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Earthworms have a pretty great reputation. They help create rich soil in your gardens, they make for a great home-composting system, they’re fantastic bait if you’re into fishing, and sometimes they crawl into a corner of your office and die—which happens to me, like… three times a week!

The problem is, much of North America has gotten along just fine without them... for thousands of years. Across much of the northern US and most of Canada, there are no native earthworms -- most likely because they were wiped out during the most recent glacial period. The ice age didn’t wipe out all North American earthworms.

There’s still nearly a hundred native species in the southern and far-western United States. But they just never really re-established themselves in the northeast. But thanks to human activity, North America now has at least 45 introduced earthworm species, mostly from Europe, that are spreading into these previously worm-free areas.

And while we might be under the impression that worms are beneficial, these introduced earthworms are actually reshaping our forests from the ground up. A study in New York in 2017 found that invasive earthworms dramatically changed the nutrient composition of the forest soils tested there. And weirdly enough, it was by increasing the availability of some nutrients, like calcium, potassium, and magnesium at shallow depths.

Now you might expect releasing nutrients from plant debris back into the soil would be a good thing -- but it isn’t.  Earthworms aren’t exactly built for speed, and for whatever reason, any earthworms that were around before the glaciers wiped them out have been super slow to move back in -- giving these ecosystems time to adjust. Without earthworms, these forest floors have a thick layer of organic material that builds up over years.  This layer helps prevent soil erosion, supports plant germination, and provides habitat for many organisms that evolved to survive in these nutrient-depleted conditions. But non-native earthworm species have spread far and wide.

They don’t travel fast on their own, but humans have introduced them repeatedly over centuries. And the activity of these invaders mixes soil layers, distributes nutrients deeper into the soil, and leaves behind nutrient-dense worm poop.  We might want that in our flower beds and veggie gardens. But native forest plants growing in the understory, near the ground, aren’t used to these conditions, and they get stressed out.

And their effect has been pretty widespread. One paper from 2020 compared 40 studies that looked at soil chemistry changes caused by introduced earthworms in diverse environments. They found that across the board, in addition to changing nutrient concentrations, these earthworms are increasing soil pH and decreasing soil water content.

And it’s not just understory plants. These changes to the soil are messing with some trees, and their natural defenses against pests.  A different 2020 study found that a high population of earthworms decreased concentrations of some defensive chemicals in balsam poplar saplings. Those chemicals would normally help protect the trees from leaf-chewing insects.

Earthworms are causing more direct harm to plants as well. Maybe you don’t exactly think of earthworms as fierce predators, but to a plant, they can be.  In one study published in Biological Invasions in 2016, invasive earthworms devoured or destroyed 73% of the seeds from six important temperate forest plant species. This means that earthworms could be directly influencing the composition of plant communities by disrupting them before they sprout.

Their impacts don’t stop at plants, either.  You might think birds would enjoy having more worms around, because they’re big juicy packages of yum.  But a study of ground-nesting ovenbirds, conducted in the northern US, found that more invasive worms meant fewer ovenbirds. It’s unclear exactly why the presence of earthworms is affecting these birds, but there are a few hypotheses.  One is that predators might be finding their nests more easily, since there’s less built up leaf litter when worms are around, spoiling their camouflage. And if that wasn’t bad enough with all these European earthworms, there’s a second wave of invasive worms native to Asia, and they’re posing a new threat to forests.  These more recent arrivals started showing up in North America in the nineteenth century.

They’re also known as jumping worms, and they are spreading largely thanks to gardening and recreational fishing. Researchers have found that these jumping worms can outcompete the existing earthworm population. Which, when one invasive species can show a different invasive species the door, that’s not a great sign.  Not only that, they alter soil composition in a unique way, even in soils previously invaded by other earthworm species, further changing the soil bacteria and fungal communities.  This adds even more complexity to an already complex issue, since we now know that different species of earthworms can cause unique threats to forest ecosystems.

We’re still learning what effects introduced earthworms have on forests, but they have the potential to be huge.  They could alter the ability of the forests to regulate CO2, decrease water quality, and help other invasive species take hold, among other unknowns.  The good news is that researchers are looking to better understand the patterns of invasion of earthworms, creating models to more accurately predict where they’re heading, which can help forest managers prepare. Unfortunately, once earthworms are introduced to a forest, it’s not possible to stop the spread. That said, we can take steps to try to stop introducing them into new environments.  Like if you’re on a camping trip, one way to help stop the spread is by cleaning your vehicle tires when changing locations, since eggs can hitch a ride.  And if you’re fishing, don’t just dump your bait when you’re done.

Ultimately, we’ll need to learn more about the impacts earthworms are having on North. American forests to best manage their spread. But those innocent little wigglers sure are a lot more trouble than I imagine you assumed!

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