Earthworm Invaders

Little-known secret: Almost every earthworm in most of the U.S. came from somewhere else. Native earthworms all but disappeared more than 10,000 years ago, when glaciers from a Pleistocene ice age wiped them out. A few survived further south. But today, virtually all earthworms north of Pennsylvania are non-native.

New earthworms began entering North America as early as the 1600s, with the first European settlers. They crossed over in root balls or the dry ballast of ships. As the British, French, Spanish and Dutch colonized the American continent, they were largely oblivious to another colonization going on under their feet. European earthworms thrived in the upper soils of forests and gardens. Native earthworms, if there were any, remained deeper underground. In the end Europe’s earthworms established an empire that would long outlive any built by its nations. Many of the earthworms most common in gardens today, including the familiar red worm Lumbricus rubellus, trace their origins back to the Old World.

Most gardeners love having earthworms in their soil. But in forests, their invasion has been far more destructive.

It’s an ironic truth—the traits that make earthworms wonderful for gardens are the same ones that make them dangerous for forests. Earthworms stir up the soil, making nutrients more accessible to flowers and vegetables. But plants in forests have evolved other ways to get nutrients from the ground. When earthworms convert them into easier-to-access forms, it can favor invasive plants that later take over the understory. In the process they have also been known to endanger birds and orchids. They can be a threat to humans as well, collapsing irrigation ditches and speeding up erosion.

In this video, SERC ecologists Melissa McCormick and Dennis Whigham, along with Purdue graduate student Yini Ma, uncover some of the dirty little secrets of earthworms beneath the soil. They’ve been joined in the effort by Katalin Szlavecz from Johns Hopkins and Timothy Filley of Purdue. Together, the team tracks how the world below ground is shaping the world above.

They’ve discovered earthworms generally do best in younger forests. The trees there, like tulip poplars and sweet gums, produce leaf litter that earthworms like to eat. They’re much less prevalent in older forests, where litter from oaks, beeches and hickories isn’t as appetizing. That means that by cutting down old forests, humans unwittingly make the ground more vulnerable to earthworms—and to other invaders as well.

But earthworms may redeem themselves in one crucial issue: climate change. Earth’s soils contain 2.5 trillion tons of carbon. As earthworms chew up and churn up the soil, researchers suspect they have some impact on its ability to store that carbon. But whether they are helpful or harmful remains unclear.

When materials decay, they release CO₂ into the atmosphere. Over the short-term, earthworms appear to speed up the process. They break down leaf litter and wood much more quickly, causing microbes to release more CO₂ than they would otherwise. But over the long term, the team suspects they could do the exact opposite. Earthworms excrete some of the soil they consume in tough pellets called casts. These casts take more time to break down—perhaps making it so the carbon stored there stays in the ground longer.

In short, the invading army of earthworms could threaten the planet just as it threatens individual forests. Or, at a critical moment, they could turn into surprising allies.

Learn more!

Find out how earthworms can transform forest soil communities and impact plant growth in the Molecular Ecology Lab.