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Multi island, multi invasive species eradication in French Polynesia demonstrates economies of scale
Available Online

Coulston, G.

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Cranwell, S.

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Derand, D.

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Ghestemme, T.

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Griffiths, R.

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Hall, T.

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Pott, M.

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Will, D.

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Zito, J.

2019
Eradication of invasive vertebrates on islands has proven to be one of the most effective returns on investment for biodiversity conservation. To recover populations of the critically endangered Polynesian ground dove (Gallicolumba erythroptera), the endangered white-throated storm-petrel (Nesofregetta fuliginosa), the endangered Tuamotu sandpiper (Prosobonia cancellata) as well as other native plant and animal species, a project was undertaken to eradicate five species of invasive alien vertebrates: Pacific rat (Rattus exulans), ship rat (R. rattus), feral cat (Felis catus), rabbit (Oryctolagus cuniculus) and goat (Capra hircus), on six islands spanning 320 km of open ocean in the Tuamotu and Gambier Archipelagos of French Polynesia. Using a ship to deliver supplies and equipment, a helicopter for offloading and bait application, and ground teams for follow up trapping and hunting, invasive vertebrates were successfully removed from five of the six islands. Pacific rats survived at one site. The project was planned and executed by a partnership consisting of international and local conservation NGO’s, working together with local communities. Combining the different eradication operations into one expedition added complexity to project planning and implementation and increased the risk of the operation failing on any one island but generated greater returns on investment allowing six islands to be targeted at significantly less cost than if each island had been completed individually. An extensive and thorough planning effort, effective relationships with local stakeholders and communities, a good operational strategy and a partnership of stakeholders that each brought complementary capacities to the project contributed to its success.
Rat eradication in the Pitcairn Islands, South Pacific: a 25-year perspective
Available Online

Brooke, M.de L.

2019
This essay offers a 25-year overview of eff orts to remove Pacific rats (Rattus exulans) from the four islands of the Pitcairn group. Following the 1991–1992 discovery that rats were severely reducing breeding success of gadfly petrels (Pterodroma spp.), Wildlife Management International proposed eradication. Eradication success was achieved using ground-based baiting on the small atolls of Ducie and Oeno in 1997, and there is now evidence of petrel recovery on Oeno, but two eradication attempts on inhabited Pitcairn (1997 and 1998) failed. By the early 2000s, the development of aerial baiting through the 1990s placed an eradication operation on the fourth island, Henderson, within reach. Preparatory fieldwork in 2009 allayed doubts in two key areas: the feasibility of maintaining a captive “back-stop” Henderson rail (Porzana atra) population, and bait uptake by crabs (Coenobita spp.). Royal Society for the Protection of Birds (RSPB) expertise secured the necessary funding of £1.5 million, and 75 tonnes of brodifacoum-containing bait were dropped in August 2011. Despite extensive mortality of free-living rails, the population, supplemented by released captive birds, returned to pre-operational levels in 2–3 years. Meanwhile those tending captive rails saw no rat sign before leaving Henderson in November 2011. Unfortunately, a rat was sighted in March 2012, and continuing rat presence confirmed in May 2012. Subsequently rat numbers have returned to pre-operational levels without any sign of population ‘overshoot’ as observed on Pitcairn. Genetic analysis suggests around 80 rats, roughly 1 in 1,000, survived the bait drop. With no evidence of imperfect bait coverage or deficiencies in bait quality or brodifacoum resistance, it seems some animals chose not to eat bait. Choice tests on Henderson Island rats suggest some rats prefer natural foods over bait. This adverse situation may have been exacerbated because, in August 2011, natural fruits were more abundant than anticipated due to drought earlier in the year. To overcome rat preference for natural food, any second Henderson attempt might benefit from more attractive bait. Without such developments, a second attempt risks another failure. Henderson’s biota will survive the delay.
Seasonal variation in movements and survival of invasive Pacific rats on sub-tropical Henderson Island: implications for eradication.
Available Online

Bond, A.L.

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Churchyard, T.

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Donaldson, A.

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Duffield, N.

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Havery, S.

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Kelly, J.

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Lavers, J.L.

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McClelland, J.T.W.

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Oppel, S.

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Proud, T.

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Russell, J.C.

2019
Invasive rodents are successful colonists of many ecosystems around the world, and can have very flexible foraging behaviours that lead to differences in spatial ranges and seasonal demography among individuals and islands. Understanding such spatial and temporal information is critical to plan rodent eradication operations, and a detailed examination of an island’s rat population can expand our knowledge about possible variation in behaviour and demography of invasive rats in general. Here we investigated the movements and survival of Pacific rats (Rattus exulans) over five months on sub-tropical Henderson Island in the South Pacific Ocean four years after a failed eradication operation. We estimated movement distances, home range sizes and monthly survival using a spatially-explicit Cormack-Jolly-Seber model and examined how movement and survival varied over time. We captured and marked 810 rats and found a median maximum distance between capture locations of 39 ± 25 m (0–107 m) in a coastal coconut grove and 61 ± 127 m (0–1,023 m) on the inland coral plateau. Estimated home range radii of Pacific rats on the coral plateau varied between ‘territorial’ (median: 134 m; 95% credible interval 106–165 m) and ‘roaming’ rats (median: 778 m; 290–1,633 m). The proportion of rats belonging to the ‘roaming’ movement type varied from 1% in early June to 23% in October. There was no evidence to suggest that rats on Henderson in 2015 had home ranges that would limit their ability to encounter bait, making it unlikely that limited movement contributed to the eradication failure if the pattern we found in 2015 is consistent across years. We found a temporal pattern in monthly survival probability, with monthly survival probabilities of 0.352 (0.081–0.737) in late July and 0.950 (0.846–0.987) in late August. If seasonal variation in survival probability is indicative of resource limitations and consistent across years, an eradication operation in late July would likely have the greatest probability of success.
Bait colour and moisture do not affect bait acceptance by introduced Pacific rats (Rattus exulans) at Henderson Island, Pitcairn Islands.
Available Online

Bond, A.L.

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McClelland, G.T.W.

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O’Keefe, S.

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Warren, P.

2019
Rodent eradications are a useful tool for the restoration of native biodiversity on islands, but occasionally these operations incur non-target mortality. Changes in cereal bait colour could potentially mitigate these impacts but must not compromise the eradication operation. Changing bait colour may reduce mortality of Henderson crakes (Zapornia atra), an endemic globally threatened flightless bird on Henderson Island, Pitcairn Islands, South Pacific Ocean. Crakes had high non-target mortality in a failed 2011 rat eradication operation and consumed fewer blue than green cereal pellets. We examined which cereal bait properties influenced its acceptance by captive Pacific rats (Rattus exulans) on Henderson Island. We held 82 Pacific rats from Henderson Island in captivity and provided them with non-toxic cereal bait pellets of varying properties (blue or green, moist or dry). We estimated the proportion of rats consuming bait using logistic generalised linear mixed models. We found no effect of sex, females’ reproductive status, bait colour or bait moisture on rats’ willingness to consume baits. Rats’ bait consumption was unaffected by cereal bait properties (colour or moisture). The use of blue bait is unlikely to affect future eradication operational success but may reduce non-target mortality of Henderson crakes. Timing cereal bait distribution in relation to precipitation may also reduce crake mortality without compromising palatability to rats.
Recovery of introduced Pacific rats following a failed eradication attempt on subtropical Henderson Island, South Pacific Ocean
Available Online

Bond, A.L.

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Churchyard, T.

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Cuthbert, R.J.

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Duffi eld, N.

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Havery, S.

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Kelly, J.

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Lavers, J.L.

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McClelland, G.T.W.

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Oppel S.

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Proud, T.

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Torr, N.

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Vickery, J.A.

2019
Rodent eradications in tropical environments are often more challenging and less successful than those in temperate environments. Reduced seasonality and the lack of a defined annual resource pulse influence rodent population dynamics differently than the well-defined annual cycles on temperate islands, so an understanding of rodent ecology and population dynamics is important to maximise the chances of eradication success in the tropics. Here, we report on the recovery of a Pacific rat (Rattus exulans) population on Henderson Island, South Pacific Ocean, following a failed eradication operation in 2011. We assessed changes in the rat population using capture rates from snap-trapping and investigated seasonality by using capture rates from live-trapping. Following the failed eradication operation in 2011, rat populations increased rapidly with annual per capita growth rates, r, of 0.48–5.95, increasing from 60–80 individuals to two-thirds of the pre-eradication abundance within two years, before decreasing (r = -0.25 – -0.20), presumably as the population fluctuated around its carrying capacity. The long-term changes in rat abundance may, however, be confounded by short-term fluctuations: four years after the eradication attempt we observed significant variation in rat trapping rates among months on the plateau, ranging from 36.6 rats per 100 corrected trap-nights in mid-June to 12.6 in late August. Based on mark-recapture, we also estimated rat density fluctuations in the embayment forest between 20.4 and 42.9 rats ha-1 within one month in 2015, and a much lower rat density on the coral plateau fluctuating between 0.76 and 6.08 rats ha-1 in the span of two months. The causes for the short-term density fluctuations are poorly understood, but as eradication operations on tropical and subtropical islands become more frequent, it will be increasingly important to understand the behaviour and ecology of the invasive species targeted to identify times that maximise eradication success.
The Rangitoto and Motutapu pest eradication - a feasibility study.
Available Online

Griffiths, Richard.

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Towns, David.

2008
The eradication of the seven remaining animal pest species remaining on Rangitoto and Motutapu was announced by the Prime Minister and Minister of Conservation in June 2006. With stoats, cats, hedgehogs, rabbits, mice and two species of rats spread across an area of 3842ha, the proposed project is the most challenging and complex island pest eradication the Department of Conservation (DOC) has ever attempted. To better understand the scale and complexity of the project, a feasibility study was undertaken. This study considered the ecological, economic and social context of the project to allow an informed decision to be made on whether or not to commit resources to further eradication planning. This document outlines the findings of the feasibility study and concludes that while a number of contingencies exist within the project, the proposed eradication is not only feasible, but has many significant benefits. No single precedent exists on which this project can be modelled and information from a wide range of sources has been required. Previous eradication and control programmes have been reviewed in conjunction with what is known about the behaviour and biology of the target species. In some cases, where information has not been available and could not be inferred, trials have been undertaken. The document has been reviewed by a number of experts both within New Zealand and overseas including DOC’s Island Eradication Advisory Group. Consultation has also been undertaken with all of the islands’ key stakeholders and communities of interest. Comments from all of these parties have been reflected in the report. Rangitoto is an iconic Scenic Reserve located just 9km from downtown Auckland City. The island is internationally significant both for its ecology and geology and is an extremely popular visitor destination served by regular ferry services. Motutapu, a Recreation Reserve, is connected to and positioned immediately to the east of Rangitoto. The island, currently managed as a pastoral farm, is noted for its extensive archaeological record but retains a diverse range of habitat types and is the focus of a community-led restoration programme. In addressing the question, ‘can it be done?’ particular attention has been paid to mice, ship rats, hedgehogs and rabbits as eradication of these species on the scale of Rangitoto and Motutapu has never been attempted. Preventing reinvasion on such highly accessible and intensively visited islands is also an enormous undertaking. It is accepted, that of all the target species, mice present the greatest risk of failure. However, while a number of mouse eradications around the world have failed, all ten attempts on islands beyond the swimming range of mice that have followed current Departmental best practice have been successful, providing confidence in the method. Rangitoto and Motutapu are a significant step up from previous operations in terms of scale but are also the logical next step to apply current techniques. Despite the unprecedented elements within the project, it is considered that the key dependencies on which eradication success relies can be met for the species targeted. Preventing reinvasion is perhaps the most important consideration of the feasibility study and the one that will ultimately determine the fate of the project. Achieving an adequate level of protection for the islands hinges heavily on changing the behaviour of all 100,000 visitors that arrive on an annual basis. Without this any investments made in removing pests will be wasted. Bringing these changes about appears possible but is contingent on a number of commitments and actions that must be put in place by both DOC and its key partners. The feasibility study also addresses what the project will take to complete and attempts to identify as many of the planning issues as possible to enable the project to be properly sized. It explores the techniques that must be used, the resources that will be required and the timeframe over which they need to be deployed. While this eradication project is the most challenging and complex to be undertaken by DOC, it also presents a significant opportunity to improve our current understanding of eradication theory and practice. If successful, the project offers outstanding benefits for conservation. The recovery of locally and nationally endangered species, the creation of a stepping stone for wildlife movement between the Hauraki Gulf and the Auckland isthmus, the potential for advocacy and education, and increased recreation and economic opportunities are just some of the likely gains. The study has shown that this project is feasible, but also that there are many significant reasons why it should proceed.