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What could be more natural than an earthworm? As discussed in a previous story (Worming into new territory), Wisconsin Natural Resources, August 2005), at least since the last Ice Age earthworms have not been part of the native mix of invertebrates that inhabited the soils across the upper Midwest. While earthworms in farm country are prized for aerating and tilling farm and garden soils, that simply is not the case in the northern hardwood forests across Minnesota, Wisconsin and upper Michigan.
Normally in these forest soils, fungi and bacteria would more slowly decompose leaf litter forming a light, spongy "duff" layer that allows seedlings and understory plants to grow slowly. Earthworms in these same soils digest both leaf litter and nutrients much more quickly. Their castings make denser, claylike pellets of the light subsoil layers. Where earthworms gain a foothold, fewer soil nutrients are available, the soil becomes heavier, more compacted, and the understory vegetation grows more sparsely, says Cindy M. Hale, research associate and environmental educator at the University of Minnesota-Duluth. Her research during the last ten years shows that different earthworm species live in different soil zones – some near the surface in leaf litter, some in the top soil layers, and some burrow deep into the subsoil. Their combined actions can leave forest soil less able to absorb water or support plant life.
Forest types are not equally vulnerable to earthworm invasion or damage. Hale's work shows that sugar maple forests like those found in the Chippewa National Forest in Minnesota and the Chequamegon National Forest in Wisconsin, are more susceptible. During the last two years, other University of Minnesota researchers examining beech forest soils at Pictured Rocks National Lakeshore in Michigan and aspen/spruce/fir soils at Voyageurs National Park in Minnesota found these different forest types contain different mixes of earthworm species. The maple forests are twice as susceptible to earthworm damage as the beech forests and four times as susceptible as the spruce/aspen forests.
More recent studies suggest that earthworms found in northern forest habitat can disrupt how tiny mycorrhizal fungi interact with plant roots, slowing down the nutrient absorption rate in fine root hairs. Other researchers are expanding these investigations of earthworms in different kinds of forested environments. DePaul University scientists are examining how earthworm disturbance may open the way for invasive plants like buckthorn to expand their range once soil is disturbed. Dennis Burton at the University of Pennsylvania is examining the pathways in which earthworms pave the way for exotic plant spread as native understory plants die back. Projects at the Smithsonian Institution are looking at maple seedling changes, and at the University of Georgia in Athens, researchers are studying how earthworm invasions may reduce food supplies available to juvenile salamanders. At Cornell University, scientists are investigating how the combined effects of worm invasion and deer browsing can cause long term changes to plant composition on the forest floor.
Given that earthworm populations would naturally spread their range at perhaps half a mile in 100 years, how have these worm populations become so mobile? People. The 16 worm species and the dusky slug that now occur across the Great Lakes region are mainly European species that were brought in accidentally as worm egg cases hatched from the soils in landscaping plants, trees, mulch and compost. Unused fishing worms cast aside near boat launches, landings and resorts established beachheads to start new worm colonies. Mail-ordered worms for vermicomposting may inadvertently contain some eggs of other worm species. Several Asian species of the genus Amynthus are spreading along the East Coast and moving westward. These species are especially wiggly and active on a fishing hook and are being marketed as "jumping" worms. They can harm both forest habitats and garden soils and have raised serious problems for some plant nurseries.
How can you help stem the wiggling invasion? Start by educating yourselves and your gardening and fishing friends. Great Lakes Worm Watch provides background information to understand earthworm behavior and identify each species. The site also offers educational materials like fact sheets, posters and brochures to explain this issue to others. A new field guide, Earthworms of the Great Lakes Region, is also available through their website. Worm Watch workshops that train volunteers to look for signs of earthworm invasions are offered periodically through the Great Lakes Worm Watch website. Workshops on worm identification and control have recently been sponsored at UW Stevens Point's Learning, Experiences & Activities in Forestry (LEAF) and the Beaver Creek Reserve.
Second, take steps to avoid introducing earthworms to new areas, especially near forested areas. Dispose of unused fishing bait in the trash. If you are moving small amounts of compost from one area to another and it is practical, freeze compost for a week to kill off live worms and worm egg casings before you use it. For larger amounts, spread the compost thin and let it freeze solid over winter before collecting it for use elsewhere. Also don't transport leaves, compost or mulch long distances to avoid spreading worms from one location to another. When planting wildflowers, trees or shrubs into forested areas, remove soil and rinse the roots in a location away from the woods before planting. And clean the dirt off of ATVs, motorcycles and other vehicles that might hold soil in their treads.
Finally, consider joining or starting a local worm watch program, especially if you live or travel to northern forest country where these worms are not part of the native mix of invertebrates. Again, the Great Lakes Worm Watch program can suggest how to get going to slow the spread of these invasive species.
David L. Sperling edits Wisconsin Natural Resources magazine.