Response of semi-desert grasslands invaded by non-native grasses to altered disturbance regimes

Similar to other biomes across the globe, grasslands in the American Southwest have experienced significant changes to their historic composition and processes (Bahre, 1991; McPherson & Weltzin, 2000). Plant communities have been shaped by disturbances such as fire, grazing, and their interactions with climate. ‘Disturbance is a change in physical or biological factors that ultimately affects the structure and function of plant communities’ (McPherson & Weltzin, 2000). Different processes come into play over long time-scales, therefore altered disturbance regimes, compared with individual disturbance events, may have novel influences on vegetation (Clark, 1996). During the last several hundred years, most grasslands of the American Southwest experienced a novel disturbance type when cattle, coincident with the extirpation of prairie dogs, became a significant influence. Ecologists have argued about the detrimental effects of livestock grazing on plants (Belsky, 1986) vs. potential benefits to plant fitness (McNaughton, 1979) with evidence from systems that evolved with grazing by large herbivores. Grazing affects grasslands directly through the School of Natural Resources, University of Arizona, Tucson, AZ, USA