Spatial Habitat Heterogeneity Influences Competition and Coexistence in an African Acacia Ant Guild

Spatial mosaics in resource productivity may facilitate competitive coexistence when species differ in their ability to exploit resource-rich vs. resource-poor conditions. In this study, I investigated the influence of a termite-generated spatial mosaic in resource productivity on the dynamics of competition and coexistence in a guild of four acacia ants that reside on Acacia drepanolobium. Near termite mounds, both new shoot production of A. drepanolobium and densities of litter-dwelling invertebrates (an important food source of the acacia ants) are higher than in “matrix” habitats between mounds. This resource variation is spatially correlated with the outcome of competition among acacia ants for host trees; near the mounds, competitively dominant species are more likely to supplant subordinates, whereas the likelihood of subordinates replacing dominants on host trees increases with distance from mounds. Variation in competitive outcomes among patch types appears to result from differential responses among acacia ant species to resource heterogeneity. Analysis of average tissue δ15N levels indicated that acacia ant colonies near termite mounds had higher ratios of animal prey in their diets than colonies in matrix areas. Bait trials and analysis of average nest size in mound and matrix habitats suggest that competitively dominant species are disproportionately successful in exploiting higher prey densities near termite mounds. Increased dietary uptake of nitrogen-rich prey may fuel more rapid colony growth in competitively dominant acacia ants, resulting in more pronounced asymmetries in colony size between dominant and subordinate species in mound areas. Because colony size asymmetry is a key determinant of interspecific competitive outcomes, colonies of subordinate species near termite mounds may be more vulnerable to aggressive take-overs by dominants. In lower-productivity matrix habitats, increased tolerance to low-resource conditions may afford subordinate species greater persistence. Overall, these results suggest that termite-induced habitat heterogeneity plays a significant role in the dynamics of the acacia ant community, and may contribute to species coexistence in this intensely competitive community.