VARIED CARNIVORE EFFECTS AND THE PREVALENCE OF INTERTIDAL ALGAL TURFS

A red algal turf covers mid-shore levels of much of the California Channel Islands. Our experiments at Santa Catalina Island show that carnivores maintain the turf by consuming juvenile mussels Mytilus spp. and associated invertebrates. Exclusion of spiny lobsters Panulirus interruptus (Randall) from comparatively wave-exposed sites caused the complete and persistent replacement of the turf by the mussel assemblage. On sites protected from wave action, the lobsters were joined by carnivorous fishes, i.e., Halichoeres semisinctus (Ayres), Oxyjulis californica (Gunther) and Semicossyphus pulcher (Ayres), and whelks, i.e., Ceratostoma nuttalli (Conrad) and Maxwellia gemma (Sowerby). Compensatory foraging shifts by the whelks prevented significant increases of mussels in lobster and fish exclosures. Exclusion of all predators at the protected site caused a moderate but statistically significant increase. Thus, experiments at different sites indicated either ''keystone'' or ''diffuse'' effects of predators. Experimental clumps of large Mytilus californianus (Conrad) persisted longer after re-exposure to predation in the post-experimental periods than did clumps of smaller M. californianus or M. edulis L. Transplants of matched cohorts indicated that the size differences of mussels in the exclosure experiments were caused by differences in shell growth rates correlated with wave exposure. We speculate that changing growth rates may influence the outcome of size-limited predation, accounting for some of the variation in the relative abundance of turf and mussels over the wave exposure gradient.