supervising scientist 164 report A preliminary risk assessment of cane toads in Kakadu National Park RA van Dam, DJ Walden & GW Begg supervising scientist national centre for tropical wetland research This report has been prepared by staff of the Environmental Research Institute of the Supervising Scientist (eriss) as part of our commitment to the National Centre for Tropical Wetland Research Rick A van Dam Environmental Research Institute of the Supervising Scientist, Locked Bag 2, Jabiru NT 0886, Australia (Present address: Sinclair Knight Merz, 100 Christie St, St Leonards NSW 2065, Australia) David J Walden Environmental Research Institute of the Supervising Scientist, GPO Box 461, Darwin NT 0801, Australia George W Begg Environmental Research Institute of the Supervising Scientist, GPO Box 461, Darwin NT 0801, Australia This report should be cited as follows: van Dam RA, Walden DJ & Begg GW 2002 A preliminary risk assessment of cane toads in Kakadu National Park Scientist Report 164, Supervising Scientist, Darwin NT The Supervising Scientist is part of Environment Australia, the environmental program of the Commonwealth Department of Environment and Heritage © Commonwealth of Australia 2002 Supervising Scientist Environment Australia GPO Box 461, Darwin NT 0801 Australia ISSN 1325-1554 ISBN 0 642 24370 0 This work is copyright Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without prior written permission from the Supervising Scientist Requests and inquiries concerning reproduction and rights should be addressed to Publications Inquiries, Supervising Scientist, GPO Box 461, Darwin NT 0801 e-mail: publications@eriss erin gov au Internet: http://www ea gov au/ssd/index html Views expressed by the authors do not necessarily reflect the views and policies of the Supervising Scientist, the Commonwealth Government, or any collaborating organisation Printed in Darwin by NTUniprint Contents Executive summary vii Acknowledgments xvi Abbreviations xvi Update 1 1 Introduction 3 1.1 Background 3 1.2 Project aim 3 1.3 Approach 3 1.3.1 Wetland risk assessment framework 3 1.3.2 Information sources 5 2 The risk assessment 6 2.1 Identification of the problem 6 2.1.1 Cane toads — overview of natural history 6 2.1.2 History of cane toad invasion in Australia 8 2.1.3 Kakadu National Park — overview of habitats and fauna 9 2.1.4 Conceptual model 20 2.2 The potential extent of cane toads in Kakadu National Park 20 2.2.1 Distribution and range expansion 20 2.2.2 Preferred habitats and environmental/bioclimatic conditions 26 2.2.3 Summary of potential extent in Kakadu National Park 32 2.3 The potential effects of cane toads in Kakadu National Park 36 2.3.1 Effects on predator species 36 2.3.2 Effects on prey species 50 2.3.3 Effects of resource competition 52 2.3.4 Cultural effects 56 2.3.5 Economic effects 57 2.3.6 Other potential effects 58 2.4 Identification of the risks 60 2.4.1 Comparison of potential effects and extent 60 2.4.2 Identification of key habitats 60 iii 2.4.3 Identification of species at risk 61 2.4.4 Cultural, socio-economic and other risks 66 2.5 Uncertainty and information gaps 67 2.5.1 Extent of cane toads in Kakadu National Park 67 2.5.2 Effects of cane toads in Kakadu National Park 68 3 Recommendations for monitoring 72 3.1 Priority habitats for monitoring 72 3.2 Priority species for monitoring 72 3.2.1 Predators 72 3.2.2 Prey 73 3.2.3 Competition 73 3.3 Priorities for addressing information gaps 73 3.4 Evaluation of past and present monitoring programs 74 3.4.1 Broad scale surveys 74 3.4.2 Ongoing monitoring programs 75 3.4.3 Other surveys or monitoring programs 75 4 Risk management and reduction 77 5 References 79 iv Figures Figure 1 Model for wetland risk assessment 4 Figure 2 An adult cane toad 7 Figure 3 The Kakadu region 10 Figure 4 The documented current west north-west range of cane toads in the Top End 22 Figure 5 Major rivers, roads and tracks in Kakadu National Park 33 Tables Table 1 Matrix of potentially adverse environmental impacts associated with the introduced cane toad (Bufo marinus) in Kakadu National Park 21 Table 2 Selected examples of habitats occupied by cane toad populations in other parts of the world 29 Table 3 Predatory groups of aquatic invertebrates in Kakadu National Park, potentially susceptible to cane toads 38 Table 4 Predatory fish species in Kakadu National Park, potentially susceptible to cane toads 39 Table 5 Predatory frog species in Kakadu National Park, potentially susceptible to cane toads 41 Table 6 Predatory lizard species in Kakadu National Park, potentially susceptible to cane toads 42 Table 7 Predatory snake species in Kakadu National Park, potentially susceptible to cane toads 44 Table 8 Predatory bird species in Kakadu National Park, potentially susceptible to cane toads 47 Table 9 Predatory mammal species in Kakadu National Park, potentially susceptible to cane toads 49 Table 10 Species in Kakadu National Park, potentially susceptible to competition with cane toads 56 Table 11 Criteria for determining risk categories and level of priority for predatory species susceptible or potentially susceptible to cane toads 62 Table 12 Risk ranking and priority status for Kakadu National Park species susceptible or potentially susceptible to cane toads 63 v vi Executive summary Background and approach Cane toads (Bufo marinus) entered the Northern Territory (NT) in 1980 from Queensland and are rapidly approaching Kakadu National Park (KNP), having recently been reported in the upper Mann River and Snowdrop Creek, approximately 15–30 km to the east of Kakadu National Park. Concern about the invasion of cane toads in Kakadu National Park has been highlighted on a number of occasions, and in 1998 participants at a workshop on the potential impacts and control of cane toads in Kakadu National Park conceded that a strategic approach for assessing and possibly minimising cane toad impacts should be developed. The first stage would be an ecological risk assessment to predict the likely extent of impacts of cane toads in Kakadu National Park and identify key vulnerable habitats and species. This information could be used to develop new monitoring programs and assess existing ones. This assessment is a direct result of Environment Australia’s concern about the potential impacts of cane toads in Kakadu National Park. The wetland risk assessment framework developed by eriss for the Ramsar Convention was used to predict key habitats and the species most at risk. The majority of the assessment involved identifying the problem, the potential extent and effects of the problem, the risk, and subsequently making recommendations on monitoring. Major information gaps relevant to predicting impacts and developing appropriate monitoring programs were also identified. The risk assessment was based on information from published and unpublished scientific and anecdotal reports. Information on Kakadu National Park was derived from relevant research projects undertaken in the Park since the early 1980s. A number of relevant Territory and Commonwealth agencies were consulted, as were relevant cane toad, native fauna and/or wildlife management experts from around Australia. Discussions were held with community members in the Borroloola and Mataranka regions to gain an indigenous/cultural perspective of the cane toad issue. Identification of the problem Since their introduction to Australia in 1935 to control sugar cane pests in Queensland, cane toads have spread naturally and with human assistance throughout much of Queensland, northern NSW and the Top End of the NT. The cane toad’s preference for certain disturbed areas means that areas of degraded natural habitat have probably helped their spread. They eat a wide variety of prey, breed opportunistically, have a far greater fecundity than native anurans, and develop rapidly particularly in warmer waters. They tolerate a broad range of environmental and climatic conditions, can occupy many different habitats and compete for resources with many native species. Most significantly, they possess highly toxic chemical predator defences, with many experimental and anecdotal reports of deaths of native predators that have attempted to consume cane toads. It is accepted that the cane toad will establish and spread rapidly in Kakadu National Park — a World Heritage area with Ramsar listed wetlands, well known for its spectacular wilderness, nature conservation values, rich diversity of habitats, flora and fauna, and cultural significance. There is serious concern that the World Heritage status of Kakadu National Park could be diminished if any of these attributes were adversely affected by cane toads. vii The potential extent of cane toads in Kakadu National Park Cane toads are likely to colonise almost every habitat type within Kakadu National Park. The saline regions of the coastal plains and deltaic estuarine floodplains will most likely support some cane toads at various times, although they are not likely to use these habitats on a permanent basis. Other less suitable areas include deep open water and/or flowing channel habitats and tidal regions of larger rivers (excluding riparian zones) which extend 70 to 80 km inland during the Dry season. The steady range expansion over the last ten years indicates that most wetland habitats are probably suitable as breeding habitat and also as Dry season refuges. Patterns of dispersal within Kakadu will probably rely on the transport corridors and the major rivers and creeks. Dispersal rates within a catchment could be up to 100 km y-1. The current location of cane toads would indicate an initial progression down the South Alligator River catchment via its sub-catchments (eg Jim Jim Creek, Deaf Adder Creek).