Since the year 1500, islands have been home to over 75% of known bird, mammal, amphibian, and reptile extinctions. The majority of these have been caused by introduced species, particularly vertebrates such as rats, mice, cats, and ungulates. Arguably, the most damaging vertebrate taxon on island ecosystems is the rodents. Mice and rats have been implicated in around half of all bird and reptile extinctions. Rodents have been introduced now to over 80% of the world’s islands. Over the last 70+ years, conservationists around the world have been working to recover species and island ecosystems from the impacts of invasive species, particularly rodents, developing systematic approaches and techniques that are guided by principles of island invasive species eradication. The eradication of rodents from islands is not only possible, but has been completed on over 600 islands, from small offshore rocks to 400,000-ha South Georgia Island, with hundreds of native species protected from the threat of extinction. Rodent eradication is becoming a mainstream tool used by managers worldwide. However, there are limits to current technologies and approaches, and, globally, we can only reach a relatively small number of islands and threatened island species. To protect and recover threatened species, conservationists must increase the scale, scope, and pace of eradication of invasive species from islands, and focus on innovation of new tools, techniques, and strategies to be allow restoration on larger and more complex islands. One of the biggest challenges to success is ensuring that the public is supportive and allows pest management tools to be used for conservation purposes. New technologies are on the horizon to improve invasive species eradications, including genetic tools and species-specific toxicants.

The removal of introduced rodents from islands can be a highly successful, and thus, important conservation tool for land managers. Non-native rodents have adverse impacts to island ecosystems. Rats alone have been implicated in approximately 40-60% of all bird and reptile extinctions globally. Introduced rodents have been removed from more than 330 islands worldwide using techniques adapted over a ~30-year time period – the use of a bait containing a rodenticide delivered by hand into bait stations or broadcast by hand or helicopter, or a combination of these techniques. Island Conservation has applied this basic approach to remove rodents from islands spanning from the deep tropics to north temperate/sub-Arctic latitudes. Across these regions, the same principle has been applied but the details customized to the local biological and sociopolitical environment. The fundamental considerations during rodent eradication planning have included: 1) delivery of a bait containing a rodenticide into every potential rat territory; 2) delivery at a time of year when the rodent population is food stressed and more likely to eat the bait; 3) minimizing or eliminating, wherever possible, risk to non-target species prone to disturbance and/or toxicant exposure; and 4) ensuring appropriate legal or regulatory compliance and acceptance by local communities. In most cases, the balance between the probability of eradication success and these considerations, particularly risk to non-target species and safe workplace seasonal time periods, are not compatible and may conflict. Thus, adjustments to the eradication plans are sometimes necessary, and are done with careful evaluation of counter-risks to eradication efficacy, non-target species, logistics, or workplace safety. We will use our projects as case studies to demonstrate the approach we take to balance the risks and mitigation, with decisions made in conjunction with our project partners. Comprehensive and transparent planning, with the involvement of experts in the eradication field, land managers, local biologists, regulators, and local community are imperative to overall project success.

Eleven years after invasive Norway rats (Rattus norvegicus) were eradicated from Hawadax Island, in the Aleutian Islands, Alaska, the predicted three-level trophic cascade in the rocky intertidal, with native shorebirds as the apex predator, returned, leading to a community resembling those on rat-free islands with significant decreases in invertebrate species abundances and increases in fleshy algal cover. Rats had indirectly structured the intertidal community via their role as the apex predator in a four-level trophic cascade. Our results are an excellent example of an achievable and relatively short-term community-level recovery following removal of invasive animals. These conservation successes are especially important for islands as their disproportionately high levels of native biodiversity are excessively threatened by invasive mammals.

More than US$21 billion is spent annually on biodiversity conservation. Despite their importance for preventing or slowing extinctions and preserving biodiversity, conservation interventions are rarely assessed systematically for their global impact. Islands house a disproportionately higher amount of biodiversity compared with mainlands, much of which is highly threatened with extinction. Indeed, island species make up nearly two-thirds of recent extinctions. Islands therefore are critical targets of conservation. We used an extensive literature and database review paired with expert interviews to estimate the global benefits of an increasingly used conservation action to stem biodiversity loss: eradication of invasive mammals on islands. We found 236 native terrestrial insular faunal species (596 populations) that benefitted through positive demographic and/or distributional responses from 251 eradications of invasive mammals on 181 islands. Seven native species (eight populations) were negatively impacted by invasive mammal eradication. Four threatened species had their International Union for the Conservation of Nature (IUCN) Red List extinction-risk categories reduced as a direct result of invasive mammal eradication, and no species moved to a higher extinction-risk category. We predict that 107 highly threatened birds, mammals, and reptiles on the IUCN Red List-6% of all these highly threatened species-likely have benefitted from invasive mammal eradications on islands. Because monitoring of eradication outcomes is sporadic and limited, the impacts of global eradications are likely greater than we report here. Our results highlight the importance of invasive mammal eradication on islands for protecting the world's most imperiled fauna.