Giant land snails in the lower North Island Ruahine Forest Park are getting a much-needed break from being eaten alive by possums.
The endangered Powelliphanta marchanti snail species, found only in the lower central North Island ranges of Ruahine, Kaweka and Kaimanawa, is in serious decline mainly due to being eaten by possums, rats and pigs.
The Department of Conservation has carried out four aerial 1080 predator control operations over the past 15 years to protect the biggest known population of snails in northern Ruahine, as well as native birds like whio/blue duck, kiwi, kākā, kākāriki and robin/toutouwai. The latest operation was earlier this year.
DOC Ranger Ruby Bennett, who led a team to monitor the snails this autumn, says there were no signs of possum-killed snails in the predator control area.
"We searched for snails in ten plots, each 100 square metres, at sites both inside and outside the predator control area.
"The difference was clear – there were no possum or rat-eaten shells in the predator control area and significantly more live snails.
"In contrast, we found a significant number of eaten shells in the area without predator control. These findings reinforce our long-term monitoring, which shows a steep decline in snails over the past 17 years in the area without predator management."
Snail monitoring is painstaking work and involves carefully searching for snails on hands and knees through leaf litter. Shells with signs of being eaten are sent to Nelson where snail experts verify the predator.
Possums leave tell-tale marks on the shells of snails they eat. They bite near the opening to sever the muscle that attaches the snail to its shell, then pull out the body.
Ruby says snail eating seems to be learned behaviour for possums. By removing them from large areas we get the extra benefit of disrupting this behaviour for possums that reinvade in between predator control operations.
From the 1970s, possum snail-eating behaviour became widespread in many mountain areas where possum-favoured vegetation was scarce. Large rivers, which kept possum populations apart, slowed the spread of this behaviour. In places like the Ruahine Ranges and Kahurangi National Park, it's been proven possible to disrupt possum snail-eating by keeping possum populations very low for a long period.
Ruby says people out tramping or 'naturing' can do their bit to help the snails.
"If you're lucky enough to see a Powelliphanta snail, admire its beauty, take a photo, but please leave it where it is. And don't take empty shells as they provide homes for other smaller snails, spiders and fungi."
