Predatory birds and ants partition caterpillar prey based on caterpillar body size and diet breadth

Background/Question/Methods The effects of multiple predator taxa on densities of a shared prey taxon can be additive or non-additive, the latter being either synergistically positive or antagonistic. Most studies show evidence for additive or antagonistic effects, but the underlying mechanisms have received limited study. One prominent hypothesis for additive effects is resource partitioning due to alternative predator foraging strategies or prey traits, such as body size. Antagonistic effects can result from intraguild predation or predator interference. In this study, we investigated multiple predator effects of birds and ants on their forest caterpillar prey. We used a field experiment in which birds and ants were excluded factorially from the branches of eight tree taxa over two years (2010-2011) in a temperate forest (Middlesex Co., CT, USA). We measured caterpillar densities across all branches twice during each year. Because multiple predator effects can vary due to prey traits, we considered bird and ant predation in relation to caterpillar body size as well as caterpillar diet breadth. We considered caterpillar diet breadth because it has been hypothesized to mediate predator-herbivore interactions as well as plant-herbivore interactions. We also asked if multiple predator effects varied among tree taxa.

Results/Conclusions Our results revealed additive effects of bird and ant predation on caterpillar density across all tree species. The additive predation resulted from partitioning of caterpillar prey based on caterpillar body size and diet breadth. Bird predation targeted large-bodied, dietary generalist caterpillars, while ant predation targeted small-bodied, dietary specialist caterpillars. From the predator perspective, partitioning of their caterpillar prey facilitates foraging efficiency and reduces competition for food. From the herbivore perspective, the ecological trade-off between bird and ant predation mediated by caterpillar body size and diet breadth promotes coexistence of herbivores with alternative life history strategies. From the plant perspective, the beneficial effects of birds and ants depend on caterpillar community structure.