California Sea Grant College Program

This thesis concerns several aspects of the structure and dynamics of a rocky intertidal platform community in southern California. This community is roughly divisible into an algae-poor, middle/upper intertidal portion and an algae rich, low intertidal portion comprising a platform reef. The community as a whole is characterized by a high degree of spatial and temporal patchiness. In the middle to upper intertidal, the friability of the sandstone bedrock has a strong effect on the outcome of competition for space between barnacles and molluscan grazers (limpets), and also modifies the influence of physical disturbances for other sessile invertebrates. The result is a greater abundance of sessile invertebrates on well consolidated substrata, compared to more friable sandstone.

Throughout the intertidal, episodic winter storms result in dramatic variation in the recruitment rates and abundances of some intertidal invertebrates. The tube building polychaete Phragmatopoma calijornica appears to respond to these intense storms by a greater reproductive output, analogous to the responses of some plants to fires in terrestrial fire-disturbed communities.

On the low intertidal platform reefs, the algal assemblage is grazed predominantly by a guild of herbivorous fishes dominated by the Kyphosidae, and to a lesser degree by other herbivores, including the shore crab, Pachygrapsus crassipes. Two kyphosid fishes, Girel/a nigricans and Hermosilla azurea feed mainly on several species of algae, including sheet-like, filamentous, and branched forms. Pachygrapsus crassipes also feeds on these algae, but exhibited even greater preference for jointed calcareous algae. Fish exclusion experiments indicated that the grazing activities of these fishes modify the structure of the algal assemblage on the reef by preferentially grazing preferred algae, mainly sheet-like and filamentous species.

The results of direct biotic interactions between the chiton, Nutta//ina kata and foliose coralline algae lead to one of two alternate community types in the low to middle intertidal. The indirect consequences of this interaction include the acilitation of the recruitment and persistence of sessile invertebrates and grazers in patches dominated by chitons. High grazing rates by N. kata indirectly decrease the competitive effects of upright algae on several invertebrates. Low grazing rates indirectly favor epiphytic algae. The outcome of this strong chiton-algal interaction thereby results in the development of one of two alternate stable states which also have positive feedback mechanisms acting to increase the rate of succession and resistance stability of each state.

Overall, the community is viewed as landscape of spatially varying intensities of physical and biological stress for community species. This environmental heterogeneity leads to a patchy distribution of biota and to spatial and temporal shifts in the dominance of species within the community.