Assessing ecological specialization of an ant-seed dispersal mutualism through a wide geographic range

Specialization in species interactions is of central importance for understanding the ecological structure and evolution of plant-animal mutualisms. Most plant-animal mutualisms are facultative and strongly asymmetric. In particular, myrmecochory (seed dispersal by ants) has been regarded as a very generalized interaction. Although some recent studies have suggested that only a few ant species are really important for dispersal, no rigorous measurement of the specialization in ant-seed dispersal mutualisms has been performed. Here, we use individual plants as basic units for replication to investigate the generalization-specialization of the herb Helleborus foetidus on its ant dispersers over a considerable part of its geographical range. We define generalization in terms of diversity components (species richness and evenness) of the ant visitor that realizes dispersal by removing diaspores. We obtain truly comparable values of ant visitor diversity, distinguishing among different functional groups of visitors and identifying incidental visitors and real ant dispersers. Using null model approaches, we test the null hypothesis that ant-mediated dispersal is a generalized mutualism. At least two premises should be confirmed to validate the hypothesis: (1) diaspores are dispersed by multiple ant-visitor species, and (2) diaspore dispersal is significantly equitable. Though up to 37 ant species visited diaspores across 10 populations, only two large formicines, Camponotus cruentatus and Formica lugubris, were responsible for the vast majority of visits resulting in dispersal in most populations and years, which strongly suggests that ant seed dispersal in H. foetidus is ecologically specialized. Interestingly, specialization degree was unrelated to dispersal success across populations. Our study offers new insights into the spatiotemporal dynamics of myrmecochory. We propose the existence of an alternative scenario to extensive generalization. In this new scenario, generalization is replaced by ecological specialization, which is determined by the intrinsic traits of the plant species rather than by the ecological context in which the interaction takes place.