The consequences of consumer diversity loss: different answers from different experimental designs

Predators are often the most vulnerable group to extinction, yet the consequences of changing predator diversity are poorly understood. One source of confusion has been different experimental designs. The multiple-predator effects literature typically employs an additive design, while the biodiversity ecosystem function literature typically uses a replacement design. Separately, these designs each detect only a subset of the changes in food web interactions caused by predator loss. Here, we measure the impact of consumer diversity on sessile marine invertebrates using a combination additive-replacement design. We couple this with a meta-analysis of previous combination experiments. We use these two approaches to explore how each design can detect different types of interactions among predators. We find that, while high diversity does lead to more negative interspecific interactions, the strength of these interactions is often weaker than negative intraspecific interactions caused by increasing the density of a single species alone. We conclude that a hybrid design is the optimal method to explore the mechanisms behind the effects of changing predator diversity. If researchers merely want to know the consequences of changing predator diversity, at a bare minimum, the experimental design must mimic the actual changes in both predator density and diversity in their system of interest. However, only a hybrid design can distinguish the consequences of shifting the balance of interspecific and intraspecific interactions within a community, an issue of great importance when considering both natural diversity loss and pest biocontrol.