Exotic rodents on Australian islands: issues paper John Parkes and Penny Fisher Landcare Research P.O. Box 40 Lincoln 7640 New Zealand Email: Parkesj @landcareresearch.co .nz James Russell University of Auckland Auckland New Zealand Keith Morris Department of Enviromnent and Conservation Western Australia Prepared for: Department of Environment and Water Resources Canberra Australia Date: June 2007 2 Contents Summary of key questions for the workshop 1. Introduction 2. The scale of the problem 3. The culprits 3.1 Ship rats 3.2 Norway rats 3.3 Polynesian rats 3.4 House mice 4. Planning rodent management 4.1 Justification - impacts of rodents on biodiversity 4.2 Feasibility plans 4.2.1 Strategies to fix the problem (a) Preventing invasions (b) Eradication and preventing reinvasions (c) Quasi-eradication (d) Sustained control (e) Do nothing 4.2.2 Tactics and costs to fix the problem 4.3 Operational plans 4.4 Audit of benefits and consequences 4.5 Predicting and managing adverse effects 4.5.1 Adverse effects of the control - non-target impacts 4.5.2 Adverse effects of rodent removal 4.4 Planning to include other pests 5. Prioritising islands 6. Legal issues 7. Education 8. Capacity building 9. Case studies 8.1 Lord Howe Island 8.2 Barrow Island 10. Key references 11. Appendices 11.1 Australian islands known to have exotic rodents 11.2 Australian islands from which exotic rodents have been eradicated 11.3 Notes on biosecurity options for different island types 3 Summary of key questions for workshop Base information 1. Should the databases on Australian islands with exotic rodents be completed? 2. Are there key gaps in information - e.g., invasion risk factors, presence of non­ targets, inhabited or not? Accountabilities 3. Which agency and who makes the decisions to plan and act against rodents in each jurisdiction? 4. Who should ' deliver' the rodent control - building agency capacity or contract out to experts? 5. What are the legal obligations and constraints in each jurisdiction? 6. What are the current laws and practice to ensure ships are free of rodents? Rodent biology 7. Are there predictable seasonal or yearly changes in food supply and breeding of rodents on different types of islands so we can apply control at the best time? 8. Are there critical knowledge gaps for the rodents' biology? Management planning 9. Which islands are candidates for eradication and which for sustained control? 10. How should the Commonwealth and States/Territories decide which insular exotic rodents to remove first? 11 . Should jurisdictions select some islands where obvious benefits to iconic species are expected as pait of wider plans to build public support for other islands? Management 12. Should island managers set up contingency response systems to counter rodent incursions and/or prophylactic control buffers around likely places of ingress? 13. What capacity is needed in agencies and among island users to enhance biosecurity? 14. What baits and toxins and which means of delivery are registered for use against rodents in Australia? 15. If sustained control is required (rather than eradication) what sequence of baits and toxins is best practice on islands? 16. What proportion or how many non-target species at risk to the baits and toxins proposed should be removed for the duration of any rodent eradication campaign? 17. Are reptiles at risk from anticoagulants? 18. When should any predicted and unmanageable adverse consequences of removing rodents cmtail the rodent eradication? 19. How should managers plan and react towards any unpredicted adverse consequences revealed once rodents are eradicated or controlled? 20. When should we include in plans the other pests that may be killed as by-catch or that are trophically linked with rodents? 21 . How can State agencies with little capacity learn from operations in other jurisdictions? 4 1. Introduction Exotic rats on islands were nominated in 2004 by World Wildlife Fund, Australia as a key threatening process under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). The Department of Environment and Water Resources (DEWR) has begun to consider this nomination with the eventual aim of producing a Threat Abatement Plan (TAP). This process includes consideration of and consultation around the various issues that will affect how exotic rodents (rats and mice) and their impacts on islands are managed by Commonwealth and State/Territory agencies, by owners of islands, and by wider stakeholders with an interest in sustainable ecosystems and biodiversity on islands. The four species of exotic rodents found on Australian islands are ship or black rats (Rctttus rattus), Norway or brown rats (Rctttus norvegicus), Polynesian or Pacific rats (Rctttus exulans), and mice (Mus musculus!Mus domesticus). This first document summarises the background information that will eventually be expanded in a formal Background Rep011 appended to the TAP. It lists the key issues and (cmTent) gaps in information (as reminders to ourselves) to act as a resource document to strncture a workshop of key stakeholders planned for 25-26 June 2007. Only a few key references are noted in this rep011. Ship rat taking fantail egg (photo David Mudge). 5 2. The scale of the problem Australia has over 8300 islands (Table 1) of which at least 122 now have one or more species of exotic rodents (Appendix 1). To date 39 populations of rodents (on 38 islands) have been eradicated (Appendix 2). On mainland Australia two invasive rodent species (ship rats and mice) are widespread - mice being almost ubiquitous while ship rats are largely absent from semi­ arid areas and the wet-dry tropics (Caughley et al. 1998). Norway rats are largely restricted to urban areas while Polynesian rats are thought to be absent from the mainland. These distributions are reflected on islands with ship rats and mice being the most common species, often occurring together (Table 2). Table 1. Status of exotic rodents on Australian islands by State/Tenitory. 1 Includes the 26 islands in the Southern Cocos/Keeling group. • j State/Territory No. of No. islands No. where References and data islands <100 000 ha rodents sources known to now have been have exotic eradicated rodents Commonwealth 37 28 1 2 Burbidge (2004) Northern Territory 398 2 0 Rankmore (2005) Western Australia 3678 22 33 Burbidge (2004) South Australia 396 11 0 Burbidge (2004) Victoria 246 10 0 Burbidge (2004) New South Wales 439 8 1 O'Neill (2006) Tasmania 300+ 26 1 Terauds (2005) Queensland 1854 15 1 Burbidge (2004) Total 7344 122 38 New Zealand 710 145+ 92 Howald et al. (2007) Galapagos 50 38 4 B. Milstead (unpubl. data) The 128 islands known to have exotic rodents range in size from < 1 ha up to 6 million ha for Tasmania (Table 2; Fig. 1). Of those less than 100 000 ha (the cut-off size for this TAP) rodents remain on at least 122 with a total area of 308 000ha. At a cost of say $200/ha to eradicate rodents it would cost $61 million to clear all islands less than 100,000 ha. X% of islands with rodents are under some sort of conservation tenure, y% are owned by Aboriginal people, while the rest are either Crown-lease or private tenures (Fig. 2). Overall planning for Crown-owned island management is further complicated in most jurisdictions by the fact that several agencies operating under different legal instruments have responsibility in most States. 6 Elsewhere in the world rodents have been eradicated from about 350 islands (Howald et al. in press and this repo11) (Table 3). Sympatric ship rats and mice have been eradicated from only five islands worldwide: Barrow in Western Australia where they were present in separate and small parts of island, Flat (253-ha in Mauritius), Rasa (60-ha in Mexico), Motutapere (46-ha in New Zealand), and Surprise (24-ha in New Caledonia). Attempts against both species on Denis and Bird Islands in the Seychelles failed to remove one or the other. Table 2. Rodent species and combinations on Australian islands of < 1000,000 ha. 1 Assuming ship rats are present on all the South Keeling Islands. Species combinations No. islands Largest island eradicated in Australia Mice only 43 Varanus (83 ha) Ship rat onl/ 58 Hermite (1022 ha) Polynesian rat only 1 Norway rat only 1 Ship rat and mice 16 Barrow but only in separate parts of island (around the camps for mice and over c. 270 ha for the rats) Norway rat and mice I Ship rat and Polynesian rat I Ship rat, Norway rat and mice I 40 35 30 "' "0= 25 -~~ 20 z0 15 10 5 0 10000+ 1000- 100 - 1000 10 -100 <10 10000 Island size class (ha) Figure 1. Size class distribution of Australian islands less than I 00 000 ha with rodents Figure 2. Propo11ions of Australian islands under different tenures Pie diagram figure to come when data available from all jurisdictions. 7 Table 3. Rodent eradications worldwide. All these largest successes were achieved using brodifacoum baits sown from the air. Species No. successful No failed Largest island with Eradications eradications successful eradication Ship rat 160 15 Hermite (Australia) l 022 ha Norway rat 104 5 Campbell (NZ) 11 300 ha Polynesian rat 55 6 Hauturu (NZ) 3083 ha Mice 30 7 Enderby (NZ) 710 ha Conclusions: • Mice and ship rats are the most common species on Australian islands. • The two occur together on at least 16 islands (see control implications). • Eradication has been attempted on only a few large islands. • Most attempts have been in Western Australia. • Most eradication operations have used ground baiting methods rather than aerial methods. • Almost all have been against ship rats. • Scaling-up to large islands and aerial baiting will require either outside contracting or capacity building by State agencies.