Asymmetries, Compartments and Null Interactions in an Amazonian Ant-Plant Community

Abstract

In the tropics, many plants offer housing and food for their specialized ant partners which, in return, offer benefit in the form of defence and/or nutrients, thus forming mutualistic bonds. Such ant-plants, also called myrmecophytes, occur together at a local scale, generating community patterns of mutualistic ant-plant associations. Here, we present the first fully quantitative description of an ant-myrmecophyte community. The study site in Central Amazonian tropical rainforest had a high myrmecophyte density of about 380 ind. ha -1 . Sixteen myrmecophyte and 25 ant species were recorded, the species abundance rank curves being highly uneven. The ant-myrmecophyte matrix was highly compartmentalized, and a Monte Carlo simulation showed that the observed pattern was not a product of chance and sample size (P < 0.0001). Cluster analyses indicated that compartments were partially explained by occurrence of the ants in phylogenetically related host plants, but not by habitat specificity. The connectance of the ant-plant community was 12%. This value seems quite low when compared with published results from other mutualistic systems (pollinator and seed-dispersor), after controlling for the total number of interacting species. The high frequency of null interactions in the ant-myrmecophyte system could not be explained by the 'phenological non-coincidence hypothesis', since both ant and plant partners occur together throughout the year. Ant-plant interactions were highly asymmetrical: ant species had fewer partners than plant species and ants were more dependent on the plants than the reverse. These asymmetries are in the opposite direction to those recorded for plant-pollinators and plant-dispersors; however, they seem to be the product of the same underlying process: differential fitness benefits between mutualistic partners. The low number of ant and plant partners per compartment, coupled with an apparently high temporal and spatial stability of ant-myrmecophyte interactions, suggests that compartments are the appropriate scale at which to investigate coevolution in ant-myrmecophyte systems.