A replicated factorial experiment was designed to test the hypothesis that manipulating inputs of water and mineral nitrogen to a semiarid grassland would disrupt existing interactions resulting in alteration of the structure of the primary producer community. Alteration of community structure was measured as either changes in growing season average biomass of 6 functional groups of plants or their relative contribution to total biomass.Additions of water greatly increased total biomass and resulted in the replacement of one of the dominant functional groups by a subordinate group. The water plus nitrogen treatment resulted in large biomass increases in two of the dominant functional groups, elimination of succulents as an important component of community structure, and establishment of several introduced weedy species. Continuation of the experiment will likely result in complete dominance of the water plus nitrogen treatment by these introduced species.Despite the large changes in community structure observed as a result of water- and nitrogen-induced stresses we conclude that the shortgrass prairie in northcentral Colorado is asymptotically stable with respect to these influences.