The eradication of introduced Australian brushtail possums, Trichosurus vulpecula, from Kapiti Island, a New Zealand nature reserve

Abstract

Kapiti Island is a nature reserve of 1965 ha, lying off the west coast of the North Island of New Zealand about 50 km north of its southern extremity. It is famous for its abundant and readily observed birdlife and has a variety of species now rare on the mainland. The island also houses the last viable population of the little spotted kiwi Apteryx oweni, the smallest of New Zealand's three species of ratites. The browsing of Australian marsupial brushtail possums Trichosurus vulpecula introduced to Kapiti in 1893 seriously threatened the island's vegetation and wildlife. Possum numbers had been reduced by trapping at various times during 1920–1968. In 1980, a programme began involving several Government management and research agencies, which resulted in the eradication of possums from the island by the end of 1986. The island was divided into blocks, each covered by a network of individually identified and mapped tracks. A combination of trapping, aerial poisoning and trained dogs was used to eradicate possums systematically from each block. During the seven years of the programme, about 21 000 possums were killed in about 1 399 000 trap nights, 4500 hours of searching with dogs, and aerial poisoning of 330 ha of steep cliffs. Details of the eradication programme are described, its costs and benefits, and comments are made on the reasons for its success, and lessons for future eradication attempts.