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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

© Andy Holdsworth, University of Minnesota
Research is underway to see if the forest floor will recover once earthworms are removed.

© Andy Holdsworth, University of Minnesota

August 2005

Worming into new territory

Inch by inch, earthworms are gaining ground in the Northwoods – where they are not naturally found.

Sophia Estante


Competing with Ice Age fungi
People move worms farther and faster than they spread naturally
How to halt the squiggling hordes
Soil profile chart

Back in 1996, biologist Cindy Hale took a field trip to the Chippewa National Forest in north-central Minnesota to examine forest floor litter, lift a few decaying logs and look for a few creepy crawlers in a clump of forest dirt.

What Hale and the other plant ecologists and biologists saw surprised them. Normally the ground in these forests has a lush understory of green plants and a thick, soft layer of duff. Here, the ground was somewhat harder; in places it was stripped of low-growing plants and tree seedlings.

The damage hadn't been brought about by grazing deer or fire. To the surprise of all, it was a consequence of exotic invaders – earthworms. "Imagine!" Hale said. "It never occurred to us that worms might be exotic."

"Exotic" is a loaded word, especially when applied to organisms Midwesterners consider among the most common of critters, friends to farmers and gardeners alike. What could be less exotic than the earthworm? But Hale and a number of other scientists are correct: Earthworms are not a natural part of the native fauna in large portions of the upper Midwest and Canada. The earthworms dug up in northern Wisconsin yards arrived only through human introduction.

One of the greatest misconceptions about earthworms, according to Lee Frelich, director of the University of Minnesota Center for Hardwood Ecology, is that they are native in northern hardwood forests. "This is not true in northern Wisconsin, Upper Michigan and all of Minnesota," he said. "All of the earthworm species in those areas are European in origin and arrived with European settlement."

Scientists are finding that, like other exotic species, earthworms may negatively change the environment. "In sites that have been invaded by earthworms for a long time, you can end up with bad erosion problems," Hale said. For years, scientists have observed environmental changes, particularly to pockets of the forest floor, but they were attributed to browsers and grazers like deer, not earthworms.

Lee Frelich, University of Minnesota
Earthworms, friends to gardeners and farmers, are not native to Wisconsin's northwoods and when introduced there, can change the soil structure and forest floor vegetation.

© Lee Frelich, University of Minnesota

"It's clear that earthworms alter plant communities," said Michael Gundale, a doctoral candidate from the University of Montana who has published on the topic of earthworm invasion. "The more complex issue is how they alter these plant communities." Gundale said one possibility is that invasive earthworms change the forest floor (the decomposing layer of leaf litter on the soil surface) and the first few inches of duff (the loose, moist roots and fine soil) by both consuming it and mixing it into the underlying soil, leaving a trail of gummy worm castings. The litter layer is important for both plant life and animal life, particularly for rare fern species like the goblin fern, which completes its entire lifecycle in the litter layer, and for salamanders and small creatures that burrow and find shelter in soft soil.

Competing with Ice Age fungi

About 10,000 years ago, the glacial ice sheets spread over northern portions of North America started to recede, leaving behind boulders, sand and glacial till. In the warmer environment, hardwood forests flourished. Nature's decomposers in the forested areas were fungi and bacteria, which built layers of moist, fine roots just under the soil surface. Scientists want to know how other organisms like earthworms can change that forest community. According to Gundale, earthworms compete with some of the fungi nearly all plant species in deciduous forests need to absorb nutrients from the soil. Moreover, different earthworm species specialize in mixing soil at the surface or wiggling between shallow soil and deeper layers.

Organisms quite beneficial in one location can be destructive in another, Hale explained. This may be the case with earthworms, which are still valuable additions in gardens and compost bins across Wisconsin and Minnesota, turning refuse into nutrient-rich soil and aerating plots of agricultural land. Let out of the garden and into the northern hardwood forests, earthworms slowly spread and strip patches of lush foliage from the forest floor. Where the low plants and duff layers give way to bare soil, Hale and her colleagues find higher worm concentrations. The worms change how nutrients cycle through forest soils, and they change the structure and composition of subsoil.

People move worms farther and faster than they spread naturally

All gardeners take note: Moving soil and transplanting plants or trees can introduce non-native species by moving worms much farther than they would spread naturally during the year. People should only introduce worms in plantings where worms are already present, or in compost piles where the worms will not survive outside the pile, said Andy Holdsworth, a scientist in disturbance ecology at the University of Minnesota.

"City gardeners need not be too concerned, but people who garden in newly developed areas or at their cabins in the Northwoods should do their best to avoid accidentally introducing earthworms with plants or soil," he said.

Terrestrial ecologists in Wisconsin note earthworms are native to the farmlands, savanna and prairie lands in southern Wisconsin, but there is little research to determine which worms were native in formerly glaciated areas.

"Given limited resources and other more pressing research questions, we're not evaluating earthworms as an exotic species, and we're not aware of other similar projects among Midwestern researchers other than these few projects in Minnesota," said Karl Martin, DNR forest ecologist. Martin was aware that Holdsworth has begun examining a few sites in western Wisconsin.

After her pivotal experience in 1996, Hale set out to study the effect of earthworms on sugar maple hardwood forests in Minnesota. She is beginning to study other types of forests as well, such as beech hardwood forests. "Each type of hardwood forest is different," she said. "What's true for one type of forest may not hold true for another type. There may not be the same types of problems in different types of forests."

Although earthworms are found in many areas, different species may be found in each location, and forests showing pockets of worm invasion where the soil and ground cover is bare in some areas may be fine just 50 feet away, given that worms spread slowly over many years.

More than 100 earthworm species have been identified, and 15 species are commonly found in the Midwest and Canada. Some live in the leaf litter layer, some dwell in the top soil layers and some, like night crawlers (Lumbricus terrestris) burrow deeper into subsoil layers. It's relatively easy to distinguish earthworm species by their size, length, color and distance from the tip of the "nose" to the collar (clitellum).

How to halt the squiggling hordes

There are still a lot of things scientists don't know about earthworms. "Right now – this research is in its very early stages – we are looking at why some plants make it after earthworms invade an area and why others don't," Hale said. "One possibility is that the plants are receiving a double hit – they are hit by both earthworms and by grazing deer. So one possible way of controlling the damage earthworms do would be to control deer populations. But we don't know yet."

Hale and her colleagues are also investigating whether or not an infested area can recover after an earthworm invasion: "We want to know, after the worms are there, is there anything that can be done?"

While scientists are searching for answers, anglers, gardeners and landscapers can do their part to help the cause by acting with more caution. Gardeners and landscapers should get their leaf litter, mulches, soil and root stocks locally to avoid transporting earthworms and worm egg cases over long distances.

Anglers unintentionally spread these exotic species, too. Many see so many earthworms for sale where they fish, they think it's okay to dump live bait into the waters or on shorelines when they are done fishing for the day.

"It's not okay," Hale says. "We find more earthworm species near the shores where people fish [in some hardwood forests]. That's where the most destruction takes place." Anglers are asked to be as careful when disposing of unused worms as they are when disposing of unused minnows, leeches and other live fish baits. The proper place for disposal is in the trash.

Hale encourages people to be aware where earthworms may be invasive, take steps to limit their spread, and join in the research to help map where the worms' ranges are spreading. Residents in Minnesota and adjoining areas of northern Wisconsin can help researchers collect data about earthworms through a Worm Watch program, which is also monitoring areas in Canada. To learn more about earthworms and Project Worm Watch, visit Great Lakes Worm Watch.

Sophia Estante is a freelance writer from Madison.