Disturbance and succession on the rocky seashore

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    Boulder field at Leo Carrillo State Park, Los Angeles Co, CA

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    Boulder field at Ellwood, Santa Barbara Co, CA 

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    Boulders overturned by winter storm waves.

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    Recently overturned boulders being colonized by early successional green alga Ulva sp., surrounded by more stable boulders covered with late successional red algae

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    The central disturbed boulder has developed a dense cover of Ulva sp.

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    Selective grazing on Ulva by the crab, Pachygrapsus crassipes, accelerates successional species replacement

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.

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    Extensive mussel (Mytilus californianus) beds dominating the mid-intertidal zone on the Bodega Head, Sonoma Co., CA

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    Large waves striking the shore on the Bodega Head

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    Recently cleared bare patch in mussel bed

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    Cleared patch colonized by dense cover of early successional green alga, Ulva sp., with bare halo zone along the mussel bed edge maintained by limpet grazing.

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    Limpets grazing at the edge of a stand of Ulva sp.

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    Sousa sampling algal cover in experimental clearings

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.