Inconsistency and variation in the development of rocky intertidal algal assemblages

Abstract

Experimentally-cleared patches were used to test hypotheses about the relative importance of broad-scale biogeographic processes and small-scale historical processes in the development of low-shore algal assemblages on wave-exposed rocky coasts of New South Wales (Australia). In addition, the applicability of generalizing from patterns of recruitment and development at one time was tested by providing similar cleared patches in the algal assemblage at three-monthly intervals and quantifying early development of the algal assemblage in these clearings fifteen times over a period of four years. The early stages of development of these assemblages differed significantly from shore to shore and time to time and there was no evidence for common broad-scale patterns of recruitment, nor for any biogeographic trend. Nevertheless, these assemblages developed from a limited common pool of species, some of which recruited fairly regularly on most shores and some of which only arrived sporadically on some shores. Despite different starting points, assemblages converged towards the surrounding assemblages on most shores, so that within less than 1–2 years, cleared areas resembled the surrounding assemblages. Although the specific changes leading to convergence differed from shore to shore, there was a general pattern of early colonizers, such as ephemeral algae and sessile animals being gradually replaced by larger perennial algae. These results demonstrate no simple seasonal nor clear-cut biogeographical patterns in the development of algal assemblages on these shores and indicated the relative importance of local influences. Results are discussed with respect to the need to do experiments at numerous places in order to examine responses of assemblages to, or recovery from, environmental disturbances.