Impact of feral pigs and other predators on macro-invertebrates, D'Urville Island

The effect of feral pigs and weka on the abundance, composition and biomass of aboveand below-ground macro-invertebrates was assessed every November over 3 years at two sites where pigs and weka, just pigs, and neither species were excluded on Attempt Hill, D'Urville Island. The large carnivorous snail Powelliphanta hochstetteri obscura increased both inside and outside the exclosures, and significantly more so within the'no pigs plus weka' treatment. The main cause of mortality of snails outside the exclosures over the 3 years was predation of adult snails by pigs (73% of 33 shells). Over the same period 34 dead snails were counted on plots inside the exclosures, but the main cause of death (82%) was predation by thrushes (or perhaps hedgehogs) on hatchling and juvenile snails. Counts and biomass estimates of soil macroinvertebrates generally showed significant changes over the 3 years and between sites, but we could detect no effect of excluding pigs or weka. Therefore, although there is clear evidence that pigs and weka eat invertbrates, the temporal and spatial variability in the numbers of invertebrates meant our limited sampling over a short period was incapable of detecting strong treatment effects.