Earthworms aren’t the soil heroes you imagine

  • A common earthworm, Lumbricus terrestris. The are more than 6,000 species of earthworms worldwide. Gabriela Braendle, Agroscope/via Flickr

For the Gazette
Published: 4/21/2020 12:49:34 PM

Most of us have a rosy image of the earthworm, that aerator of soil without whom our gardens would grievously suffer. Our myths suggest that worms turn infertile soil into fertile compost, fluffy loam.

In fact, the opposite is true: The worms are in rich soil because that’s where the compost is. Earthworms eat the nutrients in the compost; they don’t enrich it. Worms eat almost everything in the soil — fungi, invertebrate life, leaves, twigs, algae, moss and microscopic life. In passing soil through their gut from one end to the other, they throw out castings that mark their paths.

In fact, worms can be more problematic than beneficent. Some worms improve soil conditions in agricultural fields by aerating the soil, while others impoverish forest soils.

The ecological or human value of worms depends on which species we’re talking about: There are about 6,000 species worldwide and more than 180 in the United States, a third of which are non-native. Not all earthworms are similar to the one we most often see, Lumbricus terrestris, or “nightcrawler,” which tends to copulate above ground at night where we easily see them and find them on rainy days wriggling around our driveways.

Different worm species inhabit different parts of the soil: Some live on the top and others in tunnels that they permanently inhabit. Some eat fresh leaf litter, others decomposed organic matter, and still others consume the minerals, fungi and other invertebrate life a foot below the surface. Different species of worms are found in all parts of America and in all climates.

Worms live everywhere in the world, except in Antarctica and the high Arctic, because no earthworm can survive under glaciers or in permafrost. The most recent glacial age in our part of the world ended about 12,000 years ago; by the time the first indigenous people arrived here, as the glaciers retreated, earthworms had been wiped out.

A large portion of the British Isles, however, was never glaciated. The English in the 1600s arrived here with that most humble hitchhiker, Lumbricus terrestris, in the roots of their plants and the ballast of their ships.

Worms radically changed the soil structure of both meadow and woodlands in this region. After the last glaciers had left, New England forests developed without worms, and leaf litter built up only slowly, decomposing through the action of fungi, insects, algae and microscopic life. Nutrients from the litter and duff went deep into the roots of trees, nourishing them more richly than today.

When earthworms arrived, they impoverished the forest soils, keeping many nutrients in the topmost soil layers and short-circuiting nutrient networks in the soil. At the same time, the newly introduced worms worked wonders to keep agricultural soils friable by aerating the soil, thus improving crop yields.

Northern forest soils do just fine without earthworms. In fact, they thrive without them. The roots of trees and other plants interact with myriad fungi that produce enzymes creating mutually beneficial results for all soil life.

The forest ecosystem is tied together by a web of interactions. Robert MacFarlane’s recent book “Underland” offers a glimpse into the wonderful “wood wide web,” describing recent research that characterizes soil as a living network dependent on a rich flora and fauna humans rarely see, much less contemplate.

Imagine what is going on underground, the interlacing networks that reflect a constantly changing equilibrium to enrich the communities of plants and animals. Imagine the pre-colonial forests of New England with their extraordinary woodland flowers of New England and how the spring blooms would have carpeted the forest floor and captured the eyes of native people.

Earthworms interfered with the forest network by consuming nutrients that would have otherwise contributed to the established soil residents. Some scientists have been experimenting with the relationship between non-native worms and invasive plants, suggesting that these two members of Eurasian woodlands are creating conditions inimical to native species — in short, creating a forest ecosystem that more closely resembles non-native communities.

Non-native earthworm species continue to expand their range, raising concerns about the long-term consequences for forest ecosystems. For example, the “jumping” worm (Amynthas spp.), introduced from East Asia in the 1930s as a bait worm, is present throughout New England and into the Midwest. According to a University of New Hampshire Extension bulletin, these worms “alter soil structure more than any other worm [and] can be extremely detrimental to native plants and animals.”

Earthworms are just one example of the law of unintended consequences. They demonstrate how a seemingly benign introduction of animals and plants can have far-reaching deleterious ecological effects.

John Sinton is adjunct professor of landscape architecture and regional planning at the University of Massachusetts Amherst. He is a co-moderator of the citizens group the Mill River Greenway Initiative and an honorary board member of the Connecticut River Conservancy. His book “Devil’s Den to Lickingwater: The Mill River Through Landscape and History” was published in 2018 by Levellers Press. For more information about jumping worms, see the UNH bulletin at tinyurl.com/earthmatters-50 and an article from the Valley News (West Lebanon, N.H.) at tinyurl.com/earthmatters-51.




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