On a sunny Saturday last winter, my friend Roberta and I headed out to the city’s Materials Recovery Facility (MRF) on Platt for a workshop on using earthworms to turn household garbage into garden compost. The setting was perfect: a classroom that overlooked the mostly automated process of recycling the city’s paper, glass, plastic, and metal trash.

The City of Ann Arbor sponsored the workshop, and staffers were on hand to oversee registration and payment. Our $25 bought a worm bin, a spray bottle, a couple cups of compost, a spatula, and a squirmy mass of Eisenia fetida, commonly known as red wigglers, or compost worms. Our instructor was Sarah Archer, CEO of Iris Waste Diversion Specialists, LLC.

Archer showed us how to make bedding from strips of newspaper, dampen it, then place a clump of worm-filled compost on top of the bedding and close the bin. Back home, we’d add fruit and vegetable scraps and eggshells, which our worms would then “vermicompost.” What could be more ecologically correct?

Yet a shadow hung over the day. When I had invited another friend, Pat, to join us, she declined for lack of time, adding, “You know that earthworms are an invasive species, don’t you?”

No, I didn’t know, and I almost didn’t believe her. But Pat is a news writer and wouldn’t be wrong about a fact. Curious, I researched it myself.

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”?

Briefly put, the Wisconsin Glaciations some 21,000 years ago covered the upper Midwest and northeastern United States, killing all native earthworms and leaving those areas completely earthworm-free for millennia. The north woods adapted to life without earthworms; then settlers reintroduced them.

Worms harm forests that evolved without them in several ways. Those that hang out in the forest duff eat the layers of leaves and needles covering the ground, creating bare soil and removing nutrients needed by native plants, including tree seedlings, that would otherwise grow there. The worms that tunnel downward disturb and mix the natural layers of soil; those that tunnel horizontally change the flow of moisture. Together, they render hostile the conditions formerly friendly to native woodland plants.

I had always thought the plentiful Lumbricus terrestris (night crawlers) squiggling in my soil were a sign of my gardening skill. Earthworms were my friends, loosening clay and leaving behind nutritious droppings. Yet Minnesota has designated earthworms (along with the emerald ash borer, gypsy moth, and mute swan) as invasive, and has an active “Contain those crawlers!” program.

The only published study of Michigan earthworms I could find was done in the western Upper Peninsula, in the Ottawa National Forest, which is dominated by sugar maples. That study compared earthworm communities in wilderness and non-wilderness sites within the ONF. It found that all the non-wilderness sites contained one to five species of exotic earthworms, while only half the wilderness sites had exotic earthworms, all of them a single species.

I got in touch with Cindy Hale in Duluth and asked whether the same problems she identified in Minnesota–drastic declines in native hardwood forests–were likely to occur in Michigan. She said: “The short answer is that the same issues that are going on in Minnesota are going on elsewhere.”

Seeking an expert closer to home, I found Jasmine Crumsey, a U-M grad student in ecology and evolutionary biology. Crumsey has researched earthworms at the U-M Biological Station in Pellston–they’re less abundant there than in Minnesota, she told me, because Michigan’s sandier soil is less hospitable–but she was not aware of any studies of earthworms closer to Ann Arbor.

So what does all this mean for Ann Arborites? Using red wigglers to compost garbage probably won’t hurt the local ecosystem, especially if they are not let loose outdoors. And there seems no need to remove the earthworms in your yard and garden (were that even possible) unless your property abuts a pristine hardwood forest. However, to the extent that local stands of hardwoods are adversely affected by earthworms, the conditions for invasives like buckthorn and garlic mustard improve, making it easier for invasives to spread to your yard and our parks.

I waited until near the end of the vermicomposting workshop to ask Archer whether earthworms are a harmful invasive species. Her answer: Yes, they are, but only in Minnesota. And anyway, the red wigglers in our worm bins are not known to survive cold winters outside a compost pile.

Unlike Minnesota, the city of Ann Arbor does not list earthworms as an invasive species. I asked Dave Borneman, Natural Area Preservation Manager for the city, if that is likely to change. “We have recently been trying to figure out how to deal with the problem here in Ann Arbor, but as of yet have no official policy on earthworms,” he said.

Though skeptical, I brought my red wigglers home. I fed them. I went on a couple of trips and forgot about them. When I finally went to the basement to check them, alas, they were dead on the concrete floor.

Apparently I had put too much moisture in the bedding, causing my arguably invasive worms to flee. When I found them they were odor free, crispy, and easy to vacuum up.