Partula and the wolf snail

Partula is a group of tree snails from islands in the South Pacific. They are small, often finely banded, and they were once diverse in the way island animals are diverse: a valley could hold a form found nowhere else. During the twentieth century that diversity collapsed. The cause is clear enough to tell plainly, because it is a kind of mistake that still gets proposed whenever a garden seems to have too many snails.

Giant African land snails had been introduced to some of these islands and had become agricultural pests. To control them, programs introduced Euglandina rosea, the rosy wolf snail, a predatory land snail from the southeastern United States and Central America. The wolf snail eats other snails. It was expected to eat the pest. It found the native Partula, which had no history with a fast predatory snail of that kind. Populations disappeared. A number of Partula species are extinct. Others persist through captive breeding, and some have been returned to forest patches where the predator is absent or held down.

The rosy wolf snail is not a villain in its home woods. It is a predator in the food web it came from. Moved, it is a different ecological fact. The same is true of the giant African land snail that prompted the introduction. Two decisions, to import a pest’s relative and then to import a hunter, did damage that neither animal does on its own ground.

The narrow lesson for a garden is enough. Do not import a predatory snail to solve a snail problem. Do not move snails between regions because a forum called them useful cleaners or useful controls. The Partula story is what “useful introduction” can look like after half a century, when the animals that remain are in breeding boxes rather than in the trees.