The sea urchin Diadema antillarum – keystone herbivore or redundant species?

Two main hypotheses have been developed for describing the role and importance of the sea urchin Diadema antillarum on reefs of the Caribbean region in both space and time. The first and most crucial is whether D. antillarum was the most important “keystone” grazer on Caribbean reefs? The alternate to this hypothesis is that D. antillarum was merely a redundant species, playing a secondary role to herbivorious fishes which only came into dominance after herbivorous fishes had been removed by man through exploitive fishing practices. To resolve this debate, we pose the question: “What is the role of Diadema antillarum in altering the composition of the benthic community on Caribbean coral reefs in time and space?” The interpretation presented herein implies that D. antillarum was and is the most important shallow-water herbivore on Caribbean coral reefs. This is contrary to a prevailing model but in substantial agreement with the historical importance of the species. If correct, the continued and future recovery of this sea urchin may be the key ingredient to facilitating coral reef recovery throughout the region. In addition, on coral reefs where the natural recovery of Diadema has been delayed or is absent (e.g. the Florida Reef Tract); measures to actively restore these urchins may be the most efficient and cost-effective tool for conserving and enhancing the ecological function of reefs. This debate also emphasizes the requirement for implementing ecologically-based, not ideologically-based management of natural systems.