In Colorado, Milchunas et al. assessed the effects of 45 years of controlled, heavy grazing by cattle and grazing exclosures on the basal cover and density of species in plant communities along catenas following periods of wet or dry years. The interactions, and relative importance, of grazing, topography, and precipitation in determining vegetation structure were examined at the levels of organization of the plant community, functional groups and species populations in the shortgrass steppe. Of the three factors, relatively greater differences in community similarity were observed between catena positions, particularly on the ungrazed treatments. Grazing was intermediate between catena positions and short-term weather in shaping plant community structure. Grazed treatments and ridgetops had a less variable species composition through fluctuations in weather. Grazing exclosures were more diverse and they did not observe large changes in canopy structure or species composition. Convergent selection pressures of semiaridity and grazing in the short-grass steppe have resulted in a community with a high degree of tolerance to both grazing pressure and yearly fluctuation in precipitation.
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