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Rats were eradicated in 2005–2006 from the islands of Canna and Sanday, Scotland (total area 1,320 ha). Poison bait was laid from December 2005 onwards and the last rat was killed in February 2006. An intensive period of monitoring over the next two years con?rmed that no rats remained on the islands. Seabirds have been monitored on Canna for nearly 50 years and some species have shown good evidence of recovery since the eradication. Other species have not recovered and this may have been due to mortality caused by food shortages or storm events which have been impacting seabirds in the region. These regional changes in pressures affecting the seabird populations make the interpretation of the impacts of the rat eradication programme much more difficult. Atlantic puffins, formerly con?ned to off shore stacks, have recolonised sites on the mainland of Canna and a count of over 2,000 was recorded in 2016. Manx shearwaters, which had ceased nesting in the monitored colony have made a slow recovery to one or two pairs in 2016. Productivity has also increased from a low of 0.2 chicks per nest in the 1990s to 0.74 in 2017. European shags nesting in boulder colonies were most susceptible to rat predation. One such colony has recovered from 45 nests in 2005 to 75 in 2016 and productivity increased from less than 0.7 chicks per nest to an average of 1.6 following eradication. Populations of shags nesting in cliff locations have shown no recovery or have declined. Mew gulls, which nest along the shoreline, have increased from ?ve to over 30 pairs. Other seabirds, such as common guillemots and black-legged kittiwakes, have shown no clear trends and are probably affected by other factors. Rabbit populations have increased on both islands, reaching an estimated 15,500 animals in 2013 that were causing considerable damage through grazing, erosion, and disturbance of archaeological remains. It is unclear whether the increase in rabbit numbers can be attributed to rat eradication. An intensive control programme has brought the rabbit population under control. While some seabirds have responded positively to the rat eradication, the response of some has been slow and others have not responded, probably as a result of regional pressures on their survival. It is important that monitoring of both seabirds and rabbits continues to track the success of this important seabird colony.

Rat eradication is a highly effective tool for conserving biodiversity, but one that requires considerable planning eff ort, a high level of precision during implementation and carries no guarantee of success. Overall, rates of success are generally high but lower for tropical islands where most biodiversity is at risk. We completed a qualitative comparative review on four successful and four unsuccessful tropical rat eradication projects to better understand the factors influencing the success of tropical rat eradications and shed light on how the risk of future failures can be minimised. Observations of juvenile rats surviving more than four weeks after bait application on two islands validate the previously considered theoretical risk that unweaned rats can remain isolated from exposure to rodent bait for a period. Juvenile rats emerging after bait was no longer readily available may have been the cause of some or all the project failures. The elevated availability of natural resources (primarily fruiting or seeding plants) generated by rainfall prior to project implementation(documented for three of the unsuccessful projects) may also have contributed to project failure by reducing the likelihood that all rats would consume sufficient rodent bait or compounding other factors such as rodent breeding. Our analysis highlights that rat eradication can be achieved on tropical islands but suggests that events that cannot be predicted with certainty in some tropical regions can act individually or in concert to reduce the likelihood of project success. We recommend research to determine the relative importance of these factors in the fate of future tropical projects and suggest that existing practices be re-evaluated for tropical island rodent eradications.

A successful ground-based eradication of black rats (Rattus rattus) was undertaken on the remote, uninhabited Shiant Isles of north-west Scotland over winter (14 October–28 March) 2015–16. The rat eradication was carried out as part of the Shiants Seabird Recovery Project, which aims to secure long-term breeding habitat for protected seabirds and to attract European storm petrels and Manx shearwaters to nest on the Shiants. Throughout the eradication operation, teams were stationed on two of the three main Shiant islands (Eilean an Tighe, Eilean Mhuire), with access to the third (Garbh Eilean) via a boulder causeway from Eilean an Tighe. Bait (Contrac® blocks containing the anticoagulant bromadiolone 0.005% w/w), was deployed in a grid of 1,183 bait stations covering all areas of the islands and sea stacks. Bait stations were set 50 m apart, with intervals reduced to 25 m in coastal areas of predicted high rat density. Difficult areas were accessed by boat and cliff s of ~120 m in height were accessed by abseiling down ropes made safe using either bolted anchors or ground stakes. The team of staff and volunteers worked through difficult conditions, deploying bait and monitoring intensively for any surviving rats using a combination of tools. The islands were declared rat free in March 2018. This ambitious and challenging project has greatly enhanced UK capacity in rodent eradications for the purposes of conservation.

Following the incursion of rats (Rattus rattus) on Taukihepa (Big South Cape Island; 93.9 km²) off southern New Zealand in 1963, and the subsequent extirpation of several endemic species, the New Zealand Wildlife Service realised that, contrary to general belief at the time, introduced predators do not reach a natural balance with native species and that a safe breeding habitat for an increasing number of ‘at risk’ species was urgently needed. Off shore islands offered the best option for providing predator free habitat but there was a limited number of predator-free islands available and most were very small. Eradicating rodents on larger islands to provide a wider range and greater area of habitats was required and hand treating these larger areas using trapping and hand application of toxicants, the only methods available at the time, proved problematic and often impossible. Helicopters had been used to distribute bait for the control of rabbits and brushtail possums in the past but eradication of any particular predator species was considered ‘not feasible’. The development of a GPS-based aircraft guidance system, a suitable bait product, specialised bait delivery systems and second-generation anti-coagulant toxicants changed that. Now islands as large as South Georgia (3,900 km²) have been treated using this method

In winter 2016, the New Zealand Department of Conservation (DOC) eradicated mice (Mus musculus) from the Antipodes Islands located at 49°S 178°E, 760 km south-east of New Zealand’s South Island. Mice were the only mammalian pest species present. They have extensively impacted the abundance and survival of invertebrates, with likely secondary impacts on endemic terrestrial birds and nesting seabird fauna. Public-private partnerships with DOC instigated the project and provided essential financial support. Baseline scientific data for operational planning and outcome monitoring were collected by a research expedition in July 2013 and project planning began in 2014. At the time of writing, this is the largest eradication of mice undertaken where mice are the sole mammalian pest species. Logistical challenges were complicated by a broad range of regulatory obligations. The expedition-style project used a ship to deliver a team and equipment to Antipodes Island where they established camp and remained until the completion of baiting. Bait spread was completed incrementally as weather allowed, comprehensively covering the islands in two separate treatments between 18 June 2016 and 12 July 2016. The last sign of mice was detected 20 days after the fi rst application of bait and the eradication of mice was confirmed by monitoring in late summer 2018. Public engagement was achieved with regular operational updates across multiple platforms and positive media coverage. Non-toxic bait trials accurately predicted some by-kill of pipit (Anthus novaeseelandiae steindachneri) but did not anticipate poisoning of some Antipodes parakeet (Cyanoramphus unicolor) and Reischek’s parakeet (Cyanoramphus hochstetteri). Known pest-free islands were not baited, providing refuge for land birds to mitigate the risk. Fledging success of Antipodean albatross (Diomedea antipodensis antipodensis) chicks was not impacted by the operation and those species that were affected had recovered by summer 2018.

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.

House mice (Mus musculus) were introduced to South Africa’s sub-Antarctic Marion Island, the larger of the two Prince Edward Islands, by sealers in the early 19th century. Over the last two centuries they have greatly reduced the abundance of native invertebrates. Domestic cats (Felis catus) taken to the island in 1948 to control mice at the South African weather station soon turned feral, killing large numbers of breeding seabirds. An eradication programme finally removed cats from the island by 1991, in what is still the largest island area cleared of cats at 290 km2. Removal of the cats, coupled with the warmer and drier climate on the island over the last half century, has seen increasing densities of mice accumulating each summer. As resources run out in late summer, the mice seek alternative food sources. Marion is home to globally important seabird populations and since the early 2000s mice have resorted to attacking seabird chicks. Since 2015 c. 5% of summer-breeding albatross fledglings have been killed each year, as well as some winter-breeding petrel and albatross chicks. As a Special Nature Reserve, the Prince Edward Islands are afforded the highest degree of protection under South African environmental legislation. A recent feasibility plan suggests that mice can be eradicated using aerial baiting. The South African Department of Environmental Affairs is planning to mount an eradication attempt in the winter of 2021, following a partnership with the Royal Society for the Protection of Birds to eradicate mice on Gough Island in the winter of 2020. The eradication programme on Marion Island will be spearheaded by the South African Working for Water programme – Africa’s biggest conservation programme focusing on the control of invasive species –which is already driving eradication projects against nine other invasive species on Marion Island.

Since 1999, the black rat (Rattus rattus) has been eradicated from 14 Italian islands, and eradication is ongoing on a further five islands. Most projects were funded by the European Union (EU) Life Programme. Over the years, eradication techniques have been improved and adapted to different situations, including aerial bait distribution on islands with large inaccessible areas, which otherwise would have relied on a manual bait distribution. A priority list of eradications on islands, which was compiled ten years ago, has been met to a large extent, as rats have been successfully eradicated from many islands of great importance to breeding seabirds. Despite some cases of re-invasion occurring in early projects, advances in biosecurity measures have allowed for eradications on islands where this was previously considered unfeasible due to a high risk of re-invasion. This paper reports on black rat eradication work performed on Italian Mediterranean islands with small villages. We show biodiversity benefits of these programmes, but also qualitatively address socio-economic and health impacts on local communities. Eradication projects have faced new obstacles, due to recent changes in legislation which complicated the application of rodenticides and made it very difficult to get permission for aerial distribution of bait on some of the priority islands.

Sand Island, Midway Atoll National Wildlife Refuge (MANWR), is home to 21% of all nesting black-footed albatross (Phoebastria nigripes) and 47% of all nesting Laysan albatross (P. immutabilis) worldwide. During the 2015–2016 nesting season predation and disturbance by non-native house mice (Mus musculus), here documented for the first time, resulted in 70 abandoned nests, 42 adult birds killed and 480 wounded. In the following nesting season the affected area increased, resulting in 242 dead adults, 1,218 injured birds and 994 abandoned nests. Mouse predation activities triggered a mouse control response to reduce mouse densities in the affected areas using multi-catch live traps, kill traps, and limited use of anticoagulant rodenticides in bait stations. In 2016–2017 we applied a pelleted cholecalciferol rodenticide, AGRID (Bell Laboratories, Madison, WI), at a rate of 20 kg/ha in all affected areas. The purpose of this study was to evaluate the efficacy of using AGRID to reduce mouse density and rate of mouse attacks on nesting albatrosses on Sand Island. Mouse attacks decreased and mouse abundance was reduced following rodenticide applications in the plots treated in December but changes in attack rates in the plots treated in January were not detectable and mouse abundance increased subsequent to treatment. The plots in the December treatments were much larger than those used in January and rainfall rate increased after December. A minimum size of treatment area may be necessary to achieve a reduction in injury rates in albatrosses. No deleterious effects were observed in non-target organisms. The casualties resulting from mouse predation (mostly Laysan albatross) represent a small proportion of the 360,000 pairs nesting on Sand Island. However, the risk to adult breeding albatrosses representing such a large fraction of the global population prompted the United States Fish & Wildlife Service to prioritise mouse control efforts.