Roadkill and weeds are graphic testimony to how roads can upset ecosystems. But because ecologists tend to steer clear of traffic, no one had quantified the total environmental impact. A new study estimates that roads directly or indirectly damage at least 20% of the land in the United States.
The U.S. boasts 6.2 million kilometers of roads, and all that gravel and asphalt adds up to an area about the size of South Carolina. To learn how much land a typical highway will affect, landscape ecologist Richard Forman of Harvard University and ecologist Robert Deblinger of the Massachusetts Division of Fisheries and Wildlife studied a four-lane highway west of Boston. Some 50,000 vehicles travel the 25-kilometer stretch, which winds through urban, suburban, and rural areas.
The pair measured roadkill of deer, habitat loss from the road's construction, traffic noise, fences and other migration barriers, exotic species, pollution of nearby water by road salt, and altered wetland drainage. They mapped these factors to depict a so-called road-effect zone--the ecological footprint of the automobile. The zone varies with the landscape, but averages 600 meters wide. Some effects such as road salt contamination in reservoirs, changes to bird nesting behavior, and the migration of moose and other large mammals extend 1 kilometer or more beyond the road, they report in the February Conservation Biology. Extrapolating from these results, the researchers calculated that roads influence approximately 20% of the continental United States. "You just can't get more than about 20 miles from a road in the contiguous United States," says Forman.
This research "represents an important step in the field of applied ecology," says mathematical ecologist Virginia Dale of the Oak Ridge National Laboratory in Tennessee. Forman hopes that ecologists and road engineers can work together to shrink the road-effect zone. Underpasses in Florida, for example, allow panthers to cross the highway. And smoother road surfaces would be less likely to disturb noise-sensitive animals such as birds.