Exotic Worms Killing Off N. American Plants

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To his surprise, Gundale said he found that the fern had disappeared from 9 of the 28 sites he studied. Those sites that lacked ferns also harbored large populations of the European earthworm Lumbricus rubellus. They also had a surface layer of just 1.5 inches (3.8 centimeters), as opposed to the three-inch-thick (7.6-centimeter-thick) layer common at worm-free sites. A thick fungi-rich forest floor is probably a requirement for the goblin fern.

"I was somewhat astounded…at how consistently L. rubellus had shown up at the extirpated locations," said Gundale.

In order to back up this finding, Gundale completed laboratory experiments with L. rubellus, which proved that the worm was able to decimate the forest floor thickness of experimental containers by 50 percent in just 60 days.

Gundale said his study is the first "to show that exotic earthworms are harmful to rare native vegetation." His findings were reported in the December 2002 issue of the journal Conservation Biology.

Plants such as ferns "are rooted in the forest floor, [therefore] if the forest floor is disturbed or destroyed, it makes sense that some of the plants would decline," said Groffman, of the Institute of Ecosystem Studies. Declines in other plants, such as the northeastern trout lily (Erythonium), have also been anecdotally linked with exotic earthworm invasions, he said.

Earthworms cause basic changes in the structure, biology, and chemistry of soil, said Patrick J. Bohlen, a soil biologist at the MacArthur Agro-Ecology Research Center in Lake Placid, Florida. "Whether that is beneficial or not, depends on the location," he said.

Some exotic earthworms, such as L. rubellus, are examples of species that take advantage of human disturbance of the environment, said Paul F. Hendrix of the University of Georgia's Department of Crop and Soils Sciences, in Athens.

Like many of the world's most widespread species—such as pigeons, rats and cockroaches—these worms thrive under human dominated conditions, and are found wherever European settlers spread, he noted.

For some reason, European earthworms seem more adaptable to a wide range of environmental conditions, than native North American species—which are less cold and dehydration resistant, Gundale said. This may account for their relative success at colonizing northern regions, he said.

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