Mechanisms of population regulation in the fire ant Solenopsis invicta: anexperimental study

1. We tested for density-dependent regulation of biomass in a population of the fire ant Solenopsis invicta and examined the mechanisms of population recovery following replicated colony removals. 2. All colonies were killed within the core area (1018 m(2)) of six plots, while six additional plots served as undisturbed controls. Over the next 5 years, colonies were mapped several times per year and the biomass of each colony was estimated from the volume of the nest-mound. 3. The average biomass and density of colonies within the removal areas gra dually converged on those of control plots and were no longer detectably di fferent after two years. Thereafter, ant biomass on experimental and contro l plots showed nearly identical seasonal and yearly fluctuations. 4. Territories of colonies surrounding the removal areas rapidly expanded f ollowing the deaths of neighbours, while average territory size on control plots showed little short-term change. 5. Significantly more new colonies were established within core areas of ex perimental plots than within core areas of control plots during the first y ear following removals. 6. The per-colony probability of movement and the net influx of colony biom ass were significantly higher in central regions of the experimental plots than in control plots during the first year. The directions of colony movem ents were clustered towards the centres of experimental plots in the first 2 years, but did not show significant directional trends on control plots. 7. In all 5 years of the study, annual mortality rates were lower for large r colonies, but the size-specific risk of mortality was not significantly a ffected by the experimental removal of competitors. 8. The growth rates of colonies, adjusted for initial size, were significan tly higher in central regions of experimental plots than in control plots d uring the first two years of the study. In all years, colony growth rates d eclined with increasing colony size. 9. These results indicate that populations of S. invicta are regulated by c ompetition among neighbouring colonies. Due to large intraspecific variatio n in colony size, the dynamics of ant populations are described more accura tely by measures of total ant biomass than by colony density alone.