Snail Mail

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Community ranGer doC tonGariro

The Powelliphanta marchanti - also known as Marchant’s snail, is a giant carnivorous snail found only in the North Island and endemic to Aotearoa. Its shell is a yellowish brown to old-gold colour that averages around 40mm in length. Many New Zealander’s have unfortunately never seen a Powelliphanta snail, however collection of their shells was made illegal in 1982.

The Marchant’s snail was discovered close beside the Rangitikei River and in the Otamateanui catchment of the southern Kaimanawa Rangers in the mid1970’s. It occupies a relatively small, discrete habitat consisting of manuka and mountain beech forest litter.

Powelliphanta snail populations are in decline due to introduced predators such as rats, pigs, hedgehogs, brushtail possums and song thrush. However, possums were determined the main cause of morality for snails here in 1998. Interestingly, the snail-eating behaviour of possums seems to be learnt, and it has been identified that possums on the true left of the Rangitikei river are predating on snails, while those on the true right are not.

In 1999, ground-based possumcontrol operations were initiated with the long-term goal of achieving a stable snail population greater than 12 per 100m2. Possum management has been sustained annually since then, with the goal of keeping possum density below 2% residual trap catch (RTC).

Overlooking the Rangitikei/Otamateanui Confluence – Southern Kaimanawa Ranges. Photo: Candace Graham

DOC Staff from left to right: Stephen Robson, Tim Quinnell, Luke Poulsen, Ian McNickle, Candace Graham, Jo Mendonca, Tim Maule. Not Pictured: Brenda Lawson & Michel Dedual. Photo: Candace Graham

‘Snailers’ moving through the first of 36 snail monitoring plots in 2019. The pink pegs identify where snails have been found. Photo: Candace Graham

To determine whether possum control has a positive effect on snail density or not, around 39 snail monitoring plots were also established in 1999 among the treatment area. These plots are only monitored once every five years, as the long-life span and slow growth rate of the snail mean that the benefits of possum control on the P. marchanti population take a long time to come to light.

In December 2019, nine DOC staff were transported into the Rangitikei/Otamateanui confluence for eight days, to conduct the fifth, five-yearly re-measure of snail numbers.

Each snail monitoring plot measures 10x10m, further divided into 5x5m quadrants labelled A, B, C and D. For each plot, information about the weather, temperature, ground, and leaf litter conditions were recorded. Searchers then spread out along the base line of the plot and moved through it together.

When a shell is found the status of the snail (alive or dead) was recorded, and if dead whether it was intact (presumed dead from natural causes) or preyed upon. Also recorded was the diameter of the shell using Vernier callipers, and in what quadrant and plot the shell was found in.

Raking through the loose soil and leaf litter with their fingers, ‘snailers’ would leave no stone unturned in their hunt for a snail. If you could imagine, patting down every inch of the landscape on your hands and knees for 8 hours per day can be quite an undertaking. Despite the tough working conditions however, the team were enthusiastic throughout, and made somewhat of a competition out of the search that kept them going for the eight long days.

In the end, the results revealed a total of 129 live snails and 92 dead snails found across all 36 plots. Stephen (better known as Nobby), found the smallest live snail of 12.1mm, and the most dead snails. Tim Maule found the biggest live snail as well as the most live snails… Snailed it!

These results are a positive increase from 3.3 snails per 100m2 in 2014, to 3.5 snails per 100m2. The density of dead snails found had also notably decreased since the last measure in 2014, and for the first time ever, there was no evidence of possum predation. This suggested that the possum control being carried out in the Rangitikei area is having a positive effect on the snail population.

There was however an increase of shells found with bird predation, and we still remain very far from achieving the target of 12 live snails per 100m2 .

The Marchant’s snail habitat is drying out, likely due to a combination of climate change/drought and impacts on their habitat by ungulates. The team noted high sika deer densities, and a significant lack of forest undergrowth caused by their browse. The dry and open understorey created by deer reduces the ground moisture and cover essential for snail survival, and the exploration of various deer control methods will be another necessary challenge that we have to face if we have any hope for the future of the forest and these precious taonga.

Jo Mendonca - The Snail Scale. Photo: Candace Graham