Earthworms, Millipedes, and Soil Carbon in the Eastern U.S.

Ubiquitous in the southeastern U.S., native earthworms are absent from the northern part of the country. It wasn’t always so, but tens of thousands of years ago glaciers crept across the land, and earthworms below them froze to death. Because earthworms are slow travelers, they have not naturally recolonized the areas where glaciers were present.

“However, non-native earthworms have moved into these areas,” says U.S. Forest Service scientist Mac Callaham. “The introduction of non-native earthworms and other animals has affected soil communities and soil processes, and adds another degree of variability and unpredictability to these systems.”

Earthworms, millipedes, and other soil invertebrates make huge contributions to nutrient cycles by eating fallen leaves and other organic matter. Callaham recently led a study on soil invertebrates and these processes. The study was published in the Northeastern Naturalist and authored by Evelyn Wenk, who formerly worked at the Forest Service Southern Research Station with Callaham.

Callaham and his colleagues are the first to survey soil invertebrates across a wide swathe of the U.S. – Missouri, Michigan, Massachusetts, and New Hampshire. The sites are far enough apart to represent gradual changes in temperature and moisture, and one goal of the study was to determine how such gradual changes affect soil invertebrates.

“There is relatively little known about how the composition of soil macroinvertebrate communities varies across temperature and moisture gradients in eastern deciduous forests,” says Callaham. “Our goal was to compare soil communities at each of these sites and evaluate the role of invasive earthworms in the sites.”

In Missouri and Michigan, scientists only detected non-native earthworms. Earthworms were totally absent from sampling sites in the other two states, although non-native worms have been found in the states before. Missouri, where temperatures are warmer and there is more biological activity in the soil and leaf litter, had more than 25 times the number of earthworms than the Michigan site.

Like all the sites studied, the Missouri site is forested. Forest soils usually have a layer of small, fragmented dead leaves and other plant parts that is somewhat mixed with the mineral soil just below it. “Missouri was the only site where this layer was close to zero,” says Callaham. “Our study suggests that non-native earthworms have almost entirely consumed this layer of the forest floor.” The changes have also reduced the amount of carbon stored in the soil.

The study also finds that non-native earthworms may be associated with declines in native millipedes. In the 1960s, other scientists had documented healthy millipede communities in Missouri forests that were only 25 miles from the sites Callaham and his colleagues studied. “However, we found no millipedes at all at the Missouri site,” says Callaham. “Their absence was a bit of a mystery, but we suspect that the very high abundances of non-native earthworms could have something to do with it.”

Detritivores such as earthworms and millipedes have significant impacts on nutrient cycling and decomposition. Earthworms, in particular, are so influential that they are sometimes called ecosystem engineers, which means that they can create, maintain, or destroy habitat that other plants and animals rely on. “Certain species of non-native earthworms can be voracious, and eat leaf litter much faster than native earthworms or millipedes,” says Callaham. “The changes they cause to soils could have profound effects on other soil animals, and even influence plant communities.”

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

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