Milchunas et al. examined plant community relationships across various scales and disturbance phenomena with the question of whether or not underlying generalities exist. They also examined the relative response of the shortgrass steppe plant community to the particular type of disturbance in relation to the history of the community. Disturbances that had a large negative impact on the density of Bouteloua gracilis and Opuntia polyacantha also resulted in the site being classified among the most perturbed. In addition, B. gracilis appeared resilient to disturbance up to a critical threshold, after which tiller density decreased rapidly. The outcome of establishment after the disturbance was agent-specific or stochastically controlled, with the exception of recolonization by warm-season species. The level of perturbation was not related to any other species or species category. The distinct phasing of species or life-forms that is often reported for successional time gradients or for environmental or geographical gradients was not observed along the authors' perturbation gradient. Several results from this study indicate a high degree of adaptation and specialization of indigenous species to short-term drought, large-herbivore grazing, and an inability to cope with pressures: 1) the relative constancy in community composition of less perturbed sites during wet/dry cycles, 2) the relatively large perturbation produced by the water and water plus nitrogen treatments, and 3) the greater similarity of ungrazed rather than heavily grazed communities to perturbed sites.
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