29th Annual Meeting and Symposium of the
Desert Tortoise Council, February 20-23, 2004
Abstracts

Roads, Invasions, and the Desert Tortoise: How Combinations of Environmental Conditions Explain Patterns of Exotic Plant Invasion

Jonathan L. Gelbard
Department of Environmental Science and Policy, University of California, Davis Davis, California

Roads are believed to be a major contributing factor to the ongoing spread of exotic plants, which in turn influences the quality of desert tortoise habitat. We examined 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 roadcut) along 42 roads stratified by the 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. Richness and cover of exotic species were both more than 50% greater, and richness of native species was 30% lower, at interior sites adjacent to paved roads than 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 invasion 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 appear most vulnerable. We suggest that researchers use combinations of environmental conditions to understand complex invasion patterns not explainable by using single factors such as the presence of roads or disturbance. This conceptual framework holds substantial promise as scientists and land managers seek to understand how and where to improve management of roadside and adjacent interior communities to prevent and minimize invasions, and, in turn, to protect sensitive species such as the desert tortoise.