Echinoid grazing as a structuring force in coral communities: Whole reef manipulations

To examine the effects of echinoid grazing on shallow water coral community structure, the entire Diadema antillarum Philippi population (> 3000 individuals) was eliminated from a patch reef in Discovery Bay, Jamaica W.I. in January 1974. All remaining regular echinoids, notably including Echinometra viridis A. Agassiz (> 7000 individuals), were removed from one-half of the same reef 6 months later.Where Echinometra was present, its grazing was highly patchy, even at densities of up to 50/m². On the other hand, the effects of grazing by Diadema were more uniformly distributed at equivalent or lower densities.Percent-cover of adult corals was enhanced in the presence of Echinometra viridis, however, it was greatly reduced in the absence of all echinoid grazing as a result of massive overgrowth by algae. Success of coral recruitment (density of coral spat) increased in the absence of Diadema due to the alleviation of biological disturbance imposed by this urchin's feeding activities. The presence of Echinometra viridis, even at the high densities naturally present on the reef, did not deter coral settlement.Coral diversity was highest in the presence of all echinoids, intermediate in the presence of E. viridis (without Diadema), and lowest in the absence of all echinoids. This was due to the disproportionately successful recruitment and resultant high relative abundance of Agaricia at low echinoid densities. Favia, Millepora, Helioseris and Acropora all exhibited universally low recruitment levels. Predation by Diadema and Echinometra is responsible for damaging larger coral colonies, particularly Porites.As echinoid species composition and density was altered, the species composition of benthic algae and associated invertebrates shifted. In this way, the array of species competing for space with coral spat shifted as did their competitive relationships. Major epifaunal competitors included spirorbid polychaetes, various foraminifera (particularly Gypsina), and sponges. Algal competitors included filamentous chlorophytes, corallines (e.g. Jania), and numerous other red algae; the species composition of algal competitors shifted through time. Competitive success in corals varied as echinoid density changed and was species-specific with respect to coral genus. Interspecific competition between coral spat (i.e., interspecific aggression via extracoelenteric digestion) was negligible under all treatments.The presence of Echinometra viridis in high densities (in the absence of Diadema) created conditions where growth and fusion in Agaricia spat were optimized. Under these conditions, fusion allows a young coral colony to attain a large enough size to survive damage incurred from grazing or from competitive overgrowth, decreasing the probability of whole-colony mortality during the early stages of development.