ASYMMETRIES, COMPARTMENTS AND NULL INTERACTIONS IN AN AMAZONIAN ANT-PLANT COMMUNITY

1. In the tropics, many plants offer housing and food for their specia lized ant partners which, in return, offer benefit in the form of defe nce and/or nutrients, thus forming mutualistic bonds. Such ant-plants, also called myrmecophytes, occur together at a local scale, generatin g community patterns of mutualistic ant-plant associations. Here, we p resent the first fully quantitative description of an ant-myrmecophyte community. 2. The study site in Central Amazonian tropical rainforest had a high myrmecophyte density of about 380 ind. ha(-1). Sixteen myr mecophyte and 25 ant species were recorded, the species abundance rank curves being highly uneven. 3. The ant-myrmecophyte matrix was highly compartmentalized, and a Monte Carlo simulation showed that the obser ved pattern was not a product of chance and sample size (P < 0 . 0001) . Cluster analyses indicated that compartments were partially explaine d by occurrence of the ants in phylogenetically related host plants, b ut not by habitat specificity. 4. The connectance of the ant-plant com munity was 12%. This value seems quite low when compared with publishe d results from other mutualistic systems (pollinator and seed-disperse r), after controlling for the total number of interacting species. The high frequency of null interactions in the ant-myrmecophyte system co uld not be explained by the 'phenological non-coincidence hypothesis', since both ant and plant partners occur together throughout the year. 5. Ant-plant interactions were highly asymmetrical: ant species had f ewer partners than plant species and ants were more dependent on the p lants than the reverse. These asymmetries are in the opposite directio n to those recorded for plant-pollinators and plant-dispersers; howeve r, they seem to be :he product of the same underlying process: differe ntial fitness benefits between mutualistic partners. 6. 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 inve stigate coevolution in ant-myrmecophyte systems.