Alien Earthworms Changing Ecology Of Northeast Forests

Only a few forest stands are known to be affected to date, according to University of Rhode Island soil scientists Josef Görres and José Amador, but they say the threat to forests is real. Most of the earthworm species found in the Northeast are not native to the area.

Görres and Amador are evaluating the environmental impact of the common nightcrawler, one of the region's 16 to 20 species of earthworms. While the spread of the worms in Rhode Island has not yet been evaluated, the researchers note that bait cups littering popular fishing spots suggest that local forests may be affected soon.

"These exotic earthworms arrived here either in plant materials imported by European settlers, from fishing bait that escaped, and some that were imported here for use in composting," Görres said. "Any native earthworms that may have been in New England thousands of years ago were crushed by the glaciers."

When earthworms move into a new area, they feed on the organic material on the forest floor and bring it down into their burrows. They feed primarily on the top layer of leaf litter, as well as on the duff – the spongy layer of decomposing vegetation beneath the leaf litter.

Görres said that while earthworms do an excellent job of recycling nutrients, "when they eat away the duff layer, all the plant seeds that germinate there, like trillium and mayflowers and wood anemone, may disappear or may not have any place to germinate. Other creatures that live in the duff and forest litter like salamanders and ground-nesting birds may be affected as well. Within a decade or two, the worms can essentially change the soil profile into something like the black mineral-rich soils that are found in many European forests."

Funded by the U.S. Department of Agriculture and the Rhode Island Agricultural Experiment Station, Görres and Amador have set up study plots in local forests to evaluate the impact of the worms. They expect that over time the leaf litter and the duff layer in the protected plots will disappear because of the voracious worms. "At some point, the number of worms that can survive in a given area will be regulated by the amount of new leaf litter that falls," said Amador. "We'll also see a change in the plant and animal communities that live there."

The researchers are also trying to determine whether the worms are increasing the amount of carbon dioxide and methane going into the atmosphere. "The leaf litter and duff layers consist almost entirely of stored carbon, so when the worms eat and process the litter and duff, they release carbon dioxide and possibly methane in their burrows," Amador said. "We're not predicting catastrophe, of course, since the total amount of the gases they release is small. But it's a previously unaccounted potential source of greenhouse gases entering the atmosphere."

All this is not to say that earthworms aren't beneficial. In fact, Amador believes that worms could be used in place of some of the fertilizers used in commercial agriculture.

"One of the tenets of organic agriculture is the reduction of inputs of synthetic fertilizers, and worms can help do this," he said. "We want to demonstrate to farmers that worms can provide a real benefit to crop production. Sustainable agriculture is all about soil quality. We want to come up with hard data about the contribution soil animals make to the quality of soil."

When worms eat the crop litter left from last year's harvest, they release nitrogen – a major component of fertilizer -- from the dead plants, which can then be used by this year's crops to help them grow.

"To make it work, we have to convince farmers not to remove the crop litter from the previous year, and discourage them from tilling the soil, because that has a detrimental effect on the worms," Amador said. "Farmers know they can till and add fertilizer and get a good crop, but how long can they do that before the soil gives out? We want to encourage a less invasive system of crop production, one in which the farmers work with what they've already got in the soil."