Earthworms’ Underground Invasion Threatens Forest Sustainability

This underground invasion has compounding impacts on the capacity of the soil to provide nutrients and sequester carbon—an important role as the world faces global climate change.

Kyungsoo Yoo, University of Delaware assistant professor of soil and land resources, and colleagues Anthony Aufdenkampe of the Stroud Water Research Center and Cindy Hale, an ecologist at the University of Minnesota Duluth, were recently awarded a three-year, $397,500 grant from the U.S. Department of Agriculture National Research Initiative (USDA-NRI) to study the quantitative coupling of the ecology of European earthworm invasion--specifically in Canada, New England and the Great Lakes region--with mineral chemical weathering and carbon cycling.

Prior to colonization, the glaciated areas of North America were devoid of native earthworms. European earthworms were first introduced to U.S. soils when immigrants brought crops from their native lands, harboring earthworm cocoons. Worms made their way to the edges of farmlands and to the forests.

In addition, these glaciated areas are pocked with small lakes; fishermen often dispose of unwanted live bait, infesting areas where native earthworms were not typically found. Unpaved logging roads through these regions also assisted the spread of non-native earthworms, as compacted soil on tires disperses cocoons and live earthworms.

“Gardeners and farmers all appreciate the beneficial effect of earthworms,” says Yoo, “However, there were systems where worms didn’t exist before and, as we have seen with other non-native invasions, there are ecosystem implications.”

According to Yoo, 10 to 20 years ago, hikers in Minnesota’s forests noticed changes in the leaf litter layer. They noticed that the leaf layer was rapidly disappearing over the years. The researchers understand why this was happening, as Hale’s doctoral research showed that non-native earthworms were slowly eating their way into the forest, mixing the litter layer into the mineral soils in the process.

“Soil scientists and agriculturalists recognize the benefits of mixing organic matter with the mineral soil in production agriculture,” Yoo says. “However, in native forests the leaf litter is essential to the survival of native trees’ seedlings. The litter layer provides protection for temperature changes and deer browse. As earthworms invade and consume the leaves, the layer and therefore the success of seedlings, is compromised.”

He adds, “This relationship has been singled out as one of the most important factors impacting the future sustainability of forests in the glaciated areas in the U.S.”

Recent studies have shown that forest composition, with earthworm invasion, can change in as little as 30 to 40 years. As early as the 1940s, researchers studied the impact of invasive earthworms on soils, in terms of the soil chemistry and physics; however, it has only been within the last decade that the focus has shifted to the impacts on the entire ecosystem.

Hale’s research shows that there are three major groups of earthworms that invade at varying times. The first to invade are the epigeic species that live in, and therefore consume, the litter layer. Endogeic groups live in the mineral soil; through their movement and feeding, they mix the organic matter from the litter layer with the shallow mineral soils. Anecic earthworms--known to most as night crawlers--are deep borrowing, pulling organic matter to depths of one meter.

It is estimated that the invading front of earthworms migrates as much as seven meters each year farther into a forest. As these waves of earthworms move into an area, they change the structure and the bio-geochemistry of the soil. Yoo has long hypothesized that active mixing of soils by organisms accelerates the mineral/chemical weathering and has an impact on carbon-mineral interactions.

“When I read Hale’s paper in the journal Ecology, I was ecstatic,” Yoo says. “I had been looking for ideal sites to test this hypothesis.”

Yoo immediately contacted Hale and started forming a research team to address this question.

“Aufdenkampe then joined us with his expertise in carbon-mineral interaction, which significantly deepens and expands the horizon of this project,” says Yoo.

He also said that support from the University of Delaware Research Foundation (UDRF) was essential to their initial field trip to collect preliminary information.

This new study will yield insight into how and how fast the ecology of earthworm invasion alters inorganic nutrient cycling via mineral weathering and the combination of organic matter and minerals and subsequent carbon storage in the soils, Yoo says.

“We are looking at the integration of biology, chemistry, and physics in a mechanical fashion. We would like to know and understand how and how fast the sustainability of forests is affected by the invasion of these nonnative species,” Yoo says. “There is often a forgotten role of soil animals in soil biogeochemistry and we hope that this study contributes to integrating the population dynamics of soil organisms and ecology with biogeochemical processes in terrestrial ecosystems.”