Earthworms, the darlings of gardeners, fishers and composters, have a dark side: some are globetrotters and when introduced to new homes can cause real problems, both above and below ground.

“Invasive earthworms are a global problem and can cause considerable changes to ecosystems,” says Mac Callaham, research ecologist with the U.S. Forest Service Southern Research Station (SRS) scientist and an internationally recognized expert on invasive earthworms.

Invasive earthworms have spread to every continent (except Antarctica), oceanic islands, and nearly every type of ecosystem on the planet—even desert oases. “Worms may be slow on their own but theyre good hitchhikers,” says Callaham. Mud containing earthworm cocoons can get stuck to the fur and feet of animals. Tornadoes transport small pieces of earthworm-containing soil, and ocean currents carry worms to exotic locales. Humans, however, are an invasive worms main ride to new regions. People spread worms accidentally through activities such as gardening, composting and dumping fishing bait.

As early as 1900, scientists noticed that some travelling worms–but not all–became invasive. Ever since then, scientists have wondered what traits lead exotic species to cause trouble in their new homes. Invasive worms may be the ironmen of the worm phylum; they can tolerate many different environments, migrate on rainy nights, and as Callaham explains, “for certain species, a single worm can produce lots of offspring, no mate required. This means that one worm can establish a population, which may then act as a hub for future invasions.” Read Callaham’s article about the global problem of introduced earthworms.

In the United States, earthworms have invaded northern regions like the Great Lakes and New England–regions where the native earthworms were killed during the last Ice Age. In these forests, earthworm invasions can have dramatic negative consequences as worms gobble up duff that took decades to accumulate, sometimes eating so much that all the leaf litter vanishes from the forest floor and only bare soil remains. This is bad news for salamanders and other creatures whose lives are spent in the layer of leaves and organic matter that normally cloaks the forest floor. Earthworm invasions also decrease the abundance and diversity of bacteria, fungi, mites and other small soil-dwelling creatures that break down old plant matter, and endanger rare ferns and trilliums that need a rich, fertile layer of duff to germinate in.

In Tennessee, Callaham and his colleagues monitor Amynthas agrestis, an exotic worm species that has colonized parts of the Great Smoky Mountains National Park. After 2 years of monitoring these worms, the researchers found that the colonies are mobile, especially when soils are wet. “In drier weather, the worms don’t spread to new areas and even retreat, causing the size of colonies to shrink. This suggests that if land managers decide to remove invasive earthworms, the most effective time for treatment may be when its dry,” says Callaham. “We know that worms can alter soil properties, organic matter and nutrient cycles, and well as plant and animal communities. We’re trying to figure out what that might look like in the Great Smoky Mountains.”

Callaham and his colleagues recently studied the interaction between invasive earthworms and millipedes. “The layer of soil where millipedes live also happens to be where the earthworms feed. We wanted to see what happens if these millipedes and exotic worms compete for food.” Callaham and his team discovered that when Amynthas was present, millipedes had shorter lifespans. “It looks like millipedes and Amynthas do compete for the same foods,” says Callaham. “However, Amynthas cocoons were not found in soil that contained millipedes, so it looks as if millipedes have some biotic resistance the exotic worms.”

Access Callahams study on the policy and management responses to earthworm invasions.

For more information, email Mac Callaham at mcallaham@fs.fed.us

Access the latest publications by SRS scientists.