Alien earthworms are invading and harming ecosystems across North America

There’s an invasion happening right under our feet, and most of us have no idea. Across North America, at least 70 imported, alien earthworm species are quietly altering our ecosystems, posing a largely overlooked threat.

That’s where Elizabeth Hadly steps in. She’s the Paul S. and Billie Achilles Professor at the Stanford School of Humanities and Sciences and the senior author of a recent study on this very issue.

“Earthworms tell the story of the Anthropocene, the age we live in,” says Hadly. “It is a story of global homogenization of biodiversity by humans, which often leads to the decline of unique local species and the disruption of native ecosystem processes.”

They built the largest-ever database of these alien earthworms and warn that we need to better understand and manage these invaders.

Earthworms — Friends or foes?

Earthworms are nature’s ultimate recyclers, quietly working their magic beneath our feet.

These wriggly creatures belong to the annelid family and thrive in soil worldwide, except in places that are too cold or dry for their taste.

They come in a variety of sizes, ranging from tiny half-inch worms to some whoppers over 10 feet long (looking at you, giant Gippsland earthworm of Australia).

Most earthworms prefer moist, nutrient-rich soil, where they spend their days munching on organic matter like dead leaves, decomposing plants, and other goodies.

Since the late 1800s, people have brought earthworms to North America from places like Asia, Europe, South America, and Africa.

Sometimes, these introductions have boosted the agricultural economy. Other times, they’ve done more harm than good.

How earthworms do their jobs

What’s cool about earthworms is their ability to transform dirt into fertile soil.

As they eat, they break down organic material and poop it out as castings, which are packed with nutrients that plants absolutely love.

They also create tiny tunnels as they burrow, which helps water and air flow through the soil — essential for healthy plant roots.

These little guys are underground architects and farmers, keeping ecosystems balanced without breaking a sweat.

Alien earthworm invasion consequences

These imported earthworms often consume aboveground leaf litter more aggressively than native species.

This change in the soil’s surface can negatively impact native plants, amphibians, and insects.

In the northern broadleaf forests of the U.S. and Canada, alien earthworms stress trees like sugar maples by altering the soil’s microhabitat.

This shift can set off a chain reaction, making it easier for invasive plants to spread.

Ironically, for a creature we associate with improving soil, some alien earthworms can actually degrade it. They may alter soil properties like nutrients, pH, and texture, leading to poorer quality crops.

Earthworm biology

Earthworms are also biologically fascinating. They breathe through their skin, so staying moist is a must for them.

They don’t have eyes, but they’re sensitive to light and vibrations, which helps them avoid predators.

Speaking of predators, they’re a favorite snack for birds, frogs, and even some mammals. And get this — earthworms are hermaphrodites, meaning each worm has both male and female reproductive organs.

When they mate, they exchange sperm, and each worm can produce cocoons filled with eggs. It’s a pretty efficient way to keep the worm world thriving!

Why alien earthworms are winning

Invasive species of all kinds are a problem, but alien earthworms especially troublesome because they have some tricks up their sleeves.

Unlike most native species, many female alien earthworms can reproduce without needing a male.

On top of that, climate change is opening up new territories for them. As permafrost melts in northern areas void of native earthworms, these invaders find new places to thrive.

Eye-opening numbers

The research team built a database of native and alien earthworms using thousands of records from 1891 to 2021. They found alien earthworms in 97% of the soils they studied across North America.

These invaders represent 23% of the continent’s 308 earthworm species and account for 12 of the 13 most widespread species.

To put that in perspective, in the U.S., only 8% of fish species, 6% of mammals, and 2% of insects and arachnids are alien.

In Canada, the situation is even more striking. The proportion of alien earthworms is three times greater than that of native ones.

Across most of the lower 48 U.S. states and Mexico, there’s about one alien earthworm for every two native species.

“These ratios are likely to increase because human activities facilitate the development of alien species that threaten native earthworm species, a phenomenon still largely overlooked,” says study lead author Jérôme Mathieu, an associate professor of ecology at the Sorbonne.

Why do these alien earthworms matter?

To sum it all up, not all alien earthworms are troublemakers. But their widespread presence and unknown impacts on various ecosystems make them worth paying attention to.

The researchers suggest several solutions, like encouraging the use of native worms for composting and fishing bait, and early detection through regular monitoring and citizen science initiatives.

By shedding light on these little-known dynamics, the study highlights the vital roles earthworms play in shaping ecosystems and how they function in landscapes dominated by humans.

“This is most likely the tip of the iceberg,” says co-author John Warren Reynolds of the Oligochaetology Laboratory and the New Brunswick Museum in Canada.

“Many other soil organisms may have been introduced, but we know very little about their impacts.”

Next time you’re out in your garden or taking a walk in the woods, remember that there’s a whole world beneath your feet that’s more complicated than it seems.

Those wriggly worms might not just be helping your plants grow — they could be part of a bigger story of ecological change that we’re only beginning to understand.

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The study was conducted by researchers from Stanford University, Sorbonne University, and other institutions.

The full study was published in the journal Nature Ecology & Evolution.

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