Rodents on islands are known to exhibit differing spatial ecology than is seen in mainland habitats and in the case of invasive rats this may affect their impacts on native species. Ship rats' (Rattus rattus) home range size and population densities were measured on Big South Cape Island/Taukihepa, an island with a dense seabird colony, near South-west Stewart Island. Home ranges for both male and female rats were much smaller than had been recorded for virtually all sites in New Zealand. Female home ranges remained at 0.06 ha through a breeding season whereas male home ranges increased in size and also overlapped with more males later in the season. Ship rat population density ranged from 6.5 ha -1 in December to 36.4 ha -1 by late summer and remained high through autumn. The peaks in measured population densities are among the highest recorded in New Zealand. High populations densities, small home ranges and heavy mean body weights are suggested to be due to high primary productivity attributable to the dense seabird population rather than because of the 'island syndrome'. Further comparisons with other New Zealand islands and mainland sites did not clearly support nor negate the 'island syndrome' in ship rats in New Zealand, although large increases in population densities on Big South Cape Island/Taukihepa did not influence home range sizes.

Den sites of 14 ship rats (Rattus rattus) were located daily during the rat breeding season on Taukihepa (Big South Cape), a seabird island southwest of Rakiura (Stewart Island). In contrast to other New Zealand studies, no arboreal dens were found. Den sites on Taukihepa were in ferns, under logs, in woodpiles, or underground in sooty shearwater (Puffinusgriseus) breeding burrows. The number of times known den sites used was positively related to the amount of leaf litter and woodpiles near the den sites. Overall, 24% of radio-tagged rats were sharing den sites on any given day. While there was considerable individual variation in the number of times den sites were used, female rats tended to reuse den sites more than males. Many rats were found in dens alone, but frequently males and females shared. Occasionally two females and one male denned together, as did two females, whereas males never denned with another male.