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.
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."
The Smithsonian Migratory Bird Center is dedicated to fostering greater understanding, appreciation, and protection of the grand phenomenon of bird migration. Founded in 1991, it is located at the Smithsonian National Zoological Park in Washington, D.C.
"Invasions of non-native earthworms related to population declines of ground-nesting songbirds across a regional extent in northern hardwood forests of North America" is co-authored by Scott R. Loss, Gerald J. Niemi and Robert B. Blair of the University of Minnesota.
Lindsay Renick Mayer | EurekAlert!
Global threat to primates concerns us all
19.01.2017 | Deutsches Primatenzentrum GmbH - Leibniz-Institut für Primatenforschung
Reducing household waste with less energy
18.01.2017 | FIZ Karlsruhe – Leibniz-Institut für Informationsinfrastruktur GmbH
An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by...
Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...
Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.
While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...
Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales
Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
19.01.2017 | Event News
10.01.2017 | Event News
09.01.2017 | Event News
20.01.2017 | Awards Funding
20.01.2017 | Materials Sciences
20.01.2017 | Life Sciences