Best practice guidelines for rat eradication on tropical islands

Abstract

a b s t r a c tInvasive vertebrates are a leading cause of extinction on islands and rats (Rattus spp.) are one of the most damaging to island ecosystems. Methods to eradicate rats from islands are well established and there have been over 580 successful eradications to date. Increasingly, rat eradications are being implemented on tropical islands, a reflection of the need to protect the threatened biodiversity in the tropics. Yet rat eradications on tropical islands fail more frequently than those in temperate climates. In an effort to identify the main reasons for the lower success rate on tropical islands and possible solutions, a workshop was convened with 34 experts in rat eradication, tropical rodent and island ecology and toxicology. The workshop focused on projects using aerial broadcast of brodifacoum, a 2nd generation anticoagulant, because this approach has provided the highest success rate for eradicating rodents from islands. The workshop participants reviewed previously identified challenges to successful rat eradications on tropical islands including increased insect and crab densities resulting in competition for bait, year round or unpredictable timing of breeding rats and increased or unpredictable availability of alternative, natural foods. They also identified a number of new, likely reasons for the lower success rate on tropical islands and provided recommendations for how to address these risks in the planning and implementation of rat eradications. While the success rate of aerial broadcast rat eradications in tropical environments is quite high at 89% (n = 47), it is hoped that by following the recommended best practices provided in this paper, future success rates will be closer to the 96.5% (n = 116) demonstrated for aerial broadcast rat eradications on temperate islands.

Wild Pigs

“…To date, 644 successful eradications of rats on islands worldwide have been reported (DIISE Partners 2016), and the great majority of these have involved roof, Norway, and Polynesian rats. In general, successful rat eradications from tropical islands (89%) have been somewhat lower than successes in temperate islands (96%; Keitt et al 2015). Some suggestions on why the lower success rate on tropical islands include (1) increased crab and insect densities resulting in competition for bait, (2) year-round and unpredictable breeding by rats, and (3) increased or unpredictable availability of alternative, natural foods (Keitt et al 2015;Holmes et al 2015b).…”

“…In general, successful rat eradications from tropical islands (89%) have been somewhat lower than successes in temperate islands (96%; Keitt et al 2015). Some suggestions on why the lower success rate on tropical islands include (1) increased crab and insect densities resulting in competition for bait, (2) year-round and unpredictable breeding by rats, and (3) increased or unpredictable availability of alternative, natural foods (Keitt et al 2015;Holmes et al 2015b). documented the attempted eradications of FIGURE 10.7 EPA-approved label for a rodenticide designed for invasive rodent eradication on islands.…”

“…In some cases, what appeared to be failed eradications may have resulted from rapid reinvasion by rats from nearby islands, suggesting the need to eradicate rats from groups of islands as an eradication unit (Savidge et al 2012). Genetic analyses of rats before and after eradications are often necessary in helping sort out the issue of reinvasion versus failed eradication (Keitt et al 2015). Numerous islandwide rodent eradications are underway or being planned.…”

“…When using rodenticides, a variety of considerations and mitigation measures are employed to avoid, reduce, or minimize nontarget hazards and environmental impacts; some include the rodenticide type, amount (Pott et al 2015), formulation, method and timing of baiting (Keitt et al 2015), captive holding and later release of some nontarget wildlife species until after the baiting operation, removal of rodent carcasses to prevent secondary poisoning, and avoidance of bait placement in aquatic systems (Witmer et al 2007b). In general, impacts to nontarget species during invasive rodent eradications should be considered in terms of populationlevel effects, rather than the effects to individuals, and the bene ts should outweigh the costs of the project implementation.…”

“…An eradication attempt that is 99% successful can ultimately result in 100% failure due to the high reproductive potential of the remaining rodent population. Because of the large commitment of resources and usually public funds in eradication efforts, the potential for failure should be minimized, and there have been a number of factors that have been identi ed that commonly in uence whether a rodent eradication attempt is successful (e.g., see Keitt et al 2015;Holmes et al 2015b). Planning and implementation components include: …”