Miles of ice and cold temperatures during the last glaciation (~10,000 years ago) made Canada and the Northern US completely inhospitable to earthworms. Earthworms naturally disperse at a very slow rate (max 10 m/year), and native worms that were confined to the southern US have not moved very far north. What we find now in the previously glacier-covered lands are European species of earthworms transported to North America by humans. Most likely, European worms arrived here through soil used for ships’ ballast or among the roots of plants moved to the new world. Once established in North America, people continued to move worms to new areas both intentionally and involuntarily with plants, in soil, and in clods of dirt stuck to tire treads and horse hooves.
After the glaciers receded from North America, a very unique forest community developed. With no large-bodied decomposers, thick layers of leaf litter (called duff) accumulated and plant and animal species took advantage of this new habitat. When European earthworms encounter these previously earthworm free forests, they chew through the thick litter rapidly with devastating effects on the forest floor. By consuming the duff layer, earthworms alter nutrient cycling, expose bare soil to erosion, and reduce populations of plants and animals using the thick litter layer as habitat. The effects of European earthworms are most apparent in parts of Northern Minnesota where the populations of invasive earthworms advance through forests like an invading army creating a striking visual of bare soil where the earthworms have been and thick litter layers soon to be consumed by the worms. Furthermore, the rare goblin fern is threatened with extinction by invasive earthworms.
Although the presence of invasive earthworms in certain forests is devastating, the story of whether earthworms are good or bad isn’t simple. Like some other invasive species (e.g., Pacific salmon in the Great Lakes) we do get some benefit from these foreign worms. In agricultural fields and gardens without leaf litter, the mixing of soil by earthworms actually improves soil porosity and fertility. Like many other invasives, once worms move from the garden to the forest there is no way to remove them. So the best we can do is slow the invasion by being careful with how we handle soil, plants, compost, and bait buckets that contain earthworms. The story of invasive earthworms doesn’t end here because new earthworms from Asia are being discovered in North America and early evidence shows that these worms are even more voracious feeders than their European brethren. So remember to contain those crawlers!
This guest contribution comes to us from Dr. David Costello, a research fellow at the Cooperative Institute for Limnology and Ecosystems Research at the University of Michigan. Dr. Costello earned his B.S. in Biology from Hobart College and his Ph.D. in Biology at the University of Notre Dame. Currently, his research focuses on how invasive species and chemical contaminants affect the structure and function of ecosystems.