Improved roads in wilderness areas spread more invasive weeds than primitive roads, while roadless areas act as refuges for native species against invasions, according to two studies from the University of California, Davis.

Cheatgrass, knapweeds and other non-native plants have invaded nearly 125 million acres of the American West. Roads promote invasion because vehicles can transport non-native seeds into uninfested areas, and disturbed roadsides give weed seeds a place to grow.

In the April issue of Conservation Biology, UC Davis doctoral candidate Jonathan Gelbard and research ecologist Jayne Belnap of the U.S. Geological Survey report a study of plants along 42 roads in and around Utah's Canyonlands National Park. The roads had varying degrees of improvement: 4-wheel-drive track, graded, improved surface (such as gravel) or paved.

They found that each step of road improvement converted an increasing area of natural habitat to roadside habitat, from which non-native weeds spread into adjacent natural ecosystems.

Compared to 4-wheel-drive tracks, paved roads had much wider roadside verges (totals of 6 feet vs. 46 feet) and far more non-native plants, both within the verges and at "interior sites" 165 feet (50 meters) away.

In the April issue of the journal Ecological Applications, Gelbard writes with a UC Davis professor of environmental science and policy, Susan Harrison, about roads' effects on inland California foothill grasslands.

Gelbard and Harrison found that in areas with typical grassland soils, non-native plants were less abundant and native plants more abundant at sites about a half-mile (1,000 meters) from roads compared to sites less than 33 feet (10 meters) from roads.

Gelbard said, "These papers are timely in light of the debate concerning protection for roadless habitats in U.S. national forests. Our findings show that roadless habitats have multiple benefits, not just for the environment, but also for the economy and our quality of life. They are not only refuges for native biodiversity, but also protect against non-native weed invasions, which are costly for ranchers and public agencies."