Eight years ago, Richard Forman, a landscape ecologist at Harvard University, had what he describes as “a little epiphany.” Forman had gained renown with his book Land Mosaics: The Ecology of Landscapes and Regions and was subsequently invited to join a national council on the impact of the U.S. transportation network on ecology. The council focused largely on climate, but Forman realized that he and his colleagues were missing the point. “It occurred to me that the most conspicuous feature of the landscape was the least known”—roads.

This nation has 3.9 million miles of public roads and counting; cumulatively, those road surfaces cover 1 percent of the land—an area equivalent in size to South Carolina. Meanwhile, a growing body of research shows that roads have an ecological effect disproportionate to their size. They divert streams, change water tables, and abet the emission of carbon dioxide, ozone, and smog. They fill plants with heavy metals and kill lichens and mosses with their dust. Every road has its roadside: a bare, often harsh netherworld that invites the growth and spread of alien plants. On the whole, Forman estimates, the ecology of one-fifth of the United States is directly affected by roads.

Paved land for cars and trucks offers virtually every scientist something to dislike. So botanists, soil chemists, population biologists, and others have recently banded together to form a new interdisciplinary field: road ecology. In late 2002 Forman joined with Dan Sperling, a transportation expert at the University of California at Davis, to publish a survey of the field, Road Ecology, that sold out in five months. Sperling is now associate director of Davis’s new road ecology center. “The public has been raising Cain over the transportation community’s lack of sensitivity to environmental issues,” Forman says. Frustration equals lawsuits; lawsuits equal fewer new roads. It’s no surprise, therefore, that the center receives much of its funding from Caltrans, California’s embattled transportation authority.

Road traffic in Belgium kills about 4 million large vertebrates a year. In tiny Denmark, the death toll includes 1.5 million mammals, 3.7 million birds, and more than 3.1 million amphibians. In the United States, as early as the 1960s, estimates suggested that cars kill a million animals a day.

In the early days, roads more or less obeyed the landscape, following rivers and forest edges. Now they’re often plunked down anywhere, carving up populations of wolves, bears, salamanders, and other animals that once roamed freely. Smaller populations fluctuate more widely in number than larger ones do and are more vulnerable to interbreeding. As a result, Forman says, roads are likely to cause local extinctions.

Roads also change the behavior of the animals that live near them. In the winter, caribou in Alaska migrate along cleared roads, making them vulnerable to trucks and wolves. Studies have found that black bears in North Carolina shift their home ranges away from busy road areas, as do grizzlies in the Rockies. Black vultures and turkey vultures, on the other hand, shift toward these areas, presumably for the ready supply of road carrion. One species of land snail actively avoids crossing roads, even narrow, unpaved ones. (Many snails aren’t just threatened by car tires but also the risk of drying up on the slow trek across hot pavement.) One remarkable study found that the heart rate of female bighorn sheep increased notably near a road, regardless of traffic.

Forman has spent weeks in the suburbs of Boston, mapping the road networks, gathering data on traffic volume, and, with local ornithologists, studying bird habitat. He hasn’t found many birds near roads. “For a quarter mile on either side of a suburban street, birds of conservation interest don’t reproduce,” he says. “You can envision these swaths around the highways of America with reduced populations.” His conclusion: “We’re wasting time spending money protecting land near busy highways, at least for birds.”