Abstract

A modest recovery of the black long-spined sea urchin, Diadema antillarum, has been reported for a few Caribbean loca-tions. 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 ma-nipulative 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 den-sities (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 den-sities, but not rugosity. On the other hand, no effects of the factors considered in this experiment (i.e. rugosity and densi-ties) 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.

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