Modeling the Phenomenon of Intertidal Mussel Beds: Local Biotic Feedback in Gradients of Physical Forcing

C.D. Robles, California State University, Los Angeles, USA

The populations of mussels and barnacles inhabiting rocky seashores are an important test bed for general ecological theory. Theory prevailing for 30 years maintains that upper shore zones of these species form only in a spatial refuge above the reach of predators, a limit set by the regime of tidal immersion. As an alternative to this static verbal model, the authors present a dynamic model, which they reconcile with concepts of engineering control theory.

Beds of the mussel Mytilus californianus are the most massive and visually striking biological structures of temperate seashores. The beds have abrupt boundaries, even though major environmental gradients are comparatively gradual. Upper and lower boundaries of the mussel zone converge as one travels from high to low wave energy locations.

These distributional features were simulated with stochastic cellular automata in which boundaries were set by equilibria between mussel productivity (recruitment and growth) and mortality (principally size-specific predation). Rates of production and mortality varied gradually with wave energy (horizontal dimension) and tidal emergence (vertical dimension), and they were modified at any point in the gradients by spatial configurations of the size classes of the mussels in the neighborhood of the point. Such neighborhood effects exerted feedback that produced clustering of individuals, thus the abruptness of boundaries. Forcing of mussel recruitment, growth, and predation by varying wave action over the horizontal dimension results in vertical shifts in the shore level at which the equilibria occur, accounting for the convergence of boundaries. The authors term these spatially structured dynamics the "adjusted equilibriums hypothesis."