Empirical test of crab-clam predator-prey model predictions: storm-driven phase shift to a low-density steady state

A dynamic systems approach can predict steady states in predator-prey interactions, but there are very few empirical tests of predictions from predator-prey models. Here, we examine the empirical evidence for the low-density steady state predicted by a Lotka-Volterra model of a crab-clam predator-prey system using data from long-term monitoring, a field survey, and a field experiment. We show that Tropical Storm Agnes in 1972 likely resulted in a phase shift to a low-density state for the soft-shell clam Mya arenaria, which was once a biomass dominant in Chesapeake Bay. This storm altered predator-prey dynamics between M. arenaria and the blue crab Callinectes sapidus, shifting from a system controlled from the bottom-up by prey resources, to a system controlled from the top-down by predation pressure on bivalves. Predator-prey models with these two species alone were capable of reproducing observations of clam densities and mortality rates, consistent with the idea that C. sapidus are a major driver of M. arenaria population dynamics. Over 40 y post-storm, M. arenaria densities hover near a low-density steady state predicted from the predator-prey model. Relatively simple models can predict phase shifts and identify alternative stable states, as shown by agreement between model predictions and field data in this system. The preponderance of multispecies interactions exhibiting nonlinear dynamics indicates that this may be a general phenomenon.