How did earthworms invade Minnesota?

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Earthworms may be helpful in a compost heap or in the soil of a vegetable garden.

But they are a destructive force in Minnesota’s hardwood forests – chomping up entire layers of the forest floor and making it inhospitable to the plants that thrived here for thousands of years.

Earthworms aren’t native to Minnesota. Every one is an invader.

Bradley Moe, who lives in Breezy Point, became curious about worms after reading an article by the late Minnesota Star Tribune reporter Jennifer Bjorhus about the endangered, “ancient and otherworldly” goblin fern.

Researchers worry the fern could become extinct in the next decade due to the worms’ damaging onslaught.

Moe wondered about the invasion’s beginnings. He asked Curious Minnesota, the Strib’s reader-powered reporting project: “How did earthworms end up in Minnesota?”

They first likely hitched a ride with European settlers, arriving by boat and sometimes by rail from ports to cities like Minneapolis, according to experts.

But they would not have advanced to remote corners of the state without help from anglers who began using them as live bait nearly a century ago.

“If you just introduced them in Minneapolis, and they moved on their own, around 20 or 30 feet a year, it would take, like, 10,000 years to get to the Boundary Waters,” said Lee Frelich, director of the University of Minnesota Center for Forest Ecology.

Instead they made the trip in decades.

There are now at least 15 different species of earthworms crawling in Minnesota dirt, according to the state’s Department of Natural Resources. None of them are native to this state or even this part of the globe.

There are some earthworms native to other parts of North America – including the Pacific Northwest and the southeastern United States. But those species didn’t crawl their way here. (It’s likely too harsh for them anyway.)

The earthworms in Minnesota came from Europe and from Asia.

The first to arrive were European earthworms. Settlers brought them along in the dirt of potted plants – both live worms and eggs that hatched later, said Frelich. St. Paul and Minneapolis were likely the epicenter of that early invasion.

Some ships also used bags of soil as ballast. They then would ditch the dirt – worms and all – in the port where they landed, he said. That included Great Lakes ports such as Duluth.

European earthworms made similar incursions even earlier in New England. But their spread is much more advanced in Minnesota (and nearby states like Wisconsin and Michigan) because of the wide distribution of so many lakes and streams, Frelich said.

That geography compounded the situation when anglers in Minnesota began using worms as bait back in the 1930s.

“People took fishing bait into every remote corner of the state, didn’t use it all and ended up dumping it on the shore, because everybody thought earthworms were good for the environment,” Frelich said.

Introduced near thousands of lakes, the creatures spread out from the shores, covering the state’s entire landscape in just decades.

“I’ve lived in this area all my life, and in the 1960s you could find earthworm-free forests all over the landscape,” Frelich said. “By the 1990s, it was probably less than 10%.”

The worms’ spread is helped along by the tire treads of ATVs and the soles of hiking boots, which pick up earthworm eggs along with mud.

There are just a few earthworm-free places in Minnesota now – including some spots in the Boundary Waters and hillsides on the North Shore.

In the 1970s, a scientist in Wisconsin first noticed that soil structure in forests was changing. The top layer of dead leaves, which before had accumulated over centuries, was gone. (Worms find dead leaves to be delicious.)

The worms also change the soil’s density. Native insects such as beetles and centipedes make little tunnels that create a spongy, absorbent soil. When earthworms drill through, the tunnels collapse and the soil becomes compacted.

A few decades later, researchers began to document an ecological chain reaction in the forest: As worms change the soil, native plants like trilliums become less abundant. This leads to soil erosion and leaching of nutrients.

With fewer plants around, deer end up decimating the ones that survive — and soon botanic invaders like buckthorn and garlic mustard move in.

Asian earthworms — often called jumping worms — arrived in Minnesota much later, in the early 2000s. Gardeners ordered them online to add to their compost, and they ended up in backyards.

They, too, can be very damaging to woodlands — and look a little different, with a light-colored band near their head. They also wiggle more than European earthworms, getting their name from the way they seem to jump out of the ground when disturbed.

Frelich first spotted a jumping worm in Minneapolis’ Loring Park in 2006.

This July, the state added them to the list of prohibited invasive species — making it a misdemeanor to “possess, import, purchase, transport or introduce” them without a permit.

The state doesn’t regulate European earthworms in the same way. Still, it is illegal in Minnesota to release most nonnative species into the wild. The DNR encourages anglers to dispose of any unused bait worms in the trash.

These measures aim to stop more worms from being introduced and preserve the few worm-free areas left. But it’s impossible to get rid of the ones that are already here, Frelich said.

Instead, the best ecologists can do is work to re-establish native plants by sowing seeds or planting seedlings. Lowering other stress factors also helps, he said.

Researchers have noticed a difference in areas where wolf populations have returned.

“If you have more wolves, you have fewer deer. And with fewer deer, you have more plants,” Frelich said. “And the plants are getting a chance. Even trilliums are recovering in areas with wolf packs in Wisconsin.”

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