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.
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.
for a cause
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."
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
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