The structure of ant-plant ecological networks: is abundance enough?

Knowledge of the mechanisms that shape biodiversity is essential to understand the ecological and evolutionary dynamics of interacting species. Recent studies posit that most of the organization of mutualistic networks is shaped by differences in species abundance among interacting species. In this study, we examined the mutualism involving plants with extrafloral nectaries and their associated ants. We show empirically that the difference in abundance among ants on vegetation partially explains the network structure of mutualistic interactions and that it is independent of ant species compositions: an ant species that is abundant usually interacts with more plant species. Moreover, nested networks are generated by simple variation in ant abundance on foliage. However, in ant-plant mutualistic networks, nestedness was higher than in networks describing the occurrence of ants on plants without a food resource. Additionally, the plant and ant species with the highest number of interactions within these networks interacted more among themselves than expected under the assumption of an abundance-based, random mixing of individuals. We hypothesize that the dominance of these ant species occurs because these ants are able to outcompete other ant species when feeding on extrafloral nectaries and because of the presence of ecophysiological adaptations to utilize liquid food.