Habitat use by kiore (Rattus exulans) and Norway rats (R-norvegicus) on Kapiti Island, New Zealand

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. At each trap site, vegetation and microhabitat variables were measured and related to rat capture at that site with the aim of identifying and describing the preferred habitat of each species in the presence of the other. The demography and productivity of each species in each habitat was measured to determine the distribution and success of rats over the island. From 12 202 corrected trap nights, 923 rats were caught (391 Norway, 518 kiore, 14 unknown). Kiore were associated with low-growing vegetation in well-drained, flat areas whereas Norway rats were associated with taller vegetation, growing in steeper areas that were poorly drained. There was no significant inverse relationship between species capture rates. The number of kiore captured varied with year, season, and habitat. Kiore were most common in grassland and kanuka (Kunzea ericoides) forest. However, kiore were larger and produced more offspring per female in kohekohe (Dysoxylum spectabile) forest. Norway rat capture also varied with year and habitat, but was unaffected by season. Norway rats reached similar sizes, and produced similar numbers of offspring per female, in all habitats. The coexistence of rats on Kapiti Island appears to be due to the variety of habitats present combined with spatial partitioning of those habitats and the presence of some habitats in which kiore are, at least seasonally, very successful.