New Zealand Journal of Ecology (2006) 30(3): 321- 333

Habitat use by three rat species (Rattus spp.) on an island without other mammalian predators

Research Article
Grant A. Harper  
  1. Department of Zoology, University of Otago, PO Box 56, Dunedin
  2. Current address: Department of Conservation, PO Box 743, Invercargill

Ship rats (Rattus rattus) were removed from sites on Pearl Island, southern Stewart Island, in 2004 and 2005, to test whether they excluded Pacific rats (R. exulans) or Norway rats (R. norvegicus) or both from podocarp-broadleaf forest. As predators can influence habitat use in rodents, Pearl Island was selected because no mammalian predators of rodents are present. Rats were trapped in two other habitats to clarify rat distribution on the island and to obtain samples for stable isotope investigation of food partitioning within habitats. The experimental removal of ship rats failed, as Pacific rats were found to share forest and shrubland with ship rats. This result contrasted with the restricted distribution of Pacific rats on Stewart Island. Ship rats were ubiquitous, and appear to have been the dominant species in podocarp-broadleaf forest on Pearl Island. The largest species, the Norway rat, was trapped only on the foreshore of Pearl Island, but on Stewart Island it is more widespread. Ship rats and Norway rats were partitioning the coastal habitat by exploiting different food sources. Stable isotopic ratios (δ15N and δ13C) in muscle samples from Norway rats revealed a strong marine signature, suggesting intensive foraging in the intertidal zone. Ship rats trapped in the same habitat exhibited mixed terrestrial and marine sources in their diet. There was little obvious partitioning between ship rats and Pacific rats in forest, except a possible delay in breeding in Pacific rats relative to ship rats. Whether Norway rats select the intertidal zone to forage, or were excluded from forest by ship rats is unknown, but competitive exclusion is likely. Estimated densities of rats were low (2.1–5.1 rats ha-1 in forest, 1.42 rats ha-1 in shrubland) and similar to other New Zealand sites with low soil fertility. Further research will be required to elucidate the roles of food quality, habitat structure and predation in facilitating habitat selection in these species.