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) we re 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 capt ure rates in comparable habitats, although Fenns caught more heavy and fewe r young rats. Ship rats (n = 1793 collected) were more abundant, heavier an d larger in native forest, regardless of logging history, than in exotic fo rest of any age. Young ship rats (age classes 1-3) were most abundant in un logged interior native forest, and in autumn and winter after summer and au tumn breeding. Capture rates declined after peaking in 1985, probably due t o reduced recruitment of young rats following lower pregnancy rates in adul t females. The irregular annual seasonal cycle of reproduction and abundanc e 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 N orway rats collected came from a single trap by the Waipapa Stream, apparen tly set near a permanent colony. Pregnant female Norway rats were trapped i n every season, suggesting year-round breeding. This implies that both spec ies can recover rapidly after control operations conducted at any time of y ear, but especially in spring and summer. Future research should include ma nipulative exploration of factors limiting ship rat abundance and Norway ra t distribution.