Environmental Impact Assessment for the eradication of black rats Rattus rattus from the outer Chagos Archipelago

EXECUTIVE SUMMARY This Environmental Impact Assessment (EIA) evaluates the likely impact of a proposed eradication of black rats from 30 islands of the outer Chagos Archipelago, Indian Ocean, by utilising aerial application of brodifacoum-laced rat bait. The project has been justified by the evidence that rats have caused considerable degradation of the terrestrial ecosystem throughout the Chagos Archipelago, along with probable flow-on impacts in the marine environment, through significant predation on the native seabirds, invertebrates and flora over the past 200 years. The background to the eradication operation is laid out, along with an outline of the operational plan to use helicopters to distribute bait, operating from a support vessel. Alternatives to the suggested techniques in the feasibility study, such as ground baiting or using a different toxin, are assessed and rejected. All possible impacts of the operation are then assessed, including impacts on native and introduced species, soil and water, humans, and wilderness values, or because of fuel spills, waste, possible biosecurity breaches or at more distant sites. Assessment of the proposed operation’s likely environmental impacts is undertaken and mitigation actions proposed where possible. The ethics of eradicating rats using a second generation anticoagulant are evaluated, but more humane methods are, like ground-baiting, not feasible on a highly isolated archipelago such as the Chagos Archipelago. There are few non-target species likely to die through primary or secondary poisoning on the islands and there are no population level impacts on non-target species. As brodifacoum is very insoluble in water toxic baiting will have ephemeral and very minor effects on soil, water and vegetation, which will result in the poison being degraded to its constituent components over the course of weeks to months by microorganisms in soil or sediment. What little bait that enters the ocean will be quickly broken up by wave action and disperse. The bait will again be quickly degraded in the marine ecosystem. The operation of two helicopters will cause most of the physical disturbance and noise associated with the eradication, with some minor transitory effects where the few ground baiting sites are. There is unlikely to be extended effects on what few nesting seabirds there may be on rat-infested islands during the operation, but care should be taken if seabird colonies are present. Ship and helicopter operations will be the primary source of atmospheric emissions, along with possible fuel spills. There is little that can be done to reduce the atmospheric emissions to any large degree, but prompt and effective suppression of fuel spills will limit any impact from this source. Management of waste is largely covered by the MARPOL convention as the operation is ship-based, and any non-biodegradable waste generated onshore will be returned to the ship for processing. Current biosecurity procedures will require some additional improvement prior to the eradication proceeding. From its inception this proposed operation has examined the potential environmental impacts and intends to incorporate these factors into future decision processes. Further development of this document will include assigning roles and responsibilities for managing environmental impacts and implementation of suggested mitigation measures. Monitoring, and responding to any environmental impacts, will be an integral part of the eradication programme, in order to maximise the benefits that will accrue from the project. The EIA concludes that the overall predictable impacts of the eradication operation are likely to be transitory and minor in nature, particularly if suggested mitigation actions are acted upon. Negative environmental impacts are likely to be ephemeral, particularly if impact mitigation measures are adhered to. Similarly, non-target species impacts should be very limited and transitory, with rapid recovery from any losses. The anticipated ecological benefits of the operation, due to the complete removal of rats, are likely to be significant. Successful eradication is expected to result in restoration of ecosystem function. It is expected there will be a substantial increase in seabird abundance over time, associated nutrient deposition, improved native forest diversity and structure, increased marine turtle nesting success and numbers, with a corresponding improvement in the diversity and resilience of marine reef species.
Harper Grant , Carr Pete , Havery Sarah , Pitman Helen .