The introduction of nonnative species in the United States has resulted in significant environmental damage and economic losses exceeding $1 billion per year. We assessed the influence of nonnative species on biological diversity in the southwestern United States in systems prone to fire using a rigorous experimental framework. Our specific objectives were to (1) determine %effects of fire season on responses of biotic communities, and (2) quantify% relationships between biological guilds before and after burning and through post-fire recovery. This experiment is taking place within grasslands and Prosopis savannas at the Fort Huachuca Military Reservation. The experiment evaluates the main and interactive effects of dominance by nonnative plants and fire season with three replicates in each of two years. Biomass of Eragrostis lehmanniana declined and persisted following burns for more than 2 years. The degree of the response was dependent on annual precipitation and fire season. Plant species richness did not change with fire treatments but remained lower on plots dominated fire. lehmanniana and higher on native-dominated plots. In general, species richness and relative abundance of small mammals decreased in the first year following fire. However, 2 years after fire, values approached that observed in unburned areas.