Measuring competition coefficients in an ant community: Implications for intraspecific adaptation load

Understanding the stable coexistence of species despite resource competition has been a central topic in ecology. Ant communities are particularly enigmatic as various species coexist despite resource overlap. Community ecology theory predicts stable species coexistence when intraspecific competition is stronger than interspecific competition, but due to their perennial and underground life, competition coefficients of ants have never been rigorously measured in the field. We tackled this problem by studying Diacamma cf. indicum, which allows for noninvasive mark-recapture of whole colonies. Several ant species coexisted at the study site where Diacamma was most abundant, and baiting experiments and stable isotope analyses suggested overlapping food niches. Consistently, per worker brood production of Diacamma colonies was significantly negatively correlated with con- and heterospecific worker densities within the foraging area, suggesting exploitative competition among the ants. In terms of net population growth, however, the estimated intraspecific competition coefficient was about five times larger than the interspecific competition coefficient. This is possibly because exploitative competition for food occurs both intra- and interspecifically, whereas interference competition occurs mostly among conspecifics. Indeed, for Diacamma worker survival, there was a significant (nonlinear) negative correlation only with the density of conspecific colonies within the foraging area. This is consistent with the observation that Diacamma rarely fought with other species, although it violently attacked conspecific aliens encountered in their nest vicinity. We interpreted these results in light of the recent theory of intraspecific adaptation load. This theory predicts that density-dependent adaptation to intraspecific conflict can intensify intraspecific competition and act to suppress per capita population growth in dominant species, thereby leading to species coexistence with overlapping resources. Our inclusive fitness model suggests that the intraspecific territorial aggression in Diacamma may be a counter-adaptation to intraspecific conflict, that is, brood abduction between conspecific colonies. This aggression pattern can cause the observed density-dependent worker mortality. Our population dynamic model indicates that such density-dependent excess mortality acting on dominant competitors can promote stable coexistence with subordinate competitors. Overall, our results support the intraspecific adaptation load theory that aims at integrating behavioral and community ecology to understand how adaptation interacts with population and community dynamics.