Pacific rat

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 relative abundance of ship rats (Rattus rattus), Norway rats (R. norvegicus), and Pacific rats (R. exulans), was measured in four vegetation types on Stewart Island/Rakiura, over six consecutive seasons. Ship rats were found in all four vegetation types and dominated in podocarp-broadleaf forest and riparian shrubland. Norway rats were most common in subalpine shrubland and Pacific rats dominated in manuka (Leptospermum scoparium) shrubland. Analysis of micro-habitat affinities for the three species showed that ship rats were habitat generalists.

The continued coexistence of ecologically similar species relies on niche separation in space or time. Four similar species of introduced rodent occur in New Zealand, but the mechanism(s) allowing them to coexist in varying species combinations throughout the country is poorly understood. In order to investigate the coexistence of kiore or Pacific rats (Rattus exulans) and Norway rats (R. norvegicus) on Kapiti Island, rats were kill-trapped in exotic grassland and four forest types between 1992 and 1996.