Just under the forest floor, an army of invaders is attacking the forests of Minnesota.
Earthworms, though benevolent in the garden, have been found to cause surprising amounts of damage in northern woods.
After three years studying this degradation, Lee Frelich, a research associate in the Department of Forest Resources, received a $318,000 grant from the National Science Foundation to continue his research.
The worms cause most of their damage when they eat the leaf litter on the forest floor, Frelich said. Small plants need this leaf layer for nutrients and to germinate seeds.
Without plants to anchor the soil, rain carries off top-soil, causing erosion, Frelich added.
This erosion has been noted in places like the Wood Rill Scientific and Natural Area and the Chippewa National Forest where Frelich has been studying the earthworms.
He goes as far as to compare the damage of the worms to the deforestation of Minnesota’s woods by the early logger barons.
“The change in the ecosystem caused by the invasion of these European earthworms is bigger than that caused by the original clear-cutting of (Minnesota’s) forests,” said Frelich.
He went on to explain that while the loggers cut the forest down, it is usually allowed to grow back to a similar form.
The seven species of foreign worms presently making new homes in forest soil are changing the very nature of the forest ecosystem by affecting nutrient distribution and eliminating a seed bed on the forest floor, he said.
David Andow, a professor of entomology at the University, said that he became interested in the study after talking to Frelich and looking at the research sites.
“What was clear to me, after just visiting the sites is that the earthworms are having a big effect on the forests,” said Andow.
Native worms have not been present in Minnesota since the ice age, Frelich said.
The worms that are causing problems are illegal immigrants from Europe, carried in the holds of clipper ships that used soil as ballast.
Frelich said the invaders hitch a ride north in boats and bait boxes. The worms sold at the bait shop are European and fishermen who dump their bait on the shore are unwittingly introducing the species to a new home.
He plans on publishing his findings this fall and more articles will follow in the years to come.
“It’s one of the most serious environmental issues (for forests) of this generation,” said Frelich.

Seth Woehrle covers environment and transit and welcomes comments at [email protected]