Home range and population density of black rats (Rattus rattus) on a seabird island: a case for a marine subsidised effect?

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.