Understanding the distribution of plant invasions on the landscape is one of the key questions in ecology and natural resource management.  Disturbance, herbivory, diversity, productivity, and limiting similarity are a few of the factors most often used to explain why communities are vulnerable to invasion.  We explored grasslands in the Western United States to determine which factors correlated most strongly with the invasion of forbs, annual grasses, and rhizomatous grasses. We sampled vegetation in 50 sites throughout Colorado and Montana located in 4 National Park units (Little Bighorn Battlefield National Monument, Glacier National Park, Grant-Kohrs Ranch National Historic Site, and Florrisant Fossil Beds National Monument).  At each site, the functional composition of vegetation was inventoried using an array of 36 m transects and 1 m² plots to describe a 1.3 acre area.  In addition to vegetation, soil aggregate stability, litter, scat, bare ground cover, and distance to road was measured at each site.

Results/Conclusions

We indentified and recorded the abundance of 25 species of invasive graminoids and forbs. We found that invasive forb abundance was best explained by distance to road, cover of litter, and cover of wild scat.  Invasive annual and rhizomatous grass abundance was more strongly correlated with soil aggregate stability and functional diversity.  Our future work will focus on more thoroughly describing anthropogenic and natural disturbance and soil chemistry and texture at all sites which may add to with a goal of increasing our predictive ability. Our results to date suggest that the factors determining the susceptibility to invasion of Western grasslands may be differ with invader functional group and identity.