Exotic Plant Invasion Risk in the Western United States

This model was constructed to model the risk of invasion by exotic plant species.
Roads may directly influence exotic plant dispersal via disturbance during road construction or via alterations in soil regimes. For example, in Californian serpentine soil ecosystems exotic plant species can be found up to 1km from the nearest road , and Russian thistle (Salsola kali), an exotic forb growing along roads, is wind-dispersed over distances greater than 4km. Roads may also indirectly facilitate the dispersal of exotic grasses, such as crested wheatgrass (Agropyron cristatum), via human seeding along road verges or in burned areas near roads as a management strategy to curb the establishment of less desirable exotic grass species.

The inputs for this model are road type , distance from road , forest - non-forest vegetation , and proximity to rural-urban and agricultural areas. Three road-based models were built based on the classification of road type and four distance risk classes. Interstates, federal and state highways received a higher risk than secondary roads because secondary roads with shallow road verges are poorer exotic plant dispersers compared to the other two road types . Because exotic plant invasion in forest areas is restricted to roads and riparian corridors , we included only the highest risk class within forested areas. Last, because urbanized and agricultural areas act as exotic plant sources, we classified the populated areas and agricultural lands as high risk. The three road models and the populated areas and agricultural land models were merged by selecting the maximum value at each pixel location from the five input grids using the MAX command in ARC/INFO.