Road improvement paves the way for weed invasions

While it is well-known that roads can spread invasive weeds, new research shows that some roads are worse than others. In Utah, areas along paved roads were far more likely to be invaded than those along 4-wheel-drive tracks. This suggests that limiting road improvements would help keep out invasive weeds.

“Each step of road improvement would appear to convert an increasing area of natural habitat to roadside habitat,” say Jonathan Gelbard, who did this work while at Duke University in Durham, North Carolina, and is now at the University of California at Davis, and Jayne Belnap of the U.S. Geological Survey in Moab, Utah, in the April issue of Conservation Biology.

Cheatgrass, knapweeds and other non-native plants have invaded nearly 125 million acres of the American West. Roads are a big part of the problem: for instance, vehicles can transport non-native seeds into uninfested areas, and clearing land during road construction gives weed seeds a place to become established. Intuitively, it makes sense that improved roads would spread weeds more than primitive roads because the former have more traffic, more exposed soil and more maintenance such as mowing and herbicide treatments, all of which can favor invasive species.

To see if non-native weeds really are more likely to invade along improved roads, Gelbard and Belnap surveyed the plants along 42 roads with varying degrees of improvement (paved, improved surface such as gravel, graded and 4-wheel-drive track) in and around southern Utah’s Canyonlands National Park. The researchers determined the cover and number of species of non-native and native plants in two areas: roadside verges (strips along the road), and “interior sites” near but not right next to roads (165 feet from the verge).

Gelbard and Belnap found that road improvement greatly increased the cover of non-native plants in roadside verges. Notably, cheatgrass cover was three times greater in verges along paved roads than along 4-wheel-drive tracks (27 vs. 9%).

In addition, verges along improved roads were also wider, ranging from about three feet on each side of 4-wheel-drive tracks to 23 feet on each side of paved roads. This means that improving roads can convert natural habitat to non-native weed-infested roadside habitat. “For example, our results suggest that improving 10 km [about 6 miles] of 4-wheel-drive tracks to paved roads converts an average of 12.4 ha [about 30 acres] of interior habitat to roadside [habitat],” say Gelbard and Belnap.

The researchers also found that improved roads had more non-native plant cover in interior sites. Again, cheatgrass cover was more than three times greater in interior sites adjacent to paved roads than in those adjacent to 4-wheel-drive tracks (26 vs. 8%).

Overall, the cover of non-native plants was more than 50% greater in interior sites adjacent to paved roads than in those adjacent to 4-wheel-drive tracks.

In addition, road improvement changed the number of both exotic and of native species in the interior community study plots: the number of exotic species was more than 50% greater and the number of native species was 30% lower.

“Our findings suggest that major opportunities remain to prevent exotic [non-native] plant invasions in this semiarid landscape by minimizing the construction of new roads and the improvement of existing roads,” say Gelbard and Belnap.

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CONTACT:
Jonathan Gelbard (jlgelbard@ucdavis.edu)
Jayne Belnap (jayne_belnap@usgs.gov)