Earthworms Threaten Minnesota Forests

Invading earthworms appear to be causing widespread loss of native forest plant species and affecting the stability of hardwood-forest ecosystems, said Hale, a graduate student in the university's department of forest resources. During the last few decades, European earthworm species have moved into hardwood forests in the northern tier of the United States.

"We have documented significant damage in the Chippewa National Forest and in isolated forest preserves in and near the Twin Cities area in southern parts of Minnesota," Hale noted.

Minnesota's hardwood forests, which developed in the absence of native earthworms after the last glaciers retreated, contain a thick forest floor that serves as a perfect rooting medium for many species of forest herbs and tree seedlings. In the 1800s European settlers arrived, bringing with them European earthworm species in potted plants. European earthworms have been part of the habitats surrounding human habitation ever since.

When earthworms invade a forested area, they consume the forest floor, and herbaceous plant diversity and tree seedling density decrease dramatically. Heavily impacted stands have been observed with only one species of native herb and virtually no tree seedlings remaining.

"Ninety-nine percent of the populations of native plant species normally found in hardwood forests, including large-flowered trilliums, yellow violets, and Solomon's seal, are destroyed in affected areas," said Frelich, a research associate in forest resources. "In many areas, the remaining bare soil is simply eroding away."

Although it is not possible to reverse the continued migration of the earthworms, there are things people can do to help the forests recover.

"It is likely earthworms will eventually spread to all the hardwood forests except those whose soils are sandy or extremely shallow,00" said Frelich, "but we certainly don't want to speed up the process. People have always been told worms are good for the environment, so at the end of fishing vacations they dump the leftover worms near the lake. They don't understand how harmful earthworms can be to a forest."

Replanting native plant species is another way to help forests recover from earthworm damage. Native plants, grown from locally harvested seeds, are now available. A small industry is developing to provide native woodland plant species, a restoration effort similar to what has been happening with prairie plants.

"We're sharing the study results now," said Hale, "so ecologists become aware of the issue, realize that the presence of earthworms is an important criterion when evaluating an area, and begin to understand what their presence means." Hale and Frelich are continuing their research by conducting a formal survey of several Minnesota hardwood forests over the next two years. The third author, Peter Reich, is a professor of forest resources.

For more information on the meeting, visit the Ecological Society of America's Web site at http://esa.sdsc.edu/snowbird2000.htm.