Individual study: Control of brushtail possums Trichosurus vulpecula, to slow the decline of Powelliphanta land snails in Nelson and Marlborough regions, New Zealand
Walker K. (2003) Recovery plans for Powelliphanta. Land Snails 2003-2013, 2003-2013
Large carnivorous land snails in the genus Powelliphanta are one of New Zealand's most spectacular invertebrates and their shells can grow up to 10 cm in diameter. They feed on native earthworms and other small invertebrates. Like many other large flightless invertebrates, their populations have been decimated by introduced mammalian and avian predators such as feral pigs Sus scrofa, rats Rattus spp. and song thrush Turdus philomelos.
In the late 1970s a serious decline was noticed in the Nelson/Marlborough Powelliphanta snail populations and by the 1980s, it was suspected that the brushtail possum Trichosurus vulpecula, introduced from Australia, was a significant predator. Increasing numbers of broken or damaged shells surrounding the snail colonies in beech Nothofagus spp. forests alerted conservationists to this possibility. Shells were damaged in a consistent fashion - yet attack by large native parrots, the kaka Nestor meridionalis and the kea N. notabilis could not be ruled out. However, kaka and kea populations were also in steady decline. Studies were required to implicate possums as the major snail predator and thereby justify control operations to enhance snail recovery.
Predator determination: To determine if brush-tailed possums were capable of feeding on Powelliphanta, a few wild possums captured from near to the wild snail colonies were kept in cages and offered snails as food. Snails were immediately consumed. Damage to the shells matched those of destroyed shells found around the wild snail colonies. Thus possums were identified as the main Powelliphanta predator and possum control programmes were initiated in 1994.
Possum control: As most snails lived in remote and rugged high country, control was principally through aerial distribution of the acute poison 1080, at 5-yearly intervals. Smaller, more accessible sites received annual ground control using a variety of trapping and poisoning techniques.
Monitoring: As individual snail taxa appeared confined to very small areas and because most were in urgent need of conservation management, few sites were left as untreated controls. Snail numbers were therefore monitored in many managed, but only a few unmanaged, sites.
Snail population trends: Snail numbers were usually so low and monitoring so labour-intensive that sample sizes sufficient to determine population trends proved difficult to obtain. However, the few sites which still had high snail numbers present when treatment began (generally low altitude sites on more fertile ground) benefited quickly from the possum control, whether control was on an intermittent or annual basis. Colonies that showed very little response were thought to be in a state of 'predator pit', that being where snail numbers were too sparse to monitor effectively, and the few predators remaining were seeking out snails as target food. However, there are signs that, even in these sites the colonies may eventually recover if given a long enough period and consistent predator control.
For further information see: www.doc.govt.nz>Conservation>Plants-and-Animals>Native-Animals>Powelliphanta-Land-Snail
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