Invasive earthworms are rapidly changing northern U.S. forests

UNITED STATES, March 7 — In the Huron Mountains of Michigan s Upper Peninsula near Lake Superior, field crews pulled earthworms from the forest floor. By weighing and measuring each worm, Xiaoyong Chen at Governors State University in Illinois showed three invaders grew in distinct ways. Across the samples, Chen found one species stayed tiny, another landed in the middle, and the largest stood out in mass. Knowing which worm grows largest, and where, helps explain why some woods lose their leaf layer while others keep it.

Under mixed tree canopies, the team found Dendrobaena octaedra , Aporrectodea longa , and Lumbricus terrestris sharing the same ground. In the 2025 article, researchers measured each worm s length and weight after drying, then linked those numbers. On average, Lumbricus terrestris reached about 2.4 inches (6.1 cm), while Dendrobaena octaedra stayed near 1.0 inch (2.5 cm) in these plots. That size spread hints at different energy needs and survival strategies, which can keep the three species living side by side.

Size did not just change a ruler reading, it also changed where each worm worked and what it disturbed. Deep burrows made by Lumbricus terrestris pulled surface leaves downward, which mixed fresh material into lower soil layers. Near the surface, Dendrobaena octaedra stayed in the leaf layer and sped fine-scale decay of twigs and leaves. Between those extremes, Aporrectodea longa moved through multiple layers, so one woodland could experience several kinds of soil change.

In many northern forests, a thick mat of fallen leaves usually protects seeds, holds moisture, and feeds microbes through winter. A 2019 review described how earthworms consume leaf litter, the fallen leaves and needles on top. Once that layer disappears, roots face bare mineral soil, and new soil layers can form closer to the surface. Without that buffer, sunlight and wind dry the surface faster, and native seedlings can struggle to anchor and stay moist.

When worms mix and pack soil, rainwater can flow differently and carry dissolved nutrients away from the rooting zone. A 2015 paper followed a Minnesota forest during earthworm spread and found losses of calcium, magnesium, potassium, and phosphorus. We always hear about earthworms being good for the soil and productivity for plants, but as it turns out, too many of them actually causes a lot of loss of nutrients like carbon and nitrogen, said Timothy Gsell, a microbiology professor at Governors State University. Losing those nutrients from topsoil can leave plants hungrier, and it can send more nutrient runoff into streams and lakes.

Human movement often creates the first jump, because worms travel in dumped soil, potted plants, compost, and fishing bait. A 2024 analysis reported 70 alien earthworm species across North America, many with larger ranges than natives. After glaciers retreated, many northern forests lacked earthworms for thousands of years, so newcomers changed soil life fast. Once a few species establish, later introductions can stack on top of them, and effects tend to grow more complex.

Body measurements gave scientists a way to estimate total worm weight in a patch, even when digging time stayed limited. Those estimates used a length-weight relationship, a math link between length and mass, built separately for each species. Forest composition and small-scale moisture changes can push growth curves up or down, which may explain differences between sites. With better growth numbers, managers can predict which invaders spread and persist, instead of guessing from a quick look.

Management starts with mapping, because wiping out every worm across a forest is rarely realistic or even possible. Isolated pockets can still stay worm-free when water or distance blocks the steady trickle of new introductions. In the UP, around the Huron Mountains, 70% of the land is occupied by invasive earthworms, said Chen. Prevention becomes the main tool, and it can mean cleaning gear, limiting soil movement, and educating anglers and gardeners.

Small crews can still pull worms from a defined spot, which helps track species and test control ideas. On the WormWatch site, scientists described pouring mustard water onto soil, then collecting worms on the surface. When we soak the soil, their mucus membranes get irritated and they want to get out of there as fast as they can, said Gsell. That method works for sampling or tiny removals, but forests cover too many acres for it to scale.

Chen s team linked body size to how worms act in soil, giving managers a clearer target than worms in general. Future fieldwork can combine those size tools with local maps, so people slow new introductions and protect remaining worm-free areas.

The study is published in Biological Invasions.