An invertebrate survey of some Pelorus Sound islands with reference to their predator status

Abstract

The view that introduced vertebrate predators have decimated the larger flightless invertebrates in New Zealand was tested by a survey of forest-dwelling, flightless invertebrates on four islands with different predator status. The islands, ranked in order of predator influence, were Maud, Stephens, Chetwode and Tawhitinui with a site on the mainland at Kauauroa Bay for comparison. Insects, slugs and millipedes were sampled by transect observations and pit-trapping over a period of two years. The availability of invertebrates to predators at different times of year is discussed and the data analysed by 3 seasons (summer, December - March; autumn, March - May; Spring, August - September). Generally Maud Island was found to have the highest numbers of individuals and species per sample. Stephens Island, the other "predator-free" site has the next highest numbers followed by Kauauroa Bay, Chetwode and Tawhitinui Islands. This ranking is closely related to the presence of predators and it is argued that tree composition may also contribute to invertebrate status in the area. Both factors are related to human involvement and a brief history of the Marlborough Sounds is presented. A literature review indicates that vertebrates can indeed affect invertebrate populations and that devastating rodent irruptions have occurred in Marlborough.