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Exotic Worms Killing Off N. American Plants

John Pickrell
for National Geographic News
January 2, 2003
 
While most earthworms are credited for their beneficial effects on soil,
recent research suggests that several invasive European earthworm
species could be causing a decline in some North American plants.

A new study suggests that one European invader may be responsible for extirpating close to 30 percent of a highly endangered and unusual goblin fern growing in Minnesota's Chippewa National Forest.

"There is growing concern about invasions of exotic earthworms, especially in [formerly] glaciated regions of North America, where there are few, if any, native species," said Peter M. Groffman of the Institute of Ecosystem Studies in Millbrook, New York.


Few native earthworms exist in the nothern-most reaches of the continental United States. Most species were forced south in the last major glaciation, which ended 10,000 years ago.

While American earthworm species have been slow to recolonize the northern-most reaches of the U.S., European earthworm species, first introduced by colonial settlers, have made themselves at home in the country's northern climes for several centuries.

European worms likely arrived in the mud-packed root-balls of transported plants or the discarded soil used as ballast in ships, said forest ecologist and study author, Michael Gundale, formerly of Michigan Technical University in Houghton. Gundale said invasive earthworm species continue to arrive in the U.S. as fishing bait or as part of composting kits used by gardeners.

Tiny and Unusual

Gundale first became interested in how earthworms might affect rare plants when he was working as a botany technician in Minnesota's Chippewa National Forest.

One of the plants he studied was the rare goblin fern (Botrychium mormo), which is found only in the surrounding Great Lakes region. These tiny and unusual plants spend the majority of their lives submerged in the forest floor, sending a few stunted leaves up briefly during the summer months. Goblin ferns are thought to gain most of their energy from fungi growing underground, instead of from sunlight.

Gundale began to wonder if the invasive earthworms might be a factor in the fern's decline when he noticed that otherwise suitable areas lacking the fern seemed to have thinner forest floors of decomposing plant litter at the surface of the soil. Other scientists working nearby, had determined that earthworms could reduce the thickness of this layer.

In order to test whether or not worms might be linked to fern declines, Gundale—now at the school of forestry at the University of Montana in Missoula—examined 28 woodland spots where the fern had previously been located. He surveyed each site for earthworms and ferns and measured the thickness of the forest floor.

Astounded

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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