Birds, Bugs, and Agroforestry
By Gregory Gough
Scientists combined data from numerous studies, in both temperate and tropical climates, on the effect of birds on insects. The studies' findings were remarkably similar: when birds were excluded from trees (through the use of netting), insects, and insect damage to trees, increased.
The effects of bird predation were noticeable even in simplified forests, such as agroforests, where a canopy of trees provides shade for coffee or cacao shrubs below. Bird predation on insects reached its peak in the winter, when an influx of migratory birds augmented the predatory nature of the resident birds.
Not surprisingly, the more birds that are present, and the greater the variety of birds, the more bugs are eaten.
This article summarizes the information in this publication:
Van Bael, Sunshine A., Philpott, Stacy M., Greenberg, Russell S., Bichier, Peter, Barber, Nicholas A., Mooney, Kailen A. and Gruner, Daniel S. 2008. Birds as predators in tropical agroforestry systems. Ecology, 89(4): 928–934
Insectivorous birds reduce arthropod abundances and their damage to plants in some, but not all, studies where predation by birds has been assessed. The variation in bird effects may be due to characteristics such as plant productivity or quality, habitat complexity, and/or species diversity of predator and prey assemblages. Since agroforestry systems vary in such characteristics, these systems provide a good starting point for understanding when and where we can expect predation by birds to be important. We analyze data from bird exclosure studies in forests and agroforestry systems to ask whether birds consistently reduce their arthropod prey base and whether bird predation differs between forests and agroforestry systems. Further, we focus on agroforestry systems to ask whether the magnitude of bird predation (1) differs between canopy trees and understory plants, (2) differs when migratory birds are present or absent, and (3) correlates with bird abundance and diversity. We found that, across all studies, birds reduce all arthropods, herbivores, carnivores, and plant damage. We observed no difference in the magnitude of bird effects between agroforestry systems and forests despite simplified habitat structure and plant diversity in agroforests. Within agroforestry systems, bird reduction of arthropods was greater in the canopy than the crop layer. Top-down effects of bird predation were especially strong during censuses when migratory birds were present in agroforestry systems. Importantly, the diversity of the predator assemblage correlated with the magnitude of predator effects; where the diversity of birds, especially migratory birds, was greater, birds reduced arthropod densities to a greater extent. We outline potential mechanisms for relationships between bird predator, insect prey, and habitat characteristics, and we suggest future studies using tropical agroforests as a model system to further test these areas of ecological theory.
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