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POSTED 31 AUG 2000 Northern forests are under attack by a silent, invisible and deadly plague: an army of introduced earthworms. By rapidly munching decomposing organic crud on the forest floor, the worms deprive native plants and tree seedlings of a place to germinate and grow. The result is a bare forest floor, lacking most spring flowers and tree seedlings.

Earthworms are a blessing in gardens, where they mix the soil, increase porosity, and leave fertile droppings, and it's always seemed logical that they would similarly benefit other soils. But according to research by Cindy Hale, a forest ecologist at the Natural Resources Research Institute at the University of Minnesota-Duluth, it's not true in ecosystems that are not "used to" worms. In the upper Midwest, all worms were extirpated (that's eco-speak for crushed or frozen!) by Ice-Age glaciers that smothered the area until about 11,000 years ago. And since worms can't travel far under their own steam, the forests have had essentially no worms. In contrast, worms reached cities and farms courtesy of settlers who brought plants and soil).

Above: A forest floor without worms (left) and with (right). Spring flowers and tree seedlings both suffer when earthworms munch the decaying litter on the ground. Courtesy University of Minnesota, Agricultural Experiment Station

The first tip-off that worms might be causing problems came as recently as 1996, when David Shadis, a soil scientist at the Chippewa National Forest in Minnesota, read a paper proposing that worms were devouring the layer of duff -- the decomposing organic matter -- that carpets many forest floors. Wondering if this could explain the absence of low-level (understory) vegetation in some sites, Hale says, "Dave, like a good soil scientist, went out with a shovel and dug some holes, and found that these sites had worms." Sites without worms, she says, looked normal.

Wormy world
Hale wormed her way into the act in 1998, when she chose the question as a Ph.D. topic. Since then, she's investigated hardwood forest -- dominated by sugar maple and basswood, where there's normally a heavy layer of duff -- in the Chippewa, and in similar ecosystems in Michigan and Wisconsin.

Unlike many ecological processes, the situation was relatively easy to study since worms are slow-moving critters, and thus are quite localized. In research presented this spring to the Ecological Society of America (but not yet published), Hale reported that within just 100 meters, the forest changes from worm-free to containing five to seven species of earthworms.

In almost every case, the heavier the worm infestation, the fewer tree seedlings and spring ephemerals -- the flowering herbs that light up a forest floor before trees put out their leaves. "You have a visible leading edge of the invasion moving through forest," says Hale. "It's like a wave, you can draw a line in the forest, on one side there are no earthworms -- or very few. On the other side are lots of worms."

Equally dramatic, a simple shovel test will show that the layer of duff disappears where worms are common. And while mature trees seem unaffected by the worms, the absence of seedlings will eventually doom the forest.

Above: Forest soil without (left) and with (right) introduced earthworms. Notice the thick layer of duff in the worm-free soil.
Courtesy University of Minnesota, Agricultural Experiment Station

Fishing for a cause
The source of the worms is fairly obvious, Hale says. "Almost without exception in northern Minnesota, they advance from lakeshores, boat ramps, and roadside ditches. There's strong anecdotal evidence that the use of worms for fishing bait is an important source of transport in rural areas." For decades, fishing folk have been exhorted to dump extra bait worms into the woods, on the theory that what's good for the garden is good for the glade. Hale, who says she once followed this misplaced advice herself, observes that worms are helpful for mixing and aerating soil in urban areas and gardens, where soil is compacted, and where plants have evolved to germinate and root in topsoil rather than duff.

It's a different story in the deep woods. "In natural habitat, in lush hardwood forest, plants have evolved in absolute absence of worms, and plant germination, establishment, and nutrient dynamics of the ecosystem are fundamentally designed to work in absence of powerful detritovores like earthworms [detritovores, naturally, eat detritus, like duff slowly decaying on the forest floor]. When you introduce exotic earthworms, by changing the decomposition and nutrient dynamics, you turn the ecosystem upside down, you change the environment for all the plants on the forest floor."

At this point, the phenomenon has been seen in many parts of the eastern deciduous forest, which stretches from New England to Minnesota.

Fished out
The research adds to a distressing picture of the impact of recreational fishing, which is not limited to the simple depletion of desirable species. The dumping of bait minnows and the deliberate stocking of fish both alter the species composition in many lakes and rivers.

And since earthworms tamper with the root of forest processes, the research also reinforces the grim picture of ecological chaos due to invasive exotic species. "Any invasive species could have larger impacts if it affects some central feature of the ecosystem," says Hale. "Earthworms are a great example because they attack the forest floor, and that's the center of nutrient cycling and plant regeneration."

Vegetation may come back, but even that may not reverse the damage. In forests that have had worms for two or three decades, the plants that have returned are European buckthorn and garlic mustard, two notorious invasive species that are causing widespread ecological destruction in northern natural areas.

-- David Tenenbaum