OVERVIEW OF THE IMPACTS OF FERAL CATS ON AUSTRALIAN NATIVE FAUNA by Chris R. Dickman Institute of Wildlife Research and School of Biological Sciences University of Sydney Prepared for the Australian Nature Conservation Agency 1996 AUSTRALIA Overview of the Impacts of Feral Cats on Australian Native Fauna by Chris Dickman The views and opinions expressed in this report are those of the authors and do not necessarily reflect those of the Commonwealth Government, the Minister for the Environment or the Director of National Parks and Wildlife. ISBN 0 642 21379 8 Published May 1996 © Copyright The Director of National Parks and Wildlife Australian Nature Conservation Agency GPO Box 636 Canberra ACT 2601 and Institute of Wildlife Research University of Sydney Sydney NSW 2006 Design and art production by BPD Graphic Associates, Canberra Cover illustration by Karina Hansen McInnes CONTENTS FOREWORD 1 SUMMARY 2 EXECUTIVE SUMMARY AND RECOMMENDATIONS 3 ACKNOWLEDGEMENTS 4 1. INTRODUCTION 5 1.1 Scope of the report 5 2. DEFINITIONS 7 3. IMPACTS OF DOMESTIC AND STRAY CATS 9 3.1 Competitive impacts of domestic and stray cats 9 3.2 Predatory impacts of domestic and stray cats 9 3.3 Amensal impacts of domestic and stray cats 13 3.4 Summary: impacts of domestic and stray cats 14 4. IMPACTS OF FERAL CATS 15 4.1 Competitive impacts of feral cats 15 4.2 Predatory impacts of feral cats 16 4.2.1 Diets of feral cats: mainland only 16 4.2.2 Diets of feral cats: islands 18 4.2.3 Hunting behaviour of feral cats 18 4.2.4 Predatory impacts: what constitutes acceptable evidence? 19 - Predatory impacts: anecdotal evidence 19 Predatory impacts: historical evidence 20 - Predatory impacts: evidence from reintroduction programs 23 - Predatory impacts: evidence from islands 26 - Predatory impacts: field experiments 32 4.3 Amensal impacts of feral cats 33 4.3.1 Pathogens 33 4.3.2 Other indirect effects 36 4.4 Summary: impacts of feral cats 37 5. FACTORS THAT MODIFY IMPACT 39 5.1 Density of cats 39 5.2 Density and distribution of native fauna 39 5.3 Intensity of predation 40 5.4 Fecundity of native species 41 5.5 Behaviour of predator and prey species 42 5.6 Interactions with other predators 43 6. WHICH NATIVE SPECIES ARE SUSCEPTIBLE TO CAT IMPACT? 45 6.1 Development of a rank-scoring system to predict susceptibility 45 6.2 Zero, low and high risk species 48 7. WHERE DO NATIVE SPECIES SUSCEPTIBLE TO CAT IMPACT OCCUR? 53 8. CRITICAL DETERMINATION OF CAT IMPACTS: PRIORITIES FOR RESEARCH 59 8.1 Priority species and localities for research 59 Priority 1: South-west Western Australia 59 Priority 2: Channel Country 59 Priority 3: Top End 60 Priority 4: Kimberley 60 Priority 5: Tasmania 60 Priority 6: Norfolk Island 61 Priority 7: Coastal Victoria 61 Priority 8: Pilbara 61 Other priorities 61 8.2 Experiments to clarify cat impacts 62 REFERENCES 65 Table 1. Anecdotal reports of predatory impacts of feral cats Felis catus on native vertebrates in Australia. 20 Table 2. Extinctions of native mammals on the mainland of Australia, pre-1900, showing date of last records. 21 Table 3. Reintroductions of native mammals (excluding macropods) to areas with and without feral cats Felis catus in Australia. 25 Table 4. Impacts of feral cats Felis catus on native terrestrial mammals and birds on islands off the coast of Australia. 27 Table 5. Status of native terrestrial mammals on islands of the Bass Strait with populations of feral cats Felis catus. 29 Table 6. Apparent effects of removal or reduction of feral cats Felis catus on terrestrial species of mammals and birds. 34 Table 7. Coincidence of non-specific pathogenic organisms recorded from cats Felis catus in Australia and from native vertebrate fauna. 35 Table 8. Population densities of feral cats Felis catus in Australia. 40 Table 9. Susceptibility of endangered native vertebrates to predation from feral cats Felis catus on the mainland of Australia. 47 Table 10. Susceptibility of vulnerable native vertebrates to predation from feral cats Felis catus on the mainland of Australia. 50 Table 11. Susceptibility of endangered and vulnerable native vertebrates to predation from feral cats Felis catus on Australian offshore islands and island territories. 51 Figure 1. Species density maps of endangered and vulnerable species of native vertebrates (mammals, birds and reptiles) at high risk of impact from feral cats Felis catus in Australia; a) endangered species, b) vulnerable species, c) endangered and vulnerable species combined. 54 Figure 2. Species density maps of endangered and vulnerable species of native vertebrates (mammals, birds and reptiles) at low risk of impact from feral cats Felis catus in Australia; a) endangered species, b) vulnerable species, c) endangered and vulnerable species combined. 56 Appendix 1. Diet of the feral cat Felis catus in different habitats in Australia. 85 Appendix 2. Diet of the feral cat Felis catus on Australian, New Zealand and Pacific region islands. 91 FOREWORD Feral cats and native wildlife - what are the impacts? This question has been asked by researchers for many years, seemingly without much progress. On the one hand, we know that feral cats have caused extinctions on some islands and have caused the failure of reintroduction programs for endangered mammals in semi-arid areas. On the other hand, cats have been present in Tasmania, on Kangaroo island and in the northern parts of Australia for at least as long as anywhere else yet these places have retained virtually the full complement of their original fauna. If a native population is declining, the cause of that decline should be identified before any remedial work is undertaken. Many different things, such as habitat modification or introduced species, could be a causal factor in such declines. As such it is highly desirable to identify the actual cause of a decline in the population before commitment is made to an extensive feral animal control campaign. A series of national overviews were commissioned by the Invasive Species Program to determine the impacts of feral animals on our native wildlife. This overview brings together all of the existing information concerning the effects of feral cats on native species and identifies key gaps in knowledge. It will be an invaluable tool in clarifying the impacts of invasive species on the Australian environment and, perhaps more importantly, provides a strategic direction for future research and control of feral cat impacts. Peter Bridgewater Chief Executive Officer Australian Nature Conservation Agency SUMMARY This report provides an overview of the impact of feral cats Felis catus on native fauna of the Pacific region, with particular reference to Australia and its island territories. In Australia, cats take a wide variety of native species of mammals, birds and reptiles, but show evident preference for young rabbits or small marsupials where these are available. Reptiles are taken primarily in and habitats, while birds often feature predominantly in the diet of cats on islands. Despite their catholic diet, population-level impacts of feral cats on native fauna have been poorly documented. There is considerable potential for competition to occur between cats and carnivorous species such as quolls and raptors, but no critical evidence has yet been adduced. There is also potential for amerisal impacts to occur, either via transmission of the pseudophyllidean tapeworm Spirometra erinacei or of the protozoan parasite Toxoplasma gondii, but evidence for deleterious effects in freeliving animals is not compelling. Direct predatory impacts have been inferred from anecdotal and historical evidence, more strongly from failed attempts to reintroduce native species to their former ranges, and most critically from the decimation of island faunas and responses of prey species following experimental removal of cats or reduction of cat numbers. Attributes of the biology of feral cats and their prey species derived from the literature review were used to develop a rank-scoring system to assess the susceptibility of native species to cat predation. Species listed federally as endangered or vulnerable were designated as being at zero, low or high risk of impact from cats according to their attribute scores, and their distributions mapped from primary sources and actual locality data. Based on the number of threatened species they contain, localities and regions within Australia were placed in order of priority for future research to clarify the precise impacts of feral cats. Although difficult and expensive to carry out, controlled and replicated field removal experiments are recommended to elucidate cat impacts in all mainland areas. Removal of cats should take place also on offshore islands and island territories, but only if pilot studies show that this will not release populations of alternative predator species such as introduced rats. If release appears likely, cats should be removed only as a component of an integrated control program that targets all relevant predators. Disclaimer Funding to compile this report was provided by the Australian Nature Conservation Agency (ANCA) under the Invasive Species Program. The views expressed in this report are not necessarily those of ANCA. EXECUTIVE SUMMARY AND RECOMMENDATIONS 1) Population and community-level impacts of feral cats on native fauna have been little studied, but evidence from several studies indicates that direct predation is the most important process involved. 2) On the Australian mainland cats can impact on nativee mammals weighing up to about 2000 g, but impact falls most heavily on smaller species, especially those weighing <220 g. Vulnerability to predation is enhanced for species occupying open or otherwise unprotected habitats, and by behavioural traits such as saltatory locomotion. Cats can impact on birds up to about 1000 g, but impact again falls most heavily on smaller species weighing <200 g.