coexistence

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.

The intermediate disturbance hypothesis has been the focus of considerable analysis in terrestrial and aquatic systems. This model predicts that species diversity will be highest at intermediate frequencies of disturbance. Despite numerous theoretical and empirical analyses, the utility of the model is still the subject of intense debate.

The continued coexistence of ecologically similar species relies on niche separation in space or time. Four similar species of introduced rodent occur in New Zealand, but the mechanism(s) allowing them to coexist in varying species combinations throughout the country is poorly understood. In order to investigate the coexistence of kiore or Pacific rats (Rattus exulans) and Norway rats (R. norvegicus) on Kapiti Island, rats were kill-trapped in exotic grassland and four forest types between 1992 and 1996.