Diadema and its relationship to coral spat mortality: Grazing, competition, and biological disturbance

Abstract

The benthic grazer Diadema antillarum Philippi (Echinoidea) has been demonstrated experimentally to contribute to the control of coral community structure in shallow water. In Discovery Bay, Jamaica, West Indies, Diadema densities were manipulated over a range of 0-64/m2 with the aid of enclosures. Grazing by Diadema under primary and post-primary succession conditions were compared.Algal percent-cover decreased as Diadema density was increased. Despite the presence of high algal cover. highest coral recruitment and diversity occurred at lowest Diadema densities, with planular settlement occurring predominantly in openly exposed micro-habitats. However, since algal growth rates greatly exceeded those of corals, space was rapidly monopolized by the former, resulting in intense competition and high coral mortality. This was particularly evident in Agaricia and Porites spp. At high Diadema densities, coral recruitment was greatly depressed in at least a genus-specific manner by intense levels of biological disturbance resulting from the echinoid's abrasive grazing activities. Favia Fragum (Esper) was especially susceptible to this perturbation. The surviving coral spat were found generally in cryptic, protected areas. Here they suffered some competitive losses to other sessile epifauna and -flora, particularly coralline algae, polychaetes and forams, which were well adapted to these physical and biological conditions. Increased sedimentation also depressed coral recruitment, replacing grazing as a limiting factor for successful settlement.Optimal conditions for coral survival, competitive success, and possibly growth were found at intermediate densities due to a balance between competition for space and biological disturbance. Diadema antillarum plays an important role in controlling the distribution and abundance of coral spat in the shallow reef community.