Experimental evaluation of Diadema antillarum herbivory effects on benthic community assemblages

Diadema antillarum is a keystone herbivore that controls algal cover on coral reefs, as reductions in this species has been associated with significant increases in algal cover. However, most of the evidence that supports this model comes from observational studies where processes have been inferred from descriptions of temporal and spatial relationships. Within this context, a manipulative experiment was conducted to test the effects of D. antillarum densities and rugosity on benthic assemblages. The experiment consisted of fencing 1m2 experimental plots (metal fence corrals) of high and low rugosity where different densities (0, 1, 5 and 10 individuals/m2) of D. antillarum were enclosed per plot. Inside these plots, three permanent and random 100cm2 photo-quadrats were taken during eight sampling times (six months) to estimate temporal change in benthic cover. Multivariate and univariate analyses indicated that: 1) presence of D. antillarum significantly decreased turf algae (from 90% to 4%) and Dictyota (from 25% to 0%) cover and increased clean substrate (from 0% to 72%) at both sites; 2) herbivory rates for low rugosity were different from high rugosity herbivory rates; 3) densities of 1 D. antillarum individual/m2 did not change algal cover; 4) a minimum of 5 D. antillarum individuals/m2 were required to maintain low algal cover on low and high rugosity experimental units. Based on these results we suggest that D. antillarum can effectively enhance coral reef health by decreasing algal cover and creating clean substrate. For restoration efforts, optimal densities of 5 D. antillarum individuals/m2 should be considered when restocking individuals to coral reefs.