Recovery of sea urchin Diadema antillarum populations is correlated to increased coral and reduced macroalgal cover

We surveyed the benthic community structure and population density of the long-spined sea urchin Diadema antillarum on the shallow fore-reefs of the Gandoca-Manzanillo Wildlife Refuge, Caribbean Costa Rica, in September and October 2004. In zones with high densities of D. antillarum (> 0.6 ind. m(-2)), the cover of non-calcareous macroalgae, known coral competitors, was low and that of live coral was high, whereas the opposite occurred in zones with low densities of D. antillarum (< 0.1 ind. m(-2)). D. antillarum density was not related to the coverage of calcareous macroalgae, which are not viewed as coral competitors. Mean density of D. antillarum was 0.2 ind. m(-2) and the total area covered by live coral was 14 %. D, antillarum density and area covered by live coral were 2 and 7 times larger, respectively, than those reported 4 yr earlier for the study site. Within the same period, the proportion contributed by non-calcareous macroalgae to total algal cover declined from similar to 79 to 48%. Results indicate that various families of scleractinian corals in the Caribbean coast of Costa Rica have increased in abundance, that non-calcareous macroalgae have declined, and that recovering D. antillarum densities are correlated with these observations.