Experimental Investigations of Disturbance and Ecological Succession in a Rocky Intertidal Algal Community

Mechanisms of ecological succession were investigated by field experiments in a rocky intertidal algal community in southern California. The study site was algal—dominated boulder field in the low intertidal zone. The major form of natural disturbance which clears space in this system is the overturning of boulders by wave action. Algal populations recolonize cleared surfaces either through vegetative regrowth of surviving individuals or by recruitment from spores. Boulders which are experimentally cleared and concrete blocks are colonized within the first month by a mat of the green alga, Ulva. In the fall and winter of the first year after clearing, several species of perennial red algae including Gelidium coulteri, Gigartina leptorhynchos, Rhodoglossum affine, and Gigartina canaliculata colonize the surface. If there is no intervening disturbance, Gigartina canaliculata gradually dominates the community holding 60—90% of the cover after a period of 2 to 3 years. If undisturbed, this monoculture persists through vegetative reproduction, resisting invasion by all other species. During succession diversity increases initially as species colonize a bare surface but declines later as one species monopolizes the space. Several contemporary theories concerning the mechanisms of ecological succession were tested. The early successional alga, Ulva, was found to inhibit the recruitment of perennial red algae. This competition for settling space is an important feature of the successional process. Ulva is the best competitor for this space; it reproduces throughout the year and quickly becomes established on newly cleared substrates. As long as these early colonists remain healthy and undamaged, they preempt colonization by perennial red algae which have highly seasonal recruitment and slower growth. Selective grazing on Ulva by the crab, Pachygrapsus crassipes, breaks this inhibition and accelerates succession to a community of long—lived red algae. Grazing by small molluscs, especially limpets, has no long—term effect on the successional sequence. Their grazing temporarily enhances the recruitment of the barnacle, Chthamalus fissus, by clearing space in the mat of algal sporelings and diatoms which develops on recently denuded rock surfaces. Where locally abundant, middle successional red algae also slow the invasion and growth of the late successional dominant, Gigartina canaliculata. This alga replaces middle successional species because it is less susceptible to damage by desiccation and overgrowth by epiphytes. The results of this study do not support either the classical facilitation model or the tolerance (competitive) model of ecological succession. Once early colonists secure the available space/light, they resist rather than facilitate the invasion of subsequent colonists. Early colonists are not killed by direct interference competition with late successional species which grow up through their canopy; rather, early colonists can successfully inhibit the recruitment and growth of these species. Successional sequences occur because species which dominate early in a succession are more susceptible to the rigors of the physical environment and to attacks by natural enemies than late successional species. Late species colonize and grow to maturity when early species are killed and space is opened. Only late in a successional sequence, when large clearings become a mosaic of small openings, does direct competition with surrounding adult plants of late successional species contribute to the decline in cover of the remaining early species. Studies of succession in a number of terrestrial and marine communities lend support to this inhibition model.