Diadema antillarum grazing effects on algal/benthic cover and diversity in La Parguera Natural Reserve
A modest recovery of the black long-spined sea urchin, Diadema antillarum, has been reported for a few Caribbean locations. D. antillarum has been proposed to be a keystone herbivore that exerts important controls on benthic assemblages. Most of the evidence supporting this keystone model, however, has been inferred from observational studies; whereas manipulative studies that test hypotheses derived from this keystone model are scarse. Consequently, a manipulative experiment was conducted to test the effect of D. antillarum densities and rugosity (substrate heterogeneity) on the algal/benthic cover (as a proxy of grazing rate) and diversity. The field experiment consisted of fencing coral heads where different densities (1, 5 and 10 individuals per m2) of D. antillarum were placed per experimental unit. Inside these experimental units, three, 10cmx10cm quadrats were permanently and randomly placed to estimate the algal/benthic cover and diversity for six months. Preliminary, non-quantitative observations have shown a decrease in algal cover associated with D. antillarum densities, but not rugosity. On the other hand, no effects of the factors considered in this experiment (i.e. rugosity and densities) have been detected on coral recruitment. It is expected that after the conclusion of this experiment, we will be able to estimate the optimal density of D. antillarum needed on a coral reef area to create positive feedbacks in the south-west part of Puerto Rico. With the information produced, D. antillarum restoration could be constituted as a plausible biocontrol mechanism to reduce algal cover and potentially increase coral recruitment substrate.