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Ecological impacts from roads may be the rule rather than the exception for most of the U.S., according to a USDA Forest Service researcher. For an article in the April issue of Frontiers in Ecology and the Environment, Kurt Riitters, from the FS Southern Research Station (SRS) unit in Research Triangle Park, NC, teamed up with EPA scientist James Wickham to measure just how close U.S. land is to the road.
In 2001, according to US Department of Transportation (DOT) data, mainland U.S. contained approximately 6.3 million kilometers of public roads, nearly a quarter of which were classified as non-local roads in rural areas. For the same year, the US Environmental Protection Agency (EPA) estimated that the same area contained 5.3 million kilometers of streams and rivers.
"The comparison of these two sets of data begins to give you an idea of the potential influence of roads on forest ecosystems," said Riitters. "Effects of roads--what we call influence zones-- extend tens to hundreds of meters from the roads themselves, altering habitats and water drainage patterns, disrupting wildlife movement, introducing exotic plant species, and increasing noise levels."
To analyze nearness of roads at the regional scale, Riitters and Wickham used four different maps of landcover, roads, watersheds, and ecoregions. After converting the road map to generate a road-distance grid, they prepared data summaries by overlaying the road-distance grid with the other three maps and tabulating road distances by landcover type, watershed, or ecoregion. The researchers made separate estimates for total land area and for forest land area, in recognition that roads play a key role in forest fragmentation in the U.S.
Results showed that 20 percent of all land area was located within 127 meters (417 feet) of the nearest road, and 50 percent was within 382 meters (1253 feet). Only 18 percent of U.S. land area was more than 1000 meters (0.6 mile) from a road, and only 3 percent was more than 5000 meters (3.1 miles) away. Overall, forest land was slightly more remote than other landcover types, but the data followed similar trends.
"Imagine that the U.S. has been divided into 8.6 billion parcels the size of a baseball diamond infield, and that you could stand on home plate in each of these parcels," said Riitters. "According to our model, in one out of every 22 cases there would be a road no further away from you than second base. In one in five cases, the road is no further away than the center field fence. While the actual size of a road influence zone depends on local circumstances, the sheer pervasiveness of roads means that few places in the U.S. are immune to their influences."
Funding for the research was provided by the Strategic Planning and Resource Assessment Staff of the USDA Forest Service, and by the Landscape Ecology Branch of the US Environmental Protection Agency.