Public Release: 

Earthworm invasion will change forests

University of Minnesota

MINNEAPOLIS / ST. PAUL--From the East Coast to the Rockies, forests in northern states and adjacent Canadian provinces are being invaded by animals that have never lived there: earthworms. A boon to gardeners, earthworms from Europe wreak destruction on the leaf litter that supports tree seedlings, salamanders, spiders, mice, voles and other species. A new survey by University of Minnesota researchers implicates human activity in the spread of earthworms and strengthens the idea that earthworms significantly change northern hardwood forest understory and that the invasion is in an advanced, but incomplete, stage. Conservation biology graduate student Andy Holdsworth will present the study at 8 a.m. Thursday, Aug. 7, in Room 204, Savannah (Ga.) International Trade and Convention Center, during the annual meeting of the Ecological Society of America.

"Earthworms are ecological engineers," said Lee Frelich, director of the university's Center for Hardwood Ecology and head of a team of researchers studying European earthworms. "They change the leaf litter and the seed bed, and everything else will change, too, from plants to insects, birds and mammals." In addition to Frelich and Holdsworth, Peter Reich, a professor of forest resources, is an author of the study.

The researchers surveyed the understory plant communities, earthworms, soils and tree composition in mature hardwood stands within the Chippewa and Chequamegon (SCHWA-me-gun) national forests in Minnesota and Wisconsin, respectively. In both places, they found that exotic earthworms tended to be found near cabins, resorts, boat landings, roads and campsites.

Looking at three types of earthworms--those that prefer leaf litter, upper soil and deep soil--the scientists noted that plots of land with all three types of earthworms had an average of 50 percent less sugar maple seedling cover and between 10 percent and 20 percent lower plant species richness when compared to plots with few or no earthworms. Where earthworms had invaded, a sedge (Carex pensylvanica) had proliferated. The sedge finds it difficult to grow in leaf litter, but when earthworms have eaten away the litter, the sedge thrives.

"The sedge can become overabundant and hard to get rid of," said Frelich. "We think it likely that after earthworms become established, the new community will be less diverse. For example, ovenbirds nest in leaf litter, so I expect them to decline."

Earthworms are native to much of the western United States and Canada and the southeast quadrant of the United States. But it remains to be seen whether those native earthworms will resist invasion from the European species, Frelich said. The European earthworms include nightcrawlers and other common earthworms found in the northern and eastern United States and adjacent Canadian provinces.

To help slow the spread of the invaders, Frelich offers the following advice: "Don't dispose of any live bait in the woods."


Lee Frelich, 612-624-3671
Paul Moore, University News Service, 612-624-0214
Martin Moen, College of Natural Resources, 612-624-0793

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