their populations are capable of spreading only about 30 feet per year—were given a number of free rides that resulted in their wider dispersal. Anglers began traveling to once-remote lakes in far greater numbers, throwing their unused, still-wriggling bait into the water after a day’s fishing. Landscapers transported infested soil to increasingly far-flung suburbs and developments. Earthworms became popular as natural composters for gardeners and small farmers. And once they established themselves in the temperate forests of North America, Fahey and other scientists say, the worms began to wreak havoc on the carefully calibrated makeup of forest-floor ecosystems. For millennia, fungi and bacteria had been the top detritivores in northern hardwood forests; together they worked to decompose the forest floor’s litter of fallen leaves, twigs, and bark. But they performed this service slowly—so slowly, in fact, that they couldn’t keep up with all that was steadily raining down upon them from above. The result was a permanent layer of organic humus known as duff (or, by scientists, as the O horizon): that soft, spongy, 4- to 5-inch-thick layer that makes the experience of walking in a healthy forest feel like padding about in a pair of slippers. SHORT T A K E
Friend or Foe? The term “invasive species” denotes any non-native
plant or animal that has begun to inflict damage on its ecosystem. But does that mean all non-native species should be viewed with suspicion and actively prevented from establishing themselves? Mark Davis, a biologist at Macalester College in St. Paul, Minnesota, doesn’t necessarily think so. In support of taking more of a wait-and-see approach to the introduction of nonnative species, Davis notes that some of them have actually proved beneficial to their ecosystems (think honeybees), and adds that “unless a species evolved in a particular site, all species are ultimately introduced.” On the other side of the debate are ecologists like Manuel Lerdau of the University of Virginia, who argue that waiting is a luxury most ecosystems can ill afford, given the gravity of the consequences should an introduced species prove to be more harmful than beneficial. To ask ecologists to withhold judgment until all the data are in—which can take hundreds of years—“set[s] an unrealistically high bar for those making management decisions,” Lerdau has written.
Within a healthily functioning O horizon, fungi create vast networks of fine mycelia that intertwine with the tiny roots of trees and plants, facilitating nutrient absorption and water uptake. Simultaneously, bacteria perform various feats of organic chemistry that help establish the soil’s nitrogen cycle. Nematodes and protozoa then graze on both the bacteria and the fungi and, in turn, get grazed upon by arthropods such as mites, springtails, beetles, and millipedes. Salamanders feed on the lot of them. Seeds buried in the duff can survive there largely undisturbed, protected from predators. Seedlings can take their time developing in these moist, mineral-rich surroundings. What Fahey and others have now come to realize is that earthworms are capable of wiping out an entire century’s worth of this ecologically crucial humus in a few years’ time. In Minnesota’s Chippewa National Forest, scientists watched as invasive worms took over plots that had recently been earthworm-free. (Their trajectory—they began near the shore of a lake and gradually made their way inland—strongly suggested an origin as fishing bait.) The worms devoured the O horizon, gobbling up fine tree roots and leaving little behind but a gritty coating of castings and a lone season’s output of leaf litter. What typically follows in such cases is a cascade of ecological damage. Bacteria and fungi begin to disappear. Gone is the O horizon’s protective seedbed. Tree roots become exposed and vulnerable, in a phenomenon some scientists have likened to gingivitis. The supply of herbaceous vegetation is greatly reduced and physically altered, diminishing its availability as a food source for salamanders and its attractiveness as a breeding site for ground-nesting birds. One study of a Wisconsin forest found that rampant earthworm activity had reduced the duff’s thickness by two-thirds and its weight by nearly 90 percent. Pre-invasion, the O horizon had been 1.8 inches thick and had tipped the scales at 3,530 pounds per acre; post-invasion, its thickness had tapered down to 0.6 inches and its weight per acre had dropped to a paltry 393 pounds. So while they might be of great service in a backyard compost pile or at the end of a hook, earthworms are basically up to no good in the American forest. Gardeners should know that, whatever it may say on the website or on the side of the box, the worms they’re purchasing are almost never “native.” Furthermore, most commercial worm dealers aren’t selling a single species of worm inside a box, but rather groupings of species. And anglers should know that most modern earthworm invasions begin at fishing docks. Unused bait worms that get dumped into the water won’t drown, and will very likely find their way to shore before long. Finally, we should all know that earthworms, once established within an ecosystem, are almost impossible to remove. At the very least, however, we can do our part to discourage their further spread into those North American temperate forests that they have yet to invade—merely by thinking twice before blithely throwing a spare nightcrawler over the side of a dock, or ordering 1,000 red wrigglers online for $19.95. I’ll bet even Charles Darwin could get behind that. Bruce Stutz is a contributing editor to OnEarth. His most recent article for the magazine was “Will Superscientists Save the Day?” (Spring 2013).
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