ENN News
Highways stop wildlife dead in their tracks
Monday, August 7, 2000
By Margot Higgins
Why didn't the mammal cross the road?
Because roads are potentially killing fields for wildlife. What's more, they've become barriers for certain mammals to reproduce with other members of their species, according to a study in the August issue of Conservation Biology.
While previous research showed that roads facilitate the spread of endangered species and increase stream sedimentation, scientists for the first time have also demonstrated that roads can isolate populations genetically.
"When a population is small and there is no immigration from outside, there is an increasing risk that the population will become extinct," explained Gabriele Gerlach, an assistant professor at the University of Konstanz in Germany and lead author of the study.
"Our study shows that highways can limit the normal exchange between local populations. When landscapes become more and more fragmented, animals living in these remaining parts become isolated and inbreeding depression can cause their extinction."
Gerlach and colleague Kerstin Musolf compared DNA variations in bank voles found on either side of three types of roads: a 20-foot railway, a 33-foot wide country road and a 164-foot-wide, four-lane highway.
Although there was no genetic difference between the voles living across the railway or the country road, voles living on either side of the four- lane highway were so distinct that the researchers concluded there was little genetic flow between them.
Bank voles were chosen for the study because they are easy to trap and regenerate one or two times a year, Gerlach said. "If you would conduct this study on deer with a generation time of three to four years you would not see this genetic effect for 75 to 100 years. The highways are not that old."
As the density of roads increases, so does the likelihood that mammals will be subdivided into small, isolated populations, she added.
"Forest carnivores are particularly vulnerable to fragmentation," said biologist Bill Ruediger of the U.S. Forest Service. "They're at risk because of their small populations, low reproduction rates and large — even huge — home ranges."
To help animals deal with the fragmentation caused by roads, the researchers support the idea of building so-called "green bridges" and wildlife underpasses.
The idea is to create a safe, natural means for wildlife to cross a road without endangering their lives. According to the Federal Highway Administration, on a typical highway in the United States carrying 20,000 vehicles a day, a car or truck passes by every four seconds. The window of time between a safe crossing and a deadly collision is short.
"The crossings should be wider than 150 feet so that threatened animals like deer, wild boars, chamois, badgers, foxes, hares, hedgehogs
can use them," Gerlach explained. "Hedges and bushes should be planted on these bridges to "lead" animals to the other side. The adjacent area should be appropriate for the animals too."