Biodiversity and the Transformation of a Tropical Agroecosystem: Ants in Coffee Plantations

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.