Edge, K-A.; D. Crouchley, P. McMurtrie, M.J. Willans,Island and A. Byrom.invasives: Eradicating eradicationstoats (Mustela erminea and) and managementred deer (Cervus elaphus) off islands in Fiordland Eradicating stoats (Mustela erminea) and red deer (Cervus elaphus) off islands in Fiordland K-A. Edge1, D. Crouchley1, P. McMurtrie1, M.J. Willans2, and A. Byrom3 1Department of Conservation, Te Anau Area Office, PO Box 29, Lakefront Drive, Te Anau 9640, New Zealand. <[email protected]>. 2The Wilderness, RD Te Anau-Mossburn Highway, Te Anau, NZ. 3Manaaki Whenua - Landcare Research, PO Box 40, Lincoln 7640, New Zealand. Abstract In 2004, the New Zealand Government allocated NZ$7.1M to eradicate stoats (Mustela erminea) and red deer (Cervus elaphus) from Fiordland’s two largest islands: Secretary Island (8100 ha) and Resolution Island (21,000 ha), in SW New Zealand. Both islands are rugged and within the swimming range of stoats and deer from the mainland. Here we apply the six strategic rules for achieving eradication to the Secretary and Resolution islands programme and use these rules as means of assessing progress five years into the campaign. For these programmes ‘eradication’ has been defined as the complete removal of the stoat and deer populations, and the establishment of long-term control to manage reinvasion. While the original eradication objectives are yet to be achieved, the planned conservation outcomes are on track; several threatened species of birds have been successfully reintroduced and the regeneration of palatable plants is apparent. The conservation importance of large islands such as Secretary and Resolution in terms of New Zealand’s commitments to international biodiversity conventions and restoration goals cannot be overstated. However, attempting mammal eradications on such large islands in close proximity to the mainland challenges conventional paradigms for eradication. These challenges are likely to be faced increasingly by other conservation managers in New Zealand and internationally. Keywords: Secretary Island, Resolution Island, eradication, restoration, strategic rules, extirpation. INTRODUCTION Fiordland National Park, in the southwest of the South and Bauza Island (480 ha) in 2002 gave managers the Island of New Zealand, contains C. 90 islands ranging in confidence to tackle much larger islands such as Secretary size from small rock stacks up to Secretary Island (8140 ha), and Resolution (Elliott et al. 2010). at the entrance to Doubtful Sound, and Resolution Island (20,860 ha) lying between Breaksea and Dusky Sounds Successful eradications of pest species from islands in (Fig. 1). The total land area of Fiordland islands exceeds Fiordland have not been limited to stoats. In 2002-2007, 40,000 ha of which over 31,000 ha has been targeted for red deer were removed from Anchor Island in Dusky pest eradication. Sound (Crouchley et al. 2011). Successful control over 50 000 ha in the Murchison Mountains (Fraser and Nugent Stoats (Mustela erminea) were first introduced into 2003) demonstrated the feasibility of reducing the deer mainland New Zealand in the late 1880s in response to population to near-zero density elsewhere in Fiordland feral rabbit (Oryctolagus cuniculus) plagues. In 1891, National Park and in habitats similar to those on Secretary Resolution Island was gazetted as one of the world’s first and Resolution Islands. ‘reserves’. Richard Henry, curator of Resolution Island, realised the value of islands that might avoid invasion by The enormous potential for pest-eradication and stoats and translocated 572 birds, mostly kiwi and kakapo, restoration on Secretary and Resolution Islands was to the island sanctuary. Stoats are competent swimmers recognised in 2004, when the New Zealand Government (Taylor and Tilley 1984) and they had invaded many of allocated NZ$7.1 million over 10 years to eradicate stoats the remote coastal islands of Fiordland only six years and deer from both islands. Further acknowledgement of after their introduction to New Zealand. By 1900, Henry their current intrinsic and potential future ecological values had confirmed the worst when he observed a stoat on came in 2007 when they were reclassified as ‘Restoration Resolution Island (Hill and Hill 1987). Stoats probably Islands’ within the Fiordland National Park Management invaded Secretary Island around the same time. Plan (2007). In 1963, the New Zealand Government designated The Department of Conservation has developed an Secretary Island a ‘Special Area’ within Fiordland National international reputation for pioneering successful single- Park due to the island’s unmodified vegetation and the species (rodent) eradications on remote islands (Cromarty complete absence of introduced browsing or grazing animals et al. 2002). The next step was to expand to a ‘successive (brushtail possums; Trichosurus vulpecula and red deer; culls’ approach spanning many years for invasive ungulate Cervus elaphus scoticus). In reality, red deer had probably and mustelid species. This approach was planned for already established at the northern end of Secretary Island Secretary and Resolution Islands and is the subject of our but it was not until 1970 that a small resident population paper. was confirmed (Mark et al. 1991). Control measures for red deer were implemented between 1970 and 1987 and GOALS AND OBJECTIVES FOR FIordland’s although hundreds of deer were killed, control did not have ‘RESTORATIon Islands’ a major impact on the population (Brown 2005). Resolution Island, also free of possums, had red deer established in IUCN (International Union for the Conservation of high numbers by 1947 (Sutherland 1957). Nature) guidelines define eradication as the complete removal of an alien invasive species (IUCN Guidelines Since 1999, the feasibility of eradicating island for the Prevention of Biodiversity Loss caused by Alien populations of stoats and managing immigration Invasive Species, May 2000) whereas a programme of from locations within stoat swimming range has been sustained control is focussed on managing the impacts of demonstrated. Eradications of stoats from Chalky Island such species through continuous or periodic population (514 ha) in 1999, Anchor Island (1130 ha) in 2001, reduction (Cromarty et al. 2002). In the operational Pages 166-171 In: Veitch, C. R.; Clout, M. N. and Towns, D. R. (eds.). 2011. Island invasives: eradication and management. IUCN,166 Gland, Switzerland. Edge et al.: Stoats and deer off Fiordland islands and restoration plans for these programmes the term be achieved without eradication, discounted future benefits ‘eradication’ referred to the complete removal of the stoat should favour the one-off costs of eradication over the and deer populations, and the establishment of long-term ongoing costs of sustained control; and 6) ideally, animals control programmes to manage reinvasion. Three goals surviving the campaign should be detectable and dealt with were established: 1) eradicate stoats and deer; 2) enhance before an increased population size becomes obvious. the ecological values of the islands for threatened species re-introductions; and 3) ensure that these islands remain The first three are regarded as crucial rules (Parkes virtually pest-free through effective island biosecurity. 1990), which, unless they are met, eradication cannot proceed. Rules 4-6 are regarded as desirable (Bomford Six strategic rules must be met in order for eradication and O’Brien 1995). For example, eradication might still to be possible (Parkes 1990; Bomford and O’Brien 1995; proceed despite social opposition. We recognised from the Parkes et al. 2002): 1) all target animals must be put at outset that reinvasion by stoats and deer was inevitable, risk to the methods being applied; 2) target species must so we adopted the alternative interpretation of Rule 3: the be killed at rates faster than their rate of increase at all probability of the pest re-establishing is manageable to densities; 3) the risk of recolonisation must be zero; 4) social near-zero (after Broome et al. 2005). and economic conditions must be conducive to meeting the critical rules; 5) where the benefits of management can Below we discuss project planning for the stoat and deer campaigns for Secretary and Resolution Islands in terms of the six rules for eradication. STOATS For both islands, it seemed possible to put all stoats at risk with existing tools, tactics and strategic planning, as was detailed in operational plans by Golding et al. (2005) and McMurtrie et al. (2008). That all animals must be put at risk to the methods being applied (Rule 1), was thus considered a priori to hold for stoats. Large numbers of stoats were removed in the knockdown on Secretary and Resolution Islands, but we have yet to achieve our objective of eradication (McMurtrie et al. 2011). A few stoats may have retained small home ranges even with the significant population reduction and have therefore never come in contact with a trap. Alternatively, a few animals may avoid entering a trap tunnel either for an extended period of time or in perpetuity (Crouchley 1994; King and Powell 2009). Rule 1, therefore, does not appear to hold for stoats on either island at the time of writing. Stoat eradication programmes on other Fiordland islands demonstrated that animals could be killed in traps faster than their rate of increase (Rule 2), even at low densities (Elliott et al. 2010). With stoats, however, the real issue is not population density per se, but the ability to respond rapidly to ‘pulsed’ events such as immigration or in-situ breeding, particularly during mast years (Wittmer et. al. 2007). Rule 2 was thus considered to hold for stoats on both islands. On Secretary Island, trapping results indicate that the stoat population is being maintained at a very low level without further decline (McMurtrie et al. 2011) so we are not meeting Rule 2. It is too early to establish the trend for Resolution Island. It was known from the outset that the risk of recolonisation by stoats would not be zero (Rule 3) on either island (Elliott et al.