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Habitat selection and social group dynamics of African elephants in Amboseli, Kenya 1982

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HABITAT SELECTION AND SOCIAL GROUP DYNAMICS OF AFRICAN ELEPHANTS IN AMBOSELI, KENYA by WILLIAM KEITH LINDSAY B.Sc., The U n i v e r s i t y of B r i t i s h Columbia, Vancouver, 1974 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA February 1982 © W i l l i a m K e i t h Lindsay, 1982 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 a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s m a y b e g r a n t e d b y t h e h e a d o f my d e p a r t m e n t o r b y 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 b e a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f z f g g / - o ^ V T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 2 0 7 5 W e s b r o o k P l a c e V a n c o u v e r , C a n a d a V 6 T 1W5 D a t e /6 MAtCH .fifz. DE -6 ( 2 / 7 9 ) i i ABSTRACT The s e l e c t i o n and use of h a b i t a t s by A f r i c a n elephants were examined i n r e l a t i o n to v e g e t a t i o n abundance and elephant s o c i a l grouping p a t t e r n s . I conducted the study i n the Amboseli b a s i n and surrounding s e m i - a r i d bushlands i n southern Kenya from November 1978 to October 1979. Elephant h a b i t a t d i s t r i b u t i o n s in the b a s i n i n 1978/79 were monitored i n a s e r i e s of 88 ground surveys, while data from 54 a e r i a l counts from 1975 to 1980 were examined f o r a longer term view. Plant d e n s i t y and biomass were monitored i n permanent v e g e t a t i o n p l o t s l o c a t e d ' i n d i f f e r e n t h a b i t a t types i n the Amboseli basin and bushlands. H a b i t a t types were d e f i n e d , d e s c r i b e d , and compared. Herbaceous ( g r a s s / f o r b ) biomass was more abundant than browse ( t w i g / l e a f ) biomass at a l l times w i t h i n h a b i t a t types, but in the dry season, the woody -layer of some h a b i t a t types, such as the swamp edge woodlands and dense bushlands, was more abundant than the herb l a y e r of other h a b i t a t s , such as the open g r a s s l a n d s . Bushlands and woodlands had hi g h e s t d i v e r s i t y of herbaceous and woody biomass. Biomass p r o d u c t i o n was s i g n i f i c a n t l y r e l a t e d to wet season r a i n f a l l i n a l l h a b i t a t s . The herb l a y e r s of swamp edge h a b i t a t s c o n s i s t e n t l y had the highest biomass and lowest n u t r i t i o n a l q u a l i t y ( e x e m p l i f i e d by crude p r o t e i n c o n t e n t ) , and were abundant w e l l i n t o the dry season. Woodland and bushland h a b i t a t types had herb l a y e r s which were only s e a s o n a l l y abundant, but of higher p r o t e i n content through the year. Elephants p r e f e r r e d woodlands (and, i n d r i e r years, bushlands) in the wet seasons and swamps in the dry seasons. Elephant numbers in the woodlands and g r a s s l a n d s were p o s i t i v e l y r e l a t e d to monthly r a i n f a l l and herb l a y e r biomass, and numbers in the wet swamps and i n the swamp edge g r a s s l a n d s and woodlands were n e g a t i v e l y r e l a t e d to the same v a r i a b l e s . There was a l a r g e amount of v a r i a n c e a s s o c i a t e d with these r e l a t i o n s h i p s . The ranking of h a b i t a t use by elephants was s i g n i f i c a n t l y c o r r e l a t e d with the ranking of herb l a y e r biomass across h a b i t a t s i n the dry season, but not i n the wet season, when l e s s abundant, but more n u t r i t i o u s , v e g e t a t i o n was a v a i l a b l e i n the woodlands and bushlands. There were no c l e a r r e l a t i o n s h i p s between elephant numbers and browse biomass or percent woody cover a c r o s s h a b i t a t types. Groups' of cows and c a l v e s were l a r g e r i n the the "rainy seasons, and i n years with higher r a i n f a l l , and smaller i n low r a i n f a l l seasons and y e a r s . Bachelor b u l l groups showed some s i m i l a r p a t t e r n s but group s i z e s and the range of v a r i a t i o n were always f a i r l y s m a l l . There was an apparent s h i f t by the l a r g e r cow/calf groups to the high biomass h a b i t a t s , and p o s s i b l y more open h a b i t a t s , with changes in p l a n t abundance in the dry season. The number of l a r g e groups was not c o r r e l a t e d with elephant d e n s i t y i n h a b i t a t types. T h i s supports the h y p o t h e s i s that l a r g e s o c i a l group s i z e i s maintained by h a b i t a t s e l e c t i o n , r a t h e r than random encounters of groups. The l a r g e r bachelor b u l l groups d i d not s h i f t h a b i t a t s i n t h i s way. Although t h e i r h a b i t a t use p a t t e r n s were s i m i l a r o v e r a l l , bachelor b u l l s used the wet swamps and open woodlands more in i v the dry season while cow/calf herds p r e f e r r e d the open woodlands more i n the wet seasons and swamp edge g r a s s l a n d s more i n the dry seasons. E x p l a n a t i o n s of these d i f f e r e n c e s r e l a t i n g to p h y s i o l o g i c a l and s o c i a l d i f f e r e n c e s between the gender c l a s s e s were o f f e r e d . Elephant h a b i t a t s e l e c t i o n s t r a t e g y appears to c o n s i s t of a " s t a b l e element" centered on the p r e d i c t a b l e dry season h a b i t a t s and an " o p p o r t u n i s t i c element" i n l e s s p r e d i c t a b l e , but more p r o f i t a b l e , wet season h a b i t a t s . V TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES ix LIST OF FIGURES x i i ACKNOWLEDGEMENTS xv Chapter 1 INTRODUCTION 1 Chapter 2 THE STUDY AREA 11 A. I n t r o d u c t i o n 11 B. Ecosystem S t r u c t u r e and Seasonal Dynamics 11 1. Geology, s o i l s , and hydrology 13 2. Climate 14 1 3. Ve g e t a t i o n 16 4. Fauna 17" 5. The elephants 18 C. Long Term Changes 19 D. Human I n f l u e n c e s 21 E. Context of the Study 23 Chapter 3 HABITAT STRUCTURE AND DYNAMICS 24 A. I n t r o d u c t i o n 24 B. Methods 27 1. R a i n f a l l records 28 2. D e f i n i t i o n and mapping of h a b i t a t types 28 3. H a b i t a t d e s c r i p t i o n and m o n i t o r i n g : Herb l a y e r ... 29 4. Ha b i t a t d e s c r i p t i o n : Woody v e g e t a t i o n .„ 33 C. R e s u l t s 37 v i 1. R a i n f a l l p a t t e r n s 37 2. H a b i t a t types d e f i n e d , mapped, and d e s c r i b e d 42 3. Dynamics of the herb l a y e r i n d i f f e r e n t h a b i t a t types 53 4. Herb l a y e r n u t r i e n t dynamics 62 D. D i s c u s s i o n 62 Chapter 4 ELEPHANT HABITAT USE AND PREFERENCE 66 A. I n t r o d u c t i o n 66 B. Methods 68 1. Counting methods 70 a. Ground counts , 70 b. A e r i a l counts 76 2. Analyses 77 • a. C o r r e c t i o n s f o r b i a s and mis s i n g counts 78 b. Comparison of ground and a e r i a l counts 81 c. Large s c a l e h a b i t a t p r e f e r e n c e 82 .d. H a b i t a t use and pr e f e r e n c e i n the Amboseli b a s i n , i n r e l a t i o n to v e g e t a t i o n and c l i m a t i c v a r i a b l e s 83 C. R e s u l t s ." 84 1. Comparison of ground and a e r i a l counts 84 2. Large s c a l e h a b i t a t c h o i c e . 85 a. Bachelor b u l l s 85 b. Cows and c a l v e s 91 3. H a b i t a t use and preference i n the Amboseli b a s i n . 96 a. Bachelor b u l l s 98 b. Cows and c a l v e s 103 v i i 4. H a b i t a t use i n r e l a t i o n to c l i m a t e and v e g e t a t i o n 113 a. Bachelor b u l l s 113 b. Cows and c a l v e s . .. 118 5. Observations of i n d i v i d u a l movements 123 D. D i s c u s s i o n 125 1. General p a t t e r n of seasonal h a b i t a t p r e f e r e n c e ...126 2. Departures from the general p a t t e r n ...131 Chapter 5 ELEPHANT SOCIAL GROUP DYNAMICS AND HABITAT USE .134 A. I n t r o d u c t i o n 134 B. Methods 1 38 1 . Analyses 139 a. C o r r e c t i o n s f o r b i a s in ground count data ....139 b. Group s i z e d i s t r i b u t i o n 139 C. R e s u l t s 141 1. Group s i z e d i s t r i b u t i o n in the Amboseli basin ....141 a. Bachelor b u l l groups 141 b. Cow/calf groups 144 2 . i H a b i t a t d i s t r i b u t i o n of elephant group s i z e 153 a. Bachelor b u l l groups 153 b. Cow/calf groups 155 D. D i s c u s s i o n 1 62 1. Bachelor b u l l groups 162 2. Cow/calf groups 163 Chapter 6 GENERAL DISCUSSION AND CONCLUSIONS 168 A. General H a b i t a t S e l e c t i o n P a t t e r n of the Amboseli Elephants . 168 B. F a c t o r s Modifying the General P a t t e r n 170 v i i i 1. The e f f e c t s of group formation 171 2. Gender d i f f e r e n c e s i n h a b i t a t s e l e c t i o n ...176 3. S p e c i a l food types 178 C. Some Management I m p l i c a t i o n s 179 D. Questions f o r Furth e r Study 181 REFERENCES CITED '..183 APPENDIX 1 195 APPENDIX 2 ..' 1 96 APPENDIX 3 1 97 APPENDIX 4 1 99 LIST OF TABLES Table 1. R a i n f a l l at v a r i o u s s t a t i o n s around.Amboseli 41 Table 2. Amboseli h a b i t a t types: areas and v e g e t a t i o n p l o t d i s t r i b u t i o n 4.2 Table 3. Herb l a y e r growth and r a i n f a l l .. . 61 Table 4. Areas of h a b i t a t s surveyed 74 Table 5. C o r r e l a t i o n of b u l l and cow/calf numbers i n a e r i a l and ground counts 85 Table 6. Goodness of f i t c h i squared values of observed b u l l h a b i t a t d i s t r i b u t i o n s vs. an expected random d i s t r i b u t i o n ...102 Table 7. Goodness of f i t c h i squared values of observed cow/calf h a b i t a t d i s t r i b u t i o n s vs. an expected random d i s t r i b u t i o n 109 Table 8. C o r r e l a t i o n of bachelor b u l l and cow/calf h a b i t a t d i s t r i b u t i o n s 109 Table 9. C o r r e l a t i o n s of b u l l d e n s i t i e s with h a b i t a t c h a r a c t e r i s t i c s 117 Table 10. C o r r e l a t i o n s of cow/calf d e n s i t i e s with h a b i t a t c h a r a c t e r i s t i c s 122 Table 11. Ranks of bachelor b u l l MGS ac r o s s h a b i t a t types i n each season 153 Table 12. C o r r e l a t i o n of bachelor b u l l d e n s i t y with MGS i n h a b i t a t types 155 Table 13. Rankings of % cow/calf groups > 25 and > 50 X a c r o s s h a b i t a t types i n each season 158 Table 14. C o r r e l a t i o n s of % cow/calf groups > 25 and > 50 with herb l a y e r biomass and % woody cover 160 Table 15. C o r r e l a t i o n s of cow/calf d e n s i t y with MGS, % groups >25, and % groups >50 161 x i LIST OF FIGURES F i g u r e 1. Map showing the l o c a t i o n and some f e a t u r e s of the Amboseli ecosystem 12 F i g u r e 2. Monthly r a i n f a l l p a t t e r n s i n 1978/79 at 01 Tukai and s i x other s t a t i o n s in the surrounding bushlands .... 38 F i g u r e 3. Mean monthly r a i n f a l l at 01 Tukai i n three p e r i o d s : Dry1, Wet, and Dry2 40 F i g u r e 4. Map of the h a b i t a t types of the Amboseli basin and nearby bushland areas 43 F i g u r e 5. Standing biomass of the woody and herbaceous v e g e t a t i o n i n the h a b i t a t types of Amboseli in January 1979 51 F i g u r e 6. D i v e r s i t y of a v a i l a b l e p l a n t biomass in the woody and herb l a y e r s of Amboseli h a b i t a t types i n January 1979 54 F i g u r e 7. Dynamics of t o t a l biomass and green biomass of the herb l a y e r of Amboseli b a s i n h a b i t a t s i n 1978/79 ... 55 F i g u r e 8. Dynamics of t o t a l biomass and green biomass of the herb l a y e r of Amboseli bushland h a b i t a t s i n 1978/79 56 F i g u r e 9. Long term dynamics of t o t a l herb l a y e r biomass i n some Amboseli h a b i t a t types 59 F i g u r e 10. Dynamics of herb l a y e r p r o t e i n content i n some Amboseli h a b i t a t types i n 1977 63 F i g u r e 11. Routes of ground-based surveys of the Amboseli basin 71 F i g u r e 12. Seasonal means of monthly t o t a l s of bachelor b u l l s seen i n ground counts of the Amboseli basin i n 1978/79 86 F i g u r e 13. Seasonal means of monthly t o t a l numbers of b u l l s a s s o c i a t i n g with cow/calf herds 88 F i g u r e 14. Seasonal mean numbers of bachelor b u l l s seen i n a e r i a l counts of the Amboseli basin i n three p e r i o d s ... 89 F i g u r e 15. Ln Bachelor b u l l numbers in a e r i a l counts vs. Ln Cumulative r a i n f a l l over the preceding 90 days i n Dry1 p e r i o d 92 F i g u r e 16. Seasonal means of monthly t o t a l s of cows and c a l v e s seen i n ground counts of the Amboseli basin i n 1978/79 93 F i g u r e 17. Seasonal mean numbers of cows and c a l v e s seen i n a e r i a l counts" of the Amboseli b a s i n i n the three r a i n f a l l p e r i o d s 94 F i g u r e 18. Ln Cow/calf numbers in a e r i a l counts vs. Ln Cumulative r a i n f a l l t o t a l over the preceding 15 days i n Dry1 p e r i o d 97 F i g u r e 19. T o t a l numbers of bachelor b u l l s seen i n each h a b i t a t type i n ground counts of the Amboseli basin i n 1978/79 99 Fi g u r e 20. Y e a r l y mean count d e n s i t i e s of bachelor b u l l s i n Amboseli b a s i n h a b i t a t types in 1978/79 101 Fi g u r e 21. Seasonal mean d e n s i t i e s of bachelor b u l l s i n h a b i t a t types as seen i n a e r i a l counts d u r i n g the three r a i n f a l l p e r i o d s 104 x i i i F i g u r e 22. T o t a l numbers of cows and c a l v e s seen i n each h a b i t a t type d u r i n g ground counts i n each season of 1978/79 106 Fi g u r e 23. Y e a r l y mean of monthly count d e n s i t i e s of cows and c a l v e s i n h a b i t a t types i n 1978/79 108 F i g u r e 24. Seasonal mean d e n s i t i e s of cows and c a l v e s i n h a b i t a t types as seen i n a e r i a l counts dur i n g the three r a i n f a l l p e r i o d s 111 F i g u r e 25. L i n e a r r e g r e s s i o n s of monthly t o t a l numbers of bachelor b u l l s in h a b i t a t types vs. t o t a l r a i n f a l l of the preceding month in 1978/79 114 Fi g u r e 26. L i n e a r r e g r e s s i o n s of bachelor b u l l monthly t o t a l s i n h a b i t a t types vs. t o t a l herb l a y e r biomass in the middle of the c u r r e n t month * 115 Fi g u r e 27. L i n e a r r e g r e s s i o n s of bachelor b u l l monthly t o t a l s i n h a b i t a t types v s . green herb l a y e r biomass mid-month 116 F i g u r e 28. L i n e a r r e g r e s s i o n s of cow/calf monthly t o t a l s i n h a b i t a t types vs. t o t a l r a i n f a l l i n the preceding month 119 Fig u r e 29. L i n e a r r e g r e s s i o n s of cow/calf monthly t o t a l s in h a b i t a t types vs. t o t a l herb l a y e r biomass in the middle of the c u r r e n t month .120 Fi g u r e 30. L i n e a r r e g r e s s i o n s of cow/calf monthly t o t a l s i n h a b i t a t types vs. green herb l a y e r biomass mid-month ...121 F i g u r e 31. Cumulative frequency d i s t r i b u t i o n of bachelor b u l l group s i z e i n the Amboseli b a s i n i n each season of 1978/79 142 x i v F i g u r e 32. Bachelor b u l l mean group s i z e , and group s i z e s which i n c l u d e d 50% and 90% of a l l b u l l s s i g h t e d , i n each season of 1978/79 1 43 Fi g u r e 33. Bachelor b u l l seasonal mean group s i z e i n a e r i a l counts d u r i n g the three r a i n f a l l p e r i o d s ..145 Fi g u r e 34. Cumulative frequency d i s t r i b u t i o n of cow/calf group s i z e i n each season of 1978/79 147 Fi g u r e 35. Cow/calf mean group s i z e , and group s i z e s which i n c l u d e d 50% and 90% of a l l cows and c a l v e s s i g h t e d , i n each season of 1978/79 149 Fi g u r e 36. Cow/calf seasonal mean group s i z e in a e r i a l counts d u r i n g the three r a i n f a l l p e r i o d s 150 Fi g u r e 37. The percent of cow/calf groups >25 and . >50, and the mean number of groups seen per count, i n each season of 1978/79 : 152 Fi g u r e 38. .Yearly mean s i z e of bachelor b u l l groups in h a b i t a t types in 1978/79 154 F i g u r e 39. Y e a r l y mean s i z e of cow/calf groups i n h a b i t a t types i n 1 978/79 156 Fi g u r e 40. Schematic diagram of the seasonal h a b i t a t d i s t r i b u t i o n of cow/calf groups which had >50 i n d i v i d u a l s ., 159 XV ACKNOWLEDGEMENTS Many people have helped me i n my f i e l d work in Kenya and t h e s i s p r e p a r a t i o n here at UBC, and I would l i k e to thank them very much. Harvey Croze introduced me to the study of elephant f o r a g i n g as part of the r e s e a r c h p r o j e c t which he began in Amboseli. He gave me u s e f u l a d v i c e , c o n t a c t s , and a p l a c e to stay, and I am very g r a t e f u l f o r h i s h e l p . I wish to thank David Western for the o p p o r t u n i t y to work with him i n Amboseli and on the elephant study. He p r o v i d e d me with a house to l i v e i n , a v e h i c l e to d r i v e , and the funds to run my f i e l d study, as w e l l as the chance to observe the workings of the savannah. He a l s o gave me u s e f u l s u p e r v i s i o n , a d v i c e , and access to h i s unpublished data on elephant d i s t r i b u t i o n s and v e g e t a t i o n m o n i t o r i n g . Cynthia Moss, my f e l l o w elephant researcher i n Amboseli, helped me in many ways, and she has my s i n c e r e a p p r e c i a t i o n . She l e t me see her data on elephant i d e n t i f i c a t i o n and s o c i a l o r g a n i z a t i o n , and we had many u s e f u l d i s c u s s i o n s on the behaviour of elephants, both a d u l t females and t u s k e r s . Stephen Cobb g r a c i o u s l y acted as my Research A s s o c i a t e at the U n i v e r s i t y of N a i r o b i , and was g e n e r a l l y encouraging. My f r i e n d and r e s e a r c h a s s i s t a n t , David Kamiti Maitumo, was enormously h e l p f u l , as he i n t r o d u c e d me to the Amboseli f l o r a and fauna, as w e l l to other f r i e n d s among the Maasai. Michael Milgroom, my co-worker and successor i n Amboseli, helped me c o l l e c t data on the elephants and v e g e t a t i o n , and made u s e f u l x v i suggestions about my r e s e a r c h . I thank these two i n d i v i d u a l s f o r making the f i e l d work a memorable part of my l i f e . I would a l s o l i k e to thank the people of 01 Tukai f o r s h a r i n g t h e i r f r i e n d s h i p with me. The wardens of Amboseli dur i n g my stay, Joseph Kioko and, l a t e r , Bob Oguya, were f r i e n d l y , c o o p e r a t i v e , and t o l e r a n t of my a c t i v i t i e s i n t h e i r park; I hope that they b e n e f i t t e d as much from my work as I d i d from t h e i r s . The s t a f f of the Research S e c t i o n of the W i l d l i f e Conservation and Management Department (as i t was at the time of my study), of the Kenya Government, notably Mr. Simon T a i t i , were a l s o c o o p e r a t i v e and h e l p f u l . I would l i k e to thank them, the O f f i c e of the P r e s i d e n t , and the Olkejuado County C o u n c i l f o r t h e i r permission to conduct f i e l d r e s e a r c h i n Amboseli. My s u p e r v i s o r , A.R.E. S i n c l a i r , o f f e r e d c o n s i d e r a b l e support, s u p e r v i s i o n , and h e l p f u l comment in my data a n a l y s i s , w r i t i n g , and seminar p r e p a r a t i o n . I thank him f o r a c c e p t i n g me as a graduate student, through the m a i l s , and f o r h i s a t t e n t i o n to my work. I r e c e i v e d u s e f u l comments on my work and seminars from C.J. Krebs, D. Shackleton, N.R. L i l e y , F. B u n n e l l , and J.N.M. Smith. Fellow graduate students were h e l p f u l i n d i s c u s s i n g my ideas and i n m a i n t a i n i n g my s a n i t y and s i x t h sense i n t a c t . P h y l l i s Lee was of i n e s t i m a b l e h e l p d u r i n g a l l the stages of t h e s i s p r e p a r a t i o n . She helped me in counting elephants, gave me u s e f u l p r a c t i c e i n the zen/art of Suzuki maintenance, and improved my a p p r e c i a t i o n of the primate l i t e r a t u r e and of x v i i s i n g l e malts. She a l s o made v a l u a b l e comments on d r a f t s of the t h e s i s and prepared most of the f i g u r e s . Her help and support p r o v i d e d a major focus i n the long process of t h e s i s b u i l d i n g . F i n a l l y , I would l i k e to thank my f a m i l y and f r i e n d s , both here and i n Kenya, for the p e r s o n a l support which has kept wind in my s a i l s d u r i n g the storms and doldrums. The f i e l d work was supported by the New York Z o o l o g i c a l S o c i e t y , and my M.Sc. programme at ^U.B.C. was supported by an NSERC Postgraduate S c h o l a r s h i p and a U.B.C. t e a c h i n g a s s i s t a n t s h i p . 1 CHAPTER 1 INTRODUCTION Elephants are dominant h e r b i v o r e s i n many savannah ecosystems of East A f r i c a , and can cause s i g n i f i c a n t changes i n the v e g e t a t i o n by t h e i r feeding a c t i v i t i e s (Laws 1970). T h i s study i s concerned with one aspect of the f o r a g i n g process i n A f r i c a n elephants, t h e i r s e l e c t i o n of feed i n g h a b i t a t s . The s e t t i n g of my study was the s e m i - a r i d Amboseli region of southern Kenya. T h i s work i s an i n i t i a l p art of a broader study of the f o r a g i n g behaviour of elephants i n r e l a t i o n to t h e i r b i o t i c and s o c i a l environments, and i t s p o s s i b l e a p p l i c a t i o n to p o p u l a t i o n l e v e l p r o c e s s e s . The i n t e r a c t i o n of elephant p o p u l a t i o n s with t h e i r h a b i t a t s in e ast, c e n t r a l , and southern A f r i c a has been the. subject of d i s c u s s i o n and co n t r o v e r s y i n s c i e n t i f i c and management c i r c l e s f o r many years (Agnew 1968, Beuchner and Dawkins 1970, Caughley 1976, Glover 1963, Hanks 1979, Laws 1970, Laws et a l . 1975, van Wyk 1969). The c o n t r o v e r s y c e n t e r s on the apparent damage that elephants i n f l i c t on woody v e g e t a t i o n . A c e n t r a l q u e s t i o n concerns the nature of p o p u l a t i o n r e g u l a t i o n by elephants: are elephant p o p u l a t i o n s capable of s e l f - r e g u l a t i o n , or i s there need f o r management i n t e r v e n t i o n to r s a v e " the elephants or t h e i r h a b i t a t s from e x t i n c t i o n ? Some r e s e a r c h e r s (notably Laws et a l . 1975) have concluded that woody p l a n t s are the c r i t i c a l dry p e r i o d food source of 2 elephants, and that elephant p o p u l a t i o n s at high d e n s i t y i n e v i t a b l y d e s t r o y t h i s resource through over-use. They argued that i n areas with l a r g e elephant p o p u l a t i o n s , the woodland d e s t r u c t i o n seen w i l l reduce the long term c a r r y i n g c a p a c i t y of the environment. Under t h i s model, the growth of most l a r g e A f r i c a n elephant p o p u l a t i o n s was being l i m i t e d by d e n s i t y - dependent feedback mechanisms o p e r a t i n g through age of females at puberty and c a l v i n g i n t e r v a l (Laws 1969), but these were a c t i n g too slowly to prevent elephant p o p u l a t i o n crashes and i r r e v e r s i b l e h a b i t a t damage. Elephant c u l l i n g was p r e s c r i b e d f o r a l l elephant p o p u l a t i o n s , with some areas r e q u i r i n g urgent a c t i o n . Other s c i e n t i s t s (Caughley 1976, Cobb 1980, H a r r i s and Fowler 1975, N o r t o n - G r i f f i t h s 1979) f e l t that, the t r e e / e l e p h a n t dynamics may i n c l u d e more components, and c o u l d p o s s i b l y i n v o l v e long term p l a n t / h e r b i v o r e c y c l e s , c l i m a t i c c y c l e s , or other i n t e r a c t i o n s d r i v i n g the systems in l e s s s t r a i g h t f o r w a r d ways. N o r t o n - G r i f f i t h s (1979) suggested that the dynamics of p l a n t / e l e p h a n t systems may be complex, and s p e c i f i c to l o c a l c o n d i t i o n s of c l i m a t e and h a b i t a t composition. I t seems l i k e l y that h a b i t a t modifying f a c t o r s such as the presence of other l a r g e h e r b i v o r e s , the extent and t i m i n g of rangeland f i r e s , and the e f f e c t s of other human a c t i v i t i e s are a l s o important i n the ecosystem dynamics. For example, in Uganda, where annual r a i n f a l l i s r e l a t i v e l y high, elephants which consumed slow- growing woody v e g e t a t i o n had a l t e r n a t e food a v a i l a b l e i n the abundant herb l a y e r that c o u l d c a r r y them through p e r i o d s of low 3 woody cover. Ugandan elephant p o p u l a t i o n s had s i g n i f i c a n t l y reduced woody cover i n t h e i r h a b i t a t s , and density-dependent mechanisms were apparently a c t i n g slowly to reduce elephant p o p u l a t i o n growth (Laws 1970). However, Laws f e l t that t h i s process was s t i l l too slow to prevent a p o p u l a t i o n c r a s h and h a b i t a t d e s t r u c t i o n . N o r t o n - G r i f f i t h s (1979) p r e d i c t e d a d i f f e r e n t outcome: slow d e c l i n e i n elephant numbers with the v e g e t a t i o n maintained in an open g r a s s l a n d s t a t e f o r a long p e r i o d u n t i l woodland r e g e n e r a t i o n c o u l d occur at low elephant d e n s i t i e s . The p o p u l a t i o n consequences c o u l d be more abrupt, but not n e c e s s a r i l y c a t a s t r o p h i c , i n a system where l i t t l e a l t e r n a t e food was a v a i l a b l e i n the herb l a y e r . In the s e m i - a r i d Tsavo ecosystem of c e n t r a l Kenya, the herb l a y e r i s r e l a t i v e l y sparse and the woody v e g e t a t i o n much reduced by heavy elephant use; l a r g e - s c a l e m o r t a l i t y of s e v e r a l thousand a d u l t females and t h e i r young occurred i n the 1971 droughts ( C o r f i e l d 1973), which r e s u l t e d i n a sharp decrease i n the p o p u l a t i o n s i z e . In order to understand the workings of s p e c i f i c e l e p h a n t / h a b i t a t systems, and of elephant p o p u l a t i o n s i n g e n e r a l , we need more i n f o r m a t i o n on the nature of elephant f o r a g i n g p a t t e r n s and p o p u l a t i o n dynamics in areas of d i f f e r e n t h a b i t a t s t r u c t u r e and composition, and of the dynamics of the v e g e t a t i o n in those areas. As yet, there are few data a v a i l a b l e on the nature of e l e p h a n t - h a b i t a t i n t e r a c t i o n s . Feeding s t u d i e s ~ t h a t have been done (Buss 1961, F i e l d 1971, F i e l d and Ross 1976, Guy 1976b) have not c o n s i d e r e d the r e l a t i v e abundance of 4 d i f f e r e n t h a b i t a t types, or food types w i t h i n h a b i t a t s , and t h e i r r e l a t i v e use. Most workers r e p o r t a f o r a g i n g s h i f t from g r a z i n g i n the r a i n y seasons to browsing on woody v e g e t a t i o n i n the dry season, and c i t e a necdotal evidence that woody p l a n t s are of higher q u a l i t y i n the l a t e dry p e r i o d . However, there have been few analyses of the p l a n t p a r t s consumed and avoided by elephants to document these statements, and no evidence to show the r e l a t i v e abundances of grass and browse at d i f f e r e n t times of year, or between d i f f e r e n t h a b i t a t s . For example, Guy (1976b) r e p o r t e d that elephants i n Zimbabwe browsed on shrubs more, but spent l e s s t o t a l time i n fe e d i n g a c t i v i t y , than d i d Ugandan elephants (as rep o r t e d by Wyatt and E l t r i n g h a m (1974)). He suggested that t h i s was due to h a b i t a t d i f f e r e n c e s between the study areas, but o f f e r e d no comparative v e g e t a t i o n data to t e s t t h i s h y p o t h e s i s . S i m i l a r l y , most of the surveys of elephant numbers and d i s t r i b u t i o n s (Beuchner et a l . 1963, Caughley and Goddard 1975, Eltr i n g h a m 1977, Laws et a l . 1975, Watson et a l . 1969) d i d not ev a l u a t e t h e i r r e s u l t s with r e f e r e n c e to q u a n t i t a t i v e estimates of p l a n t abundance (but see Cobb (1976)). Most vegetation-based s t u d i e s of woodland use by elephants (Anderson and Walker 1974, Laws et a l . 1975, van Wyk 1969) were not done with r e f e r e n c e to elephant h a b i t a t use and food s e l e c t i o n p a t t e r n s . E v a l u a t i o n of the r e s u l t s of such s t u d i e s , and comparison between r e s u l t s from d i f f e r e n t areas and time p e r i o d s i s t h e r e f o r e d i f f i c u l t , i f not i m p o s s i b l e . In the cases where some of these f a c t o r s have been c o n s i d e r e d , as by Croze 5 (1974a,b), the c o n c l u s i o n i s o f t e n that elephant use of woody v e g e t a t i o n i s not the u l t i m a t e cause of woodland d e c l i n e . The f o r a g i n g behaviour and h a b i t a t use s t r a t e g i e s of elephants are i n t e r e s t i n g from a t h e o r e t i c a l viewpoint, as w e l l as from the conservation/management p e r s p e c t i v e . As the l a r g e s t of present-day t e r r e s t r i a l animals, elephants represent an extreme example of the l a r g e g e n e r a l i s t h e r b i v o r e . Much of the f i e l d work in f o r a g i n g theory u n t i l f a i r l y r e c e n t l y has focussed on small p r e d a t o r s , and l a r g e h e r b i v o r e s have been n e g l e c t e d . An a d d i t i o n a l dimension in f o r a g i n g behaviour i s the i n t e r a c t i o n of the s o c i a l and p h y s i c a l environments to produce compromises in f o r a g i n g a c t i v i t y . Elephants have a s t r u c t u r e d and dynamic s o c i a l o r g a n i z a t i o n (Douglas-Hamilton 1972, Moss 1981), which should p r o v i d e an o p p o r t u n i t y to examine s o c i a l e f f e c t s on, and consequences of f o r a g i n g . With a few exceptions (Belovsky 1978, Glander 1981), most resear c h i n f o r a g i n g theory has been done with p r e d a t o r s (Krebs et a l . 1981), seed-eaters (Reichman 1981), or n e c t a r - f e e d i n g s p e c i a l i s t s (Gass and Montgomerie 1981). Many of the f i n d i n g s in t h i s area are c u r r e n t l y too simple or a r t i f i c i a l f o r a p p l i c a t i o n to w i l d p o p u l a t i o n s of g e n e r a l i s t h e r b i v o r e s , which may need to o b t a i n a balance of n u t r i e n t s or a v o i d t o x i c compounds in t h e i r c h o ice of a d i e t ( F r e e l a n d and Janzen 1974, Westoby 1974). E m p i r i c a l r e s u l t s from f i e l d s t u d i e s of l a r g e h e r b i v o r e s are c l e a r l y needed to improve e x i s t i n g models, or to r e p l a c e them, i f necessary, with a d i f f e r e n t approach. A h i e r a r c h i c a l approach may be the most reasonable way to 6 approach the study of f o r a g i n g processes (Dawkins 1976, Gass and Montgomerie 1981, Johnson 1980). For example, a given animal s p e c i e s i s d i s t r i b u t e d a c r o s s i t s g e o g r a p h i c a l range, a s p e c i f i c p o p u l a t i o n of the spe c i e s occupies a region of land, i n d i v i d u a l members of the po p u l a t i o n have home ranges, and w i t h i n those home ranges, choose h a b i t a t s i n which to spend time. For a f o r a g i n g h e r b i v o r e , there are f u r t h e r l e v e l s of c h o i c e : what food type to s e l e c t , which i n d i v i d u a l p l a n t s p e c i e s , and what s p e c i f i c p l a n t p a r t s . These c h o i c e s may be made on the b a s i s of r e l a t i v e " p r o f i t a b i l i t y " r a t e of n u t r i e n t intake per u n i t c o s t -- of food items (Krebs 1978), and of the r e l a t i v e abundance and d i s p e r s i o n of p r o f i t a b l e food items i n d i f f e r e n t h a b i t a t patches, which may change s e a s o n a l l y i n p r e d i c t a b l e or u n p r e d i c t a b l e ways. Most of the models which have been developed i n the theory of h a b i t a t "patch" s e l e c t i o n (Charnov 1976, Lev i n s 1968, Rosenzweig 1979), i n v o l v e o p t i m a l i t y c r i t e r i a , and o f t e n deal with c o e x i s t i n g c o m p e t i t i v e s p e c i e s . The models developed are s i m p l i s t i c and ignore p o t e n t i a l l y c r i t i c a l c o m p l i c a t i n g f a c t o r s such as l e a r n i n g and sampling other h a b i t a t s (Werner et a l . 1981), the r i s k of p r e d a t i o n , or s o c i a l i n t e r a c t i o n s (Myers et a l . 1981). F r e t w e l l and Lucas (1970) put forward a theory of h a b i t a t s e l e c t i o n based on the " s u i t a b i l i t y " of h a b i t a t types. They d e f i n e d " s u i t a b i l i t y " as the a v a i l a b i l i t y of resources such as food or mating o p p o r t u n i t i e s i n a given h a b i t a t , r e l a t i v e to the d e n s i t y .of animals i n that h a b i t a t and i n the p o p u l a t i o n . The theory p r e d i c t s that h a b i t a t s w i l l be c o l o n i z e d i n order of 7 s u i t a b i l i t y , with s u i t a b i l i t y d e c l i n i n g as the animal d e n s i t y i n c r e a s e s i n each h a b i t a t i n t u r n . The p r e d i c t i o n that h a b i t a t s are c o l o n i z e d i n order of s u i t a b i l i t y with i n c r e a s i n g p o p u l a t i o n d e n s i t y was t e s t e d and supported f o r minnows by F r a s e r and S i s e (1980). They suggested that food abundance might be the resource which d e f i n e d h a b i t a t s u i t a b i l i t y f o r the minnows. In t h i s study, I w i l l examine food a v a i l a b i l i t y as a major cue f o r elephant h a b i t a t s e l e c t i o n . Elephants have a l a r g e a b s o l u t e requirement f o r food, estimated at about s i x percent of t h e i r body weight per day (Laws et a l . 1975), and are bulk feeders which need to process l a r g e q u a n t i t i e s of food -- about 150 to 200 kg, f r e s h weight, per day (Guy 1975). They feed f o r l a r g e p a r t s of the day -- some 50 to 75 % of the t o t a l day le n g t h (Guy 1976a, Wyatt and Elt r i n g h a m 1974). I w i l l ask two q u e s t i o n s : which h a b i t a t patches ( v e g e t a t i o n communities) are chosen, and why those p a r t i c u l a r ones? My proposed e x p l a n a t i o n i s that r e l a t i v e food a v a i l a b i l i t y i s of c e n t r a l importance i n elephant h a b i t a t s e l e c t i o n . S o c i a l c o n s i d e r a t i o n s c o u l d p o s s i b l y a f f e c t the use of h a b i t a t s by elephants. As Alexander (1974) noted, group membership g e n e r a l l y c o n f e r s an automatic cost of feeding or r e p r o d u c t i v e c o m p e t i t i o n to i n d i v i d u a l members. For an i n d i v i d u a l , the b e n e f i t s d e r i v e d from j o i n i n g a group -- f o r example, s a f e t y from p r e d a t o r s , i n c r e a s e d r e p r o d u c t i v e success, improved f o r a g i n g a b i l i t y -- must outweigh these c o s t s . Rubens'tein'.„•( 1.978) reviewed t h e o r i e s and e m p i r i c a l evidence of the c o s t s and b e n e f i t s of group l i v i n g . In some of the examples 8 he d i s c u s s e d ( l a r g e l y d e a l i n g with b i r d s ) , f o r a g i n g success was improved by s o c i a l grouping, and l a r g e r groups formed i n p e r i o d s of food s c a r c i t y or under c e r t a i n types of resource p a t c h i n e s s . However, i n l a r g e A f r i c a n ungulates, bigger s o c i a l groups g e n e r a l l y form when food resources are widespread and abundant (Leuthold 1977a). It would appear that competition i n f e e d i n g — e i t h e r i n t e r f e r e n c e competition and s u p p l a n t a t i o n from l o c a l i z e d food sources, or e x p l o i t a t i o n competition by neighbouring group members (Post 1981) -- c o u l d be an important f a c t o r i n causing s o c i a l groups to s p l i t up d u r i n g food s c a r c i t y or p a t c h i n e s s . As w e l l , when a group i s feeding on clumped food items or i n dense cover, i t may be d i f f i c u l t f o r i n d i v i d u a l s to maintain c o n t a c t with other group members, or- to keep up with the group's movement. I t i s p o s s i b l e that in some l a r g e h e r b i v o r e s , feeding h a b i t a t s are chosen so that s o c i a l groups w i l l not be f o r c e d to break up (Jarman and Jarman 1979). In t h i s case, other b e n e f i t s of group l i v i n g , such as p r o t e c t i o n from p r e d a t i o n , w i l l be r e t a i n e d and the co s t s of f e e d i n g competition reduced. Elephants have complex l e v e l s of s o c i a l i n t e r a c t i o n between i n d i v i d u a l s and d i f f e r e n t degrees of s o c i a l grouping, from small bachelor b u l l groups and m a t r i l i n e a l f a m i l y u n i t s of females and j u v e n i l e s , to l a r g e aggregations (Douglas-Hamilton 1972, Moss 1981). The tendency to group i s s e a s o n a l l y dynamic: Leuthold (1976) found that group s i z e i n elephants was p o s i t i v e l y r e l a t e d to r a i n f a l l , as were t h e i r movement p a t t e r n s (Leuthold and Sale 1973). I suggest that the tendency of elephants to form s o c i a l 9 groups i s important i n modifying t h e i r h a b i t a t s e l e c t i o n . The d i f f e r e n c e s i n s i z e , r e p r o d u c t i v e b i o l o g y , and s o c i a l o r g a n i z a t i o n between male and female elephants suggests that there c o u l d be d i f f e r e n c e s i n t h e i r r e s p e c t i v e use of h a b i t a t s . C l u tton-Brock et a l . ( i n press) demonstrate that the s u r v i v a l and r e p r o d u c t i v e s t r a t e g i e s of male and female red deer are markedly d i f f e r e n t and that t h i s r e s u l t s in observable d i f f e r e n c e s i n f o r a g i n g p a t t e r n s . D i f f e r e n c e s between the h a b i t a t p r e f e r e n c e p a t t e r n s shown by bachelor b u l l and f e m a l e / j u v e n i l e herds of A f r i c a n b u f f a l o were noted by S i n c l a i r (1977). Because of the strong sexual dimorphism in body s i z e and s o c i a l o r g a n i z a t i o n seen i n elephants, I expect that there should be s i m i l a r d i f f e r e n c e s in h a b i t a t s e l e c t i o n by b u l l and fa m i l y u n i t herds. To summarize, the main qu e s t i o n s that I ask in t h i s study ar e : 1. What h a b i t a t s are used by elephants? Which are s e l e c t e d and which avoided? 2. Are h a b i t a t c h o i c e s r e l a t e d to the abundance of herb l a y e r and/or woody l a y e r food in those habi t a t s ? 3. Do elephant f a m i l y u n i t s and l a r g e r aggregations s h i f t h a b i t a t use to maintain l a r g e group s i z e i n the face of changing food abundance? 4. Are there d i f f e r e n c e s i n h a b i t a t use between bachelor males and the m a t r i l i n e a l f a m i l y u n i t s ? My approach w i l l be e x p l o r a t o r y , but I w i l l use general 10 hypotheses as a framework f o r my a n a l y s e s . T h i s t h e s i s i s d i v i d e d i n t o s e c t i o n s which d e a l s e p a r a t e l y with the composition and dynamics of h a b i t a t types (Chapter 3), h a b i t a t use and s e l e c t i o n by elephants (Chapter 4), and the dynamics of elephant s o c i a l groups in r e l a t i o n to h a b i t a t use (Chapter 5). In Chapter 2, I w i l l d e s c r i b e the study area and cont e x t , while i n Chapter 6, I w i l l summarize and the main p o i n t s of the study and present my c o n c l u s i o n s and s p e c u l a t i o n s . 11 CHAPTER 2 THE STUDY AREA A. I n t r o d u c t i o n The study was conducted i n Amboseli N a t i o n a l Park and i t s environs i n s o u t h - c e n t r a l Kenya, East A f r i c a . The s t r u c t u r e , seasonal dynamics, and long term changes of the Amboseli ecosystem have been summarized by Western (1975) and Western and van Praet (1973), and d e s c r i b e d i n d e t a i l i n Western (1973). In t h i s chapter, I w i l l b r i e f l y d e s c r i b e the p h y s i c a l , c l i m a t i c , and b i o t i c c h a r a c t e r i s t i c s of the study area, with s p e c i a l r e f e r e n c e to the elephant p o p u l a t i o n . I w i l l conclude by d e f i n i n g the temporal and s p a t i a l s c a l e of the c u r r e n t study. B. Ecosystem S t r u c t u r e and Seasonal Dynamics Western (1973) s e t s the s p a t i a l boundaries of the Amboseli ecosystem at the e x t r e m i t i e s of l a r g e h e r b i v o r e ( c h i e f l y wildebeest) m i g r a t i o n s . T h i s area comprises an e c o l o g i c a l u n i t of some 3000 sq. km., i n c l u d i n g the c e n t r a l Amboseli b a s i n , which covers about 600 sq. km., and surrounding bushlands (see F i g u r e 1). The name "Amboseli" i s the Maasai word f o r " s a l t y s o i l " , which d e s c r i b e s the s a l i n e lake bed of the b a s i n . In t h i s t h e s i s , I w i l l f o l l o w Western's p r a c t i c e of a p p l y i n g the name to the e n t i r e system. The ecosystem as d e f i n e d l i e s between-2°15' and 3° S. l a t i t u d e and 36°50* and 37°45' E. l o n g i t u d e , and while the bulk of the area i s found w i t h i n Kenya, 12 Figure 1. Amboseli National Park and the surrounding bushlands. The major seasonal rivers are noted, as are the locations of ra infa l l stations. Principal swamps are indicated by shading. Contour lines represent intervals of approximately 65m., and the elevation of 01 Tukai is roughly 1170 m. 13 the southern l i m i t s of the system reach up the slopes of Mt. K i l i m a n j a r o , a few k i l o m e t r e s a c r o s s the border i n Tanzania. j_. Geology, s o i l s , and hydrology A v a r i e t y of g e o l o g i c a l and h y d r o l o g i c a l processes have shaped the p h y s i c a l and chemical s t r u c t u r e of the Amboseli landscape, producing a p a t t e r n of great d i v e r s i t y w i t h i n a f a i r l y small area (Western 1973). Precambrian metamorphic rock u n d e r l i e s the e n t i r e area, and has remained exposed to p h y s i c a l and chemical weathering at the s u r f a c e i n the north'and northwest. The s o i l s which developed in these areas are r e d d i s h , w e l l - d r a i n e d l a t e r i t e s on the h i l l s and r i d g e s , and dark, sandy c l a y s i n the drainage l i n e s . The K i l i m a n j a r o v o l c a n i c s emerged in the l a t e P l i o c e n e and e a r l y P l e i s t o c e n e , c r e a t i n g the mountain massif and ash f a l l s and b a s a l t i c l a v a flows a c r o s s the metamorphic s u r f a c e . V o l c a n i c - d e r i v e d s o i l s are e i t h e r w e l l - d r a i n e d dark c l a y s , o f t e n o v e r l a i n with l a v a boulders, or f i n e r drainage c l a y s ; both have a high mineral content. The K i l i m a n j a r o lava flows d i v e r t e d and then blocked a l a r g e , a n c i e n t r i v e r system, which formed Lake Amboseli as a c l o s e d drainage b a s i n , some 40 km. north of the mountain. The lake p e r i o d i c a l l y d r i e d and f l o o d e d u n t i l i t s f i n a l d e s i c c a t i o n at. the beginning of the Recent Epoch. Since that time, the former lake b a s i n has been dry, except f o r a low western p o r t i o n , the present "lake bed", that may h o l d . shallow s u r f a c e water d u r i n g p e r i o d s of high r a i n f a l l or high water t a b l e . The 1 4 basin has g r a d u a l l y accumulated sediments from both the v o l c a n i c and metamorphic systems, c r e a t i n g a complex p a t t e r n of a l l u v i a l and l a c u s t r i n e s o i l s . S a l i n e and a l k a l i n e lake bed s o i l s border the shallow a l k a l i n e s o i l s of the c e n t r a l p l a i n s , and deeper a l k a l i n e s o i l s , with s a l i n i t y i n c r e a s i n g towards the basin c e n t e r , are t r a n s i t i o n a l with the southern v o l c a n i c s . M i l d l y a l k a l i n e dark s o i l s have developed around permanent water sources. R i v e r s i n the region are seasonal; s u r f a c e water flows only d u r i n g and s h o r t l y a f t e r r a i n y p e r i o d s , but subsurface flow may continue f o r c o n s i d e r a b l y l o n g e r . A number of underground s p r i n g s from the K i l i m a n j a r o watershed emerge in the Amboseli ba s i n and i n i s o l a t e d bushland areas to the e a s t , and here water flows f o r short d i s t a n c e s a c r o s s the s u r f a c e on a more or l e s s continuous b a s i s . In r a i n y seasons, s u r f a c e water i s widespread, in drainage l i n e s and sumps, r i v e r s , and p o o l s , but the K i l i m a n j a r o s p r i n g s are the only n a t u r a l permanent water sources i n the e n t i r e ecosystem. Borehole development in the 1940's has provided a number of permanent water sources for l i v e s t o c k in the bushland ar e a s . 2 . C1i ma t e R a i n f a l l i s p o s s i b l y the s i n g l e most important c l i m a t i c f a c t o r i n t h i s s e m i - a r i d region ( P r a t t and Gwynne 1977). Annual r a i n f a l l recorded in the Amboseli basin averages 350 mm. or l e s s (Western 1975), although both the amount and timing of 1 5 r a i n f a l l can vary from year to year. T h i s regime i s t y p i c a l of l a r g e areas of c e n t r a l East A f r i c a ; Amboseli i s i n c l u d e d i n E c o l o g i c a l Zone V, Semi-Arid bushed g r a s s l a n d , by P r a t t , Greenway, and Gwynne (1966) in t h e i r East A f r i c a n rangeland c l a s s i f i c a t i o n . Most of the r a i n f a l l s between November and May; i n an "average" year there i s u s u a l l y a p e r i o d of low r a i n f a l l i n p a r t s of January, February, and March. T h i s "mid r a i n s " or "short dry season" separates the "short r a i n s " (November,December) from the "long r a i n s " (March,April,May). In wetter years, such as 1977 to 1979, the short dry season may not appear, and the short r a i n s , mid r a i n s , and long r a i n s merge i n t o one long wet season. A long dry season occurs from May or June u n t i l October . or November duri n g which l i t t l e or no r a i n f a l l s . East A f r i c a n r a i n f a l l p a t t e r n s are h i g h l y v a r i a b l e , and d i f f e r e n t authors have suggested regimes of r e g u l a r l y c y c l i c ( P h i l l i p s o n 1975, Wood and L o v e t t 1974), or merely random (Pennycuick and N o r t o n - G r i f f i t h s 1976) f l u c t u a t i o n . While the l a t t e r authors found that v a r i a n c e obscured a l l but annual, and p o s s i b l y , 5-year c y c l e s , in the r a i n f a l l of S e r e n g e t i ( i n northern T a n z a n i a ) , they suggested there might be a more widespread tendency i n East A f r i c a towards c y c l i c i t y i n s p e c i f i c l o c a l i t i e s . Anecdotal evidence suggests p o s s i b l e 5- and 10-year drought c y c l e s i n Amboseli, but records are too l i m i t e d to allow f i r m c o n c l u s i o n s . R a i n f a l l i s a l s o s p a t i a l l y v a r i a b l e . The slopes of 1 6 K i l i m a n j a r o r e c e i v e more r a i n than the lower e l e v a t i o n s ; the Amboseli b a s i n and nearby bushlands are in the "rainshadow" of K i l i m a n j a r o . As w e l l , rainstorms are t y p i c a l l y very l o c a l i z e d and one area may r e c e i v e over 70 mm. of r a i n i n one day while an area l e s s than a ki l o m e t r e away r e c e i v e s none (pers. obs.). 3_. V e g e t a t i o n A mosaic of v e g e t a t i o n communities has developed on the complex s u b s t r a t e of s o i l , hydrology, and r a i n f a l l p a t t e r n s of Amboseli. I w i l l d e s c r i b e the v e g e t a t i o n s t r u c t u r e and dynamics in g r e a t e r d e t a i l i n Chapter 3, but w i l l g ive a b r i e f i n t r o d u c t i o n here. Western (1975) recognized seven broad v e g e t a t i o n types i n Amboseli: lake bed and a l k a l i n e p l a i n s g r a s s l a n d s , sparse and dense woodlands, swamp, and sparse and dense bushlands. "Sparse" and "dense" are r e l a t i v e terms: "sparse" v e g e t a t i o n types have a woody cover of 1 - 2% or l e s s , while "dense" types g e n e r a l l y run from 5 - 20% woody cover. The g r a s s l a n d s , woodlands, and swamps are l a r g e l y r e s t r i c t e d to the Amboseli b a s i n , while bushland types occur on the surrounding r i d g e s and s l o p e s . Because of the high a l k a l i n i t y and/or s a l i n i t y of i t s s o i l s , the f l o r a of the Amboseli b a s i n i s f a i r l y simple. Grasses tend to be s a l t - t o l e r a n t p e r e n n i a l s , while the few woody spe c i e s are l a r g e l y evergreen shrubs or A c a c i a t r e e s . The bushlands, by c o n t r a s t , have g r e a t e r v a r i e t y in both herbaceous 1 7 and woody v e g e t a t i o n (Western 1975). The bushland pastures are g e n e r a l l y of higher n u t r i t i o n a l q u a l i t y and s i m i l a r biomass when compared with most of the s a l t - t o l e r a n t b a s i n swards, and may respond more q u i c k l y to the onset of r a i n f a l l (Western 1975). My c a s u a l o b s e r v a t i o n s suggested that woody p l a n t s respond to seasonal r a i n f a l l i n a manner s i m i l a r to herb l a y e r s p e c i e s , but with p r o d u c t i v i t y and green l e a f r e t e n t i o n extended longer i n t o the dry p e r i o d s due to deeper root systems. Swamp edge v e g e t a t i o n i s in l e a f and p r o d u c t i v e f o r most of the year (Pellew 1980, per s . obs.). 4. Fauna The l a r g e mammal community of Amboseli i s abundant and d i v e r s e : hippopotamus, r h i n o c e r o s (now becoming r a r e ) , elephant, 16 s p e c i e s of w i l d ungulates, four l a r g e p r e d a t o r s , and s e v e r a l s p e c i e s of primates. Two migrant ungulate s p e c i e s , wildebeest and zebra, dominate the w i l d l i f e community i n both numbers (the po p u l a t i o n s of both s p e c i e s i n c l u d e approximately 4000 animals) and biomass (Western 1975). Domestic l i v e s t o c k -- c a t t l e , sheep, goats, and donkeys of the indigenous p a s t o r a l i s t s , the Maasai, c o n s t i t u t e a major p r o p o r t i o n of the l a r g e mammal biomass; as much as 75% i n recent t i m e s . v The people and t h e i r l i v e s t o c k have long been an important component of the ecosystem, i n evident c o e x i s t e n c e with the w i l d fauna f o r thousands of years (Western 1973). Western (1975) has c a t e g o r i z e d the l a r g e h e r b i v o r e s -as 18 e i t h e r dependent or independent of s u r f a c e d r i n k i n g water. The most common water-dependent h e r b i v o r e s -- wildebeest, zebra, and Thomson's g a z e l l e -- concentrate i n the basin i n the dry season and migrate outwards to bushland areas with the r a i n s . Western hypothesized that t h i s e x p a n s i o n / c o n c e n t r a t i o n ranging p a t t e r n allows the migrants to make use of higher q u a l i t y bushland grasses when su r f a c e water i s widespread, and to r e t u r n to the poorer pastures near permanent water in the dry seasons. The Maasai l i v e s t o c k f o l l o w a s i m i l a r seasonal movement p a t t e r n , although t h e i r dry-season range has been somewhat extended i n t o the bushlands by water development. Other water-dependent sp e c i e s -- impala and waterbuck f o r example -- r a r e l y leave the v i c i n i t y of the swamps, while water-independent h e r b i v o r e s such as eland and oryx are found i n low d e n s i t i e s throughout the ecosystem at a l l times of year. 5. The elephants The Amboseli elephant p o p u l a t i o n s i z e has been estimated at 580 animals (Poole and Moss 1981), and the m a j o r i t y of i n d i v i d u a l s have been i d e n t i f i e d and monitored s i n c e 1974 (Moss 1980). The p o p u l a t i o n has been f a i r l y s t a b l e f o r at l e a s t the l a s t 12 years, with a recent b i r t h pulse under favourable h a b i t a t c o n d i t i o n s . The elephants' seasonal movements are s i m i l a r to those of the other water-dependent migratory s p e c i e s . They are g e n e r a l l y seen i n the dense bushlands, woodlands, and swamps, and i n the l a t t e r most commonly (Western 1975). 19 The only predator which s e r i o u s l y threatens A f r i c a n elephants i s man, and i n Amboseli t h i s t h r e a t i s not severe at present (see below). When compared with other p o p u l a t i o n s , the Amboseli elephants can be c o n s i d e r e d f a i r l y " n a t u r a l " and u n d i s t u r b e d . C. Long Term Changes Western and van Praet (1973) d i s c u s s long term h a b i t a t changes in Amboseli. They d e s c r i b e d a dramatic d e c l i n e i n the A c a c i a xanthophloea , or fever t r e e , woodlands from 1950. Over 90% of the t r e e s had disappeared w i t h i n 17 years, a process which i s c o n t i n u i n g at the present day. I n i t i a l l y blamed on Maasai . l i v e s t o c k o v e r g r a z i n g and/or elephant damage, the woodland d e c l i n e was c o r r e l a t e d to a r i s e i n the water t a b l e and the r e l e a s e of s o l u b l e s a l t s to the t r e e s ' s e n s i t i v e root systems. S o i l s a l t content was p o s i t i v e l y a s s o c i a t e d with t r e e death and with the spread of a s a l t - t o l e r a n t bush, Suaeda monoica . Elephant damage has merely a c c e l e r a t e d the woodland d i e - o f f . The fever t r e e d e c l i n e has meant a l o s s of. h a b i t a t f o r woodland-dependent animal s p e c i e s such as monkeys, c e r t a i n a n t e l o p e s , and l e o p a r d . T h i s kind of h a b i t a t change i s not unique h i s t o r i c a l l y . E a r l y r e p o r t s by European e x p l o r e r s d e s c r i b e d a high water t a b l e with e x t e n s i v e swamps, few t r e e s , and widespread S. monoica in the Amboseli basin i n the l a t e 1800's (Thomson 1887, von Hohnel 1894). By the 1930's, dense young fever t r e e stands were found 20 i n the Amboseli basin (Johnson 1935), and healthy mature stands were widespread i n the 1960's when the recent d e c l i n e was f i r s t n o t i c e d . Western and van Praet (1973) suggested that the water t a b l e f l u c t u a t i o n s and the r e t r e a t and expansion of the fever t r e e woodlands co u l d be p e r i o d i c i n nature, a consequence of p o s s i b l e long term r a i n f a l l c y c l e s . The Amboseli elephant p o p u l a t i o n may have been a f f e c t e d by the long term h a b i t a t changes. The e a r l y e x p l o r e r s made no mention of elephants i n the Amboseli area. They recorded most of the other l a r g e game sp e c i e s now seen i n Amboseli, and noted elephants i n other l o c a l i t i e s , so i t seems l i k e l y that elephants were l e s s conspicuous r e s i d e n t s of the Amboseli basin in the l a t e 1800's. Stigand (1909) s t a t e d that elephants c o u l d be found on the slopes of K i l i m a n j a r o and i n the marshes of the Amboseli basin i n the e a r l y 1900's, while some years l a t e r , P e r c i v a l (1924) t a l k e d of elephants m i g r a t i n g from the basin swamps to northern bushland areas. Elephants were seen i n Amboseli basin swamps and woodlands by Johnson (1935) and i n the northern bushlands near the Seleng a i River by de W a t t e v i l l e (1935), concurrent with the spread oi fever t r e e s i n the b a s i n . The v e r b a l h i s t o r y of the Amboseli Maasai t e l l s a s i m i l a r s t o r y (Western 1973). They say that fever t r e e woodlands were l i m i t e d and elephants r a r e i n the basin at the turn of the century, but that both have i n c r e a s e d s i n c e that time. However, i t i s unclear whether the apparent c o r r e l a t i o n between the extent of fever t r e e woodlands and elephant numbers i n the Amboseli b a s i n i s c a u s a l l y connected or merely c o i n c i d e n t a l (see 21 Spinage's comments below). It Western and van Praet's (1973) hypothesis of p e r i o d i c h a b i t a t and f a u n a l changes l i n k e d to long term c l i m a t i c c y c l e s i s c o r r e c t , then the s t r u c t u r e of the Amboseli ecosystem c o u l d be c o n t i n u a l l y changing r a t h e r than s t a t i o n a r y at a s t a b l e e q u i l i b r i u m . In t h i s case, change should be c o n s i d e r e d a "normal" f e a t u r e of the area, i n s t e a d of an unusual p e r t u r b a t i o n . D. Human In f l u e n c e s Human i n f l u e n c e s on the Amboseli system have come from two sources: the indigenous Maasai herdsmen, and Arab and European c o l o n i s t s and v i s i t o r s . As noted p r e v i o u s l y , the Maasai and t h e i r predecessors have long been an i n t e g r a l p a r t of ecosystem dynamics. Since they hunt w i l d l i f e e x c l u s i v e l y f o r sport or s e l f - d e f e n s e , except d u r i n g severe droughts, t h e i r d i r e c t impact on the w i l d h e r b i v o r e p o p u l a t i o n s has been f a i r l y l i m i t e d . The a g g r e s s i v e nature and m i l i t a r y s k i l l s of the Maasai have kept other t r i b a l groups from the area, and t h e i r own numbers had, u n t i l r e c e n t l y , been l i m i t e d by d i s e a s e , drought, and p r a c t i c a l c o n s i d e r a t i o n s of domestic l i v e s t o c k husbandry. C o n d i t i o n s were such that the biomasses of w i l d l i f e and l i v e s t o c k were roughly equal. However, water development, medical s e r v i c e s , and famine r e l i e f have allowed both human and l i v e s t o c k p o p u l a t i o n s to increase s h a r p l y s i n c e the 1940's. The w i l d h e r b i v o r e p o p u l a t i o n s have 22 not yet s u f f e r e d , but competition between l i v e s t o c k and w i l d l i f e f o r forage c o u l d now become c r i t i c a l d u r i n g severe drought p e r i o d s when the h a b i t a t s of the Amboseli basin are the l a s t food source f o r both groups. W i l d l i f e management a u t h o r i t i e s have attempted to reduce t h i s c o n f l i c t by s e p a r a t i n g the dry season ranges of w i l d and domestic stock. A small l i v e s t o c k - f r e e zone in the c e n t r a l Amboseli b a s i n formed the nucleus of a l a r g e r game reserve in the area from 1947. In 1977, a p r o j e c t to provide f i n a n c i a l compensation, more water, and a l t e r n a t e g r a z i n g for l i v e s t o c k was completed, and the Maasai agreed to remain o u t s i d e a 400 sq. km. area of the basin gazetted as Amboseli N a t i o n a l Park (Western 1977). European man has a f f e c t e d the ecosystem - i n d i r e c t l y through the impacts of development on the Maasai l i f e s t y l e as noted above, and more d i r e c t l y through hunting and tourism. Hunting pressure on elephants was r e p o r t e d l y intense i n the mid to l a t e 1800's and p o p u l a t i o n s may have been d r a s t i c a l l y reduced over huge areas of East A f r i c a i n an e x t e n s i v e Arab i v o r y trade, which was a c c e l e r a t e d , then c u r t a i l e d , by the a r r i v a l of B r i t i s h (Spinage 1973). Spinage suggested that widely r e p o r t e d i n c r e a s e s of elephant p o p u l a t i o n s s i n c e the turn of the century c o u l d be a r e s u l t of the c o l l a p s e of the i v o r y trade and the r e l e a s e of elephant p o p u l a t i o n s from the heavy hunting m o r t a l i t y . The Amboseli elephants may have been a f f e c t e d - i n t h i s way. More r e c e n t l y , government c o n t r o l l e d hunting blocks i n the northern seasonal range removed small 23 numbers of elephants and other Amboseli w i l d l i f e , but t h i s source of m o r t a l i t y probably had a l i m i t e d impact on the animal p o p u l a t i o n s . Hunting of elephants was banned i n Kenya in 1973, and of a l l w i l d l i f e i n 1977. Elephant poaching i n Amboseli o c c u r r e d f o r a short p e r i o d from 1974 to 1976, reducing the p o p u l a t i o n s l i g h t l y . T h i s a c t i v i t y has now been v i r t u a l l y e l i m i n a t e d , through e f f e c t i v e a n t i - p o a c h i n g enforcement and c o o p e r a t i o n with the l o c a l Maasai. The impact of l a r g e numbers of t o u r i s t s -- more than 80,000 a n n u a l l y -- on the ecosystem has been s u r p r i s i n g l y small (Western 1977). The major e f f e c t has been to c r e a t e a l a r g e economic i n c e n t i v e f o r w i l d l i f e c o n s e r v a t i o n , e s p e c i a l l y s i n c e the Maasai began to r e c e i v e b e n e f i t s d i r e c t l y (Western and Henry 1979). E. Context of the Study My study took p l a c e d u r i n g 1978 and 1979, a p e r i o d of high r a i n f a l l f o l l o w i n g a drought in 1976. In my a n a l y s i s , I make use of longer term h a b i t a t and elephant data c o l l e c t e d by Western from 1975 through 1980; I helped i n t h i s data c o l l e c t i o n d u r i n g my stay i n Amboseli. Because the Amboseli basin i s the focus of the elephant p o p u l a t i o n ' s ranging a c t i v i t y , and f o r l o g i s t i c a l reasons, I concentrated my r e s e a r c h e f f o r t s on t h i s r e l a t i v e l y small area. 24 CHAPTER 3 HABITAT STRUCTURE AND DYNAMICS A. I n t r o d u c t i o n In t h i s chapter, I w i l l d e s c r i b e the h a b i t a t types of Amboseli, t h e i r composition and seasonal dynamics. An animal s p e c i e s ' " h a b i t a t " was d e f i n e d by F r e t w e l l and Lucas (1970, p. 17) as "any p o r t i o n of the s u r f a c e of the e a r t h where the s p e c i e s i s able to c o l o n i z e and l i v e " . They f u r t h e r d e f i n e d h a b i t a t s as areas which are " e s s e n t i a l l y homogeneous with respect to the p h y s i c a l and b i o l o g i c a l f e a t u r e s which we b e l i e v e to be most r e l e v a n t to the behavior and s u r v i v a l of the s p e c i e s " . S i n c l a i r (1977, p. 53) d e f i n e d h a b i t a t s f o r A f r i c a n b u f f a l o e s as v e g e t a t i o n types which provide f o r the b a s i c needs of the animals -- needs such as food, water, p r o t e c t i o n from c l i m a t i c extremes, and p r o t e c t i o n from p r e d a t i o n . I w i l l use the term " h a b i t a t type" synonomously with " v e g e t a t i o n community" in a l l d i s c u s s i o n that f o l l o w s , f o r although elephants r a r e l y c o l o n i z e and l i v e w i t h i n a s i n g l e p l a n t community, a d i s t i n c t v e g e t a t i o n community c o u l d be c o n s i d e r e d i n t e r n a l l y "homogeneous" in terms of such b i o l o g i c a l f e a t u r e s as food type abundance, composition, and q u a l i t y , r e l a t i v e to other communities. Western's (1973) s t u d i e s and my own i n i t i a l c a s u a l o b s e r v a t i o n s suggested that the p l a n t communities of Amboseli d i f f e r c o n s i d e r a b l y i n t h e i r s t r u c t u r e , biomass standing crop, and seasonal dynamics. In my study of elephant h a b i t a t s e l e c t i o n as a l e v e l of f o r a g i n g c h o i c e , I want to examine the abundance of p l a n t 25 m a t e r i a l i n each h a b i t a t which i s a v a i l a b l e as p o t e n t i a l food f o r the ele p h a n t s . I assumed that a l l herb l a y e r p l a n t s , mainly grasses and f o r b s , were a v a i l a b l e , while the a v a i l a b i l i t y of woody browse r e q u i r e d a more s p e c i f i c d e f i n i t i o n . From the r e s u l t s of other workers (Guy 1976a, Laws et a l . 1975, Wing and Buss 1970) and my own o b s e r v a t i o n s , i t appeared that elephant browsing of woody p l a n t s i s co n c e n t r a t e d on l e a d i n g twigs and t h e i r l e a v e s . Croze (1974b) estimated the maximum feeding height f o r elephants on woody v e g e t a t i o n to be 6 m. I t h e r e f o r e c o n s i d e r e d the t w i g - p l u s - l e a f biomass of a l l woody s p e c i e s below that height to be p o t e n t i a l l y a v a i l a b l e as elephant food. Bark i s another major food type found on woody p l a n t s , but, f o r p r a c t i c a l reasons, I d i d not attempt to estimate bark biomass i n the present study. Elephants a l s o eat the f r u i t s and flowers of shrubs and t r e e s (pers. obs.), but these are r e l a t i v e l y s carce and ephemeral, and I d i d not attempt to monitor t h e i r abundance. The s e a s o n a l i t y of food a v a i l a b i l i t y i s an important aspect of the f o r a g i n g options faced by elephants. R a i n f a l l i s a c r i t i c a l l y important c l i m a t i c f a c t o r a f f e c t i n g v e g e t a t i o n p r o d u c t i v i t y and biomass i n s e m i - a r i d regions ( P h i l l i p s o n 1975, Whittaker 1970), such as Amboseli. D i r e c t q u a n t i t a t i v e r e l a t i o n s h i p s between r a i n f a l l and grass p r o d u c t i o n were noted in S e r e n g e t i , Tanzania, by S i n c l a i r (1977). I would t h e r e f o r e expect to f i n d that the primary p r o d u c t i o n of herb l a y e r v e g e t a t i o n i n Amboseli dur i n g a time p e r i o d i s c l o s e l y r e l a t e d to the r a i n f a l l i n g during or immediately preceding that p e r i o d . Other f a c t o r s that c o u l d modify the r e l a t i o n s h i p between 26 r a i n f a l l a n d h e r b l a y e r p r o d u c t i v i t y a r e t h e s i t e c o n d i t i o n s - - s p e c i e s c o m p o s i t i o n a n d a b u n d a n c e o f t h e v e g e t a t i o n , p h y s i c a l a n d c h e m i c a l s o i l c h a r a c t e r i s t i c s — , p a s t h i s t o r y , a n d t h e e f f e c t o f d i s t u r b a n c e s s u c h a s f i r e a n d h e r b i v o r y . I c o m p a r e how t h e 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 w i t h d i f f e r e n t l o c a l s o i l , h y d r o l o g i c a l , a n d f l o r a l c o n d i t i o n s r e s p o n d t o t h e s a m e s e a s o n a l r a i n f a l l r e g i m e . W e s t e r n ' s l o n g t e r m h a b i t a t d a t a w i l l b e a n a l y s e d t o c o m p a r e t h e r e s u l t s f r o m my s t u d y p e r i o d t o t h o s e o f p e r i o d s w i t h d i f f e r e n t r a i n f a l l p a t t e r n s , t o g e t a l o n g e r t e r m , p e r s p e c t i v e o n h a b i t a t d y n a m i c s . I d i d n o t a t t e m p t t o e x a m i n e t h e e f f e c t s o f d i s t u r b a n c e o n t h e v e g e t a t i o n . F i r e d i d n o t a f f e c t a n y o f t h e a r e a s I w a s c o n c e r n e d w i t h d u r i n g t h e c o u r s e o f my s t u d y , a n d w h i l e I d i d m a k e n o t e o f h e r b i v o r e c o n s u m p t i o n o n a r o u g h p r e s e n c e / a b s e n c e s c a l e , I d i d n o t a p p r o a c h t h i s q u e s t i o n q u a n t i t a t i v e l y . My d i s c u s s i o n o f h a b i t a t s i n c l u d e s t h e c o m p o s i t i o n a n d a b u n d a n c e ( s t a n d i n g d r y m a s s ) o f t h e h e r b a n d w o o d y ( s h r u b a n d t r e e ) l a y e r s o f t h e d e f i n e d c o m m u n i t i e s . I a l s o d i s c u s s t h e d y n a m i c s o f s o m e h e r b l a y e r c h a r a c t e r i s t i c s - - t h e s t a n d i n g c r o p b i o m a s s o f g r e e n ( p h o . t o s y n t h e t i c ) p l a n t m a t e r i a l a n d a l l p l a n t m a t e r i a l ( g r e e n p l u s s t r u c t u r a l ) - - t h r o u g h t h e s e a s o n s a n d b e t w e e n y e a r s . S h r u b a n d t r e e p h e n o l o g y a n d p r o d u c t i v i t y a r e m o r e c o m p l e x a n d w e r e c o n s i d e r e d b e y o n d t h e s c o p e o f t h i s s t u d y . T h e d e n s i t y a n d b i o m a s s o f a l l b u t t h e m o s t p r o l i f i c w o o d y p l a n t s w e r e t a k e n a s c o n s t a n t d u r i n g . t h e o n e - y e a r p e r i o d o f my f i e l d w o r k . W o o d y p l a n t d e n s i t y a n d b i o m a s s w e r e n o t m o n i t o r e d i n o t h e r p e r i o d s . 27 The h y p o t h e s e s I use as a framework f o r d e s c r i b i n g A m b o s e l i v e g e t a t i o n d y n a m i c s a r e : H1: The s t a n d i n g c r o p b i o m a s s of b o t h t h e h e r b a c e o u s and woody l a y e r s d i f f e r s between h a b i t a t t y p e s i n A m b o s e l i . H2a: The p r o d u c t i o n of h e r b l a y e r b i o m a s s i n - t h e d i f f e r e n t h a b i t a t s i s p o s i t i v e l y r e l a t e d t o t h e amount of r a i n f a l l r e c e i v e d . H2b: T h i s r e l a t i o n s h i p w i l l d i f f e r between h a b i t a t t y p e s , and between d i f f e r e n t c l i m a t i c p e r i o d s . B. Methods The t a s k of v e g e t a t i o n s a m p l i n g c an be o v e r w h e l m i n g , and even w i t h t e c h n i q u e s c u r r e n t l y a v a i l a b l e , i s o f t e n t e d i o u s and t i m e - c o n s u m i n g . As w i t h any s y s t e m of s a m p l i n g , a compromise between t i m e and e n e r g y e x pended and t h e l e v e l of a c c u r a c y and p r e c i s i o n r e q u i r e d must be a c h i e v e d . F o r t h e p u r p o s e s of c o m p a r i s o n between h a b i t a t s i n t h e c u r r e n t s t u d y , and w i t h i n l o g i s t i c a l l i m i t a t i o n s , I u s e d a f a i r l y b r o a d l e v e l of a p p r o x i m a t i o n . In g e n e r a l , a s t a n d a r d e r r o r of 10 % of t h e mean was c o n s i d e r e d a c c e p t a b l e f o r most of t h e v e g e t a t i o n measurements t a k e n . 28 J_. R a i n f a l l records R a i n f a l l has been recorded with a Kenya M e t e o r o l o g i c a l Department raingauge near the Park headquarters at 01 Tukai i n the center of Amboseli b a s i n on a continuous d a i l y b a s i s s i n c e 1974. V a r i o u s other gauges d i s t r i b u t e d around the area have operated s p o r a d i c a l l y and l e s s r e l i a b l y f o r v a r y i n g p e r i o d s . I attempted to v e r i f y the o p e r a t i o n of a number of these d u r i n g 1978 and 1979, f o r an examination of the s p a t i a l v a r i a b i l i t y i n r a i n f a l l p a t t e r n s . For the purposes of s i t e comparison and herb l a y e r dynamics, I c o n s i d e r a " r a i n f a l l year" to begin i n October/November with the onset of the short r a i n s a f t e r the long dry season. 2. Def in i t ion and mapping of habi t a t types In h i s comprehensive study of the- Amboseli ecosystem and i t s g r a z i n g h e r b i v o r e s , Western (1973) d e f i n e d 28 unique v e g e t a t i o n types based on an a n a l y s i s of f l o r i s t i c abundance. Basing my c l a s s i f i c a t i o n on the dominant s p e c i e s of the woody canopy and herb l a y e r s , and on p h y s i c a l c h a r a c t e r i s t i c s , such as the presence of s u r f a c e water, I took Western's o r i g i n a l 28 p l a n t communities and grouped them i n t o 14 h a b i t a t types. In g e n e r a l , I lumped together a number of v e g e t a t i o n types which Western c o n s i d e r e d separate on the b a s i s of more s u b t l e d i f f e r e n c e s i n herb l a y e r s p e c i e s composition. I recognize the s u b j e c t i v e nature of t h i s method of community d e f i n i t i o n 29 (Whittaker 1970), but I f e e l that the sharp boundaries between d i f f e r e n t g e o l o g i c a l and h y d r o l o g i c a l c o n d i t i o n s which occur i n the Amboseli basin and i t s environs are r e f l e c t e d i n d i f f e r e n c e s i n the f l o r a l composition of those areas. An a d d i t i o n a l check on my community d e f i n i t i o n s was provided by Lee (1981). In a c l u s t e r a n a l y s i s of woody s p e c i e s composition i n Amboseli woodlands, she found a s s o c i a t i o n s of pl a n t s p e c i e s which were very s i m i l a r to my more s u b j e c t i v e d e f i n i t i o n s of h a b i t a t types. Western (1973) had mapped the v e g e t a t i o n of c e n t r a l Amboseli at a s c a l e of 1:50,000, using an a e r i a l photomosaic, and a d j u s t i n g community boundaries on the ground by "s u c c e s s i v e approximation" (Poore 1962). I used the same technique to update Western's map. Because of h a b i t a t changes due to water t a b l e f l u c t u a t i o n s -- a l t e r a t i o n s of swamp courses and the boundaries of swamp edge h a b i t a t s , and the s h r i n k i n g of the fever t r e e woodlands -- and h e r b i v o r e consumption of woody p l a n t s , some of the p r e v i o u s l y d e f i n e d communities had disappeared, changed, or had s h i f t e d t h e i r d i s t r i b u t i o n s by the time of my study. I i n c o r p o r a t e d these changes i n the ve g e t a t i o n map used f o r the present study. 3_. Habi t a t d e s c r i p t i o n and m o n i t o r i n g : Herb l a y e r I l o c a t e d permanent v e g e t a t i o n p l o t s i n each h a b i t a t type (except the young fever t r e e swamp edge woodlands -- see below) by d r i v i n g to s u b j e c t i v e l y d e f i n e d " r e p r e s e n t a t i v e areas" of each h a b i t a t type, t o s s i n g a marker, and then l o c a t i n g the p l o t c e n t e r at a p o i n t a random d i s t a n c e and d i r e c t i o n from the < 30 m a r k e r . W e s t e r n h a d e s t a b l i s h e d 13 s u c h p l o t s i n 1 9 7 6 ; I r e t a i n e d 12 o f t h e s e , a n d a d d e d 16 m o r e f o r t h e p e r i o d o f my s t u d y i n 1 9 7 8 / 7 9 , g i v i n g me a t o t a l o f 28 p l o t s . S i n c e t h e h e r b l a y e r o f t h e y o u n g f e v e r t r e e w o o d l a n d s w a s e s s e n t i a l l y t h e s a m e a s t h e s w a m p e d g e g r a s s l a n d s ( S e g ) , a n d s i n c e t h e t w o h a b i t a t t y p e s w e r e g e n e r a l l y c l o s e l y a s s o c i a t e d i n l o c a l i t y , I u s e d h e r b l a y e r d a t a f r o m t h e l a t t e r t o d e s c r i b e t h e y o u n g f e v e r t r e e h a b i t a t t y p e . T h e n u m b e r a n d l o c a t i o n o f p l o t s i n h a b i t a t t y p e s w e r e s e l e c t e d a s a c o m p r o m i s e b e t w e e n t h e e x t e n s i v e c o v e r a g e o f l a r g e a r e a s u n d e r l i m i t a t i o n s o f t i m e a n d t r a v e l c o s t s , a n d t h e a c c u r a t e r e f l e c t i o n o f t h e s t r u c t u r e a n d c h a n g e s o f t h e h a b i t a t t y p e s . T h e d i s t r i b u t i o n o f p l o t s i n h a b i t a t t y p e s c a n b e f o u n d i n T a b l e 2 ( s e e b e l o w ) . T h e h e r b l a y e r s p e c i e s c o m p o s i t i o n , r e l a t i v e a b u n d a n c e a n d d i v e r s i t y w e r e e s t i m a t e d a t e a c h p e r m a n e n t p l o t b y a s l a n t i n g p i n f r a m e m e t h o d ( M c N a u g h t o n 1 9 7 9 a , D . W e s t e r n p e r s . c o m m . ) . T h e s e a t t r i b u t e s w e r e d e t e r m i n e d a t t h e t i m e o f h i g h g r e e n b i o m a s s , i n J a n u a r y 1 9 7 9 . A f r a m e w i t h f i v e s l a n t i n g p i n s w a s p l a c e d a t s i x t e e n r e g u l a r l y s p a c e d l o c a t i o n s a r o u n d t h e c e n t e r o f e a c h p l o t , g i v i n g a t o t a l o f 80 p i n s p e r p l o t . T h e n u m b e r o f " h i t s " p e r s p e c i e s - - p o i n t s w h e r e p l a n t s w e r e i n t e r c e p t e d b y a s l a n t i n g p i n - - w a s r e c o r d e d f o r e a c h p i n a t a g i v e n f r a m e l o c a t i o n . A r e g r e s s i o n d e r i v e d f r o m c l i p p e d q u a d r a t s r e l a t e d h i t s - p e r - p i n t o d r y b i o m a s s i n g / m 2 ( s e e A p p e n d i x 1 ) . T h e r e l a t i v e a b u n d a n c e o f a p l a n t s p e c i e s w a s c a l c u l a t e d a s t h e p r o p o r t i o n o f t o t a l b i o m a s s t h a t was e s t i m a t e d f o r e a c h i n d i v i d u a l s p e c i e s . D i v e r s i t y o f t h e h e r b l a y e r w a s e s t i m a t e d 31 by the Shannon-Weaver index ( P i e l o u 1966). P l a n t s p e c i e s were i d e n t i f i e d with a r e f e r e n c e c o l l e c t i o n of Amboseli f l o r a which I updated and maintained. The o r i g i n a l p l a n t i d e n t i f i c a t i o n s were made by the s t a f f of the Herbarium of the Kenya A g r i c u l t u r a l and F o r e s t r y Research O r g a n i z a t i o n i n N a i r o b i . Herb l a y e r biomass was estimated, f o r monitoring purposes, by a d i f f e r e n t method. At each permanent p l o t l o c a t i o n , a c i r c u l a r area 20 m. i n diameter was sampled a c c o r d i n g to a method d e v i s e d by D. Western (pers. comm.). Measurements were made of l e a f height and percent ground cover -- the l a t t e r estimated with a 10-pin v e r t i c a l frame as percent of the p i n s i n t e r c e p t i n g a p l a n t . S i x t y p a i r e d measurements were made per p l o t at r e g u l a r l y spaced i n t e r v a l s along r a d i i from the p l o t c e n t e r . T h i s g e n e r a l l y gave mean values of - height and cover with a c c e p t a b l e standard e r r o r s . A c a l i b r a t i o n r e g r e s s i o n based on c l i p p e d quadrats r e l a t e d the product of height and cover to t o t a l dry biomass in g/m2 (see Appendix 1). T h i s method of biomass e s t i m a t i o n gave s i m i l a r r e s u l t s to the s l a n t i n g p i n method d e s c r i b e d above, but was qu i c k e r and more p r e c i s e . The S. c o n s i m i 1 i s g r a s s l a n d s were monitored on only three occasions at widely spaced i n t e r v a l s -- both green and t o t a l biomass changed very l i t t l e d u r i n g the course of my study. The biomass of green herb l a y e r v e g e t a t i o n was estimated with a hand-held spectrophotometer; the t h e o r e t i c a l background and p r a c t i c a l use of t h i s machine are d e s c r i b e d elsewhere (McNaughton 1979a,. Pearson et a l . 1976). B a s i c a l l y , the machine measures l i g h t r e f l e c t e d by p h o t o s y n t h e t i c pigments at 32 two wavelengths, and the r a t i o of these two measures i s l i n k e d through a c a l i b r a t i o n r e g r e s s i o n to the dry mass, in g/m2, of green v e g e t a t i o n present (see Appendix 1). Although some r e s e r v a t i o n s have been expressed concerning the r e p e a t a b i l i t y of measurements under v a r y i n g l i g h t , shade, s o i l , and p l a n t c o n d i t i o n s (F. B u n n e l l , p e r s . comm.), my r e s u l t s appeared c o n s i s t e n t between s i m i l a r p l o t s on the same day and w i t h i n the same p l o t at subsequent measurement. Ten measurements were made per p l o t ; t h i s g e n e r a l l y gave an a c c e p t a b l y low standard e r r o r . I d i d not attempt to monitor green mass i n the wet swamps, because the meter was v u l n e r a b l e to water damage; when necessary for some an a l y s e s (see Chapters 4 and 5), I made crude estimates of the green mass. V i s u a l estimates of the percent of p l a n t matter removed by g r a z i n g h e r b i v o r e s were a l s o made at each p l o t by separate o b s e r v e r s . I n t e r - o b s e r v e r agreement was high, and g e n e r a l l y a s i n g l e observer's estimate was recorded. I measured t o t a l and green herb l a y e r biomass, and estimated consumption at the permanent p l o t s at i n t e r v a l s of roughly 4 - 6 weeks; sampling i n t e n s i t y was i n c r e a s e d during p e r i o d s of more r a p i d change i n the r a i n y seasons. Western's o r i g i n a l 12 p l o t s have been monitored from A p r i l 1976 up to the present, while the p l o t s I e s t a b l i s h e d during my study were monitored from November 1978 to October 1979. In a d d i t i o n to biomass dynamics of the herb l a y e r , I am a l s o i n t e r e s t e d in the dynamics of forage q u a l i t y . I d i d not c o l l e c t data on herb l a y e r n u t r i e n t contents d u r i n g my f i e l d 33 work, but D. Western (pers. comm.) has p r o v i d e d the r e s u l t s from proximate analyses of herb l a y e r samples from h i s 12 p l o t s d u r i n g 1977. I w i l l present and e v a l u a t e a summary of these data. 4. Habi t a t d e s c r i p t i o n : Woody vegetat ion Techniques f o r the e s t i m a t i o n of the biomass standing crop of woody v e g e t a t i o n are s t i l l at an e a r l y stage of development (Rutherford 1979). Most c u r r e n t techniques operate i n two stages: an estimate of p l a n t d e n s i t y i s coupled with estimates of the dry mass of i n d i v i d u a l p l a n t s based on dimension/weight equations (Stanek and State 1978, Whittaker and Marks 1975). I used t h i s two-stage method, at a f a i r l y crude l e v e l of approx imat i on. For the e s t i m a t i o n of p l a n t d e n s i t y , I used a m o d i f i e d B i t t e r l i c h s t i c k or " o c c l u s i o n quadrat". The technique was d e s c r i b e d by Cooper (1957, 1963). The method i n v o l v e s s i g h t i n g along a c a l i b r a t e d s t i c k with a c r o s s b a r as i t i s swung h o r i z o n t a l l y through 360° around the observer, and r e c o r d i n g a l l i n d i v i d u a l s of each s p e c i e s with canopy diameters that appear wider than the c r o s s b a r . A "cover percentage f a c t o r " based on gauge geometry gi v e s the percent ground cover f o r each s p e c i e s c o n s i d e r e d . I extended a m o d i f i c a t i o n of ,this technique (Cooper 1963) to allow e s t i m a t i o n of p l a n t d e n s i t y i n stems per h e c t a r e . I measured a l a r g e number ( g e n e r a l l y about t h i r t y - f o u r ) of i n d i v i d u a l p l a n t s , chosen at random, of each of 36 of the more 34 common woody sp e c i e s i n Amboseli. Canopy diameter, h e i g h t , and depth were measured f o r each i n d i v i d u a l . Species mean values g e n e r a l l y had an ac c e p t a b l y low standard e r r o r , s i n c e stands of many s p e c i e s appeared to be f a i r l y even-aged. For s p e c i e s with wider v a r i a n c e i n s i z e , I subd i v i d e d the s p e c i e s i n t o two or more broad s i z e c l a s s e s and made a d d i t i o n a l measurements f o r each c l a s s . Using mean values of canopy diameter, I c a l c u l a t e d the c i r c u l a r ground area covered by the "mean i n d i v i d u a l " of each s p e c i e s (or s p e c i e s s i z e c l a s s ) and from t h i s , a "computation f a c t o r " r e l a t i n g gauge count to p l a n t d e n s i t y i n numbers/hectare (see Appendix 3). Cooper (1963) cautioned on the use of t h i s m o d i f i e d technique; e r r o r can enter i n t o the method when small d i f f e r e n c e s i n canopy measurement r e s u l t i n p r o p o r t i o n a l l y l a r g e r changes i n d e n s i t y e s t i m a t e s , e s p e c i a l l y with the smaller shrubs. For the second stage of biomass e s t i m a t i o n , I used r e g r e s s i o n a n a l y s i s of woody dry mass on p l a n t dimensions to estimate the biomass of i n d i v i d u a l p l a n t s . Since I was i n t e r e s t e d in the woody p l a n t m a t e r i a l a v a i l a b l e as elephant food, I d e r i v e d a r e g r e s s i o n f o r t w i g - p l u s - l e a f dry mass on measurable p l a n t dimensions. Taking the dimension f i g u r e s (canopy diameter and height) I had c o l l e c t e d f o r each shrub and t r e e s p e c i e s , I l o c a t e d approximately "mean i n d i v i d u a l s " of 22 s p e c i e s , and then measured, harvested, d r i e d , and weighed them. The h a r v e s t i n g of the few deciduous woody s p e c i e s in Amboseli was done at maximum l e a f f l u s h , g e n e r a l l y i n the r a i n y seasons, while evergreen 35 p l a n t s were taken dur i n g other times of year as w e l l . A l l p l a n t samples were separated i n t o flowers and f r u i t , l e a f , l e a f - b e a r i n g twig, and woody stem (branches and mainstem) f r a c t i o n s which were then oven-dried and weighed. With small shrubs, the e n t i r e p l a n t was c o l l e c t e d , while for l a r g e r p l a n t s subsampling approximations were used. In the l a t t e r case, two observers, myself and an a s s i s t a n t , estimated the p r o p o r t i o n of canopy occupied by a s e l e c t e d l a r g e branch. T h i s branch was c o l l e c t e d , and, s i n c e observer agreement was good, was weighed as a f r a c t i o n a l sample of the whole p l a n t . The dimension and dry mass values f o r the 22 s p e c i e s sampled (see Appendix 2) were s u b j e c t e d to r e g r e s s i o n a n a l y s i s (Whittaker and Marks 1975), and the r e s u l t i n g b e s t - f i t equations were a p p l i e d to a l l the woody s p e c i e s of known mean dimensions. Canopy diameter gave the best estimate of t w i g / l e a f dry mass in the m u l t i - s p e c i e s r e g r e s s i o n s (see Appendix 1). The t w i g / l e a f biomass of the mean i n d i v i d u a l of each s p e c i e s or s i z e c l a s s w i t h i n a s p e c i e s was then estimated f o r the measured mean i n d i v i d u a l of each of the 36 s p e c i e s and the s i z e c l a s s e s w i t h i n s p e c i e s . (see Appendix 3). Values for t w i g / l e a f biomass d e n s i t y , i n kg/ha, were c a l c u l a t e d by m u l t i p l y i n g p l a n t d e n s i t y by dry mass values of each s p e c i e s i n the community. Woody p l a n t percent cover, numerical d e n s i t y , and biomass d e n s i t y were estimated at each of the permanent p l o t l o c a t i o n s at the same time as the herb l a y e r s p e c i e s composition sampling. In a d d i t i o n , a number of e x t r a "temporary p l o t s " were added on a random b a s i s i n some of the h a b i t a t types ( i n c l u d i n g the young 36 fever t r e e woodlands) of the Amboseli b a s i n f o r the purpose of woody p l a n t measurement. I d i d not attempt to r e c o r d seasonal changes i n woody biomass f o r most h a b i t a t types because my ext e n s i v e approach and the approximations employed would not p i c k up any but the gr o s s e s t changes in woody dry mass. Since I d i d not n o t i c e any marked changes in woody p l a n t d e n s i t i e s , through c a s u a l o b s e r v a t i o n s , i n the year of my f i e l d work, I f e l t t h i s approach was j u s t i f i e d . Most of the woody s p e c i e s i n the Amboseli b a s i n are evergreen, so that t w i g / l e a f mass was probably f a i r l y constant through the seasons, f o r bas i n h a b i t a t s , at l e a s t . In the swamp edge community, shrub growth was dramatic and d e n s i t y changes were recorded at i n t e r v a l s , but f o r a l l other h a b i t a t s , woody biomass was taken as constant f o r the p e r i o d of the study. T h i s i s undoubtedly an o v e r s i m p l i f i c a t i o n , and f u r t h e r work on woody p l a n t phenology i n these s e m i - a r i d areas i s c l e a r l y needed. Shannon-Weaver d i v e r s i t y was c a l c u l a t e d f o r woody s p e c i e s on the b a s i s of a v a i l a b l e biomass c o n t r i b u t i o n ( i e . , twig p l u s l e a f , below 6m.). I c a l c u l a t e d mean values of herb and woody l a y e r c h a r a c t e r i s t i c s at the v e g e t a t i o n p l o t s f o r each h a b i t a t type, and used the mean values i n a l l subsequent a n a l y s e s . 37 C. R e s u l t s J_. R a i n f a l l p a t t e r n s R a i n f a l l i s v a r i a b l e i n Amboseli i n both space and time. F i g u r e 2 shows monthly r a i n f a l l recorded at s i t e s around Amboseli from November 1978 to October 1979, the '78/'79 " r a i n f a l l year". The general seasonal p a t t e r n was s i m i l a r f o r the v a r i o u s l o c a l i t i e s : r a i n f a l l began in l a t e October or e a r l y November and continued u n t i l May or June with weak nodal peaks in December/January and April/May and a long dry p e r i o d from June to September. However, Table 1 shows that c o r r e l a t i o n s between monthly r a i n f a l l at Ol Tukai and at other s i t e s ranged from good (at Lemeipoti, r=.854, p<.01, n=12) to f a i r (at Namanga, r=.509, n.s., n=l2). The degree of c o r r e l a t i o n v a r i e d in a manner not simply r e l a t e d to the d i s t a n c e of s i t e s from 01 Tukai (rs=-.029, n.s., n=6, Spearman's rank c o r r e l a t i o n ( S i e g e l 1956)). When I compared d a i l y r a i n f a l l records f o r 1978/79 from 01 Tukai and nearby Sinya Mine, the c o r r e l a t i o n was low (r=.457, P<.01, but n=365), i n d i c a t i n g that short-term ( i e . , d a i l y ) r a i n f a l l p a t t e r n s can be q u i t e d i f f e r e n t over f a i r l y small d i s t a n c e s , while over a longer time i n t e r v a l , such as a month, the d i f f e r e n c e s can tend to even out. R a i n f a l l p a t t e r n s a l s o vary markedly through time, as r e v e a l e d by a n a l y s i s of longer term r a i n f a l l data from 01 T u k a i . Annual r a i n f a l l at 01 Tukai f o r the l a s t seven years f e l l i n t o three p e r i o d s (see F i g u r e 3). A f a i r l y dry p e r i o d from 1974 to 1977 with an mean of about 230 mm. t o t a l annual r a i n f a l l 38 F i g u r e 2. Monthly r a i n f a l l p a t t e r n s i n 1978/79 at 01 Tukai and s i x other s t a t i o n s i n the surrounding bushland ( l o c a t i o n s given i n F i g u r e 1). Note the v a r i a b i l i t y i n r a i n f a l l p a t t e r n between the d i f f e r e n t l o c a l i t i e s . 39 100H Tukai Sinya Ilassit n. 200-i 50H 200 H \r 150- _ 100H Namanga Booster 100- < BO- OT IL-0 1 fl N D J F M A M J J A S O 150H 100H 50- Lemeipoti in Kitendeni N D J F M A M J J A S O N D J F M A M J J A S O 40 100- 50H J] 1974-77 Mean Annual Total 22 6-9 mm 1001 E E < or 50H 1977 - 79 463-8 mm 1 100H 50H 1979 -80 252-8 mm 2 f l J Q N D J F M A M J J A S O SR MR LR R A I N S ED MD LD DRY Figure 3. Mean monthly ra infa l l at 01 Tukai in three periods: Dryl- INOV. 1974 to Oct. 1977, Wet- Nov. 1977 to Oct. 1979, and Dry2- Nov. 1979 to Oct. 1980. Note the differences in the amount and temporal patterning of ra infa l l between periods. In the Wet period, mean annual ra in fa l l was much higher, and almost unimodal, when compared with the lower, bimodal pattern of the two Dry periods. Seasons are defined as: SR- short rains, MR- mid rains, LR- long rains , ED- early dry, MD- mid dry, LD- late dry. 41 Table 1. R a i n f a l l at v a r i o u s s t a t i o n s around Amboseli from October 1978 to October 1979. R a i n f a l l T o t a l Annual C o r r e l a t i o n D i s t a n c e from S t a t i o n Rainfall(mm) with 01 T u k a i ( r ) 01 Tukai(km) 01 Tukai 551.0 Sinya Mine 405.1 Namanga 630.1 Booster Pump 595.9 Lemeipoti 572.6 K i t e n d e n i BH 565.7 I l a s s i t 1640.8 1.000 0 0.757 ** 27 0.509 n.s. 55 0.601 * 10 0.854 ** 28 0.659 * 15 0.747 ** 50 S i g n i f i c a n c e l e v e l s of C o r r e l a t i o n with 01 Tukai ( r ) : * p < 0.05 ** p < 0.01 Spearman's rank c o r r e l a t i o n of r with D i s t a n c e from Ol T u k a i : r s=-0.029, n.s., n=6 i n c l u d e d a drought which culminated in October 1976. The f o l l o w i n g two years (which i n c l u d e d my study p e r i o d d u r i n g 1978/79) were much wetter by comparison, with an annual mean t o t a l of 465 mm., while more r e c e n t l y i n 1979/80, a dry year (255 mm. t o t a l ) was recorded. The d i s t i n c t l y bimodal p a t t e r n -- short r a i n s , short dry season, long r a i n s -- of the e a r l i e r dry p e r i o d was r e p l a c e d by a more c l u s t e r e d , almost unimodal p a t t e r n in the wet years, and the bimodal p a t t e r n had not completely returned by the more recent dry p e r i o d . T h i s was, i n e f f e c t , a n a t u r a l experiment i n s o i l water a v a i l a b i l i t y f o r p l a n t growth. The low r a i n f a l l p a t t e r n i s a p p a r e n t l y more " t y p i c a l " of Amboseli c l i m a t e -- i t has a short recurrence i n t e r v a l (Western 1973) . 42 2. H a b i t a t types d e f i n e d , mapped, and d e s c r i b e d The h a b i t a t types I d e f i n e d f o r t h i s study are l i s t e d i n Table 2 with the approximate area of the Amboseli ecosystem Table 2. Amboseli h a b i t a t types: Approximate areas in the Amboseli ecosystem, and the number of f i x e d v e g e t a t i o n p l o t s i n each. H a b i t a t Type Area(km 2) Number of p l o t s GRASSLAND A l k a l i n e g r a s s l a n d 130 3 Lake bed g r a s s l a n d 120 2 S. c o n s i m i l i s g r a s s l a n d 31 1 WOODLAND A. t o r t i l i s woodland 43 3 Fever t r e e woodland 43 1 Salvadora/Suaeda shrubland 33 2 Palm/Fever t r e e woodland 5 " 1 Young Fever t r e e woodland 4 SWAMP Swamp edge g r a s s l a n d 10 6 . Permanent swamp 15 1 BUSHLAND Bushed g r a s s l a n d 725 1 Open bushland North 1000 1 Open bushland South 700 1 Dense bushland North 100 1 Dense bushland South 120 2 A. drepanolobium bushland 20 2 covered by each. The d i s t r i b u t i o n of the h a b i t a t types of Amboseli can be seen i n the v e g e t a t i o n map, F i g u r e 4, which •focuses on the Amboseli b a s i n . The s p a t i a l complexity of h a b i t a t d i s t r i b u t i o n r e f l e c t s the mosaic nature of s o i l and h y d r o l o g i c a l c o n d i t i o n s i n the Amboseli environment. A 43 F i g u r e 4. Map o f t h e h a b i t a t t y p e s of t h e A m b o s e l i b a s i n and n e a r b y b u s h l a n d a r e a s . The m a r g i n of t h e b a s i n i s i n d i c a t e d by t h e s o l i d d a r k l i n e . The a b b r e v i a t i o n s u s e d f o r h a b i t a t t y p e s a r e d e f i n e d as i n t h e t e x t , and w i l l be u s e d t h r o u g h o u t t h e t h e s i s : B a s i n h a b i t a t s Gr A l k a l i n e g r a s s l a n d Lk Lake bed g r a s s l a n d Con S. c o n s i m i l i s g r a s s l a n d Tw A. t o r t i l i s w o odland Xw F e v e r t r e e woodland Ss S a l v a d o r a / Suaeda s h r u b l a n d Pax P a l m / F e v e r t r e e swamp edge woodland Yax Young f e v e r t r e e swamp edge woodland Seg Swamp edge g r a s s l a n d Swp Permanent wet swamp B u s h l a n d h a b i t a t s Bgr ' Bushed g r a s s l a n d Obn Open b u s h l a n d , N o r t h Obs Open b u s h l a n d , S o u t h Dbn Dense b u s h l a n d , N o r t h Dbs Dense b u s h l a n d , S o u t h Adb A. d r e p a n o l o b i u m b u s h l a n d Grassland O 1̂ Con -Woodland • Tw K3 ss M Yax Swamp MM Seg v~n swp — Bushland L̂ ZJ Bgr ED Obn Obs. E£j Dbn £53 Dbs EZ1 Mb -p. 45 consequence of t h i s complexity i s that a great v a r i e t y of h a b i t a t c o n d i t i o n s can be encountered w i t h i n f a i r l y short d i s t a n c e s , e s p e c i a l l y i n the v i c i n i t y of the b a s i n . A b r i e f d e s c r i p t i o n of each h a b i t a t type f o l l o w s . G r a s s l a n d h a b i t a t types A l k a l i n e g r a s s l a n d , Gr With v i r t u a l l y no woody dominated by p e r e n n i a l Sporobolus s e a s o n a l l y ephemeral fo r b s on the c e n t r a l Amboseli b a s i n . cover, these g r a s s l a n d s are spp. grasses and i n c l u d e a few shallow a l k a l i n e s o i l s of the • Lake bed g r a s s l a n d s , Lk T h i s v e g e t a t i o n type occupies the present lake bed of the Amboseli b a s i n , p a r t s of which are s e a s o n a l l y inundated with r a i n water. At d i f f e r e n t s i t e s on the lakebed, one f i n d s a s i n g l e s a l t - t o l e r a n t grass s p e c i e s , (Psilolemma j a e g e r i ) , a dense, mixed s p e c i e s grass community, or a s u r f a c e barren of " v e g e t a t i o n , depending on l o c a l s o i l chemistry and water regime. Sporobolus c o n s i m i l i s g r a s s l a n d , Con . G e n e r a l l y found in dense, s i n g l e s p e c i e s stands of the t a l l (1 to 2 m.), coarse g r a s s , Sporobolus c o n s i m i l i s , t h i s h a b i t a t type i s l o c a t e d i n f a i r l y s m a l l , s c a t t e r e d s i t e s of poor drainage on c l a y s o i l s at grassland/swamp, g r a s s l a n d / l a k e bed, or grassland/woodland t r a n s i t i o n zones. 46 Woodland h a b i t a t types A c a c i a t o r t i l i s woodlands, Tw T h i s v e g e t a t i o n type occurs along the southern edge of the Amboseli b a s i n on t r a n s i t i o n a l v o l c a n i c / l a c u s t r i n e s o i l s . The woody l a y e r i s dominated by mature Acac i a t o r t i l i s , the "umbrella t r e e " , with a v a r i e t y of woody shrubs and herbs, mostly evergreen, i n the under s t o r y . A. t o r t i l i s canopy cover i s v a r i a b l e , but averages around 4 - 5 percent. The p a l a t a b l e and p r o d u c t i v e grass, Cynodon p l e c t o s t a c h y u s , dominates the herb l a y e r , which a l s o i n c l u d e s Sporobolus spp. and a number of annual grasses and f o r b s , which are found d i r e c t l y under the shade canopy of the umbrella t r e e s . Acac i a xanthophloea woodlands, Xw Much reduced in extent (as I d e s c r i b e d i n Chapter 2), the remaining Acac i a xanthophloea , or fever t r e e , woodlands s u r v i v e on the deeper a l k a l i n e s o i l s of the eastern Amboseli b a s i n and in i s o l a t e d patches near water i n the c e n t r a l and western b a s i n . Although s i m i l a r i n many ways to the A. t o r t i 1 i s woodlands, the fever t r e e woodlands have fewer herb and shrub s p e c i e s , a Sporobolus -dominated herb l a y e r , and a t r e e l a y e r composed e x c l u s i v e l y of A. xanthophloea i n v a r y i n g s t a t e s of s i z e and h e a l t h , which averages a canopy cover of about 2 percent. The s a l t bush, Suaeda monoica , now occurs as a major understory s p e c i e s over much of t h i s h a b i t a t type. 47 Salvadora/Suaeda shrublands, Ss These shrublands represent areas of the c e n t r a l and western Amboseli b a s i n formerly occupied by A. xanthophloea woodland, but now devoid of a l l but the o c c a s i o n a l s e e d l i n g fever t r e e s , and c u r r e n t l y dominated by the shrubs, Salvadora p e r s i c a and Suaeda monoica . The herb l a y e r " i s s i m i l a r to that of the A. xanthophloea woodlands, although s p e c i e s composition and biomass may have changed somewhat si n c e the disappearance of the canopy t r e e s . Date palm/fever t r e e swamp edge woodlands, Pax Oc c u r r i n g i n a s m a l l , i s o l a t e d area around a c l u s t e r of swamp sources i n the c e n t r a l Amboseli b a s i n , t h i s h a b i t a t type has a woody l a y e r composed of low-growing Phoenix r e c l i n a t a , or date palms, and fever t r e e s . Canopy cover, i n c l u d i n g date palms and fever t r e e s , averages about 20 percent. The growth of l a r g e woody herbs -- e s p e c i a l l y Abut iIon mauritianum -- has been p r o l i f i c i n recent years, while the grass/herb l a y e r i s t y p i c a l of swamp edges (see below). Young Acac i a xanthophloea swamp edge woodlands, Yax Th i s type occurs i n s c a t t e r e d patches along the margins of swamps i n the Amboseli b a s i n . I t i s b a s i c a l l y swamp edge g r a s s l a n d (see below), with.a f a i r l y dense (about 15 percent) cover of young and maturing fever t r e e s , which f l o u r i s h i n these areas of low s o i l s a l i n i t y and year-round ground water supply. The palm/fever t r e e woodlands and the young f e v e r t r e e woodlands 48 c o u l d be co n s i d e r e d as both woodland and swamp edge h a b i t a t types. Swamp and swamp edge h a b i t a t types Swamp edge g r a s s l a n d , Seg Swamp edge gr a s s l a n d s occur along the edge of permanent swamps i n the Amboseli b a s i n , where water flows below the s o i l s u r f a c e f o r much of the year. The herb l a y e r i s dominated by a dense l a y e r of Cynodon d a c t y l o n , or Bermuda grass , with a few cr e e p i n g herbs found at ground l e v e l . Woody herbs and shrubs develop l o c a l i z e d t h i c k e t s i n areas of high s o i l moisture, e s p e c i a l l y in the l a r g e Olokinya swamp i n the e a s t - c e n t r a l b a s i n . Permanent swamp, Swp In the areas of s p r i n g sources where water flows over the s o i l s u r f a c e i n pools or channels, t a l l coarse sedges, such as Cyperus immensus and C. papyrus, and succul e n t c r e e p i n g herbs have developed in dense mats of f l o a t i n g v e g e t a t i o n . The d i s t r i b u t i o n and extent of t h i s h a b i t a t type i s somewhat v a r i a b l e , depending on the output of s p r i n g water and the d i r e c t i o n and volume of water flow as a f f e c t e d by Park managers. Bushland h a b i t a t types Bushed g r a s s l a n d , Bgr 49 E x t e n s i v e areas of the l a t e r i t i c r i d g e s north of the basin are covered by s e a s o n a l l y a r i d bushed g r a s s l a n d . I t i s s p a r s e l y wooded ( l e s s than 1 percent cover) with B a l a n i t e s spp. and Acac i a m e l l i f e r a , or w a i t - a - b i t thorn, shrubs. The low, but p r o d u c t i v e , herb l a y e r i s a mixture of Sporobolus spp. and other annual g r a s s e s , and a number of annual herb s p e c i e s . Open bushland, north, Obn T h i s d i v e r s e type which i s found on t r a n s i t i o n a l s o i l types and drainage zones i n the r i d g e s and v a l l e y s north of the Amboseli b a s i n , i s c h a r a c t e r i z e d by a v a r i e t y of woody shrubs -- A. m e l l i f e r a , C o r d i a spp., Commiphora spp., B a l a n i t e s spp., and others -- which combine to make up a canopy cover of about 10 percent. The herb l a y e r i s composed of a v a r i e t y of annual and p e r e n n i a l herbs and g r a s s e s . Open bushland, south, Obs The southern open bushlands occur on the v o l c a n i c r i d g e s and l a v a boulder f i e l d s to the south and east of the b a s i n . Shrub cover, mainly A. m e l l i f e r a , amounts to l e s s than 2 percent, but low woody herbs such as T r i u m f e t t a and H i b i s c u s spp. are abundant. Both annual and p e r e n n i a l herbs and grasses are common. Dense bushland, north, Dbn In drainage areas and l o c a l i t i e s of. higher r a i n f a l l near h i l l ranges northeast of the b a s i n , f a i r l y dense (over 15 50 percent cover) mixed stands of A. m e l l i f e r a , S. p e r s i c a , and AbutiIon spp. are found. Grasses are a patchy mixture of t a l l , c oarse p e r e n n i a l s and short p a l a t a b l e annuals. Annual and p e r e n n i a l herbs are f a i r l y abundant. Dense bushland, south, Dbs Areas of t r a n s i t i o n a l v o l c a n i c s o i l s south of the Amboseli b a s i n woodlands host f a i r l y dense -- cover about 15 percent stands of A. m e l l i f e r a and A. nubica with other smaller shrubs i n the understory. The herb l a y e r i s dominated i n patches by Cynodon p l e c t o s t a c h y u s , and i n other patches a mixed community of annual and p e r e n n i a l herbs and grasses i s found. Acac i a drepanolobium bushland, Adb In p o o r l y d r a i n e d areas at the southern basin margin, i s o l a t e d stands of f a i r l y dense Acac i a drepanolobium , or w h i s t l i n g thorn, mixed with B a l a n i t e s g l a b r a are found. T h i s h a b i t a t type has a d i v e r s e and p r o d u c t i v e herb l a y e r with a v a r i e t y of annual grasses and herbs. The woody and herbaceous biomass of the d i f f e r e n t h a b i t a t types, as of January, 1979, are compared in F i g u r e 5. The hi g h e s t ' v a l u e s of herb l a y e r biomass (range: 20,000 to 30,000 kg/ha) were in the swamp (Swp), swamp edge, g r a s s l a n d (Seg), and swamp edge woodland (Pax, Yax) h a b i t a t s . A l s o abundant at that time of year, the mid r a i n s , were the herb l a y e r s of the fever t r e e and A. t o r t i l i s woodlands (Tw,Xw) (17,000 to 20,000 kg/ha), 100. A) Woody Layer CM O < \ o t o t o < z 1 C H BJHerb Layer 100 H Total Green Gr Lk Con Tw Xw Ss FaxYax SegSwp Bgr ObnObsDbnDls Adb Grassland—J Woodland 1 Swamp-i Bushland 1 F i g u r e 5. S tand ing biomass (dry we ight ) o f the woody ( t w i g + l e a f ) and herbaceous (green m a t t e r and t o t a l dry mass) v e g e t a t i o n i n the h a b i t a t types o f Ambosel i i n January 1979. Note t h a t the s c a l e i s l o g a r i t h m i c . There a re marked d i f f e r e n c e s between h a b i t a t s i n a v a i l a b l e p l a n t b iomass . In a l l h a b i t a t s , herb l a y e r biomass (both green and t o t a l mass) i s h i g h e r than t w i g / l e a f mass. Note t h a t green mass was not measured f o r t h e Swp p l o t . 52 and the denser bushlands (Dbn, Dbs, Adb) and the open southern bushlands (Obs) (about 15,000 kg/ha). The S. c o n s i m i l i s (Con) g r a s s l a n d patches had a high t o t a l biomass (40,000 kg/ha), but very low green mass (900 kg/ha), i n d i c a t i n g that much of the p l a n t biomass i n t h i s h a b i t a t was dry "standing hay" and s t r u c t u r a l m a t e r i a l , even d u r i n g the r a i n s . T h i s was a l s o the case, at a lower biomass l e v e l , i n the lake-bed g r a s s l a n d s (Lk). The a l k a l i n e g r a s s l a n d s (Gr), Salvadora / Suaeda shrublands (Ss ) , bushed g r a s s l a n d s (Bgr), and open northern bushlands (Obn) had r e l a t i v e l y lower t o t a l biomass in the herb l a y e r (1500 to 4500 kg/ha), with green mass making up a higher p r o p o r t i o n of the t o t a l . For these l a t t e r h a b i t a t s , the herb l a y e r appeared to have more p h o t o s y n t h e t i c m a t e r i a l and l e s s s t r u c t u r a l m a t e r i a l per u n i t of p l a n t biomass. Twig/leaf biomass was most abundant i n the swamp edge woodlands (Pax, Yax), where young fever t r e e s c o n t r i b u t e d much of the mass (about 1000 kg/ha), the A. t o r t i l i s woodlands (750 kg/ha), where understory shrubs were abundant, and a l l bushland h a b i t a t s (1000 to 1500 kg/ha), except f o r the bushed g r a s s l a n d (170 kg/ha). Twig/leaf biomass was low (190 kg/ha), i n the swamp edge g r a s s l a n d s (Seg), and absent from a l l other g r a s s l a n d s (Gr, Lk, Con) and the wet swamps (Swp). In a l l h a b i t a t s , a v a i l a b l e t w i g / l e a f mass was c o n s i d e r a b l y l e s s than the t o t a l dry mass on o f f e r i n the herb l a y e r , d u r i n g the mid r a i n s . In some bushland communities, (Obn, Dbn, Dbs) t w i g / l e a f mass was f a i r l y high, almost comparable' to herb l a y e r green mass, but not t o t a l dry mass. 53 When d i v e r s i t y of a v a i l a b l e biomass in h a b i t a t s i s compared (as i n F i g u r e 6), the woodland h a b i t a t s (Tw, Xw, Ss) and a l l bushland h a b i t a t s appeared to have high d i v e r s i t y of woody s p e c i e s , while the bushlands a l s o had r e l a t i v e l y high herb l a y e r d i v e r s i t y . O v e r a l l , the bushlands had a g r e a t e r d i v e r s i t y of p l a n t s p e c i e s i n both the woody and herb l a y e r a v a i l a b l e as p o t e n t i a l foods f o r the elephants d u r i n g the r a i n s . 3. Dynamics of the herb l a y e r in d i f f e r e n t h a b i t a t types F i g u r e s 7 and 8 summarize the dynamics of green and t o t a l biomass of the herb l a y e r of the d i f f e r e n t h a b i t a t types of the b a s i n and bushlands f o r the 1978/79 p e r i o d . The swamps (Swp), swamp edge g r a s s l a n d s (Seg), and swamp edge woodlands (Pax, Yax) maintained high green and t o t a l dry mass through the year, while d r i e r h a b i t a t s of the Amboseli b a s i n and some bushland types showed wider f l u c t u a t i o n s . The lake bed g r a s s l a n d s had a high t o t a l mass, and low, f l u c t u a t i n g green mass throughout the year. The a l k a l i n e g r a s s l a n d s and Salvadora / Suaeda shrublands showed s i m i l a r changes in both green and t o t a l mass, dropping to low values ( l e s s than 500 kg/ha) in the dry season, although the l a t t e r maintained higher biomass throughout. The A. t o r t i l i s woodlands showed wider f l u c t u a t i o n s , r a p i d l y d e veloping green and t o t a l mass e a r l y in the r a i n s , and dropping back down to f a i r l y low l e v e l s (900 kg/ha t o t a l biomass) by the l a t e dry season. The fever t r e e woodlands had a f a i r l y high t o t a l biomass (8000 to 15,000 kg/ha) throughout. The denser bushland h a b i t a t s maintained a high t o t a l dry 54 1-5n V-OH 0-5H A J Woody Layer oc Ul > a m < T. O CO 2-OH 1-5H 1-OH 0-5H B) Herb Layer Gr Lk Con Tw Xw Ss FuxYax SegSwp Bar ObnObsDbnDbsAdb GrasslandH Woodland 1 Swamp-i Bushland 1 Figure 6. Diversity (Shannon-Weaver index) of available plant biomass in the woody and herb layers of Amboseli habitat types in January 1979. The bushland types had high diversity in both woody and herb layers, while the basin woodlands (Tw,Xw,Ss) were also fairly diverse. 55 10000n 1000H 100H 1 0 H O »A iooo« t o A) Total Dry Mass Swp 100H 10H B) Green Mass O N D J F M A M J J A S O N 1978 1979 Figure 7. Dynamics of total biomass and. green biomass of the herb layer of Amboseli basin habitats in 1978/79. The scale is logarithmic. Peak biomass for most habitats was reached in January/February 1979. The biomass of green vegetation declined more quickly than the total biomass. Note that the swamps and swamp edges (Pax.Seg) maintained high total and green biomass throughout the year. 56 A) Total Dry Mass 1000H 100H 1CH CM 2 1 \ o 5 1000-, B l G r e e n M a S S i o o H 1<H 0 N D 1978 J F M A M J J A S 0 1979 Figure 8. Dynamics of total biomass and green biomass of the herb layer of Amboseli bushland habitats in 1978/79. As with basin habitats , peak biomass was reached in January/February 1979. Most bushland types showed a sharp biomass increase with the rains and a gradual decline in the dry season. 57 mass through the year, while the bushed g r a s s l a n d (Bgr) had the most widely v a r y i n g biomass of the bushland h a b i t a t types, d e c r e a s i n g to low l e v e l s (70 kg/ha) in the l a t e dry season. Two of the northern bushland types, the open (Obn) and dense (Dbn) northern bushlands, a c t u a l l y showed a d e c l i n e i n herb l a y e r dry mass with the onset of the short r a i n s , l a r g e l y because dry standing grass from the p r e v i o u s dry season was knocked down with the f i r s t r a i n s . Green mass showed a steady i n c r e a s e with r a i n f a l l i n these h a b i t a t s . Most h a b i t a t s reached peak green mass in December or January; some bushland h a b i t a t s (Bgr, Dbn, Adb) peaked l a t e r i n February. T h i s appeared to represent the maximum p o i n t of herb l a y e r growth, apparently r e l a t e d to the p h e n o l o g i c a l changes of l e a f f l u s h , f l o w e r i n g , and subsequent seed-set. A f t e r t h i s p o i n t , the green mass d e c l i n e d . T o t a l grass/herb biomass plateaued at roughly t h i s time, but remained high f o r some months to f o l l o w i n most h a b i t a t s . At t h i s p o i n t , the grasses were slowly d r y i n g out, and, presumably, t r a n s l o c a t i n g n u t r i e n t s to t h e i r root systems (McNaughton 1979b). The t o t a l biomass of the herb l a y e r in most h a b i t a t types remained f a i r l y high u n t i l the dry season, when consmuption by elephants and the migrant g r a z i n g h e r b i v o r e s (wildebeest, zebra, and l i v e s t o c k ) began to reduce biomass l e v e l s . In the bushed g r a s s l a n d s (Bgr), consumption by domestic l i v e s t o c k reduced t o t a l biomass l e v e l s promptly a f t e r the r a i n s . The migrant h e r b i v o r e s moved to bushland areas d u r i n g the r a i n s (as d e s c r i b e d i n Chapter 2), and used the basin h a b i t a t s e x t e n s i v e l y i n the dry season, i n a 58 sequence from a l k a l i n e (Gr) and lake bed (Lk) gr a s s l a n d s through the woodlands (Tw, Xw) and Salvadora / Suaeda shrublands (Ss) to swamp edge grasslands (Seg) and woodlands (Pax, Yax). They reduced the herb l a y e r biomass s u b s t a n t i a l l y i n the gr a s s l a n d s and woodlands as the dry season continued, although d u r i n g my study p e r i o d , they d i d not venture deeply i n t o the swamp edge h a b i t a t s . The longer term data from Western's monitoring (see F i g u r e 9) of h i s permanent p l o t s shows how seasonal herb l a y e r dynamics can vary under d i f f e r e n t r a i n f a l l regimes. During the drought p e r i o d l e a d i n g up to 1977/78, the herb l a y e r s of a l l h a b i t a t s , i n c l u d i n g the swamps, f l u c t u a t e d d r a m a t i c a l l y , reaching low l e v e l s by the end of the dry season as the l a r g e h e r b i v o r e s made ex t e n s i v e use of a l l the bas i n h a b i t a t s . In subsequent wetter years (1977 to -1979), o s c i l l a t i o n s i n biomass were damped somewhat and occurred at a higher l e v e l . The swamp edge h a b i t a t s (Pax, Yax, Seg) were used l e s s by the p l a i n s g r a z e r s during the dry seasons, and these h a b i t a t s b u i l t up a biomass " c a p i t a l " d u r i n g t h i s p e r i o d . In the recent p e r i o d of low r a i n f a l l i n 1980, green and t o t a l biomass values dropped back down in a l l h a b i t a t s except the swamp edges, which r e t a i n e d the biomass c a p i t a l developed d u r i n g the years of higher r a i n f a l l . I a n a lyzed the r e l a t i o n s h i p between r a i n f a l l and a simple estimate of herb l a y e r biomass p r o d u c t i o n i n d i f f e r e n t h a b i t a t s by p l o t t i n g t o t a l dry mass measured in the l a t e dry season and e a r l y to mid r a i n s —- the growth phase a g a i n s t cumulative r a i n f a l l at 01 Tukai from the onset of r a i n s to the date of I 59 Figure 9. Long term dynamics of total herb layer biomass in some Amboseli habitat types. During the dry period before Nov. 1977, the biomass in a l l habitats was f a i r l y low, and showed wide fluctuations during the rains . In the Wet period, 1977/79, herb layer biomass increased in a l l habitats, and fluctuations were at a higher l eve l . In subsequent dry year, 1979/80, biomass was reduced again in the grasslands, woodlands, and bushlands, but has remained high in the swamp edges (Pax,Seg). 60 v e g e t a t i o n sampling. These r e g r e s s i o n s were based on only a few data p o i n t s (only three to f i v e measurements at the p l o t s before peak biomass), and the r a i n f a l l records from only one s t a t i o n . Even so, many of the r e g r e s s i o n s were s i g n i f i c a n t , and the c o e f f i c i e n t s of determination were g e n e r a l l y high (see Table 3). There were marked d i f f e r e n c e s i n the slopes of the r e g r e s s i o n s between r a i n f a l l and dry mass i n the d i f f e r e n t h a b i t a t s d u r i n g 1978/79. The bushlands (Obn, Obs, Dbn, Dbs), woodlands (Tw, Xw), and swamps (Swp) had a high r a t e of biomass growth with r a i n f a l l , while the bushed g r a s s l a n d (Bgr) and Salvadora / Suaeda shrublands (Ss) i n c r e a s e d at a more moderate r a t e , and the a l k a l i n e and lake-bed g r a s s l a n d s (Gr, Lk) and swamp edge h a b i t a t s (Seg, Pax, Yax) had a f a i r l y slow response to r a i n f a l l . The r e l a t i o n s between herb l a y e r growth" i n many h a b i t a t s and r a i n f a l l appear to be d i f f e r e n t in the longer term data from before and a f t e r 1978/79 (see Table 3). The seasonal dynamics of the a l k a l i n e g r a s s l a n d s (Gr) and bushed g r a s s l a n d s (Bgr) remained f a i r l y s i m i l a r through the d i f f e r e n t r a i n f a l l p e r i o d s , with some increase i n the slope of the r a i n f a l l / g r o w t h r e l a t i o n in the wet years. Swamp edge g r a s s l a n d s (Seg) had the steepest response to r a i n f a l l in the d r i e r p e r i o d s , when biomass f l u c t u a t i o n s were more extreme. Herb l a y e r growth i n the woodlands, represented by the Salvadora / Suaeda shrublands (S s ) , showed a marked in c r e a s e from the e a r l y drought (pre 1977) to the wet years (1977-1979), and a subsequent decrease in the dry p e r i o d of 1979/80. It appears that the c u r r e n t s t a t e of herb l a y e r v e g e t a t i o n can have a strong i n f l u e n c e on i t s 61 Table 3. R e l a t i o n s h i p of herb l a y e r biomass growth to r a i n f a l l r a i n f a l l , showing r e g r e s s i o n c o e f f i c i e n t s of t o t a l biomass vs. Cumulative r a i n f a l l from the s t a r t of the short r a i n s u n t i l peak growth, i n d i f f e r e n t h a b i t a t types and d i f f e r e n t years ( i n g/m /mm r a i n f a l l ) . r 2 i s given i n parentheses, with s i g n i f i c a n t r e g r e s s i o n s noted. HABITAT RAINFALL YEAR TYPE 1976/77 1977/78 1978/79 1979/80 GRASSLAND A l k a l i n e 0 .48( .731 ) 0 .34( .929)** 0 .60( .960) 0. 42( .896) Lake bed 1 .71 ( .994)* 0 .26( .956)** 1 .06( .876) -1 . 14( .455) WOODLAND A. t o r t . - - - 5 .77( .718) - -Fever - - - - 2 .85( .542) - -S/S shrub 0 • 83( .781 ) 1 .94( .927)** 1 .54( .947) 0. 46( .852) Palm 9 ..77( .999)** 1 .32( .859)* 0 .27( .034) 4. 52( .899) SWAMP Edge gr. 7 .63( .972) 2 . 1 1 ( .940)** 1 .25( .302) 4. 43( .525) Swamp — — — — 5 .56( .906) — — BUSHLAND Bush gr. 0 .87( .759) 0 .50( .631 ) 1 . 14( .984)* 0. 93( .782) Open N. - - - - 4 .22( .999),** - - Open S. - - - - 3 • 27( .675) - - Dense N. - - - - 6 .49( .870) - -Dense S. - - - - 4 .08( .601 ) - -A. drep. 2 .74( .257) n = 3 5 3 3 S i g n i f i c a n c e of r 2 : * p < .05 ** p < .01 62 p o t e n t i a l f o r growth with r a i n f a l l . 4_. Herb l a y e r n u t r i e n t dynamics Data c o l l e c t e d by D. Western i n 1976 (pers. comm.) show the dynamics of herb l a y e r crude p r o t e i n content f o r some h a b i t a t types (see F i g u r e 10). Woodland (Ss) herb l a y e r had the highest p r o t e i n content a l l through the year, and v a r i e d s u b s t a n t i a l l y through seasonal changes i n phenology: 23 to 12 percent crude p r o t e i n (% C P . ) from long r a i n s to l a t e dry season. The p r o t e i n content of the herb l a y e r s of both the a l k a l i n e g r a s s l a n d s (Gr) and bushed g r a s s l a n d s (Bgr) was f a i r l y high, and a l s o decreased, although l e s s d r a m a t i c a l l y , i n the dry season: 18 to 8 % C P . The lake bed gra s s l a n d s (Lk) and, e s p e c i a l l y , swamp edge gr a s s l a n d s (Seg) had low, f a i r l y constant p r o t e i n content through the year: 8 to 5 % C P . D. Di s c u s s i o n The h a b i t a t types I d e f i n e d f o r Amboseli f a l l i n t o two broad c a t e g o r i e s : those with ground water and more or l e s s continuous, though s e a s o n a l l y v a r y i n g , p o t e n t i a l f o r primary p r o d u c t i o n , and those dependent on seasonal r a i n f a l l f o r the s o i l water necessary f o r p l a n t growth. The f i r s t group i n c l u d e s the swamps (Swp), swamp edge g r a s s l a n d s (Seg), and swamp edge woodlands (Pax, Yax), and the second group i n c l u d e s the A. t o r t i l i s (Tw) and fever t r e e (Xw) woodlands, the a l k a l i n e (Gr) and lake bed (Lk) g r a s s l a n d s , and the s e m i - a r i d bushlands. 63 ! APR MAY. JUN JUL AUG SEP 0 C T ! F i g u r e 10. The dynamics o f herb l a y e r c rude p r o t e i n content i n c e r t a i n Ambosel i h a b i t a t types i n 1977 ( a f t e r Western ( p e r s . comm.)) . The wood lands , r e p r e s e n t e d by the Sa lvadora/Suaeda s h r u b l a n d s , S S , had h i g h h e r b i l a y e r p r o t e i n content i n the r a i n s , which dropped i n the dry season t o moderate l e v e l s . The herb l a y e r o f a l k a l i n e g r a s s l a n d s , GR, and bushed g r a s s l a n d s , BGR, showed s i m i l a r , but lower l e v e l , seasona l crude p r o t e i n c o n t e n t . The l a k e bed g r a s s l a n d s , LK , and swamp edge g r a s s l a n d s , SEG, had low p r o t e i n c o n t e n t throughout the y e a r . ! I 64 The l a t t e r "rain-dependent" h a b i t a t s have herb l a y e r s with r e l a t i v e l y lower biomass and higher p r o t e i n content than the former ground-water h a b i t a t s . In the rain-dependent h a b i t a t s , biomass and q u a l i t y were more d i r e c t l y l i n k e d to r a i n f a l l p a t t e r n s (see below), and thus were more v a r i a b l e on a seasonal or longer term b a s i s . The bushland h a b i t a t s were more p r o d u c t i v e i n the r a i n s , and had more d i v e r s e and n u t r i t i o u s p l a n t m a t e r i a l on o f f e r than most of the basin h a b i t a t s , except fo r the A. t o r t i l i s and fever t r e e woodlands during wet y e a r s . There were marked d i f f e r e n c e s i n o v e r a l l s t r u c t u r e and herb l a y e r dynamics between the h a b i t a t types of Amboseli. T o t a l biomass of the herb l a y e r was g r e a t e r than or equal to t w i g / l e a f biomass w i t h i n h a b i t a t s at a l l times of year during 1978/79. At peak herb l a y e r biomass, t h i s d i f f e r e n c e was g r e a t e r than an order of magnitude in a l l h a b i t a t types, except the Salvadora / Suaeda shrublands (Ss) and the open northern bushlands (Obn). However, in the l a t e dry season of 1979, there was g r e a t e r biomass i n the woody l a y e r of the more densely wooded h a b i t a t types -- the bushlands and swamp edge woodlands -- than i n the herb l a y e r of other h a b i t a t s --. the a l k a l i n e g r a s s l a n d s , Salvadora / Suaeda shrublands, A. t o r t i l i s woodlands, and bushed g r a s s l a n d s . During low r a i n f a l l y e a r s , when the herb l a y e r abundance in many p l a n t communities i s reduced to very low l e v e l s by h e r b i v o r e consumption, the twig and l e a f m a t e r i a l i n wooded h a b i t a t types c o u l d have g r e a t e r r e l a t i v e importance. The crude p r o t e i n of twigs and e s p e c i a l l y leaves of woody p l a n t s can be r e l a t i v e l y high; values range from 10 to over 30 65 p e r c e n t c r u d e p r o t e i n h a s b e e n r e p o r t e d b y v a r i o u s a u t h o r s ( D o u g a l l e t a l . , 1 9 6 4 , F i e l d 1 9 7 1 , P e l l e w 1980) f o r d i f f e r e n t p a r t s o f w o o d y p l a n t s p e c i e s f o u n d i n A m b o s e l i . T h i s m a t e r i a l , w h e n s u f f i c i e n t l y a b u n d a n t , c o u l d p r o v i d e a v a l u a b l e f o o d s o u r c e f o r e l e p h a n t s . I t a p p e a r s t h a t h e r b l a y e r v e g e t a t i o n i n t h e r a i n f a l l - d e p e n d e n t h a b i t a t s ( g r a s s l a n d s , w o o d l a n d s , a n d b u s h l a n d s ) o f A m b o s e l i i s p o t e n t i a l l y p r o d u c t i v e o n l y d u r i n g t h e r a i n y s e a s o n m o n t h s o f N o v e m b e r t o M a y , w i t h l i t t l e o r n o p r o d u c t i o n d u r i n g t h e r e s t o f t h e y e a r , w h e n t h e r e i s n o r a i n f a l l . T h i s c o n t r a s t s w i t h t h e s i t u a t i o n s e e n i n t h e g r a s s l a n d s o f S e r e n g e t i ( M c N a u g h t o n 1 9 7 9 a , b ) w h i c h r e c e i v e l i g h t r a i n f a l l i n t h e d r y s e a s o n s , a n d a r e p o t e n t i a l l y p r o d u c t i v e , e s p e c i a l l y w h e n g r a z e d , a t m o s t t i m e s o f y e a r . I t i s a l s o c l e a r t h a t g r a s s b i o m a s s a n d p r o d u c t i o n v a r i e s c o n s i d e r a b l y b e t w e e n h a b i t a t s a n d b e t w e e n y e a r s , a n d i s a f f e c t e d b y b o t h t h e f l u c t u a t i n g r a i n f a l l , w h i c h s t i m u l a t e s b i o m a s s p r o d u c t i o n , a n d c o n s u m p t i o n b y t h e d i f f e r e n t h e r b i v o r e s i n t h e s y s t e m , w h i c h r e d u c e s t h e h e r b l a y e r b i o m a s s . T h e q u a n t i t a t i v e v a l u e s o f t h e s e r e l a t i o n s h i p s a r e c u r r e n t l y u n d e r s t u d y ( D . W e s t e r n , p e r s . c o m m . ) . 66 CHAPTER 4 ELEPHANT HABITAT USE AND PREFERENCE A. I n t r o d u c t i o n In t h i s chapter, I w i l l examine elephant h a b i t a t use with r e f e r e n c e to seasonal changes. The approach w i l l focus on the Amboseli basin and i t s mosaic of h a b i t a t types. Elephants use the whole range of b a s i n h a b i t a t s i n a given season, month, or even on the same day. A c e n t r a l assumption I make i s that a l l areas and h a b i t a t types i n the Amboseli basin are a v a i l a b l e to elephants at a l l times of the day and year. I make a f u r t h e r assumption that my p e r c e p t i o n and d e f i n i t i o n of separate " h a b i t a t types" i s congruent with the way elephants p e r c e i v e and use them. The f i r s t assumption i s most probably v a l i d , i n that a l l areas of the Amboseli b a s i n are p o t e n t i a l l y f r e e f o r elephant use. The second assumption i s more a r b i t r a r y , and l e s s l i k e l y to be t r u e — my p o s i t i o n i n t h i s case i s that my d e f i n i t i o n s were based on b r o a d - l e v e l s t r u c t u r a l and f l o r i s t i c d i f f e r e n c e s between s p e c i f i c s i t e s that I c o u l d i d e n t i f y and measure (see Chapter 3). In the f o l l o w i n g s e c t i o n s , I w i l l i d e n t i f y the h a b i t a t types s e l e c t e d and avoided by e l e p h a n t s . Elephants spend a l a r g e p r o p o r t i o n of t h e i r time f e e d i n g ; a range of 50 to 75 percent of t o t a l d a i l y a c t i v i t y has been repo r t e d i n the l i t e r a t u r e , and reviewed by Guy (1976b). With t h i s i n mind, I w i l l examine food, or p l a n t m a t e r i a l a v a i l a b l e as food, f o r elephants, as a p o t e n t i a l b a s i s of t h e i r h a b i t a t s e l e c t i o n . H a b i t a t occupancy by elephants w i l l thus be examined with 67 r e f e r e n c e to v e g e t a t i o n abundance in the d i f f e r e n t h a b i t a t types. Many s t u d i e s of elephant d i s t r i b u t i o n (Caughley and Goddard 1975, E l t r i n g h a m 1977, Lamprey 1963, Leuthold and Sale 1973, N o r t o n - G r i f f i t h s 1975) found that the p o p u l a t i o n s were non- randomly d i s t r i b u t e d , with respect to g e o g r a p h i c a l f e a t u r e s or broad h a b i t a t c a t e g o r i e s . The work of Leuthold (1977b), which l i n k e d movements and h a b i t a t use s t r a t e g i e s to seasonal r a i n f a l l , suggested that the v e g e t a t i o n a v a i l a b l e i n h a b i t a t s might be an important determinant of elephant movements. Leuthold showed that w i t h i n p o p u l a t i o n s , b u l l s and f a m i l y u n i t s had a p p a r e n t l y d i f f e r e n t h a b i t a t use s t r a t e g i e s , although he d i d not suggest reasons for these d i f f e r e n c e s . In t h i s chapter, I w i l l t e s t the general hypotheses t h a t : H1. Elephant h a b i t a t d i s t r i b u t i o n i s nonrandom. H2. There are s i g n i f i c a n t r e l a t i o n s h i p s between elephant numbers and the biomass of the herb and woody l a y e r s i n the d i f f e r e n t h a b i t a t types. H3. There are d i f f e r e n c e s i n h a b i t a t use between bachelor b u l l and cow/calf herds. In the d i s c u s s i o n , I w i l l use my o b s e r v a t i o n s to suggest a general s t r a t e g y of h a b i t a t s e l e c t i o n by the e l e p h a n t s . 68 B. Methods Survey counts were the b a s i c t o o l I used in e v a l u a t i n g elephant h a b i t a t d i s t r i b u t i o n i n the Amboseli ecosystem. The survey data were c o l l e c t e d i n two ways: I made a s e r i e s of i n t e n s i v e ground-based counts i n the year November 1978 - October 1979, while D. Western has done a e r i a l t o t a l counts as par t of h i s ex t e n s i v e monitoring programme of the Amboseli ecosystem s i n c e 1975. I c o l l a b o r a t e d i n the c o l l e c t i o n and a n a l y s i s of the l a t t e r d a t a s e t . The a e r i a l and ground count data were c o l l e c t e d i n d i f f e r e n t ways: I w i l l analyse them d i f f e r e n t l y and use them f o r d i f f e r e n t purposes. Basic c o n c l u s i o n s about elephant h a b i t a t use w i l l be d e r i v e d using the ground count data, while the a e r i a l count data w i l l give an h i s t o r i c a l p e r s p e c t i v e . T o t a l counts of elephants i n the Amboseli basin were attempted because other r e s e a r c h e r s who used sample count estimates of elephant p o p u l a t i o n s i z e had encountered problems of accuracy and p r e c i s i o n . Because elephant p o p u l a t i o n s have low d e n s i t i e s and contagious d i s t r i b u t i o n s (Eltringham 1977), sample estimates are u s u a l l y of q u e s t i o n a b l e accuracy and have wide confidence i n t e r v a l s (Croze 1972, Watson et a l . 1969) unless the sampling f r a c t i o n i s l a r g e or the counting e f f o r t i s s t r a t i f i e d i n t o many regions of d i f f e r e n t elephant d e n s i t y . As w e l l , the a c t u a l number of animals counted i s u s u a l l y low, making f i n e r s c a l e a n a l y s i s of h a b i t a t d i s t r i b u t i o n dependent on only a few s i g h t i n g s . On the other hand, t o t a l counts s u f f e r the disadvantage of a bia s e d tendency to underestimate, and lack 69 confidence l i m i t s to i n d i c a t e the p r e c i s i o n of the estimate ( N o r t o n - G r i f f i t h s 1978). However, i f the t o t a l counts are done under the same c o n d i t i o n s i n a s e r i e s of counts, the values obtained can be compared with each other, to i n d i c a t e trends at l e a s t , with some degree of assurance. Surveys were focussed on the Amboseli b a s i n s i n c e i t i s a f a i r l y small area of the ecosystem where elephant use i s conc e n t r a t e d through much of the year; in e f f e c t , t h i s was a s t r a t i f i c a t i o n of survey e f f o r t . In a d d i t i o n to survey counts, I made c a s u a l o b s e r v a t i o n s of elephant movements, feeding behaviour, and i n d i v i d u a l group i d e n t i t y on an o p p o r t u n i s t i c b a s i s , throughout the study area and p e r i o d . Cynthia Moss has been observing the s o c i a l behaviour and monitoring the demography of the Amboseli elephant p o p u l a t i o n s i n c e 1974. She has i d e n t i f i e d a l l the i n d i v i d u a l members of the p o p u l a t i o n , and continues to rec o r d the s t a t u s of i n d i v i d u a l s as members of s t a b l e cow/calf f a m i l y u n i t s , or as independent, f r e e l y a s s o c i a t i n g a d u l t b u l l s . I a s s i s t e d her in t h i s work i n 1978/79, and I i n c l u d e some of her p u b l i s h e d and unpublished r e s u l t s , with her p e r m i s s i o n . In t h i s s e c t i o n , I d e s c r i b e the methods I used to monitor and analyse elephant h a b i t a t d i s t r i b u t i o n s . 70 K Count ing methods a. Ground counts In the ground surveys, I t r i e d to o b t a i n t o t a l counts as fa r as were p o s s i b l e from a ground-based v e h i c l e ; a Suzuki f o u r - wheel d r i v e "jeep" was the v e h i c l e I used for most of the ground counts. I designed the surveys to cover each area and h a b i t a t type of the Amboseli b a s i n i n each part of the day at l e a s t . o n c e a month, so that t o t a l daytime use of the b a s i n by elephants would be sampled. I attempted to complete e i g h t survey c i r c u i t s of the Amboseli b a s i n in each month. Each c i r c u i t c o n s i s t e d of an ea s t e r n (three hours t r a v e l time) and a western (three-hour) loop (see F i g u r e 11). E i t h e r loop c o u l d be t r a v e l l e d two ways, "clo c k w i s e " or " c o u n t e r - c l o c k w i s e " as viewed on the map. T h i s meant there were four p o s s i b l e six-hour routes which c o u l d be done in e i t h e r the morning or the af t e r n o o n , or a t o t a l o f . e i g h t p o s s i b l e c i r c u i t s i n the f u l l d a y l i g h t p e r i o d of 6 am. to 6 pm. I a r b i t r a r i l y d i v i d e d the d a y l i g h t hours i n t o four equal time b l o c k s : TBIk1 - 6 am. to 9 am., TBIk2 - 9 am. to 12 noon, TBlk3 - 12 noon to 3 pm., TBIk4 - 3 am. to 6 pm. V i s i t s would thus be made to every area of the Amboseli basin twice (once i n each of two d i r e c t i o n s ) per month in each time b l o c k . Surveys were done at roughly 3 to 4 day i n t e r v a l s ; the order of survey routes was chosen at random i n each month. Night-time survey counts were not p o s s i b l e , but I attempted to observe elephant a c t i v i t i e s and movements a f t e r dark by 71 F i g u r e 11. Routes of ground-based surveys of the Amboseli b a s i n , showing the West and East Loops of the six-hour c i r c u i t . — G r a s s l a n d CI] Gr — W o o d l a n d m TW Swamp E H Seg B u s h l a n d Bgr E21 Obn 73 f o l l o w i n g f a m i l y u n i t s through a number of moonlit n i g h t s . Counts were done each month of the r a i n f a l l year from November 1978 to October 1979. I was p r i n c i p a l observer on counts u n t i l mid-September, but o c c a s i o n a l l y I had an e x t r a observer as a s p o t t e r . A co-worker, Michael Milgroom, completed counts for me i n September and October; we had compared separate counts of elephant groups fo r the month of August and observer agreement was high. For v a r i o u s reasons, some surveys were missed i n three months: only four counts were done in November 1978, seven i n December, and f i v e i n October 1979. A t o t a l of 88 from a p o s s i b l e 96 counts was s u c c e s s f u l l y completed. In d e s i g n i n g routes, I made use of roads, d i r t t r a c k s , and c r o s s - c o u n t r y d r i v i n g , when necessary. The v e h i c l e was stopped at r e g u l a r i n t e r v a l s along the route and the sur-rounding area scanned from the roof with b i n o c u l a r s . I made use of two low h i l l s in the western basin as lookout p o i n t s which g r e a t l y i n c r e a s e d the d e t e c t a b i l i t y of elephants i n the surrounding a r e a s . Elephant groups l o c a t e d , but too f a r away to count a c c u r a t e l y , were approached and counted; I returned to the survey route a f t e r r e c o r d i n g the group. In the more open h a b i t a t types, such as the a l k a l i n e , lake-bed, and S. c o n s i m i l i s g r a s s l a n d s , fever t r e e woodlands, S. monoica shrublands, and swamp edges, v i s i b i l i t y was v i r t u a l l y u n r e s t r i c t e d and the whole h a b i t a t area was c o n s i d e r e d surveyed. In h a b i t a t s with dense woody cover -- the A. t o r t i l i s woodlands, date palm/fever t r e e and young fever t r e e swamp edge woodlands -- or t a l l sedges, as i n the permanent swamps, I estimated the " v i s i b i l i t y p r o f i l e " 74 ( N o r t o n - G r i f f i t h s 1978) of the " t r a n s e c t " represented by the survey route with a ster e o r a n g e f i n d e r . In these l a t t e r h a b i t a t types, a subset of the t o t a l h a b i t a t area was observed (see Table 4 f o r areas of h a b i t a t s surveyed), but I do not conside r Table 4. Areas of h a b i t a t types surveyed in a e r i a l and ground counts of the Amboseli b a s i n . The p r o p o r t i o n of the t o t a l area surveyed i n ground counts which was occupied by each h a b i t a t type i s shown, i n c l u d i n g and e x c l u d i n g the lake bed g r a s s l a n d s . HABITAT AERIAL COUNTS GROUND COUNTS Type Area(km*) Area(km i) P. of T o t a l P. of T o t a l ( i n c . Lake) (exc. Lake) GRASSLAND A l k a l i n e 130.6 Lake bed 85.9 S. c o n s i m i l i s 30.5 130.6 85.9 30.5 0.3559 0.2342 0.0830 0.4648 0. 1084 WOODLAND A. t o r t i l i s Fever t r e e S/S shrubs Palm/Fever Young Fever SWAMP Edge g r a s s . Swamp 42 43 33 5 4 10, 14 9 2 1 .2 2 10, 7 20.7 43.2 33. 1 2. 1 3.5 0.0564 0.1177 0.0902 0.0056 0.0096 0.0271 0.0201 0.0737 0.1537 0. 1 178 0.0074 0.0126 0.0354 0.0263 T o t a l 400.2 367.0 0.9998 1.0001 t h i s a s t a t i s t i c a l l y v a l i d sample of those h a b i t a t types, for reasons I d i s c u s s below. During each survey, every separate elephant group encountered was recorded. D e f i n i t i o n of "separate"- groups was a r b i t r a r y : groups ( i n c l u d i n g s i n g l e animals) more than about 50 75 m. apart were c o n s i d e r e d "separate". T h i s i s probably a reasonable, f u n c t i o n a l d e f i n t i o n of separate f o r a g i n g groups, s i n c e the d i s t a n c e between elephants w i t h i n groups was g e n e r a l l y only a few metres. For each group I noted: time block, l o c a t i o n , h a b i t a t type, a c t i v i t y -- f e e d i n g , or one of seven non-feeding c a t e g o r i e s of the m a j o r i t y of the group, group type, and group s i z e . If the group was a cow/calf herd, I attempted to count the number of a s s o c i a t e d a d u l t b u l l s , but i t was not always p o s s i b l e to d i s t i n g u i s h the s m a l l - and medium- s i z e d independent b u l l s from cow/calf group members, e s p e c i a l l y in l a r g e a g g r e g a t i o n s . I estimated group s i z e by counting the group s e v e r a l times u n t i l I reached a c o n s i s t e n t f i g u r e . For a small p r o p o r t i o n of the 656 cow/calf groups encountered (15%), an a c c u r a t e count was c l e a r l y not p o s s i b l e because of h a b i t a t s t r u c t u r e (11 % ) _ - - high grass or shrubs which h i d smaller j u v e n i l e s -- or because the elephants were standing i n a t i g h t l y bunched group (3 %) which a l s o h i d immature animals from view, or both ( 1 % ) . In these cases, I counted the v i s i b l e elephants and a note was made f o r b i a s c o r r e c t i o n in l a t e r a n a l y s i s (see " C o r r e c t i o n s for b i a s " below). Two a d d i t i o n a l sources of b i a s a f f e c t e d the counting of elephants i n d i f f e r e n t h a b i t a t s . The survey routes were not chosen at random and avoided c e r t a i n areas where v e h i c l e t r a v e l was d i f f i c u l t , but where elephants may have been present or absent i n a nonrandom way. The e f f e c t of t h i s b i a s i s u n c l e a r , but may have l e d to an underestimate of elephant numbers in the 76 denser h a b i t a t s . A second source of b i a s was that the r e l a t i v e d e t e c t a b i l i t y of small groups or s i n g l e i n d i v i d u a l s in the denser h a b i t a t s would have been lower than f o r l a r g e , more conspicuous groups. T h i s would tend to reduce both the t o t a l number of elephants seen, and the r e l a t i v e r e p r e s e n t a t i o n of small groups (see Chapter 5). I t was not p o s s i b l e to c o r r e c t for these two b i a s e s . b. A e r i a l counts Western used a small fixed-wing a i r c r a f t , u s u a l l y a Cessna 185, i n the a e r i a l counts, a c t i n g as both p i l o t and primary observer. A search p a t t e r n of p a r a l l e l l i n e s was flown over the woodlands, g r a s s l a n d s , and swamp courses of the b a s i n , at an a l t i t u d e above the ground of about 300 f t . Elephant v i s i b i l i t y i n v i r t u a l l y a l l h a b i t a t s was good to e x c e l l e n t . Woody cover approached 20 % i n only a few small and s c a t t e r e d areas of woodland, so that canopy cover d i d not hide many elephants from view, and the areas of a l l h a b i t a t types overflown were co n s i d e r e d to be surveyed completely (see Table 4). A l l the counts were flown in l a t e morning, g e n e r a l l y between 10 and 11 a.m., and took about one hour of • f l y i n g time. O c c a s i o n a l l y , a d d i t i o n a l observers were used as s p o t t e r s and re c o r d e r s -- I served i n t h i s c a p a c i t y f o r many of the f l i g h t s i n 1978 and 1979. On each count, the l o c a t i o n of every separate elephant group was recorded on an a i r p h o t o or v e g e t a t i o n map of the b a s i n . The type of group -- bachelor b u l l or cow/calf "breeding 77 herd" -- was a l s o noted. 'Western began the a e r i a l counts i n A p r i l 1975, and surveyed at f a i r l y r e g u l a r i n t e r v a l s (almost monthly) from October 1975 to the present; counts up to December 1980 were c o n s i d e r e d i n the present a n a l y s i s . In some of the e a r l y counts, only t o t a l elephant numbers were recorded, without s e p a r a t i o n i n t o group gender type or, in some counts, separate groups. Of the t o t a l of 61 counts s i n c e A p r i l 1975, 54 were c o n s i d e r e d " c l a s s i f i e d counts" f o r use i n the present a n a l y s e s . An i n e v i t a b l e b i a s was b u i l t i n t o both the ground and a e r i a l survey programs. Because surveys c o u l d be done in d a y l i g h t hours only, the r e s u l t s were bi a s e d towards those h a b i t a t s that elephants used i n the daytime. Wyatt and E l t r i n g h a m (1974) noted that elephants fed e x t e n s i v e l y at n i g h t , and not "necessarily in the same p l a c e s as they d i d ' i n the daytime. Any r e l a t i o n s h i p s between daytime h a b i t a t occupancy and v e g e t a t i o n c h a r a c t e r i s t i c s which are d e r i v e d i n the present a n a l y s i s are thus only p a r t of the t o t a l p i c t u r e of elephant h a b i t a t use, and must be viewed with t h i s b i a s i n mind. 2. Analyses The b a s i c s t a t i s t i c s c a l c u l a t e d for the ground count data were monthly t o t a l numbers of elephants seen a c r o s s a l l times of day. In each month, there were (or should have been) the same number ( e i g h t ) of counts, so that monthly t o t a l s were used as comparable u n i t s of p o p u l a t i o n d i s t r i b u t i o n . For ground count a n a l y s e s , seasons were s t r i c t l y d e f i n e d as e q u a l - s i z e d two month p e r i o d s : short r a i n s - Nov./Dec., mid r a i n s - Jan./Feb., long 78 r a i n s - Mar./Apr., e a r l y dry - May/June, mid dry - July/Aug., l a t e dry - Sept./Oct. For some a n a l y s e s , counts were pooled as seasonal sums of monthly t o t a l s , d e r i v e d from 16 counts each, or as seasonal means of two monthly t o t a l s per season. The a e r i a l counts were each taken as an "instantaneous" t o t a l count of the basin taken at a s p e c i f i c time of day, the l a t e morning, and thus i n d i v i d u a l counts were d i r e c t l y comparable. They can be d i v i d e d i n t o three groups r e f l e c t i n g three r a i n f a l l p e r i o d s : Dry1 - A p r i l 1975 to November 1977 (n=l9 c l a s s i f i e d c o u n t s ) , Wet - December 1977 to October 1979 (n=20 c o u n t s ) , and Dry2 - November 1979 to December 1980 (n=l5 c o u n t s ) . The number of counts i n each season, or i n each year or r a i n f a l l p e r i o d , was not always the same. A l l of the- f o l l o w i n g analyses were done s e p a r a t e l y for bachelor b u l l and cow/calf groups. a. C o r r e c t ions f o r bi a s and m i s s i n g counts In my a n a l y s i s of the ground count data, I made an attempt to c o r r e c t f o r the underestimation of numbers in cow/calf group records that I knew were bia s e d because of h a b i t a t s t r u c t u r e and/or group bunching (see Counting Methods, above). I a l s o attempted to " f i l l i n the gaps" f o r those months with m i s s i n g surveys. An ac c u r a t e r e c o r d was kept of the numbers, age s t r u c t u r e , and demography of the whole, or at l e a s t a very l a r g e segment of the whole, Amboseli elephant p o p u l a t i o n (C. Moss 1980, p e r s . comm.). It was p o s s i b l e to estimate the p r o p o r t i o n of d i f f e r e n t 79 s i z e c l a s s e s that c o u l d be found i n the cow/calf p o p u l a t i o n on a monthly b a s i s , and t h e r e f o r e the p r o p o r t i o n of animals that c o u l d be hidden by shrubs and grass of known h e i g h t , or by elephants i n d i f f e r e n t s i z e c l a s s e s . The c o r r e c t i o n f a c t o r f o r each h a b i t a t in each month c o u l d then be d e r i v e d as f o l l o w s : i f N ( t o t a l ) = N(seen) + N(hidden) then 1 = N ( s e e n ) / N ( t o t a l ) + N ( h i d d e n ) / N ( t o t a l ) and N ( s e e n ) / N ( t o t a l ) = 1 - P(hidden) N ( t o t a l ) = 1/[1 - P(hidden)] * N(seen) and C o r r e c t i o n Factor = - P ( h i d d e n ) ] , where N(x) = Number(x), and P(x) = P r o p o r t i o n ( x ) . For counts of i n d i v i d u a l groups which I had recorded as p o t e n t i a l l y biased by hidden animals, I m u l t i p l i e d the number a c t u a l l y seen by the a p p r o p r i a t e C o r r e c t i o n F a c t o r f o r the h a b i t a t and month of the count r e c o r d , and rounded the product up to the nearest whole number. The c o r r e c t i o n f a c t o r s were d e r i v e d from p o p u l a t i o n s t a t i s t i c s , and would not n e c e s s a r i l y r e f l e c t the p r o p o r t i o n s of animals i n d i f f e r e n t s i z e c l a s s e s which a c t u a l l y occurred in every i n d i v i d u a l group encountered, e s p e c i a l l y i f the group was a small sample of the p o p u l a t i o n . However, as a c o r r e c t i o n f o r the " p o s s i b l e " number of animals missing from the observer's view i n any given group, the e r r o r in e s t i m a t i o n should be randomly d i s t r i b u t e d a c r o s s the group s i z e s c o r r e c t e d i n t h i s way, with no c o n s i s t e n t b i a s (unless groups with many small 80 c a l v e s were more l i k e l y to be found in shrubs and dense g r a s s ) . I d i d not attempt to check the accuracy of t h i s c o r r e c t i o n f a c t o r technique. C o r r e c t i o n f a c t o r s f o r the number of animals i n a group which might be hidden by dense grasses or shrubs were c a l c u l a t e d f o r each h a b i t a t type (see Appendix 4). I used the average height of dense grass or shrubs w i t h i n h a b i t a t s to estimate which s i z e c l a s s e s of elephants would be hidden. T h i s allowed me to c a l c u l a t e the a p p r o p r i a t e C o r r e c t i o n F a c t o r s f o r each h a b i t a t in each month, as o u t l i n e d above. C o r r e c t i o n f a c t o r s f o r bunching, a l s o found i n Appendix 4, were c a l c u l a t e d f o r each month, as above. The assumption was made that i n a t i g h t l y bunched group of elephants, approximately one q u a r t e r of- a l l animals with shoulder h e i g h t s of about 200 cm. or l e s s (mean a d u l t female shoulder height i s approximately 250 cm., Laws et a l . 1975) would be l o c a t e d behind, not to e i t h e r s i d e or i n f r o n t o f , a d u l t s , and would not be v i s i b l e to a ground-based observer. Croze (1972) followed s i m i l a r reasoning when he c o r r e c t e d c a l f number estimates i n a Ugandan elephant p o p u l a t i o n . T h i s c o r r e c t i o n may s t i l l underestimate numbers i n l a r g e r , t i g h t l y bunched herds, since many of the small animals c o u l d then be hidden i n the middle of the group. In a very few group r e c o r d s , c o r r e c t i o n f o r both h a b i t a t s t r u c t u r e and bunching was necessary. In t h i s case, I c a l c u l a t e d the p r o p o r t i o n hidden by shrubs and grass f i r s t , then the a d d i t i o n a l p r o p o r t i o n hidden by other elephants, and summed the two. Since there were only a few i n s t a n c e s of t h i s double 81 b i a s o c c u r r i n g , I c a l c u l a t e d C o r r e c t i o n F a c t o r s f o r the necessary h a b i t a t s and months (see Appendix 4). For months with m i s s i n g surveys, count f i g u r e s were c o r r e c t e d upwards i n the a p p r o p r i a t e time b l o c k s depending on the s p e c i f i c -- morning or afternoon -- routes missed. Monthly sums in the time blocks with missing counts were m u l t i p l i e d by the i n v e r s e of the p r o p o r t i o n of completed counts i n the time b l o c k s . For example, i n December, one morning count was missed, or three of a p o s s i b l e four counts were completed. The monthly sums fo r the morning time b l o c k s , T B l k l and T B l k 2 , were m u l t i p l i e d by 4 /3 , and rounded up to the nearest whole number. The sums from a l l the time b l o c k s , i n c l u d i n g the " c o r r e c t e d " ones, were added together to give a c o r r e c t e d monthly t o t a l number of elephants observed. T h i s was done s e p a r a t e l y f o r cow/calf and b u l l monthly t o t a l s . The c o r r e c t e d values f o r elephant numbers are used i n a l l the f o l l o w i n g a n a l y s e s . b. Comparison of ground and a e r i a l counts The r e s u l t s from a e r i a l and ground counts, as noted e a r l i e r , may not be d i r e c t l y comparable. The ground counts were i n d i c e s r a t h e r than absolute p o p u l a t i o n e s t imates, and monthly t o t a l s r e f l e c t e d elephant occupancy of the basin a c r o s s the e n t i r e daytime p e r i o d , while a e r i a l counts r e f l e c t e d basin occupancy at a s p e c i f i c time of day, and were done l e s s f r e q u e n t l y , though f o r a longer p e r i o d . To examine the s i m i l a r i t y of r e s u l t s from the two survey methods, I c a l c u l a t e d the product-moment c o r r e l a t i o n of seasonal 82 m e a n s f r o m a e r i a l s u r v e y s w i t h g r o u n d - b a s e d s e a s o n a l s u m s f o r t h e w h o l e d a y a n d f o r t h e l a t e m o r n i n g t i m e b l o c k ( T B l k 2 ) , t h e t i m e w h e n a e r i a l c o u n t s w e r e f l o w n . c . L a r g e s c a l e h a b i t a t p r e f e r e n c e L a r g e s c a l e h a b i t a t c h o i c e - - b u s h l a n d s v s . b a s i n — i n 1 9 7 8 / 7 9 w a s e x a m i n e d b y a n a l y s i s o f v a r i a n c e o f m o n t h l y t o t a l n u m b e r s i n t h e b a s i n a c r o s s t h e s e a s o n s . I l o o k e d a t t h e p a t t e r n i n o t h e r y e a r s b y A N O V A o f a e r i a l c o u n t s a c r o s s s e a s o n s . T h e r e l a t i o n s h i p s b e t w e e n t o t a l n u m b e r s i n t h e b a s i n a n d r a i n f a l l , a n d t o t a l b i o m a s s a n d g r e e n b i o m a s s i n t h e h e r b l a y e r o f i n d i v i d u a l h a b i t a t t y p e s w e r e a s s e s s e d b y l i n e a r r e g r e s s i o n . F o r g r o u n d c o u n t s , I u s e d t o t a l r a i n f a l l f o r t h e m o n t h p r e c e d i n g t h e c o u n t m o n t h , w h i l e f o r a e r i a l c o u n t s , I c a l c u l a t e d c u m u l a t i v e r a i n f a l l f o r d i f f e r e n t l a g p e r i o d s ( 5 , 1 0 , 1 5 , 2 0 , 3 0 , 4 0 , 5 0 , 6 0 , 7 0 , 8 0 , a n d 90 d a y s ) p r e c e d i n g t h e c o u n t d a t e . W e s t e r n ( 1 9 7 5 ) u s e d a s i m i l a r t e c h n i q u e , a n d f o u n d t h a t l i v e s t o c k a n d w i l d g r a z e r n u m b e r s i n t h e A m b o s e l i b a s i n h a d t h e s t r o n g e s t ( n e g a t i v e ) c o r r e l a t i o n s w i t h r a i n f a l l o v e r t h e p r e c e d i n g 40 t o 50 d a y s . H e r b l a y e r b i o m a s s v a l u e s - - m i d - m o n t h f o r g r o u n d c o u n t s a n d d a t e - o f - c o u n t f o r a e r i a l c o u n t s - - w e r e i n t e r p o l a t e d f r o m t h e r u n n i n g r e c o r d o f p e r m a n e n t p l o t m e a s u r e m e n t s ( C h a p t e r 3 ) . i 83 d. H a b i t a t use and p r e f e r e n c e i n the Amboseli b a s i n , i n r e l a t i o n to v e g e t a t i o n and c 1 i m a t i c v a r i a b l e s H a b i t a t use and p r e f e r e n c e by elephants, as seen i n the ground count s e r i e s , was analyzed with a goodness of f i t c h i squared technique, combined with a " f a m i l y " of confidence i n t e r v a l s (Neu et a l . 1974). For each season, I compared the frequency d i s t r i b u t i o n of elephants observed i n h a b i t a t types with an expected random d i s t r i b u t i o n . Assuming that a l l areas surveyed were e q u a l l y a v a i l a b l e to a l l elephants, a random d i s t r i b u t i o n would have elephant numbers occur in h a b i t a t s in p r o p o r t i o n to the area of the h a b i t a t s . I c a l c u l a t e d the expected random d i s t r i b u t i o n f o r ground counts by m u l t i p l y i n g the t o t a l number counted in the whole basin i n a given season by the p r o p o r t i o n of the basin area surveyed which occ u r r e d i n each h a b i t a t type (see Table 4). B o n f e r r o n i normal s t a t i s t i c s ( M i l l e r 1966) were used to c o n s t r u c t simultaneous confidence i n t e r v a l s about the i n d i v i d u a l observed p r o p o r t i o n s i n h a b i t a t s f o r comparison with expected p r o p o r t i o n s . S e l e c t e d and avoided h a b i t a t types were i d e n t i f i e d as expected values f o r h a b i t a t s in each season f e l l o u t s i d e the confidence l i m i t s about the observed v a l u e s . I compared the d i s t r i b u t i o n of bachelor b u l l and cow/calf numbers a c r o s s h a b i t a t types i n each season using Spearman's rank c o r r e l a t i o n ( S i e g e l 1956). Ha b i t a t d i s t r i b u t i o n s as seen in the a e r i a l count s e r i e s are presented f o r comparative purposes, but not subjected to the above a n a l y s e s . The a e r i a l count data probably do not r e f l e c t 84 f i n e s c a l e h a b i t a t u s e v e r y a c c u r a t e l y b e c a u s e o f t h e i r l o w s a m p l i n g i n t e n s i t y a n d t h e g r e a t v a r i a n c e i n d a i l y e l e p h a n t h a b i t a t d i s t r i b u t i o n s . T h e r e l a t i o n s h i p s b e t w e e n n u m b e r s o b s e r v e d o n g r o u n d c o u n t s a n d h e r b l a y e r b i o m a s s w i t h i n h a b i t a t t y p e s w e r e e x a m i n e d t h r o u g h l i n e a r r e g r e s s i o n . M o n t h l y t o t a l n u m b e r s i n h a b i t a t s w e r e a n a l y s e d u s i n g l i n e a r r e g r e s s i o n o n t o t a l r a i n f a l l o f t h e p r e c e d i n g m o n t h , a n d m i d - m o n t h h e r b l a y e r t o t a l d r y m a s s a n d g r e e n m a s s v a l u e s i n t e r p o l a t e d f r o m t h e p e r m a n e n t p l o t r e c o r d . I c o m p a r e d t h e d i s t r i b u t i o n o f e l e p h a n t d e n s i t y w i t h t h e d i s t r i b u t i o n o f p l a n t b i o m a s s d e n s i t y - - t o t a l a n d g r e e n h e r b l a y e r d r y m a s s , w o o d y t w i g - p l u s - l e a f d r y m a s s , a n d h e r b l a y e r p l u s w o o d y b i o m a s s — a c r o s s h a b i t a t t y p e s i n e a c h s e a s o n u s i n g S p e a r m a n ' s " r a n k c o r r e l a t i o n . C . R e s u l t s ! • C o m p a r i s o n o f g r o u n d a n d a e r i a l c o u n t s T h e c o r r e l a t i o n s b e t w e e n g r o u n d a n d a e r i a l s e a s o n a l b a s i n c o u n t s a r e s h o w n i n T a b l e 5 . C o r r e l a t i o n s w e r e p o o r b e t w e e n a e r i a l c o u n t s a n d g r o u n d c o u n t s f o r a l l t i m e b l o c k s p o o l e d ; t h e c o r r e l a t i o n s w e r e b e t t e r w h e n g r o u n d c o u n t s i n t i m e b l o c k 2 a l o n e w e r e c o m p a r e d w i t h a e r i a l c o u n t s , a n d b u t w e r e s t i l l n o t s i g n i f i c a n t a t p = . 0 5 . I t a p p e a r s t h a t t h e a e r i a l c o u n t s w e r e n o t a n a c c u r a t e s a m p l e o f t o t a l d a y t i m e o c c u p a n c y o f t h e A m b o s e l i b a s i n b y e l e p h a n t s , b u t w e r e b i a s e d t o w a r d s t h e t i m e o f d a y w h e n t h e y w e r e c o l l e c t e d . T h e g r o u n d c o u n t s may g i v e a m o r e 85 Table 5. C o r r e l a t i o n s of a e r i a l seasonal means and ground- based seasonal t o t a l s of bachelor b u l l and cow/calf numbers in 1978/79, showing low c o r r e l a t i o n when a l l time blocks are c o n s i d e r e d , and higher c o r r e l a t i o n when when the l a t e morning time block alone i s used. No c o r r e l a t i o n s were s i g n i f i c a n t at p=.05, n=6. TIME BLOCKS CORRELATION COEFFICIENTS Bachelor b u l l numbers Cow/calf numbers A l l time blocks 0.2510 0.6777 Time block 2 0.6042 0.7864 ( l a t e morning) complete p i c t u r e of t o t a l daytime h a b i t a t use, f o r the 1978/79 study p e r i o d , at l e a s t . However, the a e r i a l counts remain u s e f u l f o r comparing the long term p a t t e r n s and d i f f e r e n c e s i n h a b i t a t use. 2. Large s c a l e habi t a t c h o i c e a. Bachelor b u l l s Seasonal mean ground counts (the means of two monthly t o t a l s per season) of 1978/79 are shown in F i g u r e 12, along with the r e s u l t s of ANOVA comparisons of numbers i n the basin between seasons. These show that t o t a l numbers of b u l l s i n the ba s i n d i f f e r e d s i g n i f i c a n t l y (exact p=.014 ) between seasons, and that there was a trend f o r b u l l numbers to i n c r e a s e i n the mid and l a t e dry season. I f the l a t e morning time block (TBlk2) alone was c o n s i d e r e d , no s i g n i f i c a n t d i f f e r e n c e (p=.283) between seasons was found. 86 p = O.OU P = 0-283 r——r T r r T " Short Mid Long Early Mid Long I R A I N S — — 1 f— DRY- — I SEASON Figure 12. Seasonal means of monthly totals of bachelor bulls counted in ground counts of the Amboseli basin in 1978/79, over the whole daylight period, A , and in the late morning time block, H . There was a s ignif icant difference between seasonal means when the whole daylight 1 period was considered - - more bulls were seen in the later dry season - - but there was no s ignif icant difference when late morning time block alone was used. Vertical bars indicate + 1 standard error of the mean. 87 The s i t u a t i o n was somewhat complicated by the changing tendency of b u l l s to a s s o c i a t e with cow/calf groups at d i f f e r e n t times of year (Poole and Moss 1981). The observed changes i n the bachelor b u l l p o p u l a t i o n may have r e s u l t e d from an i n c r e a s e in the a b s o l u t e number of b u l l s e n t e r i n g the b a s i n , or from a r e l a t i v e i n c r e a s e i n the number of b u l l s j o i n i n g bachelor herds, i n s t e a d of cow/calf groups. F i g u r e 13 shows that the l a t t e r argument i s u n l i k e l y : the numbers of of both young/medium and l a r g e a d u l t b u l l s a s s o c i a t i n g with cow/calf groups ap p a r e n t l y i n c r e a s e d from the r a i n s to the l a t e dry season, suggesting an abs o l u t e i n c r e a s e in numbers. T h i s r e s u l t may be due, at l e a s t p a r t i a l l y , to a counting b i a s : I may have underestimated the numbers of b u l l s -- e s p e c i a l l y young and medium b u l l s -- a s s o c i a t i n g with l a r g e cow/calf herds i n e a r l y , r a i n y season counts because of a lack of experience i n d i f f e r e n t i a t i n g the b u l l s from a d u l t females. The numbers of b u l l s with cow/calf groups was, however, always f a i r l y s m a l l . The a n a l y s i s of a e r i a l count data (see F i g u r e 14) shows that there were n o n - s i g n i f i c a n t (p=.053, ANOVA) seasonal d i f f e r e n c e s in basin occupancy by bachelor b u l l s i n the Dry1 p e r i o d , no seasonal d i f f e r e n c e s (p=.328) i n the Wet years of 1977/79, and s i g n i f i c a n t d i f f e r e n c e s (p=.040) in the Dry2 p e r i o d . The p a t t e r n of seasonal d i f f e r e n c e s was s i m i l a r i n the Dry1 and Dry2 p e r i o d s : low numbers i n the r a i n s , with i n c r e a s e s in the dry season. The same p a t t e r n was seen i n the f u l l - d a y ground counts of 1978/79. The r e l a t i o n s h i p between ba s i n occupancy by b u l l herds and 88 80 -i CL Z5 O CU CD SI 60.-o+ X r- A O H 3 CD z 20H < L U Young/Med, • Large — r - 1— Short Mid \ — R A I N S T T Long Early — r ~ Mid -DRY- Long SEASON Figure 13. Seasonal means of monthly total numbers of bulls associating with cow/calf herds. The numbers of both young/medium, EI , and large, • , bulls seen associating with breeding herds increased towards the late dry season. 8.9 F i g u r e 14. Seasonal mean numbers of bachelor b u l l s seen i n a e r i a l counts of the Amboseli basin i n the three r a i n f a l l p e r i o d s . Note the n e a r l y s i g n i f i c a n t seasonal d i f f e r e n c e s i n Dry1 p e r i o d , n o n - s i g n i f i c a n t d i f f e r e n c e s in Wet p e r i o d , and s i g n i f i c a n t d i f f e r e n c e s i n Dry2 p e r i o d . V e r t i c a l bars i n d i c a t e + 1 standard e r r o r . SEASON 91 r a i n f a l l i n the Dry1 years i s shown i n F i g u r e 15; bachelor b u l l numbers were most s t r o n g l y , and n e g a t i v e l y , r e l a t e d to cumulative r a i n f a l l over the 90 days preceding the counts (r 2=.39, p<.0l, n=l9). Regressions with r a i n f a l l f o r other l a g p e r i o d s were a l s o s i g n i f i c a n t , but l e s s s t r o n g l y so. The r e g r e s s i o n s were not s i g n i f i c a n t with the other v a r i a b l e s , such as herb l a y e r biomass i n h a b i t a t types, i n Dry1 p e r i o d , or between b u l l numbers and r a i n f a l l or h a b i t a t v a r i a b l e s i n other time p e r i o d s (Wet, Dry2). No s i g n i f i c a n t r e l a t i o n s h i p s were found between the number of bachelor b u l l s seen i n the basin d u r i n g ground counts and r a i n f a l l or herb l a y e r biomass. b. Cows and c a l v e s F i g u r e 16 shows seasonal mean (means of two months .per season) cow/calf numbers i n the basin in ground counts. Mean numbers in the basin d i d not vary s i g n i f i c a n t l y between seasons in 1978/79 (p=.600, ANOVA), with no apparent seasonal t r e n d . I f ground counts f o r time block 2, the time of a e r i a l counts, are compared, there was s t i l l no s i g n i f i c a n t d i f f e r e n c e (p=.146), nor apparent trend, although the mid r a i n s count was low, and numbers were higher in the l a t e dry season. Seasonal means from the a e r i a l counts (see F i g u r e 17), were s i g n i f i c a n t l y d i f f e r e n t between seasons dur i n g the Dry1 (p=.044) and Wet (p=.005) p e r i o d s , but no s i g n i f i c a n t d i f f e r e n c e s appeared during Dry2 p e r i o d (p=.942). The p a t t e r n of seasonal change i n cow/calf numbers d u r i n g Dry1 p e r i o d p a r a l l e l e d the b u l l s i t u a t i o n , with numbers low i n the short and long r a i n s , 92 F i g u r e 15 . Log B a c h e l o r b u l l numbers i n a e r i a l counts o f the A m b o s e l i - b a s i n were n e g a t i v e l y r e l a t e d t o Log Cumulat ive r a i n f a l l over the 90 days p r e c e d i n g each c o u n t , d u r i n g tne D r y l p e r i o d , 1975-1977. 93 2 4 0 0 - 1 Short Mid Long Early Mid Long | RAINS 1 | D R Y — > — 1 SEASON F i g u r e 16 . Seasonal means o f monthly t o t a l s o f cows and c a l v e s seen i n ground counts o f the Ambosel i b a s i n i n 1978/79, over the whole d a y l i g h t p e r i o d , © , and i n the l a t e morning t ime b l o c k , E i . There were no s i g n i f i c a n t d i f f e r e n c e s between seasona l means i n e i t h e r c a s e . V e r t i c a l bars i n d i c a t e + 1 s tandard e r r o r . gure 17. Seasonal mean numbers of cows and c a l v e s seen i n a e r i a l counts of the Amboseli basin i n the three r a i n f a l l p e r i o d s . There were s i g n i f i c a n t d i f f e r e n c e s between seasonal mean numbers i n the Dry1 and Wet p e r i o d s -- more cows and c a l v e s were seen i n the basin d u r i n g the dry season than d u r i n g the r a i n s . There were no such d i f f e r e n c e s i n the Dry2 p e r i o d . V e r t i c a l bars i n d i c a t e + 1 standard e r r o r . 300T 200 H 100H A ) DRY1: 1975- 77 p = 0 0 U B) W E T : 1977-79 30 0-1 200-1 1 0 0 H IS. l i r j& p =0-005 i n T A O O H 300H 200H 100H C) DRY2 : 1979- 80 - a o - NJ K —I 1 r - Shor t Mid Long I RAINS 1 g p= 0-942 — 1 1 1 Early Mid Long | DRY : 1 SEASON 96 and higher i n the l a t e dry season. In the mid r a i n s , which i n Dry1 p e r i o d was a c t u a l l y a short dry season, r e l a t i v e l y high numbers of cows and c a l v e s occupied the b a s i n . In the Wet p e r i o d , the p a t t e r n was somewhat d i f f e r e n t , with r e l a t i v e l y low cow/calf numbers i n mid and long r a i n s , and hi g h numbers at a l l other times. High cow/calf numbers- were observed i n a e r i a l counts of the basin through a l l seasons of the year i n the Dry2 p e r i o d , 1979/80. These p a t t e r n s may apply only to the l a t e morning time block, as d i s c u s s e d above. The r e l a t i o n s h i p between r a i n f a l l and cow/calf occupancy of the basin in a e r i a l counts d u r i n g the Dry1 p e r i o d i s shown in F i g u r e 18. Cow/calf numbers were most s t r o n g l y r e l a t e d to cumulative r a i n f a l l over the preceding 15 days i n the Dry1 p e r i o d (r 2=.54, p<.00l, n=l9). Regressions with r a i n f a l l f o r d i f f e r e n t l a g p e r i o d s were a l s o s i g n i f i c a n t , but l e s s s t r o n g l y so. No s i g n i f i c a n t r e l a t i o n s h i p s between cow/calf numbers and r a i n f a l l were found f o r the Wet and Dry2 p e r i o d s . Other r e g r e s s i o n s between cow/calf numbers i n a e r i a l counts and herb l a y e r biomass were not s i g n i f i c a n t . T o t a l cow/calf numbers i n ground counts were not s i g n i f i c a n t l y r e l a t e d to e i t h e r r a i n f a l l or herb l a y e r biomass. 3. H a b i t a t use and p r e f e r e n c e i n the Amboseli b a s i n 97 Figure 18. Log Cow/calf numbers in aerial counts of the Amboseli basin were negatively related to Log Cumulative rainfall total over the 15 days preceding each count, during the Dryl period, 1975-1977. 98 a. Bachelor b u l l s The numbers of bachelor b u l l s seen i n the d i f f e r e n t h a b i t a t types in each season of 1978/79 (ground counts) are shown i n F i g u r e 19. Monthly count d e n s i t i e s (number counted per month / km2) i n each h a b i t a t averaged over the whole year are shown i n F i g u r e 20. No elephants were ever observed on the lakebed g r a s s l a n d , so only nine h a b i t a t types were c o n s i d e r e d i n i n i t i a l a n a l y s i s . The expected d i s t r i b u t i o n s based on occupancy p r o p o r t i o n a l to h a b i t a t area are a l s o p l o t t e d i n Fi g u r e 19, as are the 95 percent simultaneous confidence i n t e r v a l s about the observed v a l u e s . H a b i t a t s s i g n i f i c a n t l y p r e f e r r e d or avoided are found when the expected value f a l l s o u t s i d e the c o n f i d e n c e l i m i t s about the, observed. In the S. c o n s i m i l i s g r a s s l a n d s , the main a c t i v i t i e s of b u l l s and breeding herds were standing and walking. Elephants were never seen feeding there, which c o u l d e x p l a i n the low d e n s i t y noted in Fi g u r e 20. Casual o b s e r v a t i o n s suggested that patches of t h i s type were commonly used as midday r e s t areas by the elephants. Elephants fed i n a l l the other h a b i t a t s , so I examined elephant d i s t r i b u t i o n s with respect to food a v a i l a b i l i t y i n those e i g h t h a b i t a t types o n l y . Values f o r o v e r a l l c h i square goodness of f i t with the expected random d i s t r i b u t i o n are shown with and without the S. c o n s i m i l i s g r a s s l a n d s i n Table 6. The observed h a b i t a t d i s t r i b u t i o n s were s i g n i f i c a n t l y non-random i n a l l seasons. A l k a l i n e g r a s s l a n d s , although used to a small extent, were avoided at most times. The woodlands (Tw, Xw, S s ) , e s p e c i a l l y 99 F i g u r e 19. T o t a l numbers of bachelor b u l l s counted i n each h a b i t a t d u r i n g ground counts i n each season of 1978 /79. The h o r i z o n t a l l i n e s represent expected numbers based on a random d i s t r i b u t i o n p r o p o r t i o n a l to the area of each h a b i t a t type. The v e r t i c a l bars represent 95% simultaneous confidence i n t e r v a l s about the observed numbers. S i g n i f i c a n t p r e f e r e n c e or avoidance of h a b i t a t types i s i n d i c a t e d when the "expected" l i n e f a l l s o u t s i d e the confidence i n t e r v a l s . G rasslands were always avoided. Woodlands (Tw, Xw) were p r e f e r r e d in the r a i n s and avoided i n dry season, while swamp and swamp edges were more s t r o n g l y p r e f e r r e d in the dry season and used to some degree throughout the year. I—» o o 101 u - i 1 2 H 10H CM 2 ft 6 H 1/1 z LU 4- CQ 2H Basin Gr Con Tw Xw Ss Pax Yax Seg Swp HABITAT TYPE F i g u r e 2 0 . The y e a r l y mean o f monthly count d e n s i t i e s ( t o t a l numbers d i v i d e d by area) o f b a c h e l o r b u l l s i n h a b i t a t types i n 1978/79. The h o r i z o n t a l l i n e i n d i c a t e s the o v e r a l l mean d e n s i t y o f b u l l s i n the Ambosel i b a s i n , w h i l e v e r t i c a l bars r e p r e s e n t + 1 s tandard e r r o r o f the mean (note t h a t p r e f e r e n c e o r avo idance was not t e s t e d h e r e ) . B u l l d e n s i t i e s i n the swamps and swamp edges were always r e l a t i v e l y h i g h . 102 Table 6. Goodness of f i t c h i squared values of observed bachelor b u l l h a b i t a t d i s t r i b u t i o n s vs. an expected random d i s t r i b u t i o n based on the areas of h a b i t a t types. These were c a l c u l a t e d f o r the d i s t r i b u t i o n a c r o s s nine h a b i t a t types ( i n c l u d i n g S. c o n s i m i l i s g r a s s l a n d ) , and e i g h t h a b i t a t types ( e x c l u d i n g S. c o n s i m i l i s g r a s s l a n d ) i n each season. A l l d i s t r i b u t i o n s were s i g n i f i c a n t l y non-random with p < . 0.01 . CHI SQUARED SEASON VALUES FOR RAINS Short Mid Long E a r l y DRY Mid Late 9 H a b i t a t types ( i n c . S. c o n s i m i l i s ) 652 696 660 462 955 2822 8 H a b i t a t types (exc. S. c o n s i m i l i s ) 605 662 586 396 872 2716 at p = . 001 , with 8 d f, with 7 d f, c h i c h i squared = squared = 26. 1 24.3 \.. t o r t i l i s woodlands, were p r e f e r r e d in the r a i n s and, except for the Salvadora / Suaeda shrublands, avoided i n the l a t e dry season. The Salvadora / Suaeda shrublands (Ss) were p r e f e r r e d at a l l times, e s p e c i a l l y d u r i n g the r a i n s . The swamp edge woodlands (Pax, Yax) and g r a s s l a n d s (Seg) and swamps (Swp) were p r e f e r r e d i n the dry seasons and were l e s s p r e f e r r e d or even avoided i n the r a i n s . The wet swamps were used by some b u l l s throughout the year, but were e s p e c i a l l y p r e f e r r e d by b u l l s in the l a t e dry season. B u l l d e n s i t i e s (see F i g u r e 20) i n the swamp edge woodlands (Pax, Yax) and the swamps (Swp) were high at a l l times; these h a b i t a t types cover small areas and r e c e i v e d a p r o p o r t i o n a l l y l a r g e amount of use. S i m i l a r p a t t e r n s were seen in the long term a e r i a l survey data, F i g u r e 21, although there i s a l a r g e amount of v a r i a b i l i t y 103 in these r e s u l t s . Dry season use of the swamps and swamp edges was more pronounced in the Dry1 and Dry2 p e r i o d s , while i n the Wet p e r i o d the elephants used the swamps l e s s o v e r a l l . In Dry1, r a i n y season woodland use was low, presumably because the b u l l s had l e f t the basin e n t i r e l y f o r the bushlands. b. Cows and c a l v e s The monthly numbers and o v e r a l l mean count d e n s i t y of cows and c a l v e s i n d i f f e r e n t h a b i t a t s are shown i n F i g u r e s 22 and 23. In Table 7, I show c h i squared values comparing observed and expected numbers across h a b i t a t types, i n c l u d i n g and e x c l u d i n g the S. c o n s i m i l i s g r a s s l a n d s . The d i s t r i b u t i o n of cow/calf groups a c r o s s h a b i t a t s was s i g n i f i c a n t l y non-random i n a l l seasons, and most s t r o n g l y so in the l a t e dry season. The p a t t e r n of h a b i t a t use by cow/calf groups was g e n e r a l l y s i m i l a r to that of bachelor b u l l s . They showed pr e f e r e n c e for the woodlands (Tw, Xw, Ss) duri n g the r a i n s , and f o r the swamps (Swp) and swamp edges (Pax, Yax, Seg) during the dry season. The d i s t r i b u t i o n of bachelor b u l l numbers ac r o s s h a b i t a t types was s i g n i f i c a n t l y c o r r e l a t e d with the d i s t r i b u t i o n of cow/calf numbers i n the same h a b i t a t types i n the mid and long r a i n s and mid dry season (see Table 8 ). The c o r r e l a t i o n s were lowest in the short r a i n s and l a t e dry season. The use of h a b i t a t s by cow/calf groups d i f f e r e d from the b u l l p a t t e r n i n a number of ways. I n d i v i d u a l s i n the breeding herds were d i s t r i b u t e d more c o n t a g i o u s l y , i e . t h e i r numbers were more concentrated in s p e c i f i c h a b i t a t s i n each season than F i g u r e 21. Seasonal mean d e n s i t i e s of bachelor b u l l s in h a b i t a t types as seen i n a e r i a l counts d u r i n g the three r a i n f a l l p e r i o d s : A Dry1 — 1975-1977 • Wet -- 1977-1979 A Dry2 -- 1979-1980. B u l l d e n s i t i e s were low i n most h a b i t a t s d u r i n g the r a i n s of Dry1 p e r i o d , as many had l e f t the basin (see above). In g e n e r a l , woodlands were used i n the r a i n s and swamps durin g the dry seasons. Use of the swamps and swamp edges appeared g r e a t e s t i n the Dry2 p e r i o d . BULL DENSITY NO./ K M 2 o 106 F i g u r e 22. T o t a l numbers of cows and c a l v e s seen i n each h a b i t a t type d u r i n g ground counts i n each season of 1978 /79. Preference or avoidance of h a b i t a t types i s i n d i c a t e d when the expected value ( h o r i z o n t a l l i n e ) f a l l s o u t s i d e the 95% simultaneous confidence i n t e r v a l s about the observed numbers (see F i g u r e 19). Grasslands were always avoided. Woodlands (Tw, Xw, Ss) were p r e f e r r e d most s t r o n g l y i n the r a i n s , while swamps and swamp edges were most s t r o n g l y p r e f e r r e d i n the dry season. 150CH 10004 500 H Grass r^i ^ VOOOn Con a UJ y— z => o o o z 500 H T r _Lfl_ jlfL o 5 0 0 - | • o u Tw rh ft 1000- 5 0 0 H Xw L f t H f l EXP Ss 1500-1 1000- 5 0 0 - m ft •A Pax 500H rrh ft ft 500-i Yax ft 500-| S W P SR MR .LR RAINS -EXP ED MD LD DRY SEASON SR MR LR RAINS ED MD LD DRY 108 240-1 6 2 >• to 2 UJ o 2 0 0 H 160H 1 2 0 H < o 1 o o 80H 40H I Basin r*rPtr*i Gr Con Tw Xw Ss Pax Yax Seg Swp HABITAT TYPE Figure 23. The yearly mean of monthly count densities of cows and calves in habitat types in 1978/79. As for bachelor bulls (see Figure 20), the horizontal line indicates overall mean density, of cows and calves in the basin, while the vertical bars represent + 1 standard error. Cow/ calf density was always fair ly high in the swamps and swamp edges,.and highest in the palm/ fever tree swamp edge woodlands (Pax). 1 09 Table 7. Goodness of f i t c h i squared values of observed cow/calf h a b i t a t d i s t r i b u t i o n s vs. an expected random d i s t r i b u t i o n based on the areas of h a b i t a t types. These were c a l c u l a t e d f o r the d i s t r i b u t i o n across nine h a b i t a t types ( i n c l u d i n g S. c o n s i m i l i s g r a s s l a n d ) , and e i g h t h a b i t a t types T e x c l u d i n g S. c o n s i m i l i s g r a s s l a n d i n each season. A l l d i s t r i b u t i o n s were s i g n i f i c a n t l y non-random with p < .001. CHI SQUARED SEASON VALUES FOR Short RAINS Mid Long E a r l y DRY Mid Late 9 h a b i t a t types ( i n c . S. c o n s i m i l i s ) .7840 5839 4501 1 3374 21 956 25429 8 h a b i t a t types (exc. S. c o n s i m i l i s ) 8340 521 2 4542 12218 21151 25838 at p = .001, with 8 df, c h i squared = 26.1 with 7 df, c h i squared = 24.3 Table 8. C o r r e l a t i o n s (Spearman's rank) of bachelor b u l l and cow/calf numbers in h a b i t a t types i n d i f f e r e n t seasons, showing s i g n i f i c a n t c o r r e l a t i o n i n the mid and long r a i n s , and mid dry season. SEASON RAINS DRY Short Mid Long E a r l y Mid Late Spearman's .070 .754* .647* .585 .685* .568 S i g n i f i c a n c e of Spearman's r 5 (n = 9): * p < .05 was the case with the b u l l s ; t h e r e f o r e , t h e i r o v e r a l l c h i squared values were much hi g h e r . The cow/calf f a m i l y u n i t s used the S. c o n s i m i l i s g r asslands more (although i r r e g u l a r l y ) , and 1 1 0 made more use of the more densely wooded h a b i t a t s (Pax, Yax), e s p e c i a l l y d u r i n g the l a t e dry season. They showed a stronger p r e f e r e n c e f o r the open Salvadora / Suaeda shrublands d u r i n g the r a i n s , but l e s s during the dry seasons, when b u l l p r e f e rence f o r t h i s h a b i t a t type i n c r e a s e d . Cow/calf groups p r e f e r r e d the wet swamps (Swp) i n the dry seasons, to a l e s s e r degree than d i d the b u l l s , and made gr e a t e r r e l a t i v e use of the swamp edge g r a s s l a n d s (Seg). The h a b i t a t use p a t t e r n s of cow/calf groups seen i n a e r i a l counts (see F i g u r e 24) from d i f f e r e n t r a i n f a l l p e r i o d s were somewhat s i m i l a r to the 1978/79 ground count p i c t u r e with a la r g e amount of v a r i a b i l i t y . In the Dry1 p e r i o d , most f a m i l y u n i t s l e f t the basin in the r a i n s and returned to the woodlands and swamps i n the dry season. The Wet p e r i o d p a t t e r n was s i m i l a r to that found i n the ground count a n a l y s i s -- woodland use was high in the r a i n s , with a dry season s h i f t to the swamps. As with the b u l l s , there were some d i f f e r e n c e s i n s p e c i f i c s of cow/calf h a b i t a t use between the ground and a e r i a l count r e s u l t s f o r t h i s p e r i o d that may have r e s u l t e d from the d i f f e r e n c e i n counting methods. The p a t t e r n in Dry2 p e r i o d was d i f f e r e n t ; l e s s use was made of the A. t o r t i l i s woodlands and more use of the swamps and swamp edges throughout the y e a r T e s p e c i a l l y d u r i n g the dry season. 111 F i g u r e 24. Seasonal mean d e n s i t i e s of cows and c a l v e s in h a b i t a t types, as seen i n a e r i a l counts d u r i n g the three r a i n f a l l periods.: A Dry1 -- 1975-1977 • Wet -- 1977-1979 A Dry2 -- 1979-1980. Cow/calf d e n s i t i e s were low i n most h a b i t a t types during the r a i n s of the Dry1 p e r i o d . In g e n e r a l , woodlands were used more i n the r a i n s , and swamps more i n the dry seasons. The swamps and swamp edges were used more by cow/calf groups in the Dry2 p e r i o d .  1 1 3 4. H a b i t a t use i n r e l a t i o n to c1imate and v e g e t a t i o n a. Bachelor b u l l s L i n e a r r e g r e s s i o n p l o t s of bachelor b u l l numbers i n ground counts a g a i n s t t o t a l r a i n f a l l of the previous month, herb l a y e r t o t a l dry mass, and herb l a y e r green mass are shown i n F i g u r e s 25 to 27. There are some c l e a r trends i n these data, although there i s a l s o a great deal of v a r i a n c e . B u l l numbers were p o s i t i v e l y r e l a t e d to r a i n f a l l and herb l a y e r biomass, i n the g r a s s l a n d s and the A. t o r t i l i s woodlands, and, i n g e n e r a l , i n v e r s e l y r e l a t e d to swamp and swamp edge herb l a y e r biomass. The fever t r e e woodlands and Salvadora / Suaeda shrublands were used by many b u l l s at a l l times of year, and the l a t t e r e s p e c i a l l y in the l a t e dry season. Hence, use of these h a b i t a t types was n e g a t i v e l y r e l a t e d to seasonal r a i n f a l l and biomass. The only r e g r e s s i o n that was s i g n i f i c a n t was b u l l numbers vs. t o t a l biomass i n the A. t o r t i l i s woodlands (see F i g u r e 26). The r e l a t i o n of b u l l numbers to green mass in h a b i t a t s was g e n e r a l l y l e s s strong than was the r e l a t i o n to t o t a l herb l a y e r biomass. The r e s u l t s of rank c o r r e l a t i o n of b u l l d e n s i t i e s with p l a n t biomass d e n s i t y across h a b i t a t types are shown in Table 9. S i g n i f i c a n t c o r r e l a t i o n s of b u l l d e n s i t i e s with herb l a y e r t o t a l biomass and green biomass occurred mainly in the dry seasons; the c o r r e l a t i o n s with t o t a l and green mass were about e q u a l l y s i g n i f i c a n t o v e r a l l . Woody biomass alone was a poor p r e d i c t o r of b u l l d e n s i t y , but when combined with herb l a y e r biomass, e s p e c i a l l y t o t a l herb l a y e r biomass, i t produced s i g n i f i c a n t Figure 25. Linear regressions of monthly totals of bachelor bulls seen in habitat types in ground counts in 1978/79 vs. total ra in fa l l of the preceding month. Slopes are positive for alkal ine grasslands, A. t o r t i l i s woodlands, and palm/fever woodlands, and negative for other woodlands and the swamps and swamp edges. No regressions were s igni f icant . 11 Figure 26. Linear regressions of bachelor bull monthly totals in habitat types vs. total herb layer biomass in the middle of the current month. Slopes are positive for alkaline grasslands, A. tort i l is woodlands, and palm/fever woodlands, and negative for other woodlands and swamps. ++ indicates a regression significant at p < .01, for the A. tor t i l i s woodlands. No other regressions were significant. 116 200 400 GREEN MASS GM / M Figure 27. Linear regressions of bachelor bull monthly totals in habitat types vs. green herb layer biomass mid-month. Slopes are positive for alkaline grasslands, A. t o r t i l i s woodlands, and palm/fever woodlands, and negative for other woodlands and swamps. No regressions were significant. 1 17 Table 9. C o r r e l a t i o n s (Spearman's rank) of b u l l d e n s i t i e s with t o t a l mass and green mass of the herb l a y e r , and t w i g / l e a f mass and % cover of the woody l a y e r across h a b i t a t types i n d i f f e r e n t seasons i n 1978/79. S i g n i f i c a n t c o r r e l a t i o n s with herb l a y e r biomass occ u r r e d i n the dry season. C o r r e l a t i o n s with t w i g / l e a f mass alone were g e n e r a l l y not s i g n i f i c a n t , but were s i g n i f i c a n t when t w i g / l e a f mass was added to herb l a y e r biomass. B u l l d e n s i t y d i d not c o r r e l a t e with % woody cover. SPEARMAN'S Rs: SEASON BULL DENSITY VS. Short RAINS Mid Long E a r l y DRY Mid Late Herb l a y e r t o t a l biomass .613 .720* .637 .780* .839** .887** Herb l a y e r green biomass .304 .577 .470 .845** • .673* .976** Woody l a y e r t w i g / l e a f mass .042 . 530 .673* . 375 . 375 .065 T o t a l herb + t w i g / l e a f mass .548 .762* .690* .762* .810* .857** Green herb + t w i g / l e a f mass .524 .619 .429 .881** .786* .857** % woody cover -.006 .601 .601 .232 .494 . 185 S i g n i f i c a n c e of Spearman's r s (n = 8) : * p * p < .05 < .01 118 c o r r e l a t i o n s across h a b i t a t s i n the long r a i n s and dry season. There were no s i g n i f i c a n t c o r r e l a t i o n s between b u l l d e n s i t y and percent ground cover by woody shrubs. b. Cows and c a l v e s Regressions of monthly cow/calf numbers in h a b i t a t types as seen i n the ground counts a g a i n s t t o t a l r a i n f a l l i n the month preceding the counts, herb l a y e r t o t a l biomass, and green biomass are shown in F i g u r e s 28 to 30. Cow/calf numbers i n the a l k a l i n e g r a s s l a n d s and woodlands (both A. t o r t i l i s and fever t r e e woodlands) were p o s i t i v e l y r e l a t e d to r a i n f a l l and herb l a y e r biomass (both t o t a l and green mass), and, in g e n e r a l , n e g a t i v e l y r e l a t e d to herb l a y e r biomass i n the swamp edge h a b i t a t s (Pax, Yax, Seg). The r e l a t i o n of cow/calf numbers in the Salvadora / Suaeda shrublands to r a i n f a l l and green mass was p o s i t i v e , but to t o t a l mass, weakly n e g a t i v e . The only r e g r e s s i o n s which were s i g n i f i c a n t were between cow/calf numbers in the swamp edge palm woodlands (Pax) and the wet swamps (Swp) and r a i n f a l l (see F i g u r e 29), and between cow/calf numbers in the A. t o r t i l i s woodlands and t o t a l herb l a y e r biomass (see F i g u r e 30). Rank c o r r e l a t i o n s of cow/calf count d e n s i t i e s with v e g e t a t i o n v a r i a b l e s a c r o s s h a b i t a t types, Table 10, were a l s o s i m i l a r to the bachelor b u l l r e s u l t s . Cow/calf d e n s i t y was s i g n i f i c a n t l y r e l a t e d to herb l a y e r biomass i n the dry season only, when the animals were using the. swamp and swamp edge h a b i t a t s . I t appears that both herb l a y e r t o t a l . b i o m a s s and RAINFALL (MM) Figure 28. Linear regressions of cow/calf monthly totals in habitat types vs. total r a i n f a l l of the previous month. Slopes are positive for alkaline grasslands, A. t o r t i l i s woodlands, Fever tree woodlands, and Salvadora/Suaeda shrublands, and negative for swamps and swamp edges. + indicates regressions s ignif icant at p < .05, for the Salvadora/ Suaeda shrublands, palm/fever woodlands, and wet swamps. I 120 F i g u r e 2 9 . Linear regressions of cow/calf monthly totals in habitat types v s . total herb layer biomass mid-month. Slopes are positive for alkaline grasslands, A. t o r t i l i s woodlands, fever tree woodlands, and wet swamps, and negative.for Salvadora/Suaeda shrublands and swamp edge habitats. + indicates a regression signif icant at p < .05, for the A. t o r t i l i s woodlands. Figure 30. Linear regressions of cow/calf monthly totals in habitat types vs. green herb layer biomass mid-month. Slopes are positive for the alkaline grasslands, A. t o r t i l i s and fever tree woodlands, and Salvado /Suaeda shrublands, and negative for the swamp edge habitats. 1 22 Table 10. C o r r e l a t i o n s (Spearman's rank) of cow/calf d e n s i t i e s with t o t a l and green mass of the herb l a y e r , and t w i g / l e a f mass and % cover of the woody l a y e r a c r o s s h a b i t a t types i n each season of 1978/79. S i g n i f i c a n t c o r r e l a t i o n s with herb l a y e r mass occu r r e d mainly i n the dry season. C o r r e l a t i o n s with t w i g / l e a f mass alone were g e n e r a l l y not s i g n i f i c a n t , but woody mass + herb l a y e r mass c o r r e l a t i o n s were s i g n i f i c a n t . C o r r e l a t i o n s with % woody cover were s i g n i f i c a n t i n the mid and long r a i n s , and l a t e dry season. SPEARMAN'S Rs: SEASON COW/CALF DENSITY VS. Short RAINS Mid Long E a r l y DRY Mid Late Herb l a y e r t o t a l biomass .494 .399 .577 .780* .780* .756 Herb l a y e r green biomass .232 .232 .482 .738* .637 .810 Woody l a y e r t w i g / l e a f mass .292 .696* .506 .577 .506 .554 T o t a l herb + t w i g / l e a f mass .429 .452 .524 .786* .786* .726 Green herb + t w i g / l e a f mass .452 .262 .405 .714* .762* .738 % woody cover .196 .720* .696* .411 .435 .649 S i g n i f i c a n c e of Spearman's r s (n=8): * p < .05 1 23 green biomass c o r r e l a t e d about e q u a l l y w e l l with cow/calf d e n s i t i e s i n h a b i t a t types, with s l i g h t l y stronger r e l a t i o n s h i p s with t o t a l dry mass. Woody biomass alone c o r r e l a t e d s i g n i f i c a n t l y with d e n s i t y i n the mid r a i n s only, but i n c o n t r a s t to the b u l l s i t u a t i o n , the c o r r e l a t i o n s were not improved by combining t w i g / l e a f biomass with herb l a y e r biomass. Cow/calf d e n s i t y c o r r e l a t e d p o s i t i v e l y with percent woody cover in h a b i t a t s i n the mid and l a t e dry season. 5_. Observations of i n d i v i d u a l movements Casual o b s e r v a t i o n s of known i n d i v i d u a l s and cow/calf f a m i l y u n i t s suggested that movement p a t t e r n s were q u i t e d i f f e r e n t f o r d i f f e r e n t i n d i v i d u a l s and f a m i l y u n i t s , and that they a l s o v a r i e d c o n s i d e r a b l y between seasons. In the general d i u r n a l f o r a g i n g p a t t e r n of many cow/calf herds and, to a l e s s e r extent, bachelor b u l l s , i n the Amboseli b a s i n , a group of one or more i n d i v i d u a l s or f a m i l y u n i t s entered the basin at i t s northern or southern edge in e a r l y to mid-morning. The group then fed i n woodlands or swamp f r i n g e s , working i t s way towards the v i c i n i t y of permanent water in the c e n t r a l basin by midday. In l a t e a f t e r n o o n , the group began moving back towards the b a s i n edge, and remained in b a s i n edge woodlands or moved f a r t h e r out i n t o the bushlands, where the elephants spent the night feeding and s l e e p i n g . The p a t t e r n of e n t e r i n g the b a s i n h a b i t a t s d u r i n g the day and l e a v i n g at night might be repeated i n f o l l o w i n g days, or the elephants might spend a day or more feeding in bushland areas before r e t u r n i n g to the b a s i n . 1 24 I n d i v i d u a l f a m i l y u n i t s appear to have f a i r l y s p e c i f i c and d i s t i n c t , but o f t e n o v e r l a p p i n g , dry season home ranges (Moss 1980, 1981), which cover d i f f e r i n g p o r t i o n s of the Amboseli b a s i n , and which might a l t e r the b a s i c p a t t e r n d e s c r i b e d above. Some fa m i l y u n i t s were r a r e l y seen i n the bas i n d u r i n g the dry season, and appa r e n t l y foraged in the s e m i - a r i d southern bushlands most of the daytime, v i s i t i n g the basin swamps at n i g h t . Other groups p e r i o d i c a l l y entered the bas i n d u r i n g the day, while s t i l l others used s p e c i f i c areas of the basin on a very r e g u l a r b a s i s . The s t r u c t u r e of home ranges changed d r a m a t i c a l l y d u r i n g the r a i n s , when elephant f a m i l y u n i t s from a l l p a r t s of the basin and i t s p e r i p h e r i e s mixed together and moved a c r o s s a l l areas of the bas i n and over l a r g e areas of bushland. The movement p a t t e r n s of i n d i v i d u a l b u l l s are l e s s w e l l documented. B u l l s appear to have s p e c i f i c home ranges which may ov e r l a p with those of other b u l l s , and these may change on a seasonal b a s i s (C. Moss, J . Poole, pers. comm.). However, the b u l l s ' ranging p a t t e r n s are ap p a r e n t l y a l t e r e d d u r i n g p e r i o d s of re p r o d u c t i v e a c t i v i t y , when they may a s s o c i a t e with cow/calf herds. Ranging and grouping behaviour may a l s o change with the age of b u l l s , as they are ap p a r e n t l y more g r e g a r i o u s when they are younger, and might have d i f f e r e n t home ranges ( J . Poole, pers. comm., i n pr e p . ) . The ranging p a t t e r n s of i n d i v i d u a l s and groups of both b u l l s and cow/calf f a m i l y u n i t s needs f u r t h e r study to examine the causes and consequences of i n d i v i d u a l d i f f e r e n c e s . 1 25 D. D i s c u s s i o n S t u d i e s of resource p r e f e r e n c e , whether the resource i s food, a p l a c e to feed, or some other o b j e c t i v e of v a l u e , g e n e r a l l y i n v o l v e a comparison of use and a v a i l a b i l i t y . However, as Jacobs (1974) noted, most i n d i c e s of preference are based on the i m p l i c i t , but o f t e n unrecognized, assumption that a l l items on o f f e r are of equal abundance. If the items are a v a i l a b l e i n unequal p r o p o r t i o n s , i t i s d i f f i c u l t to r e a l i s t i c a l l y demonstrate p r e f e r e n c e of items which are r e l a t i v e l y common (unless use i s very h i g h ) , or avoidance of items which are r e l a t i v e l y r a r e (unless use i s very low). Johnson (1980) echoed t h i s p o i n t , and added a f u r t h e r c a u t i o n f o r the study of h a b i t a t or d i e t p r e f e r e n c e . He suggested that i t i s b e t t e r to regard use and choice as r e l a t i v e d e c i s i o n s between the number of d i f f e r e n t items found on o f f e r at a p a r t i c u l a r time and p l a c e , r a t h e r than as a b s o l u t e p r e f e r e n c e . The d e f i n i t i o n of " d i f f e r e n t " resources such as h a b i t a t types, can be a r b i t r a r y or v a r i a b l e , yet can i n f l u e n c e the outcome of an a n a l y s i s which compares use and a v a i l a b i l i t y on an a b s o l u t e s c a l e . He recommended that ranked use and a v a i l a b i l i t y be averaged f o r i n d i v i d u a l s i n a sample, and that s e l e c t i o n or r e j e c t i o n be e v a l u a t e d on the b a s i s of d i f f e r e n c e between ranks. In the c u r r e n t study, I d i d not r e c o r d the use of h a b i t a t s by i n d i v i d u a l elephants, but r a t h e r by the elephant p o p u l a t i o n as a whole. H a b i t a t s were not a l l e q u a l l y abundant — the areas occupied by d i f f e r e n t h a b i t a t types d i f f e r e d g r e a t l y (see Table 4). I used rank comparisons in l o o k i n g at the d i s t r i b u t i o n of 1 26 elephant numbers with p l a n t biomass ac r o s s h a b i t a t s , and I viewed the observed d i f f e r e n c e s between h a b i t a t use by elephants and a v a i l a b i l i t y as r e l a t i v e p r e f e r e n c e between the d e f i n e d h a b i t a t types. J_. General p a t t e r n of seasonal h a b i t a t p r e f e r e n c e It would appear t h a t , as a general r u l e , elephants s e l e c t h a b i t a t s f o r the most p r o f i t a b l e f o r a g i n g o p p o r t u n i t i e s , w i t h i n l i m i t s imposed by t h e i r requirements for s u r f a c e water. Western (1975) and Leuthold(1977b) showed that the a v a i l a b i l i t y of s u r f a c e water r e s t r i c t e d the number of h a b i t a t s a v a i l a b l e to elephants i n dry seasons. Elephants concentrate in the v i c i n i t y of permanent water sources i n the dry seasons, and can venture f a r t h e r a f i e l d during' r a i n y p e r i o d s . The survey data analyzed in the c u r r e n t study supported these o b s e r v a t i o n s in the Amboseli ecosystem, notably in the pre-1977 drought y e a r s . During that p e r i o d , the cow/calf groups responded very s t r o n g l y to short term r a i n f a l l (over two weeks), by moving out of the b a s i n h a b i t a t s in a p r e f e r e n c e f o r the d i v e r s e and p r o d u c t i v e bushlands, and r e t u r n i n g promptly a f t e r the r a i n s . The bachelor b u l l herds appeared to respond to r a i n f a l l by e m i g r a t i o n , and to drought by immigration, over a much longer term (three months) -- however, t h i s r e s u l t may be a spurious s t a t i s t i c a l a r t e f a c t , s i n c e the e f f e c t s of r a i n f a l l , d e s i c c a t i o n , and h e r b i v o r e consumption on both s u r f a c e water and herb l a y e r biomass abundance would change c o n s i d e r a b l y d u r i n g such a long p e r i o d (Western 1975). The l a r g e s c a l e movement 127 p a t t e r n s of bachelor b u l l s may be complicated by t h e i r seasonal a s s o c i a t i o n with cow/calf groups. During the years f o l l o w i n g the 1977/78 r a i n s , the elephants stayed c l o s e to the Amboseli b a s i n f o r much of the year ( a l l areas of the Amboseli basin are w i t h i n a few ki l o m e t r e s of water). T h i s " n a t u r a l experiment" suggested that l i m i t a t i o n of water a v a i l a b i l i t y alone was not s u f f i c i e n t to e x p l a i n the la r g e s c a l e h a b i t a t use p a t t e r n s , and that the r e l a t i v e a v a i l a b i l i t y of herb l a y e r v e g e t a t i o n (which i s most responsive to short term r a i n f a l l ) i n d i f f e r e n t h a b i t a t s , was a l s o important. Herb l a y e r v e g e t a t i o n may be the most p r o f i t a b l e food source f o r elephants w i t h i n h a b i t a t types through much of the year. Some p r e l i m i n a r y s t u d i e s of elephant fe e d i n g behaviour (Guy 1976, Wyatt and Eltrin g h a m 1974), suggested that elephant feeding r a t e may be higher on herb l a y e r v e g e t a t i o n than on woody browse. Woody p l a n t m a t e r i a l i s o f t e n of higher q u a l i t y than the grasses of the herb l a y e r , but i t i s a l s o much l e s s abundant in most h a b i t a t types, at most times of year (see Chapter 3). For an animal l i k e the elephant, which needs to maintain a l a r g e and f a i r l y constant r a t e of n u t r i e n t intake, the herb l a y e r of most h a b i t a t types would probably provide the best f o r a g i n g o p p o r t u n i t i e s through much of the year, unless i t s abundance i s too low to allow a high intake r a t e , or i t s q u a l i t y i s too low f o r a net gain i n n u t r i e n t s . P r o f i t a b i l i t y would be highest in h a b i t a t s where the herb l a y e r i s of the highest q u a l i t y , and where i t i s s u f f i c i e n t l y abundant to allow a high intake r a t e . I have no data on the feeding r a t e of elephants at 1 28 d i f f e r e n t herb l a y e r biomass l e v e l s , and t h i s i s an area which needs f u r t h e r work. In Amboseli, herb l a y e r q u a l i t y i s not n e c e s s a r i l y d i r e c t l y r e l a t e d to p l a n t greenness per se. The r e l a t i o n s h i p s between elephant numbers and herb l a y e r biomass were more s i g n i f i c a n t f o r t o t a l biomass i n the herb l a y e r , than green mass, suggesting that the greenness of v e g e t a t i o n was l e s s c r i t i c a l as a fe e d i n g cue. T h i s c o n t r a s t s with the r e p o r t e d importance of green grass to other l a r g e h e r b i v o r e s , such as the A f r i c a n b u f f a l o ( S i n c l a i r 1977). In the general p a t t e r n , elephants use the woodlands or bushlands i n the r a i n s when t h e i r herb l a y e r s , which have the hi g h e s t n u t r i e n t content, are abundant enough f o r p r o f i t a b l e f o r a g i n g . When the.herb l a y e r of these h a b i t a t s becomes reduced through consumption i n the dry season, or, in the case of the bushlands, s u r f a c e water a l s o becomes l i m i t i n g , they concentrate in the ground water h a b i t a t s -- swamp (Swp), swamp edge (Seg), and swamp edge woodlands (Pax,Yax). In these h a b i t a t s , the herb l a y e r biomass i s high, but of lower q u a l i t y . It may become p r o f i t a b l e to feed on woody p l a n t m a t e r i a l at t h i s time. In the extended p e r i o d of low r a i n f a l l i n the Dryl p e r i o d p r i o r to 1977, elephants t r a v e l l e d out to northern bushland areas f o r r a i n y season f e e d i n g , and returned to the woodlands and swamps in the dry season. In such low r a i n f a l l p e r i o d s , herb l a y e r p r o d u c t i o n in r a i n f a l l - d e p e n d e n t h a b i t a t s i s r e l a t i v e l y low and biomass f a l l s to low l e v e l s under h e r b i v o r e consumption i n the dry season. Swamp edge biomass was a l s o 1 29 f a i r l y low by the end of the dry season. Bushland herb l a y e r s appear to respond to the e a r l y r a i n s more q u i c k l y than the basin h a b i t a t s , and would be r e l a t i v e l y more a t t r a c t i v e under these c o n d i t i o n s . In the years of higher r a i n f a l l , 1977 to 1979, most of the elephants moved much l e s s in the r a i n s ; they stayed i n the v i c i n i t y of the basin through much of the year. I suggest that t h i s was because of the i n c r e a s e d abundance of high q u a l i t y woodland herb l a y e r ( r e l a t i v e to food q u a l i t y i n the swamps). They d i d not have to t r a v e l to the usual bushland h a b i t a t s in the wet season. In the low r a i n f a l l year of 1979/80, the cow/calf groups d i d not resume the movement p a t t e r n of the p r e v i o u s drought, p o s s i b l y because of a c o n s e r v a t i v e h a b i t a t use s t r a t e g y , or because the swamps and swamp edge herb l a y e r s were s t i l l abundant and r e l a t i v e l y more a t t r a c t i v e as feeding s i t e s than the d i s t a n t bushlands. I would p r e d i c t that a f t e r a longer p e r i o d of extended drought, when basin swamp, swamp edge, and woodland herb l a y e r s are s i g n i f i c a n t l y reduced by the end of the dry season, the elephants would r e t u r n to a basin/bushland l a r g e - s c a l e movement p a t t e r n . Leuthold (1977b) noted that i n areas of higher r a i n f a l l w i t h i n the Tsavo ecosystem, cow/calf home ranges were s m a l l e r , and he s p e c u l a t e d that t h i s may have been due to g r e a t e r l o c a l food a v a i l a b i l i t y . T h i s system might a l s o operate between years of d i f f e r e n t r a i n f a l l i n the same area, such as i n Amboseli. The d i f f e r e n c e i n d i s t r i b u t i o n across h a b i t a t , types between bachelor b u l l s and cows and c a l v e s was g r e a t e s t in the e a r l y 1 30 r a i n s and l a t e dry season. The e a r l y r a i n s were times when the cow/calf groups were using the Salvadora / Suaeda shrublands and S. c o n s i m i l i s g r a s s l a n d s much more than the b u l l s . During the l a t e dry season, the cow/calf groups used the swamp edge g r a s s l a n d s much more than d i d the b u l l s , while the b u l l s used the swamps more than d i d the f a m i l y u n i t s . D i f f e r e n c e s i n h a b i t a t use by bachelor b u l l s and breeding herds c o u l d be due to metabolic d i f f e r e n c e s i n food requirements between the sexes (see Chapter 6, General D i s c u s s i o n ) , or to d i f f e r e n c e s i n s o c i a l behaviour (see Chapter 5). H a b i t a t d i s t r i b u t i o n s of the b u l l s and cow/calf groups were s i m i l a r through much of the r e s t of the year. Bachelor b u l l herds showed some degree of l a r g e - s c a l e movement to bushlands dur i n g the wet season in a l l c l i m a t i c p e r i o d s . T h e i r eventual dry season c o n c e n t r a t i o n i n the basin was probably d i c t a t e d by water a v a i l a b i l i t y . Over the whole gender c l a s s , b u l l s were l e s s s e l e c t i v e of h a b i t a t s than the cow/calf herds. It i s l i k e l y that Amboseli b u l l s are more mobile than cow/calf groups, and have l a r g e r home ranges than do f a m i l y u n i t s . Leuthold and Sale (1973) and Leuthold (1977b) d e s c r i b e d t h i s for elephants in the nearby Tsavo West N a t i o n a l Park. Family u n i t m o b i l i t y and h a b i t a t s e l e c t i o n might be a f f e c t e d by the presence of small c a l v e s or other slow-moving i n d i v i d u a l s in the groups. Average p a t t e r n s of h a b i t a t use shown by the p o p u l a t i o n may not a c c u r a t e l y r e f l e c t h a b i t a t use by i n d i v i d u a l groups of cow/calves or b u l l s . Cow/calf groups (and p o s s i b l y b u l l groups) 131 remain w i t h i n s p e c i f i c home ranges i n the dry season, and tend to a v o i d each other (Douglas-Hamilton 1972, Leuthold 1977b, Moss 1981). P a r t i t i o n i n g of the Amboseli basin and i t s surroundings i n t o home ranges al l o w s access f o r some groups and pre c l u d e s access f o r others to the Amboseli b a s i n . The " a v a i l a b i l i t y " of h a b i t a t s to c e r t a i n groups may be decided by t h e i r c h o i c e , or lack of c h o i c e , i n home ranges. More work must be done on the movements of i n d i v i d u a l s and f a m i l y u n i t s , and the u n d e r l y i n g c a u s a l mechanisms of the apparent p a r t i t i o n i n g of the ecosystem. The development of models which are more s o p h i s t i c a t e d than simple l i n e a r r e l a t i o n s h i p s may be needed to e x p l a i n more of the v a r i a n c e in elephant h a b i t a t use p a t t e r n s . 2. Departures from the gen e r a l p a t t e r n Some h a b i t a t s with s u b s t a n t i a l herb l a y e r biomass were not v i s i t e d (lake bed g r a s s l a n d s ) , or were not used f o r feeding ( )u S. c o n s i m i l i s g r a s s l a n d s ) . Others were used f o r f o r a g i n g , yet had low biomass ( a l k a l i n e g r a s s l a n d s , Salvadora / Suaeda sh r u b l a n d s ) . Lakebed g r a s s l a n d s had a low q u a l i t y herb l a y e r , f a i r l y low biomass d e n s i t y , and they are l o c a t e d some d i s t a n c e away from permanent water sources (see F i g u r e 4). They are not near any woodlands nor the t r a d i t i o n a l d i u r n a l f o r a g i n g routes from bushlands to swamps. They were app a r e n t l y not worth a " s p e c i a l t r i p " f o r f o r a g i n g e l e p h a n t s , and were even given l i t t l e a t t e n t i o n d u r i n g r a i n y season movements. 1 32 The S. c o n s i m i 1 i s g r a s s l a n d s are somewhat of a s p e c i a l case, but one which may p o i n t to the r e l a t i v e nature of h a b i t a t p r e f e r e n c e . I never saw elephants feeding i n t h i s h a b i t a t type, yet occupancy, e s p e c i a l l y by cow/calf groups, was f a i r l y high i n the e a r l y r a i n s . The t a l l , c oarse g r a s s , S. c o n s i m i l i s , undoubtedly has a high f i b r e and low p r o t e i n content, and i s l i k e l y to be an u n p r o f i t a b l e food p l a n t . I would have expected elephants to a v o i d t h i s h a b i t a t f o r f e e d i n g . Yet, a c c o r d i n g to other observers (D. Western, C. Moss pers. comm.), elephants fed here d u r i n g the drought years before 1976, when the t a l l , abundant grass may have been more a t t r a c t i v e than the g r e a t l y reduced herb l a y e r s of nearby p l a n t communities. I can only s p e c u l a t e on the reasons why elephants, and p a r t i c u l a r l y cow/calf groups, would use the areas f o r midday r e s t p e r i o d s , s i n c e they o f f e r n e i t h e r shade nor surface water dur i n g much of the year. Patches of t h i s h a b i t a t type are g e n e r a l l y found near swamp edges or woodlands and are thus near favoured f e e d i n g h a b i t a t s . T a l l grass may a f f o r d some cover to smaller i n d i v i d u a l s of cow/calf groups when they are r e s t i n g . I o c c a s i o n a l l y observed d i s t u r b e d cow/calf herds run to patches of S. c o n s i m i l i s where they h a l t e d and g r a d u a l l y calmed down, suggesting that the elephants f e l t somehow " s a f e r " in those areas than i n other p l a c e s . The a l k a l i n e g r a s s l a n d s c o n s i s t e n t l y had the lowest herb l a y e r biomass d e n s i t y ( i t never rose above 200 g/m2) and i t s grasses were of low to intermediate n u t r i t i o n a l q u a l i t y . The a l k a l i n e g r a s s l a n d s were probably the l e a s t p r o f i t a b l e h a b i t a t 133 used by elephants f o r f o r a g i n g . My c a s u a l o b s e r v a t i o n s suggested that the only areas where elephants ventured out onto the a l k a l i n e g r a s s l a n d s were adjacent to woodlands, Salvadora / Suaeda shrublands, or swamps, and there may have been a "boundary e f f e c t " (Lamprey 1963) t a k i n g p l a c e . The Salvadora / Suaeda shrublands were p r e f e r r e d through most of the year by f a m i l y u n i t s and b u l l s , yet had a r e l a t i v e l y low herb l a y e r biomass d e n s i t y . One p o s s i b l e reason f o r the high amount of use of t h i s h a b i t a t c o u l d be i t s l o c a t i o n towards the center of the b a s i n (see F i g u r e 3) near the midpoint of the general daytime f o r a g i n g path d e s c r i b e d i n S e c t i o n 4.3.4 above. T h i s l o c a t i o n c o u l d i n c r e a s e the l i k e l i h o o d of the daytime c o u n t i n g b i a s d e s c r i b e d i n the Methods s e c t i o n ; c e n t r a l basin h a b i t a t s might be used l e s s over the whole 24 hour day, yet be more represented i n my d a y l i g h t - b i a s e d survey counts. It may a l s o be that the open, l i g h t l y wooded nature of t h i s h a b i t a t type i s genuinely a t t r a c t i v e to cow/calf herds f o r s o c i a l reasons of group cohesion, as I w i l l d e s c r i b e i n Chapter 5. Another p o s s i b i l i t y i s a "boundary e f f e c t " ; many patches of t h i s h a b i t a t type are adjacent to swamp areas which elephants used f o r d r i n k i n g in the r a i n y season and f o r feeding and d r i n k i n g i n the dry season. 134 CHAPTER 5 ELEPHANT SOCIAL GROUP DYNAMICS AND HABITAT USE A. I n t r o d u c t i o n In the preceding a n a l y s i s of elephant h a b i t a t s e l e c t i o n , I t r e a t e d a l l the i n d i v i d u a l s i n the p o p u l a t i o n as i f they chose h a b i t a t types independently of the presence of other animals. However, the s o c i a l o r g a n i z a t i o n of elephants i s h i g h l y s t r u c t u r e d (Moss 1981), and s o c i a l s t r u c t u r e has been suggested as p o t e n t i a l l y important i n a f f e c t i n g the use of space and h a b i t a t s i n other animals (Fairbanks and B i r d 1978, F r e t w e l l and Lucas 1970, Hoffmeyer 1973). One aspect of s o c i a l o r g a n i z a t i o n i s the formation and s p l i t t i n g of groups of i n d i v i d u a l s at d i f f e r e n t times of year. These processes have been d i s c u s s e d f o r hamadryas baboons (Kummer 1968) and gelada baboons (Dunbar and Dunbar 1975), and r e l a t e d to t h e i r f e e d i n g ecology and h a b i t a t use, with l a r g e r groups forming in times and p l a c e s of food abundance. S i n c l a i r (1977) d i s c u s s e d seasonal group f i s s i o n i n A f r i c a n b u f f a l o herds. He suggested that females and t h e i r young remain i n l a r g e herds to reduce the impact of p r e d a t i o n , and that the herds may break up i n the dry season when food becomes d i s t r i b u t e d i n patches so small that e n t i r e l a r g e herds cannot feed i n them. Leuthold (1976) found that group s i z e i n elephants was p o s i t i v e l y r e l a t e d to seasonal r a i n f a l l , as i n other l a r g e h e r b i v o r e s (Leuthold 1977a). He a l s o proposed that high woody cover a c t s to reduce group cohesion, as food items would be p a t c h i l y d i s t r i b u t e d , and the i n d i v i d u a l s feeding on these l o c a l i z e d food sources would 135 t h e r e f o r e be unable to maintain contact with a l a r g e , mobile group. Jarman and Jarman (1979) suggested that i t i s important f o r female impalas attempt to maintain s o c i a l group cohesion at a l l times, p o s s i b l y as a response to the constant t h r e a t of p r e d a t i o n . However, feeding competition as food items become scarce and patchy in the dry season c o u l d act to d i v i d e these s o c i a l groups. The impalas appear to s h i f t t h e i r h a b i t a t use to stay in areas where they can maintain l a r g e group s i z e . When they reach the l a s t h a b i t a t with abundant food sources, and resources d e c l i n e there, group members become more widely separated, then break up i n t o smaller groups. Under an a l t e r n a t e approach, the f o r a g i n g success of i n d i v i d u a l s or f a m i l y groups c o u l d be c o n s i d e r e d of g r e a t e r importance i n h a b i t a t s e l e c t i o n than other b e n e f i t s d e r i v e d from the formation of l a r g e groups (as reviewed by Alexander 1974, Bertram 1978, and Rubenstein 1978). In t h i s view, l a r g e r groups would be expected to form when i n d i v i d u a l s or small groups randomly encounter each other while f o r a g i n g in the same area, or c o n gregating at favored food sources, at any time of year. Group encounter rate should be p r o p o r t i o n a l to animal d e n s i t y i n the h a b i t a t , which might be i n v e r s e l y r e l a t e d to the h a b i t a t a r ea. Large groups would be l e s s l i k e l y to form where the h a b i t a t area was l a r g e , the r e l a t i v e d e n s i t y of animals was low, and the chances of encountering another group was a l s o low. At any time when i n d i v i d u a l f o r a g i n g e f f i c i e n c y was compromised by group membership, as when food items became r e l a t i v e l y rare or 1 36 patchy and feeding competition w i t h i n the group i n c r e a s e d , the group would be expected to break up. Elephants are gr e g a r i o u s and t h e i r s o c i a l o r g a n i z a t i o n i s m a t r i l i n e a l . The b a s i c u n i t in elephant s o c i e t y i s the cow/calf f a m i l y u n i t , of one or more a d u l t females with t h e i r j u v e n i l e o f f s p r i n g (Douglas-Hamilton 1972). The ad u l t females i n a fa m i l y u n i t are thought to be c l o s e l y r e l a t e d ; the females are mothers and daughters, h a l f - s i s t e r s (or p o s s i b l y , i n some cases, f u l l s i s t e r s (Dublin i n p r e s s ) ) , and h a l f - or f u l l c o u s i n s . Males leave the fa m i l y u n i t s at about the age of puberty, and become members of a d u l t b u l l s o c i e t y . The a d u l t b u l l s have a more l o o s e l y s t r u c t u r e d system of short term a s s o c i a t i o n s (Croze 1974a), although longer term s o c i a l t i e s may be present (Poole i n p r e p . ) . J u v e n i l e females remain i n the fami l y u n i t s , where they e v e n t u a l l y r a i s e t h e i r own young. When f a m i l y u n i t s become l a r g e , they ap p a r e n t l y d i v i d e .to form two or more d i s t i n c t u n i t s , which r e t a i n s o c i a l bonds, and which a s s o c i a t e together f r e q u e n t l y . T h i s d i v i s i o n of l a r g e f a m i l y u n i t s i n t o separate groups i s thought to be the o r i g i n of "extended f a m i l y u n i t s " (Douglas-Hamilton 1972) which are o f t e n seen i n fo r a g i n g a g g r e g a t i o n s . I w i l l use the more c o n s e r v a t i v e term of "bond group" (Moss 1981) to d e s c r i b e f a m i l y u n i t s which are r e g u l a r l y seen together, u n t i l t h e i r k i n s h i p t i e s have been c l e a r l y e s t a b l i s h e d . Elephant f a m i l y u n i t s , and the p o t e n t i a l l y r e l a t e d "bond" groups, r e p o r t e d l y form l a r g e herds i n the r a i n y seasons i n some se m i - a r i d r e g i o n s . Examples of t h i s s e a s o n a l i t y i n group s i z e 1 37 have been r e p o r t e d f o r l o c a l i t i e s i n Uganda (Laws et a l . 1975), Kenya (Leu t h o l d 1976), and Tanzania (A.R.E. S i n c l a i r , p e r s . comm.) while i n other areas of Tanzania and Uganda, elephant herd s i z e was not s i g n i f i c a n t l y d i f f e r e n t between seasons (Douglas-Hamilton 1972, E l t r i n g h a m 1977). In t h i s chapter, I w i l l attempt to d i s t i n g u i s h between two general a l t e r n a t i v e hypotheses as I examine the p o t e n t i a l e f f e c t s of s o c i a l o r g a n i z a t i o n on h a b i t a t s e l e c t i o n by the Amboseli e l e p h a n t s : H1a: Elephants have a tendency to form l a r g e aggregations at a l l times, and choose h a b i t a t s that allow them to maintain l a r g e group s i z e . Under t h i s h y p o t h e s i s , the number of l a r g e groups should remain high through'the seasons," and they should move to h a b i t a t s with abundant herb l a y e r biomass and low woody cover to permit the lowest f e e d i n g c o m p e t i t i o n . The p r o p o r t i o n of animals forming l a r g e groups should remain f a i r l y constant through the seasons, u n t i l food sources become l i m i t e d d u r i n g the dry season. Then, in t h e i r f i n a l h a b i t a t , group s i z e should d e c l i n e as the food supply dwindles. Group s i z e should not be d i r e c t l y r e l a t e d to elephant d e n s i t y i n h a b i t a t types; l a r g e groups c o u l d form where d e n s i t y i s e i t h e r high or low. H1b: Elephants are p r i m a r i l y concerned with f o r a g i n g e f f i c i e n c y and have a secondary tendency to 1 38 aggregate when they encounter other elephants, i f f o r a g i n g e f f i c i e n c y i s not s a c r i f i c e d . In t h i s case, there c o u l d be a c o r r e l a t i o n between elephant d e n s i t y and l a r g e group numbers i n h a b i t a t s , when food d e n s i t y i s a l s o high i n those h a b i t a t s . Large groups should not form where the d e n s i t y of elephants i s low; small groups c o u l d be found i n h a b i t a t s with e i t h e r high or low elephant d e n s i t y . T h i s hypothesis p r e d i c t s no c l e a r t rend i n l a r g e group numbers across seasons: h a b i t a t s h i f t s should not maintain l a r g e group s i z e , but should be pr o v i d e the best f o r a g i n g o p p o r t u n i t i e s f o r i n d i v i d u a l s or f a m i l y u n i t s . These hypotheses w i l l be examined s e p a r a t e l y f o r b u l l s and cow/calf groups. B. Methods For a n a l y s i s of elephant s o c i a l s t r u c t u r e , I used the same set of data as in Chapter 4: the ground and a e r i a l survey counts. In t h i s chapter, the data of i n t e r e s t are the s i z e s and h a b i t a t d i s t r i b u t i o n of elephant groups. The a n a l y s i s i s con c e n t r a t e d on ground count data, but some a e r i a l count data are presented for longer term comparison. A l l analyses were done s e p a r a t e l y f o r b u l l and cow/calf herds. 1 39 !• Analyses a. C o r r e c t ions f o r b i a s i n ground count data The f i g u r e s I used f o r group s i z e were c o r r e c t e d f o r bi a s e s due to h a b i t a t s t r u c t u r e and group bunching, as i n Chapter 4. I d i d not c o n s i d e r i t v a l i d , or necessary, to make c o r r e c t i o n s f o r months with missing counts, s i n c e I wanted to compare the r e l a t i v e d i s t r i b u t i o n of the observed group s i z e s , not ab s o l u t e numbers of animals. Each o b s e r v a t i o n of an elephant group was taken as a separate independent r e c o r d , and i n most cases, o b s e r v a t i o n s i n each season 'were pooled f o r a n a l y s i s of group s i z e p a t t e r n s . S i g h t i n g s of 1027 b u l l groups and 656 cow/calf groups were recorded i n the ground surveys of 1978/79, while a t o t a l of 492 b u l l groups and 524 cow/calf groups were recorded in the a e r i a l survey s e r i e s from 1975 to 1980. b. Group s i z e d i s t r i b u t i o n I examined the r e l a t i v e d i s t r i b u t i o n of group s i z e i n ground counts across the whole Amboseli basin in d i f f e r e n t seasons as a s e r i e s of cumulative percent frequency d i s t r i b u t i o n s which I compared with the Kolmogorov-Smirnov two- sample t e s t ( S i e g e l 1956). I then c a l c u l a t e d the a r i t h m e t i c mean of b u l l and cow/calf group s i z e s (MGS) i n each season across the basin and w i t h i n each h a b i t a t type f o r both ground and a e r i a l counts. In the Dry1 p e r i o d a e r i a l counts, the abs o l u t e number of elephants using the b a s i n , e s p e c i a l l y 140 cow/calf groups, was low in the r a i n s (see Chapter 4), and mean group s i z e (MGS) values f o r the r a i n y season i n t h i s p e r i o d should viewed with t h i s r e s e r v a t i o n i n mind. The v a r i a n c e a s s o c i a t e d with MGS values was in most cases very high, e s p e c i a l l y f o r cow/calf groups. For t h i s reason, I c a l c u l a t e d s t a t i s t i c s r e f l e c t i n g the d i s t r i b u t i o n of cow/calf group s i z e in a more meaningful way: the percent of groups l a r g e r than 25 and l a r g e r than 50 i n the whole basin and in each h a b i t a t type. These were c a l c u l a t e d as the percent of a l l basin groups >25 and >50 which were seen in each h a b i t a t , and a l s o the percent of the groups counted i n each h a b i t a t which were >25 and >50. I chose these values s i n c e the l a r g e s t cow/calf f a m i l y u n i t i n the Amboseli p o p u l a t i o n i n c l u d e d about 25 i n d i v i d u a l s , while the l a r g e s t "bond group" (of p o t e n t i a l l y r e l a t e d f a m i l y u n i t s ) had about 45 members (C. Moss pers. comm.). Groups c o n t a i n i n g more i n d i v i d u a l s than these values were most l i k e l y a ggregations l a r g e r than f a m i l y u n i t s in the f i r s t case, and l a r g e r than "bond groups" i n the second. The d i s t r i b u t i o n of b u l l and cow/calf MGS in h a b i t a t types were compared to elephant d e n s i t y , herb l a y e r and woody l a y e r biomass, and percent woody cover i n h a b i t a t s by Spearman rank c o r r e l a t i o n . The percent of a l l cow/calf groups >25 and >50 which occurred i n each h a b i t a t were c o r r e l a t e d with cow/calf d e n s i t y and herb l a y e r and woody l a y e r t o t a l biomass in comparisons across h a b i t a t s , while the percent of the groups in each h a b i t a t which were >25 and >50 were c o r r e l a t e d with percent woody cover w i t h i n h a b i t a t types. 141 C. R e s u l t s j_. Group s i z e d i s t r i b u t i o n i n the Amboseli b a s i n Bachelor b u l l groups At a l l times, the frequency d i s t r i b u t i o n of b u l l group s i z e was s t r o n g l y skewed to small groups (Figure 31). F i f t y percent of a l l s i g h t i n g s were of one to two animals, while 75 (short r a i n s ) to 90 ( e a r l y dry) percent of a l l s i g h t i n g s were of groups of four animals or l e s s . The general p a t t e r n seen across the cumulative percent d i s t r i b u t i o n s of d i f f e r e n t seasons was the tendency for groups to be smaller and the d i s t r i b u t i o n more s t r o n g l y skewed i n the dry seasons. However, t h i s g eneral p a t t e r n d i d not vary c o n s i s t e n t l y and s e q u e n t i a l l y through a l l seasons; f o r i n s t a n c e , the e a r l y dry season d i s t r i b u t i o n was more skewed than those of the mid and l a t e dry seasons. There was a s i g n i f i c a n t (p<.05, Kolmogorov-Smirnov t e s t ) d i f f e r e n c e between the frequency d i s t r i b u t i o n s f o r the short r a i n s and the e a r l y dry seasons, but a l l other d i s t r i b u t i o n s were not s i g n i f i c a n t l y d i f f e r e n t from each other. B u l l mean group s i z e (MGS) across the whole basin i n d i f f e r e n t seasons can be seen i n F i g u r e 32. A l s o shown are the group s i z e s which i n c l u d e d 50 and 90 percent of the b u l l s recorded. An a n a l y s i s of v a r i a n c e showed that MGS d i f f e r e d s i g n i f i c a n t l y (p=.0002, n=l027 groups) between seasons, although the a c t u a l range of values was not very great ( i e . 1 . 8 to 3.4). A f t e r an i n i t i a l d e c l i n e from the short r a i n s to e a r l y dry 142 100 80' 60H I - z tu o oc LU 0_ O 4oH 2 0H SEASON T 6 T 8 10 BULL GROUP SIZE Figure 31. Cumulative frequency distr ibution of bachelor bull group size as seen in ground counts of the Amboseli basin in each season of 1978/ 79. There was a trend, however inconsistent, for groups to be smaller the dry season, although bachelor bull groups were f a i r l y small at a l l times. 143 7 - , 6 -J 5 H UJ A M t o § 3 cc o ZD CO 2 -J 1 -A 90 % • _ Mean O p =0.0002 n=1027 -a • 5 0 % Short Mid Long Early Mid I RAINS 1 I D R Y - Late SEASON Figure 32. Bachelor bull mean group s ize , o , and group sizes which included 50%, • , and 90%, n , of a l l bulls sighted during ground counts in 1978/ 79. Note that mean group size was s ign i f i cant ly different between seasons, although the range of values was small. There was no apparent trend in bull MGS through the seasons, after an i n i t i a l decrease from short to mid rains. 1 44 season, b u l l MGS i n c r e a s e d s l i g h t l y towards the l a t e dry season. Analyses of the a e r i a l survey data ( F i g u r e 33) show that b u l l MGS d i d not vary g r e a t l y between seasons and r a i n f a l l p e r i o d s , with f a i r l y l a r g e a s s o c i a t e d v a r i a n c e at a l l times. The Wet p e r i o d (1977 to 1979) a e r i a l counts have a s i m i l a r p a t t e r n to the ground count r e s u l t s : a decrease in b u l l MGS from the short r a i n s f o l l o w e d by an i n c r e a s e towards the l a t e dry season. An a n a l y s i s of v a r i a n c e r e v e a l e d s i g n i f i c a n t d i f f e r e n c e s (p=.0l n=l69 groups) in b u l l MGS between seasons f o r t h i s p e r i o d . The two low r a i n f a l l p e r i o d s (Dry1, 1975 to 1977, and Dry2, 1979 to 1980) both had i r r e g u l a r , n o n - s i g n i f i c a n t seasonal p a t t e r n s , and t h e i r MGS valu e s were somewhat lower than those of the Wet p e r i o d . b. Cow/calf groups The cumulative percent frequency d i s t r i b u t i o n s of cow/calf group s i z e i n d i f f e r e n t seasons are shown i n F i g u r e 34. F i f t y percent of a l l s i g h t i n g s were of groups of 20 animals or l e s s , while cow/calf groups of 50 or l e s s made up 75 (short r a i n s ) to 90 ( l a t e dry) percent of the group s i g h t i n g s . The seasonal d i s t r i b u t i o n s , l i k e those of the b u l l groups, were s t r o n g l y skewed towards smaller groups, with skewness i n c r e a s i n g i n the dry season. However, the seasonal trend of i n c r e a s i n g group s i z e skewness was more r e g u l a r f o r the cow/calf groups than i t was f o r the b u l l s . Small groups became i n c r e a s i n g l y more common from the short r a i n s through to the l a t e dry season. No comparisons between frequency d i s t r i b u t i o n s were s t a t i s t i c a l l y 1 45 Fi g u r e 33. Bachelor b u l l mean group s i z e seen i n a e r i a l counts d u r i n g the three r a i n f a l l p e r i o d s . V e r t i c a l bars i n d i c a t e + 1 standard e r r o r of the mean. There were s i g n i f i c a n t d i f f e r e n c e s between means i n the Wet p e r i o d only, and the range of values was never g r e a t . Group s i z e s were s l i g h t l y higher i n the Wet p e r i o d , 1977-1979. WET I- I Zr—4  P=°'01 X X N=169 i : r DRY 2 I- 1- ^ 1 — i 1 r Short Mid Long I RAINS 1 — i — — r Early Mid I DRY- p = 0-10 n = 201 Long SEASON 147 ~ i r~ i r~ i T — r~ 1 20 40 60 80 100 120 140 160 COW-CALF GROUP SIZE Figure 34. Cumulative frequency dis tr ibut ion of cow/calf grouD size seen in ground counts in each season of 1978/79. There was a consistent trend . for groups to be smaller from rains to late dry season. 1 48 s i g n i f i c a n t . The trend of cow/calf MGS i n ground counts (Figure 35) was a steady, s i g n i f i c a n t (p=.0l2, n=656 groups) decrease from the short r a i n s to. mid dry season, with a s l i g h t i n c r e a s e in the l a t e dry season. The group s i z e s which i n c l u d e d 50 and 90 percent of the observed cows and c a l v e s showed a s i m i l a r seasonal d e c l i n e . In the a e r i a l count data, cow/calf MGS decreased from the short r a i n s to the mid and l a t e dry season i n both the Wet and Dry2 p e r i o d s ( F i g u r e 36). In the Dry1 p e r i o d , the r a i n y season counts recorded small MGS v a l u e s ; t h i s r e s u l t appeared because most of the cow/calf herds had l e f t the Amboseli b a s i n e n t i r e l y in the r a i n s d u r i n g t h i s p e r i o d (see Chapter 4). The var i a n c e a s s o c i a t e d with the cow/calf MGS val u e s was high at a l l times,'and seasonal d i f f e r e n c e s were s i g n i f i c a n t i n a n a l y s i s of v a r i a n c e (p=.0l, n=l99) only i n the Dry2 p e r i o d . Note that cow/calf MGS i n the dry season was g e n e r a l l y lower in both the Dryl and Dry2 p e r i o d s than i n the i n t e r v e n i n g Wet p e r i o d . F i g u r e 37 shows that a c r o s s the b a s i n , the p r o p o r t i o n of l a r g e aggregations, (those l a r g e r than 50 a n i m a l s ) , d e c l i n e d s h a r p l y from the e a r l y r a i n s to mid r a i n s and then decreased more g r a d u a l l y towards the l a t e dry season. The p r o p o r t i o n of groups l a r g e r than fam i l y u n i t s (more than 25 i n d i v i d u a l s ) d e c l i n e d in a s i m i l a r , though l e s s s i g n i f i c a n t , f a s h i o n , and remained at a higher l e v e l than % of groups >50 throughout the year. The mean number of groups recorded per count i n c r e a s e d s t e a d i l y through the same seasons. 149 100-1 80H 60-H tu M to 1 AO- < 5 o o 20-\ 90 % O Mean p=0012 n=656 5 0 % ~~I——i 1 r — — i 1— Short Mid Long Early Mid Late I - R A I N S 1 | — D R Y H SEASON Figure 35. Cow/calf mean group s ize , o , and group sizes which included 50%, • , and 90%, • , of a l l cows and calves sighted during ground counts 1978/79. Mean group size was s ignif icantly different between seasons, with a consistent trend of decreasing MGS towards the late dry season. 1 50 F i g u r e 36. Cow/calf mean group s i z e as seen i n a e r i a l c o u n t s over the t h r e e r a i n f a l l p e r i o d s . V e r t i c a l b a r s i n d i c a t e + 1 s t a n d a r d e r r o r . There were s i g n i f i c a n t d i f f e r e n c e s between means i n the Dry p e r i o d o n l y . In Dry1 p e r i o d , group s i z e s were s m a l l d u r i n g the r a i n s , p r o b a b l y because t h e r e were few c o w / c a l f groups u s i n g the b a s i n a t t h i s time (see Chapter 4 ) . i DRY1 40 20 H i r ^ P = 0-21 n=T74 1 T 60 -i 4 0 H 20H WET p=0-64 n =151 60 - i 40 H 20- DRY 2 p = 0014 J n = 199 SEASON 152 A ) 20-1 in CM A t o C L r> o cc •o o 10H r s =-0-71 © — — © B) 1 2 n o in A to C L ZD O ce o 9 10H 8 H 6H r s = 1-00 A / r -30 i r-—r~ Short Mid Long i RAINS i Early Mid DRY- Late t- z o o or UJ a. CO 20 UP  o DC O d z z < UJ 10 • SEASON Figure 37. The percent of cow/calf groups > 25 and > 50, and the mean number of groups per count, as seen in each season in ground counts of the Amboseli basin. The trend of decreasing % of large groups was s ignif icant at p < .05 for groups > 50, but not for groups > 25. The trend of increasing numbers of groups seen per.count was highly s igni f icant . 153 2. H a b i t a t d i s t r i b u t i o n of elephant group s i z e a. Bachelor b u l l groups The o v e r a l l d i s t r i b u t i o n of b u l l MGS a c r o s s the h a b i t a t types of the Amboseli b a s i n (see F i g u r e 38) was f a i r l y even; l a r g e r b u l l MGS was found i n the A. t o r t i l i s woodlands (Tw) while lower MGS occurred i n the swamp edge g r a s s l a n d s (Seg), and permanent swamps (Swp). Table 11 shows the seasonal rank d i s t r i b u t i o n of b u l l MGS across h a b i t a t s . There are few c o n s i s t e n t p a t t e r n s in these Table 11. Ranking of bachelor b u l l MGS a c r o s s h a b i t a t types i n each season (read v e r t i c a l l y i n columns). Ranks were assigned as f o r Spearman's c o r r e l a t i o n , with low MGS values having low ranks. Average values were given to t i e d ranks. There was no c o n s i s t e n t p a t t e r n in the ranks of b u l l MGS a c r o s s h a b i t a t s between seasons. RANK ACROSS SEASON HABITAT RAINS DRY TYPES Short Mid Long E a r l y Mid Late A l k a l i n e g r a s s . S. c o n s i m i 1 i s gr. A. t o r t i l i s woods Fever t r e e woods S/S shrubs Palm/Fever woods Young Fever woods Swamp edge gr a s s . Swamp 7 2.5 8.5 3 5 3 9 8 7 6 2.5 5.5 8 4 4 5 7 8.5 1 9 5.5 4 1 2 2 6 1 5.5 4 1 .5 5.5 9 5.5 9 8 1 .5 8 5 8 5 6 4 1 3 7 3 2 9 4 7 5.5 7 1 3 r e s u l t s . B u l l MGS was both l a r g e and small i n the g r a s s l a n d s , woodlands, and swamp edges in both r a i n y and dry seasons, with no c l e a r trends v i s i b l e . The only c o n s i s t e n t r e s u l t was that 154 3 i 2 - 4 LU M L O O L ZD O C C o z < 3 CD 1 -H p = 0006 n = 1027 i Gr Con Tw Xw Ss Pax Yax Seg Swp HABITAT TYPE Figure 38. The yearly mean size of bachelor bull groups seen in habitat types in ground counts. Differences were s ign i f i cant , although the range in group size was low. Vert ica l bars indicate +1 standard error . The largest groups were seen in the A. t o r t i l i s woodlands, and the smallest were seen in the wet swamps. 1 55 b u l l groups were g e n e r a l l y s m a l l e r i n the permanent swamps. T h i s echoes the year-round r e s u l t shown i n F i g u r e 38. Because of the v a r i a b i l i t y , there was no s i g n i f i c a n t c o r r e l a t i o n between b u l l MGS and such h a b i t a t c h a r a c t e r i s t i c s as herb l a y e r t o t a l biomass or percent woody cover. B u l l MGS i n h a b i t a t types was p o s i t i v e l y c o r r e l a t e d with the d e n s i t y of b u l l s i n the same h a b i t a t s only i n the mid r a i n s (see Table 12). MGS was n e g a t i v e l y c o r r e l a t e d with b u l l d e n s i t y Table 12. C o r r e l a t i o n s (Spearman's rank) of bachelor b u l l d e n s i t y with MGS acr o s s h a b i t a t types i n each season. There i s no c o n s i s t e n t p a t t e r n i n these c o r r e l a t i o n s . SPEARMAN'S SEASON Rs RAINS DRY Short Mid Long E a r l y Mid Late B u l l D e n s i t y .033 .754* • .232 -.628* -.617* .445 vs. MGS S i g n i f i c a n c e of Spearman's r s (n=9, t i e s c o r r e c t e d ) : * p < .05 in h a b i t a t s in the e a r l y and mid dry season. Thus, there were no c l e a r c o r r e l a t i o n s between b u l l MGS and d e n s i t y across h a b i t a t types at most times of year. b. Cow/calf groups The year-round d i s t r i b u t i o n of cow/calf MGS across the basi n h a b i t a t s i s seen i n F i g u r e 39. There i s some suggestion that cow/calf group s i z e was i n v e r s e l y r e l a t e d to the woody 156 40-1 30H U J M CO 0_ O or o < LU s: L U _ J < O i =S o o 20H 1 0- p = 0-083 n = 656 Gr Con Tw Xw Ss Pax Yax Seg Swp HABITAT TYPE F i g u r e 39. The y e a r l y mean s i z e of cow/ca l f groups seen i n h a b i t a t types i n ground c o u n t s . V e r t i c a l bars i n d i c a t e + 1 s t a n d a r d e r r o r . D i f f e r e n c e s were not s i g n i f i c a n t , and the range o f v a l u e s was h i g h . G e n e r a l l y , the l a r g e s t groups were seen i n the a l k a l i n e g r a s s l a n d s , f e v e r t r e e wood lands , Salvadora/Suaeda s h r u b l a n d s , and the swamp edge g r a s s l a n d s . 157 cover of h a b i t a t types. Some of the h a b i t a t s with low woody cover, such as the fever t r e e woodlands, Salvadora / Suaeda shrublands, and the swamp edge grasslands had r e l a t i v e l y high cow/calf MGS on a year-round b a s i s , while other, more densely wooded h a b i t a t s -- the A. t o r t i l i s woodlands, palm / fever t r e e swamp edge woodlands, and young fever t r e e woodlands -- had small e r o v e r a l l MGS v a l u e s . Cow/calf MGS was a l s o small i n the permanent swamps and the S. c o n s i m i l i s g r a s s l a n d s . The seasonal rank d i s t r i b u t i o n s of the percent of cow/calf groups >25 and >50 in h a b i t a t s are seen in Table 13. There i s a trend in the r e l a t i v e occupance of h a b i t a t s by l a r g e groups, which may be more c l e a r l y seen i n the schematic diagram, F i g u r e 40. Aggregations l a r g e r than 50 animals were found most o f t e n in the open Salvadora / Suaeda shrublands and fever t r e e woodlands, and i n the A. t o r t i l i s woodlands i n the r a i n y seasons. In the dry seasons, the l a r g e aggregations were found i n c r e a s i n g l y i n the swamp edge (palm/fever t r e e ) woodlands and, by the l a t e dry season, i n the swamp edge g r a s s l a n d s , while r a r e l y i n the woodlands and shrublands. Spearman rank c o r r e l a t i o n s of percent of groups >25 and >50 with herb l a y e r biomass and percent woody cover i n h a b i t a t types are shown i n Table 14. Herb l a y e r t o t a l biomass c o r r e l a t e d p o s i t i v e l y with the percent of groups >50 o c c u r r i n g i n each h a b i t a t i n the l a t e dry season. Other c o r r e l a t i o n s with p l a n t biomass were not s i g n i f i c a n t . However, there was an i n t e r e s t i n g t rend in the r e l a t i o n s h i p s with herb l a y e r biomass. The non- s i g n i f i c a n t c o r r e l a t i o n s of percent of groups >25 with herb 1 58 Table 13. Rankings of % cow/calf groups > 25 and > 50 ac r o s s h a b i t a t types i n each season. The ranking system i s the same as was used f o r bachelor b u l l MGS (see Table 11). There i s a c o n s i s t e n t p a t t e r n f o r l a r g e cow/calf groups to s h i f t from using the woodlands i n the r a i n s to using the swamp edge .woodlands and gr a s s l a n d s i n the dry season. A) COW/CALF GROUPS: % > 25 RANK ACROSS SEASON HABITAT RAINS DRY TYPES Short Mid Long E a r l y Mid Late A l k a l i n e g r a s s . 6. .5 S. c o n s i m i l i s gr. 7. ,5 A. t o r t i l i s woods 3. ,5 Fever t r e e woods 5 S/S shrubs 9 Palm/Fever woods 6 Young Fever woods 1 . ,5 Swamp edge gr a s s . 7, .5 Swamp 1 . .5 6 3 2 2 1 .5 1 .5 3 1 2 4 6 9 5.5 4 1 .5 8 7.5 7 6.5 7 9 7.5 8.5 9 8.5 4 1 5.5 8 3 7 5.5 3.5 2 6 4 3 8.5 6.5 8.5 1 .5 5.5 3.5 5 5 B) COW/CALF GROUPS: % > 50 RANK ACROSS SEASON HABITAT RAINS DRY TYPES Short Mid Long E a r l y Mid Late A l k a l i n e g r a s s . S. c o n s i m i l i s gr. A. t o r t i l i s woods Fever t r e e woods S/S shrubs Palm/Fever woods Young Fever woods Swamp edge grass. Swamp 5 2.5 2 7 2.5 4.5 3 7 7 .5 5 8 9 9 9 7.5 5 2.5 2 2 5.5 4.5 8 5.5 2 3 2.5 6 4 2.5 1 .5 1 .5 5 4 4 2.5 1 .5 8.5- 8 3.5 8.5 7 3.5 6.5 9 2 4 • 2.5 4 6.5 6 1 1 .5 2.5 4 159 CO < Swamp -• Swamp Edge Young Fever Palm Woods < cc $ Tortilis < Woods 2: o CO Fever Woods Shrub H Grass H Consimilis H I Short I — 1 1 Mid Long RAINS- 1 Early -~T~ Mid •DRY- Late SEASON © 30*% 0 10-20 * 20-30 0 0-10 Figure 40. Schematic diagram of the seasonal habitat distr ibution of cow/calf groups which had > 50 individuals . The highest proportion of these large groups shifted from the woodland habitats during the rains to swamp edge woodlands (palm/fever tree) and grasslands during the dry season. 1 60 Table 14. C o r r e l a t i o n s (Spearman's rank) of % cow/calf groups > 25 and > 50 with t o t a l biomass of the herb l a y e r and % cover of the woody l a y e r a c r o s s h a b i t a t types i n each season. Few s i g n i f i c a n t c o r r e l a t i o n s were found, although c o r r e l a t i o n s were g e n e r a l l y more p o s i t i v e i n the dry season. SPEARMAN'S Rs ; % GROUPS > 25 VS. A) COW/CALF GROUPS % > 25 SEASON Short RAINS Mid Long E a r l y DRY Mid Late Herb l a y e r t o t a l biomass 273 -.515 . 329 -.085 1 45 .307 % woody cover 1 27 .428 .048 802' .371 .524 B) COW/CALF GROUPS: % > 50 SPEARMAN'S Rs: % GROUPS > 50 VS. Short RAINS Mid Long SEASON E a r l y DRY Mid Late Herb l a y e r t o t a l biomass .077 .026 .137 .030 .406 .852** % woody cover .204 .494 .315 .594 .690* .601 S i g n i f i c a n c e of Spearman's r s (n=8, t i e s c o r r e c t e d ) : * p < .05 ** p < .01 161 l a y e r biomass were negative d u r i n g the r a i n s and became i n c r e a s i n g l y p o s i t i v e i n the dry season. Percent woody cover i n h a b i t a t s c o r r e l a t e d p o s i t i v e l y with the percent of groups i n each h a b i t a t >25 i n the e a r l y dry season and with percent of groups i n each h a b i t a t >50 i n the mid dry season. There was no c l e a r t rend a c r o s s the seasons. Cow/calf d e n s i t y c o r r e l a t e d p o s i t i v e l y with the percent of groups >25 i n each h a b i t a t i n the short r a i n s and l a t e dry season (Table 15), and with the percent of groups >50 i n each Table 15. C o r r e l a t i o n s (Spearman's rank) of cow/calf d e n s i t y with MGS, % groups > 25, and % groups > 50 across h a b i t a t types i n each season. There are no c o n s i s t e n t p a t t e r n s i n these r e s u l t s , except that d e n s i t y c o r r e l a t e d with % of groups > 25 and > 50 i n the l a t e dry season. SPEARMAN'S Rs: COW/CALF DENSITY VS. SEASON RAINS DRY Short Mid Long E a r l y Mid Late Mean Group S i z e % Groups > 25 % Groups > 50 .150 .266 -.033 .675* .426 .239 .587 .272 .194 -.383 -.417 .422 .510 .227 .372 .520 .654* .834** S i g n i f i c a n c e of Spearman's r s (n=9, t i e s c o r r e c t e d ) : * p < .05 ** p < .01 h a b i t a t in the l a t e dry season o n l y . A l l other c o r r e l a t i o n s ( f o r example, d e n s i t y vs. MGS i n a l l seasons, or % >25 and % >50 i n other seasons) were not s i g n i f i c a n t . 1 62 D. D i s c u s s i o n J_. Bachelor b u l l groups Although there were seasonal d i f f e r e n c e s i n the s i z e of bachelor b u l l s o c i a l groups, there were no c l e a r seasonal trends in group s i z e , save that groups were ap p a r e n t l y l a r g e s t in the e a r l y r a i n y season, and lower during the r e s t of the year. Mean group s i z e was never p a r t i c u l a r l y high, ranging from one to four animals in d i f f e r e n t h a b i t a t s . B u l l group s i z e was s i g n i f i c a n t l y r e l a t e d to b u l l d e n s i t y i n h a b i t a t types in the mid r a i n s , but was n o n - s i g n i f i c a n t l y or i n v e r s e l y r e l a t e d i n other seasons. There was no c l e a r seasonal s h i f t or p a t t e r n i n h a b i t a t use by l a r g e or small b u l l groups. From these r e s u l t s , i t appears that there i s no general tendency f o r b u l l s to form l a r g e bachelor herds, and that t h e i r h a b i t a t s e l e c t i o n i s l i t t l e a f f e c t e d by a need to stay in l a r g e groups with other b u l l s . There i s a tendency for b u l l s to form small groups, the s i z e of which may be somewhat l a r g e r i n r a i n y seasons or high r a i n f a l l y e a r s . S i n c l a i r (1977) suggested that by l e a v i n g the l a r g e female herds, male b u f f a l o c o u l d have access to e x t r a food resources in small patches u n a v a i l a b l e to the l a r g e r groups. T h i s b e n e f i t may be balanced by the t h r e a t of p r e d a t i o n on i n d i v i d u a l animals in small groups. However, b u l l h a b i t a t s e l e c t i o n may a l s o be i n f l u e n c e d by f o r a g i n g and r e p r o d u c t i v e s t r a t e g i e s which i n v o l v e a s s o c i a t i n g with cow/calf herds at d i f f e r e n t times of year (Poole and Moss 1981, J . Poole in p r e p . ) . 163 2. Cow/calf groups The v a r i a n c e seen i n cow/calf group s i z e s r e f l e c t s the f l e x i b l e , v a r i a b l e nature of elephant s o c i a l dynamics. My c a s u a l o b s e r v a t i o n s i n d i c a t e d that herds l a r g e r than "bond groups" stayed together f o r r e l a t i v e l y b r i e f p e r i o d s i n some cases only a few hours, i n other cases f o r a p e r i o d of days when compared with the longer term a s s o c i a t i o n s of A f r i c a n b u f f a l o ( S i n c l a i r 1977) or impala (Jarman and Jarman 1979) herds. I t appeared that the l a r g e aggregations u s u a l l y broke up in the evenings and during the course of the n i g h t , o f t e n r e - forming the next day with the same or somewhat d i f f e r e n t f a m i l y u n i t composition. Groups c o u l d a l s o form and break up at any p o i n t d u r i n g the d a y l i g h t hours. When seasonal group s i z e i n d i c e s are compared, group formation should be viewed as a s t a t i s t i c a l tendency by the elephants to form l a r g e or small groups of f a m i l y u n i t s , r a ther than as the cohesion or f i s s i o n of fixed-membership groups. The p a t t e r n of cow/calf herd formation and break up i s markedly d i f f e r e n t from that of bachelor b u l l s . Cow/calf herds were o b v i o u s l y much l a r g e r than b u l l herds at a l l times of the year -- the l a r g e s t breeding herd recorded i n ground surveys i n c l u d e d 335 i n d i v i d u a l s , while the l a r g e s t bachelor b u l l herd numbered only 17 animals. The p a t t e r n of cow/calf group s i z e d i s t r i b u t i o n across seasons and h a b i t a t types showed c l e a r trends i n c o n t r a s t to the b u l l group p a t t e r n of i n c o n s i s t e n t v a r i a t i o n . Cow/calf MGS i n ground counts over the whole b a s i n d e c l i n e d 164 s t e a d i l y from the r a i n s through to the l a t e dry season. T h i s p a t t e r n was a l s o seen i n the long term a e r i a l count data f o r Wet (1977 -1979) and Dry2 (1979 - 1980) p e r i o d s . The p r o p o r t i o n of la r g e ( s i z e >50) aggregations seen in ground counts dropped sh a r p l y a f t e r the e a r l y r a i n s , and d e c l i n e d g r a d u a l l y through the dry season, while the t o t a l number of groups (which i n c l u d e d many smaller groups) formed i n c r e a s e d s t e a d i l y . The p r o p o r t i o n of aggregations with more than 25 members showed a more gradual seasonal d e c l i n e , m a i n t a i n i n g a higher l e v e l throughout the year. Large groups i n the r a i n y season were seen, but not recorded i n basin a e r i a l counts, dur i n g the Dry1 (1975-1977) p e r i o d , s i n c e most of the cow/calf groups l e f t the basin e n t i r e l y i n the r a i n s . I t appears that l a r g e aggregations formed in the r a i n y seasons of a l l "years s t u d i e d . However, i n the two low r a i n f a l l p e r i o d s (before 1977 and a f t e r 1979), MGS dropped s u b s t a n t i a l l y lower i n the dry season, as compared with the i n t e r v e n i n g Wet p e r i o d . T h i s supports the idea that group membership i s l i m i t e d by food abundance, e s p e c i a l l y of the herb l a y e r , which I showed was s e n s i t i v e to annual r a i n f a l l p a t t e r n s (see Chapter 3). Most of the l a r g e cow/calf groups formed i n the savannah woodlands (Tw, Xw, Ss) i n the r a i n s and e a r l y dry seasons, and in the swamp edge g r a s s l a n d s and swamp edge woodlands in the mid and l a t e dry seasons (see F i g u r e 40). T h i s p a r a l l e l s the seasonal s h i f t i n cow/calf h a b i t a t use demonstrated i n Chapter 4. - Group s i z e , however, was not simply c o r r e l a t e d with the de n s i t y of animals using the h a b i t a t types through the year. 165 The l a r g e groups were found i n the h a b i t a t s with r e l a t i v e l y low d e n s i t i e s of elephants in the r a i n y season; i t was only i n the l a t e dry season that d e n s i t y c o r r e l a t e d with percent of l a r g e groups. The l a t t e r c o r r e l a t i o n c o u l d be a consequence of a number of l a r g e groups producing a high d e n s i t y i n the small swamp edge h a b i t a t s , r a t h e r than high d e n s i t i e s of elephants r e s u l t i n g i n the formation of l a r g e groups. T h i s r e s u l t supports the hypothesis that elephants g e n e r a l l y tend to form l a r g e aggregations and s h i f t t h e i r h a b i t a t use to allow l a r g e group formation; i t does not support the a l t e r n a t e hypothesis that l a r g e groups form a f t e r random encounters. S i g n i f i c a n t c o r r e l a t i o n s between the percent of groups >50 and h a b i t a t c h a r a c t e r i s t i c s such as herb l a y e r t o t a l biomass were found i n the l a t e dry season. However, the p a t t e r n seen was s i m i l a r to that of simple numerical h a b i t a t p r e f e r e n c e shown in Chapter 4. C o r r e l a t i o n s were i n c r e a s i n g l y p o s i t i v e towards the l a t e dry season. I t appears that l a r g e groups form where the v e g e t a t i o n i s of high n u t r i t i o n a l q u a l i t y , as w e l l as simply abundant. The smaller group s i z e s seen i n the dry seasons of low r a i n f a l l years suggest that the abundance of n u t r i t i o u s food, or of " p r o f i t a b l e f o r a g i n g " , may l i m i t the group s i z e s that can form. An i n v e r s e r e l a t i o n s h i p between woody cover and group s i z e , as proposed by Leuthold (1977a), was not c l e a r l y demonstrated i n the c u r r e n t study. L e u t h o l d suggested that food source p a t c h i n e s s i n woody v e g e t a t i o n types would d i s r u p t group cohesion. However, i n s e m i - a r i d areas such as Tsavo, where he 166 worked, herb l a y e r cover i s i n v e r s e l y r e l a t e d to woody cover ( N o r t o n - G r i f f i t h s 1979, P r a t t and Gwynne 1977). In the Amboseli swamp edge woodlands, high herb l a y e r cover and high woody cover c o i n c i d e d . The d i s t r i b u t i o n of food items f o r g r a z i n g animals would be much l e s s patchy i n the dense swamp edge woodlands than in s e m i - a r i d bushlands, and group s i z e c o u l d be expected to remain l a r g e . However, the year-round p a t t e r n of cow/calf group s i z e i n h a b i t a t types d i d suggest that l a r g e groups were seen more o f t e n i n the l e s s woody h a b i t a t s , so Leuthold's suggestion might have some degree of v a l i d i t y i n Amboseli. My impression from c a s u a l o b s e r v a t i o n s was that l a r g e elephant aggregations i n wooded areas were more widely d i s p e r s e d and sometimes became fragmented, but that they re-formed i n t o l a r g e w e l l - d e f i n e d herds when they emerged onto more open g r a s s l a n d s or swamp edges. My r e s u l t s do not support the hypothesis of group formation by random encounter i n areas of hig h d e n s i t y . Notably, a high p r o p o r t i o n of the l a r g e groups formed i n the savannah woodlands and shrublands d u r i n g and a f t e r the r a i n s where d e n s i t i e s were not high compared to swamp edge h a b i t a t s , but where abundant, n u t r i t i o u s grass was a v a i l a b l e . The hypothesis of l a r g e group formation and h a b i t a t s e l e c t i o n to maintain l a r g e group s i z e i s more s t r o n g l y supported. However, not a l l the p r e d i c t i o n s of t h i s hypothesis were met. S p e c i f i c a l l y , mean group s i z e d e c l i n e d evenly through the seasons, as d i d the percent of l a r g e •groups i n the basin p o p u l a t i o n , i n s t e a d of being maintained at a constant high l e v e l u n t i l the dry season. I t may be that 167 h a b i t a t s h i f t i n g cannot maintain the i n t e g r i t y of the very l a r g e and temporary aggregations, but may apply more e f f e c t i v e l y through time at the l e v e l of the i n t e r m e d i a t e - s i z e d "bond groups". I n d i v i d u a l f a m i l y u n i t s might d i f f e r i n t h e i r tendencies towards aggregation, as p r e l i m i n a r y o b s e r v a t i o n s of i d e n t i f i e d females would suggest. The f a m i l y u n i t s might a l s o d i f f e r i n t h e i r tendency to a v o i d s o c i a l i n t e r a c t i o n s or f o r a g i n g c o m p e t i t i o n with other elephants, and to r e t u r n to t h e i r d i s t i n c t dry season home ranges, r e s u l t i n g from d i f f e r e n t i a l success i n f o r a g i n g or other a c t i v i t i e s i n the l a r g e groups. The general p a t t e r n s of i n t e r a c t i o n between group formation and h a b i t a t s e l e c t i o n are suggested by the c u r r e n t study. However, the answers to more s p e c i f i c q u e s t i o n s of i n d i v i d u a l group behaviour, and the importance of d i f f e r e n t l e v e l s of s o c i a l s t r u c t u r e i n f o r a g i n g and h a b i t a t s e l e c t i o n must come from f i n e r s c a l e s t u d i e s at the i n d i v i d u a l l e v e l . 168 CHAPTER 6 GENERAL DISCUSSION AND CONCLUSIONS A. General H a b i t a t S e l e c t i o n P a t t e r n of the Amboseli Elephants L e u t h o l d (1977b) viewed the s t r a t e g y of h a b i t a t use by elephants as a " p u l s a t i n g system". T h i s system has a " s t a b l e element" centered on dry season home ranges near water sources and an " o p p o r t u n i s t i c element" i n v o l v i n g movements to ephemeral, l o c a l i z e d water and food sources s c a t t e r e d a c r o s s the bushlands i n the wet season. T h i s s t r a t e g y c o u l d be observed i n a c t i o n at the l e v e l of l a r g e - s c a l e s h i f t s i n the seasonal h a b i t a t d i s t r i b u t i o n of the p o p u l a t i o n , or at the l e v e l of seasonal movements by i n d i v i d u a l elephants. I examined h a b i t a t use by elephants at the broad l e v e l i n the c u r r e n t study, and my r e s u l t s support Leuthold's h y p o t h e s i s . My b r o a d - l e v e l e x p l a n a t i o n of h a b i t a t s e l e c t i o n by elephants in Amboseli i s that they choose the h a b i t a t s where herb l a y e r v e g e t a t i o n of the hig h e s t q u a l i t y i s s u f f i c i e n t l y abundant to permit a high intake r a t e . S e l e c t i o n of h a b i t a t s or patches with higher than average food abundance and p o t e n t i a l f o r e f f i c i e n t f o r a g i n g has been p r e d i c t e d by f o r a g i n g theory (Pyke, P u l l i a m , and Charnov 1977), and found e m p i r i c a l l y i n f i e l d s t u d i e s of b i r d s (Goss-Custard 1981, Royama 1970, Zach and F a l l s 1979). When viewed from the l e v e l of the p o p u l a t i o n , the Amboseli elephants appear to concentrate t h e i r daytime h a b i t a t use i n the swamps and swamp edge woodlands of the Amboseli b a s i n during the l a t e dry season, the p e r i o d when food resources and water 169 sources are most l i m i t i n g ( S i n c l a i r 1975, Western 1975). These are areas where the herb l a y e r i s abundant but of r e l a t i v e l y low n u t r i t i v e v a l u e . The dry season h a b i t a t s vary i n t h e i r extent of woody cover: i n the swamps and swamp edge g r a s s l a n d s , there i s l i t t l e or no woody cover, while i n the swamp edge woodlands, the woody cover i s r e l a t i v e l y dense. The dry season h a b i t a t s are thus areas of p r e d i c t a b l e , abundant, but f a i r l y low q u a l i t y herb l a y e r food a v a i l a b i l i t y , and of d i f f e r i n g t w i g / l e a f biomass d e n s i t y . During the wet season, the elephants extend t h e i r h a b i t a t use i n t o the woodlands and g r a s s l a n d s of the Amboseli b a s i n , and acro s s the bushland areas surrounding the b a s i n . They spend comparatively l e s s time f e e d i n g in the swamps and swamp edges at t h i s time. In the h a b i t a t types used i n the wet seasons, the herb l a y e r i s of higher n u t r i t i o n a l q u a l i t y than i n the swamps, and i s p r o d u c t i v e and abundant d u r i n g the r a i n s . Woody cover i n these h a b i t a t s v a r i e s from open (the a l k a l i n e g r a s s l a n d s ) to moderately dense (the dense bushlands i n the north and south). The abundance and q u a l i t y of herb l a y e r biomass in the wet season areas depends l a r g e l y on l o c a l r a i n f a l l , which i s v a r i a b l e on s p a t i a l and temporal s c a l e s . The best f o r a g i n g s i t e s in the wet season are t h e r e f o r e more u n p r e d i c t a b l e i n space and time than the ground-water swamps of the Amboseli b a s i n . Surface water a v a i l a b i l i t y in the bushlands o u t s i d e the Amboseli b a s i n i s a l s o s p a t i a l l y and temporally u n p r e d i c t a b l e (Western 1975). Based on the l i m i t e d set of o b s e r v a t i o n s I made durin g the 1 70 course of the present study, and my a n a l y s i s of Western's longer term a e r i a l count data, I can make some t e n t a t i v e p r e d i c t i o n s about wet season movements. I suggest that the elephant p o p u l a t i o n as a whole w i l l tend to move f a r t h e r out i n t o bushland areas i n the wet season dur i n g extended p e r i o d s of low annual r a i n f a l l (250 mm. or l e s s ) , and w i l l remain in or near the basin h a b i t a t s d u r i n g p e r i o d s of higher annual r a i n f a l l (300 mm. or more). T h i s view suggests that the elephants have no inherent reason to make long d i s t a n c e m i g r a t i o n s i n the wet seasons except to search f o r the best f o r a g i n g s i t e s . B. F a c t o r s Modi f y i n g the General P a t t e r n Any a p r i o r i d e f i n i t i o n of p o t e n t i a l f o r a g i n g p r o f i t a b i l i t i e s i n d i f f e r e n t h a b i t a t s which i s based on simple measurements of herb l a y e r biomass i s probably i n a c c u r a t e and o v e r l y s i m p l i s t i c . In a s o c i a l , g e n e r a l i s t h e r b i v o r e such as the elephant, there are- many p o s s i b l e c o m p l i c a t i n g f a c t o r s . These i n c l u d e : 1. The e f f e c t s of s o c i a l group formation on the f o r a g i n g e f f i c i e n c y of i n d i v i d u a l s . 2. The e f f e c t s of gender d i f f e r e n c e s on the f o r a g i n g e f f i c i e n c y and needs of i n d i v i d u a l s . 3. The d i f f i c u l t y of d e s c r i b i n g the "food a v a i l a b i l i t y " in a h a b i t a t type in general terms, such as average abundance and q u a l i t y , when there are l o c a l d i f f e r e n c e s i n p l a n t abundance and q u a l i t y w i t h i n the h a b i t a t . The need of a l l 171 animals to maintain a " n u t r i t i o n a l balance" c o u l d mean that a l a r g e h e r b i v o r e such as an elephant might have s p e c i f i c search p a t t e r n s to search out c e r t a i n p l a n t types or a v o i d others found i n d i f f e r e n t p l a n t communities (Westoby 1974). I w i l l d i s c u s s these f a c t o r s as they r e l a t e to elephant h a b i t a t s e l e c t i o n . !• T h e ef f e c t s of group format ion Alexander (1974) notes that the c o s t s to an i n d i v i d u a l of j o i n i n g a group i n c l u d e i n c r e a s e d competition f o r mates or food. The b e n e f i t s from group membership must outweigh these c o s t s . Bertram (1978), Rubenstein (1978), and Wrangham (1980) d e s c r i b e p o s s i b l e f o r a g i n g b e n e f i t s which c o u l d accrue through group membership. These g e n e r a l l y i n v o l v e i n c r e a s e d success i n the d e t e c t i o n , capture, or h a n d l i n g of prey. They focus l a r g e l y on p r e d a t o r s , however, or other animals which e x p l o i t h i g h l y n u t r i t i o u s food items d i s t r i b u t e d i n a patchy, u n p r e d i c t a b l e way in time and space. For l a r g e g e n e r a l i s t h e r b i v o r e s , the d e t e c t i o n , capture, and h a n d l i n g of t h e i r r e l a t i v e l y low q u a l i t y and widely d i s t r i b u t e d food items i s u n l i k e l y to be enhanced by s o c i a l group formation, and i s more l i k e l y to be reduced (Jarman and Jarman 1979). Other, non-feeding b e n e f i t s must outweigh the c o s t s of t h i s reduced f o r a g i n g e f f i c i e n c y . Foraging e f f i c i e n c y i s reduced by competition with other animals in . the group; e i t h e r d i r e c t l y by i n t e r f e r e n c e 172 competition and s u p p l a n t a t i o n from l o c a l i z e d food sources (Leuthold 1977a, Post 1981), or i n d i r e c t l y by e x p l o i t a t i o n c o m petition as neighbouring animals reduce the food supply f o r those f o r a g i n g behind them (Jarman and Jarman 1979, Post 1981). As the food a v a i l a b l e f o r a given group of i n d i v i d u a l s decreases, the group p r o g r e s s i o n rate and i n t e r - i n d i v i d u a l d i s t a n c e i n c r e a s e s . T h i s a c t s to reduce the cohesiveness of the group, and makes i t more l i k e l y that the group w i l l break up. Of the many b e n e f i t s that have been suggested to favour membership i n a group, I suggest that the f o l l o w i n g c o u l d be p o t e n t i a l l y important f o r e l e p h a n t s . 1. Reduced or d i l u t e d p r e d a t i o n r i s k f o r a d u l t s and j u v e n i l e s through the s e l f i s h herd concept (Hamilton 1971), or through group d e f e n s e . (Douglas-Hamilton 1972, Kruuk 1972). J u v e n i l e elephants, e s p e c i a l l y smaller c a l v e s , are p o t e n t i a l prey f o r l a r g e p r e d a t o r s such as l i o n s . More important might be the t h r e a t of p r e d a t i o n by man, the only predator of a d u l t as w e l l as j u v e n i l e animals. Man has probably been a predator of elephants f o r tens of thousands of years (Howell and B o u r l i e r e 1964), and p o s s i b l y longer (Isaac and Crader 1981). I t would appear that group formation f o r p r o t e c t i o n of j u v e n i l e s and d i l u t i o n of the t h r e a t to i n d i v i d u a l s from human p r e d a t i o n c o u l d be an e v o l u t i o n a r y pressure promoting the formation of l a r g e groups at a l l times of the year. 2. Female mate c h o i c e i n a s t r a t e g y analogous to that shown by female elephant s e a l s (Cox and LeBoeuf 1977), which encourages male-male c o m p e t i t i o n . In s e m i - a r i d r e g i o n s , o e s t r u s 1 73 in female elephants occurs through the year, but peaks i n the wet and e a r l y dry seasons. Large a d u l t b u l l elephants i n r e p r o d u c t i v e c o n d i t i o n (termed "musth" by Poole and Moss ( 1 9 8 1 ) ) , tend to a s s o c i a t e with cow/calf aggregations and f i g h t to c o n t r o l access to the oestrous females. Observations of female elephants in o e s t r u s (C. Moss, pers. comm., pers. obs.), suggest that they might i n c i t e male competition by aggregating in a few l a r g e groups and drawing the a t t e n t i o n of a l l "musth" males in the v i c i n i t y . T h i s would improve the chances that any i n d i v i d u a l female would be mated by the " f i t t e s t " b u l l . 3. B e n e f i t s f o r j u v e n i l e o f f s p r i n g of i n c r e a s e d s u r v i v a l and a c c e l e r a t e d s o c i a l development through a l l o m o t h e r i n g , or care by non-mothers, (Hrdy 1976), and of more r a p i d s o c i a l i z a t i o n of immatures through an i n c r e a s e d a v a i l a b i l i t y of p l a y p a r t n e r s (Fagen 1981) c o u l d be r e a l i z e d i f a d u l t females with young a s s o c i a t e d with other s i m i l a r a d u l t s . A l l o m o t h e r i n g of young j u v e n i l e s by o l d e r but subdominant females, has been observed i n elephant f a m i l y u n i t s by Douglas-Hamilton (1972) and reviewed by Dublin ( i n p r e s s ) . G e n e r a l l y , but not always, the a l l o m o t h e r s are thought to be kin of the i n f a n t s (reviewed by Hrdy 1976). 4. A v a r i a t i o n of the i n f o r m a t i o n center idea (Krebs 1974, Ward and Zahavi 1973), where group members c o u l d b e n e f i t from the accumulated experience i n h a b i t a t s e l e c t i o n of the o l d e r m a t r i a r c h s , might improve the f o r a g i n g success of i n d i v i d u a l group members. I n d i v i d u a l s or f a m i l y u n i t s would continue to stay with an aggregation as long as i n d i v i d u a l f o r a g i n g 174 e f f i c i e n c y remained high. T h i s reason f o r group formation might be expected to operate at a l l times of year, although the g r e a t e r u n p r e d i c t a b i l i t y of food and water sources i n wet seasons might favour group formation more at that time. With the data a v a i l a b l e at present, i t i s not p o s s i b l e to d i s t i n g u i s h which, i f any, of the v a r i o u s p o t e n t i a l reasons f o r group formation are most important. The reasons f o r forming bond groups might be d i f f e r e n t from those which favour the formation of f a m i l y u n i t s on the one hand, and l a r g e aggregations on the other. For example, the t h r e a t of human pr e d a t i o n should s e l e c t f o r l a r g e groups at a l l times, while under the female mate c h o i c e h y p o t h e s i s , l a r g e groups should form only when many females are l i k e l y to come i n t o o e s t r u s . S e l e c t i o n f o r a l l o m o t h e r i n g o p p o r t u n i t i e s might favour the formation of f a m i l y u n i t s and, p o s s i b l y , bond groups, but probably not l a r g e a g g r e g a t i o n s . The reasons f o r group formation f o r females and j u v e n i l e s are l i k e l y d i f f e r e n t ' than for bachelor b u l l s , and are undoubtedly d i f f e r e n t again f o r b u l l s i n "musth". B u l l s i n musth would have a g r e a t e r tendency to a s s o c i a t e with cow/calf groups than with other b u l l s , and, s i n c e b u l l s are not i n v o l v e d i n the r e a r i n g of young, they would not have the same i n c e n t i v e f o r group formation as a d u l t females. Since i t appears that there are a number of p o t e n t i a l l y important reasons f o r elephants to form l a r g e aggregations at most times of year, i t seems that elephants may need to compromise f o r a g i n g e f f i c i e n c y to s a t i s f y these other g o a l s . 175 T h i s s i t u a t i o n has been reviewed f o r w i l d b i r d s (Zach and Smith 1981). However, f o r a g i n g may u l t i m a t e l y l i m i t elephants' a b i l i t y to form l a r g e groups at times when food resources are l i m i t e d , and the non-foraging goals may have to be s a c r i f i c e d . I t i s p o s s i b l e f o r h e r b i v o r e s to maintain l a r g e group s i z e through seasonal changes by choosing feeding h a b i t a t s where l a r g e groups can remain cohesive, i e . , where food i s widespread and abundant (Jarman and Jarman 1979). The fragmentation of l a r g e groups elephants through time may not be random; i t i s p o s s i b l e that "bond groups" may p e r s i s t f o r a longer p e r i o d i n t o the dry season than do aggregations of groups with no obvious s o c i a l t i e s , p o s s i b l y because of t h e i r (suggested) r e l a t e d n e s s . My r e s u l t s show that there was a tendency f o r numbers of l a r g e cow/calf groups to form i n favourable h a b i t a t s on- a seasonal b a s i s , although there was a l s o a steady d e c l i n e i n the number of l a r g e groups s i g h t e d through the r a i n s to l a t e dry season. The h a b i t a t s where l a r g e groups were found changed through the seasons, from the open Salvadora / Suaeda shrublands, and fever t r e e woodlands in the r a i n s to swamp edge gr a s s l a n d s and woodlands by the l a t e dry season. T h i s suggests that the formation of l a r g e groups i s important to a c e r t a i n segment of the cow/calf p o p u l a t i o n at l e a s t , and that the elephants c o u l d be making a compromise between f o r a g i n g and s o c i a l c o n s i d e r a t i o n s i n t h e i r s e l e c t i o n and use of h a b i t a t s . Bachelor b u l l elephants had no apparent tendency to form l a r g e groups, nor d i d t h e i r l a r g e r groups tend to s h i f t h a b i t a t s in a c o n s i s t e n t way. 176 2. Gender d i f ferences i n h a b i t a t s e l e c t ion In g e n e r a l , h a b i t a t s e l e c t i o n and use was s i m i l a r f o r b u l l s and cow/calf groups but there are a l s o some d i f f e r e n c e s , as noted i n Chapter 4. B u l l s were more evenly d i s t r i b u t e d across h a b i t a t s , and s e l e c t e d the coarse grass swamp h a b i t a t s more than the female groups. These d i f f e r e n c e s may have been due to displacement of the b u l l groups by cow/calf herds, or may have r e s u l t e d from an a c t i v e c h o i c e by the b u l l s to a v o i d competition .with the cow/calf herds at a l l times of year. The d i f f e r e n c e s might a l s o be c o r r e l a t e d with d i f f e r e n c e s i n the metabolism and s o c i a l o r g a n i z a t i o n of b u l l s and adult female l e a d e r s of the cow/calf groups. Demment (1978) d i s c u s s e d sexual dimorphism i n s i z e and re p r o d u c t i v e f u n c t i o n with re s p e c t to metabolism and f o r a g i n g requirements i n baboons. The l a r g e r male baboons have a lower metabolic energy requirement per un i t of food p r o c e s s i n g c a p a c i t y (based on gut volume) than females, but have a l a r g e r a b s o l u t e requirement. They should, t h e r e f o r e , feed on the more abundant, but lower q u a l i t y , p l a n t m a t e r i a l . T h e i r smaller s i z e and the a d d i t i o n a l n u t r i t i o n a l s t r e s s of pregnancy and l a c t a t i o n c o u l d r e q u i r e females to s e l e c t a higher q u a l i t y d i e t . C l u t t o n - Brock et a l . ( i n press) f o l l o w e d s i m i l a r reasoning in t h e i r d i s c u s s i o n of red deer f o r a g i n g . In the Amboseli elephants, the coarse grasses of swamp and swamp edge were probably d i g e s t e d more e a s i l y by the b u l l s . The l a r g e r b u l l s needed a grea t e r a b s o l u t e intake r a t e , but were able to compensate by e a t i n g low q u a l i t y forage. T h i s c o u l d 1 77 e x p l a i n why b u l l s were seen f e e d i n g i n the swamps more o f t e n than the cow/calf groups. Because t h e i r s o c i a l o r g a n i z a t i o n i s so d i f f e r e n t , the e f f e c t of the s o c i a l environment on f o r a g i n g e f f i c i e n c y should a l s o be d i f f e r e n t between b u l l and cow/calf groups. B u l l groups are s m a l l , and i n d i v i d u a l s w i t h i n the groups are widely separated and probably not c l o s e l y r e l a t e d . D i r e c t i n t e r f e r e n c e c o m p e t i t i o n would have l e s s e f f e c t on p r o g r e s s i o n and feeding r a t e s w i t h i n b u l l groups, s i n c e t h i s f a c t o r may have a l r e a d y acted to prevent l a r g e group formation. Members of cow/calf a g g r e g a t i o n s , on the other hand, must c o n t i n u a l l y move with the group i n search of new food sources, and both i n t e r f e r e n c e and e x p l o i t a t i o n competition would a f f e c t members d i r e c t l y . The h a b i t a t s s e l e c t e d by cow/calf groups which "allow feeding by l a r g e r groups ( i e . , the s i t e s which are more open and where food items are more evenly d i s t r i b u t e d ) might be d i f f e r e n t from the areas s e l e c t e d by b u l l s . For example, cow/calf groups made gr e a t e r r e l a t i v e use of the open Salvadora / Suaeda shrublands in the r a i n s and swamp edge g r a s s l a n d s i n the dry season than d i d the b u l l s . One might suspect that when b u l l s a s s o c i a t e with the m a t r i a r c h a l groups, they are more i n t e r e s t e d i n mating than f e e d i n g , but they would be a f f e c t e d by the same r e s t r i c t i o n s of m a i n t a i n i n g contact with the group as are females and j u v e n i l e s . 1 78 3. S p e c i a l food types The d e s c r i p t i o n of h a b i t a t herb and woody l a y e r s i n simple terms such as " t o t a l dry mass", or "percent woody cover" ignores the complexity of s p e c i e s composition and the needs of the elephants to choose s p e c i f i c high value food types, or a v o i d p a r t i c u l a r l y u n p a l a t a b l e ones. For example, in the wet season, the swamp edge herb l a y e r contained s u c c u l e n t creepers that the elephants a p p a r e n t l y sought as a food item (pers. obs.). S i m i l a r c r e e p i n g v i n e s were found on woody shrubs in the swamp edge young fever and palm woodlands. In the mid dry season of 1979, the A. t o r t i l i s woodlands produced a mast crop of high p r o t e i n seed pods, while e a r l i e r i n the year, the P. r e e l i n a t a palm t r e e s produced f r u i t pods. The elephants were seen f e e d i n g on these r e l a t i v e l y r a r e , but presumably favoured and n u t r i t i o u s items. Were the h a b i t a t s c o n t a i n i n g them s e l e c t e d as f o r a g i n g s i t e s because of these items? At t h i s l e v e l of complexity, a more d e t a i l e d study of d i e t s e l e c t i o n i s needed to answer t h i s quest i o n . In a s i m i l a r v e i n , the low p a l a t a b i l i t y of c e r t a i n food types which may c o n t a i n t o x i n s , d i g e s t i b i l i t y reducing compounds, or p h y s i c a l defences c o m p l i c a t e s the d e s c r i p t i o n of h a b i t a t f e e d i n g o p t i o n s . For example, many of the p l a n t s i n the shrub and t r e e l a y e r of the woodlands and bushlands are evergreen, while others are deciduous. Croze (1974b) found that in g e n e r a l , the b u l l elephants he s t u d i e d were not s e l e c t i v e i n most of t h e i r f e e d i n g on woody s p e c i e s , although a few p l a n t s were s i g n i f i c a n t l y p r e f e r r e d or avoided. I d i d not examine t h i s 179 q u e s t i o n i n the present study, but c a s u a l o b s e r v a t i o n s suggested that many shrub species were fed upon while others were never or only r a r e l y touched. The p o s s i b i l i t y that the elephants sought d i f f e r e n t , s p e c i f i c food types f o r s p e c i f i c needs such as d i g e s t i b l e energy, p r o t e i n , or minerals ( f o r example, sodium (Weir 1972)), c o u l d a f f e c t the r e l a t i v e value of d i f f e r e n t h a b i t a t s . In Amboseli, the bark of A c a c i a xanthophloea , or fever t r e e , was ap p a r e n t l y a t t r a c t i v e to elephants through much of the year the sap i s known to have a high sugar content (Hausfater and Bearce 1976) -- , although the incide n c e of bar k i n g seemed to incr e a s e i n the dry season. These t r e e s were found i n the swamp edge woodlands and the fever t r e e savannah woodlands, and c o u l d have a f f e c t e d t h e i r s e l e c t i o n of these h a b i t a t s . C. Some Management I m p l i c a t i o n s It appears that the Amboseli elephants c o u l d be b u f f e r e d from the severe e f f e c t s of long term drought by t h e i r use of the swamp and swamp edge h a b i t a t s i n the Amboseli b a s i n . The dense herb l a y e r cover in these h a b i t a t s , which i s pr o d u c t i v e even d u r i n g dry times, might make the Amboseli elephants l e s s r e l i a n t on woody v e g e t a t i o n f o r drought p e r i o d maintenance, when compared with the elephant p o p u l a t i o n s of completely a r i d r egions such as Tsavo. As N o r t o n - G r i f f i t h s (1979) suggests f o r elephant p o p u l a t i o n s with a food source a l t e r n a t i v e to woody p l a n t s , the Amboseli elephants might slowly d e c l i n e through reduced recruitment at high d e n s i t y , rather than e x p e r i e n c i n g 180 the major a d u l t m o r t a l i t y seen i n Tsavo. The elephants c o u l d probably reduce the woody cover i n the basin c o n s i d e r a b l y i n the process, and perhaps maintain the system i n an open shrub g r a s s l a n d c o n d i t i o n , or f o l l o w the p o s s i b l y c y c l i c e f f e c t s of water t a b l e f l u c t u a t i o n s on woodlands. However, t h i s process might be m o d i f i e d by the l a r g e p o p u l a t i o n s of zebra, w i l d e b e e s t , and l i v e s t o c k which a l s o depend on the swamp edges as a food source in drought p e r i o d s . E x p l o i t a t i o n competition between elephants and the other l a r g e h e r b i v o r e s would l i k e l y occur at t h i s p o i n t , and i t i s d i f f i c u l t to say which p o p u l a t i o n would s u f f e r from t h i s c o m petition the most. More i n f o r m a t i o n i s c l e a r l y needed on the impact of elephants and other f a c t o r s on woodland and herb l a y e r dynamics. For the sake of the elephant p o p u l a t i o n alone, I would not c o n s i d e r management i n t e r v e n t i o n through c u l l i n g or hunting to be necessary at the present time. However, other management p r i o r i t i e s may be important, such as the maintenance of woodland areas f o r a e s t h e t i c reasons, or f o r the p r e s e r v a t i o n of woodland-dependent s p e c i e s such as r h i n o c e r o s , c e r t a i n a n t e l o p e s , or monkeys. A more d e t a i l e d study of elephant impact on the woodlands would be necessary, and p o s s i b l y management re d u c t i o n of elephant numbers might then be j u s t i f i e d . However, any removal of elephants through c r o p p i n g should be accompanied by a r e s e a r c h program to monitor the e f f e c t s of such an a c t i o n on h a b i t a t dynamics and on the s o c i a l s t r u c t u r e and p o p u l a t i o n dynamics of the elephants and other l a r g e h e r b i v o r e s . 181 D. Questions f o r Furth e r Study Many of the suggestions put forward i n t h i s d i s c u s s i o n can be only t e n t a t i v e at t h i s p o i n t , s i n c e my approach was at a very broad l e v e l . Some q u e s t i o n s r a i s e d , and which c l e a r l y need f u r t h e r study are presented below: 1 . What i s the f o r a g i n g e f f i c i e n c y i n d i f f e r e n t h a b i t a t s , on d i f f e r e n t food types, with d i f f e r i n g abundance of i n d i v i d u a l elephants i n separate age/gender c l a s s e s ? 2. What are the e f f e c t s of s o c i a l grouping on f o r a g i n g e f f i c i e n c y ? How do the e f f e c t s d i f f e r f o r d i f f e r e n t age and gender c l a s s e s ? 3. Are there measureable d i f f e r e n c e s i n h a b i t a t s e l e c t i o n and d i e t between i n d i v i d u a l s , or i n d i v i d u a l f a m i l y u n i t s , which have d i f f e r e n t h a b i t a t use p a t t e r n s ? Are the d i f f e r e n t h a b i t a t use p a t t e r n s of separate f a m i l y u n i t s or bachelor b u l l s a f f e c t e d by s o c i a l i n t e r a c t i o n s such as spacing behaviour? What are the consequences of any such d i f f e r e n c e s i n terms of s u r v i v a l or r e p r o d u c t i v e success? 4. What are the d i r e c t e f f e c t s on the v e g e t a t i o n , both woody and herb l a y e r , of elephant feeding? Is the impact d i f f e r e n t when the elephants are i n l a r g e vs. small groups? 5. What are the d i g e s t i b l e n u t r i e n t c o ntents of d i f f e r e n t elephant food types, i n d i f f e r e n t 182 p h e n o l o g i c a l stages? What are the minimum maintenance requirements of elephants f o r v a r i o u s c r i t i c a l n u t r i e n t s , such as d i g e s t i b l e energy and p r o t e i n ? 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Foraging and t e r r i t o r i a l i t y of male ovenbirds (Aves: P a r u l i d a e ) in a heterogeneous h a b i t a t . J . Anim. E c o l . 48:32-52. Zach, R. and J.N.M. Smith. 1981. Optimal f o r a g i n g i n w i l d b i r d s ? i n : A.C. Kamil and T.D. Sargent ( e d s . ) . Foraging Behavior. E c o l o g i c a l , E t h o l o g i c a l , and P s y c h o l o g i c a l Approaches. Garland STPM Press, New York. pp. 95-109. 195 APPENDIX 1 Regressions Used f o r Biomass E s t i m a t i o n 1 . T o t a l biomass, herb l a y e r ( in g_/m2 ) : S l a n t i n g p i n i n t e r c e p t (D. Western, pers. comm.) Dry mass = 8.630 + 67.745 * (mean no. h i t s / p i n ) r z = 0.882 2 . T o t a l biomass , herb l a y e r ( i n g_/mz ) : C i r c u l a r p l o t (D. Western, p e r s . comm.) Log(Dry mass) = 1.02 * Log(height * cover) - 0.38 r2- not given 3 . Green biomass , herb l a y e r ( i_n 3/m2- ) : Spectrophotometer (McNaughton, 1979) Green mass = 109.8 * Rat i o - 83.8 r2- = 0.956 4. Twig/leaf biomass, woody l a y e r ( i n kg) Ln(Twig/leaf mass) = 2.1177 * Ln(Canopy Diameter) - 0.7317 r a = 0.960 196 APPENDIX 2 Woody Species Measured f o r C a l i b r a t i o n of the Biomass E s t i m a t i o n Regression Tree/Shrub Measured Measured Twig/Leaf Species Canopy Canopy Biomass Diameter(m) Volume(m 3) (kg) Cadaba f a r i n o s a C apparis tomentosa Maerua t r i p h y l l a Suaeda monoica Sericocomopsis h i l d e b r a n d t i i S. p a l l i d a Acac i a drepanalobium A. m e l l i f e r a A. nubica A. t o r t i l i s , mature A. xanthophloea, mature Azima t e t r a c a n t h a Salvadora p e r s i c a B a l a n i t e s g l a b r a Commiphora a f r i c a n a Pluchea o v a l i s Tr iplocephalum h o i s t i i Lycium europaeum Solanum incanum: Canopy Diameter < 50 cm C D . > 50 cm Wi than i a somni f e r a D i c l i p t e r a a l b i c a u l i s Phoenix r e c l i n a t a 1 .86 4.89 1 .787 2.57 8.82 2.480 3.03 15.14 13.362 2.74 5.72 4. 1 00 0.96 0.33 0.364 0.91 0.33 0.678 3.30 29.08 1 5.922 6.04 77.36 22.671 3.69 20.85 8.805 1 6.60 2239.96 197.716 18.20 4032.41 136.777 2.34 6.88 2.538 3.06 1 3.97 4 . 1' 1 5 2.99 24. 53 42.268* 3.38 28.71 6.233 1 . 22 1 .60 0.602 1 .29 1.41 0.626 1 .91 4.87 1 .349 0.28 0.04 0.029 1.01 1.12 0.589 1 . 64 2.75 0.997 1 .85 2.37 1 .333 3.59 39.48 21.434* Note: Species i n d i c a t e d by * had u n u s u a l l y l a r g e t w i g / l e a f biomass to canopy dimensions, and were not used i n the m u l t i - s p e c i e s r e g r e s s i o n s . 1 97 APPENDIX 3 Computation Factors and Estimated Twig/Leaf Mass for the "Mean I n d i v i d u a l " of Each Woody Spec ies Tree/Shrub Mean Computation Estimated Species Canopy Factor Twig/Leaf Diameter(m) Biomass(kg) Cadaba f a r i n o s a 2. .00 6. ,37 2. 088 Capparis tomentosa 2. .42 4 . ,35 3. 1 26 Maerua t r i p h y l l a 2. .96 2 . ,91 4. 790 Trianthema ceratosepala 1 . ,48 1 1 . ,63 1 . 104 Suaeda monoica 2. ,72 3. ,44 4. 004 Achryanthes aspera: Canopy Diameter < 50 cm 0. ,30 282. ,94 0. 028 CD. > 50 cm 0. .96 27. ,63 0. 441 Sericocomopsis h i I d e b r a n d t i i 1 . ,01 24. ,96 0. 491 S . pa 11 i da 0. .87 33. ,64 0. 335 Triumfetta flavescens 0. .78 41 . ,86 0. 284 Hibiscus f l a v i f o l i u s 0. ,38 1 76. 35 0. 062 Ab u t i l o n "grandiflorum' 0. .48 110. .52 0. 102 • A. mauritianum: CD. < 50 cm 0. ,26 376, .70 0. 028 CD. > 50 cm 1, .79 7, .95 1. 651 Acacia drepanalobium 2. .79 3, .27 15. 922 A. m e l l i f e r a 5, .91 0, .73 20. 713 A. nubica 3, .82 1 , .75 8. 220 A. t o r t i l i s : mature 16, .21 0, .10 0. 000 young 4, .28 1 , .39 10. 458 A. xanthophloea: mature 15, .27 0 . 1 1 17. 862 young 6, .61 0 . 58 23. 091 s a p l i n g 4, .22 1 .43 10. 049 seedling 0, .33 233 .84 0. 037 Azima tetracantha 2, .36 4. .57 2. 965 Salvadora p e r s i c a 3, .04 2. .76 5. 068 Appendix 3 continues on the next page 198 APPENDIX 3 continued Computation F a c t o r s and Estimated Twig/Leaf Mass f o r the "Mean I n d i v i d u a l " of Each Woody Species Tree/Shrub Mean Computation Estimated Species Canopy F a c t o r Twig/Leaf Diameter(m) Biomass(kg) B a l a n i t e s g l a b r a B. p e d i c e l l a r i s Commiphora a f r i c a n a E rlangea somalensis Helichrysum glumaceum Dicoma tomentosa Pluchea b e q u a e r t i P. o v a l i s P. s o r d i d a T r i p l o c e p h a l u m h o i s t i i Lyc i um europaeum Solanum incanum; C D . < 50 cm C D . > 50 cm Withania somnifera B a r l e r i a acanthoides D i c l i p t e r a a l b i c a u l i s Leucas pododi skos Phoenix r e c l i n a t a 2.81 3.22 42.268* 2.68 3.55 34.000 3.55 2.02 7. 165 0.69 53.49 0.174 0.63 64. 1 6 0.174 0. 30 282.94 0.026 0.80 39.79 0.300 1 .28 1 5. 54 0.81 1 0.54 87.33 0. 1 30 1 .27 15.79 0.213 1 .79 7.95 1 .651 0.28 .324.81 0.032 1 .02 24.39 0.502 1 .36 1 3.77 0.923 0. 42 1 42.86 0.077 1 .78 8.05 1 .631 0.40 159. 1 5 0.068 3.24 2.43 21.434* Note: 1. Twig/leaf biomass was estimated f o r t r e e and shrub canopy a v a i l a b l e below a height of 6m., as c a l c u l a t e d from mean p l a n t dimensions. 2. Species i n d i c a t e d with * had unusually high t w i g / l e a f biomass r e l a t i v e to canopy dimensions, so a c t u a l measured t w i g / l e a f biomass was used f o r these s p e c i e s . 199 APPENDIX 4 C o r r e c t i o n F a c t o r s f o r Ground Counts A) C o r r e c t i o n F a c t o r s f o r V e g e t a t i o n S t r u c t u r e MONTH HABITAT TYPE S. consim. A. t o r t . Fever S/S Swamp grass . woods woods shrub edge gr. November 1978 1 .012 1 .199 1 .087 1 . 1 54 1 .021 December 1 .024 1 .213 1 .100 1 . 167 1 .033 January 1979 1 .036 1 .227 1 .113 1 .181 1 .045 February 1 .048 1 .241 1 . 1 26 1 .195 1 .058 March 1 .066 1 .263 1 . 1 45 1 .216 1 .076 A p r i l 1 .078 1 .278 1 . 1 58 1 .230 1 .088 May 1 .096 1 .299 1 . 1 78 1 .250 1 .113 June 1 .115 1 .322 1 .198 1 .272 1 . 1 32 J u l y 1 .121 1 .326 1 .201 1 .276 1 . 1 39 August 1 . 1 27 1 .333 1 .208 1 .283 1 . 1 45 September 1 . 1 33 1 .341 1 .214 1 . 290 1 .151 October 1 . 1 39 1 .348 1 .221 1 .297 1 . 1 57 B) C o r r e c t i o n F a c t o r s for Group Bunching MONTH ALL HABITAT TYPES November 1978 1 . 1 34 December 1.136 January 1979 1 . 1 38 February 1 . 1 40 March 1 . 1 42 A p r i l 1.143 May 1 . 1 48 June 1.149 J u l y 1 . 1 53 August 1 . 152 September 1 . 1 54 October 1 . 1 53 200 APPENDIX 4 continued C o r r e c t i o n F a c t o r s f o r Ground Counts C) C o r r e c t i o n F a c t o r s f o r Both V e g e t a t i o n S t r u c t u r e and Group Bunching MONTH HABITAT S. consim. A . t o r t . S/S Swamp gr a s s . woods shrub edge gr November 1978 1.182 December January 1979 February March A p r i l 1.260 May 1.259 June 1 .457 J u l y 1.474 August September October

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