Population dynamics of the ship rat Rattus rattus L. in the Orongorongo Valley, New Zealand

The population dynamics of ship rats (Rattus rattus L.) were studied in podocarp/hard-wood and hard beech (Nothofagus truncata) forest in the Orongorongo Valley, near Wellington, from 1971 to 1998. Rat density (indexed by captures in 116 snap traps over 3 nights per quarter) varied up to five-fold between consecutive years. Fruiting of hinau (Elaeocarpus dentatus) and hard beech did not drive the fluctuations as had been suggested. Average density increased over the first 15 years of the study (from 2.3 +/- 0.3 to 7.5 +/- 0.8 per 100 trap-nights (mean SE)), and then declined; this mirrored a trend in cat density driven by external factors. Breeding was largely restricted to spring (November) and summer (February). Breeding rate (proportion pregnant or lactating) was density-dependent, particularly in older rats. Litter size (based on live embryos and new uterine scars) increased between spring (4.4 +/- 0.4) and summer (5.5 +/- 0.3) (mean SE), but did not vary with density or age. Winter breeding was rare (2% pregnant or lactating in August). Hinau fruitfall did not predict the proportion breeding in autumn (May), or survival in winter-spring (i.e., population change between August and November). Varying predation intensity was the most likely explanation for the population trend. We review other New Zealand studies and question the assumption that ship rat populations are "eruptive", except possibly when predation is eliminated.