Invasive earthworms reshape Huron Mountains forest floors

A survey of invasive earthworms has identified three sharply contrasting growth patterns, distinguishing a large soil-mixing species from smaller leaf-litter feeders.

These contrasts help explain faster leaf decomposition and shallower forest floors, changes that can hinder native seedlings.

Worms in Huron Mountains

Field teams collected earthworms from the forest floor in Michigan’s Huron Mountains, on the Upper Peninsula near Lake Superior.

Xiaoyong Chen of Governors State University in Illinois weighed and measured every specimen, revealing that three invasive species develop in distinct ways.

Chen’s samples showed that one species remained very small, another was intermediate in size, and the largest was notably heavier.

Identifying which worms become largest, and where they occur, can clarify why certain woods lose their leaf layer whereas others retain it.

Three invaders compared

Beneath mixed tree canopies, the researchers found Dendrobaena octaedra, Aporrectodea longa, and Lumbricus terrestris occupying the same forest floor.

For the 2025 study, they recorded the length and dry weight of each earthworm before examining the relationship between the two measurements.

Within these plots, Lumbricus terrestris averaged roughly 6.1 cm in length, whereas Dendrobaena octaedra remained close to 2.5 cm.

This variation in size points to differing energy demands and survival approaches, potentially allowing all three species to coexist.

Different bodies, different digging

Body size affected more than simple measurements: it also influenced where the worms operated and what they disrupted.

The deep tunnels created by Lumbricus terrestris draw fallen leaves downwards, mixing fresh organic matter into lower layers of soil.

Closer to the surface, Dendrobaena octaedra remains within the leaf litter, accelerating the fine-scale breakdown of leaves and twigs.

Aporrectodea longa works between these two extremes, moving through several layers and allowing a single woodland to undergo multiple forms of soil change.

Worms remove forest litter

Across many northern forests, a deep layer of fallen leaves normally shields seeds, retains moisture and sustains microbes during winter.

A 2019 review outlined how earthworms eat leaf litter: the fallen needles and leaves lying on the surface.

After this layer has gone, roots are exposed to bare mineral soil, while new soil layers may develop nearer the surface.

Without this protective cover, sunlight and wind dry out the ground more quickly, making it harder for native seedlings to take root and remain moist.

Nutrients drain away

As worms mix and compact soil, they can alter rainwater flow and allow dissolved nutrients to be carried beyond the root zone.

A 2015 study tracked a Minnesota forest as earthworms spread and recorded 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.

The loss of these nutrients from topsoil can leave plants with fewer resources and increase nutrient runoff into lakes and streams.

How invasive worms spread

People often enable the initial spread, as worms can be transported in discarded soil, potted plants, compost and fishing bait.

A 2024 analysis identified 70 alien earthworm species in North America, with many occupying broader ranges than native species.

Many northern forests were without earthworms for thousands of years after glaciers receded, meaning that arrivals could rapidly transform soil ecosystems.

When a small number of species have become established, subsequent introductions can build upon them, making the effects increasingly complex.

Predicting worm invasion

Measurements of body size allowed researchers to estimate the overall weight of worms in an area, even where time for digging was limited.

These calculations relied on a length-weight relationship-a mathematical connection between body length and mass-developed separately for each species.

Tree composition and local variations in moisture can shift growth curves upwards or downwards, possibly accounting for differences among sites.

More reliable growth data can help managers forecast which invasive worms will spread and persist rather than relying on a brief visual assessment.

Protecting worm-free forests

Management begins with mapping, since eliminating every earthworm throughout a forest is seldom realistic or feasible.

Separated areas may nevertheless remain worm-free when water or distance prevents a continual flow of new introductions.

“In the UP, around the Huron Mountains, 70% of the land is occupied by invasive earthworms,” said Chen.

Prevention is therefore the principal tool, including cleaning equipment, reducing soil movement, and informing anglers and gardeners.

How mustard flushes worms

Small teams can still remove worms from a specific location, helping them monitor species and assess possible control measures.

Scientists on the WormWatch site described applying mustard water to soil and then gathering the worms that emerge at 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.

The technique is useful for sampling or very small-scale removal, but forests are far too extensive for it to be used at scale.

What comes next

By connecting body size with the ways worms behave in soil, Chen’s team has given managers a more precise focus than earthworms generally.

Further field research could combine these size-based tools with local mapping to slow new introductions and safeguard the remaining worm-free places.