Predicting Physical Disturbance in a Changing Environment: Field Test of a Biomechanical Approach

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9633070 Denny Disturbance plays a central role in non-equilibrium theories of community structure, in which high species diversity and species coexistence are attributed to processes of stochastic recruitment in a heterogeneously perturbed, patchy environment. The disturbance regime of the rocky intertidal system offers an advantage for study in that it is characterized by chronic high intensity physical disturbance due primarily to a single factor -- the hydrodynamic forces imposed by wave action. This controlling nature of "wave exposure" is one reason the intertidal zone serves as a model system in experimental ecology. Dr. Denny's lab has recently been able to combine the results of earlier work in wave statistics and hydrodynamics with empirical measurements of the strength of intertidal organisms in a quantitative, biomechanical method that predicts physical disturbance as a function of time and the "waviness" of the sea. Preliminary applications of this approach have shown that its predictions are reasonably accurate, but a rigorous test of the approach has not been conducted, which is the impetus for this project. Extensive field measurements will be conducted of the parameters that serve as inputs into the biomechanical approach, and these data will be used to predict the week-to-week rate of disturbance of each of six representative species of plants and animals at a series of sites along the shore. These predictions will then be compared to the rates of disturbance measured empirically at these same sites. The goodness of fit between prediction and empirical measurement can then be calculated to provide evidence of the overall predictive ability of the biomechanical approach. The inexpensive instruments developed in these studies will make it possible for intertidal ecologists to repeat the basic steps of this approach (measuring maximal wave height, measuring habitat-specific maximal water velocities), and thereby should facilitate the use and testing of the biomechanical method at a wide variety of sites. If these tests of the biomechanical approach confirm its ability to predict species-specific rates of disturbance, the method can be a valuable tool for understanding the ecological consequences of both seasonal and year-to-year variability in wave exposure, and for the prediction of how rates of disturbance will change if large-scale climatic change causes longterm shifts in the waviness of the ocean.