Tuesday, April 23, 2024

Trail drying up for native snails

New Zealand has some of the most spectacular land snails in the world, but they’re sliding to extinction and climate change is a major factor, the Department of Conservation has warned.

The latest threat classification report on New Zealand’s carnivorous land snails shows that of the 109 species assessed, 48 have declined in status while only six have improved.

The report’s expert panel has highlighted the urgent need for action to control exotic browsers and predators, protect habitat and address climate change, if many of New Zealand’s giant land snails are to survive.

DOC Science Advisor and leader of the report panel, Dr Kath Walker says if the high rate of decline continues, carnivorous land snail populations will crash by a further 95% over the next few decades.

“Right now, 43 types of carnivorous land snails are classified as Nationally Critical, the final status before extinction,” she said.

Wainuia clarki has gone from Nationally Vulnerable to Nationally Critical due to a rat invasion on its previously secure island habitat on Lake Taupō, while its mainland colonies suffer from predation by thrushes and hedgehogs, and drier forest floor conditions.

Te Paki kauri snail has gone from Nationally Vulnerable to Nationally Critical due to predation by rats and pigs, and increasingly severe and frequent summer droughts, making it harder for these moisture-dependent snails to survive and breed successfully.

Twenty-eight taxa of Powelliphanta, some of the largest, most brightly coloured and beautiful land snails in the world, have declined to Nationally Critical status. 

Dr Walker says climate change is killing Powelliphanta and other snail species by reducing soil moisture in summer.

“High numbers of feral pigs, goats, deer, possums and hares are exacerbating climate change problem by drying, removing and degrading the leaf litter that nourishes snail habitat and their earthworm prey.”

“A warming climate also means rats are invading some Powelliphanta species’ mountain-top homes, which previously were predator-free.

“There is a bright spot; two Powelliphanta taxa in western Golden Bay have improved from Nationally Endangered to Nationally Vulnerable thanks to predator control. Although an increase in feral pig predation has compromised their resurgence, numbers are still higher than before possum and rat control programmes were initiated,” she said.

Dr Walker says for many years DOC has kept the number of possums low in some of the places where possums threaten snail survival.

“More recently, we have trialled ways to keep rat numbers in large remote areas continuously low. However, the steep rate of snail decline has brought to the fore urgent measures such as constructing fences that exclude pigs, deer, goats, hares, hedgehogs, weka, and – where possible – rats from small areas of snail habitat.”

More information on the threat classification system and a list of threat classification reports is available on DOC’s website: New Zealand’s threat classification system.

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