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Earthworms Endangering Rare Northern Ferns
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Earthworms may be icky, but they've always had a good reputation among everyone from gardeners to anglers.
That viewpoint may soon change, thanks to Michael Gundale, who completed his MS in Biological Sciences at Michigan Tech in spring 2002. His work suggests that earthworms are radically changing the forest floor in the northern U.S., imperilling some rare native plants in the process.
Especially at risk is the tiny goblin fern, which is almost always buried in the forest floor, the layer of decomposing litter on top of the soil.
While earthworms may seem as American as fishing for catfish, they aren't native to the northern U.S., including Minnesota, where Gundale conducted the research for his master's thesis. About 10,000 years ago, glaciers pushed any native earthworms southward. Now, the only worms in Minnesota are European immigrants.
The problem earthworm appears to be Lumbricus rubellus, commonly known as the red wriggler. These worms live and feed in the forest floor and are raised commercially, primarily for composting. The northern fisherman's favored bait, nightcrawlers, live deep in the soil and only make the occasional foray up to the surface to eat fresh litter. Crawlers don't have a measureable effect on either the forest floor or the goblin fern.
Gundale studied 28 sites in Minnesota's Chippewa National Forest where goblin ferns had been documented. They also grow in Wisconsin and Michigan, including the Upper Peninsula.
Gundale found that the fern had disappeared from nine of the sites, and that these local extinctions were linked to two factors: the presence of red wrigglers and a thinner forest floor. On average, the forest floor was three inches deep at sites free of the earthworm, while the forest floor areas with the wrigglers was only half that deep.
Gundale's thesis served as the basis for his paper "Influence of Exotic Earthworms on the Soil Organic Horizon and the Rare Fern Botrichium mormo," which was published in the December issue of Conservation Biology. He concludes that the red wrigglers are a serious threat to the goblin fern and suggests that, with their habit of devouring the forest floor, they could endanger other rare fern species that share the same habitat.
So where do the worms come from? Probably from vehicles, says Professor Janice (say Jan-EESE) Glime, Gundale's advisor. "These aren't worms dumped by fishermen," she said. "They probably come in on tires. Egg cases could easily be introduced accidently from a chunk of mud stuck to a tire." Evidence: The sites with the most earthworms tend to be near roadways.
It's unusual for a graduate student, especially a master's student, to develop such a groundbreaking thesis. "It's a new idea," Glime said. "I wasn't even aware of the connection, even though all of the annelids are non-native species." Gundale is pursuing a doctorate at the University of Montana.
"I think it is just the tip of the iceberg," Gundale said. "In the future, I want to see if there is any relationship between exotic plant species and earthworm invasions. In other words, are non-native plant invasions enhanced by the presence of these earthworms?"
His discovery also serves as a cautionary tale on the accidental damage humans inflict upon other species.
"Such innocent things we do," Glime said, "that have impacts we never imagine."
MORE ABOUT THE GOBLIN FERN
The goblin fern, aka Botrychium mormo, goblin moonwort, grape fern and little goblin, is one of a group of tiny ferns known as moonworts. It occasionally grows to three or four inches tall, but is usually not even seen above the forest floor. Because it gets little or no sunlight, scientists speculate that the goblin fern derives some of its energy from an unknown, fungus-related mechanism.
Rare even within its natural range, it has been found in Minnesota, Wisconsin and parts of Michigan, including a few counties in the Upper Peninsula.
The goblin fern is considered a threatened species in Michigan and is a federal "species of concern." For more information and a picture, visit http://www.msue.msu.edu/mnfi/abstracts/botany/botrychium_mormo.pdf
CONTACT INFO: Gundale, mjgundal@yahoo.com |
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