All the large native land snails and many of the smaller ones are endangered, and numbers fell dramatically in the latter part of the 20th century. They have been threatened by changed habitats, introduced predators (both mammals and molluscs), and possibly by introduced diseases. Human collectors are also a threat, and in 1982 it was made illegal to take Powelliphanta snails and their shells.
There is strong evidence that predation by rats and pigs has taken a high toll on kauri snails, Powelliphanta and flax snails. Hedgehogs and mice may also be implicated. The Department of Conservation has developed a recovery plan for Powelliphanta snails, primarily controlling predators.
In 2006, around 1,500 rare Powelliphanta snails were moved to safety from the Stockton mine area on the West Coast. Sixty had radio transponders and antennae glued to their shells. This allows Department of Conservation staff to track the snails – but it may interfere with mating. ‘The snails climb on top of each other when they mate, and it might disturb them that there’s this bulge where the diode is,’ said scientist Ingrid Gruner. 1
Powelliphanta near the Stockton mine
The Powelliphanta ‘Augustus’ snail (named for its habitat, on Mt Augustus) is only known in a 5-hectare area near the Stockton mine on the South Island’s West Coast. Coal producer Solid Energy plans to strip-mine the area. There was controversy in 2006 when the Minister of Conservation gave the company permission to move the entire snail population to a new location 800 metres away. The expense was considerable, and there has been public debate over whether the snails should have been left undisturbed.
Threat from the garden snail
Cantareus aspersus, the common garden snail, poses a threat to native land molluscs as well as to some rare plants. Unlike native snails and slugs, it eats herbaceous vegetation as well as seedlings. It is highly gregarious, and groups can monopolise plant litter and food resources, as well as leaving mucus and faeces. They can also spread disease and parasites.
Although the garden snail is seen mainly as a pest to the gardener, there is concern that it may spread into sensitive habitats and damage them.