The Biology of African Savannahs THE BIOLOGY OF HABITATS SERIES This attractive series of concise, affordable texts provides an integrated overview of the design, physiology, and ecology of the biota in a given habi- tat, set in the context of the physical environment. Each book describes practical aspects of working within the habitat, detailing the sorts of stud- ies which are possible. Management and conservation issues are also included. The series is intended for naturalists, students studying biological or environmental science, those beginning independent research, and professional biologists embarking on research in a new habitat. The Biology of Rocky Shores Colin Little and F. A. Kitching The Biology of Polar Habitats G. E. Fogg The Biology of Lakes and Ponds Christer Brönmark and Lars-Anders Hansson The Biology of Streams and Rivers Paul S. Giller and Bjorn Malmqvist The Biology of Mangroves Peter F. Hogarth The Biology of Soft Shores and Estuaries Colin Little The Biology of the Deep Ocean Peter Herring The Biology of Lakes and Ponds, Second ed. Christer Brönmark and Lars-Anders Hansson The Biology of Soil Richard D. Bardgett The Biology of Freshwater Wetlands Arnold G. van der Valk The Biology of Peatlands Håkan Rydin and John K. Jeglum The Biology of Mangroves and Seagrasses Peter Hogarth The Biology of African Savannahs Bryan Shorrocks The Biology of African Savannahs Bryan Shorrocks Environment Department University of York 1 3 Great Clarendon Street, Oxford OX2 6DP Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide in Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With offices in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries Published in the United States by Oxford University Press Inc., New York © Bryan Shorrocks 2007 The moral rights of the author have been asserted Database right Oxford University Press (maker) First published 2007 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose the same condition on any acquirer British Library Cataloguing in Publication Data Data available Library of Congress Cataloging in Publication Data Data available Typeset by Newgen Imaging Systems (P) Ltd., Chennai, India Printed in Great Britain on acid-free paper by Biddles Ltd., King’s Lynn ISBN 978–0–19–857065–3 978–0–19–857066–0 (pbk) 10987654321 For Jo, who also loves Africa This page intentionally left blank Preface This book is an introduction to African savannahs and their biology, concentrating on large mammal ecology, behaviour and conservation. Although it is a book on savannahs I hope that it sets out its ideas within a general framework of ecological and behavioural ideas. In my 40 years as an ecologist I have worked on both temperate and tropi- cal terrestrial ecosystems. I have carried out field surveys, field experiments and built mathematical and computer models. I believe all these elements are essential for understanding Nature, and they all appear in this book. You cannot simply watch wildlife populations and communities in order to understand how they function. You have to employ field manipulations, either natural or artificial, to tease out the possibilities. You have to describe what you see not just in words but in mathematical models, to allow your less precise verbal suspicions to be tested. And savannah ecology must fit comfortably within the framework of general ecological ideas. There can be no special pleading or special mechanisms that other ecologists just don’t understand. I have tried to present such an encompassing view of savannah ecology and behaviour. African savannahs are magical places. The brightness of the light, the intensity of the colours, and the beauty and excitement of the wildlife, and of course the people. These savannahs are worth preserving for future generations and I hope, in a small way, this book will generate an interest that helps them survive. I would like to thank all the people that have helped in my savannah work in East Africa, in particular the people at the Mpala Research Centre, in Laikipia, Kenya. Their help and insights have been invaluable during my frequent stays there. Finally my thanks to Paul Ward who, over several beers, over several nights, while running an undergraduate field course in Derbyshire reawakened my latent interest in Africa and its wildlife. Bryan Shorrocks June 2007 This page intentionally left blank Contents 1 Savannahs 1 Distribution world-wide 2 African savannahs 10 2 The vegetation 29 Rainfall, plant biomass, and the grass–tree mixture 29 Morphology and life history 32 Grasses 40 Trees 43 Local vegetation patterns: the Serengeti–Mara ecosystem 57 3 The animals 64 The insects 64 The birds 66 The mammals 68 4 Single species populations 113 Estimating numbers 114 What changes numbers? 127 Population models 136 Other species, in other areas 141 5 Species interactions 155 Predator–prey type interaction ( ) 156 Competitive interactions (ϪϪ) 180 Mutualistic interactions (ϩϩ) 196 6 The savannah community and its conservation 205 Energy flow and food webs 206 Assembly rules 219 x CONTENTS Island biogeography 224 Conserving savannah ecosystems 229 REFERENCES 240 INDEX 255 1 Savannahs Savannahs constitute one of the largest biomes of the world, comprising about 20 per cent of the land surface. Stated simply, they are tropical and subtrop- ical grasslands, with scattered bushes and trees. Most savannah occurs in Africa, with a smaller amount in South America, India, and Australia. In Africa the trees tend to be deciduous, while in South America and Australia they tend to be evergreen. Savannahs occur around the equator (between the Tropic of Capricorn and the Tropic of Cancer), where it is warm, but relatively dry. Most experience seasonal drought and the vegetation is influenced by rainfall, soil type, grazing, browsing, and fire. The word savannah, or savanna, is probably derived from a sixteenth century Spanish word zavanna, meaning ‘treeless plain’.It was recorded in 1535, by the Spanish historian Gonzalo Fernandez de Oviedo, as coming originally from Carib, a language spoken in northern South America and the Caribbean. Savannah therefore shares its origin with other well known words such as barbecue, cannibal and papaya. Like many plants found in warmer and dryer regions, the dominant grasses in savannahs tend to be C4 plants. Only in some very wet environ- ments do C3 grass species become abundant. The terms C3 and C4 refer to the type of CO2 trapping mechanism (photosynthesis) used by the plant. C4 plants have evolved a secondary carbon fixation pathway. CO2 is first com- bined into a 4-carbon compound, in the mesophyll cells of the leaf, and then passed to the cells around the leaf veins where the CO2 is released at high concentrations. It then enters the usual photosynthetic carbon reduction (PCR) pathway or Calvin-Benson Cycle, used by C3 plants. C4 plants are capable of utilizing higher light intensities than C3 plants, have greater max- imum photosynthesis, and use less water in the process. Because of these C4 grasses, the photosynthetic efficiency of many savannahs is very high. However, C4 plants are extremely poor quality food for most herbivores, ver- tebrate or invertebrate, (Caswell et al. 1973) unless the animal can break down cellulose. Intriguingly, both ungulates and termites, which are com- mon in many savannah systems, have a symbiotic gut flora that produces an enzyme, cellulase, that can digest this plant cell-wall constituent. 2 THE BIOLOGY OF AFRICAN SAVANNAHS Distribution world-wide Figure 1.1 shows how global temperatures and precipitation have an influ- ence on the major biomes, and Figure 1.2 shows a map of the world with the major areas of savannah indicated. In total they occupy some 23 mil- lion km2 (Cole 1986). Mean annual rainfall typically varies between about 20 and 150 cm with 60 to 90 per cent of the year’s rain falling in a short period of a few months. Although climatic factors, such as the annual tem- perature and the annual amount of rain, are not the only determinants of the savannah biome, they do potentially, have a major effect. The com- puter model, BIOME 3, predicts a distribution of savannahs, based solely on climatic factors, almost identical to that of Figure 1.2 (Haxeltine and Prenrice 1996). On a world scale therefore, savannahs tend to occupy a climatic region between deserts and tropical forests, a picture that is seen very clearly in Africa. Here the huge savannah area surrounds the tropical forests of the Congo basin and to the north is bordered by the Sahara desert and to the south by the Kalahari desert. This climatic position is seen clearly in Figure 1.1. Because savannahs are defined as grasslands with varying amounts of tree cover, they also sit between grasslands and tropical sea- sonal forests in Figure 1.1. However, the boundaries between these biomes are never very clear cut, and grassland and seasonal forest frequently merge into savannah.