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Manipulative field experiments were used to test for the existence of competitive interactions between three abundant herbivore species in a shallow back—reef zone. Densities of the territorial threespot damselfish, Eupomacentrus planifrons, and two sea urchins, Diadema antillarum and Echinometra viridis, were altered through addition and removal experiments conducted within patches of staghorn coral, Acropora cervicornis. Removals of each urchin species resulted in population increases of the other urchin. Additions of each urchin species tended to inhibit increases of the other species. Removals of damselfish resulted in increases of the other species. Removals of damselfish resulted in increases in Diadema density within 24 h, while Echinometra density increased during the 3rd d following the perturbation. Dry biomass of algae on ceramic tiles in the lower strata of damselfish removal patches was reduced by 62% as compared with algal biomass in these strata in the presence of damselfish. Wire mesh cages were utilized over a 2—mo period to test the effect of predation on competition at the herbivore level. No significant alteration of species numbers was observed in the absence of predation. Therefore, predation did not appear to affect significantly the distributions and abundances of the adult members of the community. Low predation levels on adults within this system do not appear to alter the competitive interactions between these herbivores. Interference competition by the threespot damselfish appears to mediate competition between the two echinoids, allowing coexistence of all species. The presence of damselfish may reduce destructive echinoid grazing pressure on the coral substrate.
Ann H. Williams (Sat,) studied this question.