First posted on 03-02-2012
In the upper Midwest, ground-nesting ovenbirds are in decline as a result of an unlikely invasive species: earthworms. Ovenbirds, which are part of the warbler family, visit the Ozarks throughout the summer months. They are difficult to spot in forested areas, but their loud song which resembles “teacher, teacher, teacher” often betrays their presence.
Scott Loss of the University of Minnesota discovered the link between earthworms and ovenbird losses during a study he conducted in Minnesota’s Chippewa National Forest and Wisconsin’s Chequamegon-Nicolet National Forest. Loss and his team revealed a direct link between reduced populations of ovenbirds and the work of European earthworms.
Earthworms are not native to North American hardwood forests, but were introduced soon after European settlers arrived. They likely arrived in ship ballast chambers or in root balls of plant material brought to North America for cultivation.
“Night crawlers and the slightly smaller red worms, have the most damaging impacts to the soil, litter layer, and plants in forests that were historically earthworm-free,” Loss says.
Earthworms thrive by consuming a rich layer of leaf litter that comprises forest floors. But this causes a rapid thinning of plants that depend on the same leaf litter for survival. Eventually, the action of earthworms spells doom for ovenbirds because the ground cover these birds depend on to conceal their nests disappears, replaced by grasses and sedges. Ovenbird nests become more easy to locate, making them vulnerable to predators. The lack of forest litter also means fewer insects for ovenbirds to consume, which forces them to expand territories.
“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.”
Since the Midwestern United States was covered by glaciers during the Ice Age, no native species of earthworm is present in the soil. Hardwood forests “developed a slow fungus-based decomposition process characterized by a deep organic litter layer on the forest floor,” according to Loss.
Ovenbirds prefer maple-basswood forests, which make up a large portion of northern U.S. forest land. Loss and his team did not find the same correlation between earthworms and ovenbird decline in red oak forests. Earlier studies also showed the harmful effects of earthworms on other animal species, such as salamanders.
Listen to an ovenbird’s song:
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