The European Invasion
In response to comments over the last few days about that non-native worm, the nightcrawler Lumbricus terrestris: That’s right, some of the most common species of worms that you might turn up in your back yard or at a bait stand are actually non-native. There are plenty of native worms in the United States, but many of them have been displaced by farming and the building of roads and cities. The European worms like the nightcrawler (which probably followed European settlers in potted plants, ship ballast, horses’ hooves, etc.) found our soil to their liking and proliferated. We continue to help the spread of these worms today by, for instance, taking them into the wilderness as fish bait.
For the most part, European worms are good for the soil and a friend to the farmer. However, anytime a native species is displaced and a non-native one takes its place, one wonders about the consequences. The Minnesota story is a perfect example.
Worms are damp creatures and the parts of the world that were under ice during the last Ice Age 10,000 years ago, like Minnesota, do not tend to have native worm populations. The forests in Minnesota evolved without worms, which means that the forest floor is covered with a deliciously spongy duff layer of slowly rotting leaves. That duff layer is crucial to the germination of young trees and tender understory plants.
But now European worms—my beloved nightcrawlers and red wigglers, among others—have moved in to the forest, thanks to the spreading of sod on nearby golf courses, the trucking in of fill, the dumping of fishing bait on the shores of lakes, and the simple fact that a worm cocoon can get lodged in an ATV tire or the sole of a hiking boot. These worms can, and do, consume the entire leaf fall of a forest in a single season. That spongy duff layer is gone, and with it, many of the fragile understory plants and tree seedlings that depended on it for germination.