9528005 Feener Invasions of exotic ant species such as the red imported fire ant (Solenopsis invicta) occur frequently and often threaten the integrity of natural ecosystems in introduced ranges. Yet, the underlying ecological mechanisms that allow some invasions to succeed and others not have not been studied quantitatively. The research in this proposal is a collaborative effort between Brazilian and U.S. ecologists designed to test several hypotheses concerning the mechanisms of competitive dominance and invasion success of the red imported fire ant. Replicate plots will be established at three sites within the native range of S. invicta in southern Brazil. Experiments in these plots will assess the ability of S. invicta to dominate co-occurring ant species aggressively, both in the presence and absence of parasitic flies specialized on S. invicta. This will be the first time that the interactions between the red imported fire ant and other ant species have been examined in the native range of S. invicta. This study also will be the first time that the relative impact and synergistic interaction of competition and parasitism have been examined experimentally in ant communities This study will have immediate applied value. It will form the groundwork for future studies of the relative success of other invasive ant species in their native and introduced ranges. Moreover, the results of this study will provide a foundation for the development of effective biological control of the red imported fire ant in the southeastern United States, an area where it is having a devastating effect on biodiversity.