Next time you roam your favorite woodlot, study the leaf litter. Is it thick and spongy, a pure joy to walk upon? Or is it just a few leaves here and there, or maybe just bare soil?
If your woodlot is in or near suburbia, organic matter covering the soil may be scant or absent. If so, blame 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 were beneficial citizens of the environment. Their burrows allowed air and water to circulate through the soil, and the castings (excrement) they leave behind enrich the soil.
We learned these lessons because they are true on farms and even in backyards. But in forests, earthworms are abundant and destructive. They destroy the leaf litter on the forest floor simply by eating it.
It wasn't always that way. When Europeans settled North America, they brought with them rocks and soil as ballast in their ships. Upon arrival they dumped the debris onshore. And many immigrants brought garden and ornamental plants that contained earthworms or egg cases. That's how the first Old World earthworms, such as nightcrawlers (Lumbricus terrestris) and red wigglers (Eisentia fetida) arrived in North America.
In pre-Columbian North America, fungi and bacteria were the primary decomposers of the 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 literally 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 nesting success of ground nesting birds such as ovenbirds is reduced.
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 cannot. 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.
I learned all this last week at the Ohio Wildlife Diversity Conference in Columbus, an annual event that attracts nearly 1,000 nature enthusiasts. Cindy Hale, research scientist at the University of Minnesota, Duluth, reviewed her research on exotic earthworms' impact on forest ecosystems. Much of her work is summarized in a fascinating booklet titled "Earthworms of the Great Lakes" ($12.95, www.kollathstensaas.com).
Hale pointed out, "No native earthworms have been documented in the Great Lakes region of North America. 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 says that today approximately $110 million worth of earthworms are imported from Canada to the U.S. each year for the fishing industry. And all are invasive exotic species.