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Habitat use by small mammals in the Southwestern Yukon : the role of competitive interactions Galindo, Carlos 1984

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HABITAT USE BY SMALL MAMMALS IN THE SOUTHWESTERN YUKON: THE ROLE OF COMPETITIVE INTERACTIONS by CARLOS GALINDO L i e . U n i v . A u t o n . M e t r o p o l i t a n a M e x i c o 1979 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department o f Z o o l o g y ) We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA September 1984 © C a r l o s G a l i n d o , 1 984 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I agree t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date Oc^O^ M DE-6 (3/81) . i i A B S T R A C T M a n y s p e c i e s o f s m a l l m a m m a l s l i v e i n t h e s o u t h w e s t e r n Y u k o n . M o s t o f t h e m l i v e i n d i f f e r e n t h a b i t a t s o r o v e r l a p v e r y l i t t l e . H o w e v e r , t h e i r u s e o f h a b i t a t s c h a n g e s a s t h e i r p o p u l a t i o n s f l u c t u a t e . I n t h i s s t u d y I i n v e s t i g a t e d how f a r c o m p e t i t i v e i n t e r a c t i o n s i n f l u e n c e t h e u s e o f h a b i t a t s a n d r e l a t i v e a b u n d a n c e o f t h e c o m m o n s p e c i e s o f m i c e . I w o r k e d i n t w o e c o t o n e s . F i r s t , t h e e c o t o n e b e t w e e n a l p i n e m e a d o w s a n d s u b a l p i n e s h r u b t u n d r a . H e r e , p o p u l a t i o n s o f s i n g i n g v o l e s ( M i c r o t u s m i u r u s ) a n d t u n d r a v o l e s ( M . o e c o n o m u s ) l i v i n g i n e a c h h a b i t a t r e s p e c t i v e l y , o v e r l a p l i t t l e . S e c o n d , t h e e c o t o n e b e t w e e n b o r e a l f o r e s t a n d s e d g e m e a d o w . H e r e , p o p u l a t i o n s o f d e e r m o u s e ( P e r o m y s c u s m a n i c u l a t u s ) a n d r e d - b a c k e d v o l e ( C l e t h r i o n o m y s r u t i l u s ) i n h a b i t i n g t h e f o r e s t , m e e t w i t h m e a d o w v o l e s ( M . p e n n s y l v a n i c u s ) l i v i n g i n t h e s e d g e m e a d o w s . I f i n t e r s p e c i f i c c o m p e t i t i o n i s i n f l u e n c i n g t h e h a b i t a t u s e a n d r e l a t i v e a b u n d a n c e o f t h e s e s p e c i e s , t h e n r e m o v a l o f o n e o f t h e m w i l l c h a n g e h a b i t a t u s e a n d a b u n d a n c e o f t h e r e m a i n i n g s p e c i e s . I n t h e f i r s t f i e l d s e a s o n , I r e m o v e d t u n d r a v o l e s t o l o o k a t t h e e f f e c t o n t h e c o n t i g u o u s p o p u l a t i o n o f s i n g i n g v o l e s . S i m i l a r l y , I r e m o v e d m e a d o w v o l e s t o l o o k a t t h e e f f e c t o n t h e c o n t i g u o u s p o p u l a t i o n o f d e e r m o u s e . I n b o t h p l a c e s I u s e d a n e x p e r i m e n t a l a n d a c o n t r o l l i v e - t r a p p i n g g r i d . I n b o t h c a s e s t h e r e m o v a l o f o n e s p e c i e s h a d n o e f f e c t o n t h e o t h e r s p e c i e s ' d i s t r i b u t i o n o r d e m o g r a p h y . I n t h e s e c o n d f i e l d s e a s o n , s i n g i n g v o l e s a n d t u n d r a v o l e s d i s a p p e a r e d f r o m t h e a r e a . M e a d o w v o l e s , i n t u r n , d e c l i n e d t o v e r y l o w n u m b e r s a n d I u s e d t h e i r n a t u r a l f l u c t u a t i o n a s a r e m o v a l e x p e r i m e n t . D e e r m o u s e p o p u l a t i o n s w e r e n o t a f f e c t e d e v e n w h e n t h e n a t u r a l d e c l i n e o f m e a d o w v o l e s w a s m o r e e f f e c t i v e o n m a i n t a i n i n g t h e s e a d g e m e a d o w f r e e o f v o l e s t h a n t h e p r e v i o u s r e m o v a l m a n i p u l a t i o n w a s . I n t h e t h i r d y e a r , m e a d o w v o l e s c o l o n i z e d t w o a r e a s o f f o r e s t w h e r e d e e r m i c e h a d b e e n a l o n e t h e p r e v i o u s t w o f i e l d s e a s o n s . R e d -b a c k e d v o l e s , i n t u r n , i n c r e a s e d f r o m v e r y l o w n u m b e r s i n f o u r g r i d s . T o l o o k a t t h e e f f e c t o f m e a d o w v o l e s o n d e e r m i c e I e x a m i n e d t h e s p a t i a l d i s t r i b u t i o n a n d a b u n d a n c e o f t h e l a t t e r s p e c i e s i n a r e a s w h e r e v o l e s w e r e a b s e n t a s w e l l a s i n t h e s a m e p l a c e s d u r i n g p r e v i o u s y e a r s . T o l o o k a t t h e e f f e c t o f r e d -b a c k e d v o l e s o n d e e r m i c e , I e x a m i n e d t h e s p a t i a l d i s t r i b u t i o n a n d n u m b e r s o f t h e l a t t e r s p e c i e s i n p l a c e s w i t h l o w a n d h i g h d e n s i t i e s o f r e d - b a c k e d v o l e s . N e i t h e r m e a d o w v o l e s n o r r e d -b a c k e d v o l e s a f f e c t e d t h e s p a t i a l d i s t r i b u t i o n o r a b u n d a n c e o f d e e r m i c e . T h e r e s u l t s o f t h i s s t u d y i n d i c a t e t h a t c o m p e t i t i v e i n t e r a c t i o n s h a v e n o i n f l u e n c e o n t h e u s e o f h a b i t a t s a n d r e l a t i v e a b u n d a n c e o f t h e c o m m o n s p e c i e s o f s m a l l mammal s p e c i e s i n t h e a r e a . H o w e v e r , a s i t i s e v i d e n t f r o m t h i s s t u d y , p o p u l a t i o n s o f s m a l l m a m m a l s i n t h e a r e a a r e e x t r e m e l y d y n a m i c . S o m e y e a r s t h e y a r e s c a r c e a n d s o m e o t h e r s t h e y a r e a b u n d a n t . S i n c e c o m p e t i t i v e i n t e r a c t i o n s may c h a n g e a s d e n s i t y i n c r e a s e s , t h e c o n c l u s i o n s o f t h i s s t u d y a r e r e s t r i c t e d t o t h e d e n s i t i e s a t w h i c h I f o u n d t h e i r p o p u l a t i o n s . i v T A B L E O F C O N T E N T S A B S T R A C T i i L I S T OF T A B L E S v i i L I S T OF F I G U R E S v i i i A C K N O W L E D G E M E N T S X S E C T I O N 1. G E N E R A L I N T R O D U C T I O N : H A B I T A T U S E BY S M A L L MAMMALS 1 S E C T I O N 2 . H A B I T A T U S E BY S I N G I N G V O L E S AND T U N D R A V O L E S : A R E M O V A L E X P E R I M E N T . 6 2 . 1 I n t r o d u c t i o n 6 2 . 2 S t u d y a r e a 8 2 . 3 M e t h o d s 11 2 . 4 R e s u l t s 17 P a t t e r n s o f h a b i t a t u s e 17 H a b i t a t u s e 17 M i c r o h a b i t a t u s e 21 R e m o v a l e x p e r i m e n t 24 S p a t i a l d i s t r i b u t i o n 25 M o v e m e n t s b e t w e e n g r i d s 31 M o v e m e n t s w i t h i n g r i d s 35 P o p u l a t i o n D y n a m i c s 35 N u m b e r s 38 R e p r o d u c t i o n 42 S u r v i v a l 42 2 . 5 D i s c u s s i o n 44 SECTION 3. HABITAT USE AND ABUNDANCE OF DEER MICE: INTERACTIONS WITH MEADOW VOLES AND RED-BACKED VOLES. 49 3.1 I n t r o d u c t i o n ....49 3.2 Study area 55 3.3 Methods 58 P a t t e r n s 58 Process 60 Trapping technique 61 Experimental design 1981 61 Experimental design 1982 62 Experimental design 1983 64 3.4 R e s u l t s 67 P a t t e r n s of h a b i t a t use 67 H a b i t a t use 67 M i c r o h a b i t a t use 71 Removal experiments 74 E f f e c t i v e n e s s of removal m a n i p u l a t i o n s 74 S p a t i a l d i s t r i b u t i o n 80 Po p u l a t i o n dynamics 85 Numbers 85 Reproduction 89 S u r v i v a l 91 I n t r o d u c t i o n experiments 93 S p a t i a l d i s t r i b u t i o n 93 P o p u l a t i o n dynamics 98 Numbers 98 Meadow vo l e s 99 v i R e d - b a c k e d v o l e s 99 R e p r o d u c t i o n 102 S u r v i v a l 106 3 . 5 D i s c u s s i o n 107 SECTION 4. COMPETITION THEORY AND THE STRUCTURE OF S C I E N T I F I C REVOLUTIONS 119 4.1 I n t r o d u c t i o n 119 4.2 The s t r u c t u r e of s c i e n t i f i c r e v o l u t i o n s 121 4.3 P a r a d i g m f o r m a t i o n 122 4.4 Normal s c i e n c e 129 4.5 C r i s i s p r e - p a r a d i g m s t a g e 131 4.6 R e v o l u t i o n 139 REFERENCES 141 v i i L I S T OF TABLES T a b l e 2.1. R e s o u r c e m a t r i x f o r s n a p - t r a p d a t a 20 T a b l e 2.2. R e s o u r c e m a t r i x f o r c o n t r o l g r i d 24 T a b l e 2.3. R e s o u r c e m a t r i x f o r e x p e r i m e n t a l g r i d 31 T a b l e 2.4. S i n g i n g v o l e s s u r v i v a l r a t e s 43 T a b l e 3.1. E x p e r i m e n t a l d e s i g n f o r 1981 62 T a b l e 3.2. E x p e r i m e n t a l d e s i g n f o r 1982 63 T a b l e 3.3. E x p e r i m e n t a l d e s i g n f o r 1983 67 T a b l e 3.4. R e s o u r c e m a t r i x f o r s n a p - t r a p d a t a 68 T a b l e 3.5. Deer mouse r e p r o d u c t i v e d a t a (1981-1 982) 89 T a b l e 3.6. Deer mouse s u r v i v a l r a t e s (1981-1982) 92 T a b l e 3.7. Use o f t r a p s t a t i o n s by p a i r s o f s p e c i e s 98 T a b l e 3.8. Deer mouse r e p r o d u c t i v e d a t a (1983) 105 T a b l e 3.9. Deer mouse s u r v i v a l r a t e s ( 1983) 106 T a b l e 3.10. Summary o f r e s u l t s 113 v i i i LIST OF FIGURES F i g u r e 2.1. Map of the study area 9 F i g u r e 2.2. L i v e - t r a p p i n g g r i d s 12 F i g u r e 2.3. Densit y i n d i c e s f o r s i n g i n g and tundra v o l e s ...18 F i g u r e 2.4. M i c r o h a b i t a t use by s i n g i n g v o l e s and tundra v o l e s on the c o n t r o l g r i d 22 F i g u r e 2.5. Number of tundra v o l e s removed 26 F i g u r e 2.6. M i c r o h a b i t a t use by s i n g i n g v o l e s on experimental g r i d 29 F i g u r e 2.7. Movements between g r i d s 33 F i g u r e 2.8. Movements w i t h i n the c o n t r o l g r i d 36 F i g u r e 2.9. Minimum number a l i v e f o r both tundra and s i n g i n g v o l e s 40 F i g u r e 3.1. Map of the study area 56 F i g u r e 3.2. L o c a t i o n of the l i v e - t r a p p i n g g r i d s 65 F i g u r e 3.3. Density i n d i c e s f o r meadow v o l e s and deer mice i n d i f f e r e n t h a b i t a t s 69 F i g u r e 3.4. H a b i t a t d i s t r i b u t i o n by deer mice and meadow v o l e s on c o n t r o l g r i d 72 F i g u r e 3.5. I n t e n s i t y of t r a p use by meadow v o l e s 76 F i g u r e 3.6. Number of meadow v o l e s removed from experimental g r i d s 78 F i g u r e 3.7. E f f e c t of removal treatments on numbers 81 F i g u r e 3.8. H a b i t a t use by deer mice 83 Fi g u r e 3.9. Changes i n minimum number a l i v e of deer mice d u r i n g 1981 and 1982 86 F i g u r e 3.10. Summary o f s p a t i a l d i s t r i b u t i o n and abundance d u r i n g 1981 94 F i g u r e 3.11. Summary of s p a t i a l d i s t r i b u t i o n and abundance d u r i n g 1982 96 F i g u r e 3.12. Minimum number a l i v e of d e e r m i c e and meadow v o l e s 100 F i g u r e 3.13. Minimum number a l i v e o f d e e r m i c e and r e d -b a c k e d v o l e s 103 F i g u r e 3.14. Summary o f s p a t i a l d i s t r i b u t i o n and abundance of d e e r m i c e and meadow v o l e s 108 F i g u r e 3.15. Summary o f s p a t i a l d i s t r i b u t i o n and abundance of d e e r m i c e and meadow v o l e s 110 X A C K N O W L E D G E M E N T S T h r o u g h o u t t h e d e v e l o p m e n t o f t h i s p r o j e c t a n u m b e r o f p e o p l e w e r e i n v o l v e d i n m a n y w a y s . T h e i r c o n s t a n t h e l p a n d f r i e n d s h i p m a d e t h e r e s e a r c h m o r e t h a n p l e a s e n t . F i r s t o f a l l , I w a n t t o t h a n k my a d v i s o r C h a r l e y K r e b s f o r h i s h e l p f u l s u g g e s t i o n s , c o n t i n u o u s e n c o u r a g e m e n t a n d h i s c o n t a g i o u s e n j o y m e n t f o r f i e l d w o r k . H i s e x c e l l e n t s e n s e o f h u m o r k e p t t h e c r e w a l w a y s i n g o o d s p i r i t s . D u r i n g t h e f i e l d w o r k I r e c e i v e d c o n s t a n t a s s i s t a n c e f r o m M a r g a r e t D e P o o r t e r , S t a n B o u t i n ' t h e k i d ' , J e a n C a r e y , S c o t t G i l b e r t , H e n r i c k , P a u l L a n g e v i n , J o y S n y d e r , J e n n i f e r R o b i n s o n , R i c h a r d W a r d a n d J o h n K r e b s . A l l o f t h e m c o n t r i b u t e d d o i n g a v a r i e t y o f j o b s a n d g r a n t e d me w i t h t h e i r s u p e r b f r i e n d s h i p . I t h a n k S i a n , M e g a n , C a r o l a n d A n d y W i l l i a m s a n d P h i l U p t o n o f t h e A r c t i c I n s t i t u t e o f N o r t h A m e r i c a f o r t h e i r h o s p i t a l i t y a n d c o m p a n y d u r i n g t h e s u m m e r s . D u r i n g t h e w i n t e r s a t t h e I n s t i t u t e o f A n i m a l R e s o u r c e E c o l o g y , S t a n B o u t i n , M a r c o R o d r i g u e z , J u a n M e r k t , D a v e B e r n a r d a n d L a u r a J a m i e s o n s p e n d a l o t o f t h e i r t i m e l i s t e n i n g t o my s p o k e n t h o u g h t s . T h e y a l l r e a d p a r t s o f t h e m a n u s c r i p t a n d m a d e c o n s t r u c t i v e c r i t i c i s m s . x i I t h a nk my s u p e r v i s o r y c o m i t t e e ; Dr. C . J . K r e b s , D r . A.R.E. S i n c l a i r , D r . L. G a s s , D r . D. C h i t t y and D r . J.N.M. S m i t h as w e l l as D r . M.J. T a i t t f o r t h e i r p a t i e n c e , c o n s t a n t m o t i v a t i o n and wi s e s u g g e s t i o n s . I thank C o n s e j o N a c i o n a l de C i e n c i a y T e c n o l o g i a (CONACYT), t h e U.B.C. A r c t i c and A l p i n e R e s e a r c h C o m i t t e e and C h a r l e y K r e b s f o r f i n a n c i a l s u p p o r t . F i n a l l y I thank f e m a l e d e e r mouse 5199 f o r h e r c o l l a b o r a t i o n , i n t e r e s t and p e r s i s t a n c e d u r i n g t h e t h r e e summers of t h e s t u d y . T h i s t h e s i s i s d e d i c a t e d t o my f a m i l y t h a t has a l w a y s g i v e n me s u p p o r t and encouragement t h r o u g h o u t my e d u c a t i o n . 1 SECTION 1. GENERAL INTRODUCTION: HABITAT USE BY SMALL MAMMALS. Many s p e c i e s o f s m a l l r o d e n t s l i v e i n t h e s o u t h w e s t e r n Yukon. N i n e s p e c i e s o f m i c e have been r e c o r d e d i n K l u a n e Lake a r e a ( K r e b s and W i n g a t e 1976). Seven s p e c i e s of v o l e s and lemmings o c c u r ; t h e y a r e c l o s e l y r e l a t e d , m o r p h o l o g i c a l l y s i m i l a r , and f e e d m a i n l y on g r e e n p a r t s o f p l a n t s . T h e i r p o p u l a t i o n s f l u c t u a t e w i d e l y i n numbers w i t h y e a r s of h i g h abundance and y e a r s of n e a r a b s e n c e . I n c o n t r a s t , t h e non-m i c r o t i n e s p e c i e s , d e e r mice and meadow jumping m i c e , a r e m a i n l y s e e d - and i n s e c t - e a t e r s and t h e i r p o p u l a t i o n s a r e more s t a b l e . Most o f t h e s e s p e c i e s l i v e i n d i f f e r e n t h a b i t a t s o r o v e r l a p v e r y l i t t l e i n t h e h a b i t a t s t h e y u s e , but t h e i r use o f h a b i t a t s and o v e r l a p c hange as d e n s i t i e s i n c r e a s e o r d e c r e a s e ( K r e b s and Wi n g a t e 1976). What f a c t o r s i n f l u e n c e t h e use o f h a b i t a t s by t h e s e s p e c i e s ? A number o f a s s o c i a t i o n s between t h e abundance of r o d e n t p o p u l a t i o n s and s p e c i f i c c h a r a c t e r i s t i c s o f t h e h a b i t a t s have been documented. F o r example h i g h e r abundance o r h i g h e r i n t e n s i t y o f use by s m a l l mammals have been c o r r e l a t e d w i t h d e n s i t y o f p a r t i c u l a r p l a n t s p e c i e s (Brown and L i e b e r m a n 1973), f l o r i s t i c g r o u p s ( C o c k b u r n 1981), s p e c i f i c f o o d t y p e s ( D r i c k a m e r 1976), f o l i a g e d e n s i t y ( R o s e n z w e i g 1973), d e n s i t y of l o g s , stumps and r o c k s , and d i v e r s i t y o f s h r u b b r a n c h i n g p a t t e r n s ( M ' C l o s k e y 1975b), and s o i l t y p e ( G h i s e l i n 1970). E x p e r i m e n t a l m a n i p u l a t i o n s have a l s o p r o v i d e d some 2 c l u e s a b o u t f a c t o r s i n f l u e n c i n g h a b i t a t s e l e c t i o n . An i n c r e a s e i n v e g e t a t i o n c o v e r i n a s h o r t g r a s s p r a i r i e r e s u l t e d i n e x c l u s i o n o f two s p e c i e s , whereas t h e a d d i t i o n o f s e e d s l e d t o t h e i n v a s i o n and c o l o n i z a t i o n of a s p e c i a l i z e d s e e d - e a t i n g s p e c i e s w h i c h had been a b s e n t b e f o r e (Abramsky e_t a l . 1979). R o s e n z w e i g (1973) a l t e r e d v e g e t a t i o n s t r u c t u r e i n a s h r u b d e s e r t community and f o u n d t h a t a d e c r e a s e i n v e g e t a t i o n c o v e r had a n e g a t i v e e f f e c t on p o c k e t mice ( P e r o g n a t h u s p e n i c i l l a t u s ) whereas k a n g a r o o r a t s (Dipodomys m e r r i a m i ) were f a v o u r e d . The a l t e r n a t i v e t r e a t m e n t , i n c r e a s i n g v e g e t a t i o n c o v e r , had t h e o p p o s i t e e f f e c t . P r i c e ( 1 9 7 8 ) , o b t a i n e d s i m i l a r r e s u l t s f o r k a n g a r o o r a t s a f t e r r e m o v i n g c o v e r . Thompson (1982) r e d u c e d p r e d a t i o n r i s k i n a s h r u b d e s e r t h a b i t a t , p r o v i d i n g a r t i f i c i a l s h e l t e r s i n open a r e a s , t o d e c r e a s e t h e amount of e x p o s u r e . Two p r e v i o u s l y - a b s e n t s p e c i e s c o l o n i z e d t h e e x p e r i m e n t a l p l o t s . On t h e o t h e r hand, c o m p l e t e r e m o v a l of s h r u b s i n a s h r u b - s t e p p e community o v e r one y e a r had l i t t l e o r no e f f e c t on f o u r s p e c i e s o f r o d e n t s ( P a r m e n t e r and MacMahon 1983). H a b i t a t s e l e c t i o n t h e o r y a t t e m p t s t o p r e d i c t t h e c o n s e q u e n c e s o f d e n s i t y on t h e d i s t r i b u t i o n o f s p e c i e s i n d i f f e r e n t h a b i t a t s ( F r e t w e l l and L u c a s 1970). H a b i t a t s a r e n o t i d e n t i c a l w i t h r e s p e c t t o t h e p h y s i c a l and b i o l o g i c a l c h a r a c t e r i s t i c s r e l e v a n t t o t h e s u r v i v a l o f i n d i v i d u a l s . T h e i r s u i t a b i l i t y i s d e t e r m i n e d by s e v e r a l f a c t o r s s u c h as f o o d s u p p l y , p r o t e c t i o n f r o m p r e d a t o r s , a v a i l a b i l i t y o f w a t e r , and n e s t s i t e s . Some of t h e s e f a c t o r s a r e d e n s i t y - d e p e n d e n t and, t h e r e f o r e , t h e s u i t a b i l i t y of h a b i t a t s d e c r e a s e s as d e n s i t y 3-i n c r e a s e s . A n u m b e r o f s i t u a t i o n s may o c c u r . I f p o p u l a t i o n d e n s i t y i s l o w , t h e m o s t s u i t a b l e h a b i t a t may n o t b e c o m p l e t e l y u t i l i z e d , a n d o t h e r h a b i t a t s w i l l n o t b e o c c u p i e d . I f p o p u l a t i o n d e n s i t y i s h i g h a n d l e s s s u i t a b l e h a b i t a t s a r e a v a i l a b l e , t h e n a l a r g e p r o p o r t i o n o f t h e p o p u l a t i o n may o c c u p y t h e l e s s s u i t a b l e h a b i t a t s . F i n a l l y , i f p o p u l a t i o n d e n s i t y v a r i e s f r o m y e a r t o y e a r , a n a r r o w e r r a n g e o f h a b i t a t s may b e o c c u p i e d i n y e a r s o f l o w a b u n d a n c e . S u c h c h a n g e s may i n d i c a t e t h e r a n k o r d e r o f h a b i t a t s u i t a b i l i t y . S o m e a u t h o r s h a v e d o c u m e n t e d a w i d e r r a n g e i n t h e u s e o f h a b i t a t a s p o p u l a t i o n s i n c r e a s e b y m o n i t o r i n g n a t u r a l f l u c t u a t i o n s ( B e n d e l l 1 9 6 1 , S t o e c k e r 1 9 7 2 , P i t e l k a 1 9 7 3 , K r e b s a n d W i n g a t e 1 9 7 6 ) a n d i n e n c l o s u r e e x p e r i m e n t s ( G r a n t 1 9 7 1 ) . M e r k t ( 1 9 8 0 ) s t u d i e d d e e r m o u s e c o l o n i z a t i o n o f t w o a r t i f i c i a l l y e m p t i e d h a b i t a t s o n a n i s l a n d . A t l o w d e n s i t i e s d e e r m i c e s e t t l e d o n l y i n o n e o f t h e h a b i t a t s , a n d a s p o p u l a t i o n d e n s i t y i n c r e a s e d t h e y u t i l i z e d t h e a d j a c e n t h a b i t a t . On t h e o t h e r h a n d , s o m e p o p u l a t i o n s d o n o t s e e m t o f o l l o w t h i s p a t t e r n ( K r e b s a n d W i n g a t e 1 9 7 6 ) . P o p u l a t i o n s o f o t h e r s p e c i e s l i v i n g a t s a t u r a t i o n d e n s i t i e s i n a d j a c e n t h a b i t a t s may a c t a s p o t e n t i a l c o m p e t i t o r s . T h e y m a y r e d u c e h a b i t a t s u i t a b i l i t y o f e a c h o t h e r ' s h a b i t a t , o r i n t e r f e r e d i r e c t l y t h r o u g h a g g r e s s i v e i n t e r a c t i o n s o r a v o i d a n c e . T r a d i t i o n a l l y , n o n - o v e r l a p p i n g c o n t i g u o u s d i s t r i b u t i o n a l p a t t e r n s a m o n g r e l a t e d s p e c i e s o r s m a l l h a b i t a t o v e r l a p h a v e b e e n i n t e r p r e t e d a s i n d i r e c t e v i d e n c e o f s t r o n g c o m p e t i t i o n 4 ( G r a n t 1972a, Brown 1973, 1975). In t h i s v i e w , i f p o p u l a t i o n s have r e a c h e d s a t u r a t i o n d e n s i t i e s , t h e n i n t e r s p e c i f i c i n t e r a c t i o n s a c t t o r e d u c e h a b i t a t e x p a n s i o n , w h i l e i n t r a s p e c i f i c i n t e r a c t i o n s a c t t o i n c r e a s e h a b i t a t u s a g e . A number of o b s e r v e d p a t t e r n s have been r e l a t e d t o c o m p e t i t i o n . H i g h s p e c i e s d i v e r s i t y has been i n t e r p r e t e d a s a r e s u l t of i n t e r s p e c i f i c c o m p e t i t i o n r e d u c i n g n i c h e b r e a d t h ( M a c A r t h u r a n d W i l s o n 1967). C l o s e l y r e l a t e d s p e c i e s a r e assumed t o compete more s t r o n g l y , s i n c e t h e i r m o r p h o l o g y and r e q u i r e m e n t s a r e s i m i l a r (Brown and W i l s o n 1956). S m a l l o v e r l a p i n h a b i t a t u s e has been v i e w e d as a p r o d u c t o f i n t e n s e c o m p e t i t i o n ( G u t h r i e 1971, G r a n t 1972a, Brown 1973, 1975). The o b j e c t i v e o f t h i s r e s e a r c h was t o i n v e s t i g a t e t o what e x t e n t c o m p e t i t i v e i n t e r a c t i o n s a r e r e s p o n s i b l e f o r t h e s p a t i a l d i s t r i b u t i o n and r e l a t i v e abundance o f t h e common s m a l l r o d e n t s p e c i e s o f t h e K l u a n e a r e a . I s t u d i e d f i v e s p e c i e s o f s m a l l r o d e n t s i n two e c o s y s t e m s . S i n g i n g v o l e s ( M i c r o t u s  m i u r u s ) l i v e i n t h e a l p i n e t u n d r a a d j a c e n t t o t h e s u b a l p i n e s h r u b t u n d r a i n h a b i t e d by t u n d r a v o l e s ( M i c r o t u s oeconomus). Deer mice ( P e r o m y s c u s m a n i c u l a t u s ) l i v e i n b o r e a l f o r e s t a d j a c e n t t o sedge meadows i n h a b i t e d by meadow v o l e s ( M i c r o t u s  p e n n s y l v a n i c u s ) . T h e s e two p a i r s o f s p e c i e s show t h e same d i s t r i b u t i o n a l p a t t e r n : c o n t i g u o u s n o n - o v e r l a p p i n g p o p u l a t i o n s d e f i n e d by a h a b i t a t change ( s m a l l h a b i t a t o v e r l a p ) . T h e i r o v e r l a p , however, v a r i e s i n r e l a t i o n t o c h a n g e s i n abundance of t h e c y c l i c p o p u l a t i o n s ( K r e b s and Win g a t e 1976). In c o n t r a s t , 5 d e e r mice and r e d - b a c k e d v o l e s ( C l e t h r i o n o m y s r u t i l u s ) use s i m i l a r h a b i t a t s ( h i g h h a b i t a t o v e r l a p ) . T h e s e s p e c i e s , t h e most common i n t h e a r e a , have t h e h i g h e r h a b i t a t n i c h e b r e a d t h s , and t h e i r o v e r l a p v a r i e s w i t h p o p u l a t i o n d e n s i t y ( K r e b s and Wi n g a t e 1976). T h i s t h e s i s i s d i v i d e d i n t o t h r e e s e c t i o n s . The f i r s t d e a l s w i t h t h e p a t t e r n s o f h a b i t a t d i s t r i b u t i o n o f s i n g i n g v o l e s and t u n d r a v o l e s and t h e r o l e o f c o m p e t i t i v e i n t e r a c t i o n s i n d e t e r m i n i n g t h e s e p a t t e r n s . S e c t i o n two d e a l s w i t h t h e d i s t r i b u t i o n a l p a t t e r n s o f meadow v o l e s and d e e r mice and t h e e f f e c t of t h e p r e s e n c e of meadow v o l e s and r e d - b a c k e d v o l e s on d e e r mouse s p a t i a l d i s t r i b u t i o n and abundance. S i n c e t h e r o l e o f c o m p e t i t i o n i n n a t u r a l c o m m u n i t i e s has l a t e l y been t h e f o c u s o f a h e a t e d c o n t r o v e r s y ( S c h o e n e r 1982, S a l t 1984), t h e l a s t s e c t i o n d e a l s w i t h t h e h i s t o r i c a l d e v e l o p m e n t o f t h e i d e a t h a t community s t r u c t u r e i s s t r o n g l y i n f l u e n c e d by c o m p e t i t i v e i n t e r a c t i o n s . 6 SECTION 2. HABITAT USE BY SINGING VOLES AND TUNDRA VOLES: A REMOVAL EXPERIMENT. 2.j_ INTRODUCTION In a r e a s where p r a i r i e v o l e s ( M i c r o t u s o c h r o g a s t e r ) and montane v o l e s ( M i c r o t u s montanus) a r e a b s e n t , meadow v o l e s (M. p e n n s y l v a n i c u s ) o c c u p y a w i d e r range o f h a b i t a t s ( F i n d l e y 1954). T h i s e a r l y o b s e r v a t i o n , c o n s i s t e n t w i t h t h e e m e r g i n g view of t h e i m p o r t a n c e of i n t e r s p e c i f i c c o m p e t i t i o n i n n a t u r a l c o m m u n i t i e s , l e d F i n d l e y (1954) t o s u g g e s t t h a t c o m p e t i t i o n among c l o s e l y r e l a t e d m i c r o t i n e s ( A r v i c o l i d a e ) was a t l e a s t p a r t l y r e s p o n s i b l e f o r s u c h d i s t r i b u t i o n a l p a t t e r n s . S i n c e t h e n , a number o f s t u d i e s , some on p a i r s o f m i c r o t i n e s p e c i e s , have been u n d e r t a k e n t o l o o k i n t o t h e r o l e t h a t i n t e r s p e c i f i c i n t e r a c t i o n s p l a y i n f l u e n c i n g t h e d i s t r i b u t i o n and abundance of t h e s e s p e c i e s ( G r a n t 1972a, 1978). The m o r p h o l o g i c and d i e t a r y s i m i l a r i t i e s among m i c r o t i n e s make them p o t e n t i a l c a n d i d a t e s f o r a s t r o n g c o m p e t i t i v e i n t e r a c t i o n . A number of m i c r o t i n e s p e c i e s use h a b i t a t s s i m i l a r l y . Many s p e c i e s o c c u p y c o n t i g u o u s a r e a s a l o n g w e l l - d e f i n e d e c o t o n e s between s t r u c t u r a l l y d i f f e r e n t h a b i t a t s ( K o p l i n and Hoffman 1968, G u t h r i e 1971, S t o e c k e r 1972, I v e r s o n and T u r n e r 1972, M o r r i s and G r a n t 1972, T u r n e r e t a l . 1975, Hawes 1976, R a n d a l l 1978). However, w h i l e most s t u d i e s have s u g g e s t e d t h a t c o m p e t i t i o n f o r s p a c e m e d i a t e d t h r o u g h a g g r e s s i v e b e h a v i o r i s 7 r e s p o n s i b l e f o r t h e d i s t r i b u t i o n a l p a t t e r n , t h e e v i d e n c e has been u n c o n v i n c i n g . F u r t h e r m o r e , o t h e r s t u d i e s have f o u n d no n e g a t i v e e f f e c t s among p a i r s o f s p e c i e s (Hawes 1976, K r e b s 1977) or have s u g g e s t e d t h a t i n some a r e a s h a b i t a t p r e f e r e n c e s may be s u f f i c i e n t t o e x p l a i n t h e p a t t e r n s documented ( D o u g l a s s 1976). I s t u d i e d two m o r p h o l o g i c a l l y s i m i l a r s p e c i e s o f m i c r o t i n e s : t u n d r a v o l e s and s i n g i n g v o l e s . T u n d r a v o l e s ( M i c r o t u s oeconomus) o c c u r i n m o i s t t u n d r a a r e a s , sedge and c o t t o n g r a s s marshes ( B a n f i e l d 1974, Youngman 1975). In t h e a r e a o f s t u d y t h e y have been f o u n d i n a l p i n e and s u b a l p i n e t u n d r a and marshes as w e l l as i n s p r u c e f o r e s t ( K r e b s and W i n g a t e 1976). S i n g i n g v o l e s (M. m i u r u s ) , on t h e o t h e r hand, a r e m a i n l y c o n f i n e d t o a l p i n e t u n d r a ( B a n f i e l d 1974, Youngman 1975) but have a l s o been r e c o r d e d a t s u b a l p i n e t u n d r a , marshes and s p r u c e f o r e s t ( K r e b s and W i n g a t e 1976). D u r i n g 1981, I s t u d i e d t h e h a b i t a t use by t h e s e s p e c i e s and t e s t e d e x p e r i m e n t a l l y t h e h y p o t h e s i s t h a t c o m p e t i t i v e i n t e r a c t i o n s i n f l u e n c e t h e i r h a b i t a t d i s t r i b u t i o n and r e l a t i v e a b u n dance. I f i n t e r s p e c i f i c i n t e r a c t i o n s a c t i v e l y i n f l u e n c e t h e s p a t i a l d i s t r i b u t i o n o f t h e s e s p e c i e s , t h e h y p o t h e s i s p r e d i c t s t h a t r e m o v a l of one s p e c i e s w i l l r e s u l t i n a h a b i t a t change and d e n s i t y i n c r e a s e by t h e o t h e r s p e c i e s . 8 2.2 STUDY AREA The f i e l d work was c a r r i e d o ut d u r i n g t h e s n o w f r e e s e a s o n f r o m A p r i l t o September 1981, i n t h e K l u a n e Ranges, 7 k i l o m e t e r s s o u t h of t h e s o u t h c o r n e r o f K l u a n e L a k e , Yukon (61 N, 138 W) ( F i g . 1 ) . I worked i n two v e g e t a t i o n z o n e s : a l p i n e t u n d r a and s u b a l p i n e s h r u b t u n d r a ( H o e f s e t §_1. 1975), w h i c h I c o n s i d e r e d t o be d i f f e r e n t h a b i t a t s . A l p i n e t u n d r a o c c u p i e s t h e m o u n t a i n t o p s above a p p r o x i m a t e l y 1500 m. D w a r f e d v a s c u l a r p l a n t s , moss and l i c h e n s d o m i n a t e t h i s community. S h r u b s a r e v e r y s c a r c e o r c o m p l e t e l y a b s e n t . Some of t h e common p l a n t s p e c i e s a r e h e a t h e r ( C a s s i o p e ) , dwarf w i l l o w ( S a l i x s p p . ) , a r c t i c l u p i n e ( L u p i n u s ) , b l u e b e r r y ( V a c c i n i u m ) , b e a r b e r r y ( A r c t o s t a p h y l o s ) , dwarf b i r c h ( B e t u l a ) and a v a r i e t y o f s e d g e s and g r a s s e s . Below t h e a l p i n e t u n d r a and above t h e t r e e l i n e , from 1100 t o 1500 m, t h e s u b a l p i n e s h r u b t u n d r a forms a b r o a d b e l t c h a r a c t e r i z e d by t a l l s h r u b s (up t o 3 m), m o s t l y w i l l o w ( S a l i x  g l a u c a and S. r e t i c u l a t a ) and dwarf b i r c h ( B e t u l a q l a n d u l o s a ) . In some a r e a s t h e c o v e r i s v e r y d e n s e , f o r m i n g i m p e n e t r a b l e t h i c k e t s . V a s c u l a r p l a n t s s u c h a s h e a t h e r • ( C a s s i o p e ) and g r a s s e s ( F e s t u c a ) a r e common i n t h e u n d e r s t o r y . 9 F i g u r e 2 . 1 . M a p o f t h e s t u d y a r e a i n s o u t h w e s t e r n Y u k o n . 10 11 2 . 3 M E T H O D S H a b i t a t u s e p a t t e r n s I u s e d s n a p - t r a p l i n e s t o d e t e r m i n e w h e r e d o t h e s p e c i e s l i v e a n d how a b u n d a n t t h e y a r e . T h e s e l i n e s c o n s i s t e d o f 20 s t a t i o n s s p a c e d a t 10 m i n t e r v a l s w i t h t h r e e ' m u s e u m s p e c i a l ' t r a p s p e r s t a t i o n ( K r e b s 1 9 6 4 , K r e b s a n d W i n g a t e 1 9 7 6 ) . T h e t r a p s w e r e b a i t e d w i t h p e a n u t b u t t e r a n d w e r e l e f t i n p o s i t i o n f o r t w o d a y s . L i n e s w e r e t r a p p e d t h r o u g h o u t t h e s u m m e r . L i v e - t r a p p i n g t e c h n i q u e I s e t t w o l o n g , r e c t a n g u l a r l i v e - t r a p p i n g g r i d s : a c o n t r o l a n d a n e x p e r i m e n t a l . E a c h o c c u p i e d n e a r l y 1 h a w i t h 102 ' L o n g w o r t h ' l i v e - t r a p s a r r a n g e d i n a 6 x 17 g r i d . T h e d i s t a n c e b e t w e e n t r a p s w a s 10 m. T h e g r i d s w e r e l o c a t e d a t t h e e c o t o n e b e t w e e n a l p i n e t u n d r a a n d s u b a l p i n e s h r u b t u n d r a , w i t h a p p r o x i m a t e l y h a l f t h e t r a p s i n e a c h h a b i t a t . T h e g r i d s w e r e s e t p e r p e n d i c u l a r t o e a c h o t h e r ( F i g . 2 ) . T h e c l o s e s t t r a p s b e t w e e n g r i d s w e r e 40 m ( 1 2 9 f t ) a p a r t . I s e t t h e t r a p s f o r t w o n i g h t s e v e r y s e c o n d w e e k a n d l e f t t h e m l o c k e d o p e n b e t w e e n t r a p s e s s i o n s t o a l l o w v o l e s t o m o v e i n a n d o u t f r e e l y . T h e t r a p s w e r e p r o v i d e d w i t h w h o l e o a t s ( a s b a i t ) a n d c o t t o n , b a t t i n g ( a s b e d d i n g ) . A l l a n i m a l s c a p t u r e d f o r t h e f i r s t t i m e w e r e e a r -t a g g e d w i t h n u m b e r e d f i n g e r l i n g f i s h t a g s . E v e r y t r a p s e s s i o n I 1 2 F i g u r e 2 . 2 . L i v e - t r a p p i n g g r i d s a t t h e e c o t o n e b e t w e e n a l p i n e a n d s u b a l p i n e s h r u b t u n d r a . E a c h g r i d c o n s i s t s o f 102 L o n g w o r t h l i v e - t r a p s a r r a n g e d i n a 6 x 17 r e c t a n g u l a r s h a p e . s 1507 4m. IN F E D C B EXPERIM E N T A L Alpine Tundra Subalpine Shrub Tundra "A-, B C O N T R O L C D • • E 1457 4m. p * CM CT ^ i o (0 N t g % O i - N (0 i f 10 O N 1 4 r e c o r d e d : s p e c i e s , t a g number, s e x , b r e e d i n g c o n d i t i o n , w e i g h t and t r a p l o c a t i o n . The b r e e d i n g c o n d i t i o n o f i n d i v i d u a l s was a s s e s s e d i n t h e f o l l o w i n g way: F o r m a l e s t h e p o s i t i o n of t h e i r t e s t e s was r e c o r d e d as e i t h e r s c r o t a l ( b r e e d i n g ) o r a b d o m i n a l ( n o t b r e e d i n g ) . F o r f e m a l e s I r e c o r d e d i f t h e i r v a g i n a was p e r f o r a t e o r n o t , n i p p l e s i z e ( s m a l l , medium, l a r g e ) , p u b i c s y m p h y s i s c o n d i t i o n (open, s l i g h t l y open, c l o s e d ) and e v i d e n t p r e g n a n c i e s . E x p e r i m e n t a l m a n i p u l a t i o n From t h e e x p e r i m e n t a l g r i d I removed a l l t u n d r a v o l e s e v e r y t r a p s e s s i o n , s t a r t i n g on t h e s e c o n d one (June 29) and c o n t i n u i n g t h r o u g h o u t t h e summer. The a i m o f t h e r e m o v a l e x p e r i m e n t was t o r e d u c e t h e p o p u l a t i o n of t u n d r a v o l e s and keep t h e s h r u b b y h a b i t a t v a c a n t f o r t h e a d j a c e n t p o p u l a t i o n o f s i n g i n g v o l e s . On t h e l i v e - t r a p p i n g g r i d s I e s t i m a t e d t h e p r o p o r t i o n o f s h r u b c o v e r ( S a l i x g l a u c a ) w i t h i n a c i r c l e o f 6m d i a m e t e r c e n t r e d a r o u n d e a c h t r a p s t a t i o n . Two p e r p e n d i c u l a r d i a m e t e r s were u s e d i n e a c h c i r c l e t o e s t i m a t e c o v e r by t h e l i n e - i n t e r c e p t method ( M u e l l e r - D u m b o i s and E l l e n b e r g 1974). I c l a s s i f i e d s t a t i o n s under f i v e c a t e g o r i e s o f s h r u b c o v e r : 100%, 75%, 50%, 25% and 0%. H a b i t a t n i c h e b r e a d t h was measured by t h e f o l l o w i n g f o r m u l a : 15 B 1 3 where B = n i c h e b r e a d t h p = p r o p o r t i o n o f i n d i v i d u a l s of s p e c i e s i w h i c h i s i j a s s o c i a t e d w i t h r e s o u r c e s t a t e j . F o r t h e s n a p - t r a p d a t a : p = r e l a t i v e number o f i n d i v i d u a l s p e r t r a p i n t e n s i t y i j ( t r a p - n i g h t s ) t i m e s 1000. F o r t h e l i v e - t r a p d a t a : p = r e l a t i v e f r e q u e n c y o f c a p t u r e s p e r t r a p i n t e n s i t y i j ( t r a p - n i g h t s ) t i m e s 1000. N i c h e b r e a d t h (B) r a n g e s f r o m 1 t o t h e number o f r e s o u r c e s t a t e s u s e d . I t measures t h e u n i f o r m i t y and d i s t r i b u t i o n o f i n d i v i d u a l s o f a s p e c i e s among t h e r e s o u r c e s t a t e s o f a m a t r i x . When a s p e c i e s u s e s o n l y one c a t e g o r y B e q u a l s 1, and when i t u s e s a l l c a t e g o r i e s e q u a l l y B e q u a l s t h e number o f c a t e g o r i e s ( L e v i n s 1968, C o l w e l l and Futuyma 1971, Southwood 1978). As a measure of h a b i t a t n i c h e o v e r l a p , I u s e d 16 C . = 1 - 0.5 p - p i h i j hj where p i j and phj are the same as f o r niche breadth f o r two d i f f e r e n t s p e c i e s . Overlap ranges from 0, when the s p e c i e s do not share resource s t a t e s to 1 when they show s i m i l a r d i s t r i b u t i o n s over a l l c a t e g o r i e s ( C o l w e l l and Futuyma 1971, Southwood 1978). The measurement of resource o v e r l a p has been a widely used t o o l i n s t u d i e s of community s t r u c t u r e . The assumption has been that the g r e a t e r the o v e r l a p , the g r e a t e r the p o t e n t i a l and i n t e n s i t y of c o m p e t i t i o n . However, the r e c o g n i t i o n of a p a t t e r n does not i n d i c a t e what process i s i n v o l v e d . C o l w e l l and Futuyma (1971) p o i n t e d out that high resource o v e r l a p can be used as evidence f o r or a g a i n s t c o m p e t i t i o n . A l t e r n a t i v e l y , s m a l l or no o v e r l a p can be used to support both arguments. C o l w e l l and Futuyma u n d e r l i n e d that to make use of o v e r l a p measurements when stud y i n g c o m p e t i t i o n , both a c t u a l and v i r t u a l niche have to be estimated. A c t u a l niche i s d e f i n e d as the breadth of resource s t a t e s (food types, h a b i t a t types, s u b s t r a t e s , e t c . ) that a p o p u l a t i o n u t i l i z e s when l i v i n g with p o t e n t i a l c o m p e t i t o r s ( p o s t c o m p e t i t i v e n i c h e ) , and v i r t u a l niche i s the breadth of such c a t e g o r i e s when competitors are absent ( p r e c o m p e t i t i v e niche) p r e f e r e n t i a l l y by a removal experiment. I used both Cole's (C7) and H u r l b e r t ' s (C8) 1 7 c o e f f i c i e n t s o f i n t e r s p e c i f i c a s s o c i a t i o n t o i n d i c a t e t h e s t r e n g t h o f t h e a s s o c i a t i o n s . B o t h r a n g e f r o m - 1 w h e n t h e r e i s a s t r o n g n e g a t i v e a s s o c i a t i o n t o +1 w h e n t h e a s s o c i a t i o n i s s t r o n g l y p o s i t i v e ( C o l e 1 9 4 9 , H u r l b e r t 1 9 6 9 ) . 2 . 4 R E S U L T S P A T T E R N S O F H A B I T A T U S E T h e e c o t o n e b e t w e e n a l p i n e t u n d r a a n d s u b a l p i n e s h r u b t u n d r a i s n o t s h a r p l y d e f i n e d . I t i s c h a r a c t e r i z e d b y a m o s a i c o f s h r u b a n d o p e n m e a d o w p a t c h e s w h e r e s i n g i n g v o l e s a n d t u n d r a v o l e s s h o w a r e m a r k a b l y s m a l l o v e r l a p i n t h e i r h a b i t a t u s e . H A B I T A T U S E C a p t u r e s f r o m t h e s n a p - t r a p l i n e s i n t h e s e t w o h a b i t a t s s h o w t h a t s i n g i n g v o l e s o c c u r m a i n l y i n t h e o p e n m e a d o w s o f t h e a l p i n e t u n d r a , b u t a r e a l s o c a u g h t a t t h e e c o t o n e w i t h t h e a d j a c e n t s h r u b b y h a b i t a t ( F i g . 2 . 3 ) . F r o m a t o t a l o f 15 a n i m a l s c a u g h t i n 1 2 0 0 t r a p - n i g h t s , 13 (87%) w e r e i n t h e o p e n m e a d o w s w h i l e t h e r e m a i n i n g 2 (13%) w e r e t r a p p e d i n t h e e c o t o n e b e t w e e n b o t h h a b i t a t s . N o s i n g i n g v o l e s w e r e c a u g h t i n t h e s u b a l p i n e s h r u b t u n d r a . C o n v e r s e l y , t u n d r a v o l e s o c c u r m a i n l y i n t h e d e n s e l y 18 F i g u r e 2.3. D e n s i t y i n d i c e s f o r s i n g i n g and t u n d r a v o l e s i n d i f f e r e n t h a b i t a t s f o r May t o August 1981. '5 per 100 Trap Nights 2 • TUNDRA V O L E S SINGING V O L E S FT ALPINE T U N D R A E G O T O N E SUBALPINE SHRUB TUNDRA 20 c o v e r e d s h r u b t u n d r a w h e r e 24 (89%) w e r e c a u g h t . O f t h e r e m a i n d e r , 2 (7%) w e r e i n t h e e c o t o n e w h i l e o n e i n d i v i d u a l (4%) w a s c a u g h t i n t h e a l p i n e t u n d r a . T h e r e s o u r c e m a t r i x f o r t h e s n a p - t r a p d a t a i n d i c a t e s t h a t t h e s e s p e c i e s h a v e s i m i l a r h a b i t a t n i c h e b r e a d t h s a n d t h a t t h e i r h a b i t a t n i c h e o v e r l a p i s s m a l l ( T a b l e 2 . 1 ) . T a b l e 2 . 1 . R e s o u r c e m a t r i x , h a b i t a t n i c h e b r e a d t h a n d h a b i t a t n i c h e o v e r l a p f o r b o t h s p e c i e s o f v o l e s . H a b i t a t n i c h e b r e a d t h ( B ) r a n g e s f r o m 1 w h e n o n l y c a t e g o r y i s u s e d t o 3 w h e n a l l c a t e g o r i e s a r e u s e d e q u a l l y a n d h a b i t a t n i c h e o v e r l a p ( C ) r a n g e s f r o m 0 t o 1. ( B a s e d o n 15 s i n g i n g v o l e s a n d 27 t u n d r a v o l e s d u r i n g 1 2 0 0 t r a p - n i g h t s ) . R E S O U R C E S T A T E S : H A B I T A T S A l p i n e E c o t o n e S u b a l p i n e t u n d r a s h r u b t u n d r a S P E C I E S S i n g i n g v o l e 2 7 . 0 8 1 1 . 1 1 0 . 0 T u n d r a v o l e 2 . 0 8 1 1 . 1 1 4 4 . 4 4 2 9 . 1 6 2 2 . 2 2 4 4 . 4 4 3 8 . 19 5 7 . 6 3 9 5 . 8 2 N i j = n u m b e r o f i n d i v i d u a l s p e r t r a p i n t e n s i t y ( t r a p - n i g h t s ) t i m e s 1 0 0 0 . H A B I T A T N I C H E B R E A D T H : S i n g i n g v o l e s 1 . 7 0 T u n d r a v o l e s 1 . 5 8 A C T U A L H A B I T A T N I C H E O V E R L A P ( P O S T C O M P E T I T I V E ) C = 0 . 2 3 21 MICROHABITAT USE A more d e t a i l e d view of the m i c r o h a b i t a t d i s t r i b u t i o n of both s p e c i e s at the ecotone i s p r o v i d e d by the a n a l y s i s of frequency of captures at l i v e - t r a p s under d i f f e r e n t cover c o n d i t i o n s on the c o n t r o l g r i d . ( F i g . 2.4). S i n g i n g v o l e s were captured more f r e q u e n t l y at l i v e -t r a p s t a t i o n s surrounded by l i t t l e (25%) or no shrub cover (Gadj.= 32.28, df.2, p<0.0l). Few animals were caught at t r a p s t a t i o n s under more than 50% shrub cover. Tundra v o l e s a l s o d i f f e r e d i n t h e i r h a b i t a t use s i g n i f i c a n t l y from random (Gadj.= 25.55, df.2, p<0.0l) but i n the o p p o s i t e d i r e c t i o n . They mostly used t r a p s with high shrub cover and avoided t r a p s i n open h a b i t a t . Tundra v o l e s use a higher number of m i c r o h a b i t a t s than s i n g i n g v o l e s on the c o n t r o l g r i d . The a c t u a l m i c r o h a b i t a t niche o v e r l a p ( p o s t c o m p e t i t i v e ) i s l a r g e r than the h a b i t a t o v e r l a p but the d i f f e r e n c e i s r e l a t i v e l y small (Table 2.2). There were more traps used e x c l u s i v e l y by e i t h e r of the two s p e c i e s than traps shared by both. Throughout the summer 23 i n d i v i d u a l s i n g i n g v o l e s and 33 i n d i v i d u a l tundra v o l e s were caught on the c o n t r o l g r i d a t o t a l of 69 and 110 times, r e s p e c t i v e l y , over 2142 t r a p - n i g h t s . From a t o t a l of 102 t r a p s t a t i o n s , 20 were used only by s i n g i n g v o l e s , 39 were used only by tundra v o l e s , 6 by both s p e c i e s and the r e s t were not used. The s p e c i e s are n e g a t i v e l y a s s o c i a t e d (x2 corr= 5.17, p<0.05) and the s t r e n g t h of the a s s o c i a t i o n i s moderate (C7= -22 F i g u r e 2 . 4 . M i c r o h a b i t a t u s e b y s i n g i n g v o l e s a n d t u n d r a v o l e s o n t h e c o n t r o l g r i d . U n d e r d i f f e r e n t d e g r e e s o f s h r u b c o v e r . T h e e x p e c t e d v a l u e i s o b t a i n e d b y a s s i g n i n g t h e c a p t u r e s p r o p o r t i o n a l l y t o t h e n u m b e r o f t r a p s i n e a c h s h r u b c a t e g o r y . SINGING V O L E S Number of Captures 40 -30 -20 10 Ld 0 > 5 0 25 0 Shrub Cover TUNDRA V O L E S 70 -60 Number of Captures 50 -40 -30 20 -10 -T A / / / / / / / / Observed V. A Expected / —I / / / / / / / > 5 0 25 0 Shrub Cover 24 T a b l e 2.2. R e s o u r c e m a t r i x , n i c h e b r e a d t h and n i c h e o v e r l a p o f s i n g i n g v o l e s and t u n d r a v o l e s on c o n t r o l g r i d ( l i v e - t r a p s ) . H a b i t a t n i c h e b r e a d t h (B) r a n g e s from 1 when o n l y one c a t e g o r y i s u s e d t o 5 when a l l c a t e g o r i e s a r e u s e d e q u a l l y . H a b i t a t n i c h e o v e r l a p (C) r a n g e s f r o m 0 t o 1. RESOURCE STATES : SHRUB COVER CATEGORIES 0 25 50 75 100 SPECIES S i n g i n g v o l e 53.22 51.58 12.82 8.65 0.0 126.27 T u n d r a v o l e 8.40 47.61 64.10 73.59 95.23 288.93 61.62 99.19 76.92 82.24 95.23 415.20 N i j = number o f c a p t u r e s p e r t r a p - n i g h t s t i m e s 1000. HABITAT NICHE BREADTH S i n g i n g v o l e s 2.78 T u n d r a v o l e s 3.97 ACTUAL HABITAT NICHE OVERLAP (POSTCOMPETITIVE) C = 0.36 .422, C8= - . 4 2 0 ) . T h e r e were n e v e r t r a p s a t u r a t i o n p r o b l e m s . Even when numbers were h i g h e s t i n b o t h s p e c i e s , o n l y 32% o f t h e t r a p s were u s e d . REMOVAL EXPERIMENT A t o t a l of 35 t u n d r a v o l e s was removed t h r o u g h o u t t h e summer from t h e e x p e r i m e n t a l g r i d . A f t e r t h e r e m o v a l of what I c o n s i d e r e d t h e r e s i d e n t p o p u l a t i o n , an a v e r a g e of 5.8 t u n d r a 25 v o l e s p e r s e s s i o n was c a u g h t ( F i g . 2 . 5 ) . Between t h e f i r s t and s e c o n d n i g h t o f e a c h t r a p s e s s i o n t h e a v e r a g e number o f a n i m a l s c a u g h t and removed d e c l i n e d f r o m 4.6 t o 1.2. T h i s r e d u c t i o n p l u s t h e f a c t t h a t t h e y were u n t a g g e d i n d i v i d u a l s , i n d i c a t e s t h a t t h e g r i d was a l m o s t f r e e o f t u n d r a v o l e s by t h e end o f e v e r y s e s s i o n . A n i m a l s c a u g h t s u b s e q u e n t l y were d i s p e r s e r s f r o m e l s e w h e r e . The r e m o v a l r e s u l t e d i n a r e d u c t i o n o f h a b i t a t use by t u n d r a v o l e s o f a t l e a s t 70% ( e s t i m a t e d from t h e number of c a p t u r e s compared w i t h t h a t on t h e c o n t r o l ) . SPATIAL DISTRIBUTION I f t u n d r a v o l e s a c t i v e l y i n f l u e n c e t h e s p a t i a l d i s t r i b u t i o n o f s i n g i n g v o l e s , t h e n t h e r e m o v a l o f t h e f o r m e r w i l l r e s u l t i n t h e use o f s u b a l p i n e s h r u b t u n d r a ( n i c h e s h i f t ; e c o l o g i c a l r e l e a s e ) by s i n g i n g v o l e s . T h i s e c o l o g i c a l r e l e a s e would be r e f l e c t e d i n any o f t h e f o l l o w i n g ways. F i r s t , an i n c r e a s e i n t h e number o f t r a p s s h a r e d by b o t h s p e c i e s , due t o t h e use o f t r a p s by s i n g i n g v o l e s o f t h o s e s i t e s from w h i c h t u n d r a v o l e s had been removed. Second, an i n c r e a s e i n t h e f r e q u e n c y o f c a p t u r e s o f s i n g i n g v o l e s i n t h o s e t r a p s f o r m e r l y u s e d by t u n d r a . v o l e s . T h i r d , a s h i f t i n t h e k i n d of t r a p s t a t i o n u s e d from open s i t e s t o t r a p s t a t i o n s under h i g h e r s h r u b c o v e r . As a c o n s e q u e n c e o f t h i s s h i f t b o t h m i c r o h a b i t a t n i c h e b r e a d t h and o v e r l a p s h o u l d be l a r g e r i n t h e a b s e n c e of t h e s u s p e c t e d c o m p e t i t o r ( v i r t u a l o r p r e c o m p e t i t i v e n i c h e b r e a d t h ) . 2 6 F i g u r e 2 . 5 . N u m b e r o f t u n d r a v o l e s r e m o v e d . T h e a v e r a g e n u m b e r o f i n d i v i d u a l s r e m o v e d d e c l i n e d w i t h i n t r a p s e s s i o n s . C l o s e d c i r c l e s i n d i c a t e t h e f i r s t n i g h t a n d o p e n c i r c l e s i n d i c a t e t h e s e c o n d n i g h t . A r r o w r e p r e s e n t s t h e b e g i n n i n g o f t h e m a n i p u l a t i o n . 16 14 Number 12 of 10 8 6 4 TUNDRA VOLES Removed 28 Were more t r a p s s h a r e d by b o t h s p e c i e s on t h e e x p e r i m e n t a l g r i d ? On the. e x p e r i m e n t a l g r i d 24 s i n g i n g v o l e s and 35 t u n d r a v o l e s were c a u g h t a t o t a l of 59 and 35 t i m e s r e s p e c t i v e l y o v e r 2142 t r a p n i g h t s . From a t o t a l o f 102 t r a p s , 20 were u s e d by M i c r o t u s m i u r u s a l o n e , 20 by M i c r o t u s oeconomus a l o n e and o n l y 3 by b o t h s p e c i e s . The r e s t o f t h e t r a p s were n o t u s e d . F i s h e r ' s e x a c t t e s t d i d n o t r e j e c t t h e h y p o t h e s i s o f i n d e p e n d e n c e (p= 0 .34). The c o e f f i c i e n t s o f a s s o c i a t i o n g i v e a m o d erate n e g a t i v e a s s o c i a t i o n (C7= -.422, C8= - . 4 2 0 ) . More t r a p s were u s e d by b o t h s p e c i e s on t h e c o n t r o l g r i d . But i n f a c t t h e number o f t r a p s u s e d by M i c r o t u s m i u r u s a l o n e and s h a r e d w i t h M i c r o t u s oeconomus i s n o t s i g n i f i c a n t l y d i f f e r e n t between e x p e r i m e n t a l and c o n t r o l g r i d s (Gadj.= 1.42, d f . 1 , p>0.10) . D i d s i n g i n g v o l e s more f r e q u e n t l y use t r a p s f r o m w h i c h t u n d r a v o l e s had been removed? S i n g i n g v o l e s were c a u g h t 54 t i m e s i n t r a p s u s e d e x c l u s i v e l y by them and 5 t i m e s i n s h a r e d t r a p s . T h e i r f r e q u e n c y o f c a p t u r e s a t s h a r e d t r a p s i s l o w e r i n t h e e x p e r i m e n t a l g r i d t h a n i n t h e c o n t r o l (Gadj.= 6.17, d f . 1 , p<0.02), and not h i g h e r as was p r e d i c t e d . - D i d s i n g i n g v o l e s i n c r e a s e t h e i r use of t h e m i c r o h a b i t a t from w h i c h t u n d r a v o l e s had been removed? A c c o r d i n g t o t h e s h r u b c a t e g o r i e s u s e d , t h e p r o p o r t i o n o f s i n g i n g v o l e c a p t u r e s a f t e r t h e r e m o v a l i s n o t d i f f e r e n t f r o m t h a t on t h e c o n t r o l g r i d (Gadj.= 1.51, d f . 2 , p>0.30) ( F i g . 2 . 6 ) . A c o m p a r i s o n o f t h e a c t u a l ( c o n t r o l ) and v i r t u a l 29 F i g u r e 2.6. M i c r o h a b i t a t use by s i n g i n g v o l e s on e x p e r i m e n t a l g r i d . The use o f s h r u b c a t e g o r i e s by s i n g i n g v o l e s i s s i m i l a r i n b o t h g r i d s r e g a r d l e s s o f t h e a b s e n c e of t u n d r a v o l e s . E x p e c t e d v a l u e s were o b t a i n e d by a s s i g n i n g c a p t u r e s p r o p o r t i o n a l l y t o t h e d i s t r i b u t i o n i n t h e c o n t r o l g r i d . 30 Observed Expected Number of Captures 40 30 20 10 A i Z L A A A. Z L > 5 0 2 5 0 Shrub C o v e r 31 ( e x p e r i m e n t a l ) m i c r o h a b i t a t n i c h e b r e a d t h of s i n g i n g v o l e s shows a l m o s t no d i f f e r e n c e between them. A c c o r d i n g l y , a c t u a l and v i r t u a l m i c r o h a b i t a t o v e r l a p a r e s t r i k i n g l y s i m i l a r ( T a b l e 2 . 3 ) . T a b l e 2.3. R e s o u r c e m a t r i x , n i c h e b r e a d t h and n i c h e o v e r l a p o f s i n g i n g and t u n d r a v o l e s f o r e x p e r i m e n t a l g r i d ( l i v e - t r a p s ) . RESOURCE STATES : SHRUB COVER CATEGORIES 0 25 50 75 100 SPECIES S i n g i n g v o l e 27.47 35.14 25.79 0,0 0.0 88.40 T u n d r a v o l e 9.15 7.93 19.84 61.90 0.0 98.82 36.62 43.07 45.63 61.90 0.0 187.22 N i j = number of c a p t u r e s p e r t r a p - n i g h t s t i m e s 1000. HABITAT NICHE BREADTH S i n g i n g v o l e s 2.94 T u n d r a v o l e s 2.23 VIRTUAL HABITAT NICHE OVERLAP (PRECOMPETITIVE) C = 0.37 In summary, t h e r e m o v a l of t u n d r a v o l e s had no e f f e c t on t h e s p a t i a l d i s t r i b u t i o n o f s i n g i n g v o l e s . MOVEMENTS BETWEEN GRIDS A few i n d i v i d u a l s o f b o t h s p e c i e s were c a u g h t on b o t h g r i d s . Members of e a c h s p e c i e s moved t o t h e a r e a i n t h e new 32 g r i d w i t h a s i m i l a r h a b i t a t t o t h a t from w h i c h t h e y came. Such a r e a s i n v o l v e d a g r e a t e r d i s t a n c e t o t r a v e l t h a n t h a t r e q u i r e d t o move t o a d i f f e r e n t h a b i t a t on t h e same g r i d . T h i s i s more a p p a r e n t i n t h e movements of s i n g i n g v o l e s , s i n c e t h e a r e a s of a l p i n e t u n d r a between t h e g r i d s were f a r t h e r a p a r t . In a l p i n e t u n d r a t h e c l o s e s t d i s t a n c e between t r a p s i n d i f f e r e n t g r i d s was 150 m (485 f t ) . T h r e e s i n g i n g v o l e s t r a v e l l e d between g r i d s . Two a d u l t f e m a l e s moved from t h e c o n t r o l g r i d t o t h e e x p e r i m e n t a l g r i d , a d i s t a n c e of 230 m (744 f t ) and 220 m (712 f t ) . An a d u l t male moved from t h e e x p e r i m e n t a l t o t h e c o n t r o l g r i d and back, t r a v e l l i n g 210 m (680 f t ) and 260 m (841 f t ) . The t r i p back was done o v e r n i g h t ( F i g . 2 . 7 ) . The movements were r e c o r d e d d u r i n g J u n e and e a r l y J u l y , and by m i d - J u l y t h e s e i n d i v i d u a l s had d i s a p p e a r e d f r o m b o t h g r i d s . S i m i l a r l y , t h r e e t u n d r a v o l e s moved from t h e c o n t r o l g r i d t o t h e e x p e r i m e n t a l g r i d , where t h e y were removed. Two a d u l t f e m a l e s and an a d u l t male t r a v e l l e d 130 m (421 f t ) , 160 m (518 f t ) and 170 m (550 f t ) r e s p e c t i v e l y . The t r i p by t h e male was done o v e r n i g h t . T h e s e movements were r e c o r d e d d u r i n g June ( F i g . 2 . 7 ) . 3 3 F i g u r e 2.7. Movements between g r i d s . C l o s e d c i r c l e s r e p r e s e n t s i n g i n g v o l e s and open c i r c l e s r e p r e s e n t t u n d r a v o l e s . The s u b i n d e x i n d i c a t e s t h e o r d e r of c a p t u r e s . B o t h s p e c i e s move between g r i d s t o t h e i r c o r r e s p o n d i n g h a b i t a t . 35 MOVEMENTS WITHIN GRIDS On t h e c o n t r o l g r i d t u n d r a v o l e s o c c u p i e d two d i f f e r e n t a r e a s , t h e west h a l f (Rows 9 t o 17) and a s m a l l p a r t of t h e e a s t c o r n e r (Rows 1 t o 4 ) . T r a p s t a t i o n s i n t h e s e a r e a s had h i g h s h r u b c o v e r . They were a b s e n t f r o m t h e open meadows a r e a between t h e s e two w h i c h was o c c u p i e d by s i n g i n g v o l e s ( F i g . 2 . 8 ) . In s p i t e o f t h e r e l a t i v e l y s m a l l d i s t a n c e between t h e a r e a s (50 m), none o f t h e t u n d r a v o l e s c a u g h t i n one a r e a were c a u g h t i n t h e o t h e r a r e a . T h i s d i s t a n c e between s h r u b h a b i t a t p a t c h e s i s r e l a t i v e l y s m a l l when compared w i t h : a) t h e d i s t a n c e between t h e most w i d e l y s e p a r a t e d c a p t u r e p o i n t s o f i n d i v i d u a l s w i t h i n e a c h a r e a (10 t o 80 m) and b) t h e d i s t a n c e s t r a v e l l e d by i n d i v i d u a l s f r o m g r i d t o g r i d w i t h i n t h e same h a b i t a t ( s u b a l p i n e s h r u b t u n d r a ) . A l l of them were l a r g e r t h a n 130 m (421 f t ) . T h e r e f o r e , b o t h s p e c i e s c a n move t o t h e a d j a c e n t h a b i t a t , s i n c e t h e movements n o r m a l l y r e c o r d e d a r e f a r l a r g e r t h a n t h e d i s t a n c e r e q u i r e d t o g e t t o t h e c o n t i g u o u s h a b i t a t . POPULATION DYNAMICS Two o r more s p e c i e s compete i f an i n c r e a s e i n any one r e s u l t s i n a c o r r e s p o n d i n g d e c r e a s e i n t h e o t h e r s ( M a c A r t h u r 1972). Even when t h e p r e d i c t i o n s from t h i s d e f i n i t i o n do not d i s c r i m i n a t e o t h e r i n t e r a c t i o n s from c o m p e t i t i o n ( p r e d a t i o n , p a r a s i t i s m ) , i t i s u s e f u l t o a n a l y z e t h e e f f e c t of one s p e c i e s g u r e 2 . 8 . M o v e m e n t s w i t h i n t h e c o n t r o l g r i d . T r a p s w i t h h i g h s h r u b c o v e r o c c u p y t w o a r e a s . T u n d r a v o l e s u s e d b o t h a r e a s ( o p e n a n d c l o s e d c i r c l e s ) b u t n o i n d i v i d u a l s w e r e r e c o r d e d i n b o t h . 3 7 CONTROL • • • • • • # # J # * # > 5 0 Shrub Cover • • • • • • • • F • • • • • • • • • 17 A O O O O F O 1 O O o o o • • • • • • • • « • • • • • • • • • « 9 17 TUNDRA V O L E S A • • • • • • • • • • • • # • SINGING VOLES • • • • F • • • • • • • 1 9 17 38 on a n o t h e r . I f one o f t h e s e s p e c i e s has a n e g a t i v e e f f e c t ( e x p l o i t a t i o n , i n t e r f e r e n c e ) on t h e s e c o n d , r e m o v a l o f t u n d r a v o l e s s h o u l d r e s u l t i n an i n c r e a s e i n numbers o f s i n g i n g v o l e s . The i n c r e a s e would be p r o d u c e d e i t h e r by h i g h e r r e p r o d u c t i v e r a t e , l o w e r m o r t a l i t y , h i g h e r i m m i g r a t i o n , l o w e r e m i g r a t i o n , o r a c o m b i n a t i o n of t h e s e p a r a m e t e r s . However, s i n c e s i n g i n g v o l e s d i d n o t u t i l i z e t h e a r e a v a c a t e d by t u n d r a v o l e s , t h e r e s h o u l d be no d i f f e r e n c e s i n t h e i r d e m o g r a p h i c p a r a m e t e r s between g r i d s . In f a c t , t h e f o l l o w i n g a n a l y s i s shows t h a t t h e p o p u l a t i o n s were r e m a r k a b l y s i m i l a r . NUMBERS I s t h e p o p u l a t i o n o f s i n g i n g v o l e s h i g h e r i n t h e a b s e n c e of a c o n t i g u o u s p o p u l a t i o n of t u n d r a v o l e s ? On t h e c o n t r o l g r i d t h e p o p u l a t i o n o f o v e r w i n t e r e d s i n g i n g v o l e s began a t a v e r y low d e n s i t y d u r i n g J u n e . The o n s e t of b r e e d i n g was not documented, as some f e m a l e s were a l r e a d y p r e g n a n t when t r a p p i n g began. The f i r s t j u v e n i l e s were t r a p p e d d u r i n g t h e s e c o n d week o f J u l y . From t h e n on, t h e p o p u l a t i o n i n c r e a s e d c o n t i n u o u s l y u n t i l t h e f i r s t week of September, r e a c h i n g 15 i n d i v i d u a l s p e r a c r e (.4 ha) when t h e t r a p p i n g e n ded. T h r o u g h o u t t h e summer a t o t a l o f 23 i n d i v i d u a l s was c a u g h t ; 8 a d u l t s (6 f e m a l e s and 2 m a l e s ) and 15 j u v e n i l e s (10 f e m a l e s and 5 m a l e s ) . The s i n g i n g v o l e p o p u l a t i o n was a l s o v e r y low on t h e 39 removal g r i d d u r i n g June. With the recruitment of j u v e n i l e s through J u l y , they i n c r e a s e d at a f a s t e r r a t e than on the c o n t r o l . N e v e r t h e l e s s , they remained at a lower l e v e l than on the c o n t r o l d u r i n g August and September. A t o t a l of 21 i n d i v i d u a l s was caught with s i m i l a r sex and age r a t i o s to those of the c o n t r o l p o p u l a t i o n : 8 a d u l t s (5 females and 3 males) and 13 j u v e n i l e s (8 females and 5 males). A comparison of the minimum number of animals known to be a l i v e on both g r i d s ( F i g . 2.9) shows that the p o p u l a t i o n of s i n g i n g v o l e s on the c o n t r o l g r i d i n c r e a s e d to a higher l e v e l than that on the removal g r i d . T h i s i s c o n t r a r y to what• would be expected i f tundra v o l e s were having a n e g a t i v e i n f l u e n c e on s i n g i n g v o l e s . The t o t a l number of s i n g i n g v o l e s caught throughout the summer i s very s i m i l a r on both g r i d s (Gadj.= 0.08, p>0.70) as are the age and sex r a t i o s (Gadj.= 0.28, df.2, p>0.50). The a d u l t c l a s s e s were pooled to o b t a i n a j o i n t c l a s s with an expected frequency g r e a t e r than 5. There was a b i a s i n the sex r a t i o towards females on both g r i d s . On the c o n t r o l g r i d 75 % of the a d u l t s and 66 % of the j u v e n i l e s were females. On the experimental g r i d 62.5 % of the a d u l t s and 61.5 % of the j u v e n i l e s were females. 4 0 F i g u r e 2.9. Minimum number a l i v e f o r b o t h t u n d r a and s i n g i n g v o l e s on t h e e x p e r i m e n t a l and c o n t r o l g r i d s . S i n g i n g v o l e s r e a c h e d a h i g h e r d e n s i t y on t h e c o n t r o l g r i d . SINGING V O L E S Minimum Number Alive 16 U • - • C O N T R O L 6-0 R E M O V A L 1 4 -12 -10 h 8 6 -4 -2 -6 M TUNDRA V O L E S Minimum Number Alive 16 14 12 1 0 8 6 4 • - • C O N T R O L o • o R E M O V A L M 42 REPRODUCTION T h e r e a r e no e v i d e n t d i f f e r e n c e s i n s i n g i n g v o l e r e p r o d u c t i o n between g r i d s . F e m a l e s were a l r e a d y p r e g n a n t by t h e f i r s t t r a p p i n g s e s s i o n on b o t h g r i d s . On b o t h e x p e r i m e n t a l and c o n t r o l g r i d s t h e f i r s t j u v e n i l e s were c a u g h t i n t h e s e c o n d week o f J u l y . T h e r e f o r e t h e o n s e t of b r e e d i n g was s y n c h r o n o u s . T h e r e seemed t o be two l i t t e r s d u r i n g t h e b r e e d i n g s e a s o n o f 1981, t h e f i r s t b o r n i n mid-June and t h e s e c o n d i n m i d - J u l y . On b o t h g r i d s h a l f o f t h e t a g g e d a d u l t f e m a l e s were c a u g h t o n l y once o r t w i c e d u r i n g J u ne and e a r l y J u l y and d i s a p p e a r e d f o r t h e r e s t o f t h e summer. I c o n s i d e r e d them t r a n s i e n t i n d i v i d u a l s . B e c a u se t h e r e s t were c a u g h t i n 3 t o 5 t r a p p i n g s e s s i o n s I c o n s i d e r e d them r e s i d e n t s . F o l l o w i n g t h e b r e e d i n g c o n d i t i o n of t h e i n d i v i d u a l r e s i d e n t f e m a l e s , I e s t i m a t e d a minimum of 3 l i t t e r s was p r o d u c e d on e a c h g r i d . The number o f j u v e n i l e s r e c r u i t e d d u r i n g J u l y and A u g u s t p e r a d u l t f e m a l e f o r t h e c o n t r o l and e x p e r i m e n t a l g r i d s was 2.5 and 2.6 r e s p e c t i v e l y , o r 5 and 6.5 when o n l y t h e r e s i d e n t f e m a l e s a r e t a k e n i n t o a c c o u n t . SURVIVAL I n d i v i d u a l s t h a t d i s a p p e a r f r o m t h e g r i d have e i t h e r d i e d or d i s p e r s e d , and t h e r e f o r e l o s s e s h e r e i n c l u d e b o t h m o r t a l i t y and e m i g r a t i o n . I c o n s i d e r e d s u r v i v a l f o r b o t h a d u l t s 43 and j u v e n i l e s . A d u l t s u r v i v a l was d i v i d e d i n t o two p e r i o d s : s p r i n g (when no j u v e n i l e s a r e p r e s e n t ) and summer (when j u v e n i l e s had been r e c r u i t e d ) . S e xes were p o o l e d f o r b o t h age c l a s s e s . T a b l e 2.4 shows minimum two-week s u r v i v a l r a t e s a v e r a g e d f o r t h e d i f f e r e n t p e r i o d s . Minimum two-week s u r v i v a l i s t h e r a t i o o f r e c a p t u r e d i n d i v i d u a l s c a u g h t i n a g i v e n week o r a f t e r t o t h e t o t a l number o f a n i m a l s r e l e a s e d two weeks b e f o r e . A d u l t s u r v i v a l was v e r y s i m i l a r between g r i d s , b o t h d u r i n g t h e s p r i n g and summer. J u v e n i l e s u r v i v a l was l o w e r on t h e e x p e r i m e n t a l g r i d . T a b l e 2.4. A d u l t and j u v e n i l e minimum two-week s u r v i v a l r a t e s o f s i n g i n g v o l e s a v e r a g e d o v e r t h e s p r i n g and summer f o r a d u l t s and o v e r t h e summer f o r j u v e n i l e s . S e xes p o o l e d . Sample s i z e (number o f d i f f e r e n t i n d i v i d u a l s ) i n p a r e n t h e s i s . CONTROL EXPERIMENTAL ADULTS S p r i n g Summer E n t i r e s e a s o n 0.25 (4) 0.75 (6) 0.58 (8) 0.30 (6) 0.65 (6) 0.53 (8) JUVENILES Summer 0.52 (15) 0.33 (13) 44 2 . 5 D I S C U S S I O N A n u m b e r o f f a c t o r s may i n f l u e n c e t h e d i s t r i b u t i o n o f t h e s e t w o m i c r o t i n e s p e c i e s . W h i l e t h e w e l l - d e f i n e d p a t t e r n o f n o n - o v e r l a p p i n g p o p u l a t i o n s c o u l d r e s u l t f r o m i n t e r f e r e n c e c o m p e t i t i o n ( G u t h r i e 1 9 7 1 ) , t h e s t r u c t u r a l d i f f e r e n c e o f t h e v e g e t a t i o n f r o m o p e n m e a d o w s t o a d e n s e c o v e r o f s h r u b s a f f e c t s m a n y o t h e r b i o t i c a n d a b i o t i c c o m p o n e n t s o f t h e c o m m u n i t y . T h e r e s u l t s f r o m t h i s s t u d y d o n o t s u p p o r t t h e h y p o t h e s i s t h a t t h e h a b i t a t d i s t r i b u t i o n o f s i n g i n g v o l e s i s i n f l u e n c e d b y c o m p e t i t i v e i n t e r a c t i o n s . T h e u s e o f t r a p s t a t i o n s i n d i c a t e d t h a t b o t h s p e c i e s s h o w e d a s t r o n g i n v e r s e d i s t r i b u t i o n a l p a t t e r n i n r e l a t i o n t o h a b i t a t s t r u c t u r e ( s h r u b c o v e r ) , b u t t h e r e m o v a l o f o n e s p e c i e s d i d n o t a f f e c t t h e d i s t r i b u t i o n o f t h e o t h e r . T h e r e s o u r c e m a t r i x a n a l y s i s s u p p o r t s t h i s r e s u l t . I f c o m p e t i t i v e i n t e r a c t i o n s w e r e i n f l u e n c i n g t h e s p a t i a l d i s t r i b u t i o n o f s i n g i n g v o l e s , t h e n t h e h a b i t a t n i c h e b r e a d t h o f t h i s s p e c i e s s h o u l d b e l a r g e r i n t h e a b s e n c e o f t u n d r a v o l e s . S i m i l a r l y , v i r t u a l n i c h e o v e r l a p s h o u l d b e l a r g e r t h a n a c t u a l n i c h e o v e r l a p . N o n e o f t h e s e p r e d i c t i o n s w e r e s u p p o r t e d . B o t h t h e h a b i t a t n i c h e b r e a d t h a n d o v e r l a p o f s i n g i n g v o l e s a r e v e r y s i m i l a r r e g a r d l e s s o f t h e r e m o v a l o f t u n d r a v o l e s . F u r t h e r m o r e , i n d i v i d u a l s o f b o t h s p e c i e s m o v e d l a r g e d i s t a n c e s w i t h i n a n d b e t w e e n g r i d s . T h i s c l e a r l y s h o w s t h a t t h e y c o u l d h a v e m o v e d t o t h e a d j a c e n t h a b i t a t . N e v e r t h e l e s s , t h e y w e r e n e v e r c a u g h t t h e r e . S e v e r a l s t u d i e s h a v e d e a l t w i t h h a b i t a t s e p a r a t i o n a n d 45 i n t e r s p e c i f i c i n t e r a c t i o n s among m i c r o t i n e s . K o p l i n and Hoffmann (1968) e x p e r i m e n t a l l y t e s t e d t h e h y p o t h e s i s t h a t h a b i t a t s e g r e g a t i o n o f meadow v o l e s (M. p e n n s y l v a n i c u s ) and montane v o l e s (M. montanus) i s m a i n t a i n e d by i n t e r s p e c i f i c i n t e r a c t i o n s . Montane v o l e s made e x p l o r a t o r y movements i n t o t h e g r a s s l a n d a f t e r t h e r e m o v a l o f meadow v o l e s . However, t h e e x p e r i m e n t was c r i t i c i z e d s i n c e t h e e x p e r i m e n t a l p l o t was f e n c e d and n o t g r a z e d by b i s o n , and had a 4 t o 5 t i m e s h i g h e r d e n s i t y t h a n t h e c o n t r o l ( G r a n t 1972a, C o n n e l l 1975, B i r c h 1979). The d i f f e r e n c e s i n h a b i t a t use by montane v o l e s i n e x p e r i m e n t a l and c o n t r o l p l o t s may have been a c o n s e q u e n c e of t h e d i f f e r e n t p o p u l a t i o n d e n s i t i e s and n o t o f t h e a b s e n c e o f meadow v o l e s . S t o e c k e r (1972) d i d t h e i n v e r s e e x p e r i m e n t of r e m o v i n g montane v o l e s a n d f o u n d t h a t meadow v o l e s u s e d t h e d r i e r h a b i t a t i n t h e a b s e n c e o f t h e f o r m e r s p e c i e s . N e i t h e r s t u d i e s were a b l e t o d e t e r m i n e whether i n d i v i d u a l s t h a t moved i n t o t h e v a c a t e d a r e a s r e m a i n e d t h e r e o r were m e r e l y t r a n s i e n t s . B o t h s t u d i e s l a c k e d r e p l i c a t e s and a d e q u a t e c o n t r o l s . The s t u d i e s s u g g e s t e d b e h a v i o r a l i n t e r a c t i o n s a s a mechanism f o r t h e h a b i t a t s e g r e g a t i o n . C o m p e t i t i v e i n t e r a c t i o n s have a l s o been i n v o k e d t o e x p l a i n h a b i t a t s e g r e g a t i o n among o t h e r p a i r s o f r e l a t e d s p e c i e s . A number of d i s t r i b u t i o n a l s t u d i e s have s u g g e s t e d t h a t a c t i v e i n t e r f e r e n c e r e s u l t s i n h a b i t a t s e g r e g a t i o n by r e d - b a c k e d v o l e s ( C l e t h r i o n o m y s ) and meadow v o l e s ( M i c r o t u s ) ( M o r r i s 1969, G r a n t 1 9 7 2 a ) . The f o r m e r l i v e i n woodlands and t h e l a t t e r i n a d j a c e n t g r a s s l a n d s ( M o r r i s 1969). E x p e r i m e n t s i n s i d e 46 e n c l o s u r e s showed t h a t C l e t h r i o n o m y s o c c u r r e d more o f t e n i n t h e g r a s s l a n d i n t h e a b s e n c e o f M i c r o t u s ( G r a n t 1969) and t h a t M i c r o t u s o c c u r r e d more o f t e n i n t h e f o r e s t i n t h e a b s e n c e o f C l e t h r i o n o m y s ( M o r r i s and G r a n t 1972). B o t h c o l o n i z a t i o n o f g r a s s l a n d h a b i t a t by r e d - b a c k e d v o l e s and of s p r u c e f o r e s t by meadow v o l e s have been documented ( I v e r s o n and T u r n e r 1972, T u r n e r e_t a_l . 1975). In b o t h c a s e s t h e c o l o n i z a t i o n t o o k p l a c e d u r i n g w i n t e r , once t h e b r e e d i n g s e a s o n was o v e r , and a n i m a l s d i s a p p e a r e d f r o m t h e c o l o n i z e d h a b i t a t a t t h e o n s e t o f b r e e d i n g . The a u t h o r s s u g g e s t e d t h a t t h e d e c r e a s e i n i n t r a s p e c i f i c a g g r e s s i o n d u r i n g t h e n o n - b r e e d i n g s e a s o n p e r m i t t e d t h e s e s p e c i e s t o c o e x i s t . However t h e i r c o n c l u s i o n i s b a s e d on a c o r r e l a t i o n . No e x p e r i m e n t was done t o c o n f i r m a c a u s a l r e l a t i o n s h i p . Hawes (1976) showed t h a t M i c r o t u s o r e g o n i was more r e s t r i c t e d i n i t s h a b i t a t use a f t e r t h e i n t r o d u c t i o n o f M. t o w n s e n d i i . However, i n t h e same s t u d y t h e r e m o v a l o f M. o r e g o n i d i d n o t i n f l u e n c e h a b i t a t u t i l i z a t i o n by M. l o n g i c a u d u s . S i m i l a r l y , K r e b s (1977) f o u n d no e v i d e n c e of n e g a t i v e d e m o g r a p h i c e f f e c t s between M i c r o t u s p e n n s y l v a n i c u s and M. o c h r o q a s t e r l i v i n g i n t h e same g r a s s l a n d a r e a s . A l t e r n a t i v e l y , t h e r e i s s u p p o r t f o r t h e h y p o t h e s i s t h a t h a b i t a t s e g r e g a t i o n i n m i c r o t i n e s i s r e l a t e d t o h a b i t a t p r e f e r e n c e s . D i f f e r e n c e s i n use o f h a b i t a t s r e l a t e d t o p h y s i o l o g i c a l t o l e r a n c e s have been documented f o r some s p e c i e s . Montane v o l e s a r e o f t e n f o u n d i n d r i e r p l a c e s (even s h r u b - s t e p p e 47 c o m m u n i t i e s ) w h i l e meadow v o l e s o c c u p y more m e s i c h a b i t a t s ( M u r i e 1971, B a n f i e l d 1974). Water b a l a n c e i n meadow v o l e s i s not w e l l a d a p t e d f o r d r y s i t u a t i o n s ( G e t z 1963). D o u g l a s s (1976) d e m o n s t r a t e d t h a t t h e s e s p e c i e s show d i f f e r e n t h a b i t a t p r e f e r e n c e s and c o n c l u d e d t h a t s u c h p r e f e r e n c e s a r e p r o b a b l y t h e most i m p o r t a n t f a c t o r r e s p o n s i b l e f o r t h e i r s e g r e g a t i o n . He s u g g e s t e d t h a t t h e r e l a t i v e i m p o r t a n c e of s o c i a l i n t e r a c t i o n s and h a b i t a t p r e f e r e n c e s may change a l o n g a g r a d i e n t o f h a b i t a t s . A t one e x t r e m e , when h a b i t a t s a r e s h a r p l y d e f i n e d , t h e d i s t r i b u t i o n w i l l be t h e r e s u l t o f h a b i t a t p r e f e r e n c e s , whereas i f t h e h a b i t a t i s u n i f o r m b e h a v i o r a l i n t e r a c t i o n s may be have a s t r o n g e r i i n f l u e n c e . I n a d d i t i o n , h a b i t a t s may become more s i m i l a r n o t o n l y b e c a u s e o f t h e r e s e m b l a n c e of t h e i r s t r u c t u r a l c h a r a c t e r i s t i c s b ut b e c a u s e o f t h e e f f e c t o f p o p u l a t i o n d e n s i t y . As t h e d e n s i t y o f a s p e c i e s i n a h a b i t a t i n c r e a s e s , i t s s u i t a b i l i t y w i l l d e c r e a s e , r e d u c i n g i t s d i f f e r e n c e f r o m l e s s s u i t a b l e h a b i t a t s ( F r e t w e l l a n d L u c a s 1970, M e r k t 1981). I f p o p u l a t i o n s o f s i n g i n g v o l e s and t u n d r a v o l e s f l u c t u a t e , t h e i r i n t e r s p e c i f i c i n t e r a c t i o n s may be c o n t i n g e n t on d e n s i t y c h a n g e s . C y c l e s i n p o p u l a t i o n s o f t u n d r a v o l e s have been documented i n F i n l a n d and A l a s k a ( T a i t t and K r e b s 1984). In t h e l a t t e r l o c a l i t y peak d e n s i t i e s r e a c h e d a p p r o x i m a t e l y 70 t o 80 v o l e s p e r h e c t a r e . W h i l e t h e r e have been no s t u d i e s on t h e demography o f s i n g i n g v o l e s , K r e b s and W i n g a t e (1976) r e p o r t e d c h a n g e s i n t h e i r abundance f r o m y e a r t o y e a r . F u r t h e r m o r e , t h e t o t a l d i s a p p e a r a n c e of b o t h s p e c i e s f r o m t h e s t u d y s i t e d u r i n g t h e 48 w i n t e r of 1981 i n d i c a t e s t h a t t h e y do f l u c t u a t e i n t h e a r e a . At p r e s e n t , however, n e i t h e r s p e c i e s has been f o u n d i n t h e a r e a i n h i g h e r numbers t h a n t h o s e r e p o r t e d h e r e ( K r e b s and W i n g a t e 1976), and t h e r e f o r e d e n s i t i e s may r a r e l y be h i g h enough f o r c o m p e t i t i v e i n t e r a c t i o n s t o o c c u r . 49 SECTION 3. HABITAT USE AND ABUNDANCE OF DEER MICE: INTERACTIONS WITH MEADOW VOLES AND RED-BACKED VOLES. 3.j_ INTRODUCTION Deer mice ( P e r o m y s c u s m a n i c u l a t u s ) and meadow v o l e s ( M i c r o t u s p e n n s y l v a n i c u s ) have a wide g e o g r a p h i c a l d i s t r i b u t i o n i n N o r t h A m e r i c a . In n o n - a r i d t e m p e r a t e h a b i t a t s t h e y d o m i n a t e r o d e n t c o m m u n i t i e s , a l o n g w i t h r e d - b a c k e d v o l e s ( C l e t h r i o n o m y s  spp.) . Deer mice a r e r e m a r k a b l y f l e x i b l e i n t h e i r h a b i t a t d i s t r i b u t i o n , r a n g i n g f r o m mature c o n i f e r o u s f o r e s t s t o open g r a s s l a n d s . Two f o r m s , p r a i r i e m i ce and woodland m i c e , have s i n c e l o n g been r e c o g n i z e d , and t h e i r p a r t i c u l a r m o r p h o l o g i e s and d e m o g r a p h i c a t t r i b u t e s have a l s o been documented (Baker 1968, R e d f i e l d e t a l . 1977, M i l l a r 1982). Red-backed v o l e s l i v e i n a v a r i e t y of f o r e s t and s h r u b l a n d h a b i t a t s , f r e q u e n t l y c o e x i s t i n g w i t h d e e r m i c e . In g e n e r a l , t h e i r h a b i t a t i s h i g h l y m e s i c , and t h e r e i s o f t e n a d e n s e c o v e r of mosses on t h e g r o u n d . H a b i t a t p r e f e r e n c e s may be s t r o n g l y i n f l u e n c e d by h i g h d a i l y w a t er r e q u i r e m e n t s (Odum 1944). I n c o n t r a s t t o t h e two p r e v i o u s s p e c i e s , meadow v o l e s o c c u r m a i n l y on open h a b i t a t s s u c h as meadows, g r a s s l a n d s , abandoned f i e l d s , f o r e s t e d ges and o p e n i n g s . W i t h s u c h h a b i t a t d i s t r i b u t i o n s , t h e s e s p e c i e s o f t e n show c o n t i g u o u s p o p u l a t i o n s s e p a r a t e d by a s h a r p change of h a b i t a t s . What f a c t o r s d e t e r m i n e t h e use of h a b i t a t s by t h e s e 50 s p e c i e s ? E a r l y o b s e r v a t i o n s on i s l a n d s , where one s p e c i e s o c c u p i e d a w i d e r r a n g e o f h a b i t a t s i n t h e a b s e n c e of t h e o t h e r s ( e c o l o g i c a l r e l e a s e ) , s t i m u l a t e d e x p e r i m e n t a l work t o t e s t t h e h y p o t h e s i s t h a t c o m p e t i t i v e i n t e r a c t i o n s between s p e c i e s were r e s p o n s i b l e f o r t h e i r h a b i t a t d i s t r i b u t i o n ( G r a n t 1972a, 1978). A f t e r a s e r i e s o f e x p e r i m e n t s i n s m a l l e n c l o s u r e s , G r a n t (1969, 1971, 1972a) and M o r r i s and G r a n t ( 1 9 7 2 ) , c o n c l u d e d t h a t meadow v o l e s i n h i b i t e d t h e use o f g r a s s l a n d h a b i t a t s by t h e f o r e s t s p e c i e s . A c c o r d i n g t o them, i n t e r s p e c i f i c i n t e r a c t i o n s t h e r e f o r e would c o u n t e r a c t h a b i t a t e x p a n s i o n t h a t r e s u l t s from i n t r a s p e c i f i c p r e s s u r e . From t h e s e e n c l o s u r e e x p e r i m e n t s and f i e l d o b s e r v a t i o n s , t h e b e l i e f t h a t i n t e r s p e c i f i c c o m p e t i t i o n f o r s p a c e i s w i d e s p r e a d i n s m a l l mammal c o m m u n i t i e s has become e n g r a i n e d ( G r a n t 1972a, 1978, Hawes 1976). However, t h e e v i d e n c e a c c u m u l a t e d on i n t e r a c t i o n s between t h e s e t h r e e g e n e r a ( P e r o m y s c u s , M i c r o t u s and C l e t h r i o n o m y s ) , i s c o n t r a d i c t o r y . In s u p p o r t of G r a n t ' s h y p o t h e s i s , R e d f i e l d e t a l . ( 1 9 7 7 ) , showed t h a t two m i c r o t i n e s ( M i c r o t u s t o w n s e n d i i and M i c r o t u s o r e g o n i ) s t r o n g l y a f f e c t p o p u l a t i o n s of d e e r mice (Peromyscus m a n i c u l a t u s ) . As soon as v o l e s were removed, d e e r m ice i n c r e a s e d i n numbers from a v e r y low l e v e l and m a i n t a i n e d a h i g h d e n s i t y u n t i l t h e v o l e s were a l l o w e d t o r e c o l o n i z e t h e a r e a . The p r e s e n c e of M i c r o t u s i n c r e a s e d t h e r a t e of d i s a p p e a r a n c e ( h i g h e r m o r t a l i t y a n d / o r h i g h e r e m i g r a t i o n ) o f r e s i d e n t d e e r mice of a l l s i z e c l a s s e s . The r e m o v a l of p r a i r i e 51 v o l e s ( M i c r o t u s o c h r o g a s t e r ) from s h o r t - g r a s s p r a i r i e r e s u l t e d i n t h e i n c r e a s e d use o f t h o s e a r e a s by d e e r mice (P. m a n i c u l a t u s ) (Abramsky e t a l . 1979). However, i n n e i t h e r s t u d y was t h e mechanism o f t h e i n t e r a c t i o n known. H a l l e t t e t a l . ( 1 9 8 3 ) , a n a l y z e d t h e use o f t r a p s t a t i o n s by d i f f e r e n t s p e c i e s of s m a l l mammals i n a l i v e - t r a p p i n g g r i d and c o n c l u d e d t h a t v o l e s ( M i c r o t u s ) , jumping mice (Zapus) and g r o u n d s q u i r r e l s ( S p e r m o p h i l u s ) i n h i b i t e d d e e r mice from u s i n g t h e a r e a . However, no e x p e r i m e n t a l m a n i p u l a t i o n of t h e i r p o p u l a t i o n s was done. In summary, w h i l e some s t u d i e s have documented n e g a t i v e i n t e r a c t i o n s between t h e s e g e n e r a , l i t t l e i s known ab o u t t h e mechanisms of i n t e r a c t i o n and t h e v a r i a b i l i t y of t h e phenomena. A l t e r n a t i v e l y , t h e s e t e m p e r a t e r o d e n t s have been d e s c r i b e d t o be s t r o n g h a b i t a t s e l e c t o r s ( M ' C l o s k e y 1975a, 1975b, M ' C l o s k e y and F i e l d w i c k 1975, M o r r i s 1983). M ' C l o s k e y (1975a,b) f o u n d t h e d i s t r i b u t i o n o f meadow v o l e s ( M i c r o t u s  p e n n s y l v a n i c u s ) and w h i t e - f o o t e d mice (Peromyscus l e u c o p u s ) s t r o n g l y a s s o c i a t e d w i t h v a r y i n g v e g e t a t i o n s t r u c t u r e ( f o l i a g e h e i g h t d i v e r s i t y ) . In h a b i t a t s where b o t h s p e c i e s c o e x i s t e d , he f o u n d no r e l a t i o n between t h e o c c u r r e n c e of M i c r o t u s and t h e d e n s i t y o f P e r o m y s c u s . He h y p o t h e s i z e d t h a t t h e h a b i t a t s e p a r a t i o n of P e r o m y s c u s and M i c r o t u s i s a c c o u n t e d f o r by d i f f e r e n c e s i n t h e s t r u c t u r a l h a b i t a t and r e g a r d e d c o m p e t i t i v e i n t e r a c t i o n s as o f l i t t l e i m p o r t a n c e . C o n s i s t e n t w i t h t h i s h y p o t h e s i s , K i n g (1983) f o u n d no c h a n g e s i n d e e r mouse demography i n two c o n s e c u t i v e y e a r s when c o e x i s t i n g v o l e s ( M i c r o t u s ) f l u c t u a t e d f r o m h i g h d e n s i t y t o c o m p l e t e a b s e n c e . 52 R e c e n t l y , M o r r i s (1983) a n a l y z e d t h e d i s t r i b u t i o n and r e l a t i v e a bundance o f s p e c i e s p a i r s i n s e v e r a l h a b i t a t s . The d i s t r i b u t i o n s of t h e s e s p e c i e s were n o t de p e n d e n t on e a c h o t h e r nor was t h e r e a c o n s i s t e n t r e l a t i o n s h i p between t h e i r r e l a t i v e a bundance a c r o s s h a b i t a t s . H i s f i n d i n g s p r o v i d e s t r o n g e v i d e n c e a g a i n s t t h e h y p o t h e s i s o f c o m p e t i t i v e i n t e r f e r e n c e between d e e r m ice (Peromyscus m a n i c u l a t u s ) and r e d - b a c k e d v o l e s ( C l e t h r i o n o m y s g a p p e r i ) , and between w h i t e - f o o t e d m i c e ( P e r o m y s c u s l e u c o p u s ) and meadow v o l e s ( M i c r o t u s  p e n n s y l v a n i c u s ) . A g a i n , no e x p e r i m e n t a l m a n i p u l a t i o n s were c a r r i e d o u t . The c o n t r a d i c t o r y e v i d e n c e c o n c e r n i n g i n t e r a c t i o n s among s p e c i e s o f t h e s e t h r e e g e n e r a i s r e f l e c t e d i n s u c c e s s i o n a l s t u d i e s . C o m p e t i t i v e i n t e r a c t i o n s have been i n v o k e d t o e x p l a i n t h e p a t t e r n s of r o d e n t s p e c i e s r e p l a c e m e n t a f t e r l o g g i n g . I t i s w e l l documented t h a t r e d - b a c k e d v o l e s , a bundant i n u n d i s t u r b e d c o n i f e r o u s f o r e s t s , d e c l i n e t h r o u g h t h e f i r s t t h r e e y e a r s a f t e r c l e a r - c u t t i n g , w h i l e d e e r m i c e , r a r e i n u n c u t c o n i f e r o u s s t a n d s , i n c r e a s e o v e r t h e same p e r i o d ( M a r t e l l 1983), but M a r t e l l c h a l l e n g e d t h e s u g g e s t i o n t h a t t h e d e c l i n e i n r e d - b a c k e d v o l e numbers i s r e l a t e d t o c o m p e t i t i v e i n t e r a c t i o n s w i t h d e e r m i c e . He h o l d s t h a t s u c h c h a n g e s i n numbers a r e r e l a t e d t o h a b i t a t c h a n g e s and p o i n t e d out t h a t i n u n s c a r i f i e d c l e a r - c u t t i n g s r e d -b a c k e d v o l e s r e m a i n w h i l e d e e r m i c e a r e a b s e n t . T h i s s u g g e s t s an i n t e r a c t i o n i n t h e o p p o s i t e d i r e c t i o n , i n w h i c h r e d - b a c k e d v o l e s a r e c o m p e t i t i v e l y d o m i n a n t . 53 T h e h i g h l y d i v e r s e c o m m u n i t y o f s m a l l m a m m a l s i n t h e s o u t h w e s t e r n Y u k o n e x e m p l i f i e s t h e d i s t r i b u t i o n a l p a t t e r n s p r e s e n t e d a b o v e ( K r e b s a n d W i n g a t e 1 9 7 6 ) . T h e d o m i n a n t r o d e n t s p e c i e s b e l o n g t o t h e s e t h r e e g e n e r a : M i c r o t u s , P e r o m y s c u s a n d C l e t h r i o n o m y s . T h e f o r m e r t w o m e e t a t t h e e c o t o n e b e t w e e n f o r e s t a n d m e a d o w , w h i l e t h e l a t t e r t w o s h a r e t h e f o r e s t h a b i t a t s t o a g r e a t e r e x t e n t . T o t e s t t h e h y p o t h e s i s t h a t c o m p e t i t i o n i s r e s p o n s i b l e f o r t h e d i f f e r e n t i a l h a b i t a t u s e ( s m a l l o v e r l a p ) a n d r e l a t i v e a b u n d a n c e b e t w e e n t h e s e s p e c i e s , I u s e d b o t h a r t i f i c i a l a n d n a t u r a l f i e l d e x p e r i m e n t s . I n t h e f i r s t f i e l d s e a s o n I a r t i f i c i a l l y r e m o v e d t h e m e a d o w v o l e p o p u l a t i o n f r o m a s e d g e m e a d o w a d j a c e n t t o t h e f o r e s t w h e r e d e e r m i c e o c c u r r e d a l o n e . I n t h e s e c o n d y e a r a d e c r e a s e i n t h i s c y c l i c p o p u l a t i o n p r o v i d e d me w i t h a n a t u r a l r e m o v a l e x p e r i m e n t : v o l e s w e r e a l m o s t c o m p l e t e l y a b s e n t f r o m t h e s e d g e m e a d o w . T h e t h i r d s u m m e r , m e a d o w v o l e s c o l o n i z e d t h e f o r e s t , a n d r e d - b a c k e d v o l e s , a b s e n t t h e p r e v i o u s t w o y e a r s , i n c r e a s e d i n n u m b e r s , p r o v i d i n g n a t u r a l i n t r o d u c t i o n e x p e r i m e n t s . T h e p r e d i c t i o n s f o r t h e s e s i t u a t i o n s f r o m G r a n t ' s h y p o t h e s i s a r e : 1) A c h a n g e i n r e s o u r c e u s e . I n t h e a b s e n c e o f m e a d o w v o l e s i n t h e c o n t i g u o u s h a b i t a t ( r e m o v a l e x p e r i m e n t ) , d e e r m i c e w i l l e x p a n d t h e i r u s e o f 54 h a b i t a t s ( n i c h e s h i f t o r e c o l o g i c a l r e l e a s e ) , s h o w i n g a l a r g e r n i c h e b r e a d t h a n d a l a r g e r n i c h e o v e r l a p . I n t h e p r e s e n c e o f m e a d o w v o l e s a n d / o r r e d - b a c k e d v o l e s i n t h e s a m e h a b i t a t ( i n t r o d u c t i o n e x p e r i m e n t s ) , d e e r m i c e w i l l r e s t r i c t t h e i r u s e o f t h e h a b i t a t , s h o w i n g a s m a l l e r n i c h e b r e a d t h a n d s m a l l e r n i c h e o v e r l a p . 2 ) A c h a n g e i n n u m b e r s . D e e r m i c e w i l l i n c r e a s e i n n u m b e r s a f t e r t h e r e m o v a l o f m e a d o w v o l e s ( r e m o v a l e x p e r i m e n t ) . T h i s m i g h t h a p p e n t h r o u g h h i g h e r i m m i g r a t i o n , l o w e r e m i g r a t i o n , h i g h e r s u r v i v a l a n d / o r h i g h e r r e p r o d u c t i o n . H o w e v e r , s i n c e t h e o v e r l a p b e t w e e n p o p u l a t i o n s i s s m a l l i t i s u n l i k e l y t h a t t h e p o p u l a t i o n n u m b e r s w i l l b e a f f e c t e d u n l e s s t h e r e i s a c h a n g e i n s p a t i a l d i s t r i b u t i o n f i r s t . D e e r m i c e w i l l d e c r e a s e i n n u m b e r s a f t e r t h e i n t r o d u c t i o n o f m e a d o w v o l e s a n d / o r r e d - b a c k e d v o l e s ( i n t r o d u c t i o n e x p e r i m e n t s ) . N u m b e r s may b e a f f e c t e d t h r o u g h d i f f e r e n t c o m p o n e n t s o f t h e d e m o g r a p h i c m a c h i n e r y : l o w e r i m m i g r a t i o n , h i g h e r e m i g r a t i o n , l o w e r r e p r o d u c t i o n a n d / o r l o w e r s u r v i v a l . I w i l l now t e s t t h e s e p r e d i c t i o n s . 55 3.2 STUDY AREA The f i e l d work to o k p l a c e a t t h e s o u t h c o r n e r of K l u a n e L a k e , Yukon T e r r i t o r y (61 N, 138 W) ( F i g . 3 . 1 ) , d u r i n g t h e s n o w f r e e s e a s o n s (May t o September) o f 1981, 1982, and 1983. The s t u d y was c a r r i e d o u t i n t h r e e d i f f e r e n t h a b i t a t s : b o r e a l f o r e s t , sedge meadow and s h r u b l a n d . The f i r s t two h a b i t a t s have been d e s c r i b e d a s v e g e t a t i o n z o n e s by H o e f s e t a l . ( 1 9 7 5 ) . S h r u b l a n d i n c l u d e s s u c c e s s i o n a l s t a g e s of d i f f e r e n t a g e s . The b o r e a l f o r e s t , d o m i n a t e d by w h i t e s p r u c e ( P i c e a  g l a u c a ) , o c c u p i e s t h e l o w e r v a l l e y s f r o m 770 m t o 1100 m. The d e n s i t y o f d i f f e r e n t s p r u c e s t a n d s and t h e p r e s e n c e o f b a l s a m p o p l a r ( P o p u l u s b a l s a m i f e r a ) and t r e m b l i n g a s p e n ( P o p u l u s  t r e m u l o i d e s ) i n some a r e a s , c r e a t e s a v a r i e t y of a s s o c i a t i o n s of dominant t r e e s p e c i e s t h a t i n f l u e n c e t h e s h r u b and h e r b u n d e r g r o w t h . T h e s e a s s o c i a t i o n s have been r e c o g n i z e d as d i f f e r e n t p l a n t c o m m u n i t i e s ( D o u g l a s 1974) o r h a b i t a t s ( K r e b s and W i n g a t e 1976). In t h i s s t u d y I d e f i n e d b o r e a l f o r e s t as one h a b i t a t . The common s p e c i e s i n t h e u n d e r s t o r y a r e s o a p b e r r y ( S h e p h e r d i a c a n a d e n s i s ) i n t h e s h r u b s t r a t u m , and l i c o r i c e r o o t (Hedysarum b o r e a l e ) , and b e a r b e r r y ( A r c t o s t a p h y l o s r u b r a and A. u v a - u r s i ) i n t h e h e r b s t r a t u m . The sedge meadow i s m o s t l y r e s t r i c t e d t o t h e m a r g i n s of t h e l a k e . The f i n e - t e x t u r e s o i l s of t h i s h a b i t a t a r e w a t e r -56 F i g u r e 3 . 1 . Map o f t h e s t u d y a r e a i n s o u t w e s t e r n c o r n e r t h e Yukon T e r r i t o r y . 57 58 s a t u r a t e d and i n some a r e a s f l o o d i n g o c c u r s d u r i n g autumn due t o t h e r i s i n g l e v e l o f t h e u n d e r g r o u n d water t a b l e ( d e t e r m i n e d by r i s i n g l a k e l e v e l ) . A v a r i e t y of s e d g e s ( C a r e x s p p . ) , c o t t o n g r a s s ( E r i o p h o r u m a n g u s t i f o l i u m ) and o t h e r m o n o c o t y l e d o n s ( J u n c u s spp.) a r e t h e most c o n s p i c u o u s p l a n t s i n t h e meadow. H o r s e t a i l s ( E q u i s e t u m p a l u s t r e ) a r e v e r y a b u n d a n t , and s m a l l w i l l o w s h r u b s (up t o 1 m) ( S a l i x g l a u c a ) a r e w i d e l y s c a t t e r e d . The s h r u b l a n d h a b i t a t i s l e s s w e l l d e f i n e d and encompasses s u c c e s s i o n a l f o r e s t s t a g e s d o m i n a t e d by t a l l w i l l o w s (I t o 3 m) ( S a l i x g l a u c a ) , b a l s a m p o p l a r and w h i t e s p r u c e i n open s t a n d s . The s h r u b s a r e common i n p a t c h e s and a l o n g c r e e k s c l o s e t o t h e sedge meadows. S o a p b e r r y and l i c o r i c e r o o t a r e abun d a n t i n t h e u n d e r s t o r y . 3.3 METHODS P a t t e r n s S i n c e p a t t e r n s o f h a b i t a t use ( h a b i t a t b r e a d t h , h a b i t a t o v e r l a p ) and r e l a t i v e abundance change from y e a r t o y e a r ( K r e b s and Win g a t e 1976), I d e t e r m i n e d h a b i t a t u t i l i z a t i o n p a t t e r n s and abundance i n d i c e s d u r i n g t h e f i r s t f i e l d s e a s o n (1981) by t h e use of s n a p - t r a p l i n e s . E a ch l i n e c o n s i s t e d of 20 s t a t i o n s w i t h t h r e e 'museum s p e c i a l 1 s n a p - t r a p s p e r s t a t i o n 59 ( K r e b s 1964, K r e b s and W i n g a t e 1976). S t a t i o n s were s p a c e d a t 10 m i n t e r v a l s . The t r a p s were b a i t e d w i t h p e a n u t b u t t e r , s e t f o r t h r e e c o n s e c u t i v e n i g h t s and c h e c k e d d a i l y . T h r o u g h o u t t h e summer I s e t 45 s n a p - t r a p l i n e s i n t h e f o l l o w i n g way: 13 a t t h e f o r e s t , 14 a t t h e sedge meadow, 10 a t t h e e c o t o n e between f o r e s t and meadow and 8 a t t h e s h r u b l a n d h a b i t a t . I u s e d t h e f o l l o w i n g f o r m u l a t o measure h a b i t a t n i c h e b r e a d t h : 1 B = i p 2 i j where B = n i c h e b r e a d t h p = p r o p o r t i o n o f i n d i v i d u a l s o f s p e c i e s i w h i c h i s i j a s s o c i a t e d w i t h r e s o u r c e s t a t e j . F o r t h e s n a p - t r a p p i n g d a t a : p = r e l a t i v e number o f i n d i v i d u a l s p e r t r a p - n i g h t i j t i m e s 1000. F o r t h e l i v e - t r a p p i n g d a t a : p = r e l a t i v e f r e q u e n c y o f c a p t u r e s p e r t r a p - n i g h t i j t i m e s 1000. N i c h e b r e a d t h (B) i s a measure of t h e u n i f o r m i t y and d i s t r i b u t i o n o f a s p e c i e s among t h e r e s o u r c e s t a t e s of a m a t r i x . 60 I t r a n g e s from 1 when a s p e c i e s u s e s o n l y one c a t e g o r y t o t h e number o f r e s o u r c e s t a t e s when i t u s e s a l l c a t e g o r i e s e q u a l l y ( L e v i n s 1968, C o l w e l l and Futuyma 1971, Southwood 1978). To measure h a b i t a t o v e r l a p ( C ) , I u s e d C = 1 - 0.5 p - p i h i j h j Where p and p a r e a s d e f i n e d above f o r two d i f f e r e n t s p e c i e s . When t h e s p e c i e s do n o t s h a r e r e s o u r c e s t a t e s , o v e r l a p i s 0 and when t h e y show s i m i l a r d i s t r i b u t i o n s i n a l l c a t e g o r i e s o v e r l a p i s 1 ( C o l w e l l and Futuyma 1971, Southwood 1978). I u s e d b o t h C o l e ' s (C7) and H u r l b e r t ' s (C8) c o e f f i c i e n t s o f i n t e r s p e c i f i c a s s o c i a t i o n t o i n d i c a t e t h e s t r e n g t h of t h e a s s o c i a t i o n s i n t h e c o n t i n g e n c y t a b l e a n a l y s i s ( C o l e 1949, H u r l b e r t 1969). P r o c e s s To i n f e r t h e p r o c e s s r e s p o n s i b l e f o r t h e p a t t e r n s o b s e r v e d I u s e d e x p e r i m e n t a l m a n i p u l a t i o n s . S i n c e t h e e x p e r i m e n t a l d e s i g n c h anged e a c h f i e l d s e a s o n , I w i l l f i r s t d e s c r i b e t h e t r a p p i n g t e c h n i q u e . T h i s was s i m i l a r f o r a l l g r i d s 61 o v e r t h e t h r e e f i e l d s e a s o n s . T h e n I w i l l e x p l a i n c h a n g e s i n e x p e r i m e n t a l d e s i g n , n u m b e r a n d s i z e o f g r i d s a n d t r a p p i n g i n t e n s i t y a c c o r d i n g t o e a c h f i e l d s e a s o n . T r a p p i n g t e c h n i q u e T h e e x p e r i m e n t s w e r e c a r r i e d o u t o n l i v e - t r a p p i n g g r i d s . I u s e d ' L o n g w o r t h ' l i v e - t r a p s s p a c e d a t 10 m i n t e r v a l s . I p r e b a i t e d t h e t r a p s f o r 5 d a y s a n d s e t t h e m f o r t w o n i g h t s e v e r y o t h e r w e e k o r m o r e o f t e n . W h o l e o a t s w e r e u s e d a s b a i t a n d c o t t o n b a t t i n g a s b e d d i n g . A f t e r e v e r y t r a p s e s s i o n t h e t r a p s w e r e l o c k e d o p e n , a l l o w i n g t h e a n i m a l s t o m o v e i n a n d o u t f r e e l y . A n i m a l s c a p t u r e d w e r e e a r - t a g g e d w i t h f i n g e r l i n g f i s h t a g s , a n d I r e c o r d e d s p e c i e s , t a g n u m b e r , s e x , b r e e d i n g c o n d i t i o n , b o d y w e i g h t a n d t r a p l o c a t i o n . B r e e d i n g c o n d i t i o n o f m a l e s w a s a s s e s s e d b y r e c o r d i n g t h e i r t e s t e s p o s i t i o n a s e i t h e r a b d o m i n a l ( n o t b r e e d i n g ) o r s c r o t a l ( b r e e d i n g ) . F o r f e m a l e s I r e c o r d e d v a g i n a l s t a t u s ( o p e n o r c l o s e d ) , n i p p l e s i z e ( s m a l l , m e d i u m o r l a r g e ) , c o n d i t i o n o f t h e p u b i c s y m p h y s i s ( c l o s e d , s l i g h t l y o p e n o r o p e n ) , a n d e v i d e n t p r e g n a n c i e s . E x p e r i m e n t a l d e s i g n 1981 D u r i n g 1981 I u s e d t w o l i v e - t r a p p i n g g r i d s : a n e x p e r i m e n t a l ( B . 8 1 ) a n d a c o n t r o l ( A . 8 1 ) T a b l e ( 3 . 1 ) . E a c h g r i d h a d 120 ' L o n g w o r t h ' l i v e - t r a p s a r r a n g e d i n a r e c t a n g u l a r g r i d o f 62 T a b l e 3.1. E x p e r i m e n t a l d e s i g n f o r 1981. G r i d T r e a t m e n t A. 81 C o n t r o l B. 81 A r t i f i c i a l r e m o v a l o f meadow v o l e s 6 x 20 t r a p s t a t i o n s , c o v e r i n g an a r e a of a p p r o x i m a t e l y 1.2 h a . B o t h g r i d s were l o c a t e d i n a s i m i l a r way a t t h e e c o t o n e between s p r u c e - p o p l a r f o r e s t and sedge meadow. O n e - t h i r d of e a c h g r i d ( c o l u m n s .14 t o 20) was on t h e f o r e s t a r e a w h i l e t h e o t h e r two-t h i r d s ( c o l u m n s 1 t o 13) were on t h e sedge meadow. The g r i d s were p a r a l l e l t o e a c h o t h e r and s e p a r a t e d by 213 m (700 f t ) . Some o f t h e s n a p - t r a p l i n e s d e s c r i b e d above were ne a r t h e l i v e -t r a p p i n g g r i d s but n e v e r c l o s e r t h a n 150 m (492 f t ) . B e g i n n i n g on t h e s e c o n d t r a p s e s s i o n I removed a l l meadow v o l e s c a u g h t a t t h e e x p e r i m e n t a l g r i d (B.81) e v e r y o t h e r week o r more o f t e n . The a i m o f t h i s m a n i p u l a t i o n was t o r e d u c e t h e i r p o p u l a t i o n t o o b s e r v e t h e e f f e c t s of s u c h r e d u c t i o n on t h e c o n t i g u o u s p o p u l a t i o n o f d e e r m i c e . E x p e r i m e n t a l d e s i g n 1982 D u r i n g 1982 I s e t f i v e l i v e - t r a p p i n g g r i d s . The two f o r m e r g r i d s were u s e d i n t h e same way as i n t h e p r e v i o u s y e a r , 63 but the treatments were r e v e r s e d . The c o n t r o l g r i d (A.81) i n 1981 became the removal g r i d i n 1982 and v i c e v e r s a . I added a new g r i d (E.82) of s i m i l a r s i z e to the pr e v i o u s and s i t u a t e d a l s o at the ecotone between the two h a b i t a t s (Table 3.2). Table 3.2. Experimental design f o r 1982. G r i d Treatment C. 82 C o n t r o l B.82 N a t u r a l removal of meadow v o l e s E.82 N a t u r a l removal of meadow v o l e s A.82 N a t u r a l + a r t i f i c i a l removal meadow v o l e s D. 82 Monitor d i s p e r s a l T h i s year, however, the p o p u l a t i o n of meadow v o l e s had become very scarce and stayed low throughout the summer. The r e f o r e , the three g r i d s are c o n s i d e r e d n a t u r a l removal experiments. The only d i f f e r e n c e between the a r t i f i c i a l removal treatment (A.82) and the other g r i d s i s that on the former I removed the few animals l i v i n g t h e r e , reducing t h i s p o p u l a t i o n even f u r t h e r and keeping the h a b i t a t empty of Mic r o t u s . The c o n t r o l g r i d (C.82) was l o c a t e d f u r t h e r away from the meadows i n an advanced s u c c e s s i o n a l stage dominated by t a l l willows and a few spruce t r e e s . I t was a r e c t a n g u l a r 4 x 1 0 g r i d occupying an area s i m i l a r i n s i z e to the f o r e s t p a r t of the removal g r i d s . I trapped t h i s g r i d from J u l y to September. A f i f t h g r i d (D.82) was set between the former g r i d s to monitor 64 d i s p e r s a l and movements between a r e a s . T h i s was a r e c t a n g u l a r 4 x 10 l i v e - t r a p p i n g g r i d , w i t h two rows i n a l o n g and t h i n p a t c h o f s h r u b l a n d p a r a l l e l t o t h e l a k e s h o r e , and t h e o t h e r two rows on t h e sedge meadow. E x p e r i m e n t a l d e s i g n 1983 D u r i n g t h e f i e l d s e a s o n o f 1983 I u s e d 6 l i v e - t r a p p i n g g r i d s . I r e d u c e d them t o a b o u t o n e - t h i r d o f t h e i r f o r m e r s i z e , l e a v i n g o n l y t h e t r a p s i n t h e f o r e s t h a b i t a t . T h e i r s i z e s r a n g e d f r o m .42 ha t o .54 ha. And t h e number o f t r a p s r a n g e d from 42 t o 60. Of t h e s e 6 g r i d s , one was c o l o n i z e d by meadow v o l e s , f o u r by r e d - b a c k e d v o l e s and .one by b o t h s p e c i e s . I compared t h e g r i d s t h a t meadow v o l e s c o l o n i z e d w i t h t h e same a r e a s i n p r e v i o u s y e a r s when t h e y were a b s e n t a s w e l l as w i t h c o n t e m p o r a n e o u s c o n t r o l s . Of t h e f o u r g r i d s where r e d - b a c k e d v o l e s i n c r e a s e d i n numbers, two s u p p o r t e d r e l a t i v e l y h i g h numbers, two s u p p o r t e d v e r y low numbers. I compared t h e s e two c o n d i t i o n s a s w e l l as w i t h g r i d s f r o m w h i c h t h e y were c o m p l e t e l y a b s e n t ( T a b l e 3 . 3 ) . F i g u r e 3.2 shows t h e l o c a t i o n of t h e l i v e t r a p p i n g -g r i d s d u r i n g t h e t h r e e f i e l d s e a s o n s . 65 F i g u r e 3.2. L o c a t i o n o f t h e l i v e - t r a p p i n g g r i d s . G r i d s A B, E, were u s e d as l o n g r e c t a n g u l a r g r i d s (6 x 20) d u r i n g 1981 and 1982. D u r i n g 1983 t h e y were r e d u c e d t h e p o r t i o n i n s i d e t h e f o r e s t o n l y ( 6 x 7 f o r A and B, and 6 x 1 0 f o r E ) . 66 t ; • : • :!E 67 Table 3.3. Experimental design f o r 1983. G r i d Treatment B.81 C o n t r o l B.82 C o n t r o l B. 83 I n t r o d u c t i o n (meadow vo l e s ) E.82 C o n t r o l E.83 I n t r o d u c t i o n (meadow v o l e s + red-backed v o l e s ) C. 83 C o n t r o l (low red-backed v o l e s ) D. 83 C o n t r o l (low red-backed v o l e s ) G. 83 I n t r o d u c t i o n (high red-backed v o l e s ) H. 83 I n t r o d u c t i o n (high red-backed v o l e s ) 3.4 RESULTS PATTERNS OF HABITAT USE Hab i t a t use The captures from snap-trapping l i n e s (8040 t r a p -n i g h t s ) i n d i c a t e d broad p a t t e r n s of h a b i t a t use by both s p e c i e s d u r i n g the summer of 1981. I caught n e a r l y equal numbers of deer mice and meadow v o l e s but i n d i f f e r e n t h a b i t a t s . Deer mice (n=l05) were caught, mainly i n the f o r e s t (47%) and s e c o n d a r i l y in the shrubland (25%) and ecotone (24%) h a b i t a t s . Very few (4%) were caught i n the sedge meadow. In c o n t r a s t , meadow v o l e s (n=l08) were h i g h l y r e s t r i c t e d to the sedge meadow (78%). Some 68 were c a u g h t i n t h e e c o t o n e (18%) and fewer (4%) i n t h e s h r u b l a n d h a b i t a t , none were c a u g h t i n t h e f o r e s t ( F i g . 3 . 3 ) . A r e s o u r c e m a t r i x a n a l y s i s u s i n g t h e s e f o u r h a b i t a t s as d i f f e r e n t r e s o u r c e s t a t e s i n d i c a t e s a l a r g e r h a b i t a t n i c h e b r e a d t h f o r d e e r m i c e (B= 3.06) t h a n f o r meadow v o l e s (B= 1.76). T h e i r h a b i t a t n i c h e o v e r l a p i s s m a l l (C = 0.313) ( T a b l e 3 . 4 ) . T a b l e 3.4. R e s o u r c e m a t r i x , h a b i t a t n i c h e b r e a d t h and h a b i t a t n i c h e o v e r l a p f o r meadow v o l e s and d e e r m i c e . H a b i t a t n i c h e b r e a d t h (B) r a n g e s from 1 when o n l y one c a t e g o r y i s u s e d t o 4 when a l l c a t e g o r i e s a r e u s e d e q u a l l y . H a b i t a t n i c h e o v e r l a p (C) r a n g e s from 0 t o 1 . RESOURCE STATES : HABITATS Meadow E c o t o n e F o r e s t S h r u b l a n d SPECIES Deer mice 1.62 13.44 21.92 18.05 55.03 Meadow v o l e s 34.14 10.75 0.0 2.77 47.66 35.76 24.19 21.92 20.82 102.69 N i j = number o f i n d i v i d u a l s p e r t r a p i n t e n s i t y ( t r a p - n i g h t s ) t i m e s 1000. HABITAT NICHE BREADTH Deer mice 3.06 V o l e s 1.76 HABITAT NICHE OVERLAP C = 0.313 69 F i g u r e 3.3. D e n s i t y i n d i c e s f o r meadow v o l e s and d e e r mice i n d i f f e r e n t h a b i t a t s f o r May t o A u g u s t 1981. 4 r N u m b e r of 3 I n d i v i d u a l s c a u g h t 2 p e r 100 T r a p N i g h t s • D E E R M I C E M E A D O W V O L E S M E A D O W E C O T O N E A F O R E S T S H R U B L A N D 71 M i c r o h a b i t a t use From t h e above a n a l y s i s , t h e r e s o u r c e s t a t e ( h a b i t a t ) t h a t showed t h e maximum o v e r l a p of b o t h s p e c i e s was t h e e c o t o n e . However, a c l o s e r a n a l y s i s o f t h e m i c r o d i s t r i b u t i o n i n t h i s a r e a r e v e a l e d p r o m i n e n t d i f f e r e n c e s . To a n a l y z e t h i s m i c r o d i s t r i b u t i o n I u s e d t h e c o n t r o l l i v e - t r a p p i n g g r i d w h i c h was s e t p e r p e n d i c u l a r t o t h e b o u n d a r y between f o r e s t and meadow. E a c h h a b i t a t had a d i f f e r e n t p r o p o r t i o n of l i v e - t r a p s . The d i s t r i b u t i o n o f b o t h s p e c i e s i n t h e s e h a b i t a t s was v e r y d i f f e r e n t f r o m t h a t e x p e c t e d i f t h e a n i m a l s were u s i n g t h e h a b i t a t s p r o p o r t i o n a l l y t o t h e number o f t r a p s s e t i n e a c h . Deer mice were a l m o s t c o n f i n e d t o t h e f o r e s t h a b i t a t (Gadj.= 130.71, p<0.005) w h i l e meadow v o l e s were n e a r l y c o n f i n e d t o t h e meadow h a b i t a t (Gadj.= 465.31, p<0.005). B o t h s p e c i e s r a r e l y u s e d t h e s h r u b l a n d h a b i t a t and were n e a r l y a b s e n t from t h e h a b i t a t o c c u p i e d by t h e o t h e r s p e c i e s ( F i g . 3 . 4 ) . A n o t h e r way t o measure t h e d i s t r i b u t i o n of t h e s e s p e c i e s i s by a n s w e r i n g t h e q u e s t i o n : Among t h e 120 t r a p s t a t i o n s a t t h e e c o t o n e , d i d d e e r mice and meadow v o l e s o c c u r i n d e p e n d e n t l y ? The answer i s no. They were a s s o c i a t e d n e g a t i v e l y (x2 c o r r = 41.16, p < 0 . 0 l ) , and t h e s t r e n g t h of t h e n e g a t i v e a s s o c i a t i o n was h i g h (C7, C8 = - 0 . 7 8 ) . In summary, d e e r mice and meadow v o l e s u s e d d i s t i n c t h a b i t a t s and showed c o n t i g u o u s p o p u l a t i o n s w i t h s m a l l o v e r l a p when t h e i r h a b i t a t s come i n t o c o n t a c t . Deer mice u s e d a w i d e r 7 2 F i g u r e 3 . 4 . H a b i t a t d i s t r i b u t i o n b y d e e r m i c e a n d m e a d o w v o l e s o n c o n t r o l g r i d . D e e r m i c e u s e d m a i n l y f o r e s t w h i l e m e a d o w v o l e s w e r e r e s t r i c t e d t o t h e m e a d o w s . E x p e c t e d v a l u e s w e r e o b t a i n e d b y a s s i g n i n g t h e c a p t u r e s p r o p o r t i o n a l l y t o t h e n u m b e r o f t r a p s i n e a c h h a b i t a t . Observed E x p e c t e d DEER MICE Number of Captures 60 40 20 y / FOREST MEADOW 600 -M E A D O W V O L E S Number of Captures 400 200 V / / / / z. FOREST MEADOW 7 4 r a n g e o f h a b i t a t s . T h e y w e r e m a i n l y f o u n d i n t h e f o r e s t b u t u s e d s h r u b l a n d h a b i t a t t o o . M e a d o w v o l e s , o n t h e o t h e r h a n d , w e r e m a i n l y c o n f i n e d t o t h e m e a d o w h a b i t a t . R E M O V A L E X P E R I M E N T S I u s e d f o u r r e m o v a l g r i d s . D u r i n g t h e f i r s t s u m m e r I u s e d g r i d B . 8 1 a s a n a r t i f i c i a l r e m o v a l g r i d . T h e n a t u r a l d e c l i n e o f t h e m e a d o w v o l e p o p u l a t i o n d u r i n g t h e s e c o n d s u m m e r p r o d u c e d n a t u r a l r e m o v a l e x p e r i m e n t s i n A . 8 2 , B . 8 2 a n d E . 8 2 . G r i d A . 8 2 w a s s u b j e c t e d t o a n a d d i t i o n a l a r t i f i c i a l r e m o v a l t o l o w e r t h e p o p u l a t i o n o f m e a d o w v o l e s e v e n f u r t h e r . E F F E C T I V E N E S S OF R E M O V A L M A N I P U L A T I O N S A m a j o r d i f f i c u l t y i n u s i n g a n d i n t e r p r e t i n g t h e r e s u l t s o f r e m o v a l e x p e r i m e n t s i s t h a t a n i m a l s l i v i n g i n s u r r o u n d i n g a r e a s c o n t i n u o u s l y i m m i g r a t e a f t e r t h e r e s i d e n t i n d i v i d u a l s h a v e b e e n r e m o v e d . T h i s i m m i g r a t i o n i n t o t h e v a c a n t a r e a w a s s t r o n g l y r e d u c e d d u r i n g t h e s e c o n d f i e l d s e a s o n ( 1 9 8 2 ) w h e n t h e v o l e p o p u l a t i o n d e c r e a s e d n a t u r a l l y . I u s e d t w o w a y s t o e v a l u a t e h o w e f f e c t i v e t h e r e m o v a l t r e a t m e n t s ( B . 8 1 , B . 8 2 , A . 8 2 ) w e r e a t k e e p i n g t h e a r e a s w i t h o u t v o l e s . a ) F i r s t , b y c o m p a r i n g t h e i n t e n s i t y o f u s e ( n u m b e r o f t r a p s t a t i o n s u s e d p e r t r a p p i n g s e s s i o n ) i n t h e s e t r e a t m e n t s 75 with that i n the c o n t r o l . The mean number of t r a p s t a t i o n s used by M i c r o t u s per t r a p p i n g s e s s i o n was 15 at B.81, 3 at B.82 and 2 at A.82. T h i s t r a n s l a t e s i n t o a r e d u c t i o n of use by M i c r o t u s of 53%, 91% and 94% r e s p e c t i v e l y , i n r e l a t i o n to the c o n t r o l g r i d i n 1981, where 32 t r a p s t a t i o n s per t r a p p i n g s e s s i o n were used on average ( F i g . 3.5). b) Second, by comparing the i n t e n s i t y of use with the a v a i l a b i l i t y of t r a p s t a t i o n s i n the meadow. The r e d u c t i o n i s even more dramatic i n r e l a t i o n to the number of tra p s a v a i l a b l e in the meadow. On the c o n t r o l g r i d , an average of 39% of the meadow t r a p s are used by Microtus per t r a p s e s s i o n while only 22%, 4% and 2% were used at B.81, B.82 and A.82 r e s p e c t i v e l y . A f u r t h e r i n d i c a t i o n of the e f f e c t i v e n e s s of the removal treatments i s given by the number of i n d i v i d u a l s removed w i t h i n t r a p p i n g s e s s i o n s . V o l e s were caught and subsequently removed e i t h e r during the f i r s t or the second night of every t r a p s e s s i o n . In both g r i d s (B.81, A.82), from which v o l e s were a r t i f i c i a l l y removed,. most were removed on the f i r s t n i g ht and very few on the second. At B.81 the mean number of v o l e s removed per t r a p s e s s i o n was 10.44, from which an average of 8.9 were removed on the f i r s t n i g h t whereas only an average of 1.54 were removed on the second. At A.82, 3.44 i n d i v i d u a l s were removed every s e s s i o n on average. The average was 2.54 and 0.9 for the f i r s t and second night r e s p e c t i v e l y . T h i s d e c l i n e of animals removed w i t h i n t r a p p i n g s e s s i o n s i n d i c a t e s that the area i s almost f r e e of v o l e s a f t e r each s e s s i o n ( F i g . 3.6). 76 F i g u r e 3.5. I n t e n s i t y of t r a p use by meadow v o l e s On average 32 percent of the tr a p s were used on the c o n t r o l g r i d . The removal at B.81 reduced t h i s number to h a l f . During 1982 both g r i d s were almost empty. Trap s e s s i o n s when no v o l e s were present are i n d i c a t e d with 0. 7 7 P E R C E N T A G E O F TRAP STATIONS UTILIZED BY 40 j - A.81 C O N T R O L 30 20 101-20 10 M J J A B.81 EXPERIMENTAL -n r u n M M E A D O W 2ol B.82 CONTROL V O L E S 10r T n Vn r r ^ ° r n ^ n M J J A o 20 10 L A.82 EXPERIMENTAL I L a m Pk^ ' M J J / x 32 x 15 x 3 r~i x 2 7 8 F i g u r e 3 .6 . N u m b e r o f m e a d o w v o l e s r e m o v e d f r o m e x p e r i m e n t a l g r i d s . D e c l i n e s i n d i c a t e t h a t m o s t v o l e s w e r e r e m o v e d o n t h e f i r s t n i g h t ( c l o s e d c i r c l e s ) . T h e g r i d s w e r e a l m o s t e m p t y a f t e r t h e s e c o n d n i g h t ( o p e n c i r c l e s ) . A r r o w s r e p r e s e n t t h e b e g i n n i n g o f t h e r e m o v a l s . Number of MEADOW V O L E S Removed 1 8 1 6 1 4 1 2 1 0 8 6 4 2 1 9 8 1 T o M x 8 . 9 x 1 .54 Number of 1 0 8 MEADOW 6 V O L E S Removed 4 -2 -80 In summary, the removal treatments were h i g h l y e f f e c t i v e . T h i s was more so during the second f i e l d season (1982), when the n a t u r a l d e c l i n e i n v o l e numbers oc c u r r e d . F i g u r e 3.7 shows the e f f e c t of the removal treatments on numbers. SPATIAL DISTRIBUTION If i n t e r s p e c i f i c i n t e r a c t i o n s a c t i v e l y i n f l u e n c e the s p a t i a l d i s t r i b u t i o n of deer mice, the decrease of v o l e numbers w i l l r e s u l t i n an i n c r e a s e use of the meadow h a b i t a t by deer mice. Did deer mice change t h e i r h a b i t a t use a f t e r the removal of meadow v o l e s ? Deer mice used mainly the f o r e s t h a b i t a t i n both g r i d s (A,B) both years (1981, 1982) ( F i g . 3.8). Only a small percentage of the t o t a l number of captures was recorded i n the meadow h a b i t a t . Deer mice occupied the same h a b i t a t on the two removal g r i d s (A.82, B.82) as they d i d on the c o n t r o l . The only s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e appeared i n t h e i r use of the meadow h a b i t a t at B.81 (Gadj= 5.50 p<0.025). S e v e r a l elements i n d i c a t e that t h i s d i f f e r e n c e was not r e l a t e d to the treatments. a) The d i f f e r e n c e o c c u r r e d i n the g r i d where the removal of v o l e s was l e a s t severe. G r i d s A.82 and B.82 had almost no v o l e s present but there was no change i n the h a b i t a t d i s t r i b u t i o n of deer mice i n these g r i d s . b) The t r a p 81 F i g u r e 3.7. E f f e c t of r e m o v a l t r e a t m e n t s on numbers. In c o m p a r i s o n w i t h t h e c o n t r o l g r i d (A.81) t h e o t h e r 3 g r i d s had a v e r y s m a l l number o f meadow v o l e s . O n l y B .81 had h i g h i m m i g r a t i o n d u r i n g A u g u s t and September. 83 F i g u r e 3.8. H a b i t a t use by deer mice. Deer mice were mainly c o n f i n e d to the f o r e s t h a b i t a t of the four l i v e -t r a p p i n g g r i d s i n s p i t e of the absence of meadow v o l e s from three of them. Only the f i r s t capture i n each s e s s i o n was taken i n t o account. A.82 DEER MICE Captures per 100 Trap Nights 14 12 1 0 T 8 6 4 2 F O R E S T MEADOW 85 s t a t i o n s used i n the meadow h a b i t a t a t B.81 were l o c a t e d v e r y c l o s e t o the f o r e s t a r e a . c) The same t r a p s t a t i o n s or o t h e r s i n the c l o s e v i c i n i t y were used d u r i n g the second season (B.82) when t h e r e was no s i g n i f i c a n t d i f f e r e n c e . d) I n d i v i d u a l s t h a t used meadow t r a p s t a t i o n s d i d not remain t h e r e . They were caught o n l y once or t w i c e and s u b s e q u e n t l y were r e c o r d e d back i n the f o r e s t . e) Most of t h e s e i n d i v i d u a l s were j u v e n i l e s p r o b a b l y a t t e m p t i n g d i s p e r s a l movements. POPULATION DYNAMICS I n t e r s p e c i f i c i n t e r a c t i o n s may a f f e c t not o n l y the s p a t i a l d i s t r i b u t i o n ( h a b i t a t use) of the s p e c i e s but a l s o t h e i r l o c a l p o p u l a t i o n d e n s i t y . R e l e a s e from a n e g a t i v e i n t e r a c t i o n may r e s u l t i n h i g h e r r e p r o d u c t i v e r a t e , h i g h e r i m m i g r a t i o n , lower m o r t a l i t y r a t e or lower e m i g r a t i o n , or a c o m b i n a t i o n of these p a r a m e t e r s . However, s i n c e the removal of the c o n t i g u o u s p o p u l a t i o n of meadow v o l e s d i d not a f f e c t the s p a t i a l d i s t r i b u t i o n of deer mice, i t was u n l i k e l y t o a f f e c t t h e i r demography. In f a c t , the p o p u l a t i o n s of Peromyscus remarkably were s i m i l a r i n both 1981 and 1982. ( F i g s . 3.9). NUMBERS In g e n e r a l , p o p u l a t i o n s of deer mice i n the study a r e a have a v e r y low d e n s i t y i n the s p r i n g , from 2 t o 16 o v e r w i n t e r e d 86 F i g u r e 3.9. Changes i n minimum number a l i v e o f d e e r mice d u r i n g 1981 and 1982 i n b o t h e x p e r i m e n t a l and c o n t r o l g r i d s . Numbers i n b o t h p l a c e s d u r i n g b o t h y e a r s were s i m i l a r . D u r i n g 1982 d e e r m i c e on t h e e x p e r i m e n t a l g r i d were l o w e r . B o t h p o p u l a t i o n s a r e s l i g h t l y l o w e r i n 1982 t h a n t h o s e i n 1981. DEER MICE Minimum Number Alive 14 12 10 8 • - • B.81 E X P E R I M E N T A L O - O A.81 C O N T R O L M 14 12 DEER MICE 10 8 Minimum Number Alive 6 •-•A.82 E X P E R I M E N T A L OO B.82 C O N T R O L M J J A s 8 8 a d u l t s p e r h e c t a r e . A d u l t f e m a l e s have one or two l i t t e r s . The f i r s t l i t t e r i s b o r n i n mid-June and becomes t r a p p a b l e i n l a t e J u n e or e a r l y J u l y whereas t h e s e c o n d l i t t e r i s b o r n i n m i d - J u l y and j o i n s t h e t r a p p a b l e p o p u l a t i o n i n e a r l y A u g u s t . The p o p u l a t i o n i n c r e a s e s , t h e r e f o r e , t h r o u g h June and J u l y and r e a c h e s i t s peak o f 24 t o 30 a n i m a l s p e r h e c t a r e i n A u g u s t . L i t t l e i s known ab o u t t h e demography i n w i n t e r . D u r i n g t h e f i r s t y e a r , d e e r mouse p o p u l a t i o n s on t h e c o n t r o l (A.81) and e x p e r i m e n t a l (B.81) g r i d s b e h a v e d v e r y s i m i l a r l y . B o t h s t a r t e d w i t h v e r y low numbers i n t h e s p r i n g and i n c r e a s e d t o a peak i n A u g u s t , t h e n d e c l i n e d i n t h e f a l l ( F i g . 3 . 9 ) . The p o p u l a t i o n on t h e r e m o v a l g r i d r e a c h e d a s l i g h t l y h i g h e r peak t h a n t h a t on t h e c o n t r o l and d e c l i n e d p r o p o r t i o n a l l y . D u r i n g t h e s e c o n d f i e l d s e a s o n t h e p o p u l a t i o n o f d e e r m ice on B.82 beh a v e d l i k e t h a t i n t h e f o r m e r y e a r , i n c r e a s i n g s t e a d i l y t h r o u g h June and J u l y t o a peak i n e a r l y A u g u s t and d e c l i n i n g s u b s e q u e n t l y . On t h e o t h e r hand, on A.82 numbers s t a y e d v e r y s t a b l e a t an a v e r a g e o f 14 p e r h e c t a r e t h r o u g h o u t t h e summer ( F i g . 3 . 9 ) . In s p i t e o f t h e h i g h e r e f f e c t i v e n e s s o f t h e meadow v o l e r e m o v a l i n 1982, b o t h p o p u l a t i o n s o f d e e r mice were s l i g h t l y l o w e r i n 1982 t h a n 1981. 8 9 R E P R O D U C T I O N T h e d e m o g r a p h i c m a c h i n e r y m i g h t b e a f f e c t e d t h r o u g h c h a n g e s i n r e p r o d u c t i o n . T a b l e 3 . 5 p r e s e n t s a c o m p a r i s o n o f t h e r e p r o d u c t i v e a t t r i b u t e s b e t w e e n d e e r m i c e o n e x p e r i m e n t a l a n d c o n t r o l g r i d s . T a b l e 3 . 5 . D e e r m o u s e r e p r o d u c t i v e d a t a ( 1 9 8 1 . a n d 1 9 8 2 ) . 1981 1 9 8 2 G R I D ( A . 8 1 ) ( B . 8 1 ) ( C . 8 2 ) ( B . 8 2 ) ( E . 8 2 ) ( A . 8 2 ) N a t u r a l + A r t i f i c i a l N a t u r a l N a t u r a l A r t i f i c i a l C o n t r o l R e m o v a l C o n t r o l R e m o v a l R e m o v a l R e m o v a l J u v e n i l e s f i r s t t r a p p e d J u l 3 J u l 3 J u l 10 J u n 2 9 J u l 8 J u l 8 E n d o f m a l e b r e e d i n g s e a s o n ( . ) J u n 18 J u l 3 J u n 17 J u l 14 J u l 14 E n d o f f e m a l e b r e e d i n g s e a s o n ( . ) A u g 1 A u g 1 J u l 2 9 A u g 11 J u l 14 A u g 11 N u m b e r o f l i t t e r s ( n u m b e r o f f e m a l e s ) 2 ( 1 ) 2 ( 2 ) 1 ( 1 ) 5 ( 5 ) 6 ( 4 ) 5 ( 4 ) (..) N u m b e r o f j u v e n i l e s r e c r u i t e d 20 22 12 15 2 6 11 (...) J u v e n i l e p r o d u c t i v i t y 10 11 12 3 4 . 3 2 . 2 ( . ) l a s t i n d i v i d u a l b r e e d i n g . ( . . ) a d u l t f e m a l e s c a u g h t i n => t w o t r a p p i n g s e s s i o n s . ( . . . ) j u v e n i l e s t a g g e d i n t h e g r i d . 9 0 I n e a r l y M a y w h e n t r a p p i n g b e g a n , a l l o v e r w i n t e r e d i n d i v i d u a l s h a d r e a c h e d s e x u a l m a t u r i t y a n d w e r e i n b r e e d i n g c o n d i t i o n . T h e r e f o r e I u s e d t h e d a t e w h e n j u v e n i l e s w e r e f i r s t c a u g h t t o c o m p a r e t h e o n s e t o f b r e e d i n g s e a s o n o n d i f f e r e n t g r i d s . T h e p o p u l a t i o n s b r e d s y n c h r o n o u s l y , a n d j u v e n i l e s w e r e f i r s t c a u g h t d u r i n g e a r l y J u l y . T o e s t i m a t e t h e e n d o f t h e b r e e d i n g s e a s o n I r e c o r d e d t h e l a s t a d u l t i n d i v i d u a l t h a t s h o w e d s i g n s o f b r e e d i n g c o n d i t i o n . F o r m a l e s t h e t e s t i s i n s c r o t a l p o s i t i o n a n d f o r f e m a l e s s i g n s o f l a c t a t i o n . T h e p o p u l a t i o n s i z e s a r e t o o s m a l l t o u s e f i f t y p e r c e n t o f t h e p o p u l a t i o n b r e e d i n g a s a c u t - o f f a s o t h e r s t u d i e s h a v e d o n e . M o s t a d u l t m a l e s d i s a p p e a r e d f r o m t h e g r i d s d u r i n g m i d - J u n e t o e a r l y J u l y ; t h u s t h e m a l e b r e e d i n g s e a s o n w a s o v e r . T h e r e w a s l i t t l e o v e r l a p b e t w e e n a d u l t m a l e s a n d j u v e n i l e s . B y t h e t i m e j u v e n i l e s b e c a m e t r a p p a b l e a d u l t m a l e s h a d d i s a p p e a r e d . T h e b r e e d i n g s e a s o n o f f e m a l e s e x t e n d e d t o l a t e J u l y a n d e a r l y A u g u s t , w h e n t h e y s t o p p e d l a c t a t i n g . B y f o l l o w i n g t h e b r e e d i n g c o n d i t i o n o f i n d i v i d u a l f e m a l e s , I e s t i m a t e d a m i n i m u m n u m b e r o f l i t t e r s p r o d u c e d . P r e g n a n c i e s w e r e d e t e r m i n e d e i t h e r b y t h e s i z e a n d s h a p e o f t h e f e m a l e s o r b y e v i d e n c e o f l a c t a t i o n . A l l a d u l t f e m a l e s p r e s e n t h a d a t l e a s t o n e l i t t e r a n d s o m e h a d t w o . J u v e n i l e r e c r u i t m e n t , t h e n u m b e r o f j u v e n i l e s t a g g e d f o r t h e f i r s t t i m e o n a g r i d , w a s v e r y s i m i l a r a m o n g g r i d s i n 1981. T h e s e c o n d s u m m e r j u v e n i l e r e c r u i t m e n t w a s l o w e r e v e n 91 w h e n m o r e a d u l t f e m a l e s w e r e p r e s e n t a n d m o r e l i t t e r s w e r e p r o d u c e d . J u v e n i l e p r o d u c t i v i t y , t h e n u m b e r o f j u v e n i l e s r e c r u i t e d p e r p r e g n a n t f e m a l e , w a s s i m i l a r o n b o t h g r i d s d u r i n g t h e f i r s t f i e l d s e a s o n . T h e s e c o n d s e a s o n , p r o d u c t i o n o f j u v e n i l e s w a s l o w o n t h e t h r e e e x p e r i m e n t a l g r i d s a n d h i g h a t t h e c o n t r o l . C o n s i d e r i n g b o t h y e a r s , t h e r e s e e m s t o b e a n e g a t i v e i n t r a s p e c i f i c r e l a t i o n b e t w e e n j u v e n i l e p r o d u c t i v i t y a n d t h e n u m b e r o f b r e e d i n g f e m a l e s ( T a b l e 3.5). I n s u m m a r y , t h e g r i d s w e r e s i m i l a r i n t h e i r r e p r o d u c t i v e a t t r i b u t e s . T h e y w e r e a l m o s t i d e n t i c a l d u r i n g t h e f i r s t s u m m e r . T h e s e c o n d f i e l d s e a s o n t h e r e w a s m o r e v a r i a t i o n b u t n o c o n s i s t e n t t r e n d . O n l y j u v e n i l e p r o d u c t i v i t y s e e m s t o s h o w a c o n s i s t e n t t r e n d . I t w a s l o w o n t h e e x p e r i m e n t a l g r i d s . H o w e v e r , t h e f a c t t h a t t h e s e t h r e e g r i d s h a d t h e h i g h e r n u m b e r o f b r e e d i n g f e m a l e s m a k e s t h e r e s u l t u n c l e a r . S U R V I V A L P o p u l a t i o n s may b e a f f e c t e d t h r o u g h c h a n g e s i n t h e r a t e o f d i s a p p e a r a n c e o f i n d i v i d u a l s . D i s a p p e a r a n c e may b e d u e e i t h e r t o m o r t a l i t y o r e m i g r a t i o n . I n t h e f o l l o w i n g a n a l y s i s t h e s e t w o s o u r c e s a r e i n d i s t i n g u i s h a b l e a n d a r e i n c l u d e d i n t h e s u r v i v a l r a t e . S u r v i v a l r a t e o f a d u l t s w a s d i v i d e d i n t o t w o p e r i o d s , 9 2 s p r i n g (May and June) and summer ( J u l y , A u g u s t and S e p t e m b e r ) , when j u v e n i l e s a r e a l s o p r e s e n t . J u v e n i l e s u r v i v a l was n o t d i v i d e d i n t o p e r i o d s . T a b l e 3.6 shows minimum two-week s u r v i v a l r a t e s a v e r a g e d f o r a d u l t s and j u v e n i l e s . T a b l e 3.6. Deer mouse s u r v i v a l d a t a ( 1 9 8 1 - 1 9 8 2 ) . Minimum two-week s u r v i v a l r a t e s a v e r a g e d o v e r t h e s p r i n g and summer f o r a d u l t s and o v e r t h e summer f o r j u v e n i l e s . S e x e s a r e p o o l e d . Sample s i z e i n p a r e n t h e s i s . 1981 1982 GRID (A.81) (B.81) (C.82) (B.82) (E.82) (A.82) N a t u r a l + A r t i f i c i a l N a t u r a l N a t u r a l A r t i f i c i a l C o n t r o l Removal C o n t r o l Removal Removal Removal ADULT SURVIVAL ( s p r i n g ) 0.66 0.92 0.76 0.7 0.9 (4) (4) (8) (6) (10) (summer) 0.3 0.8 1.0 0.73 0.37 0.76 (1) (4) (2) (4) (3) (5) JUVENILE SURVIVAL (summer) 0.5 0.67 0.66 0.61 0.61 0.36 (21) (24) (19) (17) (29) (17) In 1981, s u r v i v a l r a t e s were h i g h e r i n t h e e x p e r i m e n t a l g r i d , f o r a d u l t s b o t h d u r i n g s p r i n g and summer, and f o r j u v e n i l e s . D u r i n g 1982, s p r i n g a d u l t s u r v i v a l was s l i g h t l y h i g h e r t h a n i n t h e p r e v i o u s y e a r . Summer a d u l t s u r v i v a l was l o w e r on t h e e x p e r i m e n t a l g r i d s t h a n on t h e c o n t r o l , and j u v e n i l e s u r v i v a l was s i m i l a r on a l l g r i d s . 9 3 F i g u r e 3.10 and 3.11 summarize t h e s p a t i a l d i s t r i b u t i o n and abundance d a t a of b o t h s p e c i e s a t two g r i d s d u r i n g 1981 and 1982. Deer mouse d i s t r i b u t i o n and abundance was s i m i l a r i n b o t h p l a c e s b o t h y e a r s r e g a r d l e s s o f t h e a b s e n c e of meadow v o l e s . INTRODUCTION EXPERIMENTS D u r i n g 1983, two f o r e s t g r i d s a d j a c e n t t o t h e meadow h a b i t a t (B.83, E.83) were c o l o n i z e d by meadow v o l e s , p r o v i d i n g an i n t r o d u c t i o n e x p e r i m e n t w i t h b o t h y e a r - t o - y e a r c o n t r o l s as w e l l a s c o n t e m p o r a n e o u s c o n t r o l s . S i m i l a r l y , r e d - b a c k e d v o l e s i n c r e a s e d f r o m b e i n g n e a r l y a b s e n t i n t h e p r e v i o u s y e a r s . I compared t h e d e m o g r a p h i c p a r a m e t e r s i n f o u r " g r i d s , two where r e d - b a c k e d v o l e s and d e e r mice were a b o u t e q u a l l y a bundant (G.83, H83) and two where r e d - b a c k e d v o l e s were v e r y s c a r c e (no more t h a n two i n d i v i d u a l s ) (C.83, D . 8 3 ) . SPATIAL DISTRIBUTION D i d t h e i n c r e a s e i n numbers of meadow v o l e s a n d / o r r e d - b a c k e d v o l e s i n f l u e n c e t h e d i s t r i b u t i o n o f d e e r m i c e ? N e i t h e r meadow v o l e s nor r e d - b a c k e d v o l e s i n f l u e n c e d t h e s p a t i a l d i s t r i b u t i o n o f d e e r m i c e . The d i s t r i b u t i o n o f t h e s e s p e c i e s was i n d e p e n d e n t on a l l g r i d s ( T a b l e 3 . 7 ) . T h e r e were no n e g a t i v e a s s o c i a t i o n s as e x p e c t e d i f i n t e r s p e c i f i c i n t e r a c t i o n s 9 4 F i g u r e 3.10. Summary of s p a t i a l d i s t r i b u t i o n and abundance d u r i n g 1981. B o t h X and Y a x e s a r e s p a t i a l c o o r d i n a t e s o f t h e g r i d s . The Z a x i s r e p r e s e n t s t h e f r e q u e n c y of c a p t u r e s s t a n d a r i z e d by t h e number of t r a p s e s s i o n s . H a b i t a t use by d e e r mice i s s i m i l a r i n b o t h e x p e r i m e n t a l and c o n t r o l g r i d s . CO cn 9 6 F i g u r e 3 . 1 1 . S u m m a r y o f s p a t i a l d i s t r i b u t i o n a n d a b u n d a n c e d u r i n g 1 9 8 2 . X a n d Y a x e s a r e c o o r d i n a t e s o f t h e g r i d s . T h e Z a x i s r e p r e s e n t s t h e f r e q u e n c y o f c a p t u r e s t a n d a r i z e d b y t h e n u m b e r o f t r a p s e s s i o n s . D e e r m i c e u s e d t h e h a b i t a t s i m i l a r l y r e g a r d l e s s o f t h e a b s e n c e o f m e a d o w v o l e s . 9 8 were i n f l u e n c i n g t h e i r d i s t r i b u t i o n . Furthermore, Fager's index of a f f i n i t y , that e l i m i n a t e s c e l l D (both s p e c i e s absent) from the contingency t a b l e s (Fager 1957), showed that voles and deer mice tended to use the same t r a p s t a t i o n s i n 4 out of 5 g r i d s . There were more t r a p s t a t i o n s used j o i n t l y than used by e i t h e r s p e c i e s a l o n e . Table 3.7. Use of t r a p s t a t i o n s by p a i r s of s p e c i e s . In a l l cases, s p e c i e s use t r a p s t a t i o n s independently ( F i s h e r exact p r o b a b i l i t y t e s t ) . However, Fager's index of a f f i n i t y ( Z ) , by e l i m i n a t i n g c e l l d from the contingency t a b l e s , shows that i n 4 out of the 5 g r i d s , t r a p s t a t i o n s are used by both s p e c i e s more o f t e n than by e i t h e r alone. GRID SPECIES FISHER'S Z= FAGER'S INDEX EXACT TEST OF AFFINITY B.83 E.83 Peromyscus x M i c r o t u s Peromyscus x M i c r o t u s 0.53 0.09 0.617 * 0.698 * E.83 G. 83 H. 83 Peromyscus x Clethrionomys Peromyscus x Clethrionomys Peromyscus x Clethrionomys P ; P P 0.57 0.18 0.87 0.484 0.632 * 0.852 * (* s i g n i f i c a n t i f l a r g e r than 0.5 a c c o r d i n g to Fager.) POPULATION DYNAMICS NUMBERS 99 Meadow v o l e s In two f o r e s t g r i d s a d j a c e n t t o t h e meadows (B.83, E.83) v o l e s ( M i c r o t u s p e n n s y l v a n i c u s ) a p p e a r e d d u r i n g mid-May and e a r l y J u ne 1983. They had been c o m p l e t e l y a b s e n t o r i n v e r y low numbers i n p r e v i o u s y e a r s . From v e r y few i n d i v i d u a l s , t h e y i n c r e a s e d t h r o u g h o u t t h e summer t o a h i g h peak i n t h e f a l l , and became more abundant t h a n d e e r m i c e . Deer m i c e i n t u r n , i n c r e a s e d as i n f o r m e r y e a r s t h r o u g h o u t t h e summer, but d i d not d e c l i n e i n t h e f a l l . A t t h e end o f t h e t r a p p i n g p r o g r a m d e e r mice were 1.5 t o 3.5 t i m e s h i g h e r t h a n i n e a r l i e r y e a r s ( F i g . 3.12) R ed-backed v o l e s In a s i m i l a r way, r e d - b a c k e d v o l e s ( C l e t h r i o n o m y s  r u t i l u s ) c o l o n i z e d t h r e e g r i d s (C.83, D.83, G.83), d u r i n g J une and e a r l y J u l y 1983. A t g r i d H.83, t h e y were p r e s e n t from t h e b e g i n n i n g o f t h e t r a p p i n g p r o g r a m . The f i r s t two g r i d s (C.83, D.83) were c o l o n i z e d d u r i n g J u n e , but n e v e r s u p p o r t e d more t h a n two i n d i v i d u a l s . A t h i r d g r i d (G.83) was c o l o n i z e d i n e a r l y J u l y and m a i n t a i n e d a medium t o low d e n s i t y o f 6 i n d i v i d u a l s (12 p e r h e c t a r e ) . The f o u r t h g r i d (H.83) had a r e s i d e n t p o p u l a t i o n of r e d - b a c k e d v o l e s t h a t grew p a r a l l e l t o t h e c o e x i s t i n g d e e r mouse p o p u l a t i o n u n t i l J u l y . Then i t m a i n t a i n e d an a v e r a g e o f 11 i n d i v i d u a l s t h r o u g h t h e summer. Deer mouse numbers on t h e s e f o u r g r i d s were much t h e 100 F i g u r e 3.12. Minimum number a l i v e o f d e e r mice and meadow v o l e s . The c o l o n i z a t i o n of t h e s e two g r i d s by meadow v o l e s had no e f f e c t on d e e r m i c e . In f a c t , d e e r mouse p o p u l a t i o n s were s l i g h t l y h i g h e r d u r i n g 1983 when v o l e s c o l o n i z e d them t h a n i n p r e v i o u s y e a r s when t h e y were a l o n e . Minimum Number Alive 32 28 24 20 16 12 8 4K •-• Peromyscus o o Microtus Clethrionomys E.82 6 % • A \ .•P - g o M J J A 1982 M J J A 1983 24 •-• Peromyscus Q 20 o-o Microtus B.81 B.82 B .83 6 •' 6 • 16 Minimum • 6 Number H O Alive 1 2 • ¥ 1 O - A A A A 8 4 • / A / V \ •:\u\/ o-oooo M J J A s M J J A S M J J A S 1981 1982 1983 1 02 same r e g a r d l e s s o f how many r e d - b a c k e d v o l e s were p r e s e n t ( F i g . 3 . 1 3 ) . REPRODUCTION I f t h e r e i s a n e g a t i v e i n t e r a c t i o n between t h e s e s p e c i e s , r e p r o d u c t i o n i n d e e r mice m i g h t be a f f e c t e d by t h e p r e s e n c e o f meadow v o l e s a n d / o r r e d - b a c k e d v o l e s i n d i f f e r e n t ways. Deer mice m i g h t show a r e d u c t i o n i n l e n g t h o f b r e e d i n g s e a s o n , number of b r e e d i n g f e m a l e s , number o f l i t t e r s , l i t t e r s i z e o r j u v e n i l e p r o d u c t i v i t y . T a b l e 3.8 compares t h e r e p r o d u c t i v e a t t r i b u t e s m e asured between c o n t r o l and e x p e r i m e n t a l g r i d s . The b e g i n n i n g o f t h e b r e e d i n g s e a s o n was not documented, s i n c e b o t h m a l e s and f e m a l e s were a l r e a d y b r e e d i n g i n e a r l y May when t r a p p i n g s t a r t e d . The f i r s t j u v e n i l e s were t r a p p e d f r o m mid-June t o l a t e J u l y . Two o f t h e g r i d s where r e d -b a c k e d v o l e s were p r e s e n t had l a t e r e c r u i t m e n t o f j u v e n i l e s . M a l e s ended b r e e d i n g s y n c h r o n o u s l y on most g r i d s . F e m a l e s , on t h e o t h e r hand, were more v a r i a b l e . The end o f l a c t a t i o n r a n g e d from m i d - J u n e t o m i d - A u g u s t , w i t h no c o n s i s t e n t t r e n d . The number o f b r e e d i n g f e m a l e s , number o f l i t t e r s and j u v e n i l e s r e c r u i t e d were h i g h e r where v o l e s were p r e s e n t . J u v e n i l e p r o d u c t i v i t y , however, was more v a r i a b l e . The h i g h e s t p r o d u c t i v i t y was o b t a i n e d a t E.83, where b o t h s p e c i e s of v o l e s were p r e s e n t . T h i s g r i d , however, was a l s o b e i n g u s e d f o r a 1 03 F i g u r e 3.13. Minimum number a l i v e of deer mice and red-backed v o l e s . The p o p u l a t i o n s of deer mice are very s i m i l a r r e g a r d l e s s of the number of red-backed v o l e s p r e s e n t . In f a c t , g r i d H.83 supported the h i g h e s t numbers of both red-backed v o l e s and deer mice. Minimum Number Alive 24 20 16 12 8 4 D . 8 3 / / O , 0---0- ... o M 24 Minimum 20| Number Alive l 6 12 • - • P e r o m y s c u s O - O C l e t h r i o n o m y s s G . 8 3 8 •o-o C . 8 3 / *' • •• ...o-.^ .-oo 8oo- o M H . 8 3 / Q lf°' o* • 0 0 M J J A S M J J A S 105 T a b l e 3.8. Deer mouse r e p r o d u c t i v e d a t a ( 1 9 8 3 ) . 1983 (C.83) (D.83) (B.83) (E.83) (G.83) (H.83) C o n t r o l s I n t r o d u c t i o n s M i c r o t u s C l e t h r i o n o m y s J u v e n i l e s f i r s t t r a p p e d Jun 1 9 J u l 2 J u n 30 Jun 23 J u l 28 J u l 9 End of male b r e e d i n g s e a s o n (.) J u l 19 J u l 21 J u l 19 Jun 23 J u l 19 J u l 19 End of f e m a l e b r e e d i n g s e a s o n (.) Jun 19 Aug 15 J u n 23 Jun 23 Aug 13 J u l 9 Number of l i t t e r s (number o f f e m a l e s ) 2 ( l ) 2 (1) 3 (2) 3 (2) 6 (3) 3 (2) (..) Number o f j u v e n i l e s r e c r u i t e d 13 18 23 34 22 18 (...) J u v e n i l e p r o d u c t i v i t y 6.5 9.0 7.6 11.3 3.6 6.0 (.) l a s t i n d i v i d u a l b r e e d i n g . (..) a d u l t f e m a l e s c a u g h t i n => two t r a p p i n g s e s s i o n s . (...) j u v e n i l e s t a g g e d on t h e g r i d . d e e r mouse f e m a l e r e m o v a l e x p e r i m e n t where a d u l t f e m a l e s were removed a f t e r t h e f i r s t l i t t e r had been r e c r u i t e d . 1 06 SURVIVAL The r a t e of disappearance of i n d i v i d u a l s might be a f f e c t e d d i r e c t l y , through b e h a v i o r a l i n t e r a c t i o n s l e a d i n g to emig r a t i o n , or i n d i r e c t l y , through i n c r e a s e d m o r t a l i t y . Table 3.9 shows minimum two-week s u r v i v a l r a t e s averaged f o r a d u l t s and j u v e n i l e s . Table 3.9. Deer mouse s u r v i v a l data (1983). Minimum two-week s u r v i v a l r a t e s averaged over the s p r i n g and summer fo r a d u l t s , and over the summer f o r j u v e n i l e s . Sexes are pooled. Sample s i z e i n p a r e n t h e s i s . 1983 (C.83) (D.83) (B.83) (E.83) (G.83) (H.83) C o n t r o l s I n t r o d u c t i o n s M i c r o t u s Clethrionomys ADULT SURVIVAL (sp r i n g ) (summer) JUVENILE SURVIVAL (summer) 0.66 0.79 0.54 (9) (9) (9) 0.92 0.68 0.8 (1) (7) (1) 0.91 0.74 0.84 (29) (27) (29) 0 .82 0 .74 0 .94 (7) (8) (10) 0 .02* 0 .81 0 .91 (6) (5) (8) 0 .79 0 .63 0 .76 (38) (24) (24) (* low value due to an experimental a d u l t female removal). Adult s u r v i v a l was d i v i d e d i n t o two p e r i o d s , s p r i n g , when only overwintered a d u l t s were present and summer, when j u v e n i l e s had been r e c r u i t e d . Adult s u r v i v a l was very s i m i l a r both during s p r i n g and summer i n a l l the g r i d s r e g a r d l e s s of the 1 07 p r e s e n c e o f v o l e s . Compared w i t h t h a t i n p r e v i o u s y e a r s ( T a b l e s 3.6 and 3 . 9 ) , s p r i n g a d u l t s u r v i v a l was s i m i l a r whereas summer a d u l t s u r v i v a l was h i g h e r i n t h e y e a r s when v o l e s were p r e s e n t . J u v e n i l e s u r v i v a l , i n t u r n , was s l i g h t l y l o w e r a t t h e g r i d s where r e d - b a c k e d v o l e s were p r e s e n t , but h i g h e r t h a n i n f o r m e r y e a r s when d e e r mice were a l o n e . F i g u r e s 3.14 and 3.15 summarize t h e s p a t i a l d i s t r i b u t i o n and r e l a t i v e abundance of d e e r m i c e i n r e l a t i o n t o th e c o l o n i z a t i o n by meadow v o l e s . Deer mouse d i s t r i b u t i o n and abundance was not a f f e c t e d by t h e c o l o n i z a t i o n o f meadow v o l e s . 3.5 DISCUSSION P o p u l a t i o n s o f d e e r mice were n o t a f f e c t e d by t h e r e m o v a l o f meadow v o l e s f r o m a d j a c e n t h a b i t a t . N e i t h e r t h e a r t i f i c i a l r e m o v a l o f M i c r o t u s d u r i n g 1981, nor t h e i r n a t u r a l d e c l i n e i n numbers i n 1982, i n f l u e n c e d d e e r mouse s p a t i a l d i s t r i b u t i o n or demography. F u r t h e r m o r e , n e i t h e r t h e c o l o n i z a t i o n o f f o r e s t h a b i t a t by meadow v o l e s n o r t h e i n c r e a s e i n numbers o f r e d - b a c k e d v o l e s d u r i n g 1983 had n e g a t i v e c o n s e q u e n c e s on t h e s p a t i a l d i s t r i b u t i o n or p o p u l a t i o n a t t r i b u t e s o f d e e r m i c e . In c o n t r a s t , p o p u l a t i o n s of d e e r m i c e seem t o p e r f o r m b e t t e r when a n o t h e r s p e c i e s i s p r e s e n t . D u r i n g 1982 when t h e d e c l i n e o f meadow v o l e s o c c u r r e d , d e e r mouse numbers were 108 F i g u r e 3.14. Summary o f s p a t i a l d i s t r i b u t i o n and abundance of d e e r m i c e and meadow v o l e s . X and Y a x e s r e p r e s e n t t h e c o o r d i n a t e s of g r i d B.83. Z a x i s r e p r e s e n t t h e f r e q u e n c y of c a p t u r e s s t a n d a r i z e d by number o f t r a p p i n g s e s s i o n s . Deer m i c e were n o t a f f e c t e d by t h e c o l o n i z a t i o n o f meadow v o l e s . 1983 O CO 110 F i g u r e 3.15. Summary o f s p a t i a l d i s t r i b u t i o n and abundance o f d e e r m i c e and meadow v o l e s . B o t h X and Y a x e s i n d i c a t e t h e c o o r d i n a t e s o f g r i d E.83. The Z a x i s i n d i c a t e s t h e f r e q u e n c y o f c a p t u r e s s t a n d a r i z e d by t h e t r a p p i n g i n t e n s i t y . Deer mice were not a f f e c t e d by t h e c o l o n i z a t i o n o f meadow v o l e s . I l l 1 1 2 s l i g h t l y l o w e r . C o n v e r s e l y , i n b o t h f o r e s t g r i d s c o l o n i z e d by meadow v o l e s d u r i n g 1983, d e e r mice r e a c h e d h i g h e r numbers t h a n i n t h e p r e v i o u s y e a r . S i m i l a r l y , t h e g r i d t h a t h e l d t h e h i g h e s t d e n s i t y o f r e d - b a c k e d v o l e s , s u p p o r t e d a l s o t h e h i g h e s t d e n s i t y o f d e e r m i c e ( T a b l e 3 .10). T h i s s u g g e s t s t h a t t h e s e s p e c i e s m i g h t be a f f e c t e d by a common e n v i r o n m e n t a l f a c t o r . The r e s u l t s o f t h i s s t u d y a r e i n agreement w i t h t h e s u g g e s t i o n f r o m o b s e r v a t i o n a l s t u d i e s t h a t c o m p e t i t i v e i n t e r a c t i o n s between P e r o m y s c u s , C l e t h r i o n o m y s and M i c r o t u s have l i t t l e i n f l u e n c e i n t h e i r h a b i t a t use and r e l a t i v e abundance ( M ' C l o s k e y 1975, 1975b, M ' C l o s k e y and F i e l d w i c k 1975, M o r r i s 1983, K i n g 1983). On t h e o t h e r hand, t h e r e s u l t s c o n t r a s t w i t h e a r l y f i n d i n g s i n e n c l o s u r e e x p e r i m e n t s ( G r a n t 1969, 1970, 1971, 1972, M o r r i s 1969, M o r r i s and G r a n t 1972) and r e m o v a l e x p e r i m e n t s ( R e d f i e l d ejb a l . 1977, Abramsky e_t a l . 1979). T h r e e p o s s i b i l i t i e s may e x p l a i n t h e d i s a g r e e m e n t between t h e s e r e s u l t s and o t h e r f i n d i n g s : F i r s t , c o m p e t i t i v e i n t e r a c t i o n s among t h e s e t h r e e g e n e r a may be c o n t i n g e n t on a number of d i f f e r e n t f a c t o r s t h a t change l o c a l l y . S e c ond, e a r l i e r s t u d i e s may have i n t e r p r e t e d t h e i r f i n d i n g s as c o m p e t i t i o n when some o t h e r p r o c e s s e s were r e s p o n s i b l e f o r t h e r e s u l t s . T h i r d , c o m p e t i t i v e i n t e r a c t i o n s may a f f e c t t h e p o p u l a t i o n s o f s m a l l mammals i n my s t u d y a r e a , b u t t h e m a n i p u l a t i o n s and n a t u r a l e x p e r i m e n t s were u n a b l e t o d e t e c t them. I w i l l d i s c u s s t h e s e t h r e e p o s s i b i l i t i e s i n t u r n . 1 13 Table 3.10. Summary of the e f f e c t s of removal of meadow v o l e s and i n t r o d u c t i o n of meadow and red-backed v o l e s on p o p u l a t i o n parameters of deer mice. REMOVALS B.81 B.82 A.82 E.82 N a t u r a l + A r t i f i c i a l N a t u r a l A r t i f i c i a l N a t u r a l Removal Removal Removal Removal H a b i t a t use Po p u l a t i o n d e n s i t y Reproduct ion J u v e n i l e p r o d u c t i v i t y S u r v i v a l Adult ( s p r i n g ) (summer) J u v e n i l e (summer) + + o o o + + o o o o o o o + + o INTRODUCTIONS B.83 E.83 Meadow v o l e s G.83 H.83 Red-backed v o l e s H a b i t a t use Po p u l a t i o n d e n s i t y Reproduction J u v e n i l e p r o d u c t i v i t y S u r v i v a l Adult ( s p r i n g ) (summer) J u v e n i l e (summer) o o o o o o o + + o o + higher than c o n t r o l o s i m i l a r to c o n t r o l - lower than c o n t r o l F i r s t , i n t e r a c t i o n s among these genera might be con t i n g e n t on a number of l o c a l f a c t o r s . The degree of s i m i l a r i t y among resources u t i l i z e d (food, h a b i t a t , nest s i t e s ) , 1 1 4 p o p u l a t i o n d e n s i t i e s i n r e l a t i o n t o r e s o u r c e a v a i l a b i l i t y , t h e i r s p e c i e s - s p e c i f i c a g g r e s s i v e b e h a v i o r ( l e v e l o f t o l e r a n c e a n d s p e c i f i c i t y ) may a l l i n f l u e n c e t h e r e l a t i v e i m p o r t a n c e o f c o m p e t i t i o n . A r e t h e s p e c i e s i n t h i s c o m m u n i t y u t i l i z i n g s i m i l a r r e s o u r c e s ? I n r e l a t i o n t o f o o d r e s o u r c e s , t h e r e i s n o . s t r o n g b a s i s t o c l a s s i f y t h e s e g e n e r a a s p o t e n t i a l c o m p e t i t o r s , s i n c e t h e i r f o o d h a b i t s a r e v e r y d i s t i n c t . D e e r m i c e f e e d m a i n l y o n a r t h r o p o d s a n d s e e d s ( M a r t e l l a n d M a c a u l a y 1 9 8 1 ) , r e d - b a c k e d v o l e s f e e d o n l i c h e n s , f u n g i a n d s e e d s ( M a r t e l l 1 9 8 1 ) a n d m e a d o w v o l e s f e e d o n g r e e n p a r t s o f g r a s s e s a n d s e d g e s ( B a n f i e l d 1 9 7 4 ) . I f we c o n s i d e r t h e p o s s i b i l i t y t h a t t h e i r d i e t v a r i e s l o c a l l y a n d t h a t f o o d o v e r l a p c o u l d b e g r e a t e r i n t h e K l u a n e s t u d y a r e a , t h e n t h e c o l o n i z a t i o n o f f o r e s t h a b i t a t b y m e a d o w v o l e s o r t h e d e n s i t y i n c r e a s e b y r e d - b a c k e d v o l e s s h o u l d a f f e c t t h e d e m o g r a p h y o f d e e r m i c e , s i n c e p o p u l a t i o n s o f t h e l a s t s p e c i e s i n t h e a r e a a r e l i m i t e d b y f o o d s u p p l y ( G i l b e r t a n d K r e b s 1 9 8 1 ) . N e v e r t h e l e s s , a s d e c r i b e d a b o v e , P e r o m y s c u s p o p u l a t i o n s w e r e u n a f f e c t e d o r e l s e c h a n g e d i n t h e o p p o s i t e d i r e c t i o n t o t h a t p r e d i c t e d b y t h e c o m p e t i t i o n h y p o t h e s i s . A n o t h e r p o t e n t i a l r e s o u r c e t h a t c o u l d b e i n v o l v e d i s n e s t s i t e s . H o w e v e r , l i t t l e i s k n o w n a b o u t n e s t s i t e r e q u i r e m e n t s a n d s p e c i f i c i t y b y t h e s e g e n e r a o r t h e a v a i l a b i l i t y o f t h i s r e s o u r c e . M o s t a u t h o r s who c l a i m t h a t c o m p e t i t i v e i n t e r a c t i o n s 1 1 5 between t h e s e g e n e r a a r e i m p o r t a n t , e m p h a s i z e c o m p e t i t i o n f o r s p a c e m e d i a t e d t h r o u g h a g o n i s t i c i n t e r a c t i o n s ( G r a n t 1972, 1978, Hawes 1976, T u r n e r e t a l . 1975, I v e r s o n and T u r n e r 1972). In t h i s s t u d y however, b o t h c o l o n i z a t i o n o f f o r e s t by M i c r o t u s and p o p u l a t i o n i n c r e a s e by C l e t h r i o n o m y s t o o k p l a c e d u r i n g t h e b r e e d i n g s e a s o n , when t h e a g g r e s s i v e b e h a v i o r o f t h e s e s p e c i e s w ould have had t h e s t r o n g e s t e f f e c t . N e v e r t h e l e s s , t h e s p a t i a l d i s t r i b u t i o n o f d e e r mice d i d n o t change i n t h e p r e s e n c e of c o e x i s t i n g s p e c i e s . S t u d i e s c o m p a r i n g i s l a n d and m a i n l a n d s i t u a t i o n s have documented d i f f e r e n c e s i n t h e f r e q u e n c y of a g g r e s s i v e b e h a v i o r among p o p u l a t i o n s o f t h e s e t h r e e g e n e r a ( H a l p i n 1981). I f i n t e r s p e c i f i c a g o n i s t i c i n t e r a c t i o n s a r e a p r o x i m a t e mechanism i n f l u e n c i n g t h e d i s t r i b u t i o n o f t h e s e g e n e r a , t h e n s u c h l o c a l v a r i a t i o n s i n b e h a v i o r would a f f e c t t h e i n t e n s i t y o f i n t e r a c t i o n s among them. D o u g l a s s (1976) s u g g e s t e d t h a t f a c t o r s i n f l u e n c i n g h a b i t a t use and o v e r l a p between meadow v o l e s and montane v o l e s may change f r o m p l a c e t o p l a c e . In a r e a s where t h e h a b i t a t s o f t h e s e v o l e s a r e s i m i l a r , b e h a v i o r a l i n t e r a c t i o n s would be l i k e l y t o o p e r a t e , whereas i n a r e a s where t h e h a b i t a t s a r e d i s s i m i l a r , t h e i r d i s t r i b u t i o n would r e f l e c t h a b i t a t p r e f e r e n c e s . I f we i n c o r p o r a t e t h e f a c t t h a t some p o p u l a t i o n s of v o l e s show m u l t i a n n u a l f l u c t u a t i o n s , t h e n i n f l u e n c i n g f a c t o r s would change from t i m e t o t i m e t o o . D u r i n g y e a r s o f low abundance, p o p u l a t i o n s would be r e s t r i c t e d t o o p t i m a l h a b i t a t s and a t peak y e a r s a v a r i e t y of h a b i t a t s would be o c c u p i e d , making i n t e r s p e c i f i c i n t e r a c t i o n s more l i k e l y t o o c c u r . The s t r o n g 1 16 n e g a t i v e i n t e r a c t i o n between Townsend's v o l e s and d e e r mice documented by R e d f i e l d e t a l . ( 1 9 7 7 ) , may be one e x t r e m e o f t h i s d e n s i t y g r a d i e n t , s i n c e t h e s e v o l e s s u s t a i n t h e h i g h e s t a v e r a g e d e n s i t i e s o f any M i c r o t u s i n N o r t h A m e r i c a ( T a i t t and K r e b s 1984). The s e c o n d e x p l a n a t i o n f o r t h e i n c o n s i s t e n c y o f f i n d i n g s i s t h a t o t h e r a u t h o r s m ight have i n t e r p r e t e d t h e i r r e s u l t s i n c o r r e c t l y . E a r l i e r s t u d i e s t h a t p r o v i d e d s u p p o r t f o r t h e i m p o r t a n c e of c o m p e t i t i o n , not o n l y among t h e s e r o d e n t s but i n community e c o l o g y i n g e n e r a l ( G r a n t 1969, 1970, 1971, M o r r i s and G r a n t 1972), e x h i b i t a number of d r a w b a c k s . They were c a r r i e d o u t i n e n c l o s u r e s t h a t were s m a l l i n r e l a t i o n t o t h e r o d e n t s ' home r a n g e s . The movements of t h e i n d i v i d u a l s i n t r o d u c e d were r e s t r i c t e d . The a r e a s were t r a p p e d o u t , and a n i m a l s f r o m e l s e w h e r e were i n t r o d u c e d a f t e r s p e n d i n g some t i m e i n c a p t i v i t y . Once i n s i d e t h e e n c l o s u r e t h e y e n c o u n t e r e d a n o v e l e n v i r o n m e n t and f o r e i g n c o n s p e c i f i e s . In one c a s e , t h e s p e c i e s ( r e d - b a c k e d v o l e ) i n t r o d u c e d was n o t f o u n d i n t h e s t u d y s i t e p r e v i o u s t o i t s i n t r o d u c t i o n . T h i s may i n d i c a t e t h a t t h e h a b i t a t was n o t s u i t a b l e t o b e g i n w i t h . S i n c e t h e r e s p o n s e s d ocumented a r e t h e r e s u l t o f t h e b e h a v i o r o f t h e a n i m a l s , t h e c o n c l u s i o n s may not r e p r e s e n t t h e n a t u r a l s i t u a t i o n f a i t h f u l l y . F u r t h e r m o r e , one of t h e e x p e r i m e n t s had no c o n t e m p o r a r y c o n t r o l ( M o r r i s and G r a n t 1972) and none were r e p l i c a t e d . In s p i t e o f a l l t h e s e d r a w b a c k s , t h e c o n c l u s i o n s from t h e s e s t u d i e s have n e v e r been q u e s t i o n e d and a r e p r o b a b l y t h e most w i d e l y q u o t e d e v i d e n c e f o r c o m p e t i t i o n among r o d e n t s . 1 17 I n r e l a t i o n t o r e m o v a l e x p e r i m e n t s c a r r i e d o u t i n m o r e n a t u r a l s i t u a t i o n s , b o t h s t u d i e s t h a t s h o w a c l e a r n e g a t i v e r e l a t i o n s h i p b e t w e e n M i c r o t u s a n d P e r o m y s c u s ( R e d f i e l d e t a l . 1 9 7 7 , A b r a m s k y e t a l . 1 9 7 9 ) u n d e r l i n e d t h e i r u n c e r t a i n t y a b o u t t h e m e c h a n i s m o f t h e i n t e r a c t i o n . T h e l a s t p o s s i b i l i t y i s t h a t c o m p e t i t i v e i n t e r a c t i o n s a r e i m p o r t a n t i n t h e s t u d y a r e a , b u t t h e s e e x p e r i m e n t s w e r e u n a b l e t o d e t e c t t h e m . I f t h e r e l a t i v e i m p o r t a n c e o f c o m p e t i t i o n c h a n g e s w i t h d e n s i t y , t h e n t h e d e n s i t i e s I f o u n d i n t h e a r e a d u r i n g t h e l a s t t h r e e y e a r s may n o t h a v e b e e n h i g h e n o u g h t o c a u s e c o m p e t i t i o n . D e e r m o u s e p o p u l a t i o n s i n t h e a r e a c h a n g e l i t t l e f r o m y e a r t o y e a r a n d a r e l i m i t e d b y f o o d ( G i l b e r t a n d K r e b s 1 9 8 1 ) ; a n d e v e n w h e n d e n s i t i e s a r e l o w m o s t a v a i l a b l e h a b i t a t i s u s e d u p b e c a u s e h o m e r a n g e s a r e l a r g e . T h e r e i s e n o u g h i n t r a s p e c i f i c p r e s s u r e f o r j u v e n i l e s t o l e a v e t h e b i r t h p l a c e a n d c o l o n i z e t h e a d j a c e n t v a c a n t h a b i t a t . B u t i n s t e a d , j u v e n i l e s d i s p e r s e l o n g d i s t a n c e s t o o t h e r s h r u b b y a n d f o r e s t e d h a b i t a t s w i t h o u t m a k i n g u s e o f t h e c l o s e r m e a d o w h a b i t a t . I t h i n k t h i s i s b e c a u s e t h e s e t w o h a b i t a t s , f o r e s t a n d m e a d o w , d o n o t m e r g e i n t o e a c h o t h e r , b u t t h e e c o t o n e i s s h a r p l y d i s c o n t i n u o u s . D e e r m i c e b o r n i n t h e f o r e s t w o u l d h a v e a s t r o n g g e n e t i c p r e f e r e n c e f o r t h i s h a b i t a t , r e i n f o r c e d b y e a r l y e x p e r i e n c e ( W e c k e r 1 9 6 3 ) . I n c o n t r a s t , m e a d o w v o l e s a n d r e d - b a c k e d v o l e s a r e 118 m u c h m o r e v a r i a b l e i n t h e i r n u m b e r s . T h i s f a c t , r e s t r i c t s t h e c o n c l u s i o n s t o t h e d e n s i t i e s a t w h i c h I f o u n d t h e m . I f t h e y r e a c h h i g h e r p o p u l a t i o n n u m b e r s t h a n t h o s e a t t a i n e d i n t h i s s t u d y , t h e n c o m p e t i t i v e i n t e r a c t i o n s m a y h a v e a s t r o n g e r i n f l u e n c e . H o w e v e r , p r e v i o u s s t u d i e s i n t h e a r e a h a v e f o u n d d e n s i t i e s s i m i l a r t o t h o s e r e c o r d e d i n t h i s s t u d y ( K r e b s a n d W i n g a t e 1 9 7 6 , G i l b e r t a n d K r e b s 1 9 8 1 ) . I n s p i t e o f t h e n u m b e r o f s t u d i e s o n h a b i t a t u s e a n d c o m p e t i t i o n a m o n g r o d e n t s , s e v e r a l a r e a s r e m a i n o b s c u r e . U n d e r w h i c h c i r c u m s t a n c e s d o i n t e r s p e c i f i c i n t e r a c t i o n s h a v e a n i n f l u e n c e o n p o p u l a t i o n s o f t h e s e s m a l l m a m m a l s ? W h a t i s t h e m a g n i t u d e o f s u c h i n f l u e n c e i n r e l a t i o n t o t h e a c t i o n o f o t h e r f a c t o r s ? W h e n i n t e r s p e c i f i c c o m p e t i t i o n d o e s o c c u r w h a t a r e t h e m e c h a n i s m s o f i n t e r a c t i o n ? I f we a r e t o u n d e r s t a n d t h e d y n a m i c s o f s m a l l m a m m a l c o m m u n i t i e s , t h e e a r l y g e n e r a l i z a t i o n t h a t c o m p e t i t i o n f o r s p a c e a m o n g d i f f e r e n t s p e c i e s o f r o d e n t s i s a w i d e s p r e a d p h e n o m e n o n ( G r a n t 1 9 7 2 , 1 9 7 8 ) o u g h t t o b e p l a c e d a m o n g a n u m b e r o f w o r k i n g h y p o t h e s e s . 119 S E C T I O N 4 . C O M P E T I T I O N T H E O R Y A N D T H E S T R U C T U R E OF S C I E N T I F I C R E V O L U T I O N S . 4.1 I N T R O D U C T I O N . D i f f e r e n t e c o l o g i c a l p r o c e s s e s i n n a t u r e c a n r e s u l t i n s i m i l a r a n d - o f t e n i n d i s t i n g u i s h a b l e t e m p o r a l a n d s p a t i a l p a t t e r n s . L i k e o t h e r p h e n o m e n a w i t h m a n y p o s s i b l e e x p l a n a t i o n s , t h i s b e c o m e s a p r i m a r y s o u r c e o f c o n t r o v e r s y f o r b i o l o g i s t s . A s a n e x a m p l e , c o n s i d e r a p o p u l a t i o n i n c r e a s i n g e x p o n e n t i a l l y . T h e r a p i d i n c r e a s e i n n u m b e r s c o u l d r e s u l t f r o m r e d u c t i o n i n t h e a b u n d a n c e o f p r e d a t o r s , c o m p e t i t o r s , p a r a s i t e s , d i s e a s e s , o r f r o m a n i n c r e a s e i n f o o d s u p p l y . S i m i l a r l y , a s p e c i e s may b e r e s t r i c t e d t o a p a r t i c u l a r h a b i t a t a s a r e s u l t o f p h y s i o l o g i c a l t o l e r a n c e s , b e h a v i o r a l s e l e c t i o n o f t h e h a b i t a t , i n t e r a c t i o n s w i t h p r e y , p r e d a t o r s , c o m p e t i t o r s , a n d p a r a s i t e s , o r o f h i s t o r i c a l f a c t o r s . I n b o t h e x a m p l e s t h e d i f f e r e n t f a c t o r s a r e n o t m u t u a l l y e x c l u s i v e , b u t may i n t e r a c t i n m a n y c o m p l e x w a y s . A t t h e c o m m u n i t y l e v e l , t h e d i s t r i b u t i o n , r e l a t i v e a b u n d a n c e , a n d r e s o u r c e u s e b y t h e v a r i o u s s p e c i e s c o n s t i t u t e t h e p a t t e r n s o f c o m m u n i t y s t r u c t u r e . F o r a l o n g t i m e e c o l o g i s t s h a v e b e e n i n t e r e s t e d i n t h e p r o c e s s e s r e s p o n s i b l e f o r s u c h s t r u c t u r e . Why a r e s o m e s p e c i e s m o r e a b u n d a n t t h a n o t h e r s ? Why d o s o m e s p e c i e s l i v e i n s o m e p l a c e s a n d n o t i n o t h e r s ? W h a t d e t e r m i n e s t h e i r f o o d s e l e c t i o n ? H o w e v e r , i n t h e i r s t u d y o f 1 20 c o m m u n i t i e s , e c o l o g i s t s have r e p r e s e n t e d two d i f f e r e n t and somewhat ext r e m e a p p r o a c h e s . Some h o l d t h e view t h a t b i o l o g i c a l c o m m u n i t i e s a r e a s s e m b l a g e s of i n d e p e n d e n t s p e c i e s w i t h few b i o t i c i n t e r a c t i o n s , r e s p o n d i n g d i f f e r e n t l y t o s t o c h a s t i c e v e n t s d e p e n d i n g on t h e i r a b i o t i c t o l e r a n c e s ( A n d r e w a r t h a and B i r c h 1954, Lawton and S t r o n g 1981). O t h e r s h o l d t h e v i e w t h a t b i o t i c i n t e r a c t i o n s s u c h as c o m p e t i t i o n and p r e d a t i o n p r o d u c e t h e o r g a n i z a t i o n o f c o m m u n i t i e s ( H u t c h i n s o n 1958, M a c A r t h u r 1972, C o n n e l l 1975, Diamond 1978, P i a n k a 1983). T h e s e two d i f f e r e n t v i e w s have n o t been e q u a l l y f a v o r e d . The b i o t i c s c h o o l has d o m i n a t e d most o f t h e t h e o r y o f community s t r u c t u r e . In f a c t , c o m p e t i t i o n has been i d e n t i f i e d as t h e backbone o f e c o l o g i c a l t h e o r y ( P i a n k a 1983), a c h i e v i n g t h e s t a t u s o f a p a r a d i g m (Kuhn 1970, S t r o n g 1980, S i m b e r l o f f 1980, 1982, Wiens 1983). The o b j e c t i v e of t h i s c h a p t e r i s t w o f o l d . F i r s t , I w i l l examine t h e h i s t o r i c a l d e v e l o p m e n t o f t h e i d e a t h a t i n t e r s p e c i f i c c o m p e t i t i o n p l a y s a major r o l e i n f l u e n c i n g community c h a r a c t e r i s t i c s . S e c o n d , I w i l l examine t h e g e n e r a l f e a t u r e s o f and a s s u m p t i o n s b e h i n d t h e e v i d e n c e o f f e r e d i n s u p p o r t o f t h i s i d e a . 121 4.2 THE STRUCTURE OF S C I E N T I F I C REVOLUTIONS. When i s t h e r e enough e v i d e n c e t o c o n f i r m a h y p o t h e s i s and use t h e knowledge a c q u i r e d a s a s o l i d f o u n d a t i o n f o r f u r t h e r i n q u i r y ? T h i s c e n t r a l q u e s t i o n i n t h e d e v e l o p m e n t o f s c i e n t i f i c t h o u g h t has been answered by p h i l o s o p h e r s i n d i f f e r e n t ways. C o n f i r m a t i o n ( p r o o f ) and r e j e c t i o n ( d i s p r o o f ) o f h y p o t h e s e s have s t o o d as f a v o r i t e c r i t e r i a f o r a l o n g t i m e . However, s i n c e n e i t h e r c a n e v e r be c o n c l u s i v e (Hempel 1966, Popper 1968), o t h e r c r i t e r i a a r e o f t e n u s e d t o d e c i d e t h e s t r e n g t h o f s u p p o r t o r t h e v a l i d i t y o f an e x p l a n a t i o n . Hempel (1966) summarized them as a b s e n c e of u n f a v o r a b l e e v i d e n c e , v a r i e t y of s u p p o r t by d i f f e r e n t t e s t s , e v i d e n c e f r o m s o u r c e s n o t p r e v i o u s l y c o n s i d e r e d , t h e o r e t i c a l s u p p o r t and r e l a t i v e s i m p l i c i t y . However, what t a k e s p l a c e i n s i d e t h e s c i e n t i f i c community i s more co m p l e x . The c r i t e r i a of a c c e p t a b i l i t y change as more i n f o r m a t i o n becomes a v a i l a b l e and t h e d i s c i p l i n e s m a t u r e . F u r t h e r m o r e , t h e s o c i a l s t r u c t u r e o f s c i e n t i f i c e n d e a v o u r c o n t r i b u t e s s t r o n g l y t o s e t a c c e p t e d s t a n d a r d s and f a c i l i t a t e o r d e l a y c h a n g e s o f c r i t e r i a . T h i s s i d e of t h e s c i e n t i f i c v e n t u r e was r a i s e d by Thomas Kuhn i n h i s book "The  S t r u c t u r e o f Sc i e n t i f i c R e v o l u t i o n s " ( 1 9 7 0 ) , where he s i m p l i f i e d some of t h e s e s o c i a l a s p e c t s a s t h e framework o f a s e q u e n t i a l d e v e l o p m e n t of s c i e n c e . I w i l l use h i s framework t o i n t e r p r e t t h e h i s t o r i c a l d e v e l o p m e n t o f t h e main i d e a s i n v o l v e d i n t h e 1 22 c o n t r o v e r s y a b o u t t h e r o l e o f c o m p e t i t i o n i n n a t u r a l c o m m u n i t i e s . T h e d e v e l o p m e n t o f s c i e n c e i s s i m p l i f i e d b y K u h n i n f o u r s e q u e n t i a l s t a g e s : P a r a d i g m f o r m a t i o n , N o r m a l s c i e n c e , P r e -p a r a d i g m c r i s i s a n d R e v o l u t i o n a r y s c i e n c e . 4 . 3 P A R A D I G M F O R M A T I O N . T h e f i r s t s t a g e i n t h e d e v e l o p m e n t o f s c i e n c e i s t h e f o r m a t i o n o f a p a r a d i g m . A p a r a d i g m i s a ' u n i v e r s a l l y ' r e c o g n i z e d s c i e n t i f i c a c h i e v e m e n t t h a t p r o v i d e s m o d e l p r o b l e m s a n d s o l u t i o n s t o t h e c o m m u n i t y o f p r a c t i t i o n e r s . B e c a u s e o f i t s o r i g i n a l i t y , t h e p a r a d i g m a t t r a c t s a g r o u p o f a d h e r e n t s t h a t a c c e p t i t a s a m o d e l o r p a t t e r n a n d e n g a g e t o r e s o l v e i t s m a n y o p e n e n d s ( K u h n 1 9 7 0 ) . A p a r a d i g m g a i n s s t a t u s , K u h n s a y s , b e c a u s e i t i s r e l a t i v e l y m o r e ' s u c c e s s f u l ' t h a n a l t e r n a t i v e m o d e l s i n s o l v i n g p r o b l e m s t h e s c i e n t i f i c c o m m u n i t y r e c o g n i z e s a s r e l e v a n t . A l t h o u g h i t may n o t b e c o m p l e t e l y o r n o t a b l y s u c c e s s f u l , n e v e r t h e l e s s , i t a t t r a c t s a g r o u p o f s c i e n t i s t s t o e n g a g e i n w o r k i n g o u t t h e c o n s e q u e n c e s o f t h e p a r a d i g m . T h e p a r a d i g m i s u s e d a s a c r i t e r i o n f o r c h o o s i n g p r o b l e m s . I t s p e c i f i e s w h i c h a r e t h e i m p o r t a n t q u e s t i o n s . I n c o m m u n i t y e c o l o g y t h i s s t a g e d e v e l o p e d m o r e s l o w l y t h a n t h e o t h e r s . T h e d e f i n i t i o n s o f h a b i t a t a n d n i c h e b y J o s e p h G r i n n e l l a n d C h a r l e s E l t o n a t t h e b e g i n n i n g o f t h i s c e n t u r y , a n d 1 23 t h e s u b s e q u e n t m a t h e m a t i c a l t h e o r y of L o t k a and V o l t e r r a ( l o g i s t i c e q u a t i o n ) , s t i m u l a t e d a number of e x p e r i m e n t s i n m i c r o c o s m s t h a t d o m i n a t e d t h e f o l l o w i n g t w e n t y y e a r s (Gause, Crombie, P a r k , Moore, B i r c h ) . But i t was n o t u n t i l 1954 t h a t two o p p o s e d v i e w s were c l e a r l y r e p r e s e n t e d . On t h e one hand, t h e ' e q u i l i b r i u m ' view o f t h e w o r l d h o l d s t h a t most s p e c i e s l i v e a t t h e i r c a r r y i n g c a p a c i t i e s , w i t h r e s o u r c e s b e i n g c o n s t a n t l y l i m i t i n g , and t h e r e f o r e c o m p e t i t i o n a c t i n g a s an e c o l o g i c a l and e v o l u t i o n a r y f o r c e s h a p i n g c o m m u n i t i e s ( L a c k 1954). On t h e o t h e r hand, t h e ' n o n - e q u i l i b r i u m ' o u t l o o k , h o l d s t h a t most s p e c i e s a r e r a r e and a r e kept below t h e l i m i t s e t by t h e i r r e s o u r c e s t h r o u g h t h e a c t i o n o f a b i o t i c f a c t o r s ( A n d r e w a r t h a and B i r c h 1954). The i d e a t h a t i n t e r a c t i o n s between s p e c i e s d e t e r m i n e some components o f community s t r u c t u r e i n e c o l o g i c a l t i m e , and i n f l u e n c e b e h a v i o r a l and m o r p h o l o g i c a l c h a r a c t e r i s t i c s of t h e i n t e r a c t i n g s p e c i e s i n e v o l u t i o n a r y t i m e , c a n be t r a c e d back t o D a r w i n ' s " O r i g i n o f S p e c i e s " ( 1 8 5 9 ) . However, D a v i d L a c k , i n h i s book "The N a t u r a l R e g u l a t i o n o f A n i m a l Numbers" ( 1 9 5 4 ) , c l e a r l y s t a t e d what he c o n s i d e r e d as e v i d e n c e f o r c o m p e t i t i o n . He p r o p o s e d f o u r a r guments a s i n d i r e c t e v i d e n c e t h a t b i r d p o p u l a t i o n s were l i m i t e d by t h e i r f o o d s u p p l y . In t h e t h i r d argument, Lack p o i n t e d o u t two d i f f e r e n t p a t t e r n s a s e v i d e n c e f o r c o m p e t i t i o n . The f i r s t p a t t e r n i s t h e p r o g e n i t o r o f t h e c o n c e p t of r e s o u r c e p a r t i t i o n i n g ( f o o d and h a b i t a t ) . " E i t h e r t h e s p e c i e s c o n c e r n e d l i v e d i n s e p a r a t e ( t h o u g h o f t e n a d j a c e n t ) h a b i t a t s , i n w h i c h c a s e t h e y m ight f e e d on s i m i l a r t y p e s of 124 f o o d , o r , i f t h e y o c c u r r e d t o g e t h e r i n t h e same h a b i t a t , t h e y f e d on d i f f e r e n t t y p e s of f o o d , d i f f e r i n g e i t h e r i n f e e d i n g methods o r t h e shape o r s i z e s of t h e i r b eaks ( L a c k 1954 p. 148)." The s e c o n d p a t t e r n i s t h e p r o g e n i t o r of t h e c o n c e p t o f c h a r a c t e r d i s p l a c e m e n t , i n v o l v i n g c o n v e r g e n c e and d i v e r g e n c e i n b o t h e c o l o g y and m o r p h o l o g y . " F u r t h e r e v i d e n c e t h a t t h e d i f f e r e n t i a t i o n of f e e d i n g s t a t i o n s has been e v o l v e d t h r o u g h c o m p e t i t i o n f o r f o o d i s t h a t , i n an a r e a where one s p e c i e s i s a b s e n t , a n o t h e r o f t e n o c c u p i e s t h e e q u i v a l e n t s t a t i o n ( L ack 1954 p. 1 4 9 ) . " ( c o n v e r g e n c e ) . "The c o n v e r s e c a s e a l s o e x i s t s , of c l o s e l y r e l a t e d s p e c i e s w h i c h n o r m a l l y l i v e i n d i f f e r e n t r e g i o n s and t a k e c l o s e l y s i m i l a r f o o d s , but w h i c h o c c u r t o g e t h e r i n a s m a l l p a r t of t h e i r r a n g e , and h e r e , but o n l y h e r e , d i f f e r m a r k e d l y . ...a d i f f e r e n c e p r e s u m a b l y e v o l v e d t h r o u g h t h e need f o r a v o i d i n g c o m p e t i t i o n f o r f o o d ( L a c k 1954 p. 149-150)..." ( d i v e r g e n c e ) . By a s s e m b l i n g t h e s e a r g u m e n t s , L a c k s t a r t e d a t r a d i t i o n s e t t i n g t h e k i n d of o b s e r v a t i o n s t h a t p r e v a i l e d f o r t h e n e x t t w e n t y y e a r s . The same y e a r as L a c k ' s p u b l i c a t i o n , A n d r e w a r t h a and B i r c h (1954) i n t h e i r book "The D i s t r i b u t i o n and Abundance of  A n i m a l s " s t a t e d a number o f c r i t i c i s m s c o n c e r n i n g L a c k ' s v i e w . T h e s e , as i t w i l l be c l e a r l a t e r , have been r e s u r r e c t e d i n t h e l a s t s e v e n y e a r s i n more e l a b o r a t e d f o r m s . The main c r i t i c i s m s A n d r e w a r t h a a n d B i r c h r a i s e d were a s f o l l o w s : a) The a s s u m p t i o n s b e h i n d t h e l o g i s t i c e q u a t i o n and b e h i n d t h e c o m p e t i t i o n e q u a t i o n s d e r i v e d from t h e f o r m e r a r e 1 25 r a r e l y met and t h e r e f o r e s u c h models a r e u n r e a l i s t i c . b) T h e r e i s an u n c r i t i c a l e x t r a p o l a t i o n f r o m t h e r e s u l t s o b t a i n e d i n m i c r o c o s m e x p e r i m e n t s ( h i g h l y a r t i f i c i a l ) t o t h e p a t t e r n s and p r o c e s s e s o b s e r v e d i n n a t u r e . R e c e n t l y t h i s p o i n t has a g a i n been r a i s e d by C o n n e l l (1975, 1983). c ) The e q u i l i b r i u m o u t l o o k p o r t r a y e d by t h e i d e a t h a t s y m p a t r i c s p e c i e s must e i t h e r o c c u p y d i f f e r e n t h a b i t a t s w i t h i n t h e same a r e a o r e x p l o i t t h e same h a b i t a t i n d i f f e r e n t ways i m p l i e s t h a t a l l s p e c i e s n o r m a l l y l i v e c r o wded up t o t h e l i m i t s o f t h e r e s o u r c e s a v a i l a b l e t o them. T h e r e i s no a l l o w a n c e f o r r a r e s p e c i e s . T h i s i s o f e x t r e m e i m p o r t a n c e , s i n c e i n A n d r e w a r t h a and B i r c h ' s v i e w r a r e n e s s i s a more u s u a l a t t r i b u t e o f s p e c i e s abundance t h a n commonness. A b i o t i c f a c t o r s m a i n t a i n t h e p o p u l a t i o n s w e l l below t h e i r r e s o u r c e l i m i t s . The i d e a t h a t v a r i a b l e e n v i r o n m e n t s p r e v e n t p o p u l a t i o n s f r o m r e a c h i n g e q u i l i b r i u m s t a t e s has been e l a b o r a t e d by Wiens ( 1 9 7 7 ) . d) The h i s t o r i c a l r o l e of c o m p e t i t i o n c a n n o t be e i t h e r p r o v e d o r d i s p r o v e d i n p r i n c i p l e . The i d e a t h a t c o m p e t i t i o n has a l r e a d y s h a p e d t h e community s t r u c t u r e and t h a t s h o r t - t e r m m a n i p u l a t i o n s c a n n e v e r document i t has o f t e n been u s e d as an a d hoc h y p o t h e s i s . C o n n e l l (1980) has p r o p o s e d an e x p e r i m e n t a l d e s i g n t o document " t h e g h o s t o f c o m p e t i t i o n p a s t " . e) I n f e r e n c e s a b o u t p r o c e s s e s f r o m p a t t e r n s a r e not e v i d e n c e f o r t h e r o l e of c o m p e t i t i o n . S p e c i f i c a l l y , A n d r e w a r t h a 1 26 and B i r c h p o i n t e d out t h a t f i n d i n g d i f f e r e n c e s i n t h e way s p e c i e s use r e s o u r c e s d o e s not c o n s t i t u t e e v i d e n c e f o r c o m p e t i t i o n . f ) A l t e r n a t i v e h y p o t h e s e s s h o u l d be g i v e n more c o n s i d e r a t i o n . T h i s l i n e of t h o u g h t has been f o l l o w e d by S t r o n g (1980) i n h i s a r t i c l e " N u l l H y p o t h e s e s i n E c o l o g y . " In s p i t e o f t h e v a r i e t y and t h o r o u g h n e s s of t h e s e c r i t i c i s m s , t h e y were o v e r l o o k e d . D u r i n g t h e l a s t s e v e n y e a r s t h e y been r e c o n s i d e r e d by s e v e r a l a u t h o r s , w i t h o u t a c k n o w l e d g i n g t h a t A n d r e w a r t h a and B i r c h r a i s e d them 30 y e a r s ago. F o l l o w i n g L a c k ' s t r a d i t i o n o f d r a w i n g i n f e r e n c e s a b o u t p r o c e s s e s f r o m t h e p a t t e r n s o b s e r v e d , Brown and W i l s o n (1956) r e v i e w e d known c a s e s o f c h a r a c t e r d i s p l a c e m e n t . They c o n s i d e r e d m o r p h o l o g i c a l , p h y s i o l o g i c a l and e c o l o g i c a l c h a r a c t e r s and i n t e r p r e t e d them as b e i n g t h e r e s u l t o f c o m p e t i t i o n . They f o u n d L a c k ' s h y p o t h e s i s s a t i s f a c t o r y and r e m a r k e d t h a t no a l t e r n a t i v e s were o f f e r e d by t h e c r i t i c s s u c h as A n d r e w a r t h a and B i r c h ( 1 9 5 4 ) . G . E . H u t c h i n s o n e l a b o r a t e d b o t h a rguments ( c h a r a c t e r d i s p l a c e m e n t and r e s o u r c e p a r t i t i o n i n g ) u s e d by L a c k (1954) i n two d i f f e r e n t p a p e r s . In t h e f i r s t p a p e r ( 1 9 5 8 ) , he f o r m a l i z e d t h e c o n c e p t o f n i c h e i n r e l a t i o n t o t h e V o l t e r r a - G a u s e p r i n c i p l e , p r o v i d i n g a c o n c e p t u a l and q u a n t i t a t i v e frame f o r r e s o u r c e p a r t i t i o n i n g s t u d i e s . Two y e a r s l a t e r , i n t h e s e c o n d 1 27 p a p e r h e f o r m a l i z e d B r o w n a n d W i l s o n ' s i d e a s o n c h a r a c t e r d i s p l a c e m e n t : " . . . w h e r e m e t r i c c h a r a c t e r s a r e i n v o l v e d we c a n g a i n s o m e i n s i g h t i n t o t h i s e x t r e m e l y i m p o r t a n t p r o b l e m b y t h e s t u d y o f w h a t B r o w n a n d W i l s o n ( 1 9 5 6 ) h a v e c a l l e d c h a r a c t e r d i s p l a c e m e n t . " ( H u t c h i n s o n 1 9 5 9 ) . I n t h i s s e c o n d p a p e r , p r o b a b l y t h e m o s t i n f l u e n t i a l p a p e r i n c o m m u n i t y e c o l o g y : " H o m a g e t o S a n t a R o s a l i a o r why a r e t h e r e s o m a n y k i n d s o f a n i m a l s ? " , H u t c h i n s o n a c c e p t e d b o t h L a c k ' s a r g u m e n t s a s w e l l a s B r o w n A n d W i l s o n ' s ( 1 9 5 6 ) , w i t h t h e f o l l o w i n g r e m a r k : " . . . I s u s c r i b e t o t h e v i e w t h a t t h e p r o c e s s o f n a t u r a l s e l e c t i o n c o u p l e d w i t h i s o l a t i o n a n d l a t e r m u t u a l i n v a s i o n o f r a n g e s l e a d s t o t h e e v o l u t i o n o f s y m p a t r i c s p e c i e s w h i c h a t e q u i l i b r i u m o c c u p y d i s t i n c t n i c h e s , a c c o r d i n g t o t h e V o l t e r r a - G a u s e p r i n c i p l e . T h e e m p i r i c a l r e a s o n s f o r a d o p t i n g t h i s v i e w a n d t h e c o r r e l a t i v e v i e w t h a t t h e b o u n d a r i e s o f r e a l i z e d n i c h e s a r e s e t b y c o m p e t i t i o n a r e m a i n l y i n d i r e c t . T h i s s u b j e c t h a s b e e n c a r e f u l l y c o n s i d e r e d b y L a c k ( 1 9 5 4 ) " . ( H u t c h i n s o n 1 9 5 9 ) . H u t c h i n s o n ( 1 9 5 9 ) a r r i v e d a t t h e i d e a o f t h e i m p o r t a n c e o f s i z e d i f f e r e n c e s ( r a t i o s ) f o r c o e x i s t e n c e w i t h t h e c r i t i c a l s i z e r a t i o o f 1 . 3 ( l i m i t i n g s i m i l a r i t y ) . T h i s i d e a h a s l e d t o t w o o t h e r c o n c e p t s ( S i m b e r l o f f a n d B o e c k l e n 1 9 8 1 ) . F i r s t , t h e i d e a t h a t t h e r e i s a m i n i m u m s i z e r a t i o c o m p a t i b l e w i t h c o e x i s t e n c e o f e c o l o g i c a l l y s i m i l a r s p e c i e s a n d , s e c o n d , t h a t t h r e e o r m o r e e c o l o g i c a l l y s i m i l a r c o e x i s t i n g s p e c i e s t e n d t o h a v e c o n s t a n t s i z e r a t i o s b e t w e e n s p e c i e s a d j a c e n t i n s i z e r a n k i n g . F u r t h e r m o r e , t h i s p a t t e r n i s s e e n a s t h e r e s u l t o f c o m p e t i t i o n . 1 28 R o b e r t M a c A r t h u r , i n a s e r i e s o f p a p e r s , p r o v i d e d a m a t h e m a t i c a l framework f o r s t u d i e s o f r e s o u r c e p a r t i t i o n i n g . M a c A r t h u r and L e v i n s ( 1 9 6 7 ) , l i n k e d t h e d a t a of r e s o u r c e p a r t i t i o n i n g s t u d i e s ( n i c h e o v e r l a p ) t o t h e c o m p e t i t i o n c o e f f i c i e n t , p r o v i d i n g a m a t h e m a t i c a l framework f o r a l o g i c a l i n c o n s i s t e n c y . The c o m p e t i t i o n c o e f f i c i e n t i s a d i m e n s i o n l e s s number u s e d i n c o m p e t i t i o n e q u a t i o n s t h a t r e p r e s e n t s t h e e f f e c t o f a h e t e r o s p e c i f i c i n d i v i d u a l a s compared t o a c o n s p e c i f i c one on t h e g r o w t h r a t e o f a g i v e n s p e c i e s p o p u l a t i o n . The g r e a t e r t h e o v e r l a p i n r e s o u r c e u s e , t h e g r e a t e r t h e c o m p e t i t i o n c o e f f i c i e n t and t h e g r e a t e r t h e i n t e n s i t y o f c o m p e t i t i o n . In a l a t e r p a p e r , he and May (1972) c o n c l u d e d t h r o u g h a model t h a t t h e r e i s a l i m i t t o n i c h e o v e r l a p i n d e p e n d e n t o f e n v i r o n m e n t a l f l u c t u a t i o n s . H u t c h i n s o n ' s two s e m i n a l p a p e r s and t h e p r o m i s i n g a p p r o a c h o f M a c A r t h u r c r y s t a l l i z e d t h e c o m p e t i t i o n i s t p a r a d i g m . B o t h e n c o u r a g e d many a d h e r e n t s t o l o o k f o r t h e s e p a t t e r n s ( r e s o u r c e p a r t i t i o n i n g and m o r p h o l o g i c a l l i m i t i n g s i m i l a r i t i e s ) i n d i f f e r e n t c o m m u n i t i e s and t o i n t e r p r e t them a s t h e r e s u l t o f c o m p e t i t i o n . 129 4.4 NORMAL SCIENCE. Once a p a r a d i g m has been f o r m a l i z e d t h e n e x t s t a g e , i d e n t i f i e d by Kuhn as n o r m a l s c i e n c e , d e v e l o p s . Normal s c i e n c e c o n s i s t s o f s c i e n t i s t s s o l v i n g t h e p u z z l e s p r o v i d e d by t h e p a r a d i g m . They c o n c e n t r a t e on t h e s c o p e and p r e c i s i o n o f i t s a p p l i c a t i o n w i t h o u t a i m i n g t o p r o d u c e major n o v e l t i e s . Kuhn c h a r a c t e r i z e d t h i s s t a g e as an a t t e m p t t o f o r c e n a t u r e i n t o t h e p r e f o r m e d and r e l a t i v e l y i n f l e x i b l e box t h a t t h e p a r a d i g m s u p p l i e s . Phenomena t h a t w i l l not f i t t h a t box a r e o f t e n o v e r l o o k e d o r e x p l a i n e d away ad h o c . The "commitments" t h a t s c i e n t i s t s a c q u i r e from t h e p a r a d i g m c o n s t i t u t e an i m p o r t a n t f e a t u r e o f t h i s s t a g e . T h e s e may be c o n c e p t u a l , t h e o r e t i c a l , i n s t r u m e n t a l and m e t h o d o l o g i c a l . Some examples o f t h e s e commitments i n community e c o l o g y a r e t h e f o l l o w i n g : a) T h e o r e t i c a l : E c o l o g i s t s w o r k i n g under t h e c o m p e t i t i o n i s t p a r a d i g m f i n d c o m p e t i t i o n t h e o r y u s e f u l t o i n t e r p r e t n a t u r e ( L o t k a - V o l t e r r a e q u a t i o n s and t h e i r a s s u m p t i o n s ) . b) C o n c e p t u a l : They make wide use of t h e c o n c e p t s s u c h as n i c h e , n i c h e d i m e n s i o n s , n i c h e b r e a d t h , n i c h e o v e r l a p , s p e c i e s p a c k i n g , r e s o u r c e p a r t i t i o n i n g (Cody and Diamond 1975). c) M e t h o d o l o g i c a l : They have a v e r y s t r o n g commitment 1 30 t o t h e m e t h o d o l o g y u s e d by t h e p r o p o n e n t s of t h e p a r a d i g m . T h i s i s l a r g e l y an o b s e r v a t i o n a l m e t h o d o l o g y . They o b s e r v e p a t t e r n s i n n a t u r e s u c h a s r e c i p r o c a l l y v a r y i n g a b u n d a n c e s , c h a r a c t e r d i s p l a c e m e n t , r e s o u r c e p a r t i t i o n i n g , n o n - o v e r l a p p i n g a d j a c e n t r a n g e s and n i c h e s h i f t s and i n t e r p r e t them i n t h e l i g h t o f t h e p a r a d i g m , o v e r l o o k i n g a l t e r n a t i v e i n t e r p r e t a t i o n s (Lawton and S t r o n g 1981). The f o l l o w i n g q u o t a t i o n i l l u s t r a t e s c l e a r l y t h i s a t t i t u d e : " T h e s e n i c h e d i f f e r e n c e s r e s u l t from c o m p e t i t i o n : t h e y a r e means by w h i c h s p e c i e s m i n i m i z e c o m p e t i t i o n i n n a t u r e . " (Diamond 1978, p.324) The s t a g e o f n o r m a l s c i e n c e i s w e l l r e p r e s e n t e d by t h e 81 s t u d i e s on r e s o u r c e p a r t i t i o n i n g r e v i e w e d by S c h o e n e r ( 1 9 7 4 ) . The i m p l i c i t o r e x p l i c i t h y p o t h e s i s of t h e s e s t u d i e s was t h a t n i c h e s were r e g u l a r l y and w i d e l y - s p a c e d o v e r one o r more d i m e n s i o n s ( o v e r d i s p e r s i o n o f n i c h e s ) . The g e n e r a l p r e d i c t i o n was t h a t o v e r l a p i n r e s o u r c e use s h o u l d be s m a l l . T h i s p r e d i c t i o n i s f u r t h e r d i v i d e d i n t o t h r e e c a t e g o r i e s : a) t h e r e s h o u l d be r e g u l a r s p a c i n g i n one d i m e n s i o n ; b) s i m i l a r i t y i n one d i m e n s i o n s h o u l d i m p l y d i s s i m i l a r i t y i n a n o t h e r ; c ) h i g h l y d i v e r s e s y s t e m s s h o u l d s e g r e g a t e i n more d i m e n s i o n s p r e s e r v i n g m i n i m a l o v e r l a p ( S c h o e n e r 1974). The s t u d i e s were c a r r i e d out by m e a s u r i n g e i t h e r a) o v e r l a p i n r e s o u r c e use ( f o o d and h a b i t a t ) and b) o v e r l a p i n t h e s i z e o f t r o p h i c s t r u c t u r e s ( i . e . jaws, b e a k s , m o u t h s ) . The c r i t i c i s m s r a i s e d by d i f f e r e n t a u t h o r s r e g a r d i n g t h i s k i n d of s t u d y w i l l be d e s c r i b e d i n t h e n e x t s e c t i o n . 131 T h e i n f l u e n c e o f M a c A r t h u r ' s s c h o o l a n d t h e k i n d o f s t u d i e s t h e y d i d a n d t h e e v i d e n c e t h e y a c c e p t e d a r e w e l l r e p r e s e n t e d i n a b o o k d e d i c a t e d t o M a c A r t h u r ( C o d y a n d D i a m o n d 1 9 7 5 ) e n t i t l e d " E c o l o g y a n d e v o l u t i o n o f c o m m u n i t i e s " . 4 . 5 C R I S I S P R E - P A R A D I G M S T A G E . A f t e r s o m e t i m e o f n o r m a l s c i e n c e t h e r e i s a n a w a r e n e s s o f a n o m a l i e s b y a s m a l l s e c t o r o f t h e r e s e a r c h c o m m u n i t y ( K u h n 1 9 7 0 ) ; t h i s i s t h e c r i s i s s t a g e . O b s e r v a t i o n s o f n a t u r a l p h e n o m e n a v i o l a t e t h e e x p e c t a t i o n s p r e d i c t e d b y t h e p a r a d i g m ; t h e r e a r e d i f f i c u l t i e s w i t h t h e t h e o r y a n d i t s a s s u m p t i o n s ; s c i e n t i s t s r e c o g n i z e l o g i c a l f l a w s , a n d i n a d e q u a t e t e s t s . T h e p a r a d i g m p l a y s a m a j o r r o l e t h e n , s i n c e t h e d i f f i c u l t i e s e n c o u n t e r e d c a n b e s e e n o n l y a g a i n s t t h e b a c k g r o u n d p r o v i d e d b y t h e p a r a d i g m . B u t e v e n i n t h e s t a g e o f c r i s i s s c i e n t i s t s d o n o t r e j e c t t h e p a r a d i g m u n l e s s t h e r e i s a c a n d i d a t e t o t a k e i t s p l a c e . S c i e n t i s t s d o n o t p r o d u c e a l t e r n a t i v e s d u r i n g " n o r m a l s c i e n c e ' b u t o n l y d u r i n g t h e p r e - p a r a d i g m s t a g e . H o w e v e r , K u h n u n d e r l i n e s , t h e h i s t o r y o f s c i e n c e i n d i c a t e s t h a t i t i s n o t v e r y d i f f i c u l t t o p r o d u c e s u c h a l t e r n a t i v e s i n t h e e a r l y d e v e l o p m e n t a l s t a g e s o f a new p a r a d i g m , p r i o r t o n o r m a l s c i e n c e . T h e t h o r o u g h n e s s o f A n d r e w a r t h a a n d B i r c h ' s ( 1 9 5 4 ) e a r l y c r i t i c i s m s r e p r e s e n t s a n e x c e l l e n t e x a m p l e o f s u c h s i t u a t i o n . 1 32 The 'pre-paradigm' p e r i o d i s marked by frequent and deep debates over l e g i t i m a t e methods, problems, standards of s o l u t i o n and p h i l o s o p h i c a l a n a l y s i s . These arguments d e f i n e schools r a t h e r than produce agreement. T h i s p o i n t has been evident i n the recent l i t e r a t u r e on community ecology, where methodological debates as w e l l as p h i l o s o p h i c a l d i s c u s s i o n s are commonplace (Strong 1980, S i m b e r l o f f 1980, C o n n e l l 1983, Wiens 1983, Diamond 1983). There i s probably no other f i e l d i n ecology where quotes from p h i l o s o p h e r s of sc i e n c e ( P i a t t , Chamberlin, Feyerabend, Popper, Lakatos, Kuhn) are as abundant as i n the community l i t e r a t u r e . Schoener (1982) has d e s c r i b e d the development of the c o n t r o v e r s y of c o m p e t i t i o n from an i n s i d e p o i n t of view. T h i s stage of r e c o n s i d e r a t i o n of theory, assumptions, methods and t e s t s i n community ecology was i n i t i a t e d with an e a r l y paper by Grant (1972b). He q u e s t i o n e d two assumptions of c h a r a c t e r displacement s t u d i e s . F i r s t , that animal d i s t r i b u t i o n s change from a l l o p a t r y to sympatry. Second, that these s t u d i e s c o n s i d e r p o p u l a t i o n s i n the a l l o p a t r i c zone as " c o n t r o l s " f o r the o b s e r v a t i o n s i n the sympatric zones. In doing t h i s , the authors assume that the only v a r i a b l e that changes i n sympatry i s the presence of the other s p e c i e s . The d i f f e r e n c e s i n m o r p h o l o g i c a l c h a r a c t e r s may be independent of c o m p e t i t i o n . Other processes such as c l i n a l v a r i a t i o n would l e a d to s i m i l a r p a t t e r n s . The p r e d i c t i o n s from such s t u d i e s do not d i s c r i m i n a t e from those from a l t e r n a t i v e p r o c e s s e s . A f t e r 1 33 r e - e x a m i n i n g t h e c l a s s i c a l e x a m p l e s o f c h a r a c t e r d i s p l a c e m e n t ( l a r g e l y o r n i t h o l o g i c a l ) , h e c o n c l u d e d t h a t m o s t o f t h e m w e r e e i t h e r w r o n g o r e q u i v o c a l l y i n t e r p r e t e d , a n d t h e r e f o r e t h e e v i d e n c e s u p p o r t i n g t h e a b o v e p h e n o m e n o n w a s w e a k . R e s o u r c e p a r t i t i o n i n g s t u d i e s , s u c h a s t h o s e d e s c r i b e d a b o v e a s n o r m a l s c i e n c e , h a v e b e e n c r i t i c i z e d b y d i f f e r e n t a u t h o r s b e c a u s e o f t h e a s s u m p t i o n s i n v o l v e d . T h e f i r s t i s t h a t t h e d e g r e e o f m o r p h o l o g i c a l s i m i l a r i t y b e t w e e n m e m b e r s o f s p e c i e s i n d i c a t e s t h e e x t e n t o f o v e r l a p i n t h e i r r e s o u r c e u s e . S t u d i e s t h a t m e a s u r e t r o p h i c s t r u c t u r e s a s s u m e t h a t p r e y s i z e i s s t r o n g l y c o r r e l a t e d w i t h t h e s i z e o f a n a n i m a l ' s b e a k , m o u t h o r j a w . H o w e v e r , t h e c r i t i c a l l i n k b e t w e e n o v e r l a p i n f e e d i n g s t r u c t u r e o r b e h a v i o r , a n d o v e r l a p i n a c t u a l u t i l i z a t i o n i s n o t s a t i s f a c t o r i l y d o c u m e n t e d i n m a n y s t u d i e s . F u r t h e r m o r e , t h e r e i s e v i d e n c e a g a i n s t i t . I n s o m e c a s e s , c o e x i s t i n g s p e c i e s o f d i f f e r e n t s i z e s h a v e b e e n f o u n d t o c o n s u m e h i g h l y s i m i l a r a s s o r t m e n t s o f p r e y t y p e s a n d s i z e s ( S m i g e l a n d R o s e n z w e i g 1 9 7 4 , L e m e n 1 9 7 8 ) . T h e r e i s a l s o s u b s t a n t i a l v a r i a b i l i t y o f t r o p h i c s t r u c t u r e ( o r b o d y s i z e ) a n d f o o d s i z e w i t h i n a n d a m o n g t a x a ( H e s p e n h e i d e 1 9 7 3 ) . S t u d i e s i n v o l v i n g l a r g e r f r a c t i o n s o f f a u n a s g o f u r t h e r w i t h t h i s a s s u m p t i o n , i . e . t h e d e g r e e o f s i z e d i f f e r e n c e s r e q u i r e d t o a l l o w c o e x i s t e n c e i s t h e s a m e a m o n g a l l c o n t i g u o u s s i z e p a i r s o f s p e c i e s w i t h i n c o m m u n i t i e s . T h e s e c o n d a s s u m p t i o n i s i n t h e r e l a t i o n b e t w e e n n i c h e o v e r l a p a n d c o m p e t i t i o n . T h e h i g h e r t h e o v e r l a p , t h e h i g h e r t h e i n t e r s p e c i f i c c o m p e t i t i o n . T h i s a s s u m p t i o n i n v o l v e s o t h e r s s u c h 1 34 a s a n e q u i l i b r i u m s t a t e w h e r e s p e c i e s a r e n e a r t h e i r c a r r y i n g c a p a c i t i e s a n d r e s o u r c e s a r e l i m i t i n g . T h i s s e c o n d a s s u m p t i o n h a s b e e n i n v a l i d a t e d f o r s o m e s y s t e m s b y s t u d i e s t h a t s h o w n o r e l a t i o n s h i p b e t w e e n o v e r l a p a n d c o m p e t i t i o n ( S c h r o d e r a n d R o s e n z w e i g 1 9 7 5 ) a n d b y i t s l o g i c a l i n c o n s i s t e n c i e s ( C o l w e l l a n d F u t u y m a 1 9 7 1 ) . L a r g e a n d s m a l l o v e r l a p c o u l d b e i n t e r p r e t e d a s i n t e n s e c o m p e t i t i o n a n d , a l t e r n a t i v e l y , t h e y c o u l d a l s o b e i n t e r p r e t e d a s l a c k o f c o m p e t i t i o n . I n 1 9 7 7 , W i e n s a s s e m b l e d a v a r i e t y o f c r i t i c i s m s o f t h e s t u d i e s t h a t f o l l o w e d t h e o b s e r v a t i o n a l t r a d i t i o n . H e e m p h a s i z e d f o u r n e g l e c t e d a s s u m p t i o n s . 1 . T h e s t u d i e s a s s u m e t h a t s e l e c t i o n i s c o n t i n u o u s a n d i n t e n s e . I f t h i s i s s o , t h e n c o m p e t i t i o n s h o u l d b e a n o n g o i n g p r o c e s s i n n a t u r e e v e n a m o n g s p e c i e s t h a t h a v e ' r e s o l v e d ' t h e i r c o e x i s t e n c e d i f f i c u l t i e s a n d s h o u l d b e r e l a t i v e l y e a s i l y t o d o c u m e n t . T h e i m p l i c a t i o n o f t h i s a s s u m p t i o n i s t h a t t h e e n v i r o n m e n t i s c o n s t a n t l y l i m i t i n g . 2 . P o p u l a t i o n s a r e a t t h e i r c a r r y i n g c a p a c i t i e s a n d s u i t a b l e h a b i t a t s a r e c o m p l e t e l y s a t u r a t e d . 3 . C o m p e t i t i o n i s t h e m a j o r s e l e c t i v e f o r c e a c t i n g u p o n t h e r e s o u r c e - u t i l i z a t i o n t r a i t s o r d e t e r m i n i n g t h e d i s t r i b u t i o n o f a s p e c i e s . P r e d a t i o n a n d e n v i r o n m e n t a l h a r s h n e s s a r e n o t t a k e n i n t o a c c o u n t . 1 35 4. The p r e d i c t e d o p t i m a l s t a t e i s a v a i l a b l e t o t h e p o p u l a t i o n i n i t s e v o l u t i o n a r y d e v e l o p m e n t . H i s t o r i c a l c o n s t r a i n t s a r e not a c c o u n t e d f o r . C o n n e l l (1975) u n d e r l i n e d t h e p r o b l e m s o f t h e m e t h o d o l o g y u s e d t o s t u d y community s t r u c t u r e . He d e s c r i b e d t h r e e d i f f e r e n t methods. F i r s t , d e s c r i p t i o n o f p a t t e r n s t o e v a l u a t e whether o r n o t t h e y c o n f o r m t o t h e p r e d i c t i o n s o f t h e m o d e l . Examples a r e g r a d i e n t a n a l y s i s , f i t t i n g r e l a t i v e a b u n d a n c e s t o m a t h e m a t i c a l m o d e l s , e x a m i n i n g d i s t r i b u t i o n a l p a t t e r n s and f o o d h a b i t s o f c l o s e l y r e l a t e d s p e c i e s t o document t h e i r o v e r l a p . He s t r e s s e d t h e l i m i t a t i o n of t h i s method f o r d i s c o v e r i n g h y p o t h e s e s . S e c o n d , n a t u r a l e x p e r i m e n t s t h a t compare d i f f e r e n t e c o l o g i c a l s i t u a t i o n s s u c h a s m o u n t a i n s and i s l a n d s . The drawback o f t h e s e s t u d i e s i s t h a t t h e y assume t h a t i n t h e i r c o m p a r i s o n s o n l y one v a r i a b l e c h a n g e s and have no r i g o r o u s c o n t r o l . T h i r d , c o n t r o l l e d f i e l d e x p e r i m e n t s . He u n d e r l i n e d t h e a d v a n t a g e s o f t h i s t h i r d method o v e r th e p r e v i o u s two, w h i c h have been p r e f e r r e d by M a c A r t h u r ' s s c h o o l (Cody 1975, Diamond 1978, 1983). C o n n e l l (1975) p o i n t e d o u t t h a t i n d i r e c t e v i d e n c e , s u c h as o b s e r v a t i o n s o f n o n - o v e r l a p p i n g a d j a c e n t r a n g e s , i n v e r s e c o r r e l a t i o n s o f a b u n d a n c e s , n i c h e s h i f t s on i s l a n d s and c h a r a c t e r d i s p l a c e m e n t , a r e open t o e x p l a n a t i o n s o t h e r t h a n c o m p e t i t i o n . The most r e c e n t c r i t i c i s m s of c o m p e t i t i o n t h e o r y and 1 36 t h e m e t h o d o l o g y u s e d t o s u p p o r t i t h a v e c o m e f r o m t h e T a l l a h a s s e g r o u p ( S i m b e r l o f f , S t r o n g , C o n n o r ) . T h e y h a v e p o i n t e d o u t m a i n l y m e t h o d o l o g i c a l e r r o r s i n i n t e r p r e t i n g o b s e r v a t i o n a l d a t a . A n e x a m p l e o f t h e s e c r i t i c i s m s i s p r o v i d e d i n L a w t o n a n d S t r o n g ( 1 9 8 1 ) . T h e y r e v i e w e d t h e i n s e c t l i t e r a t u r e f o r c o m m u n i t y p a t t e r n s t h a t h a d b e e n a t t r i b u t e d t o c o m p e t i t i o n , s u c h a s : l a r g e a r e a s s u p p o r t m o r e s p e c i e s t h a n s m a l l a r e a s ; c o m p l e x h a b i t a t s s u p p o r t m o r e s p e c i e s t h a n s i m p l e h a b i t a t s ; p e r s i s t e n t h a b i t a t s s u p p o r t m o r e s p e c i e s w i t h n a r r o w e r n i c h e s t h a n e p h e m e r a l h a b i t a t s ; e c o l o g i c a l l y s i g n i f i c a n t s i z e d i f f e r e n c e s e x i s t w i t h i n a n d b e t w e e n s p e c i e s i n a g u i l d ; c o m m u n i t i e s b e c o m e ' s a t u r a t e d ' w i t h s p e c i e s i n e c o l o g i c a l t i m e ; d e n s i t y c o m p e n s a t i o n o c c u r s i n d e p a u p e r a t e d g u i l d s ; s p e c i e s c o m p e t e m o r e s t r o n g l y w i t h r e l a t i v e s ; s p e c i e s l i v i n g i n t h e s a m e h a b i t a t a r e s e p a r a t e d i n a n o t h e r n i c h e d i m e n s i o n ; s i m i l a r s p e c i e s s h o w a l t i t u d i n a l z o n a t i o n ; s p e c i e s r i c h n e s s c h a n g e s i n l a t i t u d i n a l g r a d i e n t s ; s p e c i e s i n a c o m m u n i t y s h o w s i m i l a r f r e q u e n c y d i s t r i b u t i o n s , w h e r e m o s t a r e r a r e a n d f e w a r e a b u n d a n t . T h e y c r i t i c i z e t h e m e t h o d o l o g i c a l a p p r o a c h o f i n t e r p r e t i n g t h e d a t a i n t h e l i g h t o f c o m p e t i t i o n w i t h o u t e v a l u a t i n g a l t e r n a t i v e e x p l a n a t i o n s ( n u l l h y p o t h e s e s ) t h a t c o u l d a c c o u n t f o r t h e s a m e p a t t e r n s . H o w e v e r t h e i r a l t e r n a t i v e h a s l i t t l e e m p i r i c a l c o n t e n t . I t d o e s n o t s u g g e s t f u r t h e r t e s t s s i n c e i t i n c l u d e s e v e r y o t h e r f a c t o r . "We s u g g e s t t h a t a t h e o r y t h a t s t r e s s e s t h a t t h e a u t e c o l o g i c a l r e s p o n s e s t o w e a t h e r , p h e n o l o g y a n d h o s t c h e m i s t r y o f i n d i v i d u a l s p e c i e s o f p l a n t s a n d i n s e c t s , t h e e f f e c t s o f i s o l a t i o n , m i g r a t i o n a n d h a b i t a t h e t e r o g e n e i t y , g e o g r a p h i c a l f a c t o r s o f 137 i n t e r s p e c i e s a s s o c i a t i o n s and t h e a c t i o n of p r e d a t o r s and p a r a s i t o i d s would u s u a l l y be s u f f i c i e n t f o r f o l i v o r o u s i n s e c t c o m m u n i t i e s . " (Lawton and S t r o n g 1981). Most of t h e e a r l y s u p p o r t f o r t h e key r o l e o f c o m p e t i t i o n came from t h e k i n d o f i n d i r e c t and c i r c u m s t a n t i a l e v i d e n c e d e s c r i b e d by C o n n e l l ( 1 9 7 5 ) . D u r i n g t h e l a s t t e n y e a r s , s e v e r a l e c o l o g i s t s have p o i n t e d o u t t h e l a c k o f e x p e r i m e n t a l s u p p o r t f o r t h e e s t a b l i s h e d v i e w c o n c e r n i n g t h e r o l e of c o m p e t i t i o n i n n a t u r e ( C o n n e l l 1975, Wiens 1977, Connor and S i m b e r l o f f 1979, S t r o n g 1980, H a i r s t o n 1981). R e c e n t l y , however, t h e e x p e r i m e n t a l a p p r o a c h has r e c e i v e d more a t t e n t i o n , and a number o f e x p e r i m e n t a l m a n i p u l a t i o n s have been p e r f o r m e d . S c h o e n e r (1983) r e v i e w e d t h e e v i d e n c e f r o m 160 e x p e r i m e n t a l s t u d i e s i n w h i c h d e n s i t i e s were m a n i p u l a t e d e i t h e r by r e m o v a l o r i n t r o d u c t i o n . S t u d i e s were c l a s s i f i e d a s showing c o m p e t i t i o n i f t h e r e were c h a n g e s i n : d e n s i t y , (number o f i n d i v i d u a l s , w e i g h t ) , s u r v i v a l , g r o w t h , s i z e , f o r a g i n g s u c c e s s , r e c r u i t m e n t r a t e , h a b i t a t s h i f t , i m m i g r a t i o n r a t e , e m i g r a t i o n r a t e , a c t i v i t y t i m e , a c t i v i t y l e v e l , e x t i n c t i o n , f o o d t y p e o r s i z e . However, when t h e s e s t u d i e s a r e l o o k e d a t more c l o s e l y , a number o f drawbacks a r e e v i d e n t . A number of them l a c k e d c o n t r o l s o r r e p l i c a t e s o r were done u n d e r h i g h l y a r t i f i c i a l c o n d i t i o n s . T h e r e f o r e , h i s c o n c l u s i o n t h a t c o m p e t i t i o n i s p e r v a s i v e s h o u l d be t a k e n c a u t i o u s l y ( G a l i n d o and K r e b s i n p r e s s ) . D u r i n g t h e l a s t t e n y e a r s t h e d i s a g r e e m e n t has become 138 a c u t e , and has l e d t o a v a r i e t y o f a r g u m e n t s among e c o l o g i s t s ( S t r o n g e t a _ l . 1984) and t o a r e c e n t r o u n d t a b l e on r e s e a r c h i n e c o l o g y and e v o l u t i o n a r y b i o l o g y ( S a l t 1984). T h e r e a r e many i m p l i c a t i o n s i n t h e d e b a t e . D i f f e r e n t c r i t i c i s m s have been r a i s e d t h a t r a n g e from t h e v a l i d i t y o f t h e a s s u m p t i o n s of c o m p e t i t i o n t h e o r y (Wiens 1977) t o t h e l e g i t i m a c y o f d i s t i n c t r e s e a r c h methods ( C o n n e l l 1975, S t r o n g 1980, S i m b e r l o f f a n d B o e c k l e n 1981). In my v i e w , t h e q u e s t i o n s i n v o l v e d i n t h e d e b a t e l a c k p r e c i s i o n and o f t e n l e a d t o c o n f u s i o n . F o r example, a number o f a r t i c l e s a s s e r t t h a t c o m p e t i t i o n p l a y s an ' i m p o r t a n t r o l e ' or i s a 'major f a c t o r ' i n s t r u c t u r i n g c o m m u n i t i e s (Brown and Bowers 1984). These s t a t e m e n t s c o u l d mean t h a t c o m p e t i t i o n e x i s t s , t h a t i t i s more i m p o r t a n t t h a n o t h e r p r o c e s s e s , o r t h a t i t o c c u r s f r e q u e n t l y . I t h i n k nobody d o u b t s t h e f i r s t i m p l i c a t i o n . The s e c o n d a l t e r n a t i v e would be t e s t e d i f we compare t h e r e l a t i v e c o n t r i b u t i o n o f c o m p e t i t i v e i n t e r a c t i o n s w i t h t h a t o f o t h e r p r o c e s s e s s u c h as w e a t h e r , p r e d a t i o n , p a r a s i t i s m , m u t u a l i s m , d i s t u r b a n c e s and i n t e r s p e c i f i c c o m p e t i t i o n ( C o n n e l l 1983). I d e a l l y t h i s i n f o r m a t i o n s h o u l d be e x t e n d e d on b o t h t e m p o r a l and s p a t i a l s c a l e s . S i n c e s u c h i n f o r m a t i o n c u r r e n t l y d o e s n o t e x i s t , t h e t e r m s 'major f a c t o r ' o r ' i m p o r t a n t r o l e ' i n t h i s s e n s e a r e t o t a l l y u n s u p p o r t e d . We a r e l e f t w i t h t h e l a s t meaning t h a t e q u a t e s i m p o r t a n c e t o f r e q u e n c y o f o c c u r r e n c e and n o t t o i n t e n s i t y o f o c c u r r e n c e . But how o f t e n does c o m p e t i t i o n o c c u r ? Even t h i s i s d i f f i c u l t t o a s s e s s , s i n c e when r e s e a r c h e r s g a t h e r s u c h i n f o r m a t i o n t h e r e i s a s e l e c t i v e b i a s a t d i f f e r e n t l e v e l s . The 139 o r g a n i s m s c h o s e n a r e u s u a l l y a b u n d a n t a n d j u d g e d a p r i o r i t o b e p o t e n t i a l c o m p e t i t o r s . T h e s t u d i e s d o n o t l a s t e n o u g h t i m e t o d o c u m e n t y e a r - t o - y e a r v a r i a b i l i t y . I f t h e s t u d i e s p r o d u c e n e g a t i v e r e s u l t s t h e y may n o t g e t p u b l i s h e d . F i n a l l y , t h e d e b a t e c o n t a i n s d i f f e r e n t c o n s i d e r a t i o n s o n t h e k i n d o f e v i d e n c e t h a t s h o u l d b e a c c e p t e d . Some e c o l o g i s t s a r e n o t s a t i s f i e d w i t h t h e r e s u l t s f r o m a r t i f i c i a l s i t u a t i o n s b u t d e m a n d t h a t t h e e x p e r i m e n t s b e d o n e i n n a t u r a l c o n d i t i o n s . Some e c o l o g i s t s a r e m o r e p r o n e t o g e n e r a l i z e f r o m c o m m u n i t y t o c o m m u n i t y t h a n o t h e r s . 4 . 6 R E V O L U T I O N . T h e l a s t s t a g e i n t h e s c i e n t i f i c d e v e l o p m e n t , y e t t o c o m e t o c o m m u n i t y e c o l o g y , i s t h e a c q u i s i t i o n o f a new p a r a d i g m . M a k i n g a n a n a l o g y w i t h p o l i t i c a l r e v o l u t i o n s , K u h n c a l l e d t h e t r a n s i t i o n t o a new p a r a d i g m a s c i e n t i f i c r e v o l u t i o n . H i s d e s c r i p t i o n i s a s f o l l o w s : A r e v o l u t i o n s t a r t s w i t h a g r o w i n g a w a r e n e s s o f t h e i n a d e q u a c i e s o f t h e f o r m e r p a r a d i g m . O n l y a f r a c t i o n o f t h e s c i e n t i f i c c o m m u n i t y p a r t i c i p a t e s . A c r i s i s s t a r t s w i t h r e s i s t a n c e f r o m t h e e s t a b l i s h e d s e c t o r , t o whom t h e c r i t i c i s m s s e e m a s a r e v o l u t i o n . F o r t h e r e b e l s i t i s j u s t a n o r m a l p a r t o f t h e d e v e l o p m e n t a l p r o c e s s . F i n a l l y , t h e new p a r a d i g m r e d e f i n e s p r o b l e m s , m e t h o d s a n d s o l u t i o n s . Some p r o b l e m s t h o u g h t t r i v i a l may now b e c o m e k e y p r o b l e m s . 140 In summary, t h e d e v e l o p m e n t o f t h i s f i e l d i s c o n s i s t e n t w i t h Kuhn's view a b o u t t h e g e n e r a l d e v e l o p m e n t of s c i e n c e . K r e b s (1978) p o i n t e d o u t a s i m i l a r agreement between Kuhn's model and t h e h i s t o r i c a l d e v e l o p m e n t o f t h e a r e a of p o p u l a t i o n r e g u l a t i o n . I t h i n k t h a t i n e c o l o g y a number o f f a c t o r s f a c i l i t a t e t h i s k i n d o f d e v e l o p m e n t . F i r s t , e c o l o g y i s a young s c i e n t i f i c d i s c i p l i n e w i t h w e l l - d e f i n e d s c h o o l s o f t h o u g h t p r o v i d i n g s t r o n g i n f l u e n c e i n t h e c h o i c e o f t h e o r i e s , t o p i c s and methods. S e c o n d , b e c a u s e o f i t s l o n g d e s c r i p t i v e t r a d i t i o n i n h e r i t e d f r o m n a t u r a l h i s t o r i a n s , t h e i n c o r p o r a t i o n of e x p e r i m e n t a l t e s t s o f e x p l i c i t h y p o t h e s e s has been s l o w . Even a t t h e p r e s e n t t i m e , e x p e r i m e n t a l d e s i g n s a r e not c o m p l e t e l y r i g o r o u s . T h i r d , i n t h e a b s e n c e o f c o n t r a r y e v i d e n c e , e a r l y f i n d i n g s become b r o a d g e n e r a l i z a t i o n s t h a t g u i d e s u b s e q u e n t r e s e a r c h and d i r e c t our o b s e r v a t i o n s . F i n a l l y , e c o l o g i s t s seem t o be c o n t e m p t u o u s o f t r a d i t i o n a l s c i e n t i f i c c r i t e r i a o f e v i d e n c e . On t h e o t h e r hand, i t may be t h a t , i n f i t t i n g t h e h i s t o r y o f t h e d e v e l o p m e n t o f t h i s i d e a i n t o t h e i n f l e x i b l e box p r o v i d e d by Kuhn's p a r a d i g m t h e o r y , I m y s e l f have o v e r l o o k e d a n o m a l i e s . I n any c a s e , B i r c h (1979) c a p t u r e s Kuhn's i d e a s i n one s e n t e n c e : "The t r o u b l e a b o u t good a n s w e r s i s t h a t p e o p l e t e n d t o l o o k a t p r o b l e m s i n terms o f a n s w e r s t h e y a l r e a d y know and w h i c h t h e y e x p e c t o r hope t o f i n d ( B i r c h 1979, p ! 9 7 ) " . 141 REFERENCES Abramsky Z., M.I.Dyer and P.D. H a r r i s o n . 1979. C o m p e t i t i o n among s m a l l mammals i n e x p e r i m e n t a l l y p e r t u r b e d a r e a s of t h e s h o r t g r a s s p r a i r i e . E c o l o g y 60:530-536. A n d r e w a r t h a H.G., and L.C. B i r c h . 1954. The d i s t r i b u t i o n and abundance o f a n i m a l s . U n i v . o f C h i c a g o P r e s s , C h i c a g o . 782 pp. B a k e r R.H. 1968. H a b i t a t s and d i s t r i b u t i o n . I n : J.A. K i n g ( e d ) . B i o l o g y o f Peromyscus ( R o d e n t i a ) . S p e c . P u b l . 2. Am. S o c . Mamm. U.S. :98-1 26. B a n f i e l d A.W.F. 1974. The mammals o f Canada. U n i v e r s i t y of T o r o n t o P r e s s , T o r o n t o . 438pp. B e n d e l l J . F . 1961. Some f a c t o r s a f f e c t i n g t h e h a b i t a t s e l e c t i o n by w h i t e - f o o t e d mouse. Can. F i e l d N a t . 75:244-255. B i r c h L.C. 1979. The e f f e c t o f s p e c i e s of a n i m a l s w h i c h s h a r e common r e s o u r c e s on one a n o t h e r ' s d i s t r i b u t i o n and abu n d a n c e . F o r t s c h r . Z o o l . , 25:197-221. Brown J.H. 1973. S p e c i e s d i v e r s i t y o f s e e d - e a t i n g d e s e r t r o d e n t s i n s a n d dune h a b i t a t s . E c o l o g y 54:775-787. Brown J.H. 1975. G e o g r a p h i c a l e c o l o g y of d e s e r t r o d e n t s . I n : Cody and Diamond ( e d s . ) . E c o l o g y and e v o l u t i o n of c o m m u n i t i e s . B e l k n a p P r e s s o f H a r v a r d U n i v e r s i t y P r e s s , C a m b r i d g e . : 315-341. Brown J.H. 1978. E f f e c t s of mammalian c o m p e t i t o r s on t h e e c o l o g y and e v o l u t i o n o f c o m m u n i t i e s . I n : Sn y d e r D.P. ( e d . ) . P o p u l a t i o n s o f s m a l l Mammals under N a t u r a l C o n d i t i o n s . P y m a t u n i n g L a b o r a t o r y o f E c o l o g y Symposium Volume 5, P i t t s b u r g , Pa. :52-57. Brown J.H., and G.A. L i e b e r m a n . 1973. R e s o u r c e u t i l i z a t i o n and c o e x i s t e n c e of s e e d - e a t i n g d e s e r t r o d e n t s i n sand dune h a b i t a t s . E c o l o g y 54:788-797. 1 42 Brown J.H., and M.A. Bowers. 1984. P a t t e r n s and p r o c e s s e s i n t h r e e g u i l d s o f t e r r e s t r i a l v e r t e b r a t e s . I n : S t r o n g D.R. , D. S i m b e r l o f f , L.G. A b e l e and A . B . T h i s t l e ( e d s ) . E c o l o g i c a l c o m m u n i t i e s . P r i n c e t o n U n i v . P r e s s , New J e r s e y :282-296. Brown W.L., and E.O. W i l s o n . 1956. C h a r a c t e r d i s p l a c e m e n t . S y s t . Z o o l . 5:49-65. C o c k b u r n A. 1981. P o p u l a t i o n r e g u l a t i o n and d i s p e r s i o n o f t h e smoky mouse, Pseudomys fumeus. I . D i e t a r y d e t e r m i n a n t s o f m i c r o h a b i t a t p r e f e r e n c e s . A u s t . J . E c o l . 6: 231-254. Cody M.L. 1975. C o m p e t i t i o n and t h e s t r u c t u r e o f b i r d c o m m u n i t i e s . P r i n c e t o n U n i v e r s i t y P r e s s , P r i n c e t o n . 318pp. Cody M.L., and J.M. Diamond ( e d s . ) . 1975. E c o l o g y and e v o l u t i o n o f c o m m u n i t i e s . 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