We have reported through the years on how Earthworms that have been arriving in North America from Europe and Asia for 500 years via shipments of plants and soil are completely wrecking havoc on our northern environs where historically earthworms were not present.............The worms consume the duff and litter on the forest floor at astonishing rates, drying soil and thus diminishing the germination of tree seedlings including Beech, Striped and red maple and various species of ferns............Concurrently, with less leaf litter on the forest floor, ground dwelling bird species like the Ovenbird are shrinking in numbers due to a decline in the plant species that they use to nest in................When a species of animal or plant is brought to foreign locales via human assisted means, biodiversity is turned upside down due to no natural control agents being available in the environment to counter the "invasion" of the exotic organism(s)

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The humble earthworm may be a threat to plant diversity in natural ecosystems, says a study just published by researchers from Université Laval and Université de Sherbrooke. Their work found an association between the presence of these European-introduced invertebrates and reductions in the abundance of certain tree and other plant species in the understory of sugar maple forests in southern Québec (Canada).

European Nightcrawlers(earthworms)

The researchers visited 40 parcels in 5 sugar maple forests in the Eastern Townships, finding earthworms in half of all the sites. Their analyses uncovered a correlation between the number of earthworms and the abundance and diversity of certain understory species. New shoots of red maple, striped maple, American beech, and two fern species became rarer as earthworm populations increased. The presence of earthworms does however seem to be good for ash trees and grasses.

"The most likely explanation is that the earthworms consume organic matter in forest litter," suggests Line Lapointe, a professor at Université Laval's Faculty of Science and Engineering and the study's lead author. "This results in soils that can't hold as much moisture, and that in turn interferes with seed germination and the ability of some species' plantlets to survive."

The threat is not urgent yet, but Dr. Lapointe believes there are grounds for concern. "Earthworms have started to change plant composition in sugar maple forests," she noted. "If nothing is done, these changes could become more pronounced and spread to other forest communities. Most of our threatened and vulnerable plant species are in fact found in the forests of southern Québec. Earthworms could make it more difficult to protect them."

species composition thins due to European earthworms rapid
decomposition of the forest floor flora

Little has so far been attempted to limit the propagation of earthworms in natural ecosystems, but one step aimed at anglers could easily be introduced. "Earthworms used for bait should never be released in the forest," said Dr. Lapointe. "Anglers who use them for fishing should pack them back out afterwards, or if that's not possible, throw them into the lake."

The earthworms found in lawns, gardens, and farmers' fields, as well as those raised for bait, all belong to species that were brought here, intentionally or otherwise, by Europeans settlers. Their geographical distribution is closely tied to human activity.

The study was published in a recent issue of Forest Ecology and Management and co-authored by Line Lapointe of Université Laval and Mélanie Drouin and Robert Bradley of Université de Sherbrooke.

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Story Source:

The above post is reprinted from materials provided by Université Laval.Note: Materials may be edited for content and length

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A recent decline in ovenbirds (Seiurus aurocapilla), a ground-nesting migratory songbird, in forests in the northern Midwest United States is being linked by scientists to a seemingly unlikely culprit: earthworms.

A new survey conducted in Minnesota's Chippewa National Forest and Wisconsin's Chequamegon-Nicolet National Forest by a research team led by Scott Loss of the University of Minnesota and the Smithsonian Migratory Bird Center has revealed a direct link between the presence of invasive European earthworms (Lumbricus spp.) and reduced numbers of ovenbirds in mixed sugar maple and basswood forests.

The results are detailed in a paper published online in the scientific journal,Landscape Ecology.

European earthworms are invading previously earthworm-free hardwood forests in North America the scientists say, and consuming the rich layer of leaf litter on the forest floor. In turn, herbaceous plants that thrive in thick leaf litter and provide cover for ground-nesting birds are thinning out, replaced by grasses and sedges.

Ovenbird

As a result, ovenbird nests are more visible and vulnerable to predators and ovenbirds searching for nesting sites reject these low-cover areas outright. Areas of reduced leaf litter also contain fewer bugs for the ovenbirds to eat, requiring them to establish larger territories, resulting in fewer birds over a given area.

The worms invading northern Midwestern forests (and forests in the northeastern U.S. and Canada) have been in the U.S. since soon after the first European settlers arrived. Loss explains the worms were brought over inadvertently in the ballast of ships, in the root balls of agricultural plants or on purpose for use in gardening. Only now is the leading edge of their continued invasion, caused mainly by logging activities and fishermen dumping their bait, reaching interior wilderness areas such as parts of the study site in the remote forests of Wisconsin and Minnesota.

"Night crawlers [Lumbricus terrestris] and the slightly smaller red worms [also called leaf worms or beaver tails, Lumbricus rubellus], have the most damaging impacts to the soil, litter layer, and plants in forests that were historically earthworm-free," Loss says.

"Everyone has probably heard at one time or another that earthworms have really positive effects in breaking down soil and making it more porous," Loss explains. "This is true in agricultural and garden settings but not in forests in the Midwest which have developed decomposition systems without earth worms."

Because the forested areas of the Midwest U.S. were once covered in glaciers, there are no native earthworm species present in the soil. "These earthworm-free forests developed a slow fungus-based decomposition process characterized by a deep organic litter layer on the forest floor," Loss says.

Earthworms feed on this layer of leaf litter and make it decompose much faster, Loss explains. "As a result, we see the loss of sensitive forest-floor species such as trillium, Solomons seal, sarsaparilla and sugar maple seedlings and a shift in dominance to disturbance-adapted species like Pennsylvania sedge."

One result is reduced nest concealment for the ovenbird and increased predation by squirrel and bird predators.

The researchers found no decline in three other species of ground-nesting birds included in their survey -- the hermit thrush (Catharus guttatus), black-and-white warbler (Mniotilta varia) and veery (Catharus fuscescens) -- nor did they find a correlation between ovenbird decline and invasive worms in other forest types, such as red oak, paper birch and aspen.

"Our results suggest that ovenbird density may decline by as much as 25 percent in maple-basswood forests heavily invaded by invasive earthworms," the researchers conclude. "Maple-basswood forests are among the preferred ovenbird habitats in the region, comprise a considerable portion of the region's woodlands…and are experiencing Lumbricus invasions across most of the northern Midwest."

Previous studies have demonstrated that invasive earthworms also are harmful to other native North American species, such as salamanders.

There is reason for concern that the overall population of ovenbirds could decline, Loss points out. "Ovenbirds migrate to Central America and the Caribbean and back every yea --a trip during which they can fly into buildings and towers or get nabbed by a cat as they rest on the ground--and they also face loss of habitat on their breeding and wintering grounds. Now, here is yet another potential threat to their survival."

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Story Source:

The above post is reprinted from materials provided by Smithsonian. Note: Materials may be edited for content and length.

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Journal Reference:

1. Scott R. Loss, Gerald J. Niemi, Robert B. Blair. Invasions of non-native earthworms related to population declines of ground-nesting songbirds across a regional extent in northern hardwood forests of North America. Landscape Ecology, 2012; DOI: 10.1007/s10980-012-9717-4