For thousands of years, there were no earthworms in the forests around the Great Lakes. Now many exotic species are invading the region, changing the way the forest floor looks and feels. WOSU's Jonathan Hickman has the story.
Little seems more quintessentially American than a worm on the end of a fish hook. But chances are, if you've gone fishing with live bait yourself, you've never actually used an American worm. Ohio State Professor Clive Edwards wrote the book on earthworms-three editions, and he's working on a fourth.
"Literally all the gardens, all the cultivated areas around urban areas, are all these species exotic species. If you go to an agricultural soil anywhere in the northern United States-when I say Northern, I say anywhere from Georgia up-you'll find nothing but lumbricidaes, these exotic species." Says Edwards.
The lumbricid-the common nightcrawler to you and me--is one of a gang of exotic earthworms that sailed to North America in ballast or in clumps of soil shipped with plants. University of Minnesota professor Cindy Hale has made a specialty of studying the spread of these invaders, which have come to dominate American soils.
[Hale] There's a suite of about 12 to 15 European and Asian earthworm species that are the "invaders" let's say. And they're not just invading in north America, they're invading globally. China, Russia, Australia, you name it, they're invading.
There are some earthworms that are native to North America, but you'll almost never find them in your yard or on the sidewalk after it rains. And there are large parts of the country where historically, there hadn't been any earthworms since the last glaciation.
"Where the ice came down, it wiped out all of these particular species, and then they gradually re-colonized northwards. But the ones that could recolonize were the ones that came from Europe." Edwards says.
For thousands of years, forests in the Great Lakes region evolved without a single earthworm. And as anyone with a composter can tell you, without worms to chew through and churn all the accumulating leaves and needles into the dirt, everything decomposes much more slowly, allowing a thick organic layer to accumulate on the forest floor.
"What we see in an earthworm free forest, there's this very spongy layer of slowly decomposing organic material, and you can literally feel that." Says Hale.
Understory plants like ferns and orchids have come to rely on that thick organic layer to provide a place to germinate and set root. For these plants, the way Hale describes an earthworm invasion is as a tiny natural disaster. The worms take that spongy layer and bury it in the dirt, leaving hard, bare soils behind. There can be less immediately visible effects as well.
[Hale] it also changes the chemistry and the structure, and even the microbial community of a lot of these forest soils.
But Edwards, who is a transplant from Europe himself, thinks of the worms as more benevolent visitors. He's not convinced that the changes earthworms bring are necessarily bad-it's just a matter of perspective.
[Edwards] If you go to any forest in Europe, they're filled with these earthworms, and you have very healthy forests. So the fact that they're in there, doesn't mean to say, they're just changing the pattern, they're not saying, it's not necessarily doing something that's bad.
And in some situations-as in the "no-till" or conservation farming, those nightcrawlers are absolutely essential:
"In that situation, the nightcrawler is the plough. Takes the organic matter down. So you know they play a very important role in some of these soils." Says Edwards.
While Cindy Hale agrees that nightcrawlers are great for gardens and farms, she's worried about the changes she's seen in northern forests. As far as that goes, her hands aren't entirely clean.
"I started an earthworm garden when I was a kid outside my kitchen window because we wanted to be able to collect them so we could go fishing. . . And the little wood lot that is next to my house right now has this what we call "nuked" appearance where there's just lots of bare soil, very few native plants, and I remember as a kid growing up that forest being just full of beautiful flowers every spring, and they're gone now." Says Hale
Since most people don't realize that the worms they're using as bait or in their composters aren't native to North America, they don't think twice about setting them loose, which helps the worms spread much more rapidly through the Great Lakes region. Hale says that if people can change their habits-tossing unused bait in the garbage, and freezing your compost before adding it to your garden-the worms will be stuck just inching along.
"Earthworms on their own don't spread very fast. Maybe 5 meters a year. If you do the math, it takes about a hundred years to go a quarter mile. That's not very fast." Says Hale.
If it's not until well into our adult years that many of us have learned that the earthworms we all know and love only became naturalized citizens within the last few hundred years, at least we can take comfort in knowing that we're not alone.
"It wasn't until I was in my grad school years that I even really realized earthworms are exotic. It's a big shocker to people's systems." Says Hale.
Jonathan Hickman, WOSU News