A boon to robins, gardeners, farmers, fishermen and even grizzlies, earthworms aerate the soil and bore underground avenues for plant roots and water. Ever since Darwin demonstrated in 1881 that the worms in an acre of land can turn living and dead vegetation into 18 tons of castings every year simply by passing the stuff through their guts, earthworms, we thought, could do no wrong.
As it happens, however, earthworms — at least in some parts of North America—have a downside. The time has come to expose them for what they are: alien invaders. In some farm areas, nitrates are getting into the groundwater via the tunnels of exotic worms. Alien invasions have occurred in tallgrass prairie, Southern grassland and along riverbanks in the Midwest. In the Pacific Northwest, aliens take over forest land that has been clear-cut.
One of the best-studied invasions is in the Northern forests. With the efficiency of the sandworms in Dune snarfing people and machines, worms in Northern forests are downing what biologists call the duff layer, the cushy bed of decomposing leaf litter that carpets the forest floor.
The duff layer is the forest's digestive system. Its microbes and fungi break organic matter down into essential elements such as nitrogen and phosphorus so that plants can absorb them to start the cycle over again. Its entangled litter creates cover and food for larger members of the community: insects, spiders, salamanders, frogs, small mammals and migratory birds. A duffless forest floor can't support spring ephemerals like bellworts, trillium, yellow violets and wild ginger. Instead, exotics such as garlic mustard can muscle in, taking up space and shading out native flowers and tree seedlings.
The alien worms work fast. "Ordinarily a tree leaf that falls to the ground in a deciduous forest takes three to five years to decompose and be incorporated into the soil [by microbes and the hyphal filaments of fungi]," Minnesota biologist Steve Mortensen has written. "In forests infested with night crawlers, this process can take as little as four weeks."
Earthworm taxonomist Sam James, a professor at Iowa's Maharishi University of Management, says that until the arrival of European colonists, the continent above the glacial line was worm-free. "When ice sheets covered much of northern North America," he says, "native earthworms were eradicated." The glacial edge runs from Washington State to Long Island, with a southerly dip below the Great Lakes and Ohio.
Although native earthworms are found below this line, the innocuous locals, which number 90 or so identified species, still haven't squirmed more than 100 miles north in thousands of years. The invaders came to this continent packed in the soil around potted plants, in ships' ballast or tucked in the hooves of livestock. Aided by their fevered reproductive rate, the official blessing of the U.S. Department of Agriculture and a reputation for results and durability with fishermen, they prospered.
Still, until the past couple of decades, exotic duff-gorging worms were apparently uncommon in undisturbed forest areas. Back in the 1960s worm expert Gordon Gates (considered by at least one biologist to be the "greatest oligochaetologist who ever lived") was the first to suggest that exotic earthworms were arriving in forests with fishermen who liberated their unused bait.
In a woodlot on the campus of the University of Minnesota, graduate student Cindy Hale pointed to telltale signs of a night crawler earthworm invasion: little piles of worm castings adjacent to "pores" bored into the balding ground. Perhaps these worms escaped from a bucket of ornamental shrubs. Or perhaps they were dumped by a fisherman. Something like this apparently transpired in the Big Woods maple and basswood area of central Minnesota, say the scientists who work there. Now in those places where the night crawlers have appeared with five other smaller exotics, both the ground vegetation and the understory are nearly gone.
There's little doubt that the pattern of these earthworm-damaged forests correlates with fishing, adds Mortensen. "We have found the evidence, a leading edge of cropped-duff radiating out from lakes, boat landings, resorts and other places where people fish." So far, signs of the disappearing duff phenomenon have been documented in selected hardwood forests in Minnesota, Wisconsin, Michigan, New York and Canada.