Diversity of a Northern Rocky Intertidal Community: The Influence of Body Size and Succession

Hypotheses about diversity in succession in rocky intertidal communities in the Saint Lawrence estuary, Canada, were tested to evaluate whether results from the study of large organisms may be extrapolated to the entire community of macroinvertebrates. Varying the lower size limit of the organisms considered in the analyses (organisms >0.5, 1, 2, 4, and 8 mm) altered the observed trends in taxonomic richness and diversity (the latter calculated in terms of abundance, H'no, and biomass, H'wt). Diversity increased through succession when only the largest organisms were considered, but H'nowas maximal in midsuccession for the >2 and >4 mm size groupings and in late succession for the >1 and >0.5 mm size groupings, and H'wtwas greatest at the second stage of succession for all other size groupings. Richness increased through succession for all size groupings. Whole-crevice manipulations (mimic crevices, modified to leave only the dominant structural taxa) were used to examine competing hypotheses about whether variation in richness and diversity through succession was a function of the structural heterogeneity provided by the dominant taxa (structural-heterogeneity hypothesis), or a function of the age of the communities per se (ecological-time hypothesis). The first is supported if the communities in mimic crevices resemble those in control crevices for each successional stage; the second is supported if mimic crevices resemble each other more than they do control crevices at the same successional stage. Within three months, control and mimic communities largely resembled each other in terms of H'no, H'wtand richness, and the trends mirrored those observed four months earlier, thus supporting the structural-heterogeneity hypothesis. Nonparametric multivariate analyses that removed the manipulated species from the analyses also supported this conclusion. Results from 12 mo after the initiation of the experiment (following the winter, during which the communities within many of the crevices were greatly altered) further supported the structural-heterogeneity hypothesis.