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. Edge 1, D. Crouchley 1, P. McMurtrie 1, M.J. Willans 2, and A. Byrom 3 1Department of Conservation, Te Anau Area Offi ce, PO Box 29, Lakefront Drive, Te Anau 9640, New Zealand.
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 confi dence 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 fi rst 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 fi rst 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 confi rmed the worst when he observed a stoat on came in 2007 when they were reclassifi ed 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 unmodifi ed 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 confi rmed (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 defi ne 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 benefi ts ‘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 fi rst 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 benefi ts 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 signifi cant 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 . 2010). However, islands >300 m from a source population on the mainland were viewed as much less likely to receive immigrants than islands closer to the source population. Given that Secretary Island is 950 m from the mainland at the narrowest point, and Resolution 520 m, the risk of stoat reinvasion was assessed as low but not zero for both islands. The eradication campaign proceeded on that basis. Central to the plan was the long term use of traps used for the initial knock-down on the islands and control on the adjacent mainland, in order to manage reinvasion. Our assumptions about the rates of stoat immigration to the islands have been challenged by the results. On Secretary Island, DNA analysis of captures to June 2008 reveals a mix of residents and immigrants (McMurtie et al . 2011). The level of immigration detected from July 2005 to June 2008 is also higher than we Fig. 1 Location of Secretary and Resolution Islands. predicted (see Elliott et al . 2010). However, unusually high
167 Island invasives: eradication and management immigration may have been due to a beech masting in 2006 records of stoats from deer trail cameras near stoat traps and a subsequent rodent and stoat plague on the mainland on Secretary Island (D. Crouchley pers. obs.). Plans for in Fiordland. During such years, there will likely be more the Secretary and Resolution Island stoat programmes did juvenile stoats dispersing from the mainland to inshore allow for the use of trained stoat-indicator dogs. We also islands, such as Secretary Island. Further genetic work to relied on the presence of deer hunters in the four years include all of the stoats captured on both islands during the following the stoat knock-down and their observations of eradication campaigns should help to refi ne estimates for stoat sign. immigration. Because the pattern of stoat captures in kill-traps on Both the stoat and deer campaigns were initiated both islands was high initially then followed by a handful following a history of successful rodent eradications on of individuals in subsequent years (Clayton et al . 2011; increasingly large islands (Towns and Broome 2003), McMurtrie et al . 2011), Rule 6 at present still holds for so public support for pest eradications in general was stoats with the caveat that information on the behaviour high (Rule 4). The deer and stoat eradication attempts and detectability of stoats at low densities is imperfect. on Secretary and Resolution Islands had strong iwi and community support, strong political support, and were DEER well-funded. Rule 4 therefore held for both pest species and islands. Furthermore, ongoing support is evident within the Rule 1 was considered to hold a priori for deer on both Department and externally since much of the funding for islands as detailed in operational plans by Crouchley et threatened species reintroductions has come from local and al. (2007) and Crouchley and Edge (2009). On Secretary national corporate sponsors. The immense conservation Island, an estimated 80% of the deer population was benefi t likely to be generated by this programme has thus removed within the planned two-year timeframe. At the generated much community interest. time of writing we are in the second year of the mop-up phase and therefore are yet to achieve eradication. We We also knew from the outset that Rule 5 would not assume that Rule 1 still holds for deer. hold for stoats on either island since we would never be able to disengage from the ongoing costs of sustained We considered that the need to kill target species faster control. However, we calculated that near-zero density than their rate of increase at all densities (Rule 2) holds for could be achieved and maintained with the same effort red deer on both islands. At the time of writing, this still regardless of the number of stoats present because all of appears to be correct. We initially assumed that the potential the infrastructure needed, including tracks, huts and traps, for reinvasion of deer onto Secretary and Resolution islands are to remain in place (and be serviced) in perpetuity. We was relatively high and that Rule 3 would not apply. This argue that Rule 5 is not relevant where: 1) the tools and assumption has since been challenged (Crouchley et al . strategies for eradication are the same as those used for 2011) because: 1) Anchor Island has received no immigrants ongoing management; 2) there was always the intention for the past four years despite its proximity to large deer to make continued use of kill-traps as detection and populations on Resolution Island and the mainland; and 2) monitoring devices; and 3) the desired outcomes remain genotyping of the Secretary Island population suggested unchanged. a small founder population of very few hinds and little subsequent reinvasion. Another interpretation of Rule 5 is that eradication should proceed in favour of control where the benefi ts of The general principle of Rule 5 was considered to the project outweigh the costs (Broome et al. 2005). For hold for red deer on both islands. However, the concept example, when compared with Secretary and Resolution of a ‘one-off’ campaign was rejected in favour of an Islands pest control to equivalent densities over 30,000 ongoing programme able to be scaled down signifi cantly ha on mainland Fiordland would be extremely expensive once the resident population had been removed to focus without producing equivalent conservation outcomes. The on limiting reinvasion or re-establishment potential. The existing ecological values of these islands, in particular assumption that Rule 5 would hold for deer was planned to Secretary Island which has never had introduced rodents, be addressed at a formal review in the second and fourth are unparalleled anywhere else in Fiordland in terms of year of each island programme. At the time of writing it is scale. unclear whether Rule 5 will hold for deer. Because Rules 1 and 2 hold for deer and the risk of reinvasion (Rule 3) is Rule 6 holds for both populations of stoats because much lower than initially thought, eradication is likely to animals surviving the original knockdown campaign be achieved for deer on Secretary and Resolution Islands were largely detected and dealt with before an increased in the future. The alternative model is control to near population size became obvious. Our assumption that kill- zero-density akin to the Murchison Mountains (Fraser and traps would provide reliable detectability was confi rmed at Nugent 2003), where deer control provides massive and high stoat densities using an alternative method (hair tubes) demonstrable conservation benefi ts (Burrows et. al. 1999; prior to the initial knock-down (Clayton et al. 2011). Spatial Tanentzap et al. 2009). detection parameters obtained for stoats on Resolution Island using hair-tubes (Clayton et al. 2011) were similar to Rule 6 was considered to hold for deer on both islands. those for other published studies (Smith et al. 2008; Efford Deer are mobile and therefore leave obvious sign in many et al. 2009). However, we do not know how detectability places even at low population densities (Forsyth et al . changes with stoat density. Foot-print tracking tunnels were 2007). In addition, a variety of tools were to be employed not used as a monitoring tool for stoats in the Secretary and in the mop-up phase to detect and cull deer (Crouchley Resolution campaigns because the large number required et. al . 2011) in order to ensure complete coverage. One (Brown and Miller 1998; Choquenot et al. 2001; King et disadvantage of the planned deer eradication campaign was al. 2007) would have been prohibitively expensive and that, unlike stoats, deer control can only be implemented logistically diffi cult due to the terrain. Furthermore, any and/or checked regularly by people; until now, devices residual stoat population is likely to contain individuals have not been available for continuous operation on the that avoid tunnels, regardless of whether they contain traps islands. However, there are now precedents for successful or tracking cards. This observation has subsequently been eradication of ungulates internationally using fi xed devices confi rmed by the presence of stoat tracks in snow along (e.g., Ramsey et al . 2009). ridgelines with traps (McMurtrie et al. 2011) and video
168 Edge et al .: Stoats and deer off Fiordland islands
GENERAL CONCLUSIONS low density populations is particularly important if there are associated threatened native species reintroduction New Zealand deservedly has a reputation for successful programmes. It also highlights an important need for eradication of invasive alien mammals from offshore many threatened species in New Zealand; to quantify islands. This reputation emerged primarily from rodent the relationship between population density of invasive eradications where the risks of reinvasion were extremely mammals (e.g., stoats) and productivity of threatened low and manageable with strict biosecurity measures (Towns species (e.g., fl edging success; Innes et al. 1999) so that and Broome 2003). The Fiordland Islands programme has extra control effort can be applied should incursions result demonstrated that it is time to further expand our horizons in re-establishment. to islands in close proximity (0.5 – 1 km) to the mainland, of considerably larger size than some previously attempted, The campaigns to eradicate stoats and deer from and where eradication attempts involve multiple invasive Secretary and Resolution Islands challenge three rules for alien mammal species. The conservation importance of eradication, and therefore may be defi ned as extirpation large islands such as Secretary and Resolution in terms of (e.g., Parkes and Panetta 2009). Regardless of defi nition New Zealand’s commitments to international biodiversity and the low density populations of deer and stoats, conventions and restoration goals cannot be overstated. the original planned conservation outcomes have not However, attempting mammal eradications on such large been compromised. For example, on Secretary Island, islands in close proximity to the mainland challenges tried- reintroduction of mohua ( Mohoua orchrocephala ), South and-true paradigms for eradication. These challenges Island robin ( Petroica australis australis ), rock wren are likely to be faced increasingly by other conservation (Xenicus gilviventris ), and the introduction of North managers in New Zealand and internationally. Island kokako ( Callaeas cinerea wilsoni ) have proceeded as planned (Wickes and Edge 2009). Each translocation The eradication programmes for stoats and deer on was undertaken with the knowledge that these species Secretary and Resolution islands do not meet Rules 3 tolerate low density stoat populations on the mainland. and 5 for eradication as defi ned by Parkes et al. (2002), Regeneration of palatable plants is already increasing in but they do fi t with the broader defi nitions as defi ned by many areas as further evidence of a substantial reduction Broome et al. (2005). The original defi nition of Rule 3 in browsing impacts by deer (Crouchley et al. 2011). Most is applicable to offshore islands, but for islands in close planned releases of other species are still likely to proceed proximity to the mainland, the concept of zero reinvasion given the results from both island programmes to date risk is an ideal but not the reality with currently available (Wickes and Edge 2009). However, translocations of tieke tools and strategies for our focal species. Ongoing control or South Island saddleback ( Philesturnus C. carunculatus ) in perpetuity becomes the only available option for stoats may not be possible because the species appears too and deer on Secretary and Resolution Islands because of sensitive to stoats at low density. the constant, although low, risk of reinvasion. This shift in emphasis from eradication to management to zero-density We suggest that the management of invasive is likely to become increasingly applicable to islands mammalian species in New Zealand sits on a continuum elsewhere as island eradication programmes worldwide from intensive one-off operations on offshore islands tackle a range of invasive species. At this point it becomes (Cromarty et al. 2002) through to ‘local elimination’ on essential to implement a strict cost-benefi t analysis (Rule the mainland (Morgan et al. 2006) (Fig. 2). The near-shore 5) of maintaining management to zero-density on an island islands fall somewhere along this ‘continuum of reinvasion in close proximity to the mainland, versus the mainland risk.’ A combination of where the programme sits on this itself, where re-invasion is quicker but the site is easier to continuum and how it fulfi ls the conservation objectives access. This requirement is especially true when funds under Rule 5 is the main consideration when attempting a are limited and the ongoing costs of management may be programme of this nature. unsustainable. The following lessons arose from the stoat and deer Our experience to date on Resolution and Secretary eradication programmes in Fiordland: Islands suggests that it is important to detect and deal with Smaller to larger scales. There are international invasive animals before population sizes increase (Rule precedents for learning from eradication of top predators and 6) for two reasons. First, the detection of individuals ungulates on islands (e.g., Ramsey et al . 2009). Before we enables managers to mount an appropriate response, as made attempts at a larger scale, developmental information is the case for deer. Second, if an established network of vital to the success of the Secretary and Resolution Islands control devices (or routine hunting) doubles as ongoing programme came from smaller Fiordland islands, including surveillance and monitoring (as is the case for stoats), the likelihood of immigration from the mainland (Elliott et then animals must be detectable at low densities, before al . 2010; Crouchley et al . 2011). the population has increased to a level at which damage becomes a problem for threatened species and the costs of Shorter to longer time frames. Eradication of stoats management increase. This need for a rapid response to from some of the smaller Fiordland islands was carried
Fig. 2 Continuum of reinvasion risk from remote offshore islands to local elimination on the mainland.
169 Island invasives: eradication and management
out on a relatively short time scale (weeks to months). A ACKNOWLEDGEMENTS key distinction in the attempted eradications of stoats from Secretary and Resolution is the planned extended time We wish to thank Allan Munn for promoting the initial frame both for pest removal and subsequent reintroductions project concepts and pursuing funding. The programme to of threatened native species (10 years). This extended time eradicate stoats from Secretary and Resolution Islands was scale links with the increased spatial scale (above) and is funded by the New Zealand government to the Department a commonsense approach to invasive species management of Conservation. Extra research was supported by on larger islands. funds from the Foundation for Research, Science and Technology (Programmes C09X0507 and C09X0909). Control tools double as surveillance devices. The tools Advice throughout the planning and implementation used in initial knock-down of stoats and deer on Fiordland phases of this programme was given by members of the islands, which involved kill-traps for stoats, and helicopter Island Eradication Advisory Group: K. Broome, A. Cox, and ground hunting for deer, are the same tools used for R. Empson, P. Cromarty, P. McClelland and I. McFadden. ongoing monitoring and surveillance of both species. This John Parkes and Keith Broome provided many useful approach is an effi cient and cost-effective use of limited comments on an earlier draft of the manuscript as did two funds that allows conservation managers the strategic anonymous referees. option of applying the same tools during the maintenance phase of the programme. REFERENCES Early adoption of new technology. As the focus for an Bomford, M. and O’Brien, P. 1995. Eradication or control for vertebrate eradication campaign shifts from population knock-down pests? 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