Ant vs. bird exclusion effects on the arthropod assemblage of an organic citrus grove

Background/Question/Methods   The experimental exclusion of predators is a major tool for uncovering interactions among species in ecological communities. However, studies excluding more than one predator group on a shared prey assemblage that allow measuring the individual effects of each group are scarce. Two of the most important predator groups of terrestrial arthropod-dominated communities are ants and birds, and meta-analytical studies have shown that their predatory effects are of similar magnitude. However, a more controversial picture emerges when one focuses on the only two studies that excluded ants and birds from the same community. In a coffee farm in Chiapas (Mexico) birds had stronger negative effects on arthropods than ants, whereas the opposite occurred in a ponderosa pine stand in Colorado. Here we report the results of a third study of this kind in a Mediterranean citrus grove in NE Spain, intended to estimate the relative effects of ant and bird exclusion on the arthropod assemblage of tree canopies. We conducted two experiments. The first one (2006) compared ant-excluded and bird-excluded with control trees, but had no trees with both predators excluded; the second experiment (2008 and 2009) had a full factorial design to allow testing the interaction between bird and ant-exclusion.

Results/Conclusions   Both the exclusion of ants and birds affected the arthropod community of the citrus canopies, but the exclusion of ants was far more important than the exclusion of birds. Indeed, almost all groups of arthropods had higher abundance in ant-excluded than in control trees, whereas only dermapterans and web-building spiders were occasionally more abundant in bird-excluded than in control trees. The reasons why ants and birds vary in the top-down control they exert on arthropod communities remain obscure, but they might be related to the species richness of insectivorous birds in a given community. A recent meta-analysis of bird predation on arthropods in tropical agroecosystems showed that no functional attribute of the bird community predicted arthropod removal better than species richness. At our site, like at the ponderosa pine stand of Colorado, the diversity of birds was lower than at the tropical coffee farm of Chiapas, and this difference could be the reason behind the more important role of ants than birds in our study.