(exotics)

Earthwormed Over

Northern forests had been bereft of native earthworms since the most recent ice age. Today they're the front lines of a slow, squiggling invasion that may be coming to a forest near you.

By Peter Friederici

Friend to anglers, ally of gardeners, squirming subject of a muddy toddler's curiosity—nothing is more dependable than the lowly earthworm. It's the king of compost, churner of soil, architect of ecosystems. No less an authority than Charles Darwin once wrote, "It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures." It may be time for some historical revisionism. The earthworm, it turns out, can be an ecological house wrecker. "Worms change how nutrients are cycled and alter the structure of the soil," says Cindy Hale, a University of Minnesota biologist who, since the late 1990s, has been studying how northern-forest ecosystems are being altered by earthworms. "They have a cascading effect on plants, animals, and soil organisms. And we know they're causing significant damage to some forests. Their effect could be really profound."

North America has native earthworms, but many of the species most abundant today crossed the Atlantic and Pacific in recent centuries, hidden among plant roots or in ships' ballast. With the help of anglers and gardeners, they have spread widely, and populations continue to creep their way across North America at the brisk pace of 15 to 30 feet a year. Now, after centuries on the march, the ecological carnage is just becoming clear: Soil cover across large swaths of forests is being denuded, native plant communities are disappearing, and with that, so is the habitat for a host of animals, such as ovenbirds and salamanders.

"They have a cascading effect on plants, animals, and soil organisms. And we know they're causing significant damage to some forests. Their effect could be really profound."

Like habitats that support a mix of native birds along with imported starlings, many ecosystems today support both native and nonnative earthworms. No one can say how much their ecological effects differ, but what is clear is that the nonnatives have had their most profound impact in some of the northern forests that stretch from New England through New York and the Great Lakes region, which have lacked native earthworms entirely since the most recent ice age. Now they're the front lines of a slow, squiggling invasion.

The idea that earthworms are suddenly foes and not friends is not easy to grasp, admits Hale. "Even a lot of ecologists aren't willing to accept that worms can have negative effects," she says. "We're taught in kindergarten that worms are good. But these are ecosystems that evolved since the last glaciation in the absolute absence of worms."

In 1995 Dave Shadis, a soil scientist for the Chippewa National Forest, noticed that the forest floor was changing rapidly near the shoreline of Leech Lake, a large and popular fishing locale in northern Minnesota. A thick layer of spongy duff was disappearing, taking with it wildflowers such as bloodroot and wild ginger. Wielding a shovel, he discovered that earthworms were present wherever the duff was vanishing.

Three years later Hale began monitoring the areas where duff was visibly giving way to bare soil; since then she has been documenting changes in soil composition and plant life—essentially charting the path of a full-blown worm invasion. On the summer day I visit the site, Hale and Andy Holdsworth, a fellow biologist at the University of Minnesota, are on their knees, their noses a few inches above the black loam, eyeing the incursion up close.

Hale pours a yellowish solution of water mixed with irritating mustard powder over a patch of the forest floor about 200 yards from the lake, and Holdsworth picks up the first emerging creepy crawlies—slugs and beetles—with a pair of forceps. Then, one after one, the worms surface from the jaundiced ground. They rise out of narrow burrows like the business ends of roots, long bodies writhing as they try to escape from the tongs.

"There's one of the species that feeds on the deeper, mineral soil," Hale says, pointing to a specimen an inch and a half long and no thicker than an alphabet-soup letter. "See how grayish the gut is? It's been feeding. And it's an Aporrectodea, an adult—they mix leaf litter into the deeper soil. And here comes a night crawler!"

After five minutes Holdsworth has extracted 30 worms—some tiny, some night crawlers plenty big enough for a bait bucket. Hale does the math: about 250 per square yard, which works out to more than a million worms per acre.

The ground here is mainly bare. In dusky light filtered through basswood and sugar maple foliage, the first fallen leaves lie tan and umber among scattered maple seedlings and low clumps of sedges. Night crawlers and the smaller species known as Lumbricus rubellus, Holdsworth explains, do exactly what gardeners like: move organic material from the surface into the soil. Here they clean the forest floor so effectively that fallen leaves vanish in a few weeks. He points out other worm sign, too, such as a smoothed boulder protruding from the forest floor. It's topped with a perfectly round cap of green moss, like a skullcap, that's separated from the soil by four vertical inches of bare rock. "I call this 'forest gingivitis,' " he says. "This rock had duff around it to that moss line. It disappeared in only a few years." Tree roots and fallen branches lie exposed.

We take a hike inland across Hale's study site. After about 50 yards the bare soil vanishes, giving way to a continuous layer of fallen leaves that hides the rocks and roots. More wildflowers appear. Worms are just beginning to invade this area. A little farther along, three and four inches of fallen leaves lend a spring to our steps.

At the end of Hale's study area, 150 yards from where we began, the plant diversity has swelled. Here grow wood anemones, bellworts, Solomon's seals, twisted-stalks. Instead of lying exposed, fallen branches and trunks are half hidden and half decayed in the duff. This is what the North Woods are supposed to look and feel like: rich, bouncy, smelling of organic matter. To show us how the place works, Hale uses a bulb planter to raise a six-inch-long soil core. The undisturbed mineral soil at the bottom is fine, gray, and silty, nothing like the coarse black workings of earthworm guts. Above it is a three-inch stack of duff, stitched together with a webbing of slender roots that don't extend into the soil at all.

This duff is key to the workings of the northern forest. In it, nutrients are slowly and consistently released by the area's native bacteria and fungi. But worms are literally eating the rooting and seeding zone right out from under the plants.

The disappearance of the duff layer has a domino effect beyond the native plants. John Maerz, an ecologist at Cornell University, has been studying how salamanders respond to the presence of earthworms in forests in central New York and eastern Pennsylvania. Earthworms, he notes, happen to be a great food source for salamanders, but they change forest habitats to something entirely unlike what salamanders need. The duff layer provides salamanders with shelter and moisture, and it's prime habitat for many of the arthropod species they prey upon. When it disappears, salamanders lose food and protective cover, and their populations sink, as Maerz has found at a number of his study sites. "As the leaf-litter layer declines, we find fewer and fewer salamanders," he says.

In many parts of North America, exotic earthworms have been present for so long that it's hard to know for certain what effect they've had. That's why the worms in your garden or compost pile probably aren't a problem: They are recycling the organic material plants need, in an environment that's adapted to them. Nonnative worms have lived for so long in some places that an ecological balance may have been reached—although, as Maerz points out, there is always the danger of newly introduced species tipping the scales. The main problem worms pose is in the places, such as Minnesota's northern forests, that aren't adapted to worm activity at all.

Perhaps most worrisome is that while earthworms move slowly, no one knows how to stop them once they've arrived. Moreover, they're easily spread—not just when anglers release their last worms at day's end or when gardeners move a plant, but also when knobby-tired trucks or all-terrain vehicles transport mud from one place to another.

Some state officials and land managers are starting to take action. The Minnesota Department of Natural Resources recently refused a request to import and sell yet another nonnative worm species, and has distributed flyers to bait shops urging anglers not to release leftover worms. Hale and her colleagues have conducted teacher workshops and overseen the development of a website that features activities, identification tips, and an interactive database in which volunteers can report worm sightings. Canadian forest managers have begun a monitoring program that relies heavily on online observations by citizen scientists in both countries.

It's going to be difficult to upend the gospel of worms as benevolent, eco-friendly creatures. But Hale and Holdsworth envision a cadre of volunteer observers who can alert forest managers to new invasions. They imagine a large-scale public-education campaign that will encourage anglers not to drop leftover worms on the ground. And one piece of good news gives them hope: Holdsworth has been surveying national forest lands in Minnesota and Wisconsin, and he has found that many areas with little fishing remain worm-free.

He has also found a change in public attitudes. "In the last few years we've noticed a lot more people who are aware of the issue," Holdsworth says. "The word is definitely getting out." That's certainly welcome news. For Charles Darwin knew that, as the worm turns, so does much of the world surrounding it'even if for very different reasons than he could have imagined.

Peter Friederici is a freelance writer whose books include The Suburban Wild (University of Georgia Press, 1999).

© 2004  NASI