Earthworm Invasion

The enemy is digging in!

An earthworm invasion, spawned by agriculture and fishing, is spreading into forests in the formerly glaciated part in North America — forests that evolved without native earthworms.

Earthworms were first introduced to these regions when immigrants imported plants from Europe. Foreign worms then made their way to the edges of yards and farms and out into the forests.

Today, fishermen continue seeding new invasion fronts when they have mercy on their worms and toss them out on the ground. Logging roads also assist the spread of non-native earthworms. — mud on tires disperses cocoons and live worms.

Years ago, hikers noticed the leaf litter layer in northern forests was rapidly disappearing. Earthworms are slowly eating their way into the forest, mixing the litter layer into the lower, mineral soils in the process.

“Soil scientists and agriculturalists recognize the benefits of mixing organic matter with the mineral soil in production agriculture,” says Kyungsoo Yoo, a professor in the Department of Plant and Soil Sciences at the University of Delaware. “However, in native forests the leaf litter is essential to the survival of native trees' seedlings…As earthworms invade and consume the leaves, the layer and therefore the success of seedlings, is compromised.” Recent studies have shown that the kinds of trees growing in a forest can change in as little as 30 to 40 years after an earthworm invasion.

There are three major groups of earthworms that invade. First come worms that live in and consume the surface leaf layer ("epigeic worms"). Next come those that live high up in the mineral soil ("endogeic worms"). These mix organic matter from the litter layer with shallow mineral soil. Finally, the night crawlers arrive, burrowing deep (they are "anecicworms"), they pull organic matter down to depths of a meter or more.

But in terms of human lifetimes, it may take a while for earthworms to have any major impact in many areas — it's estimated that the invading front advances only about seven meters a year.

At that rate it will be a long time before they reach the primordial forest where the hand of man has never set foot (my favorite mixed metaphor!).