My mother gardens with a serious green thumb, and so as a kid, I spend summers playing around the vegetable garden. She taught me about bugs, which ones were good friends, like sociable ladybugs and graceful garden spiders. She also introduced me to enemies, like the fat tomato horn worms and the dusty green broccoli moths. From my mother, I learned that earthworms help gardeners by breaking down organic matter in the soils into nutrients that the plants can use, and breaking up the substrate with their wriggling to create spaces for roots to use.
Last week, my mother called me with terrible news. “Did you know earthworms are not native?? They are INVASIVE earthworms, and they are really bad for the forest! Did you know about this?”
She felt betrayed to learn that the creatures she has long appreciated are not the innocent, beneficial native species she always assumed that they were. The bad news came from a biologist and gardener friend of my mother’s, who heard an presentation on the damaging effects on hardwood forests of invasive earthworms at a recent conference. The expert, Cindy Hale, is a researcher at the University of Minnesota, Duluth, and she’s been studying the impacts of invasive earthworms since she completed her PhD on the issue in 2004.
Hale is not concerned with earthworms in gardens and farm fields, although they are not native to the Midwest. Earthworms do perform beneficial processes in the soil, and they were native and familiar to the European immigrants who settled and starting farming in the midwest. There are also earthworms species native to the southern US. However, when these exotic worms of European ancestry moved from the farm fields and gardens into the adjacent forest communities, that’s where they begin to wreck havoc. Havoc!
In a hardwood forest, like those in Northern Wisconsin and Minnesota, a thick organic layer of slowly decomposing leaf litter builds up on top of the soil. This litter layer provides a home for tree and shrub seedlings to send their first roots. Earthworms act as detritivores, eating organic litter and breaking it down. They act much fast than the natural decomposition processes mediated by fungi and bacteria in these forests, destroying the litter layer so that it can no longer serve as a supportive substrate for seedlings. When seedlings struggle to establish, it can change the long term dynamic of the forest ecosystem.
Most of the earthworms now found in the northern hardwood forests of Wisconsin and Minnesota were probably live bait, released by fisherman along the lake shore. Hale continues to study the patterns of earthworm invasion into new forest areas, and luckily, without human transport, the worms can’t cover that much ground very quickly. Fisherman are strongly encouraged not to dump leftover bait, to prevent further invasions.
It’s hard to believe that a species so familiar to me could be doing so much damage to forests I love, but Hale’s data is very convincing. Now, I just need to get in touch with her for an interview!