A Graphical Model of Keystone Predators in Food Webs: Trophic Regulation of Abundance, Incidence, and Diversity Patterns in Communities

I analyze a model of species interactions involving species that compete for a single resource and share a common "keystone predator" to study the "bottom-up" effects of productivity (potential carrying capacity of the resource) and the "top-down" effects of factors that affect the death rate of the predator on diversity, abundance, and distribution patterns in the resulting assemblages. The model predicts that coexistence of such species will occur at intermediate productivity (and at intermediate death rates on the top predator) and that superior resource exploiters will dominate at low productivity (and high predator death rates), whereas predator-resistant forms will dominate at high productivity (and low predator death rates). In this model, predator and resource densities are "buffered" against variation in productivity, but the densities of species at the intermediate trophic level are not. Given a large "pool" of potential such species, the model predicts a replacement series involving multiple pairs of species ranging from good resource exploiters to predator-resistant forms as productivity increases or predator death rates decrease. In such a case, coexistence is most likely among the more similar forms, but abundances of similar species are predicted to be negatively correlated. Furthermore, the overall density of organisms at all trophic levels is correlated with variation in productivity. Spatiotemporal heterogeneity modifies these predictions by allowing more than two species to coexist in the presence of a single resource and a single predator and by permitting positive covariation in the abundances of coexisting competitors as well. These models show the critical role of species compositional turnover in determining food web responses to bottom-up and top-down regulation by productivity and variation in predator death rates. The models also predict unimodal diversity versus productivity curves that depend in part on the degree of prey specificity by the top predator.