Three-Way Interactions: Barnacles, Limpets, and Algae in a Sonoran Desert Rocky Intertidal Zone

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Abstract
Field experiments and observations were used to evaluate the interactions among barnacles, limpets, and algae in a rocky intertidal community in the northern Gulf of California. Competition for space, apparently via space preemption, between the barnacle Chthamalus anisopoma and an encrusting alga of the genus Ralfsia was consistently evident. Grazing by the limpet Collisella strongiana limited algal abundance and indirectly increased the abundance of Chthamalus; no direct effect of Collisella on Chthamalus was detectable. High percent cover of Chthamalus reduced both algal and limpet abundances. Chthamalus affected Ralfsia both directly, through space preemption, and indirectly, by limiting limpet densities. In one experiment, these opposing effects were equivalent, resulting in no net effect of the removal of Chthamalus on Ralfsia. Results also indicated a negative feedback loop: removal of Chthamalus increased limpet densities, which suppressed any increase in Ralfsia, enhancing the recovery of Chthamalus. The above interactions imply a beneficial indirect effect of Ralfsia on Collisella, arising from the prevention of Chthamalus settlement by Ralfsia. Circumstantial evidence in this study and direct evidence (in Dungan 1984) suggested that predation on Chthamalus by the gastropod Acanthina angelica is also of major importance in this system. These interactions appeared to explain some aspects of temporal variation in community structure. This community was characterized by strong, highly interdependent interactions. Both direct and indirect effects of species on each other were repeatedly evident and operating on similar time scales.