The risks, costs and benefits of using brodifacoum to eradicate rats from Kapiti Island, New Zealand

In 1996 an eradication operation against two species of rats (Rattus norvegicus and R. exulans was conducted on Kapiti Island (1965 ha) and its small offshore islands. Trials with non-toxic baits had been carried out to help determine the risks to non-target species, and research was undertaken to collect baseline data for measuring the response of vegetation, invertebrates, reptiles and birds to the removal of rats. Talon 7-20 bait (containing 0.002% brodifacoum) was distributed over Kapiti Island in September and October by helicopter and by hand, while bait stations were used on the offshore islands. Impacts on birds and reef fish were investigated. Although there were non-target bird deaths as a result of the poisoning operation, post-eradication monitoring indicated that the toxin had no deleterious effect on breeding and most losses would be rapidly made up by recruitment of new individuals into the breeding population. There was no evidence that reef fish were negatively affected. The successful removal of rats has apparently resulted in a significantly improved survival rate for stitchbirds (Notiomystis cincta) and saddlebacks (Philesturnus carunculatus). Benefits to other taxa are expected and will be documented as follow-up studies are completed.