Invading earthworms threaten rare U.S. fern. (Worm Attacks).

In the ecological equivalent of the dreaded Klez Worm burrowing into computers around the world, European earthworms are eating enough leaf litter in North American forests to put a rare fern at risk of extinction.

An unusual study reports that the goblin fern (Botrychium mormo), an elusive species that pokes up from thick leaf litter on a forest floor, has disappeared from 9 out of 28 patches surveyed in Minnesota's Chippewa National Forest. Michael J. Gundale, now at the University of Montana in Missoula, also found that the normal forest carpet of fallen leaves was thin in all nine spots, and in eight of them, the forest floor was wriggling with the earthworm Lumbricus rubellus. In a lab test, these 3-to-4-centimeter-long worms proved capable of reducing a forest carpet to a balding remnant, Gundale reports in the December Conservation Biology.

"This is the first paper that looks at the response of a native plant to exotic, invasive earthworms," says Gundale.

Another chronicler of earthworm invasions, Patrick Bohlen of Archbold Biological Station in Lake Placid, Fla., welcomes the study. Although he and other scientists have studied what earthworms do to soil, "very little research has focused on the effects on plants," he notes.

North America north of a line from Massachusetts to Iowa has no native earthworms, Bohlen explains. Scientists presume that the last big glaciers creeping down from Canada wiped out any wormy ancestors, and southern species haven't advanced far into the territory.

When European settlers colonized the New World, earthworms came, too. Worms could have hitchhiked in soil used for ship ballast or in the root balls of plants. Even today, commercial bait worms escape their fate and take up residence around resorts.

Farmers have traditionally regarded earthworms as their friends because these burrowers aerate soil and can speed the release of nutrients as they eat fallen leaves. Bohlen says that his research shows that worms' effects on soil nutrients can get complicated.

Gundale suspected that earthworms could be quite a shock to a forest that hadn't hosted any for thousands of years. To see how forest plants might react, Gundale revisited sites where surveyors had found goblin ferns during the past 6 years. He found no significant link between disappearances of goblin ferns and the presence of a small exotic earthworm, Dendrobaena octaedra. However, the bigger L. rubellus was indeed associated with the disappearances. In places with this worm, the leaf litter was about half the thickness of the cushion in forest spots with no earthworms. In only 3 of the 11 sites with L. rubellus did the fern persist.

To determine whether worms could actually cause the thinning--instead of just moving into low-litter spots--Gundale raised worm colonies in buckets of leaves and soil in his lab. The earthworms did indeed consume the upper layer of litter and reduce it to castings that mixed in with the soil below.

These observations support the hypothesis that the ferns are dependent on a leafy cushion on the forest floor and "that the removal of this [layer] by exotic earthworms may lead to the extinction of this species," warns Gundale.