Patterns and causes of intertidal community structure around the coast of Southern Africa

Includes bibliographical references.A biogeographic analysis was undertaken using data for intertidal rocky-shore communities obtained from quantitative biological surveys made around the southern African coast. Substantial differences in species composition, abundance and trophic structure were found between 15 different rocky shores along ca. 5000 km of coast. These differences allowed the division of South African coast into three major biogeographic provinces. The large-scale variations were due to geographical differences in the physical environment including temperature, nutrients and primary production. Large-scale and local-scale vertical and horizontal zonation of intertidal communities were investigated. Local-scale variations were largely due to the effect of differential wave action, but also included the vertical gradient of desiccation. Gradients of wave action generate divergence of the mid-to-low zone communities, whereas in the upper shore communities converge due to the desiccation gradient. The consistent repetition of the vertical and horizontal patterns of patterns of distribution for both community biomass and species richness lead to the conclusion that the physical processes involved in the creation of local patterns also operate over a large geographical scale. The patterns of community structure on southern African rocky shores are investigated in relation to in-situ primary production and micronutrient concentrations. A conspicuous gradient of intertidal primary production was found