The red imported fire ant, Solenopsis invicta, was accidentally introduced to North America over 60 years ago and has spread throughout the southeastern United States. We document the biogeographic consequences of this invasion. We censused ground-foraging ant communities on a 2000 km transect from Florida through New York that passed through invaded and intact biotas. Native ant species density peaks at mid-latitudes in the eastern United States, and the location of this peak corresponds to the range limit of S. invicta. In uninvaded sites, ant species co-occur less often than expected by chance. In the presence of S. invicta, community structure converges to a random pattern. Our results suggest that the effects of S. invicta on native ant communities are pervasive: not only does the presence of S. invicta reduce species density at local scales, it alters the co-occurrence patterns of surviving species at a biogeographic scale.