Regional variation in the colonization of experimental substrates by sessile marine invertebrates: Local vs. regional control of diversity

The degree to which local factors interact with a broader regional pool of species to control species diversity on various spatial scales continues to be debated. For marine systems, recruitment has been identified as a critical element uniting local communities within regions. Most studies have addressed this question over large biogeographic scales encompassing multiple regions. Regional species pools are usually defined as fixed and the degree to which this pool is found on a local scale is often used as a measure of regional influence. Usually the degree to which local communities vary temporally and how this relates to the regional pool is not examined. Given that recruitment links local sites within regions and is a temporally dynamic process, it is important to include temporal variation in the regional vs. local control debate; especially given that regional pools themselves are changing much more rapidly with the invasion of new species. Here the results of a study of the colonization of experimental panels at 28 sites within a region encompassing 200 km of the California coast are used to address this question. This study examined experimentally the temporal and spatial variation in species and contrasted intraregional and local factors. At 22 of these sites, all within 15 km of each other, identical colonization experiments were followed for as long as 3 years. Results of these experiments demonstrated that there was some regional control through the seasonal variation in recruitment, but that strong local effects greatly altered the regional input. Substrates of the same size at the same station usually accumulated the same number of species, but among stations mean diversity ranged from 10 to 50 species per substrate. Differences between sites were a function of more localized recruitment than expected and normal post-recruitment processes of competitive dominance, disturbance, and predation that affected the rates at which species were lost. The colonization of most substrates resulted in their attaining an asymptotic or equilibrium diversity maintained by a balance between species immigration and local extinction. Nevertheless, on most substrates diversity fluctuated much more than predicted by an equilibrium model. The similarity in species richness on substrates at the same site regardless of initial exposure time, the large differences among substrates from different sites, the similar pattern of differences among the sites for panels of different sizes, and the resilience of communities to disturbance and their return to the diversity characteristic of a site all argue for strong local factors controlling the diversity within the region.