The degree to which disturbance facilitates exotic plant invasion depends on the availability of exotic propagules and their ability to colonize disturbed patches. Remnant plant communities located in fragmented urban areas may experience a greater influx of exotic propagules than comparatively intact communities located in rural areas. Consequently, urban communities in which natural disturbance regimes still operate may be more susceptible to exotic invaders than rural communities subject to the same disturbance regime. The conversion of native grasslands for urban use along the Colorado Front Range has created remnant grasslands inhabited by ground-dwelling black-tailed prairie dogs (Cynomys ludovicianus), an important natural disturbance agent in native grasslands. If remnant patches are subject to a greater influx of exotic plant propagules than intact grasslands, prairie dogs should facilitate exotic invaders more in urban areas than in rural areas. We sampled vegetation and seed banks on prairie dog colonies and paired off-colony sites in urban and rural areas of northeastern Colorado in June 2007, and we measured the degree to which prairie dogs disturb soil on urban versus rural colonies.
Results/Conclusions

Urban prairie dog colonies were more highly disturbed than rural colonies (urban 498 m²/ha; rural 238 m²/ha; P = 0.04). Exotic plant species richness was significantly greater in urban (4.25) than rural (0.625) areas (P = 0.0005), regardless of association with prairie dog colonies. Rural prairie dog colonies had fewer exotic species (1.25) than urban colonies (3.25; P = 0.001). Off-colony sites in rural areas had no exotic species, while off-colony sites in urban areas had the highest exotic species richness (5.25) of any sampling location (P < 0.05 for all site comparisons). Although urban colonies had lower exotic species richness than paired off-colony sites, exotics had higher cover on colonies (on colony, 19.3%; off colony, 4.6%; P = 0.002). Of the five most abundant species in the soil seed bank, only one was exotic (Verbascum thapsus), germinating almost exclusively from soils located on urban prairie dog colonies. These findings suggest that urban location is a main factor contributing to high exotic species richness, and that prairie dogs do not always facilitate an increase in exotic richness. However, where exotic species are numerous, prairie dogs can facilitate their spread in terms of vegetation cover. While natives appear to be more abundant in the seed bank than exotics, soils on prairie dog colonies may harbor a significant number of propagules for some exotic species.