State College, Pa. — From the Poconos in Pennsylvania to the forests of Alberta, there is an invasion under way — and underground.
Exotic species of earthworms from Europe and Asia are devouring the leaf litter that is so vital to northern forests, altering soil conditions, enabling the spread of invasive plant species and changing the food chain for forest animals.
It is not a new problem — explorers often used soil as ballast for their ships so earthworms probably have been arriving as long as foreign visitors have come to North America. Only recently, however, have scientists begun to understand the effect earthworms are having on their newfound ecosystems.
"It's a big issue for forest health," said Mike Blumenthal, forest health supervisor for the Pennsylvania Bureau of Forestry.
Fran Lawn, right, assistant director of land restoration at the Schuylkill Center for Environmental Education, and Anne Todd Bockarie, assistant professor of the Biology School of Science and Health at Philadelphia University, count Asian earthworms in a forested area at the centre in Philadelphia. (H. Rumph Jr./AP)
Internet Links
- Archbold Station
- Cornell University Invasive Plants site
- Pennsylvania Bureau of Forestry
- Schuylkill Center for Environmental Education
The idea of earthworms as environmental enemy seems as foreign as the earthworms themselves. But because glaciers eliminated earthworms from vast reaches of North America thousands of years ago, forests across the northern part of the continent have evolved without them.
"In agricultural settings, in gardens, everything that our grandmothers told us is true: They are good for the soil, they aerate the soil, they turn over organic matter, they break down and loosen the soil, they allow moisture," said Dennis Burton, director of land restoration at the Schuylkill Center for Environmental Education in Philadelphia.
"But when you put them into a forest situation, that's where the problems begin."
That's because in a forest, earthworms quickly devour the leaf litter and other surface material that form an integral part of the ecosystem.
In doing so, the earthworms reduce acidity and boost the nutrients available in the soil. In a garden, that's great; but most of the plant species in northern forests have evolved to do well in the low-nutrient, high-acid soils — conditions that had also prevented weeds and other invasive species from gaining a foothold.
At sites in New York and in the Poconos in Pennsylvania, John Maerz and colleagues from Cornell University found that areas with earthworm infestations also were more likely to have invasive plant species.
Mr. Maerz also found that the presence of earthworms often signalled a sharp decline in salamander populations. By eating the leaf litter, worms destroy the habitat that housed tiny insects and arthropods that salamanders fed on.
"We think that the earthworms are affecting the (food) chain below them," he said. "And there are lots of things, at least historically, that would have fed on salamanders — snakes, shrews, thrushes, screech owls, everything."
Most foreign worm infestations are found in urban forests, such as the one that the Schuylkill Center manages, and in residential areas where worms might have arrived in the root balls of ornamental plants.
Increasingly, however, invasive worms are being found in more remote forest systems, accidental deposits along logging roads and hiking trails, and escapers from fishermen's bait batches, said Patrick Bohlen, biologist and director of research at the Archbold Biological Station in Lake Placid, Fla.
Once the worms are entrenched, trying to get rid of them would take a "Herculean effort," Mr. Bohlen said, "and you'd probably end up harming other species as well."
Despite the effects that foreign earthworms are having, Mr. Maerz suggests that they might eventually find a peaceful equilibrium in North American forests.
"In places like Europe, where these earthworms are from, they have forests with lots of leaf litter and lots of herbs and plants that survive there in the presence of earthworms," Mr. Maerz said. "So are we maybe looking at a situation where we just have to be patient?
"The system may come into some degree of balance."
