Home ranges and interactions of kiore (Rattus exulans) and Norway rats (R. norvegicus) on Kapiti Island, New Zealand

Two species of rat (kiore or Pacific rat Rattus exulans and Norway rat R. norvegicus) coexisted on Kapiti Island (1965 ha) until 1996, when they were simultaneously eradicated. I radio-tracked rats of both species from June 1996 to September 1996, when the first of two aerial poison drops occurred. The aim of the study was to describe the home-range parameters of both species of rat in an area of grassland where they coexisted. Radio-tagged kiore occupied overlapping home ranges that varied from 26 to 89 m in diameter. Norway rats occupied larger home ranges (218–916 m in diameter), which overlapped the home ranges of both other Norway rats and kiore. Jacobs’ indices of cohesiveness indicated that kiore and Norway rats might avoid each other, despite the fact that their home ranges overlapped. Patterns of habitat use suggest that kiore prefer denser habitats than Norway rats. Differing habitat preferences, possibly driven by higher predation risk for kiore in open habitats, might be sufficient to explain coexistence of rodents on Kapiti Island. Further research is needed to elucidate the relative importance of diet, home range use, and predation risk in permitting multi-species assemblages in New Zealand.