EXPERIMENTAL ANALYSIS OF PROCESSES UNDERLYING THE STRUCTURE OF A ROCKY INTERTIDAL COMMUNITY IN THE NORTHERN GULF OF CALIFORNIA (BARNACLES, PREDATION, COMPETITION, MUTUALISM).

Abstract

Field experiments and observations were used to define the functional roles and relationships of species in the rocky mid-intertidal community at Pelican Point in the northern Gulf of California. This approach provided resolution of the influences of grazing, predation, competition, and abiotic factors on community structure, and of the direct and indirect effects involved in species interactions. Competition for space, apparently via space pre-emption, between the barnacle Chthamalus anisopoma and encrusting algae of the genus Ralfsia was consistently evident. Grazing by the limpet Collisella strongiana was shown to limit algal abundance and bring about the replacement of Ralfsia by Chthamalus. Chthamalus also excluded Collisella from the rock surface. Predation on Chthamalus by the thaidid gastropod Acanthina angelica increased the abundances of both Ralfsia and Collisella. Some of the temporal variations in community structure observed during this study were clearly linked to variations in the abundances of Acanthina and Collisella and the above interactions. This relatively simple community, existing in what seems a rigorous physical environment, was characterized by strong, highly interdependent biological interactions. Indirect effects were consistently important in species interactions. An intriguing result of this study was the emergence of indirect mutualism between Acanthina and Collisella; this interaction appears to contribute to the persistence and continuing influences of both consumer species, and hence may be of major importance in the organization of this community. The zonation of the barnacles Chthamalus anisopoma and Tetraclita stalactifera at Pelican Point and elsewhere in the Gulf was examined in a test of recent ideas relating ecological and evolutionary patterns in barnacles to morphology and competition for space. Experiments and observations indicated the restriction of Tetraclita to the upper part of the shore by competition from Chthamalus, with Tetraclita able to survive above Chthamalus by virtue of greater tolerance to exposure. These results were in direct opposition to the presumed competitive dominance of large, rapidly-growing, tubiferous barnacles like Tetraclita. Comparisons with results from other shores suggested that numerical dominance goes hand-in-hand with competitive dominance in acorn barnacles. Morphological differences appear to be of minor importance.