Nature’s Gardener or Forest Invader?

From farms and gardens to the forest floor, earthworms are dramatically changing the world beneath our feet—for better and for worse

02-01-2009 // Hannah Schardt

IF YOU ARE AMONG the millions of Americans who keep a garden, you already know that earthworms deserve more affection and gratitude than their lowly reputations might indicate. Worms mix and aerate soil, making it a more appealing place for seeds to germinate and for plants to grow. Their tunnels create pathways for roots to follow. And in ways that are still becoming clear to scientists, worms increase the population of beneficial microbes in the soil. These microbes in turn make the nutrients in organic matter more accessible to plants.

“What comes out of the back end of an earthworm is hundreds—probably thousands—of times more microbially active than what goes in,” says Clive Edwards, a soil ecologist at Ohio State University.

But some recent research indicates there may be a darker side to nature’s gardeners. A number of recent studies indicate that earthworms may be to blame for the decline of northern hardwood forests. A surprising natural history lesson helps explain why: Nearly all of the earthworms in what would become the northern United States were wiped out by a massive glacier that receded at the end of the last ice age, some 11,000 years ago. That means for millennia the forests of Minnesota, Wisconsin, Michigan and other northern states were virtually worm-free, leaving them to grow a thick, diverse understory. The arrival of European settlers, however, brought dozens of nonnative earthworm species to the New World. The voracious invertebrates—including the pinkish gray night crawlers so familiar to gardeners and anglers—arrived on rootstock and other agricultural cargo and soon made themselves at home on the continent.

Some forests colonized by European and Asian worms changed dramatically as a result. The very soil-altering abilities that make these worms invaluable to farmers and gardeners also create conditions less suitable for certain native plants. Without earthworms to quickly digest them into soil, leaves gather in a spongy layer known as duff on the forest floor. The layer nurtures small plants such as wildflowers, ferns and seedlings. Once introduced, worms turn duff into soil, making the forest floor less hospitable to native plants that have thrived there for thousands of years.

A 2002 study by Michigan Technological University found that the goblin fern, which lives mostly underground and grows only in the upper Great Lakes region, was disappearing from a number of sites in Minnesota’s Chippewa National Forest where nonnative earthworms had colonized the soil. The forest floor at study sites where earthworms were present was only half as thick as that at worm-free sites.

More recently, studies have also indicated that earthworms may be partly to blame for the spread of ragweed, a particularly tenacious, invasive plant that produces highly allergenic pollen. Colleagues of Edwards at Ohio State University found that night crawlers—which, unlike most earthworms, build permanent burrows—drag ragweed seeds into their homes, in effect planting them beneath the soil. According to the study, seedlings that sprout from night crawler burrows are more likely than others to grow into large, healthy plants.

So does this mean that the invertebrates are actually villains? Not quite, says Edwards. For one thing, some of the ragweed seeds buried in night crawler burrows either rot, becoming food for the worms, or germinate and quickly die because they are so deep underground. So even though some ragweed plants result from being “planted,” on balance “we’re not sure whether earthworms promote or suppress” the spread of ragweed, he says.

While damage to forests is troubling, as the goblin fern study proves, Edwards maintains that efforts to eradicate the worms are ultimately far more detrimental than the worms themselves. Since pesticides are bad at distinguishing between invasive earthworms and native creepy-crawlies, they often end up doing more harm than good.

In effect, earthworms, even nonnative ones, are here to stay—and that’s a good thing, says Edwards. For decades, he and his colleagues have researched innovative ways to put the animals to work in vermiculture, the cultivation of earthworms for use in composting. Most home gardeners do not yet breed their own worms or produce their own vermicompost, though that practice is growing in popularity as the word spreads and home vermicomposting becomes simpler. For those who want the benefits of worm compost without caring for thousands of wrigglers in their garage, a number of companies are filling the growing demand by manufacturing soil additives, potting soil and “worm tea”—a liquid fertilizer made by steeping vermicompost in water—in large-scale worm composting operations.

This all makes earthworms the ultimate recyclers. Table scraps, lawn waste, even old newspapers—earthworms turn it all into valuable garden compost.

Of course, even without human intervention, earthworms manage to do a pretty good job of cultivating soil. A decade ago, Edwards and his colleagues conducted experiments in fields, adding and removing earthworms to see how the animals’ presence affected crop growth. “We found that they increase soil fertility immensely,” he says—lofty praise for the lowly worm.

Hannah Schardt is a senior associate editor at this magazine.

Fact: Worm Production

It has been estimated that an earthworm can produce its own weight in castings—the fertile dung that makes the invertebrates so valuable in the garden—every 24 hours.

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Earthworm Facts

“It may lie 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.” —Charles Darwin, 1881

Charles Darwin was right about many things—not least of which is the role of earthworms in the environment. Here are a few facts about the ubiquitous and frequently underestimated invertebrates:

  • Studies have found as many as 350 earthworms occupying a single square meter of forest floor.

  • Earthworms may help keep greenhouse gases out of the atmosphere by feeding on forest litter and digesting it beneath the surface of the soil. Carbon dioxide, which is emitted by decomposing organic matter, is instead stored underground in the form of worm castings.

  • Some earthworms have the ability to regenerate lost segments, but that doesn’t mean you should go around chopping them in half. Most of the invertebrates will die if they lose more than a small portion of their bodies—and very few, if any, will survive as two separate worms.

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