Species Profile: Earthworms

On the sidewalk, in the garden, or on the end of a fishing hook, everyone can recognize an earthworm.  What few realize is that many common earthworms were not always in North America; most are from Europe.  Amazingly, the common earthworm was poorly understood until just the last couple decades, and we are learning that a critter with a sterling reputation in gardens can be pretty damaging in our forests.

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.

One common earthworm, the European nightcrawler Lumbricus terrestris. Photo credit: Wikimedia Commons

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.


A self-proclaimed modern day nomad, I was born in the Philippines, grew up in southern California—lived all over really—and now I do research as a graduate student in Indiana and the Pacific Northwest. Professionally speaking, my current area of focus is speciation and ecological genetics and genomics, and a common theme in my various projects over the years is evolution in agricultural systems. As a recipient of a GLOBES—an interdisciplinary program funded by the National Science Foundation (NSF) through the Integrative Graduate Education, Research, and Traineeship grant (IGERT) —fellowship, I have received training which has better enabled me to view and ponder topics without my science hat on. I take great joy in eating, cooking, and experiencing nature through various activities. I love to travel and anticipate many local and international invasivore field trips! Though my research interests do not directly involve the study of invasive species, I have had my fair share of negative encounters with environmentally noxious organisms in the midst of doing field research. I carry around a machete with which to combat my gnarly “Himalayan” foes, and my machete and I have raised more than a few eyebrows. Apart from my personal vendetta against these deliciously juicy pests, I feel that there are great advantages to linking our awareness of the natural world to our culture, and Invasivore is an avenue to do just that. I feel that an increase in general knowledge of invasive species will be of great benefit to the field of invasive species and conservation biology. Knowledge and awareness will lead to action, and action will lead to results!

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