Relative contribution of ecological and biological attributes in the fine-grain structure of ant-plant networks

Background. Ecological communities of interacting species analyzed as complex networks, revealed that species dependence on their counterpart is more complex than expected at random. For ant-plant networks (mediated by extrafloral nectar), links among species are asymmetric (nested), forming a core of generalist species. Proposed factors affecting network organization include encounter probability (species abundances, habitat heterogeneity), behavior, phylogeny and body size. While the importance of underlying factors that influence structure of ant-plant networks have been separately explored, simultaneous contribution of several biological and ecological attributes inherent to the species, guild or habitat level have not been addressed. Methods. For a tropical seasonal site we recorded frequency of pairwise ant-plant interactions mediated by extrafloral nectaries, attributes of interacting species, habitat attributes, cover of plants with EFNs, and studied the resultant network structure. We addressed for the first time the role of mechanistic versus neutral determinants at the “fine-grain” structure (pairwise interactions) of ant-plant networks, studying the simultaneous contribution of several plant, ant, and habitat attributes in prevailing interactions as well as in overall network topology (community). Results. Our studied network was highly-nested, non-modular, with core species in general having high species strengths (higher strength values for ants than plants) and low specialization; plants had higher dependences on their counterparts. The significant factor explaining network and fine-grain structure was habitat heterogeneity in vegetation structure (open vs. shaded habitats), with no evidence of neutral (abundance) effects. Discussion. Core ant species are relevant to most plants species at the network, the latter depending more on the former, core ants showing adaptations to nectar consumption and deterrent behavior, suggestive of potential biotic defense at a community scale. At our study site spatiotemporal heterogeneity is so strong, that emerges at community-level structural properties, depicting influence of abiotic factors in facultative mutualism. Frequent occurrence of morphologically-diverse EFNs at all habitats suggests plasticity in plant strategies for biotic defense provided by ants.