Marine invertebrates in an algal succession. III. Mechanisms linking habitat complexity with diversity

Abstract

A diverse community of marine invertebrates is associated with suceessional algal mats in an intertidal boulder field in southern California. Species richness and abundance of invertebrates increased from early to middle suceessional stages, then remained similar into the later stage. These changes are paralleled by those in the physical structure (biomass and surface area) of the algae. Increased complexity of the algal physical structure probably influenced the associated invertebrate community through several mechanisms. (1) It decreased mortality caused by predation from fish and crabs. (2) It reduced the severity of physical stresses, primarily wave shock. (3) It increased the accumulation of those individuals and species transported passively by wave action. (4) In mobile species, selection of algal substrates was largely based on physical aspects of algal structure. While no species tested appeared to select particular algal species. all actively chose algal clumps of high biomass. All four mechanisms result in the accumulation of greater numbers of individuals in algal mats with higher biomass and surface area. Because a larger sample of the available pool of individuals is collected, more species are found in a given area of algal mat when the structure is more complex. The suceessional patterns of change in species richness of the invertebrate community seem to result from this sampling phenomenon. Laboratory experiments and correlations in abundance between pairs of species gave no evidence of any competitive interactions. Any apparent “structure” in this assemblage seems to be the result of the summation of individual responses to changing algal physical structure during succession.