Disturbances in desert ecosystems can be either chronic or acute in effect, and natural or anthopomorphic in origin. We hypothesize that that severity of effects of an acute disturbance is shaped by ecosystem structure. We designed a long-term study to test this hypothesis. Within a grassland/shrubland ecotone we have applied livestock grazing (a 24 h defoliation of standing crop within a 365 d period) during either winter dormancy or summer growth periods. These grazing treatments are applied in either mesquite present or mesquite removed 0.5 ha inclosures. We expect that vegetation dynamics of the mesquite present treatment will demonstrate a threshold of mesquite at approximated 8% canopy cover that will result in persistent shrub dominated community irrespective of season of defoliation.