EXPERIMENTAL STUDIES OF SPECIES-SPECIFICITY IN CECROPIA-ANT RELATIONSHIPS

Strict coevolution requires that interactions among organisms be speci es-specific. We assessed the relative roles of host- and habitat-speci ficity in determining the match between a genus of myrmecophytic trees and a guild of obligate plant-ants in the moist tropical forests of M adre de Dies, Peru. Four locally coexisting but habitat-restricted Cec ropia species were cultivated in screen tents until all plants had dev eloped myrmecophytic traits. Saplings were then placed within replicat e blocks of each of two habitat types: riversides and small forest lig ht gaps. Colonization events were recorded every 3 d between June and August of 1992, and queens were later removed from stem internodes for identification and brood censuses. A similar experiment, conducted in September through November of 1993, included just two species of Cecr opia hosts. Effects of host species and habitat on queen colonization rates were evaluated by log-likelihood goodness-of-fit tests and conti ngency table tests. For three ant species, we also conducted queen pre ference experiments to compare queen behaviors across a range of host plants. Differences among ants in the extent of habitat-specificity vs . host-specificity provide evidence for multiple evolutionary routes t o obligate association with Cecropia. Habitat-specificity exceeded hos t-specificity in Azteca ovaticeps (Dolichoderinae), for which queen pr eference experiments revealed no significant discrimination among host s. This extreme riverside specialist is thought to have descended from generalist live-stem nesters in second-growth habitats. In Azteca aus tralis, host-specificity was strong, and in this species only, directe d toward hosts where brood production was most successful. Conflicting habitat associations in the two experiments indicated the weakness or absence of a consistent habitat affiliation in Azteca australis and s uggested that colonization frequencies were influenced instead by prox imity to foundress sources. Close relatives of A. australis live in ex posed carton nests, which may have been positioned ancestrally on key resource plants, e.g., those producing lipid- and amino-acid-rich pear l bodies. Pachycondyla luteola (Ponerinae) exhibited both strong habit at and host associations and may have undergone pairwise coevolution w ith its forest-gap-dwelling primary host. Queens of Camponotus balzani (Formicinae), possibly a recent and secondary associate of Cecropia, were overrepresented in forest gap habitat but were host generalists, underrepresented only on a host with extremely small internodes. Appar ently greater host-specificity in C. balzani at later stages of colony establishment may be due to differential post colonization mortality on the various hosts. Attack of ant queens by parasitoid wasps was str ongly concentrated in the linear riverside habitat and weak to absent in the patchily distributed forest gap habitat. Due to lower rates of either parasitoid attack or other forms of queen mortality, Camponotus balzani experienced greater success in the forest gap habitat, where it was overrepresented in colonization experiments. Historical coincid ences and preadaptations appear to have strongly influenced pairings b etween Cecropia species and their obligate plant-ants and account for much of the ''apparent'' niche partitioning observed in the system. Sp ecies-specificity seems to be determined mainly by coincident habitat affiliations of ants and plants (''coordinated dispersal'') and by pre adapted capacities of ants to distinguish among host-plant species. Mu ltiple mechanisms for species-specificity may be characteristic of rel ationships in which associates disperse separately from one another (i .e., show horizontal transmission). Our results are consistent with th e view that coadaptation and co-cladogenesis are more likely in system s where dispersal of associates is tightly coupled.