The feasibility of eradicating black rats Rattus rattus from the outer Chagos Archipelago

EXECUTIVE SUMMARY This feasibility study assesses whether black rats, Rattus rattus, can be eradicated from 34 islands, comprising some 1735ha, of the Chagos Archipelago, British Indian Ocean Territory (BIOT), in the central Indian Ocean. The archipelago has c. 300,000 pairs of breeding seabird comprised of 18 species, of which ~ 94% nest solely on rat-free islands. There are 10 designated and two proposed IUCN Important Bird and Biodiversity Areas (IBAs) and numerous strict nature reserves and other protected areas. However, outer islands of the Chagos Archipelago does not meet anywhere near their full ecological and environmental potential due to the presence of black rats on 34 of the 55 islands that form the terrestrial landmass. Invasive black rats should be eradicated in order to improve the archipelago’s biological value. The proposed operation would remove rats from 1735ha, or 83% of the total land area (2,100ha), meaning the entire outer Chagos Archipelago would be rat-free. After an unsuccessful attempt to eradicate rats from 263ha Eagle Island on the western Great Chagos Bank in 2006, a subsequent operation in 2014 successfully eradicated rats from Vache Marine (13ha), and two islets in the Salomon atoll. An archipelago-wide rat eradication operation is now proposed which would be the largest rat eradication carried out on wet-tropical islands to date. This study concludes that the eradication of rodents will have significant ecological benefits for the archipelago but presents numerous challenges, mainly due to its isolation. There is no guarantee that a rat eradication will be successful on all the islands, especially as tropical islands have had a slightly higher failure rate than for temperate sites (Russell & Holmes 2015). However, with appropriate focus on pre-operation trials, due care with operational execution, and effective biosecurity across the 34 target islands, the likelihood of a successful eradication appears high. It is suggested that aerial application of rat toxin is the only method likely to achieve success and has a history of being carried out on similar islands elsewhere. An operation would be contingent on chartering a suitable ship and the use of ship-borne helicopters, with a voyage duration of up to two months. The conclusion of this study is that with adequate planning and implementation and a postmonitoring programme with the ability to deal with unsuccessful islands, the eradication of black rats is feasible. Outcomes can be sustained through the implementation of effective island biosecurity, which will need to be in place well before the operation proceeds. The estimated cost for completing the planning and implementation of the eradication is US$4,876,530 (including 20% contingency). The cost of on-going biosecurity for the island will be determined when updating the existing interim biosecurity plan.
Harper Grant , Carr Pete , Havery Sarah , Pitman Helen .