Food Availability, Sea Urchin Grazing, and Kelp Forest Community Structure

Abstract

Unlike most previously reported kelp forest, which exhibit long—term persistence over large spatial scales, the kelp forest community on the west end of San Nicolas Island, California, occurs as a dynamic patchwork of barren areas characterized by grazing sea urchins and an algal assemblage consisting of upright and encrusting coralline algae, and kelp—dominated areas characterized by high densities of perennial brown algae, including the giant kelp Macrocysits pyrifera. In this study we examine the factor that regulate the grazing of sea urchins (Strongylocentroutus franciscanus), which in turn determines the structure of this community. In the barren area, drift algae sparse, sea urchins were poorly nourished, occupied open, unprotected microhabitats, and actively grazed the substratum. In the kelp—dominated are, drift algae were abundant, sea urchins were well nourished, moved little, occupied cracks and crevices, and probably fed on the drift algae. Early in the study substantial recruitment of brown macroalgae occurred in both sites, and the barren area gradually transformed into a kelp—dominated area. Concomitant with this change, the abundance of drift algae in the barren area increased. Urchins in this area abandoned open microhabitats for protected crevices and pockets, and sea urchin grazing intensity was reduced to levels characteristic of the kelp—dominated area. From these observations we present a qualitative model in which the transformation from one configuration to another is triggered by a behavioral switch in the mode of feeding of red sea urchins. This switch is controlled by the availability of drift algae, which in turn is dependent upon the abundance of attached macroalgae. The adult algal standing stocks appear to be regulated by prevailing hydrographic conditions, which are either favorable or unfavorable for kelp recruitment, survivorship, and growth. In this model grazing intensity is independent of urchin density and therefore provides a new view of how kelp forest communities function.