For decades, marine biologists and ecologists have marveled at the Caribbean coral reefs, and have tried to understand the longevity of these apparently fragile ecosystems. The past fifteen years have seen some dramatic changes in these coral reef communities, and many of these changes have revolved in one way or another around the participation of the black sea urchin Diadema antillarum Philippi. This slow-moving grazer has played a part in some dramatic and catastrophic shifts in the coral reefs of the Caribbean, and will have to play another role if these reefs are to recover.

D. antillarum has been a victim of some major disturbances, originating from both human and climatic causes. Hurricanes and other natural disturbances have been especially relentless in the past two decades, breaking up centuries-old coral reefs that provide D. antillarum's habitat (Hughes 1989). Human pollution, coral collecting, and overfishing have severely upset the balance of coral reef ecosystems (Hughes 1994). People are simultaneously destroying the reefs and destroying the means of maintaining the reef. This continuous onslaught is bringing the Caribbean coral reefs to a point from which recovery will be difficult or impossible.

The deleterious long-term effects of overfishing became apparent during the summer of 1983 and the year that followed. Urchins that had lost their chief predators (Carpenter 1984) and competitors for food (Hay & Taylor 1985) were growing almost without control. Their population densities had reached the point where little else but urchins could be seen on the coral reefs. Suddenly, a contagious pathogen spread through D. antillarum populations throughout the Caribbean (Lessios et al. 1984a). In less than a month, entire populations of thousands of urchins were wiped out (Hughes et al. 1985). In less than a year, more than 95% of all the D. antillarum individuals in the Caribbean were killed (Carpenter 1988). Such an abrupt change from overwhelming density to near absence set the reef systems into a series of unstable fluctuations.

Most notably, the algae which were once kept in check by the herbivory of fish and urchins (Carpenter 1981) now had no control mechanism. Herbivorous fish could not proliferate quickly enough to fill the void left by the mass mortality of urchins (Carpenter 1990b). Turf algae and macroalgae began to grow densely over the surface of the coral reefs. Now these algae are choking out the living coral organisms and making it difficult or impossible for the reefs to repair and regenerate themselves (Hughes et al. 1987). Furthermore, the dominating presence of algae has drastically altered the basic patterns of nutrient and energy flow in the coral reef ecosystems. Where a balanced grazing system of benthic invertebrates and controlled algae once existed, there is now tremendous overproduction by algae, which export biomass in the form of uneaten macroalgae on a dangerously large scale (Carpenter 1990a).

This ecological disaster has occurred through the combined effects of human and nonhuman disturbances, but it seems that now only intense human intervention can bring the reefs back to a healthy, balanced state. If nothing else, people need to stop compounding the negative effects of this imbalance by their carelessness. Controls on pollution, fishing, and boating are necessary parts of any attempt to restore some semblance of the delicately balanced, thriving ecosystems that existed just a few years ago in the coral reefs of the Caribbean.

The goals of this thesis are to determine the factors that led to the mass mortality of D. antillarum, to assess the significance of this mass mortality in the Caribbean coral reefs, and to propose a strategy for human intervention that will help to restore the former glory of the reefs. The feasibility of restoring the reefs to their exact former state will be considered along with the alternative viewpoint that a new stable state needs to be achieved on the reefs. An important part of either restoration or simple stabilization is to protect natural controls that will prevent future fluctuations of this extreme degree. Even if scientists were able to restore populations of all the reefs' many organisms to former levels, that would not help the reef ecosystems unless major destabilizers are suppressed.