Disturbance and succession on the rocky seashore

My early research examined the effects of disturbance on the structure and dynamics of rocky seashore communities. My dissertation study was conducted in an intertidal boulder field on the seashore at Ellwood Beach, CA, near the UC Santa Barbara campus.  The top surfaces of these boulders provide discrete habitat patches for a variety of algae and sessile invertebrate species.  When they are overturned by winter storm waves, unoccupied space on their undersides becomes available for colonization, initiating a predictable successional sequence that ends in a low diversity climax state dominated by a single species of vegetatively reproducing red algae.  Rates of disturbance by waves vary among boulders, being inversely related to their mass. Employing a mix of long-term sampling data and controlled field experiments, the study demonstrated that intermediate levels of disturbance maintain local species diversity in these algal/invertebrate assemblages.  I also tested three alternative models of successional species replacement that had been formalized by Connell and Slatyer (1977, American Naturalist 111:1119-1144). The Inhibition Model best explained temporal changes in algal species composition: early colonizing species inhibited the recruitment of later arriving species, but early species suffered differentially from herbivory and physical stress, allowing later successional species to replace them over time.

After starting my position at UC Berkeley, I studied how disturbance shapes community structure in the more continuous intertidal habitat of the Bodega Head in Sonoma Co., CA.  Here, in the mid-tidal zone, mussel beds hold most of the primary space in the absence of disturbance, but strong forces imposed by large winter storm waves tear holes in the mussel bed, exposing patches of bare rock surface of various sizes and shapes.   These patches are recolonized by algae and sessile invertebrates that exhibit a sequence of successional replacements through time before the gap is closed by mussels.  I experimentally demonstrated that clearing size interacted with grazing by limpets at patch edges to shape patterns of algal succession and species composition during patch regeneration.  These patterns were also influenced by species-specific variation in local propagule supply and dispersal, as revealed by patterns of correlation between the adult cover of an algal species at nested distances from the edge of an experimental clearing and the rate of recruitment of that species to the cleared patch.  Local abundance and recruitment rates were unrelated for ephemeral, early successional species that widely disperse large numbers of very small spores, while later successional, perennial species, which produce fewer, but larger, propagules that disperse relatively short distances exhibited a strong relationship between their local cover and rates of recruitment to new clearings.