On the Coral, Fish, and Diadema antillarum Love Triangle: A Case Study of Two Contrasting Caribbean Reef Systems

Abstract

Anthropogenic and natural factors have adversely impacted Caribbean reefs in recent decades, resulting in coral bleaching, phase-shifts, and general large-scale reef degradation. In the years following the 1983 Diadema antillarum mass mortality that eradicated 93-99% of all long-spined sea urchin populations in the Caribbean, Diadema populations have failed to make a resurgence, and overharvested fish stocks have remained largely depleted, resulting in massive region-wide macroalgal blooms. In light of this current crisis, a better understanding of the relative impacts of fish assemblages and echinoid densities in promoting Caribbean reef health is crucial in facilitating coral conservation and recovery efforts. Using field data collected from two contrasting reef systems in Honduras, Banco Capiro and Utila island, I examined the effects of Diadema antillarum densities and fish assemblages on coral cover. At Banco Capiro, I found that uncharacteristically high Diadema densities likely accounted for higher coral cover levels than those found in Utila. In Utila, I instead found a significant correlation between fish functional diversity and coral cover. To explain the higher Diadema densities in Banco Capiro, I propose that stochastic occurrences determined which sites reached critical Diadema densities to overcome Allee effects, and that positive feedback loops reinforced coral and Diadema densities once these levels had been reached. Evidence of ecological succession along with strong correlations between echinoid density and coral recruitment support this hypothesis and emphasize the importance of Diadema antillarum in facilitating coral growth. However, while Banco Capiro has more coral cover, it also accommodates significantly less coral diversity, fish species diversity, and fish functional diversity compared to Utila, leading to the conclusion that coral cover alone is an incomplete metric of reef health, an important finding considering the widespread use of coral cover as a direct proxy for reef health in the past. Finally, I suggest that many of these diversity disparities may be due to inherent differences in our sampled reef systems, and not necessarily due to higher Diadema densities, and that robust populations of herbivorous fish, Diadema antillarum, and coral polyps are necessary to maximize reef resilience in the Caribbean, a feat that can only be accomplished through possible Diadema transplantations, decreased fishing pressure, and the creation of additional Marine Protected Areas.