Little earthworms causing big trouble in forests

PARK RAPIDS, Minn. (AP) -- The lowly earthworm is causing trouble in Minnesota forests.

Sure, gardeners love them. But the worms eat the leaves that litter the forest floor, causing disturbances in the ecosystem, said Becky Marty, the resource manager at Lake Itasca State Park.

Marty pointed out the evidence of earthworm infestations as she walked through a forest near the park's Bear Paw campground. After digging up a few, she picks up dry, compacted earth with little moisture and only a year's accumulation -- instead of several years -- of decomposing leaves.

"People fish with earthworms and when they're done, they say, 'We can't kill them!"' she Marty said. "They throw them on the ground where they make their home."

Steve Mortensen, a biologist who has studied earthworms at Leech Lake Reservation in Cass Lake said earthworms can break down leaves in four weeks that would have taken four to six years to decompose. The worms consume the tender parts of the fallen leaves, converting the plant material into mineral soil.


The process, he said, is "great in the garden, but not great in hardwood forests."

The quick consumption means tree saplings and seedlings die because earthworms expose their roots. Hardened soil replaces the leaves, causing erosion and water quality problems. The earthworms also decrease the amount of soil nutrients that some microscopic animals use as food sources, says Lee Frelich, a forest researcher at the University of Minnesota in St. Paul.

"For animals that live on the ground, there's no cover, no food, no shelter from the rain or cold," Marty said. These mammals and other species affected by the infestations include amphibians such as salamanders, oven birds that nest on the ground and arachnids, she said.

"Left without their habitat, the animals could be eaten by predators" or could leave infested forest areas for safer regions, Frelich said. And since certain wildflowers such as the goblin fern can survive only by using the leaves' nutrients, the earthworm invasion might cause their extinction, he said.

University of Minnesota graduate student Cindy Hale, working with Frelich and other forestry professors, won a four-year, $330,000 grant to study the impact of earthworms in hardwood forest, especially Chippewa National Forest.

The studies show non-native worms have invaded hardwood forests, mainly consisting of maple and basswood trees, in the southern half of Minnesota over the past several decades. Frelich said that earthworms in Minnesota, which are not native to the state, were brought over with German, Swedish and other European settlers with the soil from potted plants.

But today, people spread earthworms by disposing them on forest floors or boat landings.

By themselves, earthworms can travel only about 30 feet a year, but with human intervention, there's no limit to their speed, Mortensen said.


Mortensen said it's OK to dump the worms in your garden or other wooded areas, if they're already there. But he said the best approach is to "put them in the trash where they won't end up in the environment."

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