Department of Conservation staff have been making a last-ditch attempt to save a colony of flax snails on the remote island of Motuopao, off Cape Maria Van Diemen.
The snails, thought to be a sub-species of Placostylus ambagiosus, number less than ten individuals. Their existence has been under threat by the native rat, kiore, which infests the 30-hectare island. Conservation staff made two trips to Motuopao in October to poison the rats, and believe they have succeeded in destroying more than 90 per cent of the population. Gut analyses of rats on the island indicated that they were eating seabird chicks as well as snails. With the rats’ removal, populations of such birds as the black-winged petrel, may be expected to increase.
Placostylus is one of two groups of large-sized land snails native to New Zealand (an example of the other is the kauri snail). Three species are recognised at present: P. bollonsi, which is found only on the Three Kings Islands, P. hongii, which occurs between the Bay of Islands and Whangarei, and has its most vigorous populations on the Poor Knights Islands, and P. ambagiosus, which clings to the extreme tip of Northland, between Cape Maria and North Cape.
These snails have a distinctive appearance: 75mm high, tall spired, and, when alive, with a rich brown exterior contrasting with an orange shell aperture. However, most specimens encountered are bleached white in the harsh sand dunes of Spirits Bay, Cape Maria, Tom Bowling Bay and elsewhere.
Dating suggests that they have been wiped out over the last 6000 years as sand has invaded flax and forest. More recently the assault has quickened, led now by hungry and careless vertebrates. Pigs and rats have devoured most, while the feet of cattle have trampled others and destroyed habitat. It is unlikely that more than a few hundred live P.ambagiosus remain, most within the confines of the North Cape Scientific Reserve.
The idiosyncracies of these beasts do not aid their survival. Devoted vegetarians, they seem to frequent the environs of just a few broadleaved plants: flax, native pepper, hangehange, karaka, puriri, etc. Not moving further than 10m/ year (and only on rainy nights) they give a new dimension to the word ‘sluggish’. Should disaster strike their small copse, there is no way that they would make it to the next one.