420 Conservation Biology, Pages 420–432 Volume 17, No. 2, April 2003 Roads as Conduits for Exotic Plant Invasions in a Semiarid Landscape JONATHAN L. GELBARD*‡ AND JAYNE BELNAP† *Nicholas School of the Environment, Duke University, Durham, NC 27708, U.S.A. †U.S. Geological Survey, Canyonlands Field Station, 2290 S. Resource Boulevard, Moab, UT 84532, U.S.A. Abstract: Roads are believed to be a major contributing factor to the ongoing spread of exotic plants. We ex- amined the effect of road improvement and environmental variables on exotic and native plant diversity in roadside verges and adjacent semiarid grassland, shrubland, and woodland communities of southern Utah (U.S.A.). We measured the cover of exotic and native species in roadside verges and both the richness and cover of exotic and native species in adjacent interior communities (50 m beyond the edge of the road cut) along 42 roads stratified by level of road improvement ( paved, improved surface, graded, and four-wheel- drive track ). In roadside verges along paved roads, the cover of Bromus tectorum was three times as great (27%) as in verges along four-wheel-drive tracks ( 9%). The cover of five common exotic forb species tended to be lower in verges along four-wheel-drive tracks than in verges along more improved roads. The richness and cover of exotic species were both more than 50% greater, and the richness of native species was 30% lower, at interior sites adjacent to paved roads than at those adjacent to four-wheel-drive tracks. In addition, environmental variables relating to dominant vegetation, disturbance, and topography were significantly correlated with exotic and native species richness and cover. Improved roads can act as conduits for the inva- sion of adjacent ecosystems by converting natural habitats to those highly vulnerable to invasion. However, variation in dominant vegetation, soil moisture, nutrient levels, soil depth, disturbance, and topography may render interior communities differentially susceptible to invasions originating from roadside verges. Plant communities that are both physically invasible (e.g., characterized by deep or fertile soils) and disturbed ap- pear most vulnerable. Decision-makers considering whether to build, improve, and maintain roads should take into account the potential spread of exotic plants. Caminos como Conductos para Invasiones de Plantas en un Paisaje Semiárido Resumen: Se piensa que los caminos son un factor importante que contribuye a la continua dispersión de plantas exóticas. Examinamos el efecto del mejoramiento de caminos y variables ambientales sobre la diver- sidad de plantas exóticas y nativas en bordes de caminos y comunidades adyacentes de pastizales semiári- dos, matorrales y bosques en el sur de Utah (E.U.A). Medimos la cobertura de especies exóticas y nativas en los bordes de caminos así como la riqueza y cobertura de especies exóticas y nativas en comunidades interi- ores adyacentes (50 m del borde del camino) a lo largo de 42 caminos estratificados por el nivel de mejor- amiento ( pavimentada, superficie mejorada, escalonada y camino para vehículos de doble tracción). En los bordes de caminos pavimentados, la cobertura de Bromus tectorum fue tres veces mayor (27%) que en bor- des a los largo de caminos para vehículos de doble tracción (9%). La cobertura de cinco especies comunes de hierbas exóticas tendió a ser menor en bordes a lo largo de caminos para vehículos de doble tracción que en bordes a lo largo de caminos mejorados. La riqueza y cobertura de especies exóticas fueron más de 50% mayor, y la riqueza de especies nativas fue 30% menor, en sitios interiores adyacentes a caminos pavimenta- dos que en los adyacentes a caminos para doble tracción. Adicionalmente, las variables ambientales relati- vas a la vegetación dominante, perturbación y topografía estuvieron significativamente correlacionadas con la riqueza y cobertura de especies exóticas y nativas. Los caminos mejorados pueden actuar como conductos ‡Current address: Department of Environmental Science and Policy, University of California, Davis, 1 Shields Avenue, Davis, CA 95616, U.S.A., email jlgelbard@ucdavis.edu Paper submitted August 21, 2001; revised manuscript accepted May 15, 2002.