Shalaway: The dark, destructive world of earthworms
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If you've never walked on a thick spongy layer of forest leaf litter, blame invasive earthworms. Though too many deer can destroy ground vegetation, earthworms exert an influence on forest ecosystems that few can imagine.
Most of us learned as children that earthworms are good. Their burrows allow air and water to circulate through the soil, and their castings (excrement) enrich the soil. We learned these lessons because they are true, on farms and even in backyards.
But in forests, earthworms are destructive. They destroy the leaf litter on the forest floor simply by eating it.
It wasn't always that way. In pre-Columbian North America, fungi and bacteria were the primary decomposers of the forest leaf litter that accumulates after each growing season. But their rate of decomposition is slower than the rate of accumulation of leaf litter, so the layer of duff increases slowly.
Add invasive earthworms, however, and the duff quickly disappears. The worms eat the duff and leave behind bare soil that easily compacts and is more difficult for plants to colonize. Seeds are more vulnerable to drying, freezing and predation by insects and small mammals. Even the success of ground-nesting birds such as ovenbirds is reduced.
The invasion of foreign earthworms began when Europeans settled North America. They brought with them rocks, soil and associated earthworms as ballast in their ships. Upon arrival, they dumped the debris on shore. And many immigrants brought garden and ornamental plants that contained earthworms or egg cases. That's how nightcrawlers (Lumbricus terrestris) and red wigglers (Eisentia fetida) arrived in North America.
These Old World earthworms can truly change the integrity of native forests. While some plants such as jack-in-the-pulpit can tolerate the changes caused by earthworms, many can not. Wildflowers such as trilliums and bloodroot and even sugar maple seedlings disappear. The result is a barren forest floor. Add too many deer, and the impact on the forest floor becomes devastating. Ground cover disappears.
Cindy Hale, a research scientist at the University of Minnesota, Duluth, reviewed exotic earthworms' impact on forest ecosystems last week at the Ohio Wildlife Diversity Conference in Columbus. Much of her work is summarized in a fascinating booklet entitled "Earthworms of the Great Lakes" (www.kollathstensaas.com).
"No native earthworms have been documented in the Great Lakes region of North America," said Hale. "When the last glacial ice sheets receded 11,000 to 14,000 years ago, the ground beneath those glaciers was left earthworm-free."
Today those forest floors are ravaged by exotic earthworms, imported intentionally as fish bait or in compost and mulch. Hale said approximately $110 million worth of earthworms are imported to the U.S. from Canada each year for the fishing industry. And all are invasive exotic species. For more information, visit www.greatlakeswormwatch.org.
First Published March 18, 2012 12:00 am