Destructive grazing by sea urchins Strongylocentrotus spp. in a central California kelp forest: potential roles of recruitment, depth, and predation

In January 1986, a sea-urchm mediated deforestation began in C a m e l Bay, California. USA, near the center of the California sea otter's range. The deforestation occurred on an offshore rocky reef where depths range from l ? to > 30 m. Both purple (Strongylocentrotus purpuratus) and red (S. franciscanus) sea urchins were abundant and probably belonged to a cohort that settled in 1984. By October 1986, nearly all non-crustose algae had been removed, as were most sessile invertebrates (excluding the cnidarians Balanophyllia elegans and Corynactis californica). Concurrent samples taken from 2 nearby forested sites at comparable depths showed no evidence of overgrazing by urchins. Populations of red sea urchins at the deforested site remained fairly stable, but purple sea urchins declined to zero by July 1989. Algal spores were available during the deforestation, but recruitment of seaweeds d d not occur until after sea urchln abundance had decllned Reasons for the decline of the purple sea urchin population are not clear, although predation by sea otters 1s unlikely. Circumstances surrounding these events indicate kelp forest deforestations may be controlled not only by mortality of adult sea urchins (from predation or disease) and fluctuation of their food supply, but also by factors affecting recruitment such a s larval abundance and early post-settlement sunrival.