Mussel bed boundaries as dynamic equilibria: thresholds, phase shifts, and alternative states

Ecological thresholds are manifested as a sudden shift in state of community composition. Recent reviews emphasize the distinction between thresholds due to phase shifts-a shift in the location of an equilibrium-and those due to alternative states-a switch between two equilibria. Here, we consider the boundary of intertidal mussel beds as an ecological threshold and demonstrate that both types of thresholds may exist simultaneously and in close proximity on the landscape. The discrete lower boundary of intertidal mussel beds was long considered a fixed spatial refuge from sea star predators; that is, the upper limit of sea star predation, determined by desiccation tolerance, fixed the lower boundary of the mussel bed. However, recent field experiments have revealed the operation of equilibrium processes that maintain the vertical position of these boundaries. Here, we cast analytical and simulation models in a landscape framework to show how the discrete lower boundary of the mussel bed is a dynamic predator-prey equilibrium, how the character of that boundary depends on its location in the landscape, and how boundary formation is robust to the scale of local interactions.