Roads and conservation just don’t mix. Already blamed for spreading invasive species and increasing sediment in steams, roads can also block gene flow in animal populations, according to new research in the August issue of Conservation Biology.
This is the first study showing that roads can isolate populations genetically. There are lots of natural barriers to gene flow in populations, including rivers, mountains, and volcanic eruptions. Biologists have speculated that roads also block gene flow because many species avoid crossing them. To facilitate animal movement across roads, European conservationists have pushed for “green bridges,” but building them has been contentious in Germany and Switzerland.
“There is always a fight during the environmental impact assessment about including green bridges, including when a new highway is built,” says Gabriele Gerlach of the Universitat Konstanz in Konstanz, Germany, who did this study with her colleague Kerstin Musolf.
To determine whether roads actually are genetic barriers and so warrant green bridges, Gerlach and Musolf compared DNA variations in small rodents called bank voles that lived on either side of three types of roadways: a 20-foot-wide railway, a 33-foot-wide country road, and a 164-foot-wide four-lane highway. These roadways were in large forested areas of southern Germany and Switzerland and were at least 25 years old, corresponding to at least 25 generations of bank voles.
While there was no genetic difference in the voles living across the railway or the country road, the voles living on either side of the four-lane highway were so distinct genetically that the researchers concluded there was little gene flow between them.
In small populations, low gene flow can decrease genetic diversity and fertility, thus increasing extinction risk. As the density of roads increases, so does the likelihood that animals will subdivide into small, isolated populations that ultimately die out. To help keep roads from blocking animal movement, Gerlach and Musolf call for more green bridges and wildlife underpasses. To encourage animals, from hedgehogs to badgers to wild boars, to use them, green bridges should be wider than 164 feet and planted with hedges and shrubs.
For more Information
Gerlach, G. and K. Musolf. 2000. Fragmentation of landscape as a cause for genetic subdivision in bank voles. Conservation Biology14(4):1066-1073.