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Invasive Earthworms—A Threat to North American Forests
Plants & Gardens News | Volume 19, Number 1 | Spring 2004
by Niall Dunne
Charles Darwin was an earthworm freak. He spent more than 40 years, on and off, observing, experimenting on, and thinking about these artful annelids. It got a little weird sometimes, like when he had his son play the bassoon to an audience of worms in the billiard room.
Darwin published his findings in a slim volume entitled The Formation of Vegetable Mould through the Action of Worms with Observations on their Habits (1881). In it, he outlined the monumental importance of earthworms to the development of soils—describing how they help plow, aerate, hydrate, and fertilize the earth. "They mingle the whole intimately together," he wrote, "like a gardener who prepares fine soil for his choicest plants."
Darwin's "vermiphilia" lives on today in the hearts of gardeners and farmers who embrace a biologically integrated approach to plant cultivation. But, regrettably, his idea that all earthworms are fundamentally beneficial is a bust. Some species, when placed in the wrong context, can become outright pests.
Indeed, a number of nonnative earthworm species have established themselves in North America and been implicated in a range of undesirable activities, including the extirpation of at least one rare plant species. Gardeners need to pay special attention, as they have been identified as one possible source of introduction for some of these troublesome interlopers.
An earthworm-invaded site (left) and a non-earthworm-invaded site (right) in the Chippewa National Forest in Minnesota. Earthworms are linked to the absence of tree seedlings in parts of the forest.
American Natives
In Darwin's defense, he was aware that earthworm activity could cause problems—such as soil erosion due to washed-away castings. Moreover, his study was limited to mainly pastoral lands in England and probably to a narrow range of earthworm species.
There are, however, thousands of earthworm species worldwide, adapted to many different environments. North America has roughly 100 native species. While this doesn't seem like a mind-blowing number, it's pretty impressive when you consider that Pleistocene glaciation killed most worm species in the northern half of the continent.
Because earthworms are extremely slow moving, colonization of the formerly frozen areas has been negligible. Our native earthworm fauna is still confined mostly to the Southeast and Pacific Northwest.
Foreign Arrivals
Since Colonial times, though, about 45 exotic earthworm species have made a home for themselves in North America. These include hardy European and Asian species that can survive the cold, unforgiving winters of our higher latitudes, many of them introduced through the importation of potted plant material for horticulture and land management. Today, there are strict laws prohibiting soil imports, but exotic earthworms are still gaining access to the country via the vermiculture industry, which imports worms for fish bait, bioremediation, and composting. (See "What's In My Worm Bin?")
Exotic earthworm populations are now established in many of our urban, suburban, and agricultural soils. They've also managed to hunker down in wild areas—thanks in large part to the misguided dumping of bait by fishermen. And this is the crux of the problem: the addition of worms to forests and grasslands that have evolved for millennia without them.
The Diet of Worms
Hardwood forests in the North are being hardest hit by the pest-earthworm invasions. The most immediate threat is from "epigeic" earthworms, which inhabit and feed on leaf litter on the soil's surface.
In healthy, undisturbed forests, a rich layer of litter covers the forest floor and is held together by webs of fungal filaments. The fungi, along with other decomposers, slowly break down the litter and release nutrients to forest plants—the key word here being "slowly." Soil pH is low, and the native plants are generally adapted to acidic conditions. The leaf litter is not just a nutrient bank; it's the medium in which forest plant seeds germinate. It also acts as a mulch, insulating the soil and protecting plants from disease and competition with weeds. In addition, it provides habitat for many small forest animals.
Invading earthworms feed voraciously on the leaf litter, breaking it down too fast and flooding the soil with nutrients, especially nitrogen. (Much of this valuable stored nitrogen may eventually be lost via runoff.) Some worm species can also neutralize the soil pH with special calciferous glands to create a more favorable environment for themselves. Both actions dramatically change the soil chemistry in the forest and interfere with plant growth. Also, as the leaf litter is consumed, bare patches appear on the forest floor, making it vulnerable to erosion and to invasion by nitrogen-craving weed plants. Leaf-litter animals are deprived of their habitat. If burrowing worms are present, a harmful mixing of soil strata can occur.
Case Studies
Some of the most compelling evidence of this phenomenon has come out of the Chippewa National Forest in Minnesota. Back in 1996, researchers discovered a link between the absence of tree seedlings and spring wildflowers in the forest and the presence of six or seven exotic earthworm species in the soil, and they've been studying it ever since.
Cindy Hale of the University of Minnesota-Duluth has spearheaded the research and even set up a web site called Minnesota Worm Watch (http://www.nrri.umn.edu/worms) to raise public awareness about the invasion problem. The site contains dramatic images of the damaging effects that European worms like Dendrobaena octaedra and Lumbricus terrestris can have on the leaf litter, soil profile, and plant health of the forest.
In 2002, Michael Gundale of Michigan Technological University published a report detailing how the epigeic bait and compost worm Lumbricus rubellus may be wiping out populations of the rare goblin fern, Botrychium mormo, and possibly other rare native plants too, in the Chippewa National Forest. Gundale credits the epigeic worm's destruction of mycorrhizal fungi in the soil as a reason for the goblin fern's decline.
John C. Maerz and colleagues from Cornell University have found strong evidence linking salamander decline in the hardwood forests of central New York and southeastern Pennsylvania to invasions by L. rubellus and Asian Amynthas species, among others. Maerz says that by eroding forest leaf litter and humus layers, the earthworms are driving down arthropod numbers and depriving juvenile salamanders of a key food supply. The Cornell team has also found strong links between exotic earthworm infestations and the invasion of forests by pest plants such as Japanese barberry (Berberis thunbergii). This possibly symbiotic connection has also been noted in New Jersey by researchers from Rutgers University.
Possible Solutions
Scientists, land managers, and conservationists are in a quandary over how to address the invasive earthworm issue. Experiments to find viable means of controlling earthworm invasions are under way. Dennis Burton, director of land restoration at the Schuylkill Center for Environmental Education in Pennsylvania, has had some success excluding Amynthas species from test plots using sulphur and pine needle mulch (which lower soil pH) and black walnuts (which are high in phytotoxins). But it remains to be seen if any control methods can be developed for viable large-scale forest restoration, restoration that doesn't threaten collateral damage to other organisms.
Given the slow migration of earthworms in soil, however, most experts agree that containing them is of lower priority than preventing new introductions. This means developing stricter laws regulating the importation of worms (which the USDA is considering) and getting the word out to the public.
"If you fish, do not dump your extra worms out on the ground," urges Burton. "Throw them entirely in the water or take them home. If you plant new trees and shrubs on your property, examine the root balls for worms and destroy any you find."
His advice to gardeners makes sense (especially to those who live near forested areas) because, although there might be controls on the importation of contaminated soils from outside the U.S., there are few regulations addressing the movement of soils within the country. It's conceivable that potted ornamental plants from nurseries in earthworm-infested areas are acting as vectors for new invasions.
Niall Dunne is the associate editor of Plants & Gardens News.