The density of indigenous invertebrates on three islands in Breaksea Sound, Fiordland, in relation to the distribution of introduced mammals

Abstract

The density of indigenous invertebrates was sampled on three islands in Fiordland which have similar soils, vegetation and climate, and presumably also originally had similar invertebrate faunas. Resolution Island (20,860 ha) now supports stoats and deer, and Breaksea Island (150 ha) has Norway rats; Gilbert Island No.6 (20 ha) is still completely free of mammals. Compared with Gilbert No.6, the densities of 13 groups of invertebrates were lower on Breaksea, and the densities of 2 groups were lower on Resolution. We interpret these differences to be the result of disturbance and predation by mammals introduced within the last 200 years.

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“…Its freedom from rats is considered to account for the high diversity and abundance of invertebrates recorded there, particularly the larger ground-dwelling species (Barratt et al, 1986). Similar results were also recorded on the small (20 ha) Gilbert No.6 Island in Breaksea Sound 40 km to the south, which remains free of all introduced mammals (Bremner et al, 1984). It is also important to note that modification of Secretary 1.…”

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“…When the density of invertebrates was estimated on this, and a nearby smaller island (Gilbert Island No. 6,20 ha) which was free of all introduced mammals, it was found that all taxonomic groups, except small weevils, had declined (Bremner et al 1984). The authors' survey of indigenous invertebrates, which were heat-extracted from litter, moss and rotten wood, showed that for every 100 invertebrates caught on Breaksea Island, 173 were found on Gilbert No.…”

“…6, but only fragments on Breaksea, suggesting that this species was recently present there but could no longer be found alive. Bremner et al (1984) concluded that 'predation by Norway rats is having a devastating effect on the population density of many invertebrate groups on Breaksea Island' and that the most vulnerable are large beetles, weta, harvestmen and 'some unexpectedly small species'.…”

“…They were recorded in Bremner et al (1984) study cited above, with 52 weta counted per 100 m 2 on Gilbert No. 6 compared with 11 per 100 m 2 on Breaksea Island.…”

Direct and indirect effects of rats: does rat eradication restore ecosystem functioning of New Zealand seabird islands?

“…Whitaker 1973Towns and Daugherty 1994;Cree et al 1995), large invertebrates (e.g. Ramsey 1978;Bremner et al 1984), and mammals (e.g. Burbidge and Manly 2002) as a result of predation by rats.…”