus Pontoppidan), r e d - t a i l e d hawks (Buteo jamaicensis Gmelin), northern shrikes (Lajjius excubitor L.) and great-blue herons (Ardea herodius L.). I made no attempt to distinguish between the pellets' of any of these species, since there was considerable s i z e overlap between some of them. Only two carnivores were known to occur on the area, f e r a l cats 92 ( F e l i s catus L.) and raccoons (Pxocv_on l o t o r L.) ; c o n t e n t s from t h e i r s c a t s were a l s o pooled. No weasels were ever caught on the t r a p p i n g area. Each avian p e l l e t was analyzed s e p a r a t e l y . The f i r s t 300 p e l l e t s were soaked i n water and picked apart. A l l o t h e r p e l l e t s and a l l scat were soaked f o r 24 hours i n 10% potassium hydroxide. The s l u r r y was then poured over a f i n e s c r e e n mesh, and washed with a g e n t l e stream of water to remove d i g e s t e d h a i r and f e a t h e r s . A l l s k u l l s , mandibles, innominate bones, e a r - t a g s , and i n c i s o r s were saved. R e s u l t s Incidence Of Predators F e r a l c a t s were o c c a s i o n a l l y seen on the study area i n both w i n t e r s . The warden from the waterfowl sanctuary d e s t r o y e d a l l c a t s and raccoons seen or caught on the sanctuary or adjacent areas. In January 1973 a f e r a l cat was l i v i n g underneath b u i l d i n g s adjacent to g r i d I . T h i s animal was trapped and removed on January 24 because i t d i s t u r b e d mouse t r a p s used i n the l i v e - t r a p p i n g program. No c a t s were observed u n t i l January and February 1974, wben another c a t began l i v i n g underneath the same b u i l d i n g s . I t t e m p o r a r i l y l e f t the area at the end of February, r e t u r n e d i n mid-March, and was destroyed by the warden i n May. In both s p r i n g s , raccoons were commom around the-sanctuary, r a i d i n g eggs from n e s t i n g waterfowl, and o c c a s i o n a l l y 93 wandering out onto the study area, and opening set traps. However, i n both springs the incidence of th i s was low. I recorded birds of prey sighted on the study area while I was trapping, and got additional information from the sanctuary (Table 3.1). The most common diurnal birds of prey during the f a l l and winter of 1972-73 and 1973-74 were short-eared owls and marsh hawks. One or two short-eared owls were sighted every time the study area was v i s i t e d . Lockie (1955) and Clark (1975) have suggested that t h i s species forms winter feeding t e r r i t o r i e s . One or two marsh hawks were also seen each v i s i t to the area and these were thought to be resident throughout the year, nesting i n the nearby s a l t marsh. Craighead arid Craighead (1956) and Schipper e t - a l . (1975) report that i n winter, t h i s species also has r e s t r i c t e d hunting areas. Red-tailed and rough-legged hawks were occasional v i s i t o r s . Barn owls and great-horned owls were never seen on the area, but were known to be roosting i n l o c a l farm buildings and in the woods on the sanctuary. One and occasionally two herons were seen hunting on the area i n winter. These birds were also known to roost i n conifers in the sanctuary, where some of their p e l l e t s ware c o l l e c t e d . The number and species of the birds of prey i n the Vancouver region differed greatly between the two years. The Vancouver Natural History Society made raptor counts of a region in the delta which included Westham Island i n December 1972 and again i n December 197 3. The numbers i n 1972 and 1973 were as follows: r e d - t a i l e d hawks 28:51; rough-legged hawks 9:30; northern shrikes 16:30; short-eared owls 2 5:49; and snowy owls 7:107. Numbers of the other species remained r e l a t i v e l y 94 Table 3.1. Estimated average number of b i r d s o f prey using the study area. Species October 1972- October- Januar A p r i l 1973 December 1973 A pr.i l Harsh Hawk 2 2 2 B e d - t a i l e d Hawk 1 1 1 Bough-legged Hawk 1 1 1 Great Blue Heron 1 1 1 Northern Shrike 1 1 Barn Owl 2 2 2 Great Horned Owl 1 1 Short-eared Owl 2 2 2 Snowy Owl 1 1 95 constant. The winter of 1973-74 may thus have been be a c y c l i c invasion year of a r c t i c raptors s i m i l a r to those described by Gross (1947) . Amount Of Predation The prey items found i n p a l l e t s and scats on the study area are shown in Table 3 . 2 . P e l l e t s c o l l e c t e d from nearby areas had s i m i l a r contents; 222 voles, 30 birds, 8 shrews (Sorex va<jrans Baird), 5 rats {Rattus norvegicus Berkenhout), 2 deermice CE__omjscus maniculatus Wagner) ; 1"'-insect, and 1 unidentified prey item. An unknown number of p e l l e t s were also analyzed by n a t u r a l i s t s from the sanctuary. J . townsecdii accounted for the greatest number of prey recovered during t h i s study. The table indicates that there were few alternative prey spacies besides the vole. Waterfowl ware p l e n t i f u l in adjacent areas, but were not heavily preyed upon. Shrews were common but at very low densities (2-6 per ha). Bats were uncommon but were occasionally trapped along ditch margins. Deermice were trapped only along dykes with trees and are believed to be at low d e n s i t i e s . P e l l e t contents from c e r t a i n birds of prey may contain fewer prey items than are actually eaten. P e l l e t s of hawks generally contain less bony material than do pallets from owls (Glading et a l . 1943 ) . Clark (1972) found that the bone formed 44% of the t o t a l p e l l e t weight i n p e l l e t s of short-eared owls compared with 17% in those of marsh hawks. Greater bone corrosion in the Falccniformes i s . thought to be due to a higher a c i d i t y in the stomach (Duke et al .S 1 9 7 5 ) . For the Falconiformes 96 Table 3.2. T o t a l number of prey c o l l e c t e d from p e l l e t s and scats.on the study area. The number of voles eaten by a v i a n predators was c a l c u l a t e d by counting only the number of p a i r e d mandibles found. The number of voles eaten by mammalian predators was c a l c u l a t e d by counting p a i r s of upper i n c i s o r s . P e l l e t Feb.-April May-Sept. Oct.-Dec. Jan.-May Tot a l Contents 1973 1973 1973 1974 J i c r o t u s 491 40 320 333 1189 tgwnsendii B i r d 17 2 31 11 61 ( u n i d e n t i f i e d ) Shrews 25 3 15 15 58 Hats 5 3 3 11 Deer Mice 5 2 7 I n s e c t s 8 8 ( u n i d e n t i f i e d ) Other 2 1 3 Scat Contents Hierotus 33 townsendii 4 31 167 235 97 the d i s p a r i t y between the number eaten and the number i n the p e l l e t s i s l i k e l y to be even g r e a t e r f o r young animals with s o f t bones. P e l l e t s from herons contained almost no bones at a l l , with the vole h a i r appearing i n t i g h t masses, so t h a t these p e l l e t s provided no i n d i c a t i o n of the number o f v o l e s eaten. T h e r e f o r e , the number of voles eaten by b i r d s of prey on the study area and surroundings (Table 3.2) r e p r e s e n t s a lower l i m i t of that a c t u a l l y eaten. In c a r n i v o r e s c a t s , although the bones of v o l e s were extremely fragmented, the upper i n c i s o r s showed very l i t t l e c o r r o s i o n . The number of p a i r s of upper i n c i s o r s i s t h e r e f o r e probably a good i n d i c a t i o n of the number of i n d i v i d u a l s a c t u a l l y consumed. Vole P o p u l a t i o n Density And Predator Im£act During the f i r s t two years, the vole population on c o n t r o l g r i d I seemed to be going through a t y p i c a l c y c l e , with numbers i n c r e a s i n g from a low i n 1971 to a peak i n 1972 ( F i g . 3.1). Numbers remained high f o r most of 1972, and than d e c l i n e d from a peak of 231 to a low of 46 animals i n A p r i l 1973. Numbers then i n c r e a s e d r a p i d l y t c another peak of 233 animals at the end of 1973, and d e c l i n e d again the next s p r i n g t o a low o f 41. There was some v a r i a b i l i t y i n numbers among the v a r i o u s g r i d s (LeDuc and Krebs 1975) but the o v e r a l l p o p u l a t i o n changes were the same. The p o p u l a t i o n during 1973-74 appeared to be going through an annual c y c l e . Tha p e r i o d s of i n t e r e s t f o r the present study were the two p o p u l a t i o n d e c l i n e s i n the s p r i n g of 1973 and 1974, and the 98 Figure 3.1 Population changes on control grid I during 1971-1974. The winter months (November - February) are shaded. 66 100 p e r i o d i n the f a l l of 1973. During the summer of 1973, t a l l g rass made p e l l e t c o l l e c t i o n almost impossible so t h a t t h i s p e r i o d was not examined. To f i n d out what p r o p o r t i o n of the disappearance during these three p e r i o d s could be a t t r i b u t e d to p r e d a t i o n , I counted the number of animals t h a t disappeared and the number of t h e i r e a r - t a g s found i n p e l l e t s or s c a t s . These f i g u r e s w i l l be reasonably c l o s e to the a c t u a l p r e d a t i o n r a t e s , i f I found the m a j o r i t y of p e l l e t s and s c a t s , and i f the predator e a t s the tag. Of the 165 tags recovered from p e l l e t s and sc a t s during the study, a l l but 8 were found i n p e l l e t s and s c a t s c o l l e c t e d on the study area. T h i s suggests that the ma j o r i t y of v o l e s eaten by predators were subsequently d e p o s i t e d on the study area i t s e l f , where I c a r r i e d out i n t e n s i v e searches. I missed some unknown f r a c t i o n of the p e l l e t s and s c a t s because I d i d not know a l l the r o o s t s , and co u l d not re c o v e r p e l l e t s from a r e a s such as d i t c h e s , ponds, and the s a l t marsh. Als o t r a n s i e n t p r e d a t o r s may have d e p o s i t e d p e l l e t s and s c a t s f a r from the study a r e a . To see i f tags were l o s t i n the feeding process, I f e d two tame s h o r t - e a r e d owls dead v o l e s . Out of 29 tagged v o l e s f e d to the owls, 24 tags were recovered i n p e l l e t s . The owls d e c a p i t a t e d some of these v o l e s and dropped t h e i r heads from perehs; i n at l e a s t one case the head was not eaten l a t e r . Since I never found severed heads i n the f i e l d , the tag l o s s i n the experiment may be an a r t i f a c t . Some b i r d s p e c i e s , such as marsh hawks, are known to p u l l h a i r from the prey before i n g e s t i n g the c a r c a s s (Clark 1972). I thoroughly searched every k i l l of t h i s nature and never found ear c a r t i l a g e or tags. During the e n t i r e 101 study, p a r t l y consumed v o l e s were very i n f r e q u e n t l y found. Kaufman (1973), using r a d i o a c t i v e pins i n s e r t e d below the neck s k i n , o b t a i n e d a r e c o v e r y r a t e i n p e l l e t s of 76% f o r barn owls and 55$ f o r s c r e e c h owls (Otus agio L . ) . He a t t r i b u t e d the d i f f e r e n c e between the two s p e c i e s to e a t i n g b e h a v i o r , the barn owl e a t i n g the prey whole and the screech owl t e a r i n g the prey to p i e c e s before consuming i t . A tag which i s f i x e d t o the body, such as an e a r - t a g , may be l o s t l e s s r e a d i l y than p i n s as i n the above experiment. Tag l o s s e s from v o l e s caught by p r e d a t o r s were probably few, and I estimate l e s s than 20$. During the d e c l i n e of 1973 only about 1$ of the tagged animals which disappeared were subsequently r e c o v e r e d i n mammalian s c a t s . The data from avian and mammalian predators were t h e r e f o r e pooled. During the d e c l i n e of 1974, a much l a r g e r p r o p o r t i o n of the disappearance was due to mammalian preda t o r s , so t h a t a d i s t i n c t i o n was made between v o l a s eaten by the two c l a s s e s of p r e d a t o r s . Table 3.3 shows that l a s s than 8% of the v o l e s disappearing... from the trapped populations d u r i n g the 1973 d e c l i n e could be a t t r i b u t e d to p r e d a t i o n . In 1974, on g r i d I , more than twice as much of the disappearance c o u l d be a t t r i b u t e d t o l o s s by p r e d a t i o n , l a r g e l y because of the cat l i v i n g adjacent to the g r i d . However i n both d e c l i n e s , the- p o p u l a t i o n on g r i d I peaked and d e c l i n e d to almost i d e n t i c a l l e v e l s . The average r a t e of d e c l i n e per week was 8$ i n 1973 and 11$ i n 1974. T h i s suggests t h a t the more i n t e n s i v e predation i n the second d e c l i n e d i d accentuate the r a t e of l o s s . . In order to examine more c l o s e l y how p r e d a t i o n a f f e c t e d s u r v i v a l of the v o l e s on g r i d I i n the f a l l and winter of 102 Table 3.3. Number of vo l e s d i s a p p e a r i n g from study area and percent recovered from predators. P e r i o d G r i d Number Percent recovered from Disappearing a l l predators Dec. 1972- fl 129 3.1 A p r i l 1973 G 204 5.9 I 230 7.4 Oct. - I 132 Dec. 10, 1973 Dec. 22, 1973-I 253 A p r i l 1974 Percent recovered from: Avian Mammalian l o t a l P r e d a tors Predators 12.9 17.4 30.3 3.2 15.8 19.0 103 1973-74, I compared the changes i n s u r v i v a l r a t e with l o s s due to p r e d a t i o n . Poor s u r v i v a l i s a r b i t a r i l y d e f i n e d t o be any r a t e below .707 per two weeks (half of the p o p u l a t i o n d i s a p p e a r i n g every f o u r weeks). Data f o r both sexes were combined because minimum s u r v i v a l r a t e s were s i m i l a r f o r most of that p e r i o d . The c o r r e l a t i o n between the minimum s u r v i v a l r a t e s and the amount of l o s s to p r e d a t i o n over the winter was good (r=.70, n=15). From October 1973 to the end of January 1974, s u r v i v a l was good, averaging .81 per 14 days ( F i g . 3.2). T h i s compared t o a minimum s u r v i v a l r a t e of .85 i n 1972-73 when predation was l e s s i n t e n s e . During February, minimum s u r v i v a l r a t e s remained high at .80 per 14 days, while l o s s of animals due to p r e d a t i o n was n e g l i g i b l e (.02). In the next 1 1/2 months, the minimum s u r v i v a l r a t e dropped d r a s t i c a l l y so that the mean f o r t h i s time was .65 per 14 days, while the l o s s due t o p r e d a t i o n was s m a l l (.10). In two of the three t r a p p i n g i n t e r v a l s no l o s s t o p r e d a t i o n was observed at a l l . T h i s c o i n c i d e d with the absence of a c a t . I conclude from these r e s u l t s t h a t p r e d a t i o n i s not necessary f o r p o p u l a t i o n d e c l i n e s to occur, s i n c e p e r i o d s of poor s u r v i v a l occur i n the absence of d e t e c t a b l e predation and p e r i o d s of good s u r v i v a l occur i n the presence o f p r e d a t i o n . To see i f the amount of p r e d a t i o n by b i r d s was s i m i l a r i n the tagged and untagged p o p u l a t i o n on the study area, I made the f o l l o w i n g c a l c u l a t i o n s . Because t h e r e was some v a r i a b i l i t y i n vole d e n s i t i e s on v a r i o u s p a r t s of the study area owing to e x c e s s i v e l o c a l f l o o d i n g i n winter, I took the average of the number of v o l e s d i s a p p e a r i n g between October 197 2 and A p r i l 1973 from g r i d s G, H, and I as an index of the number d i s a p p e a r i n g 104 Figure 3.2 A) Minimum survival rate per 14 days for both sexes combined. The horizontal l i n e i s drawn at a rate of .707, below which half the population disappears every 4 weeks. B) The number of animals disappearing every 14 days. The blacked portions indicate the number of animals known to have been eaten by predators. 106 from the entire area. In 1973-74, the number of voles disappearing from the entire study area was estimated from the number disappearing from grid I and correcting t h i s downward by a factor (.88) eguivalent to the amount that grid I had been reduced in 1972-73 to make i t egual to the average for that time. Table 3.4 shows that, i n both winters, raptors ate at le a s t about 11% of the voles estimated to have been l o s t from the whole area. The table also shows that they ate only 6% of the voles l o s t from the trapped population (difference between t o t a l and tagged predation i n 1972-73 X2=12. 55, P<.001; difference i n 1973-74 X 2 = 6,05, P<.05). How can we explain this difference? 1) The percent recovery of the tagged voles may be too low. This could occur i f one assumes that for each vole caught on the area and deposited elsewhere, another vole i s caught elsewhere and deposited on the area, and that t h i s type of interchange between areas i s high. A great number of tagged voles would therefore be deposited elsewhere. For reasons already stated, I believe that t h i s error was small. 2) The percent recovery of a l l voles may be too high. This could occur i f a segment of the population, such as the young, i s not trappable but i s subject to predation. Evidence supporting t h i s explanation i s presented below. 3) The tags may have been l o s t . For reasons stated previously, I also believe that t h i s l o s s i s low. 107 Table 3.4, Comparison cf avian predation on the entire vole population with that on the fagged portion of the population. Winter 1972-73 Winter 1973-74 Number of voles disappearing 348* per hectare Number of 12.963 hectares Total number of 4510 voles disappearing Number of voles 491 found i n p e l l e t s Percent of t o t a l number of voles 10.9$ disappearing which were recovered i n pe l l e t s Percent of tagged voles disappearing 6.1$ which were recovered i n p e l l e t s 4912 12.15* 5966 658 11.0* 6.5% 1 Obtained from the average of three trapping grids - G, H, I. 2 Obtained from the number of voles disappearing frm grid I, corrected for heterogeneity of habitats. 3 area occupied by road, buildings, ditches, and low density expermimental grids excluded. * In addition to 3 , one new low density experimental grid excluded. 108 0_ Predation To determine i f predators were s e l e c t i v e l y removing certain classes of voles from the population, I removed over 900 animals from the population during the study for autopsy purposes. After autopsy, the s k u l l and innominate bones of each vole were cleaned by dermestid beetles.- I recorded seven measurements for each vole whenever possible: mandibular height, mandibular length (both according to the method of Lidicker and MacLean, 1969), innominate length, rostrum length from the outer edge of the i n c i s o r s to the end of the mandibular tooth row, ischium length from the posterior angle of the ischium to the nearest edge of the acetabulum, pubis length from the ventral apex of the pubis to the nearest edge of the acetabulum, and pubis width at the thinnest part (the l a t t e r three measurements are described i n Dunmire, 1955). These measurements were chosen because they could also be taken from most of the voles consumed by birds. No measurements were made from voles eaten by carnivores as few whole bones were found. Mandibular height, innominate length, and rostrum length were eliminated af t e r preliminary analysis indicated that they were not useful for predicting either sex or body weight. Sex Ratio To predict sex of unknown animals, I used a stepwise discriminant analysis program (Program BHD 07M, Health Sciences Computing F a c i l i t y , UCLA). Measurement (in mm) of pubis length and pubis width were s u f f i c i e n t to predict sex i n the known 109 sample 96.55$ of the time. The c l a s s i f i c a t i o n function i s as follows: X=19.38(pubis width)-1.751(pubis length)+1.390 with animals having an X > 0 being male and X < 0 being female. A higher percentage of males (5.9%) than females (1.1$) were m i s c l a s s i f i e d . In males, a higher proportion of the small animals was mi s c l a s s i f i e d than of the large animals (X2=21.21, df 3, P<.001), ind i c a t i n g that differences between males and females were more d i s t i n c t i n larger animals, flisclassification of females was not more pronounced i n small animals (X2=4.49, df 3, . 10>P>.25). To correct f o r the m i s c l a s s i f i c a t i o n of animals, I added 4$ of the animals c l a s s i f i e d as females to those c l a s s i f i e d as males. The data for t h i s section i s broken down into three periods to coincide with major changes i n population trends and to coincide with the way the p e l l e t and scat data were coll e c t e d . The data used from the control grid covers the following periods: 1) Winter 1973 (October 15, 1972 - A p r i l 30, 1973) 2) F a l l 1973 (September 17 - December 24, 1973) 3) Winter 1974 (January 7 - A p r i l 15, 1974) To see i f predators s e l e c t i v e l y removed one sex from the population, I compared the sex r a t i o of resident animals on grid I to: 1) the sex r a t i o of a l l voles recovered from tha p e l l e t s of avian predators; and 2) the sex r a t i o of tagged animals eaten by avian and carnivore predators. The resident sex r a t i o of the 110 control population was the minimum number of animals of each sex known to be a l i v e at every trapping period pooled over the entire period. Table 3.5 shows that there i s no consistent trend i n the s e l e c t i v i t y of either c l a s s of predators for one sex. In winter 1972-73, males were s e l e c t i v e l y eaten by both mammalian and avian predators i n both the tagged sample and in the untagged samples. In winter 1974, the sex r a t i o of both the tagged samples eaten by the two classes of predators and the t o t a l sample eaten by avian predators was not s i g n i f i c a n t l y d i f f e r e n t from the sex r a t i o on the control. There are a number of possible explanations for t h i s difference between years. One explanation i s that because the sample sizes were small (less than 15 i n three samples) there would be a greater probability that by chance alone the values would deviate from the real values; The other explanation i s that because I had to lump a l l the data for 1972-73 into one period, t h i s may have obscured any difference there may have been between f a l l and winter. However, the - populations in the two years appear to be guite d i f f e r e n t with regard to sex r a t i o . Body Height Distributions To sea i f there was selection f o r certain s i z e classes in the sample caught by avian predators, I used multiple regression analysis to develop eguations to predict the body weight of voles I was able to sex. The sample of standards was divided into the two sexes and into four blocks of data corresponding to the periods of breeding and non-breeding in which the 111 Table 3.5. Sex r a t i o comparison of v o l e s on the c o n t r o l with that of voles eaten by p r e d a t o r s . Data presented as the number of males per female. The t o t a l avian sample i n c l u d e s a l l voles which c o u l d be sexed. The pooled tagged samples i n c l u d e s tha tagged samples eaten by avian and mammalian pred a t o r s . Period C o n t r o l G r i d T o t a l Avian Sample Poole d Tagged Sample Tagged Mammalian Sample Tagged Avian Sample Winter 1972-73 .58 .77* 1.29** 2.0 (2063) (362) (48) (9) 1.17* (39) F a l l 1973 . 91 (1233) .88 (357) 1.6 5 (45) 1.14 (15) 2. 00 (30) Winter 1974 .90 (895) . 89 (104) .74 (66) .78 (58) . 60 (8) * P < .05 f o r h y p o t h e s i s of equal sex r a t i o to t h a t found cn c o n t r o l g r i d ** p < .01 112 i n d i v i d u a l s were c o l l e c t e d . Because the innominate bone was used to predict sex, and because I could not assume that a s k u l l and a set of innominate bones i n a p e l l e t necessarily belonged to the same vole, weight of the sexed animals was predicted only from measurements made on the innominate bones. A cube root transformation was done on body weight in the standards to l i n e a r i z e the eguations. In males there was no s i g n i f i c a n t difference between the four blocks of data when the three measurements explaining the greatest amount of v a r i a b i l i t y were used (F=.09, df 12, 437). The equation for the pooled data was: ^ /Body Weight = 1.02 3 + . 1926 (pubis width) / o f Males . + . 1584 (pubis length) + . 1523 (ischium length) The equation had a c o e f f i c i e n t of multiple determination of ,73. In the female standards, there were s i g n i f i c a n t differences between regressions for reproductive and non-reproductive periods. Because most of the predation I was able to detect occurred in the non-breeding period, I used only the female standards c o l l e c t e d during that time to develop multiple regression eguations. As there were s i g n i f i c a n t differences between the female standards i n the two non-breeding periods, a separate multiple regression was developed for the 1972-73 data and for the 1973-74 data. The 1972-73 multiple regression was: 3/Body weight = 1.7262 + .1366(pubis length) So£ Females + . 0662 (ischium length) with a c o e f f i c i e n t of multiple determination of .57. The equation for the 1973-74 data was: VBody Weight = 1.5148 + .1427(pubis length) /of Females • . 0973 (ischium length) with a c o e f f i c i e n t of multiple- determination of .72. In both 113 cases, pubis width was not s i g n i f i c a n t and was eliminated from the regressions. The mean body weight of animals from the control grid was obtained by summing the body weights of every animal caught during a given period and dividing by the t o t a l number of animals. This was therefore a mean cumulative body weight. Table 3.6 shows that the t o t a l sample of animals eaten by avian predators i n both males and females was s i g n i f i c a n t l y lower i n a l l periods by 2.8 - 13.7 g than the mean body weight i n the same period on the control grid (Mann-Whitney U-test). There was no s i g n i f i c a n t difference i n mean body weights between the tagged sample eaten by carnivores and the tagged sample eaten by avian predators so only the pooled data are given rn Table 3.6. The pooled tagged sample had mean body weights that were consistently higher than the mean body weights of the t o t a l sample eaten by avian predators. This difference was s i g n i f i c a n t i n f a l l 1973 in males and i n females i n f a l l 1973 and winter 1974. The mean body weight of the pooled tagged sample was s i m i l a r to the mean body weights on the control, except for f a l l 1973 and winter 1974 i n males, when they were they were lower. This suggests that with regard to body weight, the tagged animals i n the control population were being caught at random by predators. It also suggests that avian predators ware either s e l e c t i v e l y catching the smaller segment of tha population which was not tagged, or our trapping was s e l e c t i v e l y catching the larger animals. The large proportion of small animals i n the sample eaten, by predators becomes even more obvious when the data are 114 Table 3.6. Mean body weight (g ± 1 SE) of voles on the control grid and those eaten by predators. Total avian sample includes a l l voles which could be sexed. Sample sizes i n parentheses. Pooled tagged sample includes a l l tagged voles eaten by avian and mammalian predators. Weights of animals eaten by avian predators were predicted by measurements made on innominate bones. Period Control Grid Total Avian Sample Pooled Tagged Sample MALES Sinter 1972-73 46.4 ± .4 (687) 43.6 ± 1.0* (148) 46.2 ± 1.8 (27) F a l l 1973 51.2 ± .5 (542) 37.5 ± .8** (159) 45.5 ± 2.4* (21) Winter 1974 52.4 ± .6 (381) 40.4 ± 1.4** (47) 45.2 ± 2.2* (29) FEMALES Winter 1972-73 40.2 ± .2 (1146) 35.8 ± .4** (216) 39.6 ± 2.0 (21) F a l l 1973 46.5 ± .4 (585) 35.8 ± .6** (199) 46.5 ± 3.1 (13) Winter 1974 41.0 ± .4 (423) 35.8 ± .9** (60) 39.7 ± 1.4 (34) * P < .05 for hypothesis that control mean weight i s to mean weight of sample eaten by predators. ** P < .001 egual 115 analyzed with a simple r e g r e s s i o n to p r e d i c t body weight based on mandibular l e n g t h . The above m u l t i p l e r e g r e s s i o n eguations were u s e f u l only i n cases i n which innominate bones were present so t h a t sex c o u l d be determined. In young v o l e s , the innominate bones are very f r a g i l e , and most of these bones do not s u r v i v e the d i g e s t i o n process of avian predators, fiandibles, on the other hand, are not di g e s t e d so much, with most remaining r e l a t i v e l y i n t a c t . In order to get a rough i n d i c a t i o n of the s i z e d i s t r i b u t i o n of the t o t a l sample eaten by avian p r e d a t o r s , I developed a simple r e g r e s s i o n using standards from the two non-breeding p e r i o d s and lumping both sexes, k cube root t r a n s f o r m a t i o n was done on body weight to l x n e a r i z e the r e g r e s s i o n . The eguation was: a/Body Height = -2.0477 + . 2986 (mandibular length) with a c o e f f i c i e n t of determination of .65. Of the 1343 i n d i v i d u a l s found i n p e l l e t s on a l l areas, only 989 c o u l d be sexed. Of the remainder, 237 (17.6%) had a p r e d i c t e d body weight of < 20 g. F i g u r e 3.3 shows t h a t the body weight d i s t r i b u t i o n of voles eaten d u r i n g the f a l l and e a r l y winter of 1973 was d e c i d e d l y i n f a v o r of small animals, whereas on the c o n t r o l g r i d the cumulative body weights f o r both sexes f o r the same p e r i o d favored l a r g e animals. The cumulative body weights on the c o n t r o l are the summation of the body weights of each animal every time i t was caught. The data from other periods showed s i m i l a r d i f f e r e n c e s between the two groups. 116 Figure 3.3 Body weight d i s t r i b u t i o n s of animals (both sexes combined) trapped on the control grid and captured by avian predators, f a l l and early winter 1973-7 4. Weight of voles eaten by predators were predicted by mandibular length. A n i m a l s on C o n t r o l A r e a N-1336 10 25 3 0 34 38 42 46 5 0 54 5 8 8 0 A n i m a l s E a t e n by A v i a n P r e d a t o r s N = 517 10 2 5 3 0 34 38 42 4 6 5 0 54 5 8 8 0 Body W e i g h t , G. 118 Predation On Tagged Voles To see i f the tagged voles eaten by predators d i f f e r e d i n some way from tagged voles not known to have been eaten, I compared these two groups with respect to the information c o l l e c t e d about the voles. S p e c i f i c a l l y , . I examined for differences between these two groups with regard to: 1) Movements between trapping periods. 2) Movements within trapping periods. 3) Mean body weight at f i r s t capture. 4) Average duration of l i f e . Measuring the difference in position of capture between adjacent trapping periods { f i r s t capture point of one period to f i r s t capture point of the next) i s a way of obtaining an index of a c t i v i t y . Animals showing greater movement may be more active and thus more vulnerable to predation. Table 3.7 shows that i n females in the various weight classes, there i s no consistent difference in movement between those animals eaten by predators and those not known to have bean eaten. The v a r i a b i l i t y within any given class i s in general so high as to obscure any differences between groups. Onfortunately, t h i s high v a r i a b i l i t y and low sample size was common to a l l the variables I looked at. Because of t h i s and the fact that there was no consistent trend in these other variables, the remaining data w i l l not be presented here. I conclude that, with regard to the above variables and the inherent constraints i n the trapping technique, both avian and mammalian predators were se l e c t i n g the voles at random from this population. 119 Table 3.7. Comparison of mean d i s t a n c e ( f t ± 1 SE) between s u c c e s s i v e p o s i t i o n s of cap t u r e . Distance measured from f i r s t c apture poi n t i n p e r i o d t to f i r s t capture point i n p e r i o d t+1. Sample s i z e s i n parentheses Period C o n t r o l G r i d Tagged Avian Sample Tagged Mammalian Sample Winter 1972-73 Adult 29.4 ± 2.7 (162) Subadult 25.3 ± 1.5 (309) J u v e n i l e 29.3 ± 19.0 (13) 44.7 ± 8.5 (8) 22.8 ± 5.4 (26) 35.4 ± 15.8 (5) 20.0 ± 6.6 (8) 12.5 (2) F a l l 1973 Adult Subadult J u v e n i l e 19.6 ± 1.4 (256) 22. 1 ± 1.7 (104) 18.4 ± 5.7 (14) 21.4 ± 4.0 (21) 1.0 (D 12.5 (2) 17.4 ± 2.6 (64) 24.7 ± 7.3 (24) 14.3 ± 5.1 (7) S i n t e r 1974 Adult 26.5 ± 2.2 20. 1 ± 10.5 (125) (3) Subadult 31.3 ± 2.7 26.4 ± 5.9 (169) (19) J u v e n i l e 19.5 ± 5.9 (8) 22.3 ± 6.0 (7) 120 Discussion This study provides no support for the hypothesis that predation i s necessary to s t a r t a decline or to iceep i t going. The vole population i n the second year, when predation was more intense, had higher rates of loss than in the f i r s t year. Predation may thus have accentuated the decline in the second year. However, during the second decline, periods of poor sur v i v a l occurred i n the absence of predation, and periods of good s u r v i v a l occurred i n the presence of predation. This suggests that some other agent was responsible for the declines. The predators associated with' the two • declines were very d i f f e r e n t . In 1972-73 the major predators were primarily birds of prey, whereas i n 1973-74 they were primarily mammalian carnivores. The in t e n s i t y of mammalian predation during certain weeks suggested that these predators could p o t e n t i a l l y depress the population. Many workers have emphasized the importance of mammalian predators which, because they lack the mobility of avian predators, are forced to stay on an area while a vole population i s declining. In winter, in areas where snow cover i s continuous and l a s t s a long time, continual pressure by weasels (justela erminea L. and M. n i v a l i s L.) i s postulated to be the major influence in depressing microtine densities i n decline periods (Thompson 1955b, Maher 1967, Fitzgerald 1972 cited i n Krebs and Myers 1974, MacLean et a l . 1974). In none of these studies, however, were the microtine populations actually studied during the winter. Rather, the importance of weasels was 1 2 1 i n f e r r e d from the presence of d i s t u r b e d m i c r o t i n e winter n e s t s . To t e s t the hypothesis that weasel p r e d a t i o n i s necessary f o r d e c l i n e s i n the n o r t h , one c o u l d e n c l o s e a m i c r o t i n e p o p u l a t i o n with a p r e d a t o r - p r o o f fence j u s t p r i o r t o an expected d e c l i n e . The f a t e of i n d i v i d u a l s during a d e c l i n e might a l s o be obtained by r a d i o a c t i v e l y t a g g i n g a m i c r o t i n e p o p u l a t i o n , thereby more a c c u r a t e l y determining the amount eaten by weasels ( H i l b o r n and Krebs 1976). However i n the l a t t e r suggestion, scavenging by foxes might confound the r e s u l t s . In areas where there i s l i t t l e o r no snow, l a r g e r c a r n i v o r e s are thought to be the important agents of m o r t a l i t y , i n i n t e n s i v e study cn p r e d a t i o n i n m i c r o t i n e s has been done by Pearson (1964, 1966, 1971). He a l s o emphasized the importance of the l e s s mobile mammalian pre d a t o r s . He contends that d u r i n g the i n c r e a s e and peak p e r i o d s of a t y p i c a l c y c l e , p r e d a t i o n tends to be independent of d e n s i t y . During the d e c l i n e , p r e d a t i o n i n t e n s i f i e s as d e n s i t i e s drop, and maintain the vole p o p u l a t i o n at low d e n s i t i e s f o r some time t h e r e a f t e r . E s s e n t i a l to the continued p r e d a t i o n pressure a t low d e n s i t i e s i s the presence of a l t e r n a t i v e prey to s u s t a i n these predators while they continue to hunt f o r the remaining v o l e s . Thus, Pearson b e l i e v e s that p r e d a t o r s help determine the p e r i o d i c i t y of the c y c l e by determining i t s amplitude and the d u r a t i o n o f the phase of low numbers. My westham I s l a n d p o p u l a t i o n of M. f o w n s e n d i i does not appear to respond as Pearson's i d e a s p r e d i c t s . C a r n i v o r e p r e d a t i o n on my study areas was s p o r a d i c because of predator removal on the a d j o i n i n g sanctuary. No weasels e x i s t e d on the study area. Severe p o p u l a t i o n d e c l i n e s occurred n o n e t h e l e s s , and 122 the d e c l i n e s I observed were more severe.than the d e c l i n e s observed i n nearby p o p u l a t i o n s of M. townsendii which have weasel predation {Krebs et a l . 1S76). The absence of continuous c a r n i v o r e p r e d a t i o n and of a l t e r n a t i v e prey s p e c i e s on Westham I s l a n d may be one of the reasons f o r the immediate i n c r e a s e i n numbers at the end of each d e c l i n e p e r i o d i n t h i s study. None of the M. townsendii p o p u l a t i o n s s t u d i e d i n the d e l t a r e g i o n have had extended p e r i o d s of low numbers (LeDuc and Krebs 1975, Krebs et a l . 1S76) . What happens to a l l the animals t h a t d isappear i n a d e c l i n e , i f p r e d a t i o n i s i n s u f f i c i e n t , as i n the present study? Very l i t t l e d i s p e r s a l to nearby vacant areas o c c u r r e d i n d e c l i n e s observed by C h i t t y and Phipps {1966), dyers and Krebs (1971), and H i l b o r n and Krebs (1S76). T h e r e f o r e , v o l e s are e i t h e r dying i n s i t u from unknown causes or they are d i s p e r s i n g long d i s t a n c e s . H i l b o r n and Krebs (1976) suggest that the l a t t e r e x p l a n a t i o n may indeed be r e s p o n s i b l e f o r one of the d e c l i n e p e r i o d s they observed. Future experiments, such as t h e f e n c i n g experiment i n S e c t i o n 2, w i l l h o p e f u l l y decide between these two a l t e r n a t i v e s . Predators are able to a f f e c t the prey p o p u l a t i o n d i r e c t l y by the removing l a r g e numbers and i n d i r e c t l y by a l t e r i n g the age and sex s t r u c t u r e of the p o p u l a t i o n , thus changing the dynamics of the p o p u l a t i o n . Sometimes one sex s u r v i v e s l e s s w e l l than the o t h e r , with males f r e g u e n t l y d e c l i n i n g before females ( C h i t t y and Phipps 1966, Krebs 1966, Krebs et a l . 1969, 1973). T h i s was a l s o the case i n t h i s study. For example, during the f i r s t two weeks of A p r i l 1974, males had a minimum s u r v i v a l r a t e of .40 123 per 14 days (N=20) while females had a minimum s u r v i v a l rate of .80 (N=25) . This poor survival i n males was associated with an e a r l i e r onset of breeding condition. Several workers have found selection by various predators for a s p e c i f i c sex which may explain some of this d i f f e r e n t i a l s u r v i v a l . Thompson (1955a) found selection by snowy owls for male brown lemmings (Lejmmus s i b i r i c u s Kerr) during the early summer, but not during the l a t e summer. He suggested that t h i s difference was due to the greater movement and thus exposure of the males i n early summer. Maclean _ t a l . (1974) found some evidence of heavy predation by weasels on breeding brown lemming females, which, i f i t were widespread, could depress the rate of increase of the population. Stendell (pers. comm.) in a study of raptor predation in C a l i f o r n i a , found that i n short-eared owls, there was no preference for either sex compared to the trapped population, whereas in barn owls, there was a preference for males. In none of the other studies on microtines have workers found selective predation on either sex (Southern and Lowe 1968, Maher 1970). In the present study, there was no consistent selection by either mammalian or avian predators for either sex between periods, although i t may be that s e l e c t i o n for a given sex was indeed occurring by d i f f e r e n t bird species but in d i f f e r e n t directions f o r d i f f e r e n t species, so that lumping the data obscured the s e l e c t i o n . A number of • workers have also looked at s e l e c t i o n by predators for various age classes. Lockie (1955) found that, i n the spring, short-eared owls took small animals i n the same proportion as the traps did. However, i n the summer, the owls were taking a much higher proportion of young than occurred i n 124 the t r a p s . In the other s t u d i e s no evidence o f s i z e s e l e c t i o n was observed (Southern and Lowe 1968, S a i n t Giron 1973, S t e n d e l l p e r s . comm.). In the present study, i n the t o t a l sample of v o l e s eaten by avian p r e d a t o r s , t h e r e was a c o n s i s t e n t s e l e c t i o n f o r s m a l l e r animals (Table 3.6, F i g . 3.3). Because p r e d a t i o n on the tagged p o p u l a t i o n appeared to be random with r e g a r d t o body weight, these r e s u l t s suggest t h a t e i t h e r the t r a p p i n g scheme was s e l e c t i n g l a r g e r animals or that avian p r e d a t o r s were s e l e c t i n g s m a l l e r animals. To d i s t i n g u i s h between these two a l t e r n a t i v e s , a technigue f o r c a t c h i n g small animals i n f a l l and winter would be necessary. Data presented i n Se c t i o n 5 i n d i c a t e t h a t Longworth l i v e - t r a p s d u r i n g the summer c a t c h mainly l a r g e r animals. I f b i r d s of prey were s e l e c t i n g young animals h e a v i l y , e s p e c i a l l y during the f a l l i n c r e a s e p e r i o d , t h i s might cause a subsegent d e c l i n e , f o r there would be few young t o r e p l a c e the o l d e r animals d i s a p p e a r i n g g r a d u a l l y over the winter from normal causes of m o r t a l i t y . S e l e c t i v e removal of the young would be removing the p r e - r e p r o d u c t i v e s and thus having a d i s p r o p o r t i o n a t e e f f e c t on the f u t u r e d e n s i t y of the p o p u l a t i o n . However, because the t o t a l p r e d a t i o n on the study area was estimated t o have been low (Tables 5.2, 5.3, 5.4), t h i s s e l e c t i o n , i f i t were o c c u r r i n g , would probably be of minor importance i n determining p o p u l a t i o n processes. E r r i n g t o n (1956) proposed t h a t predators do not remove i n d i v i d u a l s from prey p o p u l a t i o n s at random, but r a t h e r remove those i n d i v i d u a l s t h a t are v u l n e r a b l e because of t h e i r p o s i t i o n i n the s o c i a l s t r u c t u r e of the p o p u l a t i o n . J e n k i n s e t a l . (1964) found support f o r t h i s argument i n red grouse, where 125 n o n - t e r r i t o r i a l , dispersing birds were s e l e c t i v e l y removed by avian and mammalian predators, while t e r r i t o r i a l birds were r e l a t i v e l y safe. Periods of dispersal i n small mammals are thought to be times when predation i s p a r t i c u l a r l y important (Blair 1953, Errington 1967). Hetzgar (1967), working on Peromyscus leucopus (Hafinesgue) and Ambrose (1972), working on Microtus pennsylvanicus (Ord), have found that animals unfamiliar with the habitat were removed more often by the avian predators. In my study , of the 40 tagged animals known fo have dispersed from one grid to another, only two did so during either of the decline periods. This suggests that most of the animals disappearing during the declines were dying i n s i t u , and that the animals disappearing because of predation were k i l l e d on the grid and so were presumably f a m i l i a r with the habitat. I t may be, however, that the reason l i t t l e dispersal was observed during the declines was because of intense predation on dispersers, so that -the majority never reached another gr i d . Selective predation on members of the vole population may be related to positions of dominance, predisposing subordinate i n d i v i d u a l s to various forms of mortality. In an experimental study on the e f f e c t of s o c i a l p o sition on s u s c e p t i b i l i t y to predation i n cotton rats (Sigmodon hispidus. Say and Ord). Roberts and Wolfe (1974) found that a f e r a l cat s e l e c t i v e l y k i l l e d dominant in d i v i d u a l s , whereas a r e d - t a i l e d hawk s e l e c t i v e l y k i l l e d subordinate animals. In M. penusylvanicus, Turner and Iverson (1973) found that the heavier animals were generally dominant. In my study, i n the tagged segment that was eaten by predators, there was no selection for l i g h t e r females, 126 whereas there was selection for l i g h t e r males (5-7 g) in the f a l l and winter of 1973-74. There ware no differences i n the demographic c h a r a c t e r i s t i c s examined between the tagged segment eaten by predators and that not known to have been eaten. Therefore t h i s study provides no support f o r Errington*s hypothesis of non-randcm predation. However, the s o c i a l position of i n d i v i d u a l s eaten by predators i s unknown. A mora rigorous test of Errington's ideas would be to obtain behavioral p r o f i l e s of individuals in large enclosures i n a manner s i m i l a r to that of Krebs (1970) and Turner and Iverson (1973) and subject these animals to predation. Predation by raptors on voles may be viewed as e s s e n t i a l l y a random process, at least with regard to the tagged population. A vole moving about i n a grass-lined runway w i l l probably not detect a hawk or owl moving s w i f t l y and s i l e n t l y above i t u n t i l i t i s too l a t e to escape. Habitat f a m i l i a r i t y and the p o s s i b i l i t y for escape may play a r o l e only i f the avian predator has a s i g n i f i c a n t chance of missing the vole on the f i r s t s t r i k e and then has to chase aft e r the vole on foot. An avian predator using sound to locate prey can miss the prey i f the prey suddenly stops moving (Konishi 1973). In t h i s type of s i t u a t i o n , young, inexperienced voles may do worse than animals f a m i l i a r with the habitat. Because mammalian predators, such as f e r a l cats, a c t i v e l y pursue th e i r prey, no prey c l a s s may be immune. 127 Summary 1. The object of t h i s study was to assess the impact and s e l e c t i v i t y of avian and mammalian predators on fi. townsendii populations, e s p e c i a l l y during decline periods. 2. Vole populations were live-trapped at two week i n t e r v a l s on a study area near Vancouver, Canada. The vole population went through a severe decline in the winter and spring of 1972-73 and again i n the winter and spring of 1973-74. 3. Intensive searches were made for a l l pe l l e t s and scats from February 1973 to May 1974, both on the main study area and on a l l nearby areas. Various species of hawks and owls were the major avian predators and f e l i d cats were the major mammalian predators. 4. Of the tagged animals disappearing, less than 8% in the decline of 1972-73 and less than '2OX in the decline of 1973-74 could be accounted for by predation. I conclude that predation i s not necessary to i n i t i a t e or maintain a decline. 5. Bone measurements were ' made from autopsy animals to develop equations to predict sex and weight. In the tagged animals, selection by predators was not consistent for either sex or for any body s i z e . In the t o t a l sample of voles eaten by avian predators, the body weights were consistently lower than that occurring on the control area. In the other l i f e history c h a r a c t e r i s t i c s examined no selection was found on tagged voles. I conclude that selection was random with regard to the tagged population but may not have been with regard to the t o t a l population. 128 _ I £ _ I Q I _.. _______ 91 MMkl _ _ _ _ _ _ _ _ _ Y O U N G ^Btroduction Changes i n the survival rate of juvenile animals from one year to the next are a c h a r a c t e r i s t i c feature of population cycles. In the snowshoe hare, Green and Evans (1940) attributed a five-year decline in numbers to poor juvenile s u r v i v a l . Adult and yearling s u r v i v a l remained f a i r l y constant u n t i l the end of the decline. In microtines, juvenile survival i s always low but i t i s worst during the decline. With multiple regression analysis, Krebs (1971) and Krebs and Myers (1974) have been able to show that juvenile s u r v i v a l i s of primary importance i n determining population growth i n Microtus ochrogaster, and i s of secondary importance i n M. c a l i f o r n i c u s and M. peiynsylyanicus. In these l a t t e r species, female survival rate and percentage of l a c t a t i n g females were found to be of primary importance. What causes these poor survival rates? I t i s believed that agonistic behavior by adult voles toward juveniles may lead to either death or emigration (Chitty and Phipps, 1966). My study was designed to test the hypothesis that agonistic behavior of adults i s responsible for poor juvenile s u r v i v a l , recruitment, and growth i n M. townsendii. Two experimental designs were used. In the spring of 1974, I carried out a preliminary experiment by cropping three experimental areas of a l l but a cohort of adult animals and t h e i r young. Although many young, most of waom were probably 129 born on the areas, entered these p o p u l a t i o n s , s u r v i v a l remained poor. To t e s t i f t h i s poor s u r v i v a l was due to the presence of a d u l t animals, I removed a l l a d u l t s . In 1975 a second experimental design was s e t up. To d i f f e r e n t i a t e between the e f f e c t s of a d u l t males and females and the e f f e c t of breeding season, I set up t h r e e experimental areas: a complete removal g r i d , a male removal g r i d , and an i n t a c t g r i d . P e r i o d i c a l l y throughout the s p r i n g , summer, and f a l l I i n t r o d u c e d c o h o r t s of young onto each area, and f o l l o w e d t h e i r progress. There w i l l be two s e c t i o n s to d e a l with each of two experiments s e p a r a t e l y . The 1974 Experiment Methods The study area was l o c a t e d on Westham I s l a n d i n the F r a s e r R i v e r d e l t a , near Vancouver, B r i t i s h Columbia. The l i v e - t r a p p i n g g r i d s were l o c a t e d on a 37-ac (15 ha) p a r c e l of f l a t p a s t ureland owned by the Department of N a t i o n a l Defence. The c o n t r o l area, g r i d I, has been d e s c r i b e d by LeDuc and Krebs (1975). T h i s g r i d had been trapped c o n t i n u o u s l y s i n c e J u l y 1971. another g r i d ( g r i d G), which had a l s o been used by LeDuc and Krebs, was not trapped between summer 1973 and February 1974, but was used as another c o n t r o l from March 1974 onwards. Three experimental g r i d s were l i v e - t r a p p e d . G r i d s L and X had been trapped s i n c e e a r l y 1973. During the winter of 1974, the v o l e s on both these g r i d s were maintained at low d e n s i t i e s 130 see Section 1). In t h i s experiment, these two grids were rep l i c a t e s . A cohort of animals (25-30) was allowed to remain on these areas throughout t h i s time, and a l l animals entering them subsequently were removed. When breeding began i n 1974, the young of t h i s cohort were also allowed to remain on the grids. I assumed that a l l animals f i r s t trapped weighing less than 40 g were the progeny of thi s cohort. The t h i r d experimental area, grid Z, was f i r s t trapped on March 6, 1974. A l l animals except a cohort of 20 animals (10 males: 10 females) were removed from t h i s grid. Because of the poor s u r v i v a l of t h i s cohort, a small number of animals (6 males: 9 females) subsequently appearing on the area were allowed to remain; a l l others continued to be removed. When breeding began, the young of these animals were also allowed to stay on the grid. This trapping scheme was followed on ,.' . -- - r . r (\ - T X grids L and X u n t i l 10 June, and on grid Z u n t i l 24 June. A l l males and non-lactating fe_jnales over 40 g were then removed. Lactating females were allowed to f i n i s h nursing, then they too were removed. Because of the large amount of immigation onto the experimental grids, trapping was done at weekly i n t e r v a l s , i n an attempt to create an environment free of older animals. Only new r e c r u i t s under 40 g were allowed to remain i n the population. These r e c r u i t s , however, did grow into the adult weight class so that a small number of adults were present in the l a t t e r weeks of t h i s experiment. This experiment was terminated on August 6. The analysis i s thus divided info two periods: Spring 1974 (March-June 10) and Summer 1974 (June 131 24 - August 6). Each of the t r a p p i n g areas except g r i d L had 100 t r a p p i n g p o i n t s arranged i n a 10 by 10 p a t t e r n ( F i g . 4.1). These areas were approximately 1.7 acre (.7 ha) i n s i z e . G r i d L was an i r r e g u l a r t r a p p i n g area with 68 t r a p p i n g p o i n t s and an area of approximately . 9 acre (.38 ha) bordered on one s i d e by a drainage d i t c h and on the other by a d i r t road. Each t r a p p i n g p o i n t was 7.6 m (25 f t ) from the next. Longworth l i v e - t r a p s were placed at every s t a t i o n and b a i t e d with oats; c o t t o n bedding was prov i d e d . Traps were set i n the aft e r n o o n every second week (except as mentioned above), checked the next morning, the next a f t e r n o o n , and again on the second morning, when they were locked open. During the summer, t r a p s were set o n l y i n the evenings t o avoid m o r t a l i t y i n the t r a p s during the heat of the day. A l l v o l e s were ear-tagged, and on each c a p t u r e the f o l l o w i n g data were recorded: t r a p l o c a t i o n ; sex; s e x u a l c o n d i t i o n ( f o r males p o s i t i o n of the t e s t e s and f o r females vagina p e r f o r a t e or not; n i p p l e s s m a l l , medium, or l a r g e ; pubic symphysis c l o s e d , s l i g h t l y open, or open; pregnant o r n o t ) ; and weight. In t h i s paper, animals are c l a s s i f i e d as a d u l t {> 43 g ) , subadult (30 g to 42 g) , or j u v e n i l e (<30 g) . Because i t i s im p o s s i b l e t o sample Micrgtus p o p u l a t i o n s randomly, I r e s o r t e d to complete" enumeration of the tra p p a b l e p o p u l a t i o n by i n t e n s i v e l i v e - t r a p p i n g ( C h i t t y and Phipps 1966, Krebs 1966). This method of demographic a n a l y s i s r e q u i r e s t h a t most of the i n d i v i d u a l s i n the p o p u l a t i o n be caught. H i l b o r n et a l . (1976) have demonstrated t h a t enumeration techniques provide 132 F i g u r e 4. 1 Map of the study area on Westham I s l a n d . C o n t r o l areas were g r i d s G and I. In 1974, g r i d s L, X, and Z were experimental areas. In 1975, the Female g r i d , tha I n t a c t g r i d , and X {Removal gr i d ) were experimental areas. Exp. was a removal g r i d maintained f o r a shor t time i n 1975. Young v o l e s were obtained from Source Areas 1 and 2 i n 1975. 134 accurate estimates of population s t a t i s t i c s , i t t r a p p a b i l i t y i s at 80$ or greater, and les s so at lower t r a p p a b i l i t i e s . Trappability i n t h i s study was calculated with the methods used by these workers. In the spring, t r a p p a b i l i t y was high for a l l areas, averaging 79$ for males and 80% for females. In the summer however, t r a p p a b i l i t y was only moderate, averaging 61% for males and 64$ for females. Results Population Density The populations on the two control grids were almost i d e n t i c a l , both i n size and rate of change of numbers. The corre l a t i o n between the numbers of animals on the two areas throughout the study i s very high (r=.96, n=11) so that only grid I i s presented in Figure 4.2. From March to the end to A p r i l , the population on grid I declined at an average rate of 16$ par week, remained constant for two trapping sessions, and then incraased again u n t i l the beginning of August, with an average gain of 8$ per week. The adult population bred throughout the entire study. From the end of March to August, 89$ of the. adult males were scrota l (range 83-96$). Breeding i n females, as indicated by l a c t a t i o n , began at the end of March, remained at less than 30$ lac t a t i n g for A p r i l , and then increased to 40-60$. Over the en t i r a study, 46$ of the females caught were l a c t a t i n g (range 15-68$). 135 Figure 4.2 Minimum number aliv e on various areas on Westham Island, 1974. A horizontal l i n e i s drawn at N=60 to simplify comparisons between areas. Grid L i s 4/7 the area of the other grids and was not corrected f o r s i z e . The arrows indicate the s t a r t of the summer removal of old adults and a l l new animals above 40 g. 136 200-Control Grid I > - 40 -< ' / it i/D i / I / Fit'* |jj U D _ 9 c jp ^ ^ 100T E 8 0 + E 6 0 c x n \ /i / / i / i O / ? 2 4 0 ^ ^ 6 8 0 T 6 0 4 0 + Y 4-4 20-/ / I / 1/ -4-/ l / I , 1 / 1 / if If -'1 10- ' Spr i n g ' 'Summer' M a r c h A p r i l May June J u l y 1974 137 On t h e e x p e r i m e n t a l g r i d s , t h e number removed d u r i n g e a c h t r a p p i n g s e s s i o n i s shown by t h e v e r t i c a l l i n e s ( F i g . 4 . 2 ) . D u r i n g most o f A p r i l and May few a n i m a l s were removed f r o m g r i d s L and X. The f i r s t l a c t a t i n g f e m a l e s w e r e c a u q h t two weeks e a r l i e r on g r i d L a n d s i x weeks e a r l i e r on g r i d X t h a n on t h e c o n t r o l g r i d s ; t h u s most o f t h e a n i m a l s e n t e r i n g t h e s e p o p u l a t i o n s a t t h i s t i m e were a n i m a l s b e l o w 40 g and were assumed t o have been b o r n on t h e a r e a . The a v e r a g e number o f a n i m a l s r e m a i n i n g a f t e r e a c h r e m o v a l was 30 a n i m a l s on g r i d L a n d 43 on g r i d X c o m p a r e d w i t h 6 3 on t h e c o n t r o l . B e c a u s e g r i d L was a p p r o x i m a t e l y h a l f a s l a r g e a s t h e o t h e r g r i d s , i t had a d e n s i t y s i m i l a r t o t h a t on c o n t r o l g r i d I . The number o f a d u l t s a l l o w e d t o r e m a i n cn g r i d X was a l w a y s l o w e r t h a n t h a t on t h e c o n t r o l . D u r i n g a t r a p p i n g s e s s i o n , g r i d X a v e r a g e d 13 a d u l t m a l e s and 12 a d u l t f e m a l e s c o m p a r e d w i t h 22 m a l e s and 25 f e m a l e s on c o n t r o l g r i d I . G r i d L a v e r a g e d 7 m a l e s and 13 f e m a l e s p e r t r a p s e s s i o n , w h i c h was a s i m i l a r d e n s i t y t o t h a t on t h e c o n t r o l when c o r r e c t e d f o r g r i d s i z e . On g r i d Z, a f t e r t h e f i r s t two t r a p p i n g s e s s i o n s , i n w h i c h a l l a n i m a l s e x c e p t f o r a c o h o r t o f 20 were r e m o v e d , few a n i m a l s e n t e r e d t h i s p o p u l a t i o n . B e c a u s e o f t h e p o o r s u r v i v a l r a t e s o f t h i s c o h o r t , a number o f a n i m a l s o v e r 40 g were a l l o w e d t o e n t e r t h e p o p u l a t i o n . L a c t a t i n g f e m a l e s were n o t c a u g h t on g r i d Z u n t i l a month a f t e r t h e y were c a u g h t on t h e c o n t r o l g r i d s and r e c r u i t m e n t r e m a i n e d l o w . F r o m mid-May o n w a r d s , an i n c r e a s i n g number o f a n i m a l s g r e a t e r t h a n 40 g e n t e r e d a l l t h r e e e x p e r i m e n t a l p o p u l a t i o n s . A f t e r t h e r e m o v a l o f a l l a n i m a l s g r e a t e r t h a n 40 g f r o m g r i d s L a n d X on J u n e 10 a n d f r o m G r i d Z o n J u n e 24, l a r g e 138 numbers of animals immigrated to these areas. Up to 50 heavy body weight animals entered these areas per week, making the pre-removal d e n s i t i e s on the experimental g r i d s s i m i l a r to those on the c o n t r o l s . A l t o g e t h e r 317 animals were removed from g r i d 1 from March to August, 379 from g r i d X, and 316 from g r i d Z. The average number of animals allowed to remain a f t e r the June removal, was 52 on g r i d I , 56 on g r i d X, and 34 on g r i d 2 compared with 111 animals on the c o n t r o l . The d e n s i t i e s on the experimental g r i d s were thus g e n e r a l l y no lower than those on the c o n t r o l g r i d s except during March and p a r t of A p r i l . During the r e s t of the study, the pre-removal d e n s i t i e s on a l l the areas except g r i d 2 became s i m i l a r , even though l a r g e numbers of animals ware removed. G r i d Z remained s l i g h t l y lower. S u r v i v a l In The Trappable P o p u l a t i o n Loss from a t r a p p i n g area can occur e i t h e r through death or e m i g r a t i o n ; the minimum s u r v i v a l r a t e s presented i n Table 4.1 do not d i f f e r e n t i a t e the two. These s u r v i v a l r a t e s are summed over each p e r i o d f o r aach c l a s s , an i n d i v i d u a l vole being t a l l i e d each time i t i s trapped. To compare these r a t e s among c l a s s e s and g r i d s , I have used chi-sguare a n a l y s i s on samples which are not completely independent. Though these comparisons are not s t r i c t l y v a l i d , they i n d i c a t e the degree of d i f f e r e n c e . F i s h e r ' s exact t e s t was used f o r s m a l l sample s i z e . I n t h i s a n a l y s i s , I am i n t e r e s t e d i n the e f f e c t of the experimental treatments on s u r v i v a l , and t h e r e f o r e w i l l only be making comparisons between the c o n t r o l g r i d s and the exparimehtal g r i d s . 139 Table 4.1. Minimum s u r v i v a l r a t e s per 14 days f o r M. tow n s e n d i i on two c o n t r o l g r i d s (G and I) and on three experimental g r i d s . Sample s i z e s i n parentheses. G r i d Adult Male Subadult J u v e n i l e Female Adult Subadult J u v e n i l e Spring G .73 (128) .55 (29) .57 (7) . 83 (180) .86 (74) . 69 (13) I .72 (152) ,69 (13) .83 (6) . 82 (179) .68 (40) .60 (5) L .76 (51) . 30 (20) 1.00 (8) . 94 (91) .86 (21) . 75 (16) X .82 (91) .44 (27) . 57 (37) . 83 (83) .97 (35) .78 (36) Z .71 (41) .67 (8) .56 (18) . 87 (94) .91 (22) .94 (17) Summer G .83 (138) I .85 (132) L .69 (50) X .95 (41) Z .62 (10) .44 (16) .67 (3) .79 (169) .47 (32) . 00 (1) . 00 (3) 1.00 (1) . 82 (180) .76 (25) .20 (5) ,60 (54) . 85 (18) .91 (74) . 78 (89) . 93 (25) .62 (34) .84 (11) .90 (128) .91 (77) . 90 (27) .78 (16) . 56 (8) 1,00 (26) .94 (26) 1 .00 (4) 140 In comparing the two c o n t r o l g r i d s withxn weight c l a s s e s w i t h i n p e r i o d s , there was no s i g n i f i c a n t d i f f e r e n c e between I and G i n e i t h e r the s p r i n g or the summer except f o r the subadult females on G which had higher minimum s u r v i v a l r a t e s i n summer than those on I. Therefore i n drawing comparisons among the c o n t r o l g r i d s and the experimental g r i d s , the s u r v i v a l r a t e s f o r each c l a s s on the two c o n t r o l g r i d s were.pooled except f o r the one d i f f e r e n c e . In the s p r i n g , the only s i g n i f i c a n t d i f f e r e n c e was between the subadult females on X which s u r v i v e d b e t t e r than those on I . The subadult males from L and X tended t o s u r v i v e worse than those from the c o n t r o l g r i d s although the d i f f e r e n c e was not s i g n i f i c a n t (.05>P>.10). In the summer, there were pronounced d i f f e r e n c e s i n the s u r v i v a l of the subadults and j u v e n i l e s on the experimental g r i d s from those on the c o n t r o l s . The cases i n which the animals on the experimental g r i d s s u r v i v e d s i g n i f i c a n t l y b e t t e r than those on the c o n t r o l s are the f o l l o w i n g : subadult males on L, X, and Z; ad u l t females on L, X, and Z; j u v e n i l e females on L, X, and Z. Subadult females on 1, X, and Z su r v i v e d b e t t e r than the subad u l t s on G but not than those on I. Only a d u l t males on L s u r v i v e d s i g n i f i c a n t l y worse than those on the c o n t r o l g r i d s . I conclude t h a t i n g e n e r a l , t h e r e was no d i f f e r e n c e i n s u r v i v a l of the c o n t r o l s and experiments i n the s p r i n g , when r e s i d e n t a d u l t s animals were allowed to remain on the experimental g r i d s . In the summer, when a l l r e s i d e n t a d u l t s were i n i t i a l l y removed and a l l a d u l t immigrants were removed from the experimental a r e a s , s u r v i v a l was g e n e r a l l y b e t t e r on the experimental g r i d s than on the c o n t r o l s . 141 E a r l y J u v e n i l e S u r v i v a l To get an index of the s u r v i v a l of the very young animals born on an area, I used the f o l l o w i n g index (Krebs 1966 and l a t e r papers): t o t a l number of new v o l e s < 40 g i n week t index= • • t o t a l number of females l a c t a t i n g i n t-4 Table 4.2 i n d i c a t e s that on the c o n t r o l g r i d s t h e number of young per l a c t a t i n g female was always l e s s than one, and recruitment only about a t h i r d as much i n summer as i n s p r i n g . In the s p r i n g , on two of the experimentals (L and X), the index was 3-5 times h i g h e r than on the c o n t r o l s . Grid Z had a s i m i l a r index to t h a t of the c o n t r o l s . In the summer, a l l the experimental g r i d s had an index t h a t was 7-13 times higher than that on the c o n t r o l g r i d s . In g e n e r a l t h e r e f o r e , r e c r u i t m e n t of young i n t o the t r a p p a b l e p o p u l a t i o n on the experimental g r i d s was much b e t t e r than cn the c o n t r o l g r i d s . D i s p e r s a l During the present study, 22 animals are known to have moved from one g r i d to another. Table 4.3 i n d i c a t e s t h a t d i s p e r s a l was r e s t r i c t e d almost e n t i r e l y to males, and t h a t i t accounted f o r a s i m i l a r amount of l o s s from a l l the g r i d s . Of the 11 males d i s p e r s i n g from the experimental g r i d s , 10 were not i n breeding c o n d i t i o n on l a s t capture on the experimental g r i d and were judged as breeding cn the new g r i d . Of the 9 males d i s p e r s i n g from the c o n t r o l s , 6 were not i n breeding c o n d i t i o n Table 4.2. Number of young trapped per l a c t a t i o n in 1974. Number of l a c t a t i n g females at t-4 in parentheses. Control Control Grid G Grid I Spring .60 .82 (53) (63) Summer .16 .30 (101) (96) Grid Grid Grid L X Z 2. 46 2.85 . 72 (26) (34) (29) 2.10 1.81 1.81 (38) (42) (16) 143 T a b l e 4.3. P e r c e n t o f known l o s s f r o m t h e g r i d s o w i n g t o d i s p e r s a l . T o t a l number o f a n i m a l s d i s a p p e a r i n g i s g i v e n i n p a r e n t h e s e s . C o n t r o l C o n t r o l G r i d G r i d G r i d G r i d G G r i d I L X z T o t a l M a l e s 7 (87) 4 (81) 9 (57) 9 (57) 4 (28) 6 (310) F e m a l e s 2 (88) 0 (92) 0 (36) 0 (37) 0 (30) 1 (283) 144 on l a s t capture on the c o n t r o l g r i d and breeding on the new area. The average weight c f d i s p e r s i n g males on l a s t c a pture was 39 g (n=20). The upper weight l i m i t of 40 g used f o r the index of e a r l y j u v e n i l e s u r v i v a l and f o r judging whether animals had been born on the e x p e r i m e n t a l areas or not may t h e r e f o r e be s l i g h t l y high. I t i s p o s s i b l e t h a t i n some cases r e c r u i t s born elsewhere were c l a s s i f i e d as having been born on the g r i d . However, r e s u l t s obtained by D. C h i t t y on an area from which a l l animals were removed every t r a p p i n g s e s s i o n from March 6 - June 26, 1974 suggest t h a t 40 g i s a reasonable upper l i m i t . Of a t o t a l of 113 animals caught on C h i t t y ' s removal a r e a , 93 were a d u l t , 19 were subadult, and only 1 was j u v e n i l e . T h i s suggests t h a t o n l y a s m a l l p r o p o r t i o n of d i s p e r s e r s are s u b a d u l t s or j u v e n i l e s . Sexual Maturity I now i n q u i r e whether the age a t sexual m a t u r i t y may be r e l a t e d to the number of a d u l t r e s i d e n t s . Since the age of v o l e s i s not known, weight i s used as an index of age. The weight a t sexual maturity f o r l i v e - t r a p p e d v o l e s was c a l c u l a t e d by means of the technigue of L e s l i e e t a l . (1945). Maturity i n males was judged by the presence of s c r o t a l t e s t e s . Maturity i n females was judged by the presence of a p e r f o r a t e v a g i n a l o r i f i c e , or of medium to l a r g e n i p p l e s i z e , or of an open pubic symphysis, or of a l i t t e r i n the t r a p . In males, the median weight a t sexual maturity d i d not 145 d i f f e r between the control and experimental grids, nor did i t d i f f e r between the spring and summer periods (Table 4,4). In females, the weight at sexual maturity was approximately the same on both control areas during both periods. On the experimental areas, there was also l i t t l e difference, either among areas or between periods. However, females on the experimental areas had body weights at sexual maturity that were consistently 3-20 g lower than those on the control qrids. Growth In previous studies on Microtus (Krebs 1966, Krebs et a l . 1969, LeDuc and Krebs 1975), growth rates were analyzed i n terms of l i n e a r regressions of average growth rates in percent per day on body weight. However, in pooling a l l the growth rate data available from M. townsendii from Hestham Island for f i v e years, I found that the relationship between growth and weight was generally c u r v i l i n e a r , with the larger animals showing l i t t l e or no growth. The growth rate data were separated according to sex and month of the year, and second degree polynomial regressions were calculated. Only mice caught at two-week or four-week i n t e r v a l s were used. To compare the growth rates of animals among grids i n t h i s study, I calculated the following r e l a t i v e growth rate index: r e l a t i v e growth = rate ^actual body weight at t + 1 \ ^•predicted body weight at t+1 100 -100 The predicted body weight was determined from the polynomial regressions described above. The r e l a t i v e growth rate of an 146 Table 4.4. Median body weight (g) at sexual maturity, along with 95% confidence l i m i t s . Control Control Grid Grid Grid Grid G Grid I I X z Spring Male 40 42 42 41 40 (37-44) (38-47) (40-44) (39-43) (39-42) Female 36 44 24 25 33 (31-43) (40-49) (17-28) (20-32) (21-54) Summer Male 41 45 42 40 41 (38-44) (40-51) (41-43) (38-41) (38-43) Female 39 37 26 24 24 (34-45) (33-42) (24-28) (21-28) (15-39) 147 animal in a given month of the year i s thus r e l a t i v e to the growth of a l l the animals, expressed as a percent. Animals having an index above zero were growing faster than average and animals having an index less than zero were growing slower than average. To compare among the r e l a t i v e growth rates of the animals on the grids, I performed an analysis of variance. Only male data have been analyzed because of the confounding e f f e c t s of pregnancy in females. The analysis was r e s t r i c t e d to animals l e s s than or equal to 50 g, since the most rapid growth occurs below t h i s weight and above this animals tend to flu c t u a t e in weight and grow only s l i g h t l y . Because of the small sample size f o r the controls i n the summer (n=3 cn grid G and n=6 on grid I ) , the summer period was eliminated from the analysis. Looking only at the spring period, I found no s i g n i f i c a n t difference among grids (F=.58, df=4). Therefore creation of low adult density areas had no influence on the growth of young in the spring of 1974. _ The presence of generally reduced numoers of resident adults on the experimental areas i n the spring of 1974 was associated with the recruitment of large numbers of young but su r v i v a l of these remained poor. The absence of resident adults on the experimental areas in the summer of 1974 was again associated with the recruitment of large numbers of young, but sur v i v a l of these young was consistently better than of those on the control grids. Adult residents are therefore one of the factors responsible for poor su r v i v a l of young i n M. townsendii. 148 Are a d u l t males and females e q u a l l y important i n a f f e c t i n g s u r v i v a l and how i s t h i s r e l a t e d t o breeding c o n d i t i o n of these a d u l t s ; To answer these q u e s t i o n s , the f o l l o w i n g experiment was performed i n 1975. The .1975 Experiment Methods The study area was the same as i n 1974, but only g r i d I was used as the c o n t r o l . Three experimental g r i d s were s e t up e a r l y i n 1975. G r i d X, which had been used by M. T a i t t as a t o t a l removal g r i d s i n c e January 1975, was taken over on May 21, and w i l l be r e f e r r e d to as the Removal g r i d . A l l animals, except those I i n t r o d u c e d , were removed at f i r s t c a p t u r e from t h i s g r i d . The Female g r i d and the I n t a c t g r i d were both s e t up on March 27 and l i v e - t r a p p e d u n t i l May 21 to get base l i n e e s t i m a t e s of d e n s i t y . From then on a l l a d u l t males were removed from the Female g r i d with the exception of those animals I i n t r o d u c e d onto the g r i d . I allowed a s m a l l number of j u v e n i l e and subadult male r e c r u i t s to remain on the area i n order examine t h e i r s u r v i v a l and growth, but these were removed as soon as they became mature. The I n t a c t g r i d was simply l i v e - t r a p p e d throughout the study i n a manner s i m i l a r to that on the c o n t r o l . At six-week i n t e r v a l s throughout the s p r i n g , summer, and f a l l , I i n t r o d u c e d cohorts of young onto the Removal g r i d , the 149 Female grid, and the Intact g r i d . A l l young remaining were removed at the end of a six-week period, when the next cohort was introduced. A t o t a l of four cohorts was introduced (Table 4.8). The young used for these introductions were obtained from four sources. Prior to the introduction of the f i r s t cohort, a series of outdoor breeding pens was set up, into which pregnant females were introduced and allowed to raise t h e i r young. These young then made up part of the f i r s t cohort. The breeding pens were abandoned thereafter because of the time i t took to maintain them and because the young were tunneling out. Young were also obtained from two areas maintained s p e c i f i c a l l y f or t h i s purpose. These were trapped p e r i o d i c a l l y with p i t f a l l traps (see Section 5) and l i v e - t r a p s . Untagged young from the Removal gr i d were also used to make up the cohorts introduced to the other two grids. For the l a s t cohort, young males removed from the Female g r i d also sere used to make up the cohort introduced to the other grids. For the f i r s t two cohorts, only young below 40 g were used. In the l a s t two cohorts, because few young below that weight were caught, the upper weight l i m i t was increased to 45 g. The trapping technigue was similar to that used i n 1974. Trapping was done every second week, except for the Removal g r i d , which was also trapped in the weeks cohorts were to be introduced, inorder to obtain young. The Female g r i d and the Removal grid were both standard 10 X 10 grids s i m i l a r to the control . Tha Intact grid was an i r r e g u l a r l y shaped grid of 106 trapping points bounded by a road, a wate r - f i l l e d d i t c h , and a 150 grass f i e l d . I t was .96 as l a r g e as the other g r i d s . Because of the high d e n s i t i e s , the c o n t r o l g r i d and the I n t a c t g r i d were each given about 50 ex t r a t r a p s , with two t r a p s at every other t r a p s t a t i o n . Only 100 t r a p s were present on the Female g r i d f o r the f i r s t two months of the study, a f t e r which 50 more were added. The Removal g r i d had only one t r a p per s t a t i o n . The data were d i v i d e d i n t o three breeding p e r i o d s . In the s p r i n g ( A p r i l 14 - June 25) most of the a d u l t s were bre e d i n g ; i n the summer (July 4 - September 17) very few were br e e d i n g ; i n the f a l l (September 26 - November 12) some a d u l t s s t a r t e d b reeding a g a i n . The animals on a l l the areas were only moderately t r a p p a b l e during most of the study. On the c o n t r o l , the t r a p p a b i l i t y i n the s p r i n g averaged 68%, i n the summer 51% and i n the f a l l 57%. The other g r i d s showed s i m i l a r t r a p p a b i l i t i e s . Part of the reason f o r the poor t r a p p a b i l i t y d uring 1975 was our i n a b i l i t y t o provide an excess of t r a p s over the number of animals. However, low t r a p p a b i l i t y was not p u r e l y r e l a t e d t o too few t r a p s being a v a i l a b l e . In the summer, when d e n s i t i e s were lower cn the Female g r i d , t r a p p a b i l i t y of females on t h i s g r i d was s i m i l a r to t h a t on the other two areas. T h i s suggests t h a t other f a c t o r s a s s o c i a t e d with the season a l s o played a r o l e , e s p e c i a l l y i n summer. 151 Results Population Density The trappable populations on westham Island were at very high densities throughout 1975 (Fig. 4.3). An average of 307 animals was known to be on the control grid throughout th i s study compared with only 132 i n 1974 for the same period. The number of animals in the trappable population on the control r e l a t i v e l y constant, as did the number on the Intact and Female grids. Throughout the study an average of 60 fewer animals per trapping session where knowntobe present on the Intact grid than on the control g r i d . During the period before the removal of males, the Female grid appeared to have fewer animals than the control grid, which was probably due to the presence of fewer traps. After the removal of males, t h i s grid averaged 172 fewer animals per trapping session than the control. The attempt to maintain the Removal grid at low numbers was only partly successful: i t s pre-removal densities averaged 78 in d i v i d u a l s , which was about a guarter the number on the control, while the post-removal densities averaged only 20. Immigration onto t h i s area was enormous. Throughout t h i s study, 984 animals were removed, of which 41% were removed during the summer non-breeding season. On the Female grid 433 animals were removed, almost a l l of them males, of which 41% again were removed in the summer non-breeding period. 152 Figure 4,3 Minimum number a l i v e on various area on Westham Island, 1975. The arrows indicate when cohorts of young were introduced onto the experimental areas. The summer non-breeding season i s shaded. 153 154 Survival In The Trappable Population The survival rates presented i n Table 4.5 were calculated i n the same way as those i n Part I. Since very few juveniles were caught, only the adult and subadult classes w i l l be discussed. The su r v i v a l of the cohorts of young introduced onto the grids was not included i n the analysis and w i l l be given l a t e r . In comparison among grids within classes, there were three s i g n i f i c a n t differences: i n spring the adult females on the control gr i d survived better than those on the Female g r i d ; i n the f a l l the adult females on the Intact and Female grids survived better than those on the control; and the subadult females on the Female grid survived better than those on the Intact grid. I conclude that even with the lower de n s i t i e s on the Female g r i d , the su r v i v a l of the females was generally similar to that on the control and Intact grids, suggesting that whatever was causing loss was affecting a l l females i n a similar manner ir r e s p e c t i v e of the presence or absence of males. The sexes generally had very s i m i l a r s u r v i v a l rates, although males were s l i g h t l y lower. The correlation between the su r v i v a l rates of males and females on the control grid was moderate (r=.60, n=14, P<.05). The c o r r e l a t i o n of s u r v i v a l rates in females between the various grids was considerably higher (control versus Intact r=.73, P<.005; control versus Female r=.75, P<.005; Intact versus Female r=.82, P<.0G1). 155 Table 4.5. Minimum survival rates per 14 days for IS* townsendii i n 1975. Sample sizes i n parentheses. Male Female Grid Adult Subadult Adult Subadult Spring control .79 (314) .82 (11) .88 (646) .60 (25) Intact . 80 (279) .46 (35) . 86 (426) .69 (35) Female . 75 (125) .48 (17) . 83 (503) .69 (3 8) Summer Control .68 (452) .65 (120) .69 (533) .73 (186) Intact .62 (416) .68 (79) .69 (438) .65 (144) Female .40 (5) .76 (29) .69 (425) .63 (107) F a l l Control .70 (315) .78 (23) .73 (250) .77 (88) Intact .74 (262) .68 (25.) .85 (191) .76 (74) Female .50 (2) .75 (4) .85 (227) .94 (33) 156 Reproduction And Early Juvenile Survival Differences i n reproductive i n t e n s i t y may influence the success of the introduced cohorts. As a test for homogeneity among grids within periods, chi-sguare analysis was employed for adult animals only. The presence of sc r o t a l testes i n males and of l a c t a t i o n in females are the best external c h a r a c t e r i s t i c s available for indicating breeding status. Each animal was t a l l i e d at every capture within a period. In the spring there were no s i g n i f i c a n t differences among any of the grids f o r either males or females (Table 4.6). In the summer, there was s i g n i f i c a n t heterogeneity i n reproductive condition i n both males and females. A s i g n i f i c a n t l y higher percentage of males on the Removal grid were i n breeding condition than on the Intact grid, which i n turn was s i g n i f i c a n t l y higher than either the Female grid or the control g r i d . This trend was evident i n females, where s i g n i f i c a n t l y fewer females were lactating on the control grid than on the other three grids. In the f a l l male reproduction was sim i l a r among grids, but a s i g n i f i c a n t l y higher percentage of females was l a c t a t i n g on the Removal grid than on the Intact grid. Female reproduction on the Intact grid was greater than on either the Female g r i d or the control g r i d . Three conclusions can be reached from these data. The in t e n s i t y of breeding on the Female grid was not affected by the selective removal of males when compared with that on the control grid. A higher proportion of the animals on the Removal grid tended to be i n breeding condition i n summer and f a l l than 157 Table 4.6. Mean proportion of s c o t a l males and l a c t a t i n g females. Sample sizes i n parentheses. Males Scrotal Females Lactating Spring Control Intact Female Removal* .91 , 93 ,89 ,75 (377) (364) (257) (114) .53 .50 .51 .43 (762) (539) (592) (125) Summer Control Intact Female Removal .18 . 26 .18 .36 (509) (447) (165) (22 3) 02 -06 ,04 ,08 (482) (399) (410) (158) F a l l Control Intact Female Removal .56 .57 .59 .60 (410) (367) (109) (79) . 15 .30 .19 .42 (426) (360) (390) (93) •Included only period from May 21 on. 158 did those on the other grids. The animals on the Intact gr i d , which can be considered e s s e n t i a l l y another control g r i d except for the addition of small numbers of young, showed generally higher breeding i n t e n s i t i e s in the summer and f a l l than control gri d I. This indicates that even on r e l a t i v e l y unmanipulated adjacent areas there i s some v a r i a b i l i t y . To get an indicati o n of the sur v i v a l of young, I used the same index as i n the 1974 experiment. Table 4.7 shows that during spring and summer very few young animals entered any of the populations. Throughout the entire study, l e s s than one young par lac t a t i n g female entered the trappable population on the cont r o l . In the f a l l , over four times as many young per l a c t a t i n g female entered the trappable population on the Intact and Female grids as entered the control population. Since the Intact population was e s s e n t i a l l y a control population except for the introduction of a small number of young, this shows that there i s a great deal of v a r i a b i l i t y between areas. In conclusion, recruitment of young onto the female g r i d was no better than that on the Intact gr i d , where density was 1.85 times higher whereas i t was generally higher than that on the control g r i d , where density was 2,2 times higher. Introduction Of Young Table 4.8 summarizes the number of young introduced in each cohort. During each series of introductions, I t r i e d to obtain as many young as possible below the upper weight l i m i t , but could only introduce about half as many young in the f i r s t and 159 Table 4.7. Number of young per lactating female in 1975. Number of l a c t a t i n g females at t-4 i n parentheses. Control Intact female Grid I Grid Grid Spring .20 .30 .44 (270) (154) (158) Summer .81 .45 .45 (139) (132) (89) F a l l .33 1.48 1.49 (27) (25) (35) 160 fourth, cohorts as i n the second and t h i r d cohorts. Altogether, 262 animals (123 males: 139 females) were introduced. The young from the cohorts introduced onto the Intact grid and the Female grid survived poorly in comparison with those introduced onto the removal grid. A two-way analysis of variance was done on duration of l i f e to show differences between grids and cohorts. Duration of l i f e on the grid as detected by trapping was calculated for each i n d i v i d u a l up t o the end of the six-week i n t e r v a l , when a l l remaining young were removed. The sexes were pooled as there was no s i g n i f i c a n t difference between them (F=1.33, df 1, P=.25). There was no in t e r a c t i o n effect between grids and cohorts. A s i g n i f i c a n t difference between grids (F=42.64, df 2, P<.001) was found, with the young on the Removal grid having a mean expectation of l i f e 3-6 times greater than the young on the Intact or Female grids. There was no s i g n i f i c a n t difference in duration of l i f e between young on the Intact and Female grids (Duncan's Multiple Range Test). Figure 4.4 shows that of the young introduced onto the Removal g r i d , over 50% were s t i l l present at the end of six weeks, while on the two other grids less than 20% were s t i l l present, with over 70% disappearing within the f i r s t week. There was also a s i g n i f i c a n t difference between cohorts (F=3.45, df 3, P=.02), with the f i r s t cohort having a s i g n i f i c a n t l y shorter duration of l i f e than the t h i r d and fourth cohorts. The second cohort was not different from the ether cohorts. This indicates that i n the periods when breeding was l e s s intense, young survived better. The poor s u r v i v a l of young on the Female grid indicates that the females have an important influence on juvenile 161 Table 4.8. Summary of the number of young introduced onto the experimental areas. The mean body weight (± 1 SE) i s also presented. Cohort Date Intact Grid Female Grid Removal Grid Number Released male : female male :female male : female 1 Hay 23 7 : 6 22.8 ± 2.3 9 : 8 24.1 ± 2 . 0 9 : 8 20.7 ± 1.9 J u l y 4 12 : 18 32.3 ± .9 15 : 16 34.2 ± .7 13 : 17 28.0 ± 1.5 August 15 13 : 13 40.1 ± .7 13 : 13 39.4 ± .8 13 : 13 38.8 ± 1.0 4 September 7 : 9 26 38.2 ± 2.0 6 : 9 34.2 ± 2.5 6 : 9 37.1 ± 2.2 Total 39 : 46 43 : 46 41 : 47 Total average 34.3 ± .9 weight 33.8 ± .8 32.1 ± 1.0 162 Figure 4.4 Survivorship curves for the pooled data from four cohorts on three d i f f e r e n t areas. Both sexes of young are combined. 0 1 2 3 4 5 Weeks 164 s u r v i v a l . To get a clearer understanding of the r e l a t i o n s h i p between the duration of l i f e of young and variables thought to influence their s u r v i v a l , I carried out multiple regression analysis using the mean duration of l i f e per cohort as the dependent variable and the following six measures as independent variables: mean number of lactating females i n the six-week period; number of la c t a t i n g females present, i n the trapping period immediately after cohort introduction; mean number of adult females; mean number of s c r o t a l males; number of scrota l males present i n the trapping period immediately a f t e r cohort introduction; and mean number of adult males. The l a t t e r three variables on the Pemale grid and a l l the variables on the Removal grid were obtained by assuming that the animals removed at two-week int e r v a l s were present on the grid f o r af least a portion of the previous two weeks and could therefore p o t e n t i a l l y influence survival of young. The number caught i n each of these classes every trapping session was summed; i t was then divided by the number of trapping periods i n the six-week i n t e r v a l . The only s i g n i f i c a n t variable of the six considered was the mean number of adult females which was negatively correlated with the mean duration of l i f e of the young (r= -.84, n=12, P<.001). Figure 4.5 shows this relationship. The young which disappeared from the grids onto which they were introduced could have either died or moved to another area. Of the 262 young introduced, 13% were recaptured elsewhere (Table 4.9). An indication that some of these young may have been homing comes from these data. Of the 13 animals appearing on the Removal g r i d , 11 had been transferred from t h i s grid i n 165 Figure 4.5 Relationship between the mean density of adult females and the mean duration cf l i f e of four cohorts of young on each of three areas (r=-.84). 166 </> 4 o c o L. D Q H Removal A Female Intact 0 20 40 60 80 100 Mean Density of Females 167 Table 4,9. Movement of young from the area of introduction to other areas. Introduction Cohort Number appearing on: Grid Number Control Removal Experimental Source Grid Grid Grid* Area 1 Intact Female Removal 1 2 4 2 3 1 3 4 Total males:females 1 male 1 female 2 males 3 males 4 females 5 females 3 males 1 male 3 females 1 female 1 male 4 females 1 male 1:0 5 : 8 6:10 1 male 1 female 2 females 1:3 *See text for description cf experimental area. 1 6 8 the f i r s t p l a c e . Of the four animals appearing on source area 1 (which was separated from the Removal by an enclosure g r i d ) , a l l had been t r a n s f e r r e d from t h i s g r i d . However, there was probably a l s o a l a r g e element of chance, with d i s t a n c e s to other g r i d s , d e n s i t i e s , and p h y s i c a l b a r r i e r s to movement to c e r t a i n g r i d s p r e d i s p o s i n g these wandering young to capture on other areas. Of the 16 animals caught on the experimental g r i d (an area maintained by D. C h i t t y as mainly a removal area and operated mainly during the p e r i o d t h a t c o h o r t 2 was i n t r o d u c e d ) , 12 had f i r s t been t r a n s f e r r e d from the Removal g r i d while the other f o u r had been t r a n s f e r r e d from the source areas near the Removal g r i d . Another i n d i c a t i o n that wandering was at l e a s t p a r t l y random i s t h a t no young were ever caught r e t u r n i n g from the Female g r i d to a home area, although the d i s t a n c e was not much g r e a t e r than f o r the young t h a t d i d r e t u r n home. However the young from the Female g r i d would have had t o c r o s s a w a t e r - f i l l e d d i t c h . I conclude from these data t h a t the d i r e c t i o n of d i s p e r s a l from the i n t r o d u c t i o n g r i d was l a r g e l y random, that both sexes d i s p e r s e d , and t h a t d i s p e r s a l occurred mainly from the I n t a c t and Female g r i d s on t o vacant areas. Sexual Maturity I now i n q u i r e whether the median weight at s e x u a l maturity was d i f f e r e n t among the c o n t r o l and the three experimental g r i d s d u r i n g the t h r e e p e r i o d s . I n i t i a l l y I wanted to compare only the weight at s e x u a l maturity f o r the i n t r o d u c e d c o h o r t s , but because few young were re c a p t u r e d on two o f the areas, a l l 169 animals were used. The start of the spring period was from the date that the f i r s t cohort was introduced. Table 4.10 shows that there are no differences between the control and Intact grids in either males or females, except i n the f a l l when females on the Intact grid matured at a s l i g h t l y lower body weight. The females on the Female grid were not di f f e r e n t from those on the control or Intact grids, but the males on t h i s grid generally matured 1-7 g lower. On the Removal g r i d , males had consistently lower weights at sexual maturity than those on the control and Intact grids but were not greatly d i f f e r e n t from the males on the Female grid. In the summer and f a l l , the females on the Removal gr i d had s i g n i f i c a n t l y lower weights at sexual maturity than did those on the other three areas. I conclude that the removal of males from the Female grid and from the Removal grid lowered the weight at sexual maturity of the males entering those areas. The females on the Female grid were not influenced by the removal of males, while the absence of females from the Removal area resulted i n lower weights at sexual maturity. These results suggest that reaching maturity may be delayed by the presence of members of the same sex. Growth Growth rates may have been influenced by the experimental treatments used. Growth rates were analyzed in the same manner as before. The analysis was carried out only on males less than or equal to 50 g. Since only the young on the Removal grid were available, the star t of the spring period began with the f i r s t 170 Table 4.10, Median body weight at sexual maturity, along with 95% confidence l i m i t s . Control Intact Female fiemoval Grid Grid Grid Grid Spring Males 52 51 47 46 (50-54) (49-54) (46-49) (44-48) Females 41 42 45 41 (39-44) (39-45) (43-47) (38-44) Summer Males 68 67 61 57 (66-70) (65-69) (58-63) (55-58) Females 64 63 67 47 (61-66) (60-66) (63-72) (45-48) F a l l Males 60 59 58 55 (58-61) (57-60) (55-60) (52-58) Females 56 49 51 42 (53-59) (46-51) (47-54) (39-46) 171 cohort introduction. ft two-way analysis of covariance was performed (grids and season). The interaction e f f e c t was not s i g n i f icant. There was a s i g n i f i c a n t difference in r e l a t i v e growth rates among grids (F=3.91, df 3, P=.009). The control grid had s i g n i f i c a n t l y lower growth rates than either the Female grid or the Removal grid (Duncan's Multiple Range Test). However, the males on the Intact grid were not different from those on the control grid or on the Female and Removal grids. The ranking of growth rates was i n the seguence: Removal > Female > Intact > con t r o l . Because the Intact grid was essentialy another control except for the introduction cf young, and growth on t h i s grid did not d i f f e r from that on the two other experimental grids, the r e s u l t s are somewhat eguivocal. There was s i g n i f i c a n t difference i n r e l a t i v e growth rates among periods (F=39.08, df 2 ,P<.001). Each of the periods was s i g n i f i c a n t l y d i f f e r e n t from the others, with the difference being in the following seguence: f a l l > summer > spring. In spring, almost no young animals entered either the control or Intact grids, so that the sample size i n t h i s period was small. Table 4.11 therefore presents only the l a s t two periods. Because there was no s i g n i f i c a n t regression between body weight and growth rate, the observed means are presented. Both periods indicate higher r e l a t i v e growth rates in males on the Female and Removal grids. This suggests that adult males on the control and Intact grids were i n h i b i t i n g growth of young males. 172 Table 4,11. Observed mean r e l a t i v e growth rate for males (< 50 g), i n percent (± 1 SE). Calculation of mean r e l a t i v e growth rates i s described i n the text. Sample sizes i n parentheses. Control Intact Female Removal Grid Grid Grid Grid Summer -2.33 ± .75 -1. 28 ± . 90 7.54 ± 2.34 3. 72 ± 1.68 (170) (129) (25) (41) F a l l 4.85 ± 1.42 3.18 ± 1.60 11.20 ± 3.02 14.62 ± 8.90 ( 5 5 ) (59) (21) (13) 173 Discussion The importance of changes in survival of young to population growth i n microtines i s readily apparent. Females have large l i t t e r s (3-5), have a short gestation period (21 days), and are capable of postpartum mating (for a review of reproduction i n microtines, see Hasler 1975). The causes of the changes in survival of young throughout microtine cycles are poorly understood, but appear to be an instrumental driving force (Krebs and Myers 1974). In t h i s study, survival of young was improved by the absence of adults. During the spring of 1974, when adult numbers were maintained at low l e v e l s on the experimental areas, recruitment of young was increased, but survival was not. This ef f e c t was es p e c i a l l y pronounced i n subadult males, where surv i v a l was generally even lower than on the control areas. Removal of a l l adults from the experimental areas i n the summer of 1974 produced an increased recruitment of young and a substantial improvement in the survival of young in both sexes. In 1975, young again survived well i n the absence of a l l adults, but survived egually poorly i n the presence or adult females only, or in the presence of both sexes. This indicates that adult females are in some way reducing survival of young. Adult males were thought to be c r i t i c a l in eithe r k i l l i n g or driving out young of both species and sexes in M. aqrestis and Clethrionojnys qlareolus i n a study by Chitty and Phipps (1966). Satts (1970) set out s p e c i f i c a l l y to improve s u r v i v a l of juveniles by removing adult overwintering males during the 174 breading season; he was u n s u c c e s s f u l . Experiments i n v o l v i n g the removal of both sexes have a l s o been c a r r i e d out. Krebs (1966) and Smyth (1968) removed of o l d e r animals t o t r y to keep p o p u l a t i o n s i n a phase of continuous i n c r e a s e . Neither was s u c c e s s f u l , owing mainly to very l a r g e immigration r a t e s , though j u v e n i l e and subadult s u r v i v a l d i d improve i n Krebs's experiment, but not i n Smyth's, I b e l i e v e the l a t t e r experiment had no e f f e c t on s u r v i v a l because of the long i n t e r v a l between t r a p p i n g ( 6 weeks versus 2 weeks i n the experiment of Krebs), so t h a t the l a r g e number of animals moving onto the area could have had time to s e t t l e and set up home ranges. In the present study, I a l s o had a l a r g e amount of immigration onto the removal area, both i n 1974 and i n 1975, but s u r v i v a l of young remained high. The f a i l u r e of both Watts (1970) and myself (1975) to improve s u r v i v a l of young by removing j u s t males, and the success of Krebs (1966) and myself (1974 and 1975) i n improving s u r v i v a l of young by removing a d u l t s of both sexes, i n d i c a t e s t h a t the importance of females i n i n f l u e n c i n g s u r v i v a l of young may be a f a i r l y g e neral m i c r o t i n e t r a i t . A s i m i l a r r e s u l t was found by R a d f i e l d et a l . 1976 who found that the index of e a r l y j u v e n i l e s u r v i v a l (used i n the present study i n Tables 2.2 and 2.7) was i n v e r s e l y p r o p o r t i o n a l to average female d e n s i t y . Aggression by a d u l t s has been shown to r e g u l a t e j u v e n i l e r e c r u i t m e n t and s u r v i v a l i n Peromyscus maniculatus ( S a d l e i r 1965, Healey 1967). Healey found that a d u l t males appeared t o be much more a g g r e s s i v e towards j u v e n i l e s than were a d u l t females, and s e r i o u s l y a f f e c t e d t h e i r s u r v i v a l . Adult females had no e f f e c t on s u r v i v a l . A l l of the b e h a v i o r a l s t u d i e s on Microtus 175 (Tamura 1966, Krebs 1970, Turner and Iverson 1973) have looked only at male - male interactions. Seme in d i r e c t evidence on the ef f e c t s of adults cn juvenile s u r v i v a l and recruitment comes from the work of Anderson (1975). She found that i n small outdoor breeding enclosures, the presence of neither the father nor of any other male had any influence on the s u r v i v a l of the l i t t e r . However, she found that maternal behavior could account for as much as 50% of the variance among mothers i n mean number of offspring recruited per l i t t e r . This suggests that females may not only be detrimental to the young cf other mothers but also to their own. Exactly when t h i s detrimental behavior occurs i s unknown. If may be associated with the end of weaning and forced disperal of the young. This would seem reasonable i f the mother were again expecting another l i t t e r . Behavioral differences i n the responses of adults to young may underly the differences i n population dynamics i n Peromyscus and Microtus. Adult male behavior may be most important in Peromyscus and adult female behavior i n Microtus. Juvenile losses tend to vary with the phase of the microtine cycle, with the poorest s u r v i v a l occurring i n the decline (for a review see Krebs and Myers 1974). If behavior of adult females i s important, does i t vary with the phase of the cycle? At present, there i s no evidence on this point, although i n males, animals i n the peak phase were found to be most aggressive (Krebs 1970). If aggression affects the survival of juveniles, one might expect to see severe wounding in the young and changes in wounding rates with the phase of the cycle. Most of the studies 176 that have examined wounding have found that i t i s r e s t r i c t e d mainly to adult reproductive males (Christian 1971, B a t z l i and P i t e l k a 1971, Lidicker 1973). In the brown lemming, Krebs (1964) found some wounding of sexually immature young, mainly i n males, which was higher i n peak and decline phases than i n the increase phase. Recent work by Rose and Gaines (19 76) and Hose (pers. comm.) indicate trends similar to those reported by the other workers, but a much higher incidence of wounding i n females and in young. Rose found that up to 56% of the females from the lowest weight class i n M. jennsylyanicus were wounded compared with 35% i n males. In M. gchrogaster. considerably less wounding was found i n the young. Wounding i n males was associated with the attainment of maturity, as i t was i n females i n M, ochrogaster. However i n females in M. £9nasylyanicus, there was no relationship between reproductive condition and wounding. In both species, wounding was generally most severe i n declining populations. Wounding of . the young of both sexes therefore occurs in some species although there appears to be a great deal of v a r i a b i l i t y among species. Wounding i s a crude index of aggression, with the actual effect of aggression being much more subtle i n seme species (Sadleir 1965). The very poor su r v i v a l rates on the control area i n the summer and f a l l of 1S75 indicate that over h a l f of the population, or about 160 animals, were disappearing every month, yet Fig. 4.4 shows that densities remained r e l a t i v e l y constant. This would reguire an enormous recruitment to offset the losses. However, Table 4.7 indicates that very few recru i t s entered the population as young. Frcm data obtained from p i t f a l l traps (see 177 Section 5) which were trapped concurrently with the live-trapping program cn control grid I and caught large numbers of young, I found that 315? of the juveniles and 5 83 of the subadults were not trapped in l i v e - t r a p s u n t i l they were adults. The rate of sexual maturation appears to be inversely related to population density (Kalela 1957, Krebs 1964, Keller and Krebs 1970). Females appear to be much more p l a s t i c i n the weight at which they mature than do males. In Table 4.4, where densities ware lower on grids X and Z in 1974 than on the controls, median weight at sexual maturity was not affected i n males, whereas in females i t was s i g n i f i c a n t l y lower than on the control grids. On grid L, which was approximately 1/2 the size of the control grids, the density was si m i l a r to that on the controls, but the females matured at body weights s i m i l a r to those on the other experimental grids. It i s possible that the s o c i a l disruption of large numbers of immigrants i s s u f f i c i e n t to a f f e c t tha weight at puberty. In 1975 the females on the Removal grid had weights at sexual maturity that, were 4-17 g lower than on the control or Intact grids compared with only 4-10 g lowar i n males. In 1975, females on the Female grid did not have lower weights at sexual maturity than on the control, while females on the Removal grid did. Males had lower weights at sexual maturity i n the absence of adult male residents on the Female grid and the Removal gr i d . Similar results have been found by Redfield et a l . (1976). These data indicate that adult males or females may i n h i b i t or retard young from reaching sexual maturity. Evidence has been found by other workers that increased 178 d i s p e r s a l i s a s s o c i a t e d with the attainment of s e x u a l maturity (Howard 1960, Smith 1968, Myers and Krebs 1971). In 1974, when d e n s i t i e s were moderate, d i s p e r s a l of tagged animals was r e s t r i c t e d l a r g e l y t o males at puberty. In 1975, when d e n s i t i e s were high, d i s p e r s a l of tagged animals occurred i n both sexes, only h a l f of which was a s s o c i a t e d with reaching puberty. These r e s u l t s suggest t h a t d i s p e r s a l i n 1975 was not as c l o s e l y a s s o c i a t e d with the attainment of s e x u a l maturity, and may have been more c l o s e l y r e l a t e d to the pressure of high d e n s i t y . The sex r a t i o of animals d i s p e r s i n g onto vacant areas has g e n e r a l l y been found to favor males (Myers and Krebs 1971, Krebs e t a l . 1976). T h i s was a l s o the case f o r animals d i s p e r s i n g i n t o reduced p o p u l a t i o n s i n 1974. In the summer of 1974 26-38% more males entered them than the c o n t r o l p o p u l a t i o n s . In c o n t r a s t , d u r i n g the peak year of 1975, the sex r a t i o of animals e n t e r i n g the removal g r i d favored females d u r i n g the two breeding p e r i o d s , but not i n the summer non-breeding p e r i o d . T h i s suggests t h a t females may have been under g r e a t e r pressure to move from occupied areas than i n the lower d e n s i t y year of 1974. T h i s i s supported by evidence from the c o n t r o l g r i d , where of the 14 animals known to have d i s p e r s e d d u r i n g the breedinq p e r i o d s , 9 were female. In the non-breeding p e r i o d , no females were known to have l e f t the c o n t r o l g r i d . On the Female g r i d during the two breeding p e r i o d s i n 1975, higher p r o p o r t i o n s of the a d u l t s e n t e r i n g the p o p u l a t i o n were male than on the other t h r e e areas. T h i s could be caused by a d u l t males a c t i v e l y seeking out females and being a b l e to move onto the Female g r i d unopposed by r e s i d e n t males. Ia the s p r i n g 179 of 1975, 52 adult males entered the Female g r i d compared with 35 on the control and 39 on the Intact grid. In the f a l l , 97 adult males entered the Female grid compared with 7 3 on the control and 77 on the Intact grid. These results suggest that at least i n males spacing behavior i s sex-specific. These r e s u l t s d i f f e r from those of Bedfield et a l . (1276) who found that fewer males were recruited to a male-deficient grid than to a control grid or to a female-deficient grid. In conclusion, more emphasis should be placed on the study of female behavior, especially as i t relates fo the s u r v i v a l and recruitment of juveniles. The experiments of Sadleir (1965) and Healey (1967) provide models to follow, although the large immigration rates observed in Microtus populations may make th e i r type of f i e l d experiments d i f f i c u l t . Summary 1. The object of t h i s study was to test the hypothesis that adult animals are one of the factors responsible for poor juvenile s u r v i v a l , recruitment, and growth. 2. In the spring of 1974, three experimental areas were maintained at reduced densities of adult residents compared to the two control areas. Recruitment of young was generally better on the experimental areas but s u r v i v a l of young was not improved. 3. In the summer of 19 74, a l l adult residents were i n i t i a l l y removed and a l l new adult r e c r u i t s caught subsequently were also removed cn the experimental areas. Both recruitment 180 and s u r v i v a l of young l a s improved. The weight at sexual maturity was reduced only i n females in both seasons on the experimental areas. 4. In 1975, cohorts of young were introduced at six week in t e r v a l s throughout the summer and f a l l onto an unmanipulated area, a male removal area, and a t o t a l removal area. Young survived best on the t o t a l removal area but equally poorly on the other two areas. Young survived better when breeding was less intense. Growth i n young males tended to be reduced i n the presence of adult males. Weight at sexual maturity tended to be higher in the presence of adults of the same sex, 5. I conclude that adult females reduce the s u r v i v a l of young and suggest that further study of female - young interactions i s necessary to f i n d out the mechanisms involved. 181 SECTION 5. THE EELIABILITY OP ENUJEE1TION TRAPPING AN jD SUBVIVAL 2 1 1 S U N G l S l jcoduction One of the assumptions i m p l i c i t i n most c a p t u r e - r e c a p t u r e s t u d i e s of s m a l l mammals i s that the animals not c a t c h a b l e i n the c o h o r t s at r i s k of capture make up only a small p r o p o r t i o n of the t o t a l p o p u l a t i o n of a c t i v e animals. This assumption i s important both i n the p o p u l a t i o n e s t i m a t i o n techniques r e q u i r i n q e q u a l c a t c h a b i l i t y of a l l members i n the p o p u l a t i o n ( J o l l y 1965) and i n the t o t a l p o p u l a t i o n enumeration technique (Krebs 1966, H i l b o r n et a l . 1976). P r o b a b i l i t y of capture i s not c o n s t a n t i n the whole p o p u l a t i o n (Young e t a l . 1952, Crowcroft and J e f f e r s 1961, Getz 1961, Tanaka 1963, Krebs 1966). T h i s may be due to e i t h e r u n f a m i l i a r i t y with t r a p s ( C h i t t y and Kempson 1949) or to dominants d e t e r r i n g subordinates from e n t e r i n g t r a p s (Kikkawa 1964, Andrzejewski et a l . 1967, Summerlin and S o l f e 1973). I t i s p o s s i b l e to t e s t the assumption of equal c a t c h a b i l i t y only by t r a p p i n g with two or more d i f f e r e n t methods and comparing the p o p u l a t i o n s captured by each type of t r a p . T h i s was the f i r s t reason f o r doing the present study. The second reason was to measure the s u r v i v a l and d i s p e r s a l r a t e s of s m a l l j u v e n i l e s immediately a f t e r weaning. J u v e n i l e s are p a r t i c u l a r l y d i f f i c u l t to capture by standard l i v e - t r a p p i n g techniques. However, changes i n the r a t e s of j u v e n i l e s u r v i v a l are of great importance i n determining the r a t e of p o p u l a t i o n 182 growth in a number of Microtus species (Krebs 1971, Krebs and Myers 1974). If juveniles avoid traps when the vole population i s at higher densities, but enter traps at lower densities, t h i s w i l l give the f a l s e appearance of low juvenile s u r v i v a l rate at higher densities. A technigue allowing one to catch large numbers of juveniles would also enable one to pin-point where mortality was occurring, whether before the end of weaning, or between weaning and the entry of the animals into the trappable population. If young disappeared before weaning t h i s might suggest improper maternal care or some maternal deficiency (Southwick 1955). If young disappeared after weaning t h i s might indicate deleterious s o c i a l interactions with adults (Sadleir 1965, Healey 1967). The technigue I used to investigate juvenile s u r v i v a l and the e f f i c i e n c y of t o t a l enumeration trapping involved the use of p i t f a l l traps concurrently with Longworth l i v e - t r a p s . Other workers (Kott 1965, Andrzejewski and Hajska 1972) have had success i n trapping t i e younger segment of the population by the use of p i t f a l l s which were continuously open. I modified the p i t f a l l trap so that i t could be prebaited when not i n use. 183 Methods The study area was located on Westham Island i n the Fraser River delta near Vancouver. The live-trapping grid used was Grid I, which served as the control i n the experiment of LeDuc and Krebs (1975). This grid had been trapped continuously since July 1971. My own study lasted from May 6 to September 26, 1975. The vegetation on the area was dominated by a grass community c o n s i s i t i n g of Agrostis alba^ JL___£__2_ rejgens_. Poa _ a t e n s i s x -nd Holcus lanatus. The grid was bordered on one side by a ditch f i l l e d with water for most of the summer and on the other three sides by grassland. The trapping area had 100 trapping points arranged in a 10 by 10 pattern. Each trapping point was 7.6 m (25ft) from the next. Because of very high densities of voles, 150 Longworth traps, instead of the usual 100 traps, were used throughout the study, with two traps at every other trap point. Traps were baited with oats; cotton s t u f f i n g was provided. In the summer, li v e - t r a p s were set every second week on Monday afternoon, checked Tuesday morning, locked shut Tuesday afternoon because of the heat, set again Tuesday evening, and checked Wednesday morning, when they were then locked open. In the spring and f a l l , when heat was not a problem, traps were l e f t set Tuesday morning for an afternoon check. During the f i r s t month of the study , I experimented with d i f f e r e n t types of p i t f a l l s u n t i l I found one which appeared to catch animals e f f i c i e n t l y and from which none could escape (Fig. 5.1). The p i t f a l l consisted of two five-pound coffee cans, one 184 Figure 5.1 Diagram of the p i t f a l l trap. (See text for a description.) protective board ] >3-8cm p^rebaiting p^latform 186 stacked on the other. The end of the upper can was cut out and served as a platform to seal the p i t f a l l trap when not i n use. This platform was attached to a short wooden rod fastened to a 25 cm x 30 cm board protecting the p i t f a l l from r a i n and heat. This board stood about 3 cm above the top of the p i t f a l l , and allowed voles to s i t on the platform underneath the board and eat the prebait. On the days that trapping was carried out, the platform was pulled out of the p i t f a l l , the p i t f a l l was cleaned and supplied with oats and cotton, and the board was inverted and placed above the p i t f a l l . One p i t f a l l was placed at every trap point at the intersection of two or three vole runways. These traps were set Wednesday afternoon, checked Thursday morning, Thursday afternoon, and Friday morning. The p i t f a l l was then closed to voles by lowering the platform i n t o i t again. Heat never became a problem. P i t f a l l s were trapped every week u n t i l mid-August, when I switched to a two-week i n t e r v a l because the number of new animals entering the populations had tapered of f . In late August, heavy rains flooded a number of the p i t f a l l s , and i n September brought the study to an end. A l l voles were ear-tagged, and, on capture, each animal's tag number, trap l o c a t i o n , sex, weight, and sexual condition were recorded. On recapture i n the same trapping session, only tag number and location were noted. In t h i s paper animals are c l a s s i f i e d as being adult (> 43 g) , subadult (30 g to 42 g) , or juvenile {< 30 g) . 187 figsuits f g i E f i a b J i i£l I w i l l be using the t o t a l enumeration technique of C h i t t y and Phipps (1966) to draw comparisons between v o l e s caught i n l i v e - t r a p s and those caught i n p i t f a l l s . This type of demographic a n a l y s i s r e q u i r e s t h a t I catch most of the i n d i v i d u a l s i n the p o p u l a t i o n . The estimate of t r a p p a b i l i t y I used was the f o l l o w i n g f o r a l l N i n d i v i d u a l s : hi r~- /number of a c t u a l c a p t u r e s f o r a given animal \ \7\ ^number of p o s s i b l e captures f o r that animal ' T r a p p a b i l i t y = H where N i s the number of v o l e s caught more than two times. In the number of a c t u a l and p o s s i b l e c a ptures, the f i r s t and l a s t times of capture are excluded from the c a l c u l a t i o n , s i n c e an animal i s n e c e s s a r i l y caught at these times. Table 5.1 g i v e s the t r a p p a b i l i t y e stimates f o r f i v e summers from l i v e - t r a p s and f o r one summer from p i t f a l l t r a p s . During the f i r s t f our years, d e n s i t i e s i n the summer were r e l a t i v e l y low, and t r a p p a b i l i t y was always over 10% except i n females i n 1974. Males c o n s i s t e n t l y had higher t r a p p a b i l i t i e s than females i n these y e a r s . In 1975 d e n s i t i e s reached very high l e v e l s . The t r a p p a b i l i t y e stimates f o r each of the two types of t r a p s were c a l c u l a t e d independently of i n f o r m a t i o n . from capture i n the other. Thus, animals known to be present from capture i n 188 p i t f a l l s but not i n l i v e - t r a p s were not included in the estimate of t r a p p a b i l i t y of animals captured in l i v e - t r a p s . This also holds for the converse. Trappability in l i v e - t r a p s was 15-4 0% le s s than i t had been in previous years, when densities were lower. Females had s l i g h t l y higher trappabities than males in 1975 i n contrast to previous years. P i t f a l l traps, since they are a multiple-capture trap, could t h e o r e t i c a l l y catch a l l animals on the area. However, t h e i r success was only half that of the l i v e - t r a p s , which indicates that voles avoided repeated capture i n p i t f a l l s . Part of the reason for t h i s lower success may be that the animals had learned to avoid capture during the 11 consecutive weeks of p i t f a l l trapping at the s t a r t of the study. Capture i n p i t f a l l s may have also been more traumatic than i n l i v e - t r a p s because of the f a i l into the p i t f a l l and the confinement with other voles (up to 7) caught at the same time. I concluded that, at these high densities, i t i s not possible to census the vole populations adequately with l i v e - t r a p s , and that p i t f a l l s are even worse. __________ D e n s i t y In 1975, Microtus townsendii numbers were extremely high compared with those in previous years. The minimum number a l i v e in the l i v e - t r a p population at the end of A p r i l , 1975 was 272 animals compared with 41 i n A p r i l , 1974, 46 i n A p r i l , 1973, and 88 i n A p r i l , 1972. Throughout the study the number of voles caught on the area by l i v e - t r a p s remained between 250 and 370 189 Table 5.1. T r a p p a b i l i t y of 8. townsendii on g r i d I d u r i n g f i v e summers. Longworth l i v e - t r a p s were used e x c l u s i v e l y i n the f i r s t f o u r summers and both Lcngworths and p i t f a l l t r a p s were used i n 1975. Sample s i z e s i n parentheses. Trap Type Year Hale; Females longworth 1971 l i v e - t r a p s 1972 1973 1974 1975 78 80 91 74 52 (6) (72) (61) (72) (244) 76 77 82 65 54 (15) (118) (78) (109) (3 30) P i t f a l l s 1975 26 (386) 29 (4 54) 190 animals. F i g u r e 5.2 shows the minimum number of animals a l i v e from those known to have entered l i v e - t r a p s , from those known to have entered p i t f a l l s , and from those known to have ent e r e d one or both t r a p types, although breeding s t a r t e d i n February, only a s m a l l p r o p o r t i o n of the a d u l t females (5%) was l a c t a t i n g d u r i n g the f i r s t 8 weeks. In A p r i l , t h i s p r o p o r t i o n i n c r e a s e d markedly, so t h a t 20-65% of the ad u l t females caught were l a c t a t i n g i n each t r a p p i n g s e s s i o n . The ma j o r i t y of the young born would, t h e r e f o r e , have been weaned when p i t f a l l t r a p p i n g was i n pro g r e s s . The p o p u l a t i o n enumerated by p i t f a l l s i n c r e a s e d enormously i n the f i r s t p art of the study, owing to the capture of both v o l e s tagged from previous capture i n l i v e - t r a p s and untagged v o l e s . From the beginning o f the study onto J u l y 1, t h i s p o p u l a t i o n i n c r e a s e d at an average r a t e of about 3336 per week compared with 3% per week i n c r e a s e i n the l i v e - t r a p p o p u l a t i o n , so that the maximum number reached i n J u l y i n p i t f a l l s was 685 compared with 319 i n l i v e - t r a p s . The r a t e of i n c r e a s e f o r the t o t a l p o p u l a t i o n a c c o r d i n g to both t r a p p i n g methods averaged about 17% per week. T h i s i n c r e a s e stopped when r e p r o d u c t i o n tapered o f f i n the beginning of J u l y . The p i t f a l l p o p u l a t i o n then went i n t o a steady d e c l i n e averaging 10% per week compared with 0.1% d e c l i n e per week i n the l i v e - t r a p p o p u l a t i o n . The maximum number of animals known to be a l i v e on the t r a p p i n g area at one time was 860 v o l e s . T h i s was 2.7 times the number i n d i c a t e d by l i v e - t r a p s and 1.3 times the number i n d i c a t e d by p i t f a l l s . The s t o c h a s t i c model of J o l l y (1965) g i v e s an estimate of 191 Figure 5.2 Population numbers of M. townsendii on g r i d I. Minimum number al i v e ( MNA ) i s given"for the population known to have entered l i v e - t r a p s , for those known to have entered p i t f a l l s , and for those known to have entered one or both trap types. M I N I M U M N U M B E R A L I V E Z6l 193 1140 ± 75(2 SE) animals present on the area when the combined trap data were used for the trapping session when the maximum number of animals were known to be a l i v e on the area. This compares with an estimate of 1163 ± 127 animals wnen only the p i t f a l l data are used and 432 ± 48 animals when only the l i v e - t r a p data are used. This suggests that the p i t f a l l trapping method may more accurately sample actual population numbers. This model assumes randomness of capture of a l l animals at r i s k of capture. By the time the rains came in September and I could no longer trap with p i t f a l l s , these traps were enumerating a much smaller proportion of the animals known to be on the area. In the penultimate trapping session, p i t f a l l s were enumerating only 53% of the animals known to be on the area while l i v e - t r a p s were now enumerating 64%. Even after trapping with p i t f a l l s had stopped, the l i v e - t r a p s were s t i l l f a i l i n g to catch a l l the tagged animals ,as those caught previously only i n p i t f a l l s took a number of weeks to enter the l i v e - t r a p population. Survival Changes in s u r v i v a l rates are important causes of population fluctuations i n microtines (Krebs and Myers 1974). In c a l c u l a t i n g these rates one assumes that the s u r v i v a l observed i n the trappable population applies to the entire population of comparable age. These rates result from deaths on the grid and emigration from i t . Chi-sguare analysis has been applied to samples which are not completely independent, with i n d i v i d u a l 194 animals having the p o s s i b i l i t y of occurring several times i n a given class. The results should therefore be viewed as an indica t i o n of the degree of difference between sets of data. Figure 5.3 shows the mean minimum sur v i v a l rates for the trappable population of males and females caught i n l i v e - t r a p s and for those caught in p i t f a l l s . Poor survival i s a r b i t a r i l y defined to be any rate below .707 per two weeks ( half of the population disappearing per four weeks). Both males and females i n the l i v e - t r a p population maintained good survival rates u n t i l the beginning of July. During the rest of the summer both males and females had poor s u r v i v a l rates. This coincided with the sta r t of i n f e s t a t i o n s of b o t f l i e s (Guterebra sp.) and of the grey f l e s h f l y (Wohlfahrtia v i g i l ) . Both of these were probably responsible for part of t h i s poor s u r v i v a l . Male s u r v i v a l rates in l i v e - t r a p s showed a moderate cor r e l a t i o n with female s u r v i v a l rates (r=.56, N=9). Over the entire study, females survived s i g n i f i c a n t l y better than males (P<.001, Table 5.2). The population trapped i n p i t f a l l s had consistently poor s u r v i v a l rates throughout the majority of the study. Minimum su r v i v a l rates of voles i n p i t f a l l s w i l l necessarily be lower than in l i v e - t r a p s because of the lower t r a p p a b i l i t y i n p i t f a l l s and therefore the minimum survival rates of animals caught i n the two trap types are not s t r i c t l y comparable. Correlation between the survival rates of males and females i n p i t f a l l s was good {r=.73, N-14). Females again survived s i g n i f i c a n t l y better than males (P<.01, Table 5.2). Survival was p a r t i c u l a r l y poor in the f i r s t part of July i n both males and females. The cause for t h i s i s unknown, since both types of p a r a s i t i c f l i e s were only 195 Figure .5.3 Mean minim-urn survival rate per 14 days for male and female M. townsendii caught in l i v e - t r a p s and i n p i t f a l l s on grid I. The horizontal l i n e i s of a survival rate of 0.7 07, below which half the population disappears every 4 weeks. 196 1.00 T M A L E S Pitfall Traps LU < M A Y J U N E J U L Y A U G U S T S E P F E M A L E S 2 .60 Z 4 0 .20 P/Y/d/^ Traps 0 M A Y J U N E J U L Y 1975 A U G U S T S E P 197 Table 5.2. Minimum survival rates per 14 day period for 13 • l£wnsendii on grid I i n two types of traps. Sample sizes in parentheses. Group liv e - t r a p s Males Females P i t f a l l s Males Females Adults .69 (713) .76 (960) .60 (843) .70 (933) Subadults .66 (146) .71 (269) .61 (480) .60 (933) Juveniles .50 (8) .64 (11) .48 (223) .55 (279) Total . 68 (867) .75 (1240) . 59 (1546) .66 (1868) 198 j u s t appearing, and r e p r o d u c t i o n was t a p e r i n g o f f with the probable r e d u c t i o n i n a g g r e s s i o n a s s o c i a t e d with b r e e d i n g . The p e r i o d s of extremely poor s u r v i v a l i n the p i t f a l l p o p u l a t i o n (mid-July) were not p a r a l l e l e d i n the l i v e - t r a p p o p u l a t i o n . In males, where the d i f f e r e n c e between the minimum s u r v i v a l r a t e between l i v e - t r a p s and p i t f a l l s was most pronounced, t h i s d i f f e r e n c e was not r e s t r i c t e d only to the young. In t h i s p e r i o d , a d u l t males i n p i t f a l l s had a s u r v i v a l r a t a of .56 per two weeks compared with a s u r v i v a l r a t e of .73 i n l i v e - t r a p s . T h i s suggests t h a t there may have been a r e a l d i f f e r e n c e between the animals caught i n tha two t r a p types which was not simply r e l a t e d to d i f f e r e n c e s i n t r a p p a b i l i t y . There was no c o r r e l a t i o n between the s u r v i v a l r a t e s of males i n l i v e - t r a p s and those i n p i t f a l l s (r=.20, N=9) . However, c o r r e l a t i o n between s u r v i v a l r a t e s of females caught i n p i t f a l l s and those caught i n l i v e - t r a p s was good (r=.70, N=9). I now i n g u i r e d whether t h e r e were d i f f e r e n c e s i n s u r v i v a l among tha waight c l a s s e s w i t h i n the two t r a p types (Table 5.2). There was no s i g n i f i c a n t d i f f e r e n c e i n mean s u r v i v a l r a t e s between a d u l t and subadult c l a s s e s w i t h i n e i t h e r sex f o r animals caught i n l i v e - t r a p s . In p i t f a l l s j u v e n i l e males had s i g n i f i c a n t l y lower s u r v i v a l r a t e s than a d u l t or subadult males. J u v e n i l e and subadult females have s i g n i f i c a n t l y lower s u r v i v a l r a t e s than a d u l t females. A number of the very young j u v e n i l e s , which were not yet weaned, d i d d i e i n the p i t f a l l s , so t hat capture i n p i t f a l l s may have depressed s u r v i v a l i n t h i s group somewhat. I have no evidence that s u r v i v a l o f post-waanlings was d e t r i m e n t a l l y a f f e c t e d by capture i n p i t f a l l s , though i t may 199 have been. The survival rate of voles between b i r t h and weaning i s p a r t i c u l a r l y d i f f i c u l t to ca l c u l a t e , because of the long i n t e r v a l between weaning and capture i n l i v e - t r a p s . The index that has been used i n other Hicrotus population studies (Krebs 1966, and l a t e r papers) i s the following: number of new voles less than 40 g at t number of l a c t a t i n g females at t - 4 For t h i s p a r t i c u l a r population, the index for l i v e - t r a p s only was very low (.34), because many of the young born on the area did not enter the l i v e - t r a p population u n t i l they were adults. The index for both technigues combined was much higher (1.47). Therefore changes i n t h i s index from year to year i n l i v e - t r a p s may not necessarily indicate that s u r v i v a l i s d i f f e r e n t , merely that t r a p p a b i l i t y i s d i f f e r e n t . Since I was able to catch large numbers of the young soon after they l e f t the nest, I made the following calculations to get a more accurate picture of survival between b i r t h and weaning and between weaning and recruitment into the traps. The minimum number of females l a c t a t i n g during the breeding period was known (510). A female was assumed to be l a c t a t i n g separate l i t t e r s i f i t was caught l a c t a t i n g at least two weeks apart. This i s probably an overestimate because a female may be nursing the same l i t t e r i n two consecutive weeks. L i t t e r size was approximately f i v e {281 pregnant females collected i n 1973 and 1974 had a mean l i t t e r size of 5.05). The average weight at 200 f i r s t capture of both males and females (<4Q q) was c a l c u l a t e d to be 27 g. This weight was converted to an age by the use of growth r a t e equations c a l c u l a t e d f o r each month of t h e year and f o r each sex on a l l data obtained on M. townsendii from Westham I s l a n d . This weight was b a c k - c a l c u l a t e d to 20 g, a t which the animal was assumed to be f o u r weeks o l d . With t h i s method, a 27 g male or female caught i n e i t h e r May or June, when the m a j o r i t y of breeding occurred, was c a l c u l a t e d to be s i x weeks o l d . T h i s may be an overestimate by up to a week i f the young reach the 20 g weight e a r l i e r . However, t h i s w i l l not e f f e c t the g e n e r a l c o n c l u s i o n s . Using t h i s average age at f i r s t c apture, I c a l c u l a t e d three p o s s i b l e combinations of s u r v i v a l r a t e s that c o u l d e x p l a i n the r e l a t i o n s h i p between the number of young born and tha number t h a t e v e n t u a l l y entered the trappable p o p u l a t i o n (Figure 5.4). The minimum s u r v i v a l r a t e of a l a r g e sample of j u v e n i l e animals was known from the t r a p p i n g data to be about 0.52 per two weeks (Tabla 5.2). In curve A, I assumed t h a t t h i s was the s u r v i v a l r a t a o f a l l post-weanlings, i n which case the pre-weanlings' s u r v i v a l r a t e would be very high (0.86). Curve A probably approximates the t r u e s i t u a t i o n most c l o s e l y , with most of the l o s s e s o c c u r r i n g j u s t a f t e r weaning. Curve B assumes constant s u r v i v a l from b i r t h to recruitment. T h i s constant s u r v i v a l i s not l i k e l y to be tha case because i t i m p l i e s a b e t t e r s u r v i v a l r a t e than t h a t observed among j u v e n i l e s ; and f o r the same reason curve C, which assumes an even worse s u r v i v a l before weaning, i s a l s o u n l i k e l y . I conclude t h a t s u r v i v a l was high before weaning and dropped d r a s t i c a l l y immediately a f t e r 201 ure 5.4 Survivorship curves for young voles between b i r t h and recruitment. The number born (2550) was estimated from the number of l a c t a t i n g females; the number of re c r u i t s was known. Three combinations of survival rates per 14 days are presented to account for a loss of 2/3 of the young from birth to average age at recruitment (42 days) : A. High survival to weaning B. Uniform su r v i v a l rate to recruitment C. Low survival rate to weaning 202 203 weaning. Body Weight Differences Animals subject to capture by liv e - t r a p s and by p i t f a l l s d i f f e r in body weight. Figure 5.5 shows the corresponding weight d i s t r i b u t i o n s of males caught in p i t f a l l s and i n l i v e - t r a p s . From May to early July, large numbers of very small voles were caught in p i t f a l l s , while l i v e - t r a p s caught only the heavier animals. P i t f a l l s during t h i s time also caught fewer heavy voles than did l i v e - t r a p s . When breeding tapered off i n early July, the weight d i s t r i b u t i o n of the animals caught in both types of traps gradually became simi l a r . The mean body weight of both males and females was about 10 g lower for those f i r s t caught i n p i t f a l l s than i t was f o r those f i r s t caught in l i v e - t r a p s (Table 5.3). Females i n both p i t f a l l s and l i v e - t r a p s were about 5 g l i g h t e r than males at time of f i r s t capture. These results suggest that there i s a considerable time-lag between f i r s t capture in these two trap types. How long does i t take for an animal f i r s t caught i n a p i t f a l l to enter a l i v e - t r a p ? Figure 5.6 shows that juvenile and subadult males tend to enter l i v e - t r a p s sooner than do juvenile and subadult females. However, adult males take s l i g h t l y longer than adult females to enter traps. Juveniles took 3 weeks longer to enter l i v e - t r a p s than did adults. Subadult females also took 3 weeks longer to enter l i v e - t r a p s than did adult females. There i s a general trend in d i c a t i n g that the heavier an animal, tha sooner 204 Figure 5.5 Weight d i s t r i b u t i o n of sales caught in p i t f a l l s and in l i v e - t r a p s . Blacked i n histograms indicate voles caught in p i t f a l l s . I 1 1 1 1 1 1 1 ^ o o o o o O C O C D ^ <M SWVcJO '1 HO IBM 206 Table 5.3. Mean body weight (g ± 1SE) a t time of f i r s t capture i n l i v e - t r a p s and i n p i t f a l l s . Sample s i z e i n parentheses. Trap Type Males Females L i v e - t r a p 47.9 ± .6 43. 3 ± .6 (333) (299) P i t f a l l s i 37.4 ± .6 32.7 ± .6 (599) (540) ^Excluding animals already tagged when p i t f a l l - t r a p p i n g began. 207 ure 5.6 Mean time between f i r s t capture i n p i t f a l l s and subseguent capture i n l i v e - t r a p s . V e r t i c a l l i m i t s i n d i c a t e 95% confidence i n t e r v a l s . 11 T 10 9 8 7 + 0 4 Juveniles • Ma les O Females Subadults Adu l ts 209 i t e n t e r s the l i v e - t r a p p o p u l a t i o n . J u v e n i l e s and subadults g e n e r a l l y must a t t a i n l a r g e r body weights before e n t e r i n g the l i v e - t r a p p o p u l a t i o n . Of the 132 j u v e n i l e s f i r s t caught i n p i t f a l l s and l a t e r caught i n l i v e - t r a p s , 1% entered l i v e - t r a p s while s t i l l j u v e n i l e s ; 621 waited u n t i l they were s u b a d u l t s ; and 31$ waited u n t i l they were a d u l t s . Of the 225 subad u l t s f i r s t caught i n p i t f a l l s and l a t e r caught i n l i v e - t r a p s , 45$ entered l i v e - t r a p s while s t i l l s ubadults; 55$ waited u n t i l they were a d u l t s . Young voles have the a b i l i t y to t r i p the t r e a d l e mechanism i f they enter a l i v e - t r a p , so t h a t t h e i r l a c k of capture at a r e l a t i v e l y e a r l y age i s not purely the r e s u l t of a mechanical b a r r i e r . P i t f a l l - t r a p p e d Voles Over 1100 v o l e s were f i r s t caught i n p i t f a l l s . I now i n g u i r e how many of these v o l e s subseguently entered l i v e - t r a p s and how they were d i s t r i b u t e d by weight c l a s s e s . Taule 5.4 shows th a t over h a l f the animals f i r s t caught i n p i t f a l l s were never r e c a p t u r e d , t h a t j u v e n i l e s were the l e a s t l i k e l y to be so rec a p t u r e d , and a d u l t s the most. Thus, the l a r g e r a vole a t f i r s t c apture, the higher the p r o b a b i l i t y of i t s e n t e r i n g the l i v e - t r a p p o p u l a t i o n . Males caught only i n p i t f a l l s had an average weight of 36.0 ± .9 g (1 SE) at f i r s t capture compared with 39.5 ± .8 g f o r males caught f i r s t i n p i t f a l l s and l a t e r i n l i v e - t r a p s . Females showed the same 4 g d i f f e r e n c e . G e n e r a l l y , the younger an animal, the worse i t s u r v i v e d (Table 5.2), so that fewer of the low body weight i n d i v i d u a l s made i r i n t o the 210 Table 5,4. Numbers of animals f i r s t caught i n p i t f a l l s and l a t e r i n l i v e - t r a p s , and those caught only i n p i t f a l l s . 8 a l e s Females T o t a l Adult Subadult J u v e n i l e Adult Subadult J u v e n i l e Caught f i r s t i n p i t f a l l s and l a t e r i n l i v e - t r a p s 118 112 58 61 113 74 536 Caught only i n p i t f a l l s 97 105 98 40 12 1 115 576 T o t a l s 215 217 156 101 234 189 1112 211 l i v e - t r a p s after capture in p i t f a l l s . It i s possible that many of the voles appearing only i n p i t f a l l s were transients or voles l i v i n g on the edge of the g r i d . I f t h i s i s so, I would expect a higher num.er of these transients per trap i n the outer than i n the inner part of the gr i d . There was no s i g n i f i c a n t difference between the proportion I expected to be l a s t caught in the outer row of p i t f a l l s (36%) i f these animals were f a l l i n g into the traps randomly and the proportion actually caught there (41%) (X2=2.82, .05 > P > .10), although there was a trend. Grid I has three edges contiguous with adjacent grassland and one edge abutting a w a t e r - f i l l e d ditch. The grass edge along the ditch i s not abrupt, but rather grades into unfavorable low moss cover before the ditch i s encountered. Therefore voles encountering the edge are not l i k e l y to run along i t as they might a fence edge, but would probably go back to grass cover again. For t h i s reason I looked only at the outer row of p i t f a l l s along the three edges facing grassland to see what proportion of voles caug_t only in p i t f a l l s were la s t caught in t h i s outer row. A s i g n i f i c a n t l y higher proportion of the voles were l a s t caught i n t h i s outer row of traps (X*=6.12, P < .025; expected 28%, observed 35%). This indicates that some of the large number of voles which never entered l i v e - t r a p s were simply animals l i v i n g on the edge of the grid, making occasional s a l l i e s into the trapped area. There i s s t i l l a large number of animals caught only i n p i t f a l l s that can not be accounted for by t h i s edge effe c t . Figure 5.7 shows the minimum survivorship curves of animals caught only in p i t f a l l s . Approximately 50% of these animals 212 ure 5 . 7 Minimum s u r v i v o r s h i p curves f o r male and female _• townsendii caught only i n p i t f a l l t r a p s and never i n l i v e - t r a p s . W E E K S A F T E R FIRST C A P T U R E ro 214 disappeared within one week. The other 50% were i n the population for at least one week i n which trapping with l i v e - t r a p s occurred, and a few were present i n the population for 17 weeks without being caught by live-traps. These animals made up a substantial segment of the t o t a l population about which nothing could be known i f one trapped only with l i v e - t r a p s . Most of these animals were either adults or subadults, as indicated by Table 5.5. Of the juveniles which were caught only i n p i t f a l l s , approximately 75% were caught only once. I next inquired how e f f i c i e n t p i t f a l l s were at catching new animals that had come into the population either by bi r t h or dispersal before they were caught by live-traps. To answer this question, I looked at the data from June 18 to the end of the study, since that was the period when the f i n a l version of the p i t f a l l trap was i n use. I f e l t that in some of the i n i t i a l versions, some large adults could have jumped out. Table 5.6 shows that 77% of a l l animals caught for the f i r s t time in l i v e - t r a p s had been previously caught in p i t f a l l s . Trapping with p i t f a l l s was therefore an e f f i c i e n t technigue to trap animals before they were known to be i n the l i v e - t r a p population. During t h i s entire period v i r t u a l l y no juveniles ( 1.7%) were caught by l i v e - t r a p s . Proportion Of The Sexes Differences i n the proportion of the sexes are useful indices of fundamental differences in performance of the two 215 Table 5.5. Percent of animals f i r s t caught in p i t f a l l s and never recaptured again. Sample sizes in parentheses are for weight classes at f i n a l capture. Males Females adult Subadult Juvenile Adult Subadult Juvenile 45 5 4 77 32 42 73 (146) (89) (65) (65) (142) (69) 216 Table 5.6. Number of re c r u i t s caught in live-traps plus percent previously caught in p i t f a l l s (June 18 to September 28, 1976). Males Females Total Adult Subadult Juvenile Adult Subadult Juvenile Total new re c r u i t s caught i n l i v e - t r a p s 255 80 113 142 600 Percent previously caught in p i t f a l l s 78 86 60 68 77 60 77 217 trap types. The proportion of males I present i n _able 5.7 are calculated in two ways: from resident animals known to be l i v i n g on the area because of capture i n a s p e c i f i c type of trap (each animal was t a l l i e d every trapping session i t was known to be i n the s p e c i f i c trapping population); and from a l l new r e c r u i t s entering the population (each animal was t a l l i e d only when i t entered the population). I looked only at the period when the f i n a l version of the p i t f a l l was in use. Three points may be noted from these data (Table 5.7). (1) The proportion of the sexes i n juvenile and subadult r e c r u i t s in p i t f a l l s was not d i f f e r e n t from 1:1. This suggests that p i t f a l l s captured younger weight classes i n proportion to t h e i r presence i n the population, i f an egual sex r a t i o i n these classes i s assumed. In contrast, a higher proportion of subadults caught i n l i v e - t r a p s were females, suggesting that young males must reach a higher body weight before entering l i v e - t r a p s . The difference between the proportion of subadult males caught in l i v e - t r a p s and those caught i n p i t f a l l s i s s i g n i f i c a n t (X*=7.45, P < .01). (2) A higher proportion of adult r e c r u i t s in p i t f a l l s were males (11%). Because the younger ages classes showed a 1:1 sex r a t i o , the high proportion of males suggests that adult males move about more than females. (3) A higher proportion of adult r e c r u i t s in l i v e - t r a p s were also males (18%) and was s i g n i f i c a n t l y higher than that i n p i t f a l l s ( X 2 = 4.00, P < .05). 218 Table 5.7. Proportion of males among r e c r u i t s and residents for i n H. townsendii in two trap types. Sample s i z e s i n parentheses. Period extends from June 18 to September 28, 1975. Recruits Besidents Juvenile Subadult adult Total A l l ages l i v e - t r a p s .50 .34* .68** .55 .44** (10) (232) (353) (595) (2683) P i t f a l l s .48 .46 .61* .53 .4 7* (174) (355) (389) (918) (5743) * P < .01 for a n u l l hypothesis of 0.5 males. ** P < .001 219 D i s p e r s a l Seme of the l a r g e number of v o l e s never e n t e r i n g l i v e - t r a p s c o u l d have emigrated from the area. But of the 576 v o l e s known to have disappeared before capture i n l i v e - t r a p s , o n l y 9 are known to have d i s p e r s e d to other areas (Table 5 . 3 ) , A t o t a l of 16 animals were known to have d i s p e r s e d from G r i d I d u r i n g t h i s time. This d i s p e r s a l i n d i c a t e s t hat even at these high d e n s i t i e s , a s m a l l number o f i n d i v i d u a l s sere s t i l l l e a v i n g the p o p u l a t i o n f o r other areas, e s p e c i a l l y to low d e n s i t y s i t e s such as the removal g r i d s . The m a j o r i t y of these i n d i v i d u a l s (87.5%) were f i r s t caught i n p i t f a l l s , and over h a l f of these were caught only i n p i t f a l l s before l e a v i n g . Most of the d i s p e r s i n g animals (62.5%) had already reached a d u l t s i z e before l e a v i n g . T h i s was true of both sexes. Almost a l l the d i s p e r s i n g i n d i v i d u a l s were caught on the removal areas, even though the C o n t r o l # 2 was c l o s e r than two of the removal areas (Table 5.8), However, as i n d i c a t e d below, the animals t h a t moved from one i n h a b i t e d area to another may have had low t r a p p a b i l i t i e s on the new areas, so that l i t t l e d i s p e r s a l might have been dete c t e d . A l u l l i n breeding o c c u r r e d during the summer, l a s t i n g from about the beginning of J u l y t o mid-September. Only two animals, both males, were known to have di s p e r s e d d u r i n g t h i s time. The other d i s p e r s e r s l e f t the area while b r e e d i n g was s t i l l i n progress. Three animals (2 males, 1 female) moved onto G r i d I from other t r a p p i n g areas. They were a l l caught f i r s t i n p i t f a l l s and subseguently i n l i v e - t r a p s (mean time i n t e r v a l between capture i n two trap types - 5 weeks). A l l were caught 220 Table 5.8. D i s p e r s a l of M. townsendii from G r i d I t o other t r a p p i n g g r i d s d u r i n g the study." Movement to other t r a p p i n g g r i d s Distance from G r i d I to other g r i d s Trap types caught i n cn G r i d I B - both types I - l i v e - t r a p s P - p i t f a l l s Number and Sex C o n t r o l #2 60 m 1 1 male 1 male Male removal g r i d 85 1 male Removal #1 25 m B* L P 3 females 1 male 2 females Removal #2 70 m Bi P P 1 male 4 males 2 females T o t a l 16 *B 111 these i n d i v i d u a l s were f i r s t caught in p i t f a l l s . 221 when breeding was i n progress on Grid I. Thus what l i t t l e d ispersal occurred appeared to be related to the reproductive state of the population. Of a l l the individuals that disappeared during the study only a very small f r a c t i o n could be accounted for by dispersal. Most animals which disappeared therefore probably died on the grid. Measuring differences i n the position of capture between adjacent trapping periods ( f i r s t capture points of one period to f i r s t capture points of the next) i s another way of obtaining an index of a c t i v i t y and minor s h i f t s i n home range. There are no s i g n i f i c a n t within-class differences between animals caught i n p i t f a l l s and those caught in l i v e - t r a p s (Table 5.9). There i s only one s i g n i f i c a n t between-class difference (subadult females move less than adult females caught i n l i v e - t r a p s ) . The ove r a l l conclusion from these data i s that the trappable animals generally move about very l i t t l e , at least at these high densities. These data, together with a v i r t u a l absence of evidence for i n t e r - g r i d movement, suggest that most of the disappearance from the trapping 'area was due to i n s i t u mortality. 222 Table 5.9. Mean distance (ft ± 1 5E) between successive positions of capture in l i v e - t r a p s and p i t f a l l s . Distance measured from f i r s t capture point i n period t to f i r s t capture point i n period t+1. Sample sizes i n parentheses. Males Females adult Subadult Juvenile Adult Subadult Juvenile l i v e - 8.6 ± .9 4.7 ± 1.6 8.8 ± 8.8 8.0 ± .7 4.3 ± 1.0 15.0 ± 10.0 traps (353) (73) (4) (527) (123) (5) p i t - 7.8 ± .9 6.4 + 1.2 11.0 ± 2.4 5.8 + .5 5.0 ± .7 5.7 ± 1.1 f a l l s (393) (246) (124) (627) (318) (154) 223 Discussion This study has indicated the d i f f i c u l t y of obtaining accurate information on population s i z e and population processes in small mammals, espe c i a l l y at high densities. Numerous workers have t r i e d to elucidate the best methods of obtaining population information (for a review see Saith et a l . 1975). Krebs and his co-workers (Krebs 1966, Krebs et a l . 1973, Krebs et a l . 1976) have resorted to t o t a l enumeration because rodent populations do not f i t the assumptions of deterministic or stochastic models of population estimation ( L e s l i e et a l . 1953, Hoff 1973). The major assumption that i s seldom met i s that a l l marked and unmarked animals of equivalent age or weight i n the population are equally catchable. The complete enumeration technique requires that the majority of the animals be counted. Through the use of a simulation model, Hilborn et a l . (1976) have demonstrated that t h i s technique may provide accurate population estimates when 80% or more of the voles are caught in each sampling period. Shen I entered my estimates of t r a p p a b i l i t y i n l i v e - t r a p s (50%) into t h i s model, the model predicted an underestimate of the population size of 20%. The actual underestimate was 6 3% below the number known to be on the area i n July. At t h i s time the young were v i r t u a l l y untrappable i n l i v e - t r a p s . In M. c a l i f o r n i c u s , Krebs (1966) felt, that he was able to enumerate 80-90% of the individuals i n populations of up to 125 - 150 per acre. Above t h i s density only about 60-80% of the population could be enumerated. In Indiana, Krebs et a l . (1969) had evidence that they could enumerate 90% of the 224 _• ___I2__s__I 75% of the H. pennsy1yan,icus (50% i n summer). A trap-out of a number of e n c l o s u r e s suggested t h a t , i n Indiana, l i v e - t r a p p i n g was a f a i r l y r e l i a b l e technique f o r !• P-.-£2q§ster. In the present study, at peak d e n s i t i e s , only 37% of the i n d i v i d u a l s known to be on the area were enumerated by l i v e - t r a p s , compared with 79% by p i t f a l l s . T h e r e f o r e , with the l i v e - t r a p p i n g set-up used i n t h i s study, I d i d not succeed i n g e t t i n g accurate estimates of v a r i o u s demographic s t a t i s t i c s . T h i s fl. townsendii p o p u l a t i o n was i n a peak phase d u r i n g 1975. The maximum d e n s i t y reached was 519 v o l e s per a c r e , which i s c o n s i d e r a b l y higher than has been enumerated f o r any other m i c r o t i n e p o p u l a t i o n . Krebs (1966) r e p o r t e d a maximum d e n s i t y of 324 voles per acre f o r H. c a l i f o r n i c u s . i n Indiana, !• _____ll__ll_£__ reached peak d e n s i t i e s of 55-6 0 per acre and _,• 2£_______er reached d e n s i t i e s of 35-40 per acre (Krebs e t a l . 1969). In B r i t a i n , M. a g r e s t i s reached d e n s i t i e s o f about 300 per acre at Lake Vyrnwy ( C h i t t y , 1952). In a l l of these other s t u d i e s only l i v e - t r a p s were used. The above e s t i m a t e s may t h e r e f o r e be much lower than the a c t u a l p o p u l a t i o n s i z e . In the present study there are at l e a s t two reasons f o r the i n a b i l i t y of the l i v e - t r a p p i n g technigue to g i v e a good d e s c r i p t i o n of demographic events i n t h i s p o p u l a t i o n . The f i r s t reason i s that there sere too few t r a p - n i g h t s a v a i l a b l e f o r a l l animals to be caught; animals were f i l l i n g t r a p s f a s t e r than they could be processed. Van Vleck (1968) found that the denser a p o p u l a t i o n , the more t r a p s t h a t are necessary to c a t c h a given percentage of the p o p u l a t i o n . The second reason may be r e l a t e d to the behavior of dominant i n d i v i d u a l s i n the p o p u l a t i o n which 225 may deter subordinates from coming near to traps or entering them, especially i n the reproductive season. Kikkawa (1964) witnessed a number of encounters at l i v e - t r a p s i n Clethrionomys glareolus, where large dominant animals attacked and chased smaller subordinates away from traps. Gliwicz (1970) also found that tha younger a vole, the lower i t s t r a p p a b i l i t y , and suggested that t h i s was related to i t s position i n the s o c i a l hierarchy. Andrzejewski et a l . (1959) found a positive r e l a t i o n s h i p between s o c i a l dominance and the readiness to enter traps i n laboratory house mice. Other workers have found si m i l a r evidence in other species ( Davis and Emlen 19 56, Andrzejewski et a l . 1967, Joula and Cameron 1974). In tha cotton rat (Siamodon his£idus), Summerlin and Wolfe (1973) found that dominant, heavier animals were more trap-prone than subordinates. These subordinates tended to avoid traps contaminated with the odor of conspecifics, whereas dominant animals did not. Recognizing the odor of certain i n d i v i d u a l s in traps may be one of the ways young M. townsendii learn to avoid traps. However, i t i s known that the young s t i l l prefer to enter d i r t y traps presumably, because they are contaminated with the odor of conspecifics, than to enter clean traps (Boonstra and Krebs in press). In t h i s study the young had to grow to a larger body size before they could enter the l i v e - t r a p population. Exactly what the mechanism i s that prevents young from entering l i v e - t r a p s at an e a r l i e r age i s unknown, but may be some form of aggression. Andrzejewski and Eajska (1972) also trapped a vole population concurrently with p i t f a l l s and l i v e - t r a p s . They found 2 2 6 that as animals got older, they entered traps more readi l y and p i t f a l l s less readily. New animals, when introduced into the population, were as hard to trap with l i v e - t r a p s as young residents. These individuals were thought to be low i n the s o c i a l heirarchy of the population. This may be part of the explanation for the long time i n t e r v a l between f i r s t capture of adult animals in p i t f a l l s and subseguent capture i n l i v e - t r a p observed i n M. townsendii. If these adult animals are indivi d u a l s which have moved from their s i t e of b i r t h , i t would presumably take them some time to establish themselves i n the s o c i a l structure of the population. The three animals, which dispersed from other trapping areas onto Grid I and were caught f i r s t in p i t f a l l s , took up to seven weeks to enter the l i v e - t r a p population. The two trapping methods sampled d i f f e r e n t segments of the vole population. Capture in p i t f a l l s probably has two aspects: an accidental one i n which animals unfamiliar with the area run down a runway and inadvertently f a l l into an open trap, and a deliberate one i n which animals f a m i l i a r with the area seek the p i t f a l l as a source of food. Capture in l i v e - t r a p s involves mainly the deliberate aspect, for an animal must f i r s t encounter the trap and then enter i t . Presumably for these reasons, p i t f a l l s caught the ycung, inexperienced animals, the transients from surrounding areas, and some residents; l i v e - t r a p s caught mainly animals which had been on the area for some time. In the younger age classes, p i t f a l l s caught animals randomly with respect to sex, whereas l i v e - t r a p s caught mainly young females. This sugests that the behavior of adults may be more e f f e c t i v e 227 i n preventing young males from entering l i v e - t r a p s , than i t i s i n preventing young females. Microtines i n general have an enormous po t e n t i a l for increase which i s seldom r e a l i z e d . One of the reasons for t h i s i s the extremely poor rate of survival of the young (Krebs and Myers 1974). These rates are p a r t i c u l a r l y d i f f i c u l t to measure d i r e c t l y , and so are generally deduced from information on l i t t e r size, pregnancy rates, and subsequent capture of young. Immigration and emigration are assumed to cancel each other out. The assumption has been made that individuals caught on the area as young animals were born there and those caught as adult animals were born elsewhere (Hilborn and Krebs 1976). This i s c l e a r l y not the case for M. townsendii i n t h i s study. Young trapped as pre-weanlings were not trapped in l i v e - t r a p s for up to 6 1/2 half months after their calculated-date of b i r t h . Watts (1970) found similar evidence i n Clethrionomys gapperi. On trapping areas from which he had removed the large adult males, juveniles entered traps at a younger age. These data suggested that i n the unmanipulated population young had presumably avoided the traps. In a number of other studies on microtines, poor recruitment of juveniles has been found and interpreted to indicate poor s u r v i v a l (Getz 1960, Golley 1961, Krebs 1964, Chitty and Phipps 1966, Krebs 1966, Krebs et a l . 1969); but trap-avoidance u n t i l an older age may be part of the reason. This type of trap-response may occur in other c r i c e t i d s . Tanton (1965) found a large part of the population of wood mice (Apgdemus sylyaticus) avoided capture during the summer, and suggested that t h e i r avoidance was due to an abundance of 228 food. Sadleir {1965), Healey {1967), and Faixbairn (1976) working on a s i m i l a r species, Per q my sous manic mat us, also found low numbers in the summer and increasing numbers i n the f a l l , but suggest that the cause of t h i s was poor juvenile s u r v i v a l during the breeding season, and good survival and recruitment at the end of the breeding season. However, i t may be that juveniles avoid capture i n the presence of reproductive adult animals, and enter the population only when breeding stops. I t i s necessary, therefore, to lock at other rodent populations at various densities with more than one trapping technique, to see i f certain segments avoid traps. Very l i t t l e d ispersal of tagged individuals was detected i n t h i s peak population. The l i t t l e that did occur was associated largely with the reproductive period. Myers and Krebs (1971) found that dispersal was most commonly associated with increase and peak periods in vole populations i n Indiana. Sexual maturation and dispersal were related in the two species they studied. Very few of the young I captured i n p i t f a l l s matured during the course of the study because of the short breeding season, so that the impetus for dispersal (presumed agonistic behavior by larger adults) was not present. In a study on the bank vole, Clethrioncmys g l a r e d u s , Mazurkiewicz and Bajska (1975) found that dispersal of young from the nest s i t e was inversely proportional to density, although they did not report what happened when the young reached puberty. The majority of the young they tagged as nestlings moved an average of 27 m at high densities compared with movements of 50-60 m at low densities. The young trapped in my study exhibited much smaller 229 movements (1-5 m). -The heavy l o s s witnessed throughout most of t h i s study was t h e r e f o r e probably due to i n s i t u m o r t a l i t y r a t h e r than movement o f f the g r i d . There i s very l i t t l e i n f o r m a t i o n i n the l i t e r a t u r e about the s u r v i v a l of pre-weanling and post-weaniing mice. In !• £eaisylvafii£iis, Getz (1960) estimated an average s u r v i v a l of 12$ f o r tha f i r s t month of l i f e . I f weaning occurs at about 21 days o f age (Hamilton 1941, Sichmond and Coaaway 1969), t h i s e s t i mate would cover both the pre-weanling and post-weanling stage. G o l l e y (1961) and Krebs s t a l . (1969) c a l c u l a t e d s i m i l a r poor s u r v i v a l r a t e s f o r t h i s s p e c i e s . T h i s trend i s a l s o found i n other s p e c i e s ( Krebs 1964, 1966, Krebs §t a l . 1969). In B r i t a i n , Godfrey (1955) found evidence that young M. a g r e s t i s s u f f e r e d severe m o r t a l i t y a f t e r l e a v i n g the nest i n a d e c l i n e year. Hoffman (1958) compared the number of a c t i v e mammae of females to tha number of p l a c e n t a l s c a r s i n order to measure n e s t l i n g m o r t a l i t y . His data showed a pre-weanling s u r v i v a l r a t e of 68$ f o r M. c a l i f org i c us and 62$ f o r M. montatags. T h i s i s comparable to my estimate of a pre-weanling s u r v i v a l r a t e of 74$ ( s u r v i v a l per 21 days). Hoffman (1958) found t h a t n e s t l i n g s u r v i v a l i n M, montanus a c t u a l l y i n c r e a s e d during a d e c l i n e , which suggested t h a t n e s t l i n g m o r t a l i t y was not contributing t o the p o p u l a t i o n d e c l i n e . I f the poor j u v e n i l e s u r v i v a l reported i n peak and d e c l i n e p eriods i s not an a r t i f a c t of t r a p p i n g procedures ( C h i t t y 1952, C h i t t y and Phipps 1966, Krebs 1966, Krebs et a l , 1969, t h i s s t u d y ) , then the above data suggest t h a t i t occurs p r i m a r i l y i n the post-weanling stage. I t may ba due to d i f f e r e n t s o c i a l c o n d i t i o n s the young experience during the 2 3 0 various phases of a population cycle. Since changes i n the rate of juvenile survival are an important driving force i n population cycles (Krebs and Myers, 1974), experiments on young adult interactions similar to those performed by Sadleir (1965) and Healey (1967) on Peromy.scus would be useful. The present study suggest several areas that should be explored further. Are the live-trapping techigues presently used to study microtines as i n e f f i c i e n t at low densities as they are at the high densities observed in t h i s study? Populations of voles at different densities should be trapped concurrently with l i v e - t r a p s and p i t f a l l s to examine t h i s guestion, Since p i t f a l l s capture the young at an early age, trapping with t h i s technigue throughout a cycle might give clues to the cause of the changes i n juvenile s u r v i v a l that have been observed (Krebs and Myers 1974). F i n a l l y , what prevents young and old animals from entering l i v e - t r a p s but not p i t f a l l s ? Observation of resident animals and introduced young and adults i n small enclosures may indicate the role of aggression, odour, or some other f a c t o r . Summary 1. The object of t h i s study was to fin d out more about the methods of t o t a l enumeration and to obtain more accurate measures of juvenile survival and dispersal after weaning. 2. A f i e l d population of Microtus townsendii was trapped concurrently with l i v e - t r a p s and with p i t f a l l traps throughout the spring and summer of 1975. The population was at peak densities, reaching a maximum of 519 voles per acre. 231 3 . The number enumerated by p i t f a l l s was up to two times larger than the number enumerated by live-traps, 4. Capture of a large number of young i n p i t f a l l s indicated that mortality was higher among pcst-weanlings than among any other age group up to adults, 5. Recruitment to l i v e - t r a p s was largely r e s t r i c t e d to animals over 40 g whereas i n p i t f a l l s , animals were generally l i g h t e r . Over half of the 1100 animals captured f i r s t by p i t f a l l s were never caught by l i v e - t r a p s , among those subsequently entering l i v e - t r a p s , many stayed away for 6-10 weeks, and some of the smaller ones stayed away even longer. 6. 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