Birds as predators in tropical agroforestry systems

Birds provide strong top-down control, but little is known about whether predatory effects vary with resource availability, habitat complexity, diversity and composition of predator assemblages, and temporal or seasonal variation in these factors. We examined effects of insectivorous birds on arthropods and plant damage using data from bird exclosure studies in agroforest and forest systems. We first compared the effects of birds in forests and agroforests. Within agroforest studies, we then compared bird effects in canopy trees vs. crop plants, during the season when migratory birds are present or absent, and examined for relationships between bird effects and bird diversity and density. A meta-analysis comparing the effects of bird predation found no difference between agroforests and forests despite simplified habitat structure and plant diversity in agroforests. Within tropical agroforests, bird reduction of arthropods was greater in the canopy than the crop layer of agroforests. Top-down effects of bird predation were especially strong when migratory birds were present in agroforests. Finally we found that birds reduced arthropod densities to a greater extent at sites where the diversity of birds, especially migratory birds, was greater. Theoretical work suggests that top-down predation effects should be stronger where food web complexity and diversity are lower and where plant productivity is greater. Our results provide many striking patterns indicative of potential relationships between predatory function and plant productivity, habitat complexity, and species diversity. We outline potential mechanisms and suggest future studies using tropical agroforests as a model system to further advance ecological theory.