SNOWBIRD, UTAH--Exotic earthworm species are wreaking ecological havoc in the northern Great Lakes region, researchers warned here on 7 August at the annual meeting of the Ecological Society of America. In the most heavily affected areas, the invading worms have devastated plants and turned a forest floor once carpeted with leaf litter into bare soil.
Some 18,000 years ago, thick ice sheets killed all the native earthworms in the northern half of North America. Recolonization by native worms has been slow, and there are still no native species in the northern part of the United States and Canada. When European colonists arrived in the East toting plants and seeds, Old World earthworms came along, too--as many as 14 species. Now ensconced in soils around cities and farms, the worms are on the march. And once again, humans--by transporting worms for fish bait and dumping out the extras, for example--are helping them spread.
To see what vanishes when worms crawl in, Cindy Hale, a forest ecologist at the University of Minnesota, Twin Cities, and her colleagues studied the advancing worm front in the Chippewa National Forest in Minnesota. During the last 2 years, the frontier of the worm invasion moved about 10 meters into the forest. The forest floor, in turn, lost about 75% of its plant cover. The invaded areas had only one or two species of forest floor plants, while 150 meters away, the worm-free zones had more than 10. And tree seedlings, usually found in densities of 100 per square meter in the worm-free zone, were almost entirely absent. It's not entirely clear yet how the worms disrupt the ecosystem, but the long-term results could be dramatic, says Hale: "A worst-case scenario is the total extirpation of a whole suite of understory plants, many of which are our most beloved spring flowers, including trilliums and spring beauties."
The findings surprised some scientists. "I've never seen somebody link a decrease in biodiversity to invasion by earthworms," says Mark Hunter, an ecologist from the University of Georgia, Athens. Nor have many other ecologists, says Hale. When she asks forest ecologists if there are worms in the soil they study, she says, "nobody knows!"