The Foraging Behaviour of the Arid Zone Herbivores the Red Kangaroo (Macropus Rufus) and the Sheep (Ovis Aries) and Its Role In
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The Foraging Behaviour of the Arid Zone Herbivores the Red Kangaroo (Macropus rufus) and the Sheep (Ovis aries) and its Role in Their Competitive Interaction, Population Dynamics and Life-History Strategies by Steven McLeod A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy University of New South Wales February, 1996 Summary (1) The foraging behaviours of the arid zone herbivores the red kangaroo (Macropus rufus) and the sheep ( Ovis aries) were studied over a four year period in arid western New South Wales. These herbivores are subject to wide temporal variation in food abundance and quality. I examined their foraging behaviour to test the hypothesis that their population dynamics could be explained from the basis of their food and energy intake. (2) Previous studies of these herbivores have hypothesised that they compete exploitatively for food. The results of these studies have been suggestive but not conclusive. I conducted a controlled removal experiment to examine the hypothesis that these herbivores interspecifically compete. There were three treatments (x2 replicates); i) sheep only, ii) red kangaroos only and iii) sheep and red kangaroos. The productivity of each herbivore in the presence and absence of its putative competitor were compared to determine any competitive effect. The results indicated that exploitative competition between sheep and red kangaroos occurs rarely. Interference competition is more common. Red kangaroos were found to consistently avoid areas used by sheep. The effect of interference competition sheep productivity was less clear and appeared to be associated with unusual circumstances such as the combination of low food availability and high red kangaroo density. Interspecific competition was asymmetric with sheep dominating. (3) Coincident with the removal experiment I solved a mechanistic model of exploitative competition between sheep and red kangaroos. The model indicated that these herbivores share most of their food resources, but were always had exclusive food resources that were not eaten by the other species. The results of the mechanistic model of exploitative competition support the conclusions of the controlled removal experiment. (4) The ability ofred kangaroos and sheep to choose an optimal diet was examined. I used a linear programming model of optimal diet choice to predict the percentage in the diet of three categories of food plants; forbs, grasses and shrubs. The herbivores were hypothesised to follow a feeding goal of either energy maximisation, time minimisation ii or being completely unselective. The model indicated that the dominant feeding goal of sheep and red kangaroos was energy maximisation. Even under extremes of environmental variation the herbivores were able to select, or closely approach, an optimal diet. However, when food availability declined the predicted diets of the alternative goals converged to the point when there was no longer a detectable difference between any predicted diet and the observed diet. ( 5) I used the linear programming model of optimal diet choice to examine the hypothesis that red kangaroo body size is dependent upon the rate of energy intake. Comparing the predictions of an individual's energy intake with its requirements I predicted the upper and lower limits of each sex' s body size, as well as the optimum body size (defined as the body size at which the ratio of net energy intake to requirement is maximised). Furthermore, I used the model to examine a proposed mechanism for the evolution of sexual dimorphism in a fluctuating environment. Specifically, I hypothesised that male red kangaroos will exceed their optimum body size, foregoing long-term survivorship for short-term dominance over other males. In contrast, females will approach optimal body size in order to maximise survivorship. The results indicated partial agreement with the hypotheses. At times when food was abundant the body size model failed to explain the upper limit to body size of either sex. As food availability declined there was closer agreement between the predictions of the body size model and observed body sizes. These results indicate that the upper limit to the body size of both sexes may be set by infrequent and unpredictable times of food limitation, whereas the lower limit seems to be independent of food availability. As predicted, the largest males exceeded the optimum body size of 50 kg and probably would not survive a period of prolonged food limitation. Females did not approach the 50 kg optimum but reached a maximum size of about 35 kg. Possible reasons for the failure of the model to predict female optimum body size are discussed. (6) I simulated the population dynamics of a typical arid zone herbivore (the red kangaroo) to examine the usefulness of the concept of herbivore carrying capacity in a variable environment. The results of the simulations indicated that the concept of carrying capacity is only valid in slightly variable plant-herbivore systems that closely iii approach equilibrium. In highly variable systems, such as the arid zone of Australia, the concept is not useful and cannot be used to predict sustainable herbivore density. (7) The results of this study indicate that simple measurements of food or energy intake can be used to examine hypotheses about exploitative competition, optimal diet choice, the coevolution of herbivores and plant communities, the evolution of body size and sexual dimorphism and the long-term population dynamics of herbivores in variable environments. iv Acknowledgments I would like acknowledge the assistance of Glenn Edwards, Adam McLean, Graeme Moss, Shane Maloney, Nicky Marlow, Gary Belovsky, Os Schmitz, Mark Ritchie, Glen Saunders, Greg Curran, David Choquenot, the staff of Fowlers Gap Station, Trevor Warburton, and Rick Taylor who helped with various aspects of the field work, reviewing earlier drafts or advice. I would also like to thank my supervisors, Terry Dawson and David Croft for their guidance through all aspects of this project. I would particularly like to acknowledge the assistance ofmy wife Lynette, who unselfishly gave me love, support and encouragement when I needed it the most. During this study I was financially supported by an Australian Postgraduate Scholarship, the RSPCA Alan White Scholarship (1992) and the Fowlers Gap Scholarship. Support for the running of the study was provided by a grant to Terry Dawson from the Australian Research Council. The rainfall data for Tibooburra was provided by the Bureau of Meteorology. V Table of Contents Summary i Acknowledgments iv Table of Contents V List of Figures X List of Tables xv CHAPTER 1: LITERATURE REVIEW The Feeding Strategies and Diet Choice of Generalist Mammalian Herbivores 1.1 Introduction 1 1.2 Diet Choice Descriptions 2 1.3 Feeding Strategy and Diet Choice 4 1.3.1 Optimal Foraging Theory 4 1.3.1.1 Contingency Models 4 1.3.1.2 Central-place Foraging 6 1.3.1.3 Risk-sensitive Foraging 8 1.3.1.4 LinearProgramming 11 1.3.1.5 Other Optimisation Models 13 1.3.2 Complementary Nutrients 14 1.3 .3 Plant Defence 15 1.3.3.1 Spines and Thorns 15 1.3.3.2 Secondary Compounds 16 1.4 Field Studies of Mammalian Herbivore Diet Choice 19 1.5 Synthesis 22 1.5.1 Why Linear Programming? 22 CHAPTER2 Thesis Introduction 2.1 Introduction 24 2.2 European settlement and the land laws 24 2.3 Increasing sheep numbers 26 2.4 The extent of land degradation 29 2.5 Changes in native herbivore communities 30 2.6 Outline of the thesis 31 2.6.1 The Study Site 33 2.6.2 Competition between sheep and kangaroos 33 2.6.3 Optimal diet selection and the evolution of arid zone herbivores 37 2.6.4 Carrying capacity 37 2.6.5 Conclusion 38 vi CHAPTER3 General Description of Study Site 3 .1 Location 39 3.2 Land Systems 40 3.3 Climate 40 3 .3 .1 Rainfall 40 3.3.1.1 Annual and Seasonal Rainfall 41 3.3.1.2 Wet Spells 41 3.3.1.3 Droughts 42 3.3.2 Temperature 42 3.3.2.1 Summer Temperatures 42 3.3.2.2 Winter Temperatures 42 3.3.3 Humidity and Evaporation 43 3.4 Soils 44 3.5 Vegetation 44 3.5.1 Low Shrubland 44 3.5.2 Perennial Grasslands 45 3.5.3 No Perennial Vegetation 45 3.5.4 Tall Open-Shrubland 46 3.6 Fauna 46 3.6.1 Monotreme Mammals 46 3.6.2 Marsupial Mammals 47 3.6.3 Eutherian Mammals 47 3.6.4 The Biology of Red Kangaroos 48 3.6.4.1 Distribution and Abundance 48 3.6.4.2 Reproduction 49 3.6.4.3 Body Size and Growth Rate 49 3.6.4.4 Movement, Social Organisation and Home Range 49 3.6.4.5 Management 50 3.6.5 The Biology of Sheep 50 3.6.5.1 Distribution and Abundance 50 3.6.5.2 Reproduction 51 3.6.5.3 Body Size and Growth Rate 52 3.6.5.4 Movement, Social Organisation and Home Range 52 3.6.5.5 Management 52 CHAPTER4 Experimentally Examining Competition Between Sheep and Red Kangaroos. 4.1 Introduction 53 4.2 Materials and Methods 54 4.2.1 Sheep 56 4.2.1.1 Statistical analyses 58 4.2.2 Red Kangaroos 58 4.2.2.1 Statistical Analyses 60 4.3 Results 62 4.3.1 Sheep 62 4.3.2 Red Kangaroos 69 vii 4.4 Discussion 73 CHAPTERS Mechanistically Modelling Competition Between Sheep and Red Kangaroos. 5 .1 Introduction 80 5.2 Materials and Methods 82 5 .2.1 Modelling 82 5.2.2 Parameterising the Model 84 5.3 Results 90 5.3.1 Parameterising the Model 90 5.4 Discussion 102 5.5 Appendixes 106 CHAPTER6 Optimal diet choice by sheep and red kangaroos.