Geographic variation and community structure in an ant-plant mutualism: Azteca and Cecropia in Costa Rica

The association between Azteca ants and Cecropia trees is the most conspicuous ant-plant mutualism in the neotropics, yet little is known about the identities or community ecology of the species involved. A survey of ant communities found in Costa Rican Cecropia trees revealed a community of Azteca species obligately associated with Cecropia, and a diverse assemblage of non-obligate ants in a variety of genera. High occupation rates of Cecropia saplings and trees, and presence of many incipient colonies in saplings vs single colonies in trees, suggest that Cecropia are a limiting resource for which ants compete. Obligate Azteca appeared competitively superior to non-obligate ants, since non-obligate ants were never found dominating mature trees. Competitive relationships between obligate Azteca were not clear; local communities contained three to four Azteca species using the same host tree species resource. Two Azteca species had a parapatric distribution, the zone of parapatry co-occurring with a similar zone of parapatry between two Cecropia species. At the boundary, the Azteca species were not host specific. Possible factors influencing the outcome of competition between ants for Cecropia trees include queen and worker behavior, priority effects, partial hostplant specialization, and habitat specialization. Explanation of current distributions may require knowledge of historical factors and stochastic effects. SPECIALIZED MUTUALISMS BETWEEN ANTS AND MYRMECOPHYTES are not isolated pairs of interacting species. They usually involve communities of many species. Myrmecophytes, as used here, are plants that produce cavities (domatia) within which specialized ants nest. Myrmecophytic Acacia trees in Central America are occupied by several Pseudomyrmex species (Janzen 1967, 1973a), at least one of which is a parasite rather than a mutualist (Janzen 1975). Leonardoxa africana in Cameroon is occupied by the mutualist Petalomyrmex phylax and the parasite Cataulacus mckeyi (McKey 1984). New World Triplaris are inhabited by sympatric species of Pseudomyrmex, and Old World Barteria by sympatric species of Tetraponera (P. S. Ward, pers. comm.). Cordia aliodora in Costa Rica is occupied by sympatric populations of Azteca longiceps and Zacryptocerus setulifer (pers. obs.). Examining the structure and spatial variation of these communities may provide insight into how specialized mutualisms evolve and are maintained. Wilson (1987) reported a very rich arboreal fauna occupying single rain forest tree canopies in Peru, and raised questions regarding the mechanisms promoting such high local species richness. Communities of myrmecophytes and their ants provide a microcosm within which to examine the community ecology of arboreal ants. By examining a specialized ant-plant interaction at the community level one can address the following questions. What is the spatial variation in community composition? Do ants exhibit hostplant specificity? Are plants a limiting resource for ants? What factors influence the outcome of competition between ants for plants? Results from a study of an ant-plant community in Peru suggest that a variety of ants do compete for plants, and that specialized inhabitants are competitively superior (Davidson et al., in