Thanks to my employer, the University of Michigan, I had instant access to two e-books: Biological Invasions Belowground and Earthworm Invasion, collections of scholarly papers published in 2006. With horror I read about the devastation earthworms have caused in the Carpathian Mountains of Romania, other parts of northern Europe, Russia, South America, Puerto Rico, and Taiwan--all safely remote.
I started to relax, but too soon. The next paper outlined earthworm-induced "forest decline syndrome" in Minnesota. At the University of Minnesota Duluth, professor Cindy Hale and her colleagues have documented dramatic changes in native hardwood forest ecosystems due to earthworms. Victims include native understory plant species--including varieties of fern, trillium, uvularia, and viola--and tree seedlings themselves. These losses open the way for invasive species such as buckthorn and garlic mustard.
Both buckthorn and garlic mustard are officially classified as invasive in Ann Arbor. Major avenues for the introduction of invasive earthworms, I learned, are the fish-bait and horticultural industries--and vermicomposting.
But how could earthworms be "invasive"?