ROUGHLY SPEAKING, there are two different classes of ecosystems, Ryan explained in our chat:

In manmade environments, such as farms and gardens, worms have proven to be helpful as soil-aerators and as detritivores, super-efficient recyclers who break down organic material and return it to the loosened-up soil.

Natural habitats, such as the hardwood forests of the Northeast and upper Midwest, were historically earthworm-free by design—meaning no native earthworm species and not meant to have earthworms, instead relying on tiny fungi and bacteria to do the recycling tasks.

“The fungus and bacteria do that job really, really slowly,” Ryan explains. “But when earthworms come into such an environment [as they did with the European settlers hundreds of years ago] they do it much, much faster.” They accelerate the order of things—and not in a good way.

The forest floor is meant to be a thick, spongy organic “duff” layer, slow to break down. Not only do earthworms make things decompose too fast, but their castings make forest soil more compacted and dense–and more mineral-rich. The altered medium is inhospitable to tree seedlings and herbaceous plants that used to thrive. Natural forest succession is interrupted, and the diversity of the plant community threatened.

The forest floor can even drop, so what used to be a tree root may suddenly find itself above soil grade—a root no longer, but a branch.

“Researchers have coined the term ‘tree root gingivitis’,” says Ryan, who can tell right away when he enters a woodland whether it has been invaded.  Big trees still tower up above, perhaps, but on the ground: maybe sedges (Carex species) or not much at all.  When the big trees die, what will happen, since there are no saplings in the community?

Asian earthworm in genus AmynthasLATELY, the story of invasive earthworms who can change environments has gotten more complicated—hence headlines such as “The Dark Side of Earthworms” and “Invasion of the Earthworms.”

Asian species in the genus Amynthus (above) have made become more widespread in American soils, including large portions of the East Coast, and lately into the Great Lakes area, with pockets in Minnesota, Ohio and Wisconsin. With help from citizen researchers, Great Lakes Worm Watch is trying to track, and help stop, their march. Why?

“These could be a gamechanger when it comes to earthworms,” says Ryan, referring to potential impact of unchecked spread. “They’re very aggressive, and can live in really high densities,” consuming a lot of organic materials in a very short time.  “Some researchers have coined it ‘Nothing Grows Here Syndrome’ where these worms have been.”

more earthworm 101

Q. Why were there mostly no earthworms here to greet settlers hundreds of years ago in much of the nation?

A.  In the last Ice Age, glaciers “scraped right down to the bedrock,” says Ryan.  In the northern tier of the country particularly, and in Canada: no native earthworms the last 11,000ish years, which is about when the last glacial period ended.

Q. How did European earthworms reach the United States?

A. “The earthworms we have known and grown up with are not originally from North America, they’re from Europe,” says Ryan.

Researchers think they came as a side effect with ballast in ships—soil and rocks were used in ships and then unloaded. Plus European plant species brought with settlers, such as lilacs or buckthorn among many, probably had earthworms in their rootballs. Settlers—noticing the Northern U.S. didn’t have earthworms–may also intentionally have brought them along for soil improvement of farms and gardens.

Today human activity–from soil in tire treads to improperly disposed-of fishing bait and more–continues to spread worms where they don’t belong. The worms sold for composting (read GLWW’s pdf), for instance, often include the aggressive Asian species.