Response of seedling communities to mammalian pest eradication on Ulva Island, Rakiura National Park, New Zealand

Norway rats (Rattus norvegicus) were eradicated from Ulva Island, Rakiura National Park, in 1996. The aim of our work was to determine if seedlings and saplings increased in density and/or species richness following this eradication. In 2003, we took advantage of eight permanent plots (5 × 5 m) that had been established on Ulva Island in 1991, by counting seedlings and saplings of woody species, including tree ferns. Over this period, total numbers of woody seedlings (< 30 cm tall), and saplings (30 cm – 2 m tall) did not increase significantly (P > 0.05). However, seedlings of two species, Dicksonia squarrosa and Pittosporum colensoi, and saplings of one species, Coprosma colensoi, did increase in numbers significantly. Little change in the species richness of seedlings or saplings was recorded. The removal of Norway rats may have been partly responsible for the recorded increases in density, though the eradication of white-tailed deer (Odocoileus virginianus borealis) from Ulva Island approximately two decades prior to this work is an important confounding factor.