It’s been known for years that European earthworms are changing the makeup of North American forests by altering the soil that trees grow in.

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Now, in a new study published last week, scientists have for the first time documented that imported worms are directly responsible for declining species diversity.

Just about all the earthworms found in Northland forests are foreign, including nightcrawlers and angleworms, and their impacts are most noticeable in vegetation on the forest floor, the study reports.

Those changes include a decline in native plant species diversity, increased non-native plants from Europe (like buckthorn) and an increase in grasses moving into forests.

The study, published in journal “Global Change Biology,” involved research by more than two-dozen scientists, including Lee Frelich, director of the University of Minnesota’s Center for Forest Ecology and an expert on Northeastern Minnesota forests.

Frelich says the non-native worms consume the duff — the decaying leaves on the forest floor — as they eat their way across the continent’s forests. They were first introduced by European settlers and, in recent decades, have spread rapidly through their use as fishing bait.

The worms alter the physical and chemical properties of soils, changing the pH, nutrient and water cycles and disrupting symbiotic relationships between soil fungi and tree roots.

“The earthworm invasion has altered the biodiversity, and possibly the functioning of the forest ecosystems, because it affects the entire food web as well as water and nutrient cycles,” said Dylan Craven of Leipzig University in Germany, lead author of the study.

The earthworms also amplify the negative effects of droughts, warming climate and deer grazing on native plants, Frelich told the News Tribune, making those stressors even worse than they would be without worms.

The recent scientific evidence backs past findings by UMD professor Cindy Hale that found the worms’ impact especially intrusive on hardwood forests. They seem to have less impact on evergreen species, probably because needles are more acidic and less tasty to the worms.

As worms eat the leaves on the forest floor, big trees survive, but many young seedlings perish, along with many ferns and wildflowers. That changes what kind of trees are in a forest.

“We’ve documented the areas of heaviest worm density have 30 percent less growth in sugar maples,’’ Frelich said of a study that looked at sites in both northern Minnesota and Wisconsin. “Another study found worms were the largest contributing factor in maple dieback.”

Tens of thousands per acre

Scientists have been studying nightcrawlers on a peninsula at Leech Lake for years and  determined that worms were first introduced in the 1950s, probably as they became popular for fishing bait. Now, there are 30,000 to 100,000 nightcrawlers per acre in that area.

In some parts of southern Minnesota, with better soils, there are up to 200,000 nightcrawlers per acre. And that doesn’t include angleworms, which can be as thick as 400,000 per acre — all of them digesting plant material and leaving the forest floor more bare.

Because earthworms live in different soil layers and their effects are cumulative, the more types of earthworms that live together in one location, the more native plant species are affected.

As worms remove the protective duff from on top of the soil, the ground becomes exposed to sunlight and the soil gets hotter and dryer. It also gets compacted, with water running off faster and not soaking in.

Frelich says the North Shore’s massive birch die-off in recent decades may have something to do with worms, although there’s been no research yet to prove it.

“We have seen some areas on the North Shore go from no worms to totally infested. That’s been the same time that area saw a long drought period and where the birch have died,’’ he said. “One of the worms’ biggest impacts has been exacerbating the impacts of drought. … It would be hard to believe they could have such an impact on maples and not impact birch as well.”

Worms also make the soil better-suited for grases to move in. Grasses can better handle dry soils — think prairies — and their fine roots can better absorb nutrients compared to tree roots.

“So now we are seeing what looks like lawns under forests where before it was just clumps of grass here and there,’’ he said. “The grasses are native. But they never thrived like this without the worms.”

Frelich, who for a decade has predicted fewer trees and more savanna in Minnesota due to climate change, says worms are already hastening that transition.

And while some species like maple are declining due to worms, other species can’t handle worm-infested soils at all.

“The goblin fern, which was never common, absolutely can’t tolerate them, they are disappearing. And it’s been very hard on trillium, on our native orchids,’’ Frelich said.

If that wasn’t enough, the new study indicates that worms help prepare the soil for invading species like buckthorn and garlic mustard which evolved over millennia in Europe right along with the worms.

Some uninfested “islands”

Like most exotic species, worms depend on people to move them into new territory. It would take a worm about 200 years to travel a half-mile. But thanks to plant lovers, gardeners and anglers who transport worms in tackle boxes and potting soil, the worms can travel hundreds of miles in just a few hours.

Minnesota is probably 90 to 95 percent infested with foreign worms. But new, as of yet unpublished research found some islands of worm-free areas, including about half the Boundary Waters Canoe Area Wilderness, mostly away from popular fishing lakes. There also are a few areas in Minnesota’s Arrowhead and in Upper Michigan’s Porcupine Mountains that are still worm-free, Frelich noted.

“It would still be good if we could keep some of these areas from being’’ infested by worms,  Frelich said. “But there’s really nothing we can do once it occurs.”

There are about 15 species of earthworms in the Northland and all of them are foreign. Over the past 30 years scientists have shed more light on earthworms’ impact on the makeup of forests that developed without big worms for 10,000 years, since the last glaciers. (There are native North American worms — tiny, white or translucent, hair-thin critters — but there are no native earthworms.)

Frelich said scientists are trying to find out why worms reduce soil nutrients in North American forests while in Sweden, where worms have been for thousands of years, they enhance forest soil nutrients. He said it appears ecosystems may adapt to worm invasions after hundreds or even thousands of years.

Worms may even be impacting what birds and animals are thriving in our forests.

“When you change the structure of the soil, which everything else is based on, the ecological cascades are going to ripple through the entire ecosystem,’’ Frelich said.

Scientists from the United States, Canada and the German Centre for Integrative Biodiversity Research at Leipzig University worked on the species diversity project, which joined research from 14 separate studies.

For more information on foreign earthworms and their impact on Northland forests, go to