DENSITY-DEPENDENT COMPETITION IN FIRE ANTS - EFFECTS ON COLONY SURVIVORSHIP AND SIZE VARIATION

1. Competition among ants can drive changes in both the density and th e sizes of colonies. Models of competitive interactions produce confli cting predictions concerning size variation; these predictions have no t previously been tested in social insect populations. 2. To measure t he effects of early competition on population dynamics of the fire ant Solenopsis invicta, 2284 incipient colonies, each grown from a single queen, were planted in replicate arrays in an irrigated field. In a s eries of three experiments, colonies were planted in square grids with either 13.5, 15, 40, 60 or 120 cm between nearest neighbours. 3. Soon after planting, active colonies organized brood raids, which ended wh en victorious colonies acquired the brood and workers of one or more n eighbours. 4. Initial colony density had strong effects upon the perce ntage of colonies engaged in raids, which ranged from 0% at the lowest density to 68.5% at the highest density. High colony density markedly increased raid complexity, colony mortality and queen migration. 5. T he complex brood raids seen at high colony density rapidly reorganized clusters of incipient colonies. Excavation of surviving colonies show ed that the number of workers per colony was more variable on plots wi th numerous raids; thus, competition by brood raiding tended to increa se the degree of size inequality among colonies. This result is consis tent with models and observations of 'asymmetric competition' among se ssile organisms. 6. During raids, queens sometimes moved to nearby nes ts where they joined or usurped the original queen. Queen usurpation w as especially likely on high-density plots. 7. Brood raids were restri cted to the first few weeks following worker emergence and were reduce d by low surface moisture, apparently because worker activity declines in dry conditions. By 10 weeks following emergence, the ants develope d intolerance of workers from neighbouring colonies, and the mechanism of competition changed from brood raids to group fighting at food res ources or foraging trails.