Changes in Urchin and Kelp Densities at Anacapa Island, California

The densities of three species of sea urchins and the giant kelp (Macrocystis pyrifera) were monitored at a site near Anacapa Island, California. Although the study site had previously supported a dense kelp bed, from 1981-1983 it was dominated by urchins and coralline algae, and Macrocystis was restricted to a narrow band in shallow water. After limited recruitment in 1984, Macrocystis recruited heavily throughout the study site in 1985, resulting in an extensive kelp bed; the recruitment event was not accompanied by a sharp decline in urchin densities. However, high existing urchin densities, accompanied by unusually heavy urchin recruitment in 1985, resulted in the gradual decline of the kelp bed. By 1987, the kelp bed was again restricted to a narrow band in shallow water.