The extraordinary abundance of ants in tropical rainforest canopies has led to speculation that numerous arboreal ant taxa feed principally as "herbivores" of plant and insect exudates. Based on nitrogen (N) isotope ratios of plants, known herbivores, arthropod predators, and ants from Amazonia and Borneo, we find that many arboreal ant species obtain little N through predation and scavenging. Microsymbionts of ants and their hemipteran trophobionts might play key roles in the nutrition of taxa specializing on N-poor exudates. For plants, the combined costs of biotic defenses and herbivory by ants and tended Hemiptera are substantial, and forest losses to insect herbivores vastly exceed current estimates.

Studies of biological diversity have focused mainly on undisturbed ecosystems, effectively neglecting potential losses due to changes in areas already altered by human intervention. In this study we test the hypothesis that measures of biological diversity change significantly with changes in agricultural practices. In particular, we examine differences in measures of ant species diversity correlated with changes in vegetational complexity associated with the modernization of Costa Rica's coffee agroecosystem. We examine patterns of within‐habitat α and between‐habitat β diversity in the ant community. Ants were sampled in 16 coffee farms falling on a gradient of vegetational and structural complexity. Percentage of shade created by the canopy was used as an index of vegetational complexity. As a partial indicator of the food resource base, arthropods were sampled using pitfall traps. The diversity (S, H', and E) of ground‐foraging ants decreased significantly with the reduction of vegetational diversity. However, no significant changes were recorded for the diversity of the ants on the surface of coffee bushes. Similarity indices (I) showed a high degree of similarity among ant communities in coffee monocultures but a low degree of similarity among farms with high vegetational diversity. We discuss several possible mechanisms leading to reduced ant diversity.

Disparities in liquid-feeding performances of major ant taxa have likely been important to resource partitioning among ants, to the nature and composition of ant partnerships with plants and sap-feeding trophobionts, and to ecological and evolutionary diversification of ant taxa. We measured performance volumetrically for individual workers of 77 ant species from lowland rain forests of Amazonia and Borneo and three key North American taxa. In trials with 9% sucrose solution, performances were strongly related to body size (and alitrunk length) and to proventricular structure at generic to subfamilial levels. Highly modified proventriculi were associated with disproportionately large load sizes in Formicinae and certain small-bodied Dolichoderinae. These same taxa also ingested liquids more rapidly during foraging than did similar-sized species with plesiomorphic proventriculi. Secondarily reduced foraging performances of several formicines likely reflect ecological or evolutionary trade-offs related to dietary specialization or anti-predator defenses. Across formicines and dolichoderines, performances differed by functional group. Relatively small loads and slow uptake characterized species tending trophobionts (mainly Hemiptera) day and night in large worker aggregations. Large loads and rapid uptake typified solitary, diurnal "leaf-foragers." Intermediate feeding performances characterized a few species that both tended trophobionts in small aggregates and frequently foraged alone.