When Worms Turn

By Beth Daley

Boston Globe  | 

April 10, 2007

Ordinarily, earthworms are considered gardeners' most trusted helpers, natural plows that churn dirt and deposit nutrient-rich beads of soil that feed plants.

But the wrigglers have a darker side.

Scientists are concerned that people, from fishermen to landscapers, are inadvertently accelerating a 350-year-old earthworm invasion in the forests of New England and the northern United States.

To be sure, it's a slow-motion invasion: Many worms spread just half a mile every century. But they are now so numerous and widespread that they are dramatically changing the forest ecosystem, devouring a layer of the forest floor that native wildflowers, beetles, and other species need to survive. The worms are also changing the soil's chemical composition, scientists say, perhaps altering the availability of nutrients that support trees like sugar maples.

"A lot of people here in New England are in the mind-set that earthworms are all good, but really, that is not always the case," said Josef H. Görres, a research associate professor at the University of Rhode Island who is studying earthworms' effects on forests and agriculture.

Scientific interest in invasive earthworms is growing. In September the journal Biological Invasions devoted a special issue to the impact of earthworms. Görres's earthworm work is supported by the US Department of Agriculture's National Research Initiative, most recently with a $75,000 grant. Federally sponsored invasive earthworm work is being done in Iowa, Georgia, Minnesota and New York.

Contrary to popular belief, the earthworms found in the gardens and forests of New England aren't native. Virtually all of the worms north and west of New Jersey were wiped out during the ice age that ended about 10,000 years ago.

European earthworms -- or their cocoons -- first hitched rides to the New World on the root balls of colonists' plants or in dirt that was used as ballast in ships to steady them on the long journey across the Atlantic Ocean.

In the 1800s, much of the region's vast forests were cut down for farmland, and worms -- clinging to plants or even plows' wheels -- were introduced to more areas of New England. A patchwork of wormed and worm-free woods was created when forests later reclaimed the farmlands.

Now scientists suspect that humans are again bringing worms into New England's remaining worm-free woods. Fishermen abandon nightcrawlers or other worm bait at fishing holes. Second homeowners are worming their property with landscape dirt and, possibly, compost piles. Hikers and campers unwittingly bring along worm cocoons, wedged in the tire treads of their cars.

"What's changing now is the explosive use of earthworms by people," said Lee Frelich, director of the University of Minnesota's Center for Hardwood Ecology. Minnesota's forests are under siege from human-transported worms, said Frelich, who with colleagues has documented widespread changes in forest floors. He's so concerned about earthworms, he's helped launch a public relations campaign to encourage fishermen to "contain those crawlers."

In gardens, worms aerate and loosen compact soil, but they have the opposite effect in forests. Most northern forests have a spongy floor of decayed leaves and other organic debris called duff -- the stuff your boot sinks down in when hiking off trail. Native ferns, wildflowers, tree seeds, beetles, and salamanders need the duff layer for nutrients and protection. The layer can also prevent erosion.

Once worms colonize an area, they can munch through the duff layer and can leave behind a homogenized hard soil layer in as little as three years. Invasive plants such as barberry and buckthorn can move in more easily, and erosion can increase. Scientists also say the worms change the chemical structure of the soil, and they are studying whether trees like sugar maples get fewer nutrients in regions colonized with the worms.

"If you lose the duff layer, it's a fundamental change to the ecosystem," said Peter Groffman, a microbial ecologist at the Institute of Ecosystem Studies in Millbrook, N.Y. He and colleagues at four other US institutions are studying earthworm invasions with a $850,000 National Science Foundation grant.

Researchers estimate that New England forests could have as many as 15 species of European and Asian earthworms, though typically no more than five species will occupy the same forest swath. Among the most common are nightcrawlers, which can burrow down as much as six feet into the earth, changing the dynamics of the forest floor. Nightcrawlers mark their entrance holes on the forest floor with a small pile of leaves and beads of soil.

For reasons scientists don't quite understand, worms can colonize an acre in numbers ranging from the tens of thousands to more than 1 million. While some researchers say the acidic nature of some New England soils may limit the worms' invasion, others say they are finding some worms well suited to acid soils.

"We have so many questions, and we really know very little in New England," said the University of Rhode Island's Görres. In addition to forests, he is also studying whether the valuable fertilizer that farmers spread on fields is being drained down wormholes when it rains.

Scientists say they want to better understand the distribution and types of earthworms in New England to confirm the anecdotal evidence that they are spreading more rapidly and which ones may be causing the most harm.

But perhaps their greatest challenge, said Patrick Bohlen, a research biologist at Florida's Archbold Biological Station , is to get the public to adjust their image of a creature Aristotle once called the "intestines" of the earth and Charles Darwin immortalized with years of research and a book.  "It's a difficult sell," said Bohlen.