The role of interspecific interactions in population regulation in rodent communities can be determined by experimental species removal. Two populations, the cotton rat (Sigmodon hispidus) and the harvest mouse (Reithrodontomys fulvescens), have been studied. Plots were maintained where such species was experimentally removed; demographic and resource utilization effects on remaining species were monitored. Density, biomass, survival, and reproductive effort of both species decreases on experimental (species removal) plots. These results suggest that positive (proto-cooperative) interactions are important population regulatory factors in this system. Niche dynamic studies, however, reveal evidence of the importance of negative (competitive) interactions; increased spatial, dietary, and microhabitat utilization were evidenced for both species on experimental plots. Alterations in intraspecific behavior on experimental plots, leading to emigration, antagonism, or nestling loss, may be responsible for these results. Interspecific interactions may serve as a buffer to intraspecific encounters, thereby enhancing population performance (protocooperation). Behavioral attributes are currently being investigated.