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 the sizes of colonies. Models of competitive interactions produce conflicting predictions concerning size variation; these predictions have not previously been tested in social insect populations. 2. To measure the 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 series 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 when victorious colonies acquired the brood and workers of one or more neighbours. 4. Initial colony density had strong effects upon the percentage 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. The complex brood raids seen at high colony density rapidly reorganized clusters of incipient colonies. Excavation of surviving colonies showed that the number of workers per colony was more variable on plots with numerous raids; thus, competition by brood raiding tended to increase the degree of size inequality among colonies. This result is consistent with models and observations of 'asymmetric competition' among sessile organisms. 6. During raids, queens sometimes moved to nearby nests where they joined or usurped the original queen. Queen usurpation was especially likely on high-density plots. 7. Brood raids were restricted to the first few weeks following worker emergence and were reduced by low surface moisture, apparently because worker activity declines in dry conditions. By 10 weeks following emergence, the ants developed intolerance of workers from neighbouring colonies, and the mechanism of competition changed from brood raids to group fighting at food resources or foraging trails.