The Effects of Disturbance Size and Frequency on a Shortgrass Plant Community

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Abstract
We examined the effects of small, patch—producing disturbances on a shortgrass plant community dominated by the perennial grass blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Griffiths). Size distributions and rates of occurrence were determined from field data and from the literature for B. gracilis plants and for three disturbance types: cattle fecal pats, western harvester ant mounds, and small animal burrows. A Spatially explicit simulation model was then used to determine the effects of each disturbance. The average turnover rates (percent affected per year) of the basal cover and density of B. gracilis plants in the simulated plots were calculated for three topographic positions and three grazing intensities. Disturbance size relative to the size of B. gracilis plants distinguished two classes of disturbances. Ant mounds and animal burrows were significantly larger than B. gracilis plants, while fecal pats were comparable in size to B. gracilis plants. The frequency of occurrence of a disturbance was found to be a good predictor of the amount of basal cover and the number of pants killed each year by each disturbance type. The fastest turnover rates for basal cover were associated with the smallest and most frequent disturbance type (fecal pats). The effects of these disturbances were found to be dependent on topographic position and grazing intensity, especially the former. Slopes and lowlands had faster turnover rates than uplands. Our results suggest that the recovery of B. gracilis is dependent on the spatial scale of the disturbance.