Some earthworm invasions spread from the lakeshore into the forest when anglers dump unused bait in the forest.
Little worms, big consequences
Ground-dwelling birds may be silenced by invasive earthworms.
One of the most beautiful sounds of the Northwoods in summer is the ethereal, fluting song of the hermit thrush. But in some areas of northern Wisconsin, this song is in danger of being silenced by an unlikely culprit: the earthworm.
Many who grew up with the idea that worms are good for the soil are surprised to learn that earthworms are not native to the Northwoods. Further south, in areas of North America that were unaffected by glaciers during the most recent Ice Age 11,000 years ago, native earthworms can still be found. However, any that were here would have been killed off by the glaciers, and the forest ecosystem that has developed since has done so in the absence of worms ó any present here now originated in other parts of the world.
Exactly how and when they were first introduced is not known, but they probably arrived with European settlers in ship ballast and imported plants. Once introduced to an area, worms extend their range very slowly on their own, at a rate of as little as five yards per year, but humans continue to transport them into new places. Their presence in an area has the potential to change the forest floor in significant ways.
Earthworms are part of a special category of creatures known as ecosystem engineers. Like human engineers designing buildings and bridges, ecosystem engineers have the ability to greatly modify their own habitats and alter the structure of the ecosystems in which they live.
The "structure" of an ecosystem, however, is more than just physical; it refers to how energy and nutrients flow between plants and animals.
As earthworms move through a forestís soil and digest dead leaves and other organic material, they have several subtle but far-reaching effects. They decrease the amount of leaf litter on the soilís surface and mix soil layers together.
They can reduce the abundance of small soil-dwelling insects, as well as of fungi called mycorrhizae that help provide nutrients to plants. This, in turn, affects what plants will thrive in an area invaded by earthworms. Grasses and their cousins the sedges, which donít rely on mycorrhizae and can tolerate disturbed soil, replace a variety of other, more sensitive types of plants, leading to lower diversity.
Earthworms and birds
What does all this mean for birds? A study conducted by University of Minnesota researchers Scott Loss and Robert Blair in northeastern WisconsinísChequamegon-Nicolet National Forest sought to answer this question, both for the hermit thrush and another species, the ovenbird.
Though not closely related, these two songbirds have important behaviors in common ó both nest and forage on the forest floor. In recent decades, populations of these ground-dwelling birds have been in decline across the northern Midwest.
Loss and Blair surveyed both sites that had been invaded by worms and sites that were still earthworm-free for the presence of hermit thrushes and ovenbirds.They found unequivocally that the population density of both birds was lower in earthworm-invaded areas.This only answered part of the question. They still wondered why ground-dwelling birds were so affected by the presence of worms. To find out, they monitored ovenbird nests in both site types over two breeding seasons. But because hermit thrushes have larger breeding territories than ovenbirds, they couldnít locate enough hermit thrush nests to have a sufficient sample for their analysis.
They recorded the type of understory vegetation and the depth of the leaf litter at the nest sites to see if they were related to nesting success. Two factors affected the survival of ovenbird chicks: first, they were less likely to survive if their nests were in areas with greater amounts of grass and sedge cover, and second, they were less likely to survive in areas with a shallower layer of leaf litter.
The researchers theorized that the nests were more visible to predators against the uniform green backdrop of grass. Moreover, the decrease in insects and other arthropods caused by a lack of leaf litter could mean that ovenbird parents had to devote more time to foraging for food and less time to keeping watch over their nestlings.
"In general, the results were along the lines of what we hypothesized," says Loss, who worked on the research as part of his graduate studies. "However, we were surprised at just how clearly different the densities of ovenbirds and hermit thrushes were between earthworm-free and invaded forests."
Loss adds that another surprise was that the grasses and sedges densely covering the forest floor in earthworm invaded areas didnít provide sufficient cover for the ovenbird nests. Finally, the researchers realized that in addition to the color contrast, the single species produced a single layer of plants that were all roughly the same height, rather than the diverse mixture of cover that would have better hidden the ground-level nests.
All of this is a classic example of what ecologists call "cascading effects." The familiar metaphor is of a series of dominoes falling. One small alteration in an ecosystem, like the introduction of nonnative worms, can cause changes that ripple outward through the food web in hard-to-predict ways. While earthworms are probably not the only factor in the decline of hermit thrushes, ovenbirds, and other ground-dwelling songbirds, the effects they have on the forest floor are certainly contributing to it.
What you can do
Once earthworms have become established in an area of forest, there is no feasible way to remove them. Instead, conservation efforts focus on limiting their spread, and you can help:
If you are interested in helping to monitor earthworms in your area, consider participating in the Great Lakes Worm Watch (www.nrri.umn.edu/worms). By working together, we can protect Wisconsinís forests from further worm invasions in the future, and the ovenbird, hermit thrush and other ground-dwelling songbirds will continue to sing for years to come.
Rebecca Deatsman is a graduate student in environmental education at the University of Wisconsin-Stevens Point and works at Conserve School in Land Oí Lakes.