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Investigation of causes of the 10-year hare cycle Trostel, Kendrick A. 1986

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INVESTIGATION OF CAUSES OF THE 10-YEAR HARE CYCLE by KENDRICK A. TROSTEL B.A., Brown U n i v e r s i t y , Providence, R.I., 1983 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming t o the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA December 1986 (2) Kendrick A. T r o s t e l , 1986 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and. study' 1 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 or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of 2^ y?^ 7, y — The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date \t fa ML DE-6(3/81) i i ABSTRACT T h i s t h e s i s combined data from a t r a p p i n g and r a d i o -t e l e m e t r y study of snowshoe hares at Kluane Lake, Yukon from January 1984 through August 1985 with data c o l l e c t e d at the same s i t e from 1977-83 (Boutin et a l . 1986; Krebs et a l . 1986) to examine p o s s i b l e causes f o r the 10-year c y c l e i n d e n s i t y of snowshoe hares. In Chapter 2 I used data on causes of m o r t a l i t y , from a r a d i o - t e l e m e t r y study of a c y c l i c snowshoe hare p o p u l a t i o n d u r i n g 1978-84, to c o n s i d e r the importance of p r e d a t i o n i n c a u s i n g the hare c y c l e . I found that p r e d a t i o n d u r i n g winter was the l a r g e s t source of m o r t a l i t y f o r snowshoe hares d u r i n g 1978-84. There was a 1-year l a g i n the response of p r e d a t i o n m o r t a l i t y to changing hare d e n s i t y . There was a 2-year l a g i n the response to changing d e n s i t y of m o r t a l i t y due to causes other than p r e d a t i o n . I i n c o r p o r a t e d t h i s i n f o r m a t i o n on causes of m o r t a l i t y i n t o a s i m u l a t i o n model, to see whether observed p r e d a t i o n m o r t a l i t y can cause changes i n d e n s i t y s i m i l a r to those of a c y c l i c p o p u l a t i o n . I f i t t e d the p r e d a t i o n m o r t a l i t y data to a f u n c t i o n i n which t o t a l p redator response c o n s i s t s of a Type II f u n c t i o n a l response and a delayed density-dependent numerical response. Using a s i m u l a t i o n model that p r e d i c t e d m o r t a l i t y r a t e s with t h i s f u n c t i o n , I produced 8-11 year c y c l e s w i t h i n parameter values measured i n t h i s study. In Chapter 3 I compared a n o n - c y c l i c snowshoe hare p o p u l a t i o n on Jacquot I s l a n d i n Kluane Lake, with a c y c l i c p o p u l a t i o n on the mainland, 40 km to the SE. I use t r a p p i n g data from both mainland and is land s i t e s , for a period that included population increase, peak, and dec l ine (1977-85) to test hypotheses of condi t ions s u f f i c i e n t to cause a hare populat ion c y c l e . I a lso presented resu l ts from a rad io -telemetry study, conducted on both mainland and i s land during a populat ion low on the mainland (1984-85). The hypothesis that high rates of recruitment followed by low rates of recruitment, i s s u f f i c i e n t to cause a cyc le was not supported. Data presented was consistent with hypotheses that any one of the fol lowing condi t ions was s u f f i c i e n t to cause the hare c y c l e : 1. High rates of surv iva l followed by low rates of s u r v i v a l , p a r t i c u l a r l y of juven i les 2. Delayed dens i ty -dependent predation 3. Per iodic food shortage. iv TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v i LIST OF FIGURES v i i ACKNOWLEDGEMENTS v i i i Chapter 1. General Introduction 1 Chapter 2. Predation and the 10-year cyc le 3 Introduction 3 Methods 5 Results 7 Key Factors 7 Density-dependent Relat ionships 9 Population Models Based on Density-dependent Mor ta l i ty 19 Linear Density-dependent Model 19 Predator Response Model 22 Discussion 29 Summary 35 Chapter 3. Comparison of a c y c l i c and a non-cyc l ic hare populat ion 37 Introduction 37 Methods 40 Results 43 Population trends 43 Recruitment 48 S u r v i v a l from t r a p p i n g s t u d i e s 48 Te lemetry 61 V e g e t a t i o n d e n s i t y 63 Weight 66 D i s c u s s i o n 67 Demographic p a t t e r n s 67 P r e d a t i o n 68 Food shortage 70 Summary 73 Chapter 4. G e n e r a l D i s c u s s i o n 76 L i t e r a t u r e C i t e d 81 Appendix 1 86 Appendix 2 93 LIST OF TABLES Table 2.1. Causes of death of r a d i o - c o l l a r e d hares 8 Table 2.2. M o r t a l i t y r a t e s of r a d i o - c o l l a r e d hared at S i l v e r Creek 10 Table 2.3. Slopes of k - f a c t o r s on o v e r a l l K 13 Table 2.4. Hare d e n s i t y on December 1 vs. Overwinter p r e d a t i o n r a t e 25 Table 3.1. M o r t a l i t y of r a d i o - c o l l a r e d hares on Jacquot I s l a n d and S i l v e r Creek areas. 1984-85 62 Table 3.2. V e g e t a t i o n d e n s i t y on Jacquot I s l a n d 64 Table 3.3. Comparison of v e g e t a t i o n d e n s i t y between areas on Jacquot I s l a n d 65 v i i LIST OF FIGURES F i g u r e 2.1. K-values from telemetry data 11 F i g u r e 2.2. P r e d a t i o n m o r t a l i t y r a t e s vs. P o p u l a t i o n s i z e . 14 F i g u r e 2.3. Non-predation m o r t a l i t y r a t e s v s . Po p u l a t i o n s i z e 16 F i g u r e 2.4. Mean recruitment index vs. P o p u l a t i o n s i z e .... 20 F i g u r e 2.5. Behavior of the predator response model 26 F i g u r e 3.1. P o p u l a t i o n s i z e on S i l v e r Creek C o n t r o l grid,summer 1977 through summer 1985 44 F i g u r e 3.2. P o p u l a t i o n s i z e on Jacquot I s l a n d food and c o n t r o l g r i d s , summer 1977 through summer 1985 46 F i g u r e 3.3. Recruitment index f o r S i l v e r Creek c o n t r o l , Jacquot I s l a n d c o n t r o l and Jacquot I s l a n d food g r i d s : 1977-84 49 F i g u r e 3.4. 28-day s u r v i v a l r a t e s f o r hares:summer-fall ... 51 F i g u r e 3.5. 28-day s u r v i v a l r a t e s f o r h a r e s : w i n t e r - s p r i n g . 53 F i g u r e 3.6. Minimum overwinter s u r v i v a l r a t e s of j u v e n i l e hares 57 F i g u r e 3.7. Minimum overwinter s u r v i v a l r a t e s of a d u l t hares 59 v i i i ACKNOWLEDGEMENTS I would l i k e to thank my s u p e r v i s o r , Tony S i n c l a i r , f o r h i s support throughout t h i s degree. I a l s o owe s p e c i a l thanks to Charley Krebs, to the W i l l i a m s f a m i l y , and to L i z Hofer and Peter Upton. Ed Buffam, Jean Carey, L i z E l l e n t o n , Ted G u l l i s o n , A l i c e Kenney, S c o t t Kindred, and D a r i e l l e T a l a r i c o helped with the f i e l d work. They deserve s p e c i a l commendation f o r l i v i n g and working with me i n small q u a r t e r s . Stan B o u t i n , Dennis C h i t t y , John F r y x e l l , H o l l y D u b l i n , Peter Morrison, B i l l N e i l l , A r t Rodgers, Jamie Smith, and Mary T a i t t o f f e r e d v a l u a b l e advice and much a p p r e c i a t e d encouragement at v a r i o u s stages of t h i s t h e s i s . Linden Haunt pr o v i d e d d i s t r a c t i o n s d u r i n g the w r i t i n g -up stages, but redeemed h e r s e l f by e x p e d i t i n g the f i n a l weeks of manuscript p r e p a r a t i o n . I thank the A r c t i c I n s t i t u t e of North America f o r use of t h e i r f a c i l i t i e s at Kluane. The support of Andy W i l l i a m s , the base manager, was i n v a l u a b l e . T h i s work was funded by N a t u r a l Sciences and Eng i n e e r i n g Research C o u n c i l grants to A.R.E. S i n c l a i r and C.J. Krebs. At v a r i o u s times the work was supported by Northern S t u d i e s T r a i n i n g grants and the Canadian N a t i o n a l Sportsmens Fund. I was supported by f e l l o w s h i p s from the U n i v e r s i t y of B r i t i s h Columbia and from the James Robert Thompson memorial fund and by a te a c h i n g a s s i s t a n t s h i p at the U n i v e r s i t y of B r i t i s h Columbia. 1. CHAPTER 1. GENERAL INTRODUCTION D e n s i t i e s of snowshoe hares (Lepus americanus) f l u c t u a t e with an 8-11 year p e r i o d i n northern b o r e a l f o r e s t s (Green & Evans 1940; K e i t h & Windberg 1978; Krebs et a l . 1986). These changes i n numbers of hares have a t t r a c t e d a t t e n t i o n because they are both r e g u l a r and dramatic, and they appear to a f f e c t p r o f oundly other organisms i n t h e i r environment. I t has been suggested, f o r example that hare p o p u l a t i o n c y c l e s a f f e c t behavior and p o p u l a t i o n dynamics of predators ( E l t o n & N i c h o l s o n 1942; K e i t h et a l . 1977) and that p e r i o d i c a l l y high hare d e n s i t i e s i n f l u e n c e s u r v i v a l ( E l t o n 1933), growth form ( K l e i n 1977), and chemical defenses ( K l e i n 1977, Bryant 1981a) of the p l a n t s on which hares feed. Many hypotheses have been proposed to e x p l a i n what causes c y c l e s i n d e n s i t i e s of hares. E x t r i n s i c f a c t o r s such as weather (Moran 1953), sun spots ( E l t o n 1924) and lunar c y c l e s ( S i i v o n e n & Koskimies 1955, A r c h i b a l d 1977) have been proposed to d r i v e the hare c y c l e . Other authors suggest that p o p u l a t i o n c y c l e s are caused by v a r i a t i o n i n behavior that r e s u l t s from changes i n d e n s i t y . These b e h a v i o r a l changes are the r e s u l t e i t h e r of d e n s i t y - i n d u c e d s t r e s s ( C h r i s t i a n 1975) or of s e l e c t i o n f o r d i f f e r e n t g e n e t i c behavior types at high and low d e n s i t i e s ( C h i t t y 1967). S t i l l other authors have suggested that food shortage (MacLulich 1937; Grange 1949; Fox & Bryant 1984) or a combination of food shortage and p r e d a t i o n ( K e i t h 1974) cause t h i s c y c l e i n numbers of snowshoe hares. In t h i s t h e s i s I present data r e l e v a n t to the hypotheses 2 that food shortage, p r e d a t i o n p r e s s u r e , or a combination of the two are r e s p o n s i b l e f o r the 10-year hare c y c l e . In chapter 2 I combine data from a r a d i o - t e l e m e t r y study conducted at Kluane Lake, Yukon while hare d e n s i t i e s were low, with data c o l l e c t e d in the same area during four years i n c l u d i n g an i n c r e a s e , peak and d e c l i n e (Boutin et a l . 1986). I c o n s t r u c t a model that uses these data to produce 10-year c y c l e s of the approximate amplitude observed i n that study area. In chapter 3 I analyze data from an 8-year t r a p p i n g study on Jacquot I s l a n d i n Kluane Lake, Yukon and on a mainland s i t e 40 km to the SE. I a l s o present the r e s u l t s of a r a d i o -telemetry study on both i s l a n d and mainland d u r i n g a year of low hare d e n s i t y . P o p u l a t i o n d e n s i t y of hares on the i s l a n d d i d not e x h i b i t the peak and d e c l i n e , c h a r a c t e r i s t i c of a c y c l i c hare p o p u l a t i o n , that occurred on the mainland. I compare the i s l a n d to the mainland to t e s t hypotheses of c o n d i t i o n s that are s u f f i c i e n t to cause a 10-year c y c l e i n numbers of hares. In the f i n a l chapter I o u t l i n e d i r e c t i o n s f o r f u t u r e res e a r c h to t e s t hypotheses of c o n d i t i o n s necessary and s u f f i c i e n t to cause a c y c l e i n numbers of hares. 3 CHAPTER 2. PREDATION AND THE 10-YEAR CYCLE I n t r o d u c t i o n P o p u l a t i o n s i z e of the snowshoe hare (Lepus americanus) f l u c t u a t e s with an 8-11 year p e r i o d i n northern b o r e a l f o r e s t s ( E l t o n & N i c h o l s o n 1942; K e i t h 1963; Krebs et a l . 1986). We now know something of the p a t t e r n s of b i r t h and death that produce these f l u c t u a t i o n s i n numbers of hares, commonly known as the 10-year c y c l e (Cary & K e i t h 1979; K e i t h et a l . 1984; Boutin et a l . 1986). K e i t h (1974) proposed a s e r i e s of ^mechanisms that might cause the demographic changes of t h i s "10-year c y c l e " . He hypothesized that shortage of winter food f o r hares i s r e s p o n s i b l e f o r the i n i t i a l d e c l i n e from peak d e n s i t i e s , and that p r e d a t i o n on hares then d r i v e s the p o p u l a t i o n to yet lower d e n s i t i e s , at which the food supply can recover. R e s u l t s from Pease e_t al. (1979) suggested that hares may have experienced food shortage d u r i n g the winter of peak hare d e n s i t i e s as w e l l as d u r i n g the winter f o l l o w i n g the peak. Hares e x p e r i e n c i n g food shortage i n an experimental treatment had lower mean winter weights, g r e a t e r weight l o s s over the winter, lower j u v e n i l e s u r v i v a l , and reduced mean n a t a l i t y (Vaughan & K e i t h 1981). K e i t h et a l . (1977) found that the predator numerical response to peak hare d e n s i t i e s was delayed i n t h e i r A l b e r t a study area. They r e p o r t e d a time l a g i n p r e d a t i o n r a t e s which they estimated from s t u d i e s of f u n c t i o n a l and numerical responses of four p r e d a t o r s of snowshoe hares i n t h e i r study 4 area. Thus there i s evidence f o r delayed density-dependent mechanisms i n the hare-predator i n t e r a c t i o n . In t h i s chapter I present data on the causes of m o r t a l i t y d u r i n g the c y c l i c low i n snowshoe hare abundance at Kluane Lake, Yukon. I combine m o r t a l i t y data c o l l e c t e d i n t h i s study with data r e p o r t e d by Boutin et a l . (1986) f o r a four-year p e r i o d that spanned a peak i n numbers and I c o n s t r u c t a model of m o r t a l i t y p a t t e r n s over a snowshoe hare c y c l e . Because p r e d a t i o n d u r i n g winter was the l a r g e s t source of m o r t a l i t y f o r hares i n my study s i t e from 1978-84, I propose the hypothesis that p r e d a t i o n alone can account f o r the c y c l i c p o p u l a t i o n dynamics observed f o r snowshoe hares. S i m u l a t i o n models can be used as t o o l s to e x p l o r e whether a p a r t i c u l a r h y p o t h e s i s , expressed as a mathematical r e l a t i o n s h i p , can produce the p o p u l a t i o n dynamics t h a t are observed i n nature. In t h i s chapter I have c o n s i d e r e d the hypothesis that p r e d a t i o n m o r t a l i t y alone, measured at my Yukon study s i t e and a c t i n g a c c o r d i n g to predator response mechanisms r e p o r t e d by K e i t h et a l . (1977) can produce p o p u l a t i o n c y c l e s of the p e r i o d and amplitude observed i n northern b o r e a l f o r e s t s . 5 Methods The study s i t e was l o c a t e d at the south end of Kluane Lake, Yukon (61° North, 138° West). The study area and the t r a p p i n g methods used are d e s c r i b e d i n Krebs et a l . (1986) and Boutin (1980; 1984). M o r t a l i t y data d u r i n g 1978-82 are from Boutin et a l . (1986), who trapped and r a d i o c o l l a r e d - h a r e s on the S i l v e r Creek g r i d (a 10x10 g r i d , 30.5 m between s t a t i o n s and t r a p s s et at a l t e r n a t e s t a t i o n s ) . In 1983-85, when hare d e n s i t i e s were lowest, I expanded t r a p p i n g e f f o r t to o b t a i n a l a r g e enough sample of r a d i o -c o l l a r e d hares. I used a 5 km2 study area that i n c l u d e d the S i l v e r Creek g r i d , and was bounded on one s i d e by the Alaska Highway, on two s i d e s by l o c a l access roads, and on the f o u r t h by Kluane Lake. Traps were set on well-used runways w i t h i n t h i s a r e a . A l l hares weighing over 700 g were given r a d i o c o l l a r s weighing 35g, and those weighing over 1000 g were given c o l l a r s weighing 35-55g. Some of the l i g h t e r r a d i o c o l l a r s were equipped to permit d e t e c t i o n of a c t i v i t y ( W i l d l i f e M a t e r i a l s I n c . ) . A l l heavy r a d i o c o l l a r s (55g) were equipped to d e t e c t m o r t a l i t y with m o t i o n - s e n s i t i v e switches that caused the r a d i o p u l s e r a t e to double i f the c o l l a r d i d not move f o r 4 hours (Advanced Telemetry Systems). Sample s i z e s were l i m i t e d by the number of r a d i o c o l l a r s a v a i l a b l e , or by the numbers of hares trapped. F i e l d techniques f o r monitoring a c t i v i t y of hares and i d e n t i f y i n g the cause of death of r a d i o - c o l l a r e d hares are d e s c r i b e d by Boutin e_t a l . (1986). R a d i o - c o l l a r e d hares that 6 were suspected to have died were tracked down using a hand-held antenna and receiver. Deaths of radio-collared hares were classified as predator k i l l s i f there were obvious signs of a chase in the snow, blood on the collar, or a predator actually feeding on the k i l l . Scavenging was probably rare because carcasses l e f t in the study area and checked periodically remained undisturbed. When entire carcasses were recovered they were skinned and examined for sub-cutaneous bruises that were taken as evidence of surplus k i l l i n g (Kruuk 1972). If the carcass was free of bruises the death was classified as due to causes other than predation. Keith et a_l. ( 1 984) argued that radio collars decrease survival of hares for a week subsequent to collaring. However, Boutin e_t a l . (1986) found l i t t l e evidence of this in the Kluane population. The proportion of hares lost within one week of collaring was never significantly higher than the proportion of hares lost within any one of the following three weeks. Therefore, I assumed that radio collaring had no effect on hare survival during the low phase of the hare cycle. I lumped a l l demographic data into two 6-month periods: winter-spring (December 1 to May 31), and summer-autumn (June 1 to November 31). Hare population densities and recruitment rates of juveniles were taken from Krebs et a l . (1986), and from the unpublished data of a long-term monitoring program. The details of summer demography are incomplete because i t is d i f f i c u l t to census juvenile hares. I began to catch juveniles in live-traps at three weeks of age, and I was unable 7 to a t t a c h r a d i o - c o l l a r s to them u n t i l they were approximately four weeks o l d . Key f a c t o r a n a l y s i s ( V a r l e y & Gradwell 1960) was used to i d e n t i f y which m o r t a l i t y f a c t o r s c o n t r i b u t e d most to changes i n t o t a l m o r t a l i t y . The k-values were c a l c u l a t e d as l o g ! o ( p o p u l a t i o n s i z e at the beginning of a season) - l o g 1 0 ( p o p u l a t i o n a f t e r m o r t a l i t y f a c t o r has a c t e d ) . P o t e n t i a l recruitment l o s s e s , k(rec) , were c a l c u l a t e d as l o g 1 0 of maximum p o t e n t i a l n a t a l i t y per female, from Cary & K e i t h ' s (1979) data, minus the l o g 1 0 of j u v e n i l e recruitment per a d u l t female, from Krebs et al_. (1986). Maximum p o t e n t i a l n a t a l i t y was taken from the maximum value obtained by Cary & K e i t h (1979). T h e r e f o r e , k(rec) was the maximum l o s s due to decreased n a t a l i t y and e a r l y j u v e n i l e m o r t a l i t y . T o t a l K was c a l c u l a t e d only f o r those years in which both summer and winter m o r t a l i t y data were a v a i l a b l e . I used the method of Podoler and Rogers (1975) to rank key f a c t o r s i n order of importance. By t h i s method, those f a c t o r s with the steepest slopes when p l o t t e d a g a i n s t t o t a l K c o n t r i b u t e d the most to changes i n o v e r a l l m o r t a l i t y . R e s u l t s Key F a c t o r s The causes of death of r a d i o - c o l l a r e d hares are presented in Table 2.1 along with "hare exposure-days", the cumulative number of days that hares with c o l l a r s were i n the f i e l d . The l a r g e number of deaths a t t r i b u t e d to unknown causes i n win t e r -8 Table 2.1 Causes of Death of Radio-Collared Hares Hare Season Year days Number of Deaths Non- Unknown Avian T e r r e s t r i a l Unknown predation predator predator predator Winter-Spring 78-79 2093 2 8 2 1 1 Winter-Spring 79-80 4417 6 12 6 2 0 Winter-Spring 80-81 4474 5 29 5 14 5 Winter-Spring 81-82 1547 10 12 3 5 1 Winter-Spring 83-84 2289 3 5 0 7 15 Winter-Spring 84-85 3011 1 3 2 5 5 Summer-Pa11 78 2870 0 4 2 1 2 Summer-Fall 79 3857 1 3 2 0 3 Summer-Fall 80 7056 4 19 4 8 3 Summer-Fall 81 4284 1 16 3 5 6 Summer-Fall 84 3213 2 2 3 1 4 Summer-Fall 85 1413 0 3 3 0 5 9 s p r i n g 1983 was due to r e c e i v e r f a i l u r e . I was unable to recover c o l l a r s u n t i l long a f t e r the hares had d i e d so i t was impossible to determine cause of death. The seasonal r a t e s of m o r t a l i t y i n Table 2.2 were c a l c u l a t e d a c c o r d i n g to Trent & Rongstad (1974) as: 1 - ((hare exposure-days - deaths) / hare exposure-days)' where t i s the number of days i n the season. P r e d a t i o n d u r i n g the winter, k(wp_) , was the most important f a c t o r i n f l u e n c i n g t o t a l m o r t a l i t y ( F i g . 2.1). T h i s o b s e r v a t i o n i s supported by the k - f a c t o r s l o p e s presented i n Table 2.3. A c c o r d i n g to the slope comparison method, p r e d a t i o n in winter was f o l l o w e d , i n order, by winter m o r t a l i t y not caused by p r e d a t o r s , p r e d a t i o n i n summer and l o s s i n r e c r u i t m e n t . Unknown m o r t a l i t y i n both summer and winter and m o r t a l i t y due to causes other than p r e d a t i o n had s l o p e s near zero and, t h e r e f o r e , d i d not e x p l a i n changes i n o v e r a l l m o r t a l i t y . Density-dependent R e l a t i o n s h i p s The r e l a t i o n s h i p s between hare d e n s i t y at the beginning of each p e r i o d and m o r t a l i t y r a t e s observed d u r i n g that p e r i o d are i l l u s t r a t e d i n F i g u r e s 2.2 and 2.3. M o r t a l i t y due to winter p r e d a t i o n changed i n a c o u n t e r - c l o c k w i s e c y c l i c f a s h i o n when p l o t t e d s e q u e n t i a l l y a g a i n s t hare d e n s i t y i n the same time p e r i o d ( t ) . M o r t a l i t y i n c r e a s e d only a f t e r hare d e n s i t i e s had begun to i n c r e a s e . S i m i l a r l y , winter p r e d a t i o n m o r t a l i t y decreased a f t e r hare d e n s i t i e s had a l r e a d y d e c l i n e d ( F i g . 2.2a), i n d i c a t i n g a time l a g . When winter m o r t a l i t y was p l o t t e d a g a i n s t p o p u l a t i o n d e n s i t y the p r e v i o u s year ( F i g . 2.2b), t h i s 10 Table 2.2 I n t e r v a l M o r t a l i t y Rates of Radio C o l l a r e d Hares I n t e r v a l Source of M o r t a l i t y Season Year Non- Pre d a t i o n Unknowi p r e d a t i o n W i n t e r - S p r i n g 78-79 0.16 0.62 0.08 Wint e r - S p r i n g 79-80 0.22 0.56 0.00 Win t e r - S p r i n g 80-81 0.18 0.84 0.18 Wint e r - S p r i n g 81-82 0.69 0 .91 0.11 Wint e r - S p r i n g 83-84 0.21 0.66 0.70 Wint e r - S p r i n g 84-85 0.06 0.46 0.26 Summer-Fall 78 0.00 0.36 0.12 Summer-Fa11 79 0.05 0.21 0.13 Summer-Fall 80 0.10 0.55 0.07 Summer-Fall 81 0.04 0.64 0.20 Summer-Fall 84 0.11 0.29 0.20 Summer-Fall 85 0.00 0.54 0.48 F i g u r e 2 . 1 . K - v a l u e s o b t a i n e d from t e l e m e t r y data p l o t t e d a g a i n s t t i m e . Winter p r e d a t i o n m o r t a l i t y , k(wp) c o n t r i b u t e s most to t o t a l m o r t a l i t y ( K ) . Kjwnp) i s p o p u l a t i o n l o s s due to w in ter n o n - p r e d a t i o n m o r t a l i t y . K(wu) i s p o p u l a t i o n l o s s due to unknown w i n t e r m o r t a l i t y . K ( r e c ) i s maximum p o p u l a t i o n l o s s due to decreased n a t a l i t y and j u v e n i l e s u r v i v a l . K(sp_) i s p o p u l a t i o n l o s s due to summer p r e d a t i o n m o r t a l i t y . K(snp) i s p o p u l a t i o n l o s s due to summer n o n - p r e d a t i o n m o r t a l i t y . K(su) i s p o p u l a t i o n l o s s due to unknown summer m o r t a l i t y . 12 3.0 2.5 2.0 1.5 1.0 0.5 0 0 0.5 0 0 0 JL 1 K k(wp) k(wnp) k(sp) k(rec) k(su) k(wu) —• k(snp) i i 77 78 79 80 81 82 83 84 85 YEAR 13 Table 2.3 Slopes of k - f a c t o r s on o v e r a l l K f a c t o r slope winter p r e d a t i o n k(wp) 0.401* P < 0. 025 winter non-predat ion k(wnp) 0.212* P < 0. 05 summer pr e d a t i o n M s p ) 0.185* P < 0. 01 recruitment k ( r e c ) 0.162 P > 0. 05 winter unknown k (wu) 0.021 P > 0. 05 summer unknown k(su) 0.013 P > 0. 05 summer non-predation k(snp) 0.008 P > 0. 05 14 F i g u r e 2 . 2 . P r e d a t i o n m o r t a l i t y r a t e vs p o p u l a t i o n s i z e p l o t t e d s e q u e n t i a l l y f o r y e a r s 1978 to 1985. A - C : w in ter p r e d a t i o n m o r t a l i t y a g a i n s t p o p u l a t i o n s i z e i n the same year ( t ) , the year be fore ( t -1 ) and two y e a r s b e f o r e ( t - 2 ) . D - F : summer p r e d a t i o n m o r t a l i t y a g a i n s t p o p u l a t i o n s i z e in the same year ( t ) , the year b e f o r e ( t -1 ) and two y e a r s b e f o r e ( t - 2 ) . 16 Figure 2.3. Non-predation mortality rate vs population size plotted sequentially for years 1978 to 1985. A-C: winter non-predation mortality against population size at time t, t-1, t-2 respectively. D-F: summer non-predation mortality against population size at time t, t-1, t-2 respectively. 0.901-N(t-2) 0.15 0.10 0.0S cc o z g < o 111 oc CL I z o z cc 1X1 h .85 . « 7 8 10 20 30 N(t) 0.15 0.10 0.05 D) •80 40 50 E) •81 10 20 30 40 50 N(t-1) 3 (0 0.15 0.10 0.05 •85 12 N(t-2) F) •81 16 20 18 t ime l a g d i s a p p e a r e d and the r e l a t i o n s h i p between p r e d a t i o n m o r t a l i t y and hare d e n s i t y gave a b e t t e r l i n e a r f i t (p<0.01) . Summer p r e d a t i o n m o r t a l i t y ( F i g . 2 . 2 d , e , f ) e x h i b i t e d a p a t t e r n s i m i l a r to t h a t of w in ter p r e d a t i o n m o r t a l i t y , a l t h o u g h the r e g r e s s i o n of p r e d a t i o n r a t e on hare p o p u l a t i o n d e n s i t y the p r e v i o u s year was not s i g n i f i c a n t (p>0.05) . M o r t a l i t y not due to p r e d a t i o n was lower than tha t due to p r e d a t i o n , and the former f o l l o w e d a d i f f e r e n t p a t t e r n w i t h r e s p e c t to hare d e n s i t y ( F i g . 2 . 3 ) . N o n - p r e d a t i o n m o r t a l i t y r e l a t i v e to hare d e n s i t y a l s o e x h i b i t e d the c o u n t e r - c l o c k w i s e c y c l e in w i n t e r ( F i g . 2 . 3 a ) , aga in s u g g e s t i n g a time l a g . The c o u n t e r - c l o c k w i s e c y c l e , however, i s dependent on a s i n g l e data p o i n t . In c o n t r a s t to m o r t a l i t y from p r e d a t i o n , m o r t a l i t y from o ther causes remained at low l e v e l s i n a l l y e a r s except the year f o l l o w i n g peak hare d e n s i t y (1981) . U s i n g r e g r e s s i o n a n a l y s i s , I found win ter m o r t a l i t y due to o ther causes was more c l o s e l y r e l a t e d to hare d e n s i t y two y e a r s e a r l i e r ( F i g . 2 .3c) (p<0.05) than to d e n s i t i e s one year e a r l i e r (the t ime l a g seen i n w in ter p r e d a t i o n m o r t a l i t y ) . Summer m o r t a l i t y from causes o ther than p r e d a t i o n was not r e l a t e d to hare d e n s i t y i n the c u r r e n t year or i n p r e v i o u s y e a r s . ( F i g . 2 . 3 d , e , f ) . Recru i tment was c a l c u l a t e d as the mean v a l u e over a l l f i v e c o n t r o l g r i d s (Krebs et a l . 1986) of the r a t i o : Number j u v e n i l e s on g r i d from June(year t) to A p r i K y e a r t+1) Number b r e e d i n g females on g r i d i n May or J u n e ( y e a r t ) 19 When recruitment rates were p lot ted against population density on June 1 (F ig . 2.4) they showed a clockwise c y c l e . Change in recruitment rate was l i n e a r l y re la ted to population density three years in the future (p<0.05). This re la t ionsh ip depended on a s ing le po in t . If I discarded the f i r s t year of data , which was based on a s ing le adult female on each of two g r i d s , there was no s i g n i f i c a n t re la t ionsh ip between density and recruitment. Population Models Based on Density-dependent Mor ta l i ty Linear Density-dependent Model I used simulat ion models to see whether any of these re la t ionsh ips between morta l i ty and density could produce a ten-year cyc le of the amplitude and period observed in my study area . I projected population s ize (N) into the future holding recruitment rates constant and applying surv iva l rates that are l i n e a r l y re la ted to populat ion s ize in previous years . Population s ize in subsequent years was ca lcu la ted by the equation: N(t+1)=N(t)*R*S (1) 20 F i g u r e 2 . 4 . Mean r e c r u i t m e n t index vs mean p o p u l a t i o n s i z e , measured over 5 t r a p p i n g g r i d s . Recru i tment index i s the number of j u v e n i l e s c a p t u r e d between b i r t h and the f o l l o w i n g A p r i l 1 per a d u l t female presen t i n May and June each summer (Krebs et a l . 1986). 3"ivN3d/snnd03a 22 where R, the p o p u l a t i o n growth r a t e , e q u a l e d 1+(recrui tment r a t e per f emale /2 ) and S, the s u r v i v a l r a t e , was a l i n e a r f u n c t i o n of p o p u l a t i o n s i z e i n a p r e v i o u s y e a r . T h u s , f or a m o r t a l i t y agent a c t i n g w i t h a one-year l a g , S = i n t e r c e p t + ( s l o p e * N ( t - 1 ) ) (2) Parameter e s t i m a t e s for s u r v i v a l a f t e r winter m o r t a l i t y due to p r e d a t i o n ( s lope=-0 .00320 , i n t e r c e p t = 0 . 4 8 8 ) d i d not produce 8-11 year c y c l e s f o r any v a l u e s of R. Parameter e s t i m a t e s for s u r v i v a l a f t e r winter m o r t a l i t y due to o ther causes ( s lope=-0.00648, i n t e r c e p t = 1 . 0 0 ) produced 8-11 year c y c l e s on ly w i t h R v a l u e s between 2.3 and 2.7 which r e p r e s e n t s on ly a s m a l l p o r t i o n of the range of v a l u e s of R measured on the c o n t r o l g r i d s . Over the e i g h t y e a r s of the s t u d y , R v a l u e s ranged from 2.13 to 6.75 (mean=3.55), a l t h o u g h these v a l u e s d i d not v a r y c o n s i s t e n t l y w i th d e n s i t y . P r e d a t o r Response Model The r e l a t i o n s h i p between prey d e n s i t y and p r e d a t i o n r a t e can be more r e a l i s t i c a l l y d e s c r i b e d by a model tha t i n c l u d e s f u n c t i o n a l and n u m e r i c a l responses of p r e d a t o r s . T h e r e f o r e I assumed a Type II f u n c t i o n a l response ( H o l l i n g 1959) and a l i n e a r d e l a y e d d e n s i t y - d e p e n d e n t n u m e r i c a l response i n a 23 Nicholson-Bailey model (1935) to determine predation rate in my simulations; In this model: predation rate=kill/N(t) (3) where N(t) is the hare population size at the beginning of the time interval in which the rate is measured. Predation rate is calculated as: (a*N(t)/( H+N(t))) * (c*N(t-l)) = ( 4 ) N(t) where a*N(t)/(H+N(t)) is the k i l l rate per predator, a Type II functional response. Here 'a' is the maximum predation rate of an individual predator and H is the hare population density at which predators' k i l l rates are half of 'a'. The second term in the numerator, c*N(t-l), is the numerical response of the predator to hare population size in the previous year, where c is a constant. This equation simplifies to: predation rate= a*c*N(t-1) H+N(t) (5) 24 A*N(t-1) or predation rate= (6) H+N(t) where A=a*c. I fitt e d the data on hare density and predation rate (Table 2.4) to this model using a non-linear estimation technique (Marquardt algorithm) to estimate A and H. The best estimates of A and H for winter predation mortality were 2.38 kills/hare (S.D.=0.48) and 8.02 hares/grid (S.D.=4.3) respectively. I also fitted data on summer hare density and predation rate to this model. Best estimates of A and H from summer mortality data were 0.859 kills/hare (S.D.=0.554) and 1.77 hares/grid (S.D.=4.84) respectively. In the simulation model, the hare population in year t+1 was calculated as: N(t+1)=N(t)*R*exp(-A*N(t-1)/(H+N(t))) (7) where R is the rate of growth of the hare population due to recruitment, as in the previous linear model. The parameter space in which 8-11 year cycles were produced by this simple model when H=8, is illustrated in Fig. 2.5. Maximum population densities on a 9.56 ha grid, for select parameter combinations are included in the figure. Increasing H produced a larger maximum N for the cycle but had l i t t l e effect 25 Table 2.4 Overwinter s u r v i v a l r a t e s v s . December 1 p o p u l a t i o n d e n s i t y on the S i l v e r Creek C o n t r o l g r i d i n the p r e v i o u s year. Year s u r v i v a l p o p u l a t i o n r a t e d e n s i t y 1978 .3839 20 1979 .4365 47 1980 .1419 95 1981 .0951 120 1983 .3833 4 1984 .5465 5 26 Figure 2.5 Behaviour of the "predator response" model. 8-11 year c y c l e s are produced with parameter combinations f a l l i n g w i t h i n the area Y. Cycles with p e r i o d g r e a t e r than 8 years or dampening c y c l e s are produced w i t h i n the area X. C y c l e s with p e r i o d g r e a t e r than 11 years are produced w i t h i n the area Z. Numbers w i t h i n the boxes are maximum p o p u l a t i o n s i z e s during of 8-11 year c y c l e s for s e l e c t e d combinations of A and R, with H=8 (see t e x t ) . "A" i s a constant that d e s c r i b e s the f u n c t i o n a l and numerical response of the predators and R i s the rate of growth of the hare p o p u l a t i o n . Observed v a l u e s of A and R at Kluane occur at the i n t e r s e c t i o n of the broken l i n e s . 27 < 28 on p e r i o d i c i t y . Decreasing H produced a sma l l e r maximum N and a s l i g h t l y s h o r t e r p e r i o d of the c y c l e . Values of H gr e a t e r than 20 or l e s s than 8 produced c y c l e s with maximum N that were o u t s i d e of the range of s p r i n g d e n s i t i e s measured by Krebs et a l . (1986). The model produced c y c l e s with an 8-11 year p e r i o d w i t h i n a f a i r l y narrow range (Y) of values of A and R. For the parameter combinations i n the re g i o n X ( F i g . 2.5), the model produced c y c l e s with p e r i o d l e s s than 8 years or dampening c y c l e s . Parameter combinations i n region Z ( F i g . 2.5) produced c y c l e s with constant amplitude and constant p e r i o d that were longer than 11 y e a r s . As R i n c r e a s e d f o r a f i x e d A, the p e r i o d of c y c l e s became longer and the amplitude i n c r e a s e d . As p r e v i o u s l y mentioned, the mean R f o r the c o n t r o l g r i d s ranged from 2.13 to 6.75 with a mean f o r the e i g h t years of 3.55. The value of A measured i n t h i s study i s 2.38(S.D.=0.48), as r e p o r t e d above. Thus, assuming that p r e d a t i o n i n winter was the only source of m o r t a l i t y , and recruitment was constant, our recorded values of A and R were w i t h i n the range i n which c y c l e s of 8-11 year p e r i o d are produced. I f I add summer p r e d a t i o n to t h i s model, using the estimated parameters r e p o r t e d above to c a l c u l a t e summer s u r v i v a l , then 8-11 year c y c l e s are produced only with values of R grea t e r than 8. T h i s model that i n c l u d e s summer p r e d a t i o n m o r t a l i t y d i d not t h e r e f o r e produce c y c l e s with r e a l i s t i c values of R. For winters i n which p r e d a t i o n r a t e s are high, prey p o p u l a t i o n d e n s i t y w i l l be much lower at the end of the winter than i t was at the beginning. Estimates of predator f u n c t i o n a l 29 response however, are based on an average f o r the e n t i r e winter, so that k i l l r a t e s at the beginning of the winter are underestimated and k i l l r a t e s at the end of the winter are overestimated. I f I address t h i s problem by using an exact s o l u t i o n to the "random predator equation" (Rogers 1972) i n the model, the estimates of A and H are not b i o l o g i c a l l y reasonable and p o p u l a t i o n dynamics produced were u n s t a b l e . F i t t i n g my m o r t a l i t y data to t h i s model r e s u l t e d i n predator a t t a c k r a t e s that were higher at the end of the winter than would be p r e d i c t e d by a Type II f u n c t i o n a l response. D i s c u s s i o n To understand what causes the 10-year hare c y c l e i t i s necessary to d e s c r i b e the mechanisms behind the demographic changes observed i n snowshoe hare p o p u l a t i o n s . The k - f a c t o r a n a l y s i s r e i t e r a t e s the c l a i m of Boutin et a_l. (1986) that p r e d a t i o n was the f a c t o r with the l a r g e s t i n f l u e n c e on snowshoe hare s u r v i v a l i n the Yukon study area d u r i n g 1977-85. However, i t does not n e c e s s a r i l y f o l l o w from t h i s that p r e d a t i o n causes the 10-year c y c l e . To show that p r e d a t i o n c o u l d cause the snowshoe hare c y c l e , there must be evidence of an i n t e r a c t i o n between p r e d a t o r s and hares that can produce f l u c t u a t i o n s i n hare d e n s i t y with an 8-11 year p e r i o d . I have shown that p r e d a t i o n i n winter i s the l a r g e s t m o r t a l i t y f a c t o r f o r snowshoe hares at Kluane, Yukon. Such winter p r e d a t i o n r a t e s can produce c y c l i c a l changes i n hare 30 d e n s i t i e s s i m i l a r to those r e p o r t e d by Krebs et a l . (1986). When H=12 (mean value from f i t t i n g procedure + 1 S.D.) i n the model that i n c l u d e d winter p r e d a t i o n , peak s p r i n g d e n s i t i e s of 30-100 hares per 10 ha ( s i m i l a r to data of Krebs et a l . (1986)) were p r e d i c t e d . In t h i s model p r e d a t i o n a l s o accounted f o r the d e c l i n e from those d e n s i t i e s to near z e r o . T h i s r e s u l t i s c o n t r a r y to K e i t h ' s (1974) hypothesis that p r e d a t i o n m o r t a l i t y alone i s i n s u f f i c i e n t to cause a d e c l i n e from peak hare d e n s i t i e s . In a more recent paper ( K e i t h e_t a_l. 1984), g r e a t e r emphasis has been given to the r o l e of p r e d a t i o n i n hare p o p u l a t i o n d e c l i n e s , although they s t i l l propose that o u t r i g h t s t a r v a t i o n i s the primary cause of death d u r i n g the f i r s t winter of the d e c l i n e . I t i s p o s s i b l e that food shortage acted i n d i r e c t l y to l i m i t hare p o p u l a t i o n s i z e , by making hares more v u l n e r a b l e to p r e d a t o r s . K e i t h et a_l. (1984) suggest that m o r t a l i t y due to p r e d a t i o n may be p a r t i a l l y compensatory. They noted that hares taken by p r e d a t o r s are i n poorer c o n d i t i o n than hares i n a randomly shot sample ( c o n d i t i o n i s measured as percent f a t i n bone marrow), suggesting that p r e d a t o r s were c a t c h i n g hares that were s t a r v i n g . S i n c l a i r et a_l. ( i n prep) are p r e s e n t l y a s s e s s i n g whether n u t r i t i o n dropped below body maintenance l e v e l d u r i n g the Kluane hare c y c l e u sing f e c a l n i t r o g e n a n a l y s i s . I f s t a r v a t i o n d i d not occur, then I would expect to see n u t r i t i o n remaining above t h i s maintenance l e v e l over the c y c l e . In a l l the s i m u l a t i o n models I have assumed the recruitment was constant i n order to t e s t whether m o r t a l i t y processes alone 31 c o u l d cause the hare c y c l e . A c c o r d i n g to the work of Cary & K e i t h (1979), r e p r o d u c t i o n v a r i e s i n a c y c l i c a l manner, preceding p o p u l a t i o n d e n s i t y change by 2 1/2 years on t h e i r Rochester, A l b e r t a study s i t e . Changes i n r e p r o d u c t i o n i n t h e i r study were l a r g e and c o u l d p o t e n t i a l l y impose a c y c l i c p a t t e r n of d e n s i t y changes on the f a l l p o p u l a t i o n . I was unable to d i r e c t l y r e l a t e r e p r o d u c t i o n to r e c r u i t m e n t . Krebs et a l . (1986) measured recruitment only as an index of r e p r o d u c t i o n . The recruitment index may precede changes i n d e n s i t y by three y e a r s , but the s i g n i f i c a n c e of t h i s r e l a t i o n s h i p i n my data r e s t s on a s i n g l e p o i n t . Thus, i t i s not c l e a r whether summer m o r t a l i t y compensates f o r changes i n r e p r o d u c t i o n , r e s u l t i n g i n v i r t u a l l y constant recruitment of j u v e n i l e s to the p o p u l a t i o n . The l a c k of i n f o r m a t i o n about j u v e n i l e hare demography may e x p l a i n why the model which i n c l u d e d summer m o r t a l i t y d i d not p r e d i c t 8-11 year c y c l e s i n numbers of hares. If pred a t o r s respond to j u v e n i l e s as w e l l as to a d u l t s , then my estimates of the p r e d a t i o n f u n c t i o n s A and H from summer s t u d i e s are inadequate. Both the f u n c t i o n a l and numerical responses of pr e d a t o r s were probably a f f e c t e d by the combined j u v e n i l e and a d u l t d e n s i t y . Density-dependent m o r t a l i t y i n winter from causes other than p r e d a t i o n (with a 2-year time lag) can produce 8-11 year c y c l e s , but only w i t h i n a narrow range of R va l u e s (2.3-2.7). The amplitude of these c y c l e s i s 1 1/2 to 2 times as l a r g e as the changes r e p o r t e d by Krebs et a l . (1986). At present, there i s only weak c i r c u m s t a n t i a l evidence f o r a mechanism by which 32 food shortage may cause non-predation m o r t a l i t y to l a g two years behind changes i n hare d e n s i t y . Bryant (1981b) and Fox & Bryant (1984) suggested that p l a n t s that have s u f f e r e d heavy browsing r e q u i r e two years to become p a l a t a b l e to hares. P r e d a t i o n m o r t a l i t y i s best d e s c r i b e d as a n o n - l i n e a r f u n c t i o n of hare p o p u l a t i o n d e n s i t y . I n c r e a s i n g d e n s i t i e s of prey t y p i c a l l y cause p r e d a t o r s both to i n c r e a s e i n number and to vary t h e i r r a t e s of consumption as prey abundance changes (Solomon 1949; H o l l i n g 1959). I measured p r e d a t i o n r a t e d i r e c t l y by monitoring hare m o r t a l i t y . Consequently, the p r e d a t i o n r a t e s apply to the e n t i r e s u i t e of hare p r e d a t o r s . These r a t e s a l s o r e f l e c t the t o t a l ( f u n c t i o n a l and numerical) response of p r e d a t o r s . T h e r e f o r e , I cannot d i s t i n g u i s h between the impact of v a r i o u s predator s p e c i e s on the hare p o p u l a t i o n , nor can I separate the f u n c t i o n a l and numerical responses which, i n c o n c e r t , produce the time l a g observed i n the p r e d a t i o n r a t e s . The estimates of p r e d a t i o n r a t e s presented here exceeded those of K e i t h e_t al_. (1977), probably because I estimated p r e d a t i o n from hare m o r t a l i t y d i r e c t l y , r a ther than e x t r a p o l a t i n g k i l l r a t e s f o r i n d i v i d u a l p r e d a t o r s . Predator s t u d i e s can underestimate k i l l r a t e s i f s c a t s or p e l l e t s are missed, i f predator numbers are underestimated, i f non-resident p r e d a t o r s are r e s p o n s i b l e f o r s i g n i f i c a n t hare m o r t a l i t y , or i f s u r p l u s k i l l i n g o c curs. K e i t h et a l . (1984), found higher p r e d a t i o n r a t e s d u r i n g the second winter of d e c l i n i n g hare p o p u l a t i o n s than those they had p r e d i c t e d from t h e i r p r e v i o u s 33 s t u d i e s of p r e d a t o r s . They a t t r i b u t e d 92% of deaths of r a d i o -c o l l a r e d hares to p r e d a t i o n d u r i n g t h i s second winter of the d e c l i n e . The p a t t e r n s of numerical and f u n c t i o n a l response that r e s u l t i n the p r e d a t i o n r a t e s I measured can be deduced from s t u d i e s of snowshoe hare p r e d a t o r s . The decrease i n numbers of coyotes, lynx, and great horned owls lagged a year behind the d e c l i n e i n hare d e n s i t y i n s t u d i e s by K e i t h e_t a l . (1977). Numbers of goshawks changed synchronously with hare d e n s i t y and numbers of r e d - t a i l e d hawks (Buteo j a m a i c e n s i s ) remained constant throughout an 8-year p e r i o d at t h e i r study s i t e near Rochester, A l b e r t a . F u n c t i o n a l responses by snowshoe hare p r e d a t o r s have been d e s c r i b e d i n two s t u d i e s . Ward & Krebs (1985) showed that d a i l y t r a v e l d i s t a n c e of lynx i n Yukon, Canada, remained r e l a t i v e l y constant at hare d e n s i t i e s ranging from 14.7 per ha to 1 per ha and then i n c r e a s e d when hares reached d e n s i t i e s below 1 per ha. Ward (1985) suggested that d a i l y capture r a t e s of snowshoe hares by lynx d e c l i n e d as lynx i n c r e a s e d t h e i r t r a v e l d i s t a n c e . K e i t h et a l . (1977), working at Rochester, A l b e r t a , found Type II f u n c t i o n a l response curves f o r great horned owl, red t a i l e d hawk and lynx, and evidence f o r a Type III f u n c t i o n a l response f o r coyotes fe e d i n g on snowshoe hares. I propose that m i g r a t i n g r a p t o r s may a l s o e x h i b i t n o n - l i n e a r f u n c t i o n a l responses to changing hare d e n s i t i e s by pausing i n t h e i r m i g r a t i o n through snowshoe hare h a b i t a t to hunt hares only when d e n s i t i e s are h i g h . 34 The t i m e - l a g i n m o r t a l i t y due to p r e d a t i o n ( t o t a l response) i n d i c a t e d i n F i g s . 2.2 and 2.3 was s i m i l a r to that found by K e i t h et a l . (1977) f o r p r e d a t i o n on snowshoe hares by lynx (Lynx canadensis) , coyote (Cards l a t r a n s ) , great horned owl (Bubo v i r g i n i a n u s ) and goshawk ( A c c i p i t e r g e n t i l i s ) . The parameters i n my model are based on m o r t a l i t y r a t e s and d e n s i t y estimates from a s i n g l e c y c l e i n hare numbers. I t i s , t h e r e f o r e , p o s s i b l e that the r e l a t i o n s h i p between p r e d a t i o n r a t e and d e n s i t y that produces c y c l e s i n t h i s model can not be g e n e r a l i z e d to represent other areas or other y e a r s . No other comparable s e r i e s of snowshoe hare m o r t a l i t y r a t e s e x i s t s f o r comparison with my measurements. My model shows that a simple i n t e r a c t i o n between snowshoe hares and t h e i r p r e d a t o r s i n winter i n the Yukon can produce c y c l i c a l p o p u l a t i o n changes. Winter p r e d a t i o n can cause the i n i t i a l d e c l i n e from peak hare d e n s i t i e s as w e l l as cause the d e c l i n e to extremely low d e n s i t i e s . F i e l d experiments are necessary to t e s t the p r e d i c t i o n of t h i s model that winter p r e d a t i o n i s s u f f i c i e n t to cause the hare p o p u l a t i o n c y c l e . If a hare p o p u l a t i o n i s prevented from e x p e r i e n c i n g food shortage and no c y c l e s are produced, the h y p o t h e s i s that p r e d a t i o n i s s u f f i c i e n t to cause the 10-year c y c l e would be f a l s i f i e d . I f , i n a d d i t i o n , a hare p o p u l a t i o n that i s p r o t e c t e d from p r e d a t i o n s t i l l e x h i b i t s a 10-year c y c l e , the hypothesis that p r e d a t i o n i s necessary to cause a 10-year c y c l e would be f a l s i f i e d . 35 Summary 1. I r e l a t e d causes of m o r t a l i t y of snowshoe hares to d e n s i t y of hares over an 8-year p e r i o d that i n c l u d e d a peak i n numbers. I then used s i m u l a t i o n modeling to examine whether these density-dependent r e l a t i o n s h i p s c o u l d produce changes i n hare d e n s i t y s i m i l a r to those observed i n my study area i n Yukon, Canada. 2. P r e d a t i o n d u r i n g winter was the l a r g e s t source of m o r t a l i t y f o r snowshoe hares at Kluane, Yukon d u r i n g 1978-84. 3. There was a one-year l a g i n the response of the p r e d a t i o n m o r t a l i t y r a t e i n winter to hare d e n s i t y . 4. There was a two-year l a g i n the response of winter m o r t a l i t y not caused by pre d a t o r s to hare d e n s i t y . 5. A simple s i m u l a t i o n model with density-dependent p r e d a t i o n produced 8-11 year c y c l e s only w i t h i n a narrow range of parameters that are i n c o n s i s t e n t with data from the Kluane r e g i o n . 6. A s i m u l a t i o n model that p r e d i c t e d winter m o r t a l i t y r a t e s u sing a delayed density-dependent numerical response and a Type II f u n c t i o n a l response by p r e d a t o r s , produced 8-11 year c y c l e s w i t h i n the range of parameter v a l u e s measured i n my study. 7. A s i m u l a t i o n model that p r e d i c t e d both summer and winter m o r t a l i t y r a t e s u sing a delayed density-dependent numerical response and a Type II f u n c t i o n a l response by p r e d a t o r s , d i d not produce 8-11 year c y c l e s w i t h i n the range of parameter values measured i n my study. Lack of data on j u v e n i l e m o r t a l i t y may be one reason f o r t h i s r e s u l t . 37 CHAPTER 3. COMPARISON OF A CYCLIC AND A NON-CYCLIC HARE POPULATION I n t r o d u c t i o n Since Green & Evans (1940) documented the r e g u l a r p o p u l a t i o n f l u c t u a t i o n s of snowshoe hares i n northern b o r e a l f o r e s t s , i n v e s t i g a t o r s have examined the necessary and s u f f i c i e n t c o n d i t i o n s to cause t h i s "10-year c y c l e " . K e i t h (1974) proposed a conceptual model i n which food shortage and delayed d e n s i t y -dependent p r e d a t i o n are both necessary to produce the c y c l e i n numbers of hares. Fox and Bryant (1984) proposed a mechanism by which the e f f e c t of changing food a v a i l a b i l i t y alone might be s u f f i c i e n t to cause the 10-year c y c l e . In chapter 2 I presented a model i n which a delayed density-dependent numerical response of p r e d a t o r s i n c o n j u n c t i o n with a Type II f u n c t i o n a l response i n t h e i r k i l l r a t e s was s u f f i c i e n t to cause the hare c y c l e . P a tterns of recruitment and s u r v i v a l that accompany the p e r i o d i c change i n numbers of hares have been documented i n d e t a i l f o r hare p o p u l a t i o n s i n Rochester, A l b e r t a ( K e i t h & Windberg 1978; Cary & K e i t h 1979; K e i t h et a l . 1984) and Kluane, Yukon (Boutin et a l . 1986; Krebs et a l . 1986). Although these s t u d i e s r e v e a l demographic changes that are c h a r a c t e r i s t i c f o r the hare c y c l e , no study has t e s t e d the hypotheses that p a r t i c u l a r components of demography are e i t h e r necessary or s u f f i c i e n t to cause the 10-year c y c l e . N o n - c y c l i c hare p o p u l a t i o n s o f f e r an o p p o r t u n i t y to t e s t hypotheses that p a r t i c u l a r p a t t e r n s of recruitment and s u r v i v a l 38 that are a s s o c i a t e d with c y c l i c p o p u l a t i o n s are s u f f i c i e n t to cause a c y c l e . Furthermore, n o n - c y c l i c hare p o p u l a t i o n s can be compared with p o p u l a t i o n s that c y c l e i n an attempt to f a l s i f y hypotheses t h a t p a r t i c u l a r mechanisms are s u f f i c i e n t to cause a c y c l e . Both K e i t h & Windberg (1978) and Krebs et a l . (1986) found evidence that r e p r o d u c t i v e r a t e s were hi g h e s t d u r i n g years of p o p u l a t i o n i n c r e a s e and lowest d u r i n g years of d e c l i n e and low d e n s i t y . I f t h i s p a t t e r n i n r e p r o d u c t i o n i s s u f f i c i e n t to cause a c y c l e , I p r e d i c t that i t w i l l not occur i n a n o n - c y c l i c p o p u l a t i o n . I f t h i s p a t t e r n i n r e p r o d u c t i o n occurs i n a non-c y c l i c p o p u l a t i o n , the p a t t e r n may be necessary, but i t can not be s u f f i c i e n t to cause a c y c l e . S u r v i v a l of hares i n c y c l i c p o p u l a t i o n s was h i g h during years of p o p u l a t i o n i n c r e a s e , d e c l i n i n g to i t s lowest l e v e l s d u r i n g the d e c l i n e phase ( K e i t h & Windberg 1978; Krebs et a l . 1986). J u v e n i l e s u r v i v a l d e c l i n e d to l e v e l s lower than those of a d u l t s u r v i v a l (Green & Evans 1940; K e i t h & Windberg 1978; Krebs et a l . 1986). If changes in s u r v i v a l s i m i l a r to these are s u f f i c i e n t to cause a c y c l e , I would p r e d i c t that they would not occur i n a n o n - c y c l i c p o p u l a t i o n . I f changes i n s u r v i v a l s i m i l a r to these occur i n a n o n - c y c l i c p o p u l a t i o n , they may be necessary, but can not be s u f f i c i e n t to cause a c y c l e . If a delayed density-dependent rate of p r e d a t i o n i s r e s p o n s i b l e f o r c y c l i c hare p o p u l a t i o n dynamics, I would expect to f i n d low r a t e s of p r e d a t i o n m o r t a l i t y d u r i n g i n c r e a s e phases and h i g h r a t e s of p r e d a t i o n m o r t a l i t y i n peak and d e c l i n e years 39 i n a c y c l i c h a r e p o p u l a t i o n . I w o u l d a l s o p r e d i c t a r e l a t i v e l y c o n s t a n t p r o b a b i l i t y o f s u r v i v a l i n t h e n o n - c y c l i c compared t o t h e c y c l i c p o p u l a t i o n . The h y p o t h e s i s t h a t a d e l a y e d d e n s i t y -d e p e n d e n t r a t e o f p r e d a t i o n i s s u f f i c i e n t t o c a u s e t h e c y c l e , w ould be f a l s i f i e d i f p a t t e r n s o f s u r v i v a l i n c y c l i c and non-c y c l i c p o p u l a t i o n s were s i m i l a r . I f p e r i o d i c f o o d s h o r t a g e i s s u f f i c i e n t t o c a u s e a h a r e c y c l e , a c y c l i c p o p u l a t i o n s h o u l d show s i g n s o f f o o d s h o r t a g e a t some p o i n t i n t h e c y c l e , w h i l e a n o n - c y c l i c p o p u l a t i o n shows no s i g n s o f f o o d s h o r t a g e . In a n o n - c y c l i c p o p u l a t i o n , I w o u l d e x p e c t t o see no impact of f o o d a d d i t i o n on numbers o r on c o n d i t i o n of h a r e s . C y c l i c p o p u l a t i o n s s u p p l i e d w i t h a d d i t i o n a l f o o d s h o u l d i n c r e a s e i n number and be i n b e t t e r c o n d i t i o n compared w i t h a r e a s w i t h o u t s u p p l e m e n t a l f o o d . The h y p o t h e s i s t h a t f o o d s h o r t a g e i s s u f f i c i e n t t o c a u s e a c y c l e would be f a l s i f i e d i f s i g n s o f f o o d s h o r t a g e e x i s t i n a n o n - c y c l i c p o p u l a t i o n . In t h i s c h a p t e r I compare a n o n - c y c l i c p o p u l a t i o n o f snowshoe h a r e s on J a c q u o t I s l a n d i n K l u a n e L a k e , Yukon, t o a n e a r b y m a i n l a n d p o p u l a t i o n t h a t e x h i b i t e d a f l u c t u a t i o n i n numbers t y p i c a l o f a 10-year h a r e c y c l e . 40 Methods For e i g h t years (summer 1977 through summer 1985) hares were l i v e - t r a p p e d on two g r i d s on Jacquot I s l a n d i n Kluane Lake as w e l l as on the S i l v e r Creek C o n t r o l (SCC) mainland g r i d 40 km to the SE. Each g r i d had 100 s t a t i o n s arranged i n a 10x10 p a t t e r n with 30.5m between s t a t i o n s . Traps were l o c a t e d at every other s t a t i o n , along well-used runways and b a i t e d with a l f a l f a cubes i n winter, and with apple and a l f a l f a i n summer. Every 3-4 weeks t r a p s were set f o r two c o n s e c u t i v e n i g h t s , then l o c k e d open u n t i l the next t r a p p i n g s e s s i o n . Jacquot i s a 5 km2 i s l a n d l o c a t e d i n the n o r t h end of Kluane Lake and separated from the mainland by a minimum d i s t a n c e of 2 km. The lake i s frozen between mid-November and mid-May, p r o v i d i n g access by t e r r e s t r i a l p r e d a t o r s and a l l o w i n g the p o s s i b i l i t y of immigration and emigration of hares. V e g e t a t i o n on the i s l a n d i s d i v e r s e , i n c l u d i n g open meadows, dense willow t h i c k e t s ( S a l i x glauca) , aspen (Populus  tremuloides) and poplar (Populus b a l s a m i f e r a ) stands, and mature white spruce f o r e s t s (Picea glauca) . V e g e t a t i o n on the g r i d s i s d e s c r i b e d by Krebs et a_l. (1986). Food on one of the i s l a n d g r i d s was supplemented from September through May each year. Rabbit chow (17% crude p r o t e i n ) was s u p p l i e d i n turkey feeders d i s t r i b u t e d throughout the g r i d and t r a p s were b a i t e d with r a b b i t chow ra t h e r than a l f a l f a . The i s l a n d food g r i d was l o c a t e d w i t h i n a 1 km2 area that i s separated from the main i s l a n d by a narrow isthmus (30m wide i n summer). Movement of tagged hares between the i s l a n d 41 food and c o n t r o l g r i d s was uncommon. Approximately 1 i n 50 hares moved from the food g r i d to the c o n t r o l and 1 i n 60 moved from the c o n t r o l to the food g r i d . Before an animal was r e l e a s e d d u r i n g a l i v e - t r a p p i n g s e s s i o n , i t s tag number, weight, l e n g t h of r i g h t h i n d f o o t , sex, and breeding c o n d i t i o n were recorded. From 1978-85, with the exception of 1982, hares trapped on the mainland g r i d (SCC) weighing more than 700g were f i t t e d with r a d i o c o l l a r s that weighed no more than 5% of hare body weight. When hare d e n s i t i e s on the mainland were low, t r a p p i n g e f f o r t to o b t a i n hares f o r r a d i o - c o l l a r i n g was expanded to i n c l u d e a 5 km2 area bounded on one s i d e by the Alaska highway, on two s i d e s by l o c a l access roads, and on the f o u r t h by Kluane Lake. From s p r i n g 1984 through summer 1985, hares caught on both Jacquot I s l a n d g r i d s as w e l l as hares trapped i n an area with abundant hare s i g n on the n o r t h end of the i s l a n d , were a l s o f i t t e d with r a d i o c o l l a r s . S i g n a l s from r a d i o c o l l a r s were monitored d a i l y from permanent towers to determine whether each hare was s t i l l a l i v e . I suspected hares had d i e d i f they showed no s i g n of movement for two c o n s e c u t i v e monitoring p e r i o d s ( a c t i v i t y c o l l a r s from W i l d l i f e M a t e r i a l s , Inc.) or i f the pulse of a m o r t a l i t y t r a n s m i t t e r (Advanced Telemetry Systems) had doubled, i n d i c a t i n g that the c o l l a r had not moved f o r 4 hours or more. T r a n s m i t t e r s were t r a c k e d on foot u s i n g a hand-held antenna. Deaths were c l a s s i f i e d as predator k i l l s i f s i g n s of a chase and k i l l , or l a r g e q u a n t i t i e s of blood or a predator feeding on the k i l l were 42 found. When an e n t i r e c a r c a s s was recovered, i t was skinned and examined fo r sub-cutaneous b r u i s e s . I f none were found, the death was c l a s s i f i e d as due to causes other than p r e d a t i o n . Trapping data from 1977-85 are from Krebs et a l . (1986). P o p u l a t i o n s i z e at each t r a p p i n g s e s s i o n and 28-day s u r v i v a l p r o b a b i l i t i e s f o r winter (December-May) and summer (June-November) on a l l three g r i d s were c a l c u l a t e d u s i n g the J o l l y -Seber method (Seber 1982). Minimum s u r v i v a l p r o b a b i l i t i e s were a l s o c a l c u l a t e d f o r a d u l t s and j u v e n i l e s s e p a r a t e l y . Standard e r r o r s can not be c a l c u l a t e d f o r minimum estimates, so c h i -square t e s t s were used to compare s u r v i v a l of a d u l t s between g r i d s and s u r v i v a l of j u v e n i l e s between g r i d s . Recruitment r a t e s were c a l c u l a t e d as number of j u v e n i l e s appearing on a g r i d between June and the f o l l o w i n g A p r i l 1, per a d u l t female present i n May or June. Hares weighing l e s s than 1 kg were c o n s i d e r e d to be j u v e n i l e s . M o r t a l i t y r a t e s f o r r a d i o - c o l l a r e d hares d u r i n g the year i n which both i s l a n d and mainland p o p u l a t i o n s were monitored were c l a s s i f i e d i n t o 4 seasons : summer (June-August), f a l l (September-November), winter (December-March), and s p r i n g ( A p r i l and May). M o r t a l i t y r a t e s were compared between sexes, seasons and areas using chi-square t e s t s . In 1985, I estimated v e g e t a t i o n d e n s i t y on the two i s l a n d g r i d s and i n the n o r t h end t r a p p i n g area both before and a f t e r leaves appeared on the v e g e t a t i o n ( s p r i n g and summer censuses, r e s p e c t i v e l y ) . The maximum d i s t a n c e from which a board c o u l d be seen, unobstructed by v e g e t a t i o n (maximum unobstructed 43 d i s t a n c e ) , was used as an i n d i c a t o r of v e g e t a t i o n d e n s i t y . I chose randomly 20 s t a t i o n s from each of the two i s l a n d g r i d s and 20 p o i n t s , each 15 m from the pre v i o u s p o i n t , w i t h i n the north end area. At each of these p o i n t s , one person h e l d a 30 cm 2 board marked i n t o 100 square d i v i s i o n s , f i r s t at ground l e v e l and again at a height of 1m. A second person then measured the d i s t a n c e at which every square on the board was covered by i n t e r v e n i n g v e g e t a t i o n , when viewed from a height equal to that of the board. These two measurements were repeated four times f o r each of the four 90° quadrants. R e s u l t s P o p u l a t i o n trends Hare p o p u l a t i o n d e n s i t y on the S i l v e r Creek c o n t r o l g r i d f o l l o w e d a p a t t e r n t y p i c a l of a c y c l i n g hare p o p u l a t i o n ( F i g . 3.1). P o p u l a t i o n s of male and female hares change synchronously (Krebs et a l . 1986), so I present p o p u l a t i o n estimates f o r both sexes combined. P o p u l a t i o n s i z e on the g r i d , i n c r e a s i n g when the study began, peaked i n the f a l l of 1980 at 138 animals. By winter 1981-82 i t had d e c l i n e d to an extremely low l e v e l (between 2 and 12 a n i m a l s ) , at which i t remained f o r three y e a r s . In c o n t r a s t , hare p o p u l a t i o n s i z e on the Jacquot I s l a n d c o n t r o l g r i d d i d not c y c l e ( F i g . 3.2). I t f l u c t u a t e d over the p e r i o d of study, reaching only h a l f the maximum a t t a i n e d by the mainland and never d e c l i n i n g to l e v e l s as low as those on the 44 Figure 3.1 Jolly-Seber population estimates for S i l v e r Creek control grid, summer 1977 through summer 1985, plotted on a log scale. 25 ha i s the estimated sampling area for a 9.56 ha square g r i d . Winter months (December-May) are shaded. S N O W S H O E H A R E - SILVER CREEK < x LO CN DC LU CL >-00 z LU Q LU CC < z < LU 100-10-I A' v Ii \t * ii / 1 •v., 77 78 79 80 1 11 • • i.« • i « I 81 82 83 84 YEAR 46 Figure 3.2. Population size on Jacquot Island control and food grids, summer 1977 through summer 1985, plotted on a log scale. 25 ha i s the estimated sampling area for a 9.56 ha square g r i d . Winter months (December-May) are shaded. SNOWSHOE HARE - JACQUOT ISLAND LO CM CC LU Q_ CO z LU Q LU CC < 100-: : \ 10 1-9 >*\t 9 \ » • • JACQUOJ CONTROL » JACQUOT FOOD it ;*-:v: ;:x:v: \ I: < LU 77 78 79 80 81 82 Y E A R 83 84 85 48 mainland d u r i n g 1981-84. Krebs et a l . (1986) have noted that p o p u l a t i o n s i z e on the Jacquot food g r i d , u n l i k e those of other food g r i d s i n t h e i r study, c l o s e l y matches p o p u l a t i o n s i z e of i t s c o n t r o l g r i d ( F i g . 3.2). The other two food g r i d s i n the study a t t a i n e d d e n s i t i e s averaging 2.4 and 2.8 times c o n t r o l d e n s i t i e s , i n c o n t r a s t to Jacquot I s l a n d where the food g r i d averaged 1.4 times the d e n s i t y of the c o n t r o l . Recruitment Because young hares were d i f f i c u l t to c a t c h , recruitment r a t e s r e p o r t e d here i n c o r p o r a t e b i r t h , j u v e n i l e s u r v i v a l , and immigration and emigration of j u v e n i l e s to the g r i d s . They are t h e r e f o r e only a rough measure of r e p r o d u c t i v e output of the g r i d p o p u l a t i o n s . The t r e n d i n recruitment r a t e s i s s i m i l a r f o r a l l three g r i d s ( F i g . 3.3). High r a t e s of recruitment i n the e a r l y years of the study d e c l i n e to lows in 1981 and 1982, and then begin to recover i n 1983 and 1984. S u r v i v a l from t r a p p i n g s t u d i e s S u r v i v a l r a t e s f o r summer-fall ( F i g . 3.4) and winter-s p r i n g ( F i g . 3.5) are from t r a p p i n g data, and t h e r e f o r e confound immigration and emigration with s u r v i v a l . T h i s c o n s i d e r a t i o n may be p a r t i c u l a r l y r e l e v a n t when comparing 49 Figure 3.3. Number of juveniles appearing on a gri d between June and the following A p r i l , per adult female present in May or June. Recruitment rate plotted for S i l v e r Creek Control, Jacquot Island Control, and Jacquot Island Food grids for 1977 through 1984. LU LU LL Q < CO OC o UJ OC 12 < 11 10 9 8 7 6 5 4 3 2 1 9 SCC (MAINLAND) • JACQUOT ISLAND CONTROL A JACQUOT ISLAND FOOD 77 78 79 80 81 82 83 84 YEAR tn O 51 Figure 3.4. 28-day survival rates for a l l hares calculated by the Jolly-Seber method during summer (June-November). UJ < GC CC CO >-< Q I 00 CM 1.6 1.5 1.4 1.3 1.2 1.1 1.0 .9 .8 .7 .6 .5 .4 .3 .2 .1 SUMMER • SCC (MAINLAND) • JACQUOT ISLAND CONTROL A JACQUOT ISLAND FOOD I II "tl 77 78 79 80 81 YEAR 82 83 84 t n ro 53 Figure 3.5. 28-day survival rates for a l l hares calculated by the Jolly-Seber method during summer during winter (December-May). LU h-< or > CE CO > < Q I OO CM 1.6 1.5 1.4 1.3 1.2 1.1 1.0 .9 .8 .7 .6 .5 .4 .3 .2 .1 WINTER • SCC (MAINLAND) • JACQUOT ISLAND CONTROL • •JACQUOT ISLAND FOOD : 77-78 78-79 79-80 80-81 81-82 82-83 83-84 84-85 YEAR 55 s u r v i v a l on an i s l a n d (where i n g r e s s and egress are presumably r e s t r i c t e d ) to tha t on a main land g r i d where movement between w i d e l y spaced areas i s more l i k e l y . I c o n s i d e r e d s u r v i v a l r a t e s to be s i g n i f i c a n t l y d i f f e r e n t i f the ± 2 s . e . bars d i d not o v e r l a p . S u r v i v a l r a t e s of males and females were not s i g n i f i c a n t l y d i f f e r e n t i n any g r i d i n any y e a r , so I combined the two sexes i n my a n a l y s i s . Winter s u r v i v a l on the Jacquot c o n t r o l and food g r i d s was not s i g n i f i c a n t l y d i f f e r e n t between g r i d s i n any y e a r . Winter s u r v i v a l on Jacquot I s l a n d food g r i d was not s i g n i f i c a n t l y d i f f e r e n t between y e a r s . Winter s u r v i v a l on the Jacquot I s l a n d c o n t r o l g r i d was s i g n i f i c a n t l y h i g h e r i n 1977 than i t was i n 1984. Summer s u r v i v a l on the i s l a n d food g r i d was s i g n i f i c a n t l y h i g h e r i n 1983 than i n e i t h e r 1977, 1978, or 1982. Summer s u r v i v a l on the i s l a n d c o n t r o l g r i d was never s i g n i f i c a n t l y d i f f e r e n t between y e a r s and d i f f e r e n c e in summer s u r v i v a l between food and c o n t r o l g r i d s was not s i g n i f i c a n t i n any y e a r . S u r v i v a l on the main land (SCC) was lowest in the w i n t e r of 1981-82, when i t was s i g n i f i c a n t l y lower than i t had been i n the w i n t e r s of 1977-78, 1978-79 and 1982-83. Summer s u r v i v a l r a t e on SCC d i d not d i f f e r s i g n i f i c a n t l y between y e a r s . SCC s u r v i v a l r a t e s were lower than those on the i s l a n d i n the f i r s t w inter of the p o p u l a t i o n d e c l i n e (1980-81) as w e l l as i n the w in ter of 1983-84. In g e n e r a l t h e n , w i n t e r s u r v i v a l of i s l a n d food and c o n t r o l g r i d hares remained r e l a t i v e l y c o n s t a n t over time and s i m i l a r between g r i d s , wh i l e w in ter s u r v i v a l of SCC hares v a r i e d both 56 between y e a r s and w i t h r e s p e c t to the i s l a n d p o p u l a t i o n . Summer s u r v i v a l d i d not f o l l o w a c l e a r p a t t e r n of d i f f e r e n c e s between y e a r s or between g r i d s . Minimum o v e r w i n t e r s u r v i v a l of j u v e n i l e s and a d u l t s were c o n s i d e r e d s e p a r a t e l y ( F i g s . 3.6 & 3 . 7 ) . J u v e n i l e s u r v i v a l was s i g n i f i c a n t l y h i g h e r on Jacquot c o n t r o l g r i d than on SCC i n 1979 and 1980 ( x 2 = 4 . 3 3 , p<0.05; x 2 = 4 . 2 9 , p< .05) . J u v e n i l e s u r v i v a l on the Jacquot food g r i d was s i g n i f i c a n t l y h i g h e r than on SCC i n 1980 and 1981 (x 2 =7 .48 , p < 0 . 0 l ; x 2 =4 .42 , p<0 .05) . J u v e n i l e s u r v i v a l on the i s l a n d food g r i d exceeded t h a t on the c o n t r o l on ly i n 1980 and 1982. A d u l t o v e r w i n t e r s u r v i v a l on main land and i s l a n d c o n t r o l g r i d s f o l l o w e d p a t t e r n s s i m i l a r to each o t h e r . S u r v i v a l was h i g h e s t in 1979 , d e c l i n i n g s h a r p l y in 1980 and 1981 and r e c o v e r i n g aga in in 1982 on the i s l a n d and in 1983 on the m a i n l a n d . With the e x c e p t i o n of the f i r s t year of the s t u d y , a d u l t s u r v i v a l was always g r e a t e r on the i s l a n d c o n t r o l g r i d than on SCC. Overwin ter s u r v i v a l on the Jacquot I s l a n d food g r i d was h i g h e r than on e i t h e r the main land or the i s l a n d c o n t r o l d u r i n g these y e a r s of d e c l i n i n g s u r v i v a l (1979-82) . A d u l t o v e r w i n t e r s u r v i v a l on the i s l a n d never d e c l i n e d to l e v e l s as low as those on SCC, but i t was s i g n i f i c a n t l y d i f f e r e n t o n l y between SCC and Jacquot food g r i d s i n 1980 and 1981 (x 2 =7 .64 , P<0.01; x 2 = 4 . 6 7 , p<0.05) . 57 Figure 3 .6 . Minimum overwinter survival for juveniles A p r i l 1 for S i l v e r Creek Control, Jacquot Island control, and Jacquot Island food grids. JUVENILES J L • SCC (MAINLAND) • JACQUOT ISLAND CONTROL A JACQUOT ISLAND FOOD J L 77 78 79 80 81 82 83 84 YEAR 59 Figure 3 . 7 . Minimum overwinter survival for adults to A p r i l 1 for S i l v e r Creek Control, Jacquot Island c o n t r o l , and Jacquot Island food grids. OVERWINTER SURVIVAL 61 Telemetry S u r v i v a l estimates obtained using telemetry d i s t i n g u i s h immigration and emigration from s u r v i v a l . However, only a small amount of i n f o r m a t i o n i s obtained on j u v e n i l e s u r v i v a l because r a d i o c o l l a r s were only a t t a c h e d to hares weighing more than 700 g. T h e r e f o r e , s u r v i v a l before approximately 8 weeks of age co u l d not be measured. Here, I use telemetry data c o l l e c t e d between s p r i n g 1984 and summer 1985 to compare r a t e s of m o r t a l i t y between i s l a n d and mainland and a l s o among areas w i t h i n the i s l a n d (Table 3.1). For non-predation m o r t a l i t y , there were no s i g n i f i c a n t d i f f e r e n c e s i n m o r t a l i t y r a t e s between sexes, seasons or areas ( x 2 , p>0.05). When c o n s i d e r i n g four seasons, 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 seasons i n both p r e d a t i o n and t o t a l m o r t a l i t y r a t e s . N e i t h e r p r e d a t i o n nor t o t a l m o r t a l i t y r a t e s were s i g n i f i c a n t l y d i f f e r e n t f o r s p r i n g and winter or summer and f a l l , so I lumped these seasons i n t o winter (winter+spring) and summer (summer+fall) f o r a l l f u r t h e r a n a l y s i s . A h e t e r o g e n e i t y c h i - s q u a r e i n d i c a t e d that male and female m o r t a l i t y r a t e s from d i f f e r e n t areas c o u l d have come from the same p o p u l a t i o n only f o r p r e d a t i o n m o r t a l i t y i n w i n t e r . T h e r e f o r e , I c o n s i d e r e d male and female m o r t a l i t y s e p a r a t e l y i n f u r t h e r a n a l y s e s . The four areas (SCC, Jacquot food, Jacquot c o n t r o l , and Jacquot n o r t h end) had s i g n i f i c a n t d i f f e r e n c e s i n p r e d a t i o n m o r t a l i t y r a t e s f o r males i n summer (x 2=13.2, p<0.005), and i n t o t a l m o r t a l i t y r a t e s f o r females i n winter (x 2=11.1, p<0.025) 62 Table 3.1 M o r t a l i t y of r a d i o - c o l l a r e d hared between January 1984 and August 1985 Winter i s from December1-May3l. Summer i s from June 1-November 31. SCC i s S i l v e r Creek C o n t r o l , JC i s Jacquot I s l a n d C o n t r o l , JF i s Jacquot I s l a n d Food, and JNE i s Jacquot I s l a n d North End. area sex SCC male female hare number of deaths season c o l l a r - d a y s non-predation p r e d a t i o n unknown winter summer winter summer 2190 1371 3110 3255 3 0 1 2 9 6 13 6 12 5 8 3 JC male winter 1252 summer 1535 female winter 787 summer 844 JF male winter 1732 summer 1327 female winter 1580 summer 1844 1 6 4 0 2 2 1 7 3 0 0 0 1 5 3 2 1 1 1 1 2 1 .3 4 JNE male winter 2480 summer 1496 female winter 1954 summer 1362 2 8 4 0 1 1 0 4 7 2 2 1 63 and f o r males i n summer (x 2=9.9, p<0.025). These d i f f e r e n c e s were due t o : 1. s i g n i f i c a n t l y g r e a t e r p r e d a t i o n and t o t a l m o r t a l i t y r a t e s on the i s l a n d compared to the mainland f o r males i n summer (x 2=9.3, p<0.005; X 2 =10.8 , p<0.005); 2. s i g n i f i c a n t l y g r e a t e r t o t a l m o r t a l i t y of females on the i s l a n d c o n t r o l g r i d compared to the north end of the i s l a n d (x 2=4.9, p<0.05); and 3. gr e a t e r t o t a l m o r t a l i t y of females on the i s l a n d non-food areas compared to the food g r i d i n winter (x 2=5.0, p<0.025). Telemetry p r o v i d e d some in f o r m a t i o n on movement of hares. I measured l o c a t i o n of f i r s t capture and l o c a t i o n of recovery of r a d i o c o l l a r s as an index of d i s p e r s a l . The mean f i r s t c apture to recovery d i s t a n c e was 417m (s.d.=459, N=75) on the i s l a n d and 376m (s.d.=279, N=55) on the mainland, d i f f e r e n c e s which were not s i g n i f i c a n t . V e g e t a t i o n d e n s i t y The maximum unobstructed viewing d i s t a n c e (Table 3.2) was s i g n i f i c a n t l y s h o r t e r i n the north end area than i t was i n e i t h e r the food or c o n t r o l g r i d s . T h i s h e l d a t both 1 m and ground l e v e l h e i g h t s , and both before and a f t e r leaves appeared (Table 3.3). S i m i l a r l y , viewing d i s t a n c e on the food g r i d was s i g n i f i c a n t l y s h o r t e r than on the c o n t r o l g r i d at both h e i g h t s f o r both censuses, with the exce p t i o n of the 1 m height d u r i n g the s p r i n g census. Table 3.2 Mean unobstructed viewing d i s t a n c e as measure of v e g e t a t i o n d e n s i t y on Jacquot I s l a n d f o r s p r i n g ( before l e a v e s appeared) and summer (with l e a v e s ) . (N=80 o b s e r v a t i o n s f o r each mean.) census height s p r i n g 1m s p r i n g ground area c o n t r o l n o r t h end food c o n t r o l n o r t h end food mean unobstructed d i s t a n c e i n m (s.d.) 8.96 (4.69) 4.48 (2.53) 8.20 (4.79) 4.02 (3.47) 1.95 (1.46) 2.99 (2.23) summer 1m c o n t r o l 8.69 (5.36) n o r t h end 4.18 (2.96) food 6.83 (4.33) summer ground c o n t r o l n o r t h end food 3.11 (3.02) 1.16 (0.85) 1.71 (1.07) Table 3.3 Comparisons of mean unobstructed viewing distances for vegetation density on Jacquot Island during spring (before leaves appeared) and summer (with leaves). census height areas t p spring 1m control vs. food 1.05 0.30 control vs. north end 7.55 <0.00 food vs. north end 6.14 <0.00 spring ground control vs. food 2.22 0.03 control vs. north end 4.91 <0.00 food vs. north end 3.53 <0.00 summer 1m control vs. food 2.42 0.02 control vs. north end 6.59 <0.00 food vs. north end 4.52 <0.00 summer ground control vs. food control vs. north end food vs. north end 3.90 5.54 3.55 <0.00 <0.00 <0.00 66 Weight Mean weights for male and female a d u l t snowshoe hares were c o n s i d e r e d s e p a r a t e l y i n t h i s a n a l y s i s because an a n a l y s i s of v a r i a n c e showed s i g n i f i c a n t d i f f e r e n c e s between the sexes . D i f f e r e n c e s were judged s i g n i f i c a n t by means of Duncan's m u l t i p l e range t e s t at the p<0.05 l e v e l (see Appendix 1 for mean w e i g h t s ) . There were no s i g n i f i c a n t d i f f e r e n c e s between years or between g r i d s for mean weight of males in any of the 4 seasons . Female weights were compared on ly i n f a l l and w i n t e r , to a v o i d the confounding e f f e c t of pregnancy . Mean win ter weight of females on Jacquot c o n t r o l and SCC g r i d s was s i g n i f i c a n t l y h i g h e r i n 1977 than i t was i n 1979 or 1981-85. Mean w i n t e r weight of females on Jacquot food g r i d was s i g n i f i c a n t l y lower i n 1981 than i t was i n any o ther year on tha t g r i d . SCC females were s i g n i f i c a n t l y h e a v i e r i n 1977 than i n any o t h e r year on that g r i d . Jacquot c o n t r o l g r i d females had s i g n i f i c a n t l y lower mean weight compared w i t h SCC in 1977 and compared wi th Jacquot food g r i d i n 1983 and 1984. Females on SCC were s i g n i f i c a n t l y l i g h t e r in 1977 and 1980, compared wi th Jacquot c o n t r o l g r i d in those y e a r s . There were no s i g n i f i c a n t d i f f e r e n c e s i n p e r c e n t weight change f o r a d u l t s or for j u v e n i l e s w i t h i n any season , between g r i d s or between y e a r s , as judged by o v e r l a p of ± 2 s . e . bars (see Appendix 2 for p e r c e n t weight change ) . 67 D i s c u s s i o n Demographic p a t t e r n s The h y p o t h e s i s that h i g h r a t e s of r e c r u i t m e n t d u r i n g the i n c r e a s e , and low r a t e s of r e c r u i t m e n t d u r i n g peak and d e c l i n e y e a r s are s u f f i c i e n t to cause a "10 -year c y c l e " i n numbers of snowshoe h a r e s , p r e d i c t s tha t t h i s p a t t e r n of r e c r u i t m e n t be presen t on the m a i n l a n d , but not on the i s l a n d . Changes in r e c r u i t m e n t r a t e s were s i m i l a r on main land and i s l a n d g r i d s . These d a t a , t h e r e f o r e , are i n c o n s i s t e n t w i t h the h y p o t h e s i s t h a t changes in r e c r u i t m e n t r a t e c h a r a c t e r i s t i c of c y c l i c hare p o p u l a t i o n s are s u f f i c i e n t to cause the c y c l e . The h y p o t h e s i s that h i g h r a t e s of s u r v i v a l d u r i n g y e a r s of p o p u l a t i o n i n c r e a s e , and low s u r v i v a l r a t e s , p a r t i c u l a r l y f o r j u v e n i l e s , d u r i n g years of p o p u l a t i o n d e c l i n e are s u f f i c i e n t to produce a c y c l e p r e d i c t s tha t t h i s p a t t e r n of s u r v i v o r s h i p be present on the m a i n l a n d , but not on the i s l a n d . Overwin ter s u r v i v a l r a t e s from t r a p p i n g i n d i c a t e d tha t d e c l i n i n g j u v e n i l e s u r v i v a l on a g r i d may be important i n c a u s i n g c y c l i c hare p o p u l a t i o n dynamics . Lower a d u l t s u r v i v a l on the ma in land compared to the i s l a n d i n d i c a t e d tha t a d u l t s u r v i v a l may a l s o p l a y a p a r t i n c a u s i n g a c y c l e on the main land and s t a b l e p o p u l a t i o n dynamics on the i s l a n d . Thus , I d i d not f a l s i f y the h y p o t h e s i s t h a t h i g h s u r v i v a l d u r i n g p e r i o d s of p o p u l a t i o n i n c r e a s e , and low s u r v i v a l , p a r t i c u l a r l y of j u v e n i l e s , d u r i n g p e r i o d s of p o p u l a t i o n d e c l i n e are s u f f i c i e n t to cause a snowshoe hare c y c l e . 68 Predat ion I f d e l a y e d d e n s i t y - d e p e n d e n t p r e d a t i o n i s s u f f i c i e n t to cause the hare c y c l e , s u r v i v a l r a t e s shou ld be h i g h d u r i n g p e r i o d s of p o p u l a t i o n i n c r e a s e and low d u r i n g p e r i o d s of d e c l i n i n g p o p u l a t i o n s i z e on the m a i n l a n d . S u r v i v a l r a t e s s h o u l d be r e l a t i v e l y s t a b l e over time on the i s l a n d . The s u r v i v a l data d i s c u s s e d above are c o n s i s t e n t w i t h the h y p o t h e s i s tha t v a r i a b l e p r e d a t i o n p r e s s u r e on the main land and cons tant p r e d a t i o n p r e s s u r e on the i s l a n d were r e s p o n s i b l e for the d i f f e r e n c e between main land and i s l a n d p o p u l a t i o n dynamics . The d i f f e r e n t s u r v i v a l p a t t e r n s on main land and i s l a n d are not n e c e s s a r i l y the r e s u l t of d i f f e r e n c e s i n p r e d a t i o n . Because s u r v i v a l e s t i m a t e s from t r a p p i n g i n c o r p o r a t e movement of hares onto and o f f of the g r i d , h i g h e r s u r v i v a l f or the i s l a n d g r i d s c o u l d i n d i c a t e e i t h e r decreased movement or lower r a t e s of m o r t a l i t y due to p r e d a t i o n or to m o r t a l i t y causes o ther than p r e d a t i o n . R a d i o - t e l e m e t r y can be used to d i s t i n g u i s h between d i s p e r s a l and d e a t h , and to determine whether deaths were due to p r e d a t i o n or to o ther c a u s e s . D u r i n g the p e r i o d i n which a r a d i o - t e l e m e t r y study was conducted on both main land and i s l a n d a r e a s , I found h i g h e r r a t e s of p r e d a t i o n m o r t a l i t y on the i s l a n d a r e a . To t e s t the h y p o t h e s i s t h a t p r e d a t i o n p r e s s u r e on the i s l a n d i s r e l a t i v e l y c o n s t a n t between y e a r s , a c o n t i n u e d r a d i o - t e l e m e t r y study of the i s l a n d p o p u l a t i o n would be n e c e s s a r y . The problem of s e p a r a t i n g the e f f e c t s of m o r t a l i t y and d i s p e r s a l in j u v e n i l e hares i s more d i f f i c u l t because t r a p p i n g 69 j u v e n i l e s d u r i n g the f i r s t month of t h e i r l i v e s i s d i f f i c u l t and j u v e n i l e s must be equipped with extremely l i g h t - w e i g h t r a d i o c o l l a r s . A great deal of u s e f u l i n f o r m a t i o n about snowshoe hare demography c o u l d be obtained i f j u v e n i l e hares c o u l d be l o c a t e d before they are weaned. Pr e d a t i o n has been the primary proximate cause of m o r t a l i t y i n snowshoe hare p o p u l a t i o n s monitored by telemetry i n peak and immediate post-peak years (Brand et a_l. 1 975; K e i t h e_t a l . 1984). P r e d a t i o n was a l s o the main cause of m o r t a l i t y d u r i n g a p o p u l a t i o n c r a s h at Kluane, Yukon (Boutin et. a l . 1986), suggesting that i n c r e a s e d p r e d a t i o n may be necessary to cause the 10-year c y c l e . K e i t h ' s 1974 model proposes that both v a r i a b l e p r e d a t i o n pressure and food shortage are necessary to cause a hare c y c l e . V a r i a b l e p r e d a t i o n pressure may be necessary to allow snowshoe hare d e n s i t i e s to reach l e v e l s high enough f o r food shortage to occur. Food shortage may never have o c c u r r e d on Jacquot I s l a n d because any " s u r p l u s " animals were removed by p r e d a t o r s . P r e d a t i o n pressure c o u l d be constant between years on the i s l a n d because the i s l a n d i s l o c a t e d i n the middle of Kluane Lake, a fly-way f o r the s p r i n g m i g r a t i o n of r a p t o r s . Constant p r e d a t i o n p r e s s u r e c o u l d a l s o occur because the i s l a n d may allow only one-way d i s p e r s a l . U n l i k e the mainland where d i s p e r s e r s may s u r v i v e i n marginal h a b i t a t s ( h a b i t a t s that do not o f f e r p r o t e c t i o n from predators i n years of peak predator d e n s i t y , but p r o v i d e adequate h a b i t a t d u r i n g other y e a r s ) , d i s p e r s i n g i s l a n d hares must c r o s s 2 km of i c e i n the winter, where they would 70 l i k e l y be removed by p r e d a t o r s . An analogous mechanism in which j u v e n i l e s d i s p e r s e d from d i s c o n t i n u o u s patches of good snowshoe hare h a b i t a t i n t o open h a b i t a t where s u r v i v a l was low, was suggested by Do lbeer & C l a r k (1975) to e x p l a i n the s t a b i l i t y of snowshoe hare p o p u l a t i o n s i n C o l o r a d o . Wol f f (1980) suggested tha t s u r v i v a l of hares at peak d e n s i t i e s was r e l a t e d to d e n s i t y of v e g e t a t i v e c o v e r . He argued that o n l y those hares l i v i n g i n areas of very dense cover c o u l d escape the h i g h p r e d a t i o n p r e s s u r e d u r i n g a hare p o p u l a t i o n d e c l i n e . There was some ev idence t h a t hares l i v i n g i n areas of the i s l a n d w i t h dense cover had lower m o r t a l i t y r a t e s than hares l i v i n g in areas of l e s s dense h a b i t a t d u r i n g 1984-85. T o t a l m o r t a l i t y of females on the c o n t r o l g r i d was s i g n i f i c a n t l y h i g h e r than on the n o r t h end, where v e g e t a t i o n d e n s i t y e s t i m a t e s were h i g h e r . Food shortage I f p e r i o d i c food shortage was s u f f i c i e n t to cause a 10-year c y c l e in numbers of h a r e s , t h e r e sh ou ld be s i g n s of food shortage on the m a i n l a n d , but not on the i s l a n d . S ign s of food shortage i n c l u d e decreased mean weight of a d u l t s and decreased weight g a i n of j u v e n i l e h a r e s . Vaughan & K e i t h (1981) r e p o r t e d d e c r e a s e d mean w i n t e r weight of a d u l t s and decreased growth r a t e s of both j u v e n i l e s and a d u l t s i n e n c l o s e d p o p u l a t i o n s that e x p e r i e n c e d food s h o r t a g e . P o p u l a t i o n s that are e x p e r i e n c i n g food shortage shou l d a l s o i n c r e a s e i n number i f supplementa l food i s p r o v i d e d , wh i l e p o p u l a t i o n s that are not l i m i t e d by food 71 w i l l not i n c r e a s e in number when g i v e n supplementa l f o o d . A l t h o u g h mean w i n t e r weight of a d u l t females on SCC was lower than on Jacquot c o n t r o l g r i d d u r i n g the peak y e a r , n e i t h e r a d u l t mean weights nor j u v e n i l e growth r a t e s d i f f e r e d between y e a r s in the c y c l i c main land p o p u l a t i o n . Male body weight changes showed no i n d i c a t i o n of food s h o r t a g e , e i t h e r on the m a i n l a n d , or on Jacquot c o n t r o l g r i d . Growth r a t e s of j u v e n i l e s and a d u l t s , measured as percent weight change, were not d i f f e r e n t between g r i d s . T h u s , body weight data p r o v i d e d l i t t l e ev idence f o r d i f f e r e n c e s i n food shortage between main land and i s l a n d . The i s l a n d food g r i d hare p o p u l a t i o n d i d not reach l e v e l s as h i g h as those on food g r i d s on the m a i n l a n d . T h i s d i f f e r e n c e may r e f l e c t food shortage on the main land that d i d not occur on the i s l a n d . The i n c r e a s e s on the main land g r i d c o u l d have been the r e s u l t of immigra t ion from s u r r o u n d i n g h a b i t a t , of i n c r e a s e d r e p r o d u c t i o n of r e s i d e n t h a r e s , or of i n c r e a s e d s u r v i v a l of food g r i d hares compared to c o n t r o l g r i d h a r e s . I f food shortage i s s u f f i c i e n t to cause a c y c l e , t h i s i n c r e a s e must r e f l e c t more than a s imple s h i f t of hares from one l o c a t i o n to a n o t h e r . The i n c r e a s e must be due to i n c r e a s e d s u r v i v a l or r e p r o d u c t i o n r a t h e r than to i n c r e a s e d immigra t ion to the l o c a l l y abundant f o o d . There i s ev idence t h a t s u r v i v a l of hares i n c y c l i n g p o p u l a t i o n s may be l i n k e d to food a v a i l a b i l i t y . Pease et a l . (1979), by d e t e r m i n i n g the minimum food requirements of hares in c a p t i v i t y and measuring the a v a i l a b l e browse on two study areas 72 d u r i n g s i x w i n t e r s , concluded that food s u p p l i e s were i n s u f f i c i e n t to prevent s t a r v a t i o n of hares on t h e i r study area d u r i n g the peak and post-peak w i n t e r s . Food shortage reduced j u v e n i l e , but not a d u l t , s u r v i v a l i n a study i n which c a p t i v e hares were d e p r i v e d of food over winter (Vaughan & K e i t h 1981). In a f i e l d experiment, s u r v i v a l of f r e e - r a n g i n g hares was improved d u r i n g two winters i n which p o p u l a t i o n s were pr o v i d e d with supplemental food (Boutin 1984). Reproduction may a l s o be i n f l u e n c e d by food shortage. Vaughan & K e i t h (1981) found that overwinter food shortage caused decreases i n r e p r o d u c t i o n by s h o r t e n i n g the breeding season and reducing mean n a t a l i t y . Immigration to i s l a n d g r i d s may be be reduced because of l i m i t e d i n g r e s s to the i s l a n d . Hares c o u l d c r o s s the i c e to the i s l a n d d u r i n g 6 months of the year, but immigrants may encounter a high r i s k of p r e d a t i o n d u r i n g the 2 km c r o s s i n g . Future r e s e a r c h e r s can t e s t the hypothesis that i n c r e a s e d p o p u l a t i o n s i z e of mainland g r i d s i s due to i n c r e a s e d r e p r o d u c t i o n r a t h e r than to i n c r e a s e d immigration. With the calcium-45 method used by Boutin (1983), i t i s p o s s i b l e to separate o f f s p r i n g of r e s i d e n t s from immigrants. If the i n c r e a s e i n mainland food g r i d p o p u l a t i o n s i s due to r e p r o d u c t i o n , I p r e d i c t that recruitment r a t e , c a l c u l a t e d as number of isotope-marked j u v e n i l e s per isotope-marked female, w i l l be g r e a t e r on the mainland food than the mainland c o n t r o l g r i d s . T h i s hypothesis would be f a l s i f i e d i f the number of isotope-marked j u v e n i l e s per isotope-marked female i s s i m i l a r 73 between mainland food and contro l g r i d s . Number of isotope-marked juveni les per isotope-marked female could a lso be compared between is land and mainland to test the hypothesis that the d i f fe rence in population response to food addi t ion between mainland and is land is due to d i f f e ren t reproductive responses rather than to d i f fe rences in immigration. Food shortage could a lso be indicated by increased d i s p e r s a l . Windberg & Keith (1976) suggested that food shortage in years of peak hare density may induce starv ing hares to d isperse . These authors argue that d ispersers should suffer higher morta l i ty ra tes . Boutin et a l . (1985) found evidence that hares may disperse in response to food shortage. The present crude measures of hare movement showed no d i f ference between is land and mainland. This i s only weak evidence, however, and i t included only movements of hares that d i e d , and hares weighing more than 700g. Therefore, further measures of d ispersa l on mainland and is land are needed. Summary 1. Snowshoe hare populat ions on Jacquot i s l a n d , Yukon did not exhib i t a f luc tua t ion in numbers t y p i c a l of the "10-year c y c l e " , whereas a nearby mainland populat ion d id exhib i t density changes i n d i c a t i v e of a hare c y c l e . 2. A snowshoe hare population on the is land that was suppl ied with extra food d id not increase to l e v e l s as high as those of populat ions supplemented with food on the mainland. 3. Recruitment rates were s imi la r between is land and 74 main land from 1977-84. 4. O v e r w i n t e r s u r v i v a l of j u v e n i l e s , i n a t r a p p i n g s t u d y , was s i g n i f i c a n t l y lower on the main land than on the i s l a n d g r i d s d u r i n g a 3 -year p e r i o d c e n t e r e d on the peak i n numbers on the m a i n l a n d . Overwin ter s u r v i v a l of a d u l t s , i n a t r a p p i n g s t u d y , was h i g h e r on both i s l a n d g r i d s than on the ma in land from 1978-84, a l t h o u g h o n l y s i g n i f i c a n t l y so f o r the i s l a n d food g r i d i n 1980 and 1981. 5. A r a d i o - t e l e m e t r y study d u r i n g a low i n numbers of hares on the main land ( s p r i n g 1984 to summer 1985) r e v e a l e d h i g h e r r a t e s of both p r e d a t i o n and t o t a l m o r t a l i t y on the i s l a n d compared to the main land for males in summer. There were no d i f f e r e n c e s between i s l a n d and main land in r a t e s of m o r t a l i t y due to causes o t h e r than p r e d a t i o n f o r males i n any season . There were no d i f f e r e n c e s between i s l a n d and main land in r a t e s of m o r t a l i t y f o r females . 6. Data p r e s e n t e d here f a l s i f i e d the h y p o t h e s i s that h i g h r a t e s of r e c r u i t m e n t d u r i n g an i n c r e a s e in numbers of h a r e s , f o l l o w e d by low r a t e s of r e c r u i t m e n t i s s u f f i c i e n t to cause a c y c l e . Data p r e s e n t e d here was c o n s i s t e n t w i t h the h y p o t h e s i s t h a t h i g h r a t e s of s u r v i v a l d u r i n g an i n c r e a s e i n numbers of h a r e s , f o l l o w e d by low r a t e s of s u r v i v a l , p a r t i c u l a r l y of j u v e n i l e s , i s s u f f i c i e n t to cause a c y c l e . 7. Data p r e s e n t e d here was c o n s i s t e n t w i t h the h y p o t h e s i s t h a t d e l a y e d d e n s i t y - d e p e n d e n t p r e d a t i o n i s s u f f i c i e n t to cause the hare c y c l e . Data p r e s e n t e d here was a l s o c o n s i s t e n t w i th the h y p o t h e s i s tha t p e r i o d i c food shortage i s s u f f i c i e n t to 75 cause the hare c y c l e . More s t r i n g e n t t e s t s of these two hypotheses were proposed. 76 CHAPTER 4. GENERAL DISCUSSION The q u e s t i o n of what causes the hare c y c l e has yet to be r e s o l v e d . In t h i s t h e s i s I have p r e s e n t e d da ta r e l e v a n t to the hypotheses that d e l a y e d d e n s i t y - d e p e n d e n t p r e d a t i o n , p e r i o d i c food s h o r t a g e , or a combinat ion of the two are r e s p o n s i b l e f o r the 10-year hare c y c l e . No s tudy to date has f a l s i f i e d any of these hypotheses . In order to t e s t the h y p o t h e s i s tha t d e l a y e d d e n s i t y -dependent p r e d a t i o n i s neces sary to cause the hare c y c l e , a hare p o p u l a t i o n must be p r o t e c t e d from p r e d a t i o n . T h i s h y p o t h e s i s w i l l be f a l s i f i e d i f a hare p o p u l a t i o n e x h i b i t s a 10-year c y c l e i n the absence of p r e d a t i o n . To t e s t the h y p o t h e s i s tha t food shortage i s neces sary to cause the hare c y c l e , a hare p o p u l a t i o n must be p r o v i d e d wi th supplementa l food so t h a t food shortage does not o c c u r . T h i s h y p o t h e s i s w i l l be f a l s i f i e d i f a hare p o p u l a t i o n e x h i b i t s a 10-year c y c l e in the absence of food s h o r t a g e . To t e s t the h y p o t h e s i s tha t both d e l a y e d d e n s i t y -dependent p r e d a t i o n and food shortage are neces sary to cause a hare c y c l e , a hare p o p u l a t i o n must be p r o t e c t e d from p r e d a t i o n and p r o v i d e d w i t h supplementa l f o o d . T h i s h y p o t h e s i s w i l l be f a l s i f i e d i f a hare p o p u l a t i o n e x h i b i t s a 10-year c y c l e i n the absence of both p r e d a t i o n and food s h o r t a g e . Demographic p a t t e r n s tha t accompany a c y c l e i n numbers of hares have been w e l l documented (Green & Evans 1940a, 1940b, 1940c; K e i t h & Windberg 1978; Krebs et a l . 1986). Each of these authors d i s c u s s e d the importance of changing j u v e n i l e s u r v i v a l in c a u s i n g the hare c y c l e . In a l l of these s t u d i e s , 77 j u v e n i l e s u r v i v a l was h i g h d u r i n g p e r i o d s of low and i n c r e a s i n g hare p o p u l a t i o n s i z e , and low d u r i n g peak and d e c l i n e y e a r s . The n o n - c y c l i c i s l a n d p o p u l a t i o n d i s c u s s e d i n Chapter 3 e x h i b i t e d h i g h and c o n s t a n t r a t e s of j u v e n i l e s u r v i v a l , whi l e j u v e n i l e s u r v i v a l in a c y c l i c main land p o p u l a t i o n was s i m i l a r to j u v e n i l e s u r v i v a l r e p o r t e d for o ther c y c l i c p o p u l a t i o n s . T h i s i s a d d i t i o n a l ev idence tha t the f a c t o r s a f f e c t i n g j u v e n i l e s u r v i v a l may be c e n t r a l in c a u s i n g the hare c y c l e . D e t a i l e d s t u d i e s of causes of j u v e n i l e m o r t a l i t y in c y c l i c and n o n - c y c l i c p o p u l a t i o n s can p r o v i d e i n f o r m a t i o n about the r o l e of changing j u v e n i l e s u r v i v a l in hare p o p u l a t i o n c y c l e s . I f i n c r e a s e d p r e d a t i o n p r e s s u r e i s necessary f o r d e c l i n i n g j u v e n i l e s u r v i v a l in y e a r s of peak and d e c l i n i n g p o p u l a t i o n s i z e , t h e r e s hou l d be i n c r e a s e d p r e d a t i o n d u r i n g the three y e a r s of peak numbers in the c y c l e . I f i n c r e a s e d j u v e n i l e m o r t a l i t y due to p r e d a t i o n i s s u f f i c i e n t to cause a c y c l e , there should be no analogous i n c r e a s e i n p r e d a t i o n i n the n o n - c y c l i c i s l a n d p o p u l a t i o n . I f food shortage i s neces sary for d e c l i n i n g j u v e n i l e s u r v i v a l i n y e a r s of peak and d e c l i n i n g hare p o p u l a t i o n s i z e , there s h o u l d be i n c r e a s e d s t a r v a t i o n m o r t a l i t y d u r i n g the three y e a r s of the peak in the c y c l e . I f such j u v e n i l e m o r t a l i t y i s s u f f i c i e n t to cause the c y c l e , t h e r e shou ld be no analogous i n c r e a s e i n s t a r v a t i o n i n the n o n - c y c l i c i s l a n d p o p u l a t i o n . K e i t h (1974) suggested tha t food shortage and i n c r e a s e d p r e d a t i o n p r e s s u r e o c c u r r i n g s e q u e n t i a l l y cause the d e c l i n i n g s u r v i v a l of hares that accompanies the p o p u l a t i o n d e c l i n e . 78 K e i t h does not s p e c i f y whether t h i s v a r i a b l e m o r t a l i t y should a f f e c t both a d u l t s and j u v e n i l e s . However, the p a t t e r n of a d u l t m o r t a l i t y found by Boutin et a l . (1986) i s not c o n s i s t e n t with t h i s model. I f t h i s p a t t e r n of m o r t a l i t y f o r j u v e n i l e s i s necessary f o r a hare c y c l e , a three-year r a d i o - t e l e m e t r y study of j u v e n i l e s , centered on the peak i n numbers of hares, should r e v e a l high s t a r v a t i o n m o r t a l i t y f o l l o w e d by high p r e d a t i o n m o r t a l i t y on the mainland. I f high s t a r v a t i o n m o r t a l i t y of j u v e n i l e s f o l l o w e d by high p r e d a t i o n m o r t a l i t y of j u v e n i l e s i s s u f f i c i e n t to cause a c y c l e , r a t e s of both p r e d a t i o n and s t a r v a t i o n m o r t a l i t y should remain constant on the n o n - c y c l i c i s l a n d p o p u l a t i o n . More r e c e n t l y , K e i t h et a l . (1984) emphasized that there might be a s y n e r g i s t i c e f f e c t between s t a r v a t i o n and p r e d a t i o n , in which s t a r v i n g hares d u r i n g peak and e a r l y d e c l i n e phases are more l i k e l y to be k i l l e d by p r e d a t o r s . Even i f the proximate cause of hare m o r t a l i t y was p r e d a t i o n , s t a r v a t i o n c o u l d be the u n d e r l y i n g cause fo r the decreased s u r v i v a l that accompanies a c y c l i c d e c l i n e . T h i s hypothesis would p r e d i c t that hare p o p u l a t i o n s t h a t do not experience food shortage would not e x h i b i t a 10-year c y c l e . R e s u l t s from S i n c l a i r et a_l. ( i n prep) i n d i c a t e that hare p o p u l a t i o n s w i l l d e c l i n e even when hares are not e x p e r i e n c i n g food shortage. These authors, by p r o v i d i n g supplemental food to hare p o p u l a t i o n s , succeeded i n m a i n t a i n i n g f e c a l p r o t e i n above the l e v e l at which weight l o s s o c c u r s . These p o p u l a t i o n s a t t a i n e d higher d e n s i t i e s than those of c o n t r o l p o p u l a t i o n s , yet food a d d i t i o n d i d not prevent a 79 d e c l i n e . I t i s not known, however, whether these fed p o p u l a t i o n s a t t r a c t e d p r e d a t o r s to the l o c a l l y h i g h hare d e n s i t i e s . I f t h i s happened, the u n n a t u r a l l y h i g h p r e d a t o r d e n s i t i e s c o u l d account for the d e c l i n e of the fed hare p o p u l a t i o n s . I f i n c r e a s e d p r e d a t i o n does cause the d e c l i n e in s u r v i v a l , p a r t i c u l a r l y of j u v e n i l e s , t h a t accompanies a p o p u l a t i o n d e c l i n e , the q u e s t i o n remains why p r e d a t i o n p r e s s u r e would be c o n s t a n t i n the n o n - c y c l i c i s l a n d p o p u l a t i o n . V u l n e r a b i l i t y of hares to p r e d a t i o n m o r t a l i t y c o u l d be a f f e c t e d by p r e d a t o r b e h a v i o r or by hare b e h a v i o r . I f b e h a v i o r of p r e d a t o r s accounts f o r c o n s t a n t p r e d a t i o n p r e s s u r e on the i s l a n d and v a r i a b l e p r e d a t i o n p r e s s u r e on the m a i n l a n d , monthly p r e d a t o r counts on both main land and i s l a n d shou ld r e v e a l a more u n i f o r m presence of p r e d a t o r s between y e a r s on the i s l a n d than on the m a i n l a n d . I f , a l t e r n a t i v e l y , d i s p e r s a l b e h a v i o r of i s l a n d hares makes them more v u l n e r a b l e than main land hares to p r e d a t i o n , I would expect to see r a d i o - c o l l a r e d hares moving a c r o s s the i c e towards the m a i n l a n d , and no ev idence of immigra t ion to the i s l a n d from the ma in land I f i n c r e a s e d s t a r v a t i o n causes the d e c l i n e i n hare s u r v i v a l , p a r t i c u l a r l y of j u v e n i l e s , that accompanies a c y c l i c d e c l i n e , the q u e s t i o n remains why p e r i o d i c food shortage does not occur on the i s l a n d . E i t h e r reduced r e c r u i t m e n t , reduced s u r v i v a l , or i n c r e a s e d e m i g r a t i o n would be neces sary to h o l d the p o p u l a t i o n below the l e v e l at which food shor tage o c c u r s . Recru i tment r a t e s were s i m i l a r between the c y c l i c main land 80 population and the non-cyclic island population. Therefore, the island population must experience either reduced survival due to predation or increased emigration towards the mainland. Evidence for either increased predation mortality or emigration could be c o l l e c t e d in an intensive, long-term r a d i o - c o l l a r study of the island hare population. 81 LITERATURE CITED A r c h i b a l d , H . L . (1977) . Is the 10-year w i l d l i f e c y c l e induced by a l u n a r c y c l e ? W i l d l i f e S o c i e t y B u l l e t i n . 5:126-129. B o u t i n , S . (1980) . E f f e c t of s p r i n g removal exper iments on the spac ing b e h a v i o r of female snowshoe h a r e s . C a n . J . Z o o l . 58:2167-2174. B o u t i n , S . (1983) . E x p e r i m e n t a l a n a l y s i s of j u v e n i l e s u r v i v a l and d i s p e r s a l i n snowshoe h a r e s . P h . D . t h e s i s . U n i v e r s i t y of B r i t i s h C o l u m b i a . B o u t i n , S . (1984) . 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E x p e r i m e n t a l a n a l y s e s of d i s p e r s a l i n snowshoe hare p o p u l a t i o n s . C a n . J . Z o o l . 54: 2061-2081. W o l f f , J . O . (1980) . The r o l e of h a b i t a t p a t c h i n e s s i n the p o p u l a t i o n dynamics of snowshoe h a r e s . E c o l . Monog. 50: 1 1 1-130. 8 6 APPENDIX 1 Mean weights of hares by season: winter (December-March) , s p r i n g ( A p r i l & May) , summer ( J u n e - A u g u s t ) , and f a l l (September-November). JC i s Jacquot I s l a n d c o n t r o l g r i d ; J F i s Jacquot I s l a n d food g r i d ; SCC i s S i l v e r Creek c o n t r o l g r i d . Females Winter Fa l l Year JC JF SCC JC JF SCC 1977 mean 1 726 1 739 1718 1652 1602 1630 s.e. (41 ) (39) (26) (45) (51 ) (34) - N 1 7 19 41 1 4 1 1 25 1978 mean 1 585 1759 1622 1 550 - 1603 s.e. (53) (84) (28) (119) - (53) N 1 0 4 36 2 0 10 1 979 mean 1 492 1 651 1559 1 561 1 526 1 481 s.e. (34) (37) (18) (45) (39) (16) N 25 21 89 1 4 1 9 1 1 4 1980 mean 1 548 1 696 1 505 1 523 1 526 1521 s.e. (64) (45) (25) (21) (22) (14) N 7 14 46 66 57 154 1981 mean 1 369 1 495 1458 1436 1 544 1507 s.e. (84) (35) (31 ) (33) (21 ) (17) N 4 23 29 27 64 94 88 Females Winter F a l l Year JC JF SCC JC JF SCC 1982 mean 1459 1554 1531 1489 1616 1589 s.e. (64) (36) (31) (60) (31) (27) N 7 22 29 8 30 38 1983 mean 1389 1758 1642 1517 1497 1490 s.e. (84) (44) (36) (64) (36) (30) N 4 15 22 7 22 32 1984 mean 1368 1736 1632 1585 1404 1511 s.e. (119) (60) (49) (47) (56) (36) N 2 8 12 13 9 22 Males Winter Spring Year JC JF SCC JC JF SCC 1977 mean 1 434 1 455 1 395 1305 1215 1245 s.e. (53) (56) (33) (118) (84) (68) N 10 9 25 2 4 6 1978 mean 1 407 1 380 1 369 1 380 1350 1323 s.e. (59) (84) (34) (43) (35) (25) N 8 4 24 1 5 23 45 1 979 mean 1 379 1 395 1 294 1 330 1 332 1253 s.e. (56) (59) (19) (42) (59) (25) N 9 8 75 1 6 8 44 1980 mean 1 364 1401 1 332 1 308 1 350 1 337 s.e. (53) (46) (23) (24) (35) (12) N 10 1 3 53 50 23 184 1981 mean 1 282 1363 1322 1 302 1316 1 345 s.e. (75) (56) (43) (22) (22) (14) N 5 9 15 58 58 1 44 Males Year 1982 mean s.e. N 1983 mean s.e. N 1984 mean s.e. N Winter JC JF 1339 1391 (75) (75) 5 5 1377 1445 (97) (59) 3 8 1284 1405 (75) (84) 5 4 SCC JC 1337 1381 (48) (37) 12 20 1426 1404 (50) (75) 1 1 5 1326 1211 (50) (75) 1 1 5 Spring JF SCC 1511 1446 (37) (26) 20 40 1 341 (68) 0 6 1354 1280 (68) (48) 6 12 M a l e s Summer F a l l Y e a r JC J F SCC JC J F SCC 1 977 mean 1517 1559 1 520 1578 1558 1568 s . e . (68) (59) (42) (56) (59) (41) N 6 8 16 9 8 1 7 1978 mean 1 467 1423 1 340 1 545 - 1459 s . e . (42) (50) (24) (84) - (46) N 1 6 1 1 47 4 0 1 3 1 979 mean 1 343 1 490 1 436 1 548 1453 1385 s . e . (59) (84) (31 ) (50) (45) (15) N 8 4 30 1 1 1 4 1 30 1 980 mean - - 1 527 1 464 1383 1440 s . e . - - (20) (26) (28) (15) N 0 0 73 40 35 126 1 981 mean 1 452 1 469 1 469 1 449 1 426 1425 s . e . (68) (75) (24) (50) (25) (22) N 6 5 50 1 1 45 60 Males Year 1982 mean s.e. N 1983 mean s.e. N 1984 mean s.e. N Summer JC JF 1450 1525 (75) (97) 5 3 1512 1517 (97) (97) 3 3 1416 1484 (75) (63) 5 7 SCC JC 1430 1470 (48) (48) 12 12 1539 1502 (59) (59) 8 8 1444 1514 (46) (35) 13 23 F a l l JF SCC 1526 1492 (59) (37) 8 20 1552 1530 (39.) (32) 18 28 1533 1518 (68) (31) 6 29 93 APPENDIX 2 Mean p e r c e n t weight change per day f o r a d u l t s and j u v e n i l e s , by season: w i n t e r ( D e c e m b e r - M a r c h ) , s p r i n g ( A p r i l and May) , summer ( J u n e - A u g u s t ) , f a l l (September-November) . Summer i s the f i r s t season of the y e a r , so t h a t w in ter 1977 i s December 1977-March 1978 and s p r i n g 1977 i s A p r i l and May, 1978. J u v e n i l e s , Summer season Males Females Year JC JF SCC JC JF SCC 1977 mean - 2.88 1 .41 - 1 .60 6.41 2s. e. - 0.05 0.00 - 0.00 0.00 N 0 2 1 0 1 1 1978 mean 3.16 2.37 2.09 3.97 3.49 2.06 2s.e. 1 .39 0.68 1 .27 1 .51 1.11 0.80 N 1 4 1 5 7 1 1 19 1 5 1979 mean 4.51 5.26 2.78 2.87 2.42 2.48 2s.e. 1.11 0.00 1.14 1 .90 1.11 1.10 N 9 1 8 7 6 1 6 1980 mean 2.17 2.18 1 .92 2.54 1 .31 1 .96 2s. e. 1 .30 1 .89 0.28 2.06 0.44 0.36 N 20 1 1 75 16 3 65 1 981 mean 6.11 1 .47 1 .88 2.31 1 .83 2.38 2s.e. 0.00 0.00 0.62 0.99 1 .39 0.54 N 1 1 20 4 4 12 J u v e n i l e s , Summer season Year 1982 mean 2s . e. N 1983 mean 2s . e. N 1984 mean 2s . e. N Males Females JC JF SCC JC JF SCC 2.45 1 .28 3 3.89 0.01 2 1.57 1.07 5.62 3.16 1.10 0.37 0.66 2.00 7 5 2 4 2.54 1.86 1.02 0.00 13 1 J u v e n i l e s , F a l l season Males Females JF SCC JC JF SCC 2.19 0.22 1.01 0.70 0.58 1.46 0.00 0.73 0.00 0.07 6 1 4 1 2 1.89 0.70 2.00 3.46 1.37 1.79 0.57 1.22 0.86 0.98 3 16 7 2 26 3.68 0.41 0.28 1.95 0.75 3.23 0.14 0.00 1.81 0.53 2 3 1 7 5 0.55 - - 0.59 0.18 - - 0.38 0 72 0 0 58 0.63 - - 0.21 0.34 - - 0.56 0 31 0 0 11 97 J u v e n i l e s , F a l l season Males Females Year JC JF SCC JC JF SCC 1982 mean 2.57 5.10 0.35 3.83 - 0.63 2s.e. 0.72 0.50 0.32 2.52 - 0.83 N 3 2 5 3 0 9 1983 mean - - 0.43 - - 0.86 2s.e. - - 0.29 - - 0.40 N O 0 13 0 0 2 1984 mean 0.58 0.44 0.62 1.23 0.86 0.13 2s.e. 0.34 0.39 0.46 0.93 0.78 0.37 N 12 8 2 8 5 2 J u v e n i l e s , Winter season Males Females Year JC JF SCC JC JF SCC 1977 mean - 0 . 0 9 - 0 . 1 0 - 0.19 0.23 -2 s . e . 0.06 0.03 - 0.00 0.06 -N 3 5 0 1 2 0 1978 mean - - 0.06 0.06 - -0.01 2 s . e . - - 0.20 0.11 - 0.19 N 0 0 5 2 0 6 1979 mean - 0 . 0 6 0.04 -0 .22 - 0 . 0 3 0.09 - 0 . 2 5 2 s . e . 0.10 0.20 0.27 0.16 0.08 0.34 N 7 3 27 5 2 55 1980 mean 0.14 - 0 . 0 9 - 0 . 3 0 0.20 - 0 . 0 6 -0 .24 2 s . e . 0.15 0.17 0.51 0.18 0.27 1 .05 N 42 27 13 15 1 1 4 1 981 mean 0.14 0.22 - - 0 . 0 5 0.28 -2 s . e . 0.25 0.33 - 0.17 0.30 -N 5 1 4 0 4 16 0 J u v e n i l e s , Winter season 99 Males Females Year JC JF SCC JC JF SCC 1982 mean 0.10 2 s . e . 0.62 -N 2 0 0 0 0 1983 mean - 0 . 0 5 0.32 0.02 - 0 . 2 5 0.43 2 s . e . 0.17 0.70 0.18 0.25 0.00 N 8 6 7 7 1 1984 mean 0.08 0.00 0.58 - 0 . 0 8 0.15 0.31 2 s . e . 0.18 0.08 1.13 0.20 0.17 0.57 N 6 8 2 8 5 3 J u v e n i l e s , S p r i n g s e a s o n Y e a r J C 1977 mean 0.03 2 s . e . 0.19 N . 2 1978 mean 0.09 2 s . e . 0.36 N 6 1979 mean - 0 . 2 6 2 s . e . 0.15 N 20 1980 mean - 0 . 1 2 2 s . e . 0.09 N 18 1981 mean - 0 . 2 7 2 s . e . ,0.20 N 7 M a l e s J F S CC J C - 0 . 0 2 - 0.61 0.10 0.00 5 1 0 - 0 . 1 7 0.36 0.53 0.11 0 21 4 0.05 - 0 . 0 9 0.20 0.34 0.26 0.27 6 17 10 - 0 . 0 3 - 0 . 2 8 0.03 0.12 0.38 0.10 2 1 9 7 - 0 . 2 6 - - 0 . 0 2 0.11 - 0.32 8 0 5 F e m a l e s J F S CC 0.02 0.16 0.00 2.10 1 4 0.12 0.47 0 13 0.46 0.30 0.53 0.30 5 16 0.32 0.45 0.22 0.69 9 6 0.21 0.28 0.11 0.16 9 2 J u v e n i l e s , S p r i n g season Year JC 1982 mean - 0 . 1 7 2 s . e . 0.24 N 3 1983 mean - 0 . 0 5 2 s . e . 0.19 N 7 1984 mean -0 .52 2 s . e . 0.00 N 1 Males JF SCC JC 0.58 0.80 2 0 0 -0 .08 -0.21 0.27 0.17 0.06 0.30 3 2 6 0.02 -0 .70 -0 .27 0.16 0.59 0.24 3 2 2 Females J F SCC 0.57 0.96 0 2 0.10 0.35 2 0 0.11 0.00 1 0 102 A d u l t s , Summer season Males Females Year JC JF SCC JC JF SCC 1977 mean - - - - - -2 s . e . - - . - - - -N 0 0 0 0 0 0 1978 mean -0 .05 0.05 1.18 - 0 . 15 -0 .20 0.06 2 s . e . 0.11 0.16 0.63 0.13 0.09 0.16 N 4 7 2 12 12 19 1 979 mean -0 .09 0. 16 0.68 0.00 0.54 0.11 2 s . e . 0.41 0.54 0.58 0.24 0.00 0.39 N 3 3 1 3 8 1 25 1980 mean 0.10 0.03 0.23 -0 .09 0.01 0.01 2s . e . 0.14 0.32 0.11 0.18 0.26 0.22 N 31 10 90 27 24 91 1 981 mean 0.24 0.15 0.97 0.11 -0.01 0.08 2s . e. 0.09 0.09 2.14 0.25 0.17 0.13 N 39 29 20 19 27 56 A d u l t s , Summer season Year JC 1982 mean 0.94 2 s . e . 1.63 N 1 1 1983 mean 0.29 2 s . e . 0.17 N 3 1984 mean 0.11 2 s . e . 0.42 N 3 Males Females JF SCC JC JF -0 .30 - 2.80 0.20 0.80 - 5.09 0.82 9 0 13 23 0 0 0 0 0.11 - 0.05 -0 .15 0.11 - 0.29 0.29 3 0 11 9 A d u l t s , F a l l season 104 Males Females Year JC JF SCC JC J F SCC 1977 mean 0.18 0.05 2 s . e . 0.14 0.00 N 2 1 0.16 0.13 4 •0. 13 0.00 1 •0.05 0.20 2 1978 mean 0.091 2 s . e . 0.31 N 7 0.80 0.75 1 5 0.10 0.71 2 -0.11 0.36 1 3 1979 mean 0.04 2 s . e . 0.00 N 1 0.09 0.11 2 •0.11 0.00 1 0.01 0.04 3 1980 mean 2s . e, N 0.00 0.09 53 0.05 0.09 51 1981 mean 0.17 1.13 -0 .12 0.20 0.15 0.01 2 s . e . 0.29 1.39 0.17 0.00 0.49 0.15 N 3 2 34 1 2 47 A d u l t s , F a l l season 1 05 Males Females Year JC JF SCC JC J F SCC 1982 mean 0.25 0.18 - 0 . 2 9 0.12 0.21 - 0 . 0 5 2 s . e . 0.04 0.02 0.19 0.15 0.18 0.00 N 3 2 3 2 3 1 1983 mean 2s . e. N 0.25 0.56 3 1984 mean 0.26 0.02 2 s . e . 0.31 0.39 N 5 2 •0.15 0.43 7 0.16 0.20 4 106 A d u l t s , W i n t e r s e a s o n M a l e s F e m a l e s Y e a r J C J F S C C J C J F S C C 1977 mean - 0 . 1 4 - 0 . 0 6 - 0.04 0.24 -2 s . e . 0.07 0.05 - 0.07 0.11 -N 7 7 0 5 7 0 1978 mean -0.21 - 0.00 0.10 - - 0 . 1 0 2 s . e . 0.10 - 0.22 0.89 - 0.15 N 3 - 5 2 - 6 1979 mean 0.03 - 0 . 0 7 - 0 . 3 3 - 0 . 0 6 0.17 - 0 . 1 9 2 s . e . 0.08 0.26 0.19 0.09 0.13 0.18 N 5 5 85 6 9 60 1 9 80 mean - 0 . 0 2 -0.19 - 0 . 1 4 - 0 . 0 2 0.08 - 0 . 2 6 2 s . e . 0.1 0 0.26 0.18 0.17 0.12 0.22 N 25 16 42 37 24 23 1981 mean -0.01 0.05 - 0 . 0 7 0.00 0.07 - 0 . 0 7 2 s . e . 0.27 0.16 0.16 0.24 0.12 0.38 N 5 22 3 1 6 23 3 A d u l t s , Winter season 107 Males Females Year JC J F SCC JC JF SCC 1982 mean 0.02 0.02 2 s . e . 0.23 0.17 N 6 4 •0.15 0.15 5 0.05 0.08 18 1983 mean 0.02 0.19 0.10 -0 .15 2 s . e . 0.36 0.38 0.00 0.06 N 8 11 1 3 0.27 -0.01 0.30 0.25 12 3 1984 mean -0 .08 2 s . e . 0.15 N 16 •0.10 0.16 7 0.36 0.32 2 108 A d u l t s , S p r i n g season Males Females Year JC JF SCC JC J F SCC 1 977 mean -0 .07 - 0 . 0 9 - 0.05 0.24 - 0 . 0 3 2s . e . 0.28 0.11 - 0.05 0.14 0.67 N 2 5 0 7 12 3 1978 mean -0 .12 -0 .20 -0 .10 0.22 0.21 0.19 2 s . e . 0.21 0.00 0.46 0.37 0.18 0.43 N 6 1 5 4 2 15 1 979 mean -0 .42 0.09 -0 .07 0.09 0.23 0.25 2s . e . 0.26 0.35 0.18 0.19 0.30 0.27 N 1 2 5 48 23 10 29 1 980 mean -0.11 - 0 . 0 7 - 0 . 1 9 0.13 0.06 -0.01 2s . e. 0.19 0.15 0.19 0.13 0.18 0.51 N 14 12 27 1 2 15 1 1 1 981 mean -0 .26 -0 .22 - 0.03 0.10 -2s. e. 0.26 0.11 - 0.22 0.07 -N 4 10 0 6 23 0 109 A d u l t s , S p r i n g season Males Females Year JC JF SCC JC JF SCC 1982 mean -0.29 -0.28 - -0.22 0.11 2s.e. 0.20 0.65 - 0.59 0.20 N 6 3 0 4 19 0 1983 mean - -0.17 - - 0.24 2s.e. - 0.13 - - 0.21 N O 9 0 0 15 0 1984 mean -0.22 -0.02 -1.35 -0.18 0.42 0.16 2s.e. 0.34 0.29 0.00 0.00 0.09 0.00 N 2 4 1 1 4 1 

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