Mechanistic analysis of interspecific competition using foraging trade-offs: implications for duck assemblages

The role of interspecific competition and resource partitioning in determining the composition of species assemblages is often controversial. In many cases data on species co-occurrence or resource use (prey or habitat) have been interpreted without a clear understanding of how, or even whether, phenotypic differences constrain performance to allow resource partitioning or how these constraints and the density of resources and competitors should shape resource selection by each species. Instead, predictions have been based on assumed constraints, possibly leading to conflicting results. One such controversy involves the role of bill morphology in mediating resource partitioning among dabbling ducks (Anas spp.). To determine whether incorrect assumptions may have contributed to this controversy, I constructed mechanistic models that predict filter-feeding performance for seven species of ducks directly from bill morphology and kinetics and compared these predictions to those of earlier studies that tested the bill morphology hypothesis. The models predicted that species should share a preference for their most profitable (primary) prey while partitioning their less profitable (secondary) prey by size. Consequently, ducks should forage in the same habitats and exhibit high overlap in prey size when competitor/resource ratios are either high or low. In contrast, earlier studies expected that resource partitioning should always be evident, which implicitly assumes that species partition their primary resources. The models also predicted that the ecological similarity of species in assemblages should increase as prey abundance and size variability declines, contrary to the expectations of an earlier study. A more consistent understanding of the mechanisms regulating assemblages of dabbling ducks, and other species, might emerge if patterns of resource use and species co-occurrence were predicted directly from a mechanistic understanding of how performance trade-offs affect resource selection in the context of varying resource and competitor densities.