Population biology of the ship rat and Norway rat in Pureora Forest Park, 1983-87

Populations of ship rats (Rattus rattus) and Norway rats (R. norvegicus) were sampled over the five years 1983-87 at Pureora Forest Park, by Fenn and rat kill-traps every three months. Fenn and rat traps recorded similar capture rates in comparable habitats, although Fenns caught more heavy and fewer young rats. Ship rats (n = 1793 collected) were more abundant, heavier and larger in native forest, regardless of logging history, than in exotic forest of any age. Young ship rats (age classes 1-3) were most abundant in unlogged interior native forest, and in autumn and winter after summer and autumn breeding. Capture rates declined after peaking in 1985, probably due to reduced recruitment of young rats following lower pregnancy rates in adult females. The irregular annual seasonal cycle of reproduction and abundance observed at Pureora is the same as that described for non-commensal ship rat populations elsewhere in New Zealand and the world. Thirty five of 43 Norway rats collected came from a single trap by the Waipapa Stream, apparently set near a permanent colony. Pregnant female Norway rats were trapped in every season, suggesting year-round breeding. This implies that both species can recover rapidly after control operations conducted at any time of year, but especially in spring and summer. Future research should include manipulative exploration of factors limiting ship rat abundance and Norway rat distribution.