Prey depletion by a predator guild suggests spatial differences in competitive ability, but not prey partitioning,...

Understanding the mechanisms by which competing species coexist is fundamental for explaining the distribution and abundance of organisms. Many coexistence mechanisms require trade-offs in the ability to deplete shared resources, but resource depletion by competing species is rarely measured, especially in communities of mobile predators. Among competing predators, differences in diet and habitat use are commonly observed, and to promote coexistence, these differences must be associated with trade-offs in the ability to suppress different prey types (prey partitioning) or to suppress prey in different habitats (spatial differences in competitive ability), respectively. We hypothesize that these two mechanisms are important for competition among three arboreal lizards common in Hawaii. We test these mechanisms using experimental monocultures to measure prey suppression and a consumer-resource model parameterized with trait and demographic data from the same experiment. We found that species differed in spatial patterns of prey suppression, supporting spatial differences in competitive ability, but species did not differ in which prey taxa they suppressed, providing no support for prey partitioning. Patterns of prey suppression were consistent with differences between species in microhabitat-specific attack rates, and including differences in the conversion of prey into offspring allowed model outcomes to better align with species' relative prey suppression. When using the model to predict competitive outcomes, Phelsuma laticauda and Anolis sagrei are expected to coexist over a wide range of microhabitat availabilities, Anolis carolinensis can coexist with P. laticauda under a wider range of microhabitat availabilities than it can with A. sagrei, and A. carolinensis cannot persist in the presence of both of the other species under any conditions. These predictions are consistent with the change in community composition that has occurred over time in the field: Anolis carolinensis has declined while the other two species have increased in distribution and abundance. Our results also demonstrate that differences in diet do not necessarily translate into resource partitioning of prey, highlighting the importance of measuring resource depletion when understanding exploitation competition.