Earthworms are not prepossessing creatures. As a child I used to find them gross and carefully avoided them when I encountered them on sidewalks, hard to do after rains when they seemed to be all over the place. But over time, I came to view them in a positive light, that though they may not win any beauty contests, they were busily working away on our behalf, enriching the ground and making it more fertile, and that their abundant presence was an indicator of healthy soil. But it seems like that entirely benign beneficial façade may be misleading, the product of a good public relations campaign by the earthworm lobby.

One has the impression that earthworms have been around from time immemorial. But that is not the case. It appears that in Ohio, for example, the ones we have now are in fact colonizing oppressors, an invasive species brought over from Europe and Asia within the last few hundred years. The original earthworm residents were largely wiped out by glaciers during the last Ice Age 12,000 years ago. Kathryn M. Flinn, a professor of biology at Baldwin Wallace University and a plant ecologist, says that, like other invasive colonizers, they are opening the door to more invasive species to infiltrate forests.

Small but mighty, earthworms transform the soil of any ecosystem they inhabit. In farm fields, we appreciate the way they recycle nutrients and aerate soil. But in forests, earthworms’ services can be less welcome.

Glaciers removed native earthworms from northeastern North America and the Great Lakes region 12,000 years ago, so the forests that now stand in these regions developed largely without earthworms. Native earthworms have only slowly returned north. Most earthworms living in these forests today were introduced from Europe and Asia – likely arriving via ships’ ballast and soils of imported plants – and they continue to spread from activities that transport soil or by dumping fishing bait.

Introduced earthworms are considered invasive in forest ecosystems because they have many negative impacts, from eliminating the forest floor layer to reducing the diversity of understory plants. In particular, species like nightcrawlers that carry surface leaf litter into deep, vertical burrows have dramatic effects on forest floors. Recently, researchers have proposed an even more nefarious role for earthworms, which interests me as a plant ecologist: They may be hidden drivers of plant invasions.

She says that while the evidence in not completely conclusive, there are strong suggestions that earthworms in forests are having a deleterious effect.

Increasingly, ecologists have noticed that invasive plants tend to dominate areas where invasive earthworms are also abundant. In a recent synthesis of 645 observations across the United States and Canada, researchers found that the cover of nonnative plant species in forests increased with increasing mass of earthworms in the soil.
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So far, evidence suggests that earthworms can indeed facilitate plant invasions. In experiments where ecologists created forest-floor habitats with and without earthworms and added seeds of invasive plants, the resulting mass of invasive plants often, but not always, increased in the presence of earthworms.

She says that the changes brought by earthworms are due to a multiplicity of factors, by what they eat, changing the pH of the soil, mobilizing nutrients, and altering the mix of bacteria, fungi and other microbes. They create a symbiotic relationship with invasive plants, in that the ones they favor tend to produce the kinds of leaf litter they love and thrive on.

So no more Mr. Nice Guy. In future when I pass an earthworm on the street, I shall merely nod coldly.