By Sharon Oosthoek

It sounds like a bad Hollywood film, but truth can be stranger than fiction. While gardeners love to see earthworms in their soil and eco-conscious apartment dwellers rely on them to compost food waste, most people do not realize that the vast majority of worms in Ontario are invasive species. Furthermore, scientists recently discovered that the earthworm’s ability to decompose organic matter makes it a growing threat to our hardwood forests, including Canada’s iconic maple trees.

The majority of the approximately two dozen species of worms we see today arrived with European settlers more than two centuries ago, in ships’ ballast and agricultural products. (Before that, only two species of worms were in Ontario.)

But the very trait that makes the worms the darling of gardeners everywhere also makes them a menace in Ontario’s hardwood forests. European worms are much better than native species at munching through leaf litter. In doing so, they alter the structure of phosphorus and nitrogen – nutrients on which northern hardwood trees and plants depend – such that they are no longer bound up with organic matter and they leach away with the rain.

A 2008 study of northern Minnesota hardwood forests found significantly smaller growth rings in maple trees from forests that contained European earthworms compared to those from worm-free forests. “Our research would apply to the hardwoods of southern Quebec and Ontario’s maple forests,” says University of Minnesota forest ecologist Lee Frelich, who worked on the study.

While European worms have been here for more than two centuries, according to Frelich it takes roughly 1,000 years for a hardwood forest to adapt to such drastic change. And, as the climate warms, these worms are thriving farther and farther north.

While worms can move five to 10 metres a year on their own, their wide dispersal is believed to be mostly due to fishermen transferring bait from one lake to another. In 2008, Trent University graduate student Stacy Gan found European earthworms on Akimiski Island in James Bay; their eggs probably arrived there in soil on the runners of float planes carrying goose hunters. Before that, worms had not been found farther north than Moosonee.