By George Stoyle

Caribbean coral reefs have experienced fundamental changes in structure, function and integrity over the past 30 years resulting in substantial loss of coral organisms and significant declines in ecosystem resilience. In ecology resilience is defined as an ecosystem's ability to recover from disturbance. Over millions of years coral reefs have adapted to occasional natural disturbances, such as hurricanes, tsunamis and rises in sea surface temperature. Indeed disturbance, at an intermediate level, acts to promote biodiversity through the processes of reorganisation and regeneration. Problems begin to occur when there is either too little or too much disturbance. Too little allows competitively superior species to dominate; too much allows only species tolerant to the disturbance to survive. Both scenarios lead to overall reductions in biodiversity which, in turn, leads to declines in ecosystem resilience. Once resilience has been lost the ecosystem has the potential to 'shift' into an alternate state whereby previously dominant species are replaced by another.

The majority of reefs throughout the Caribbean are now dominated by numerous species of algae. These algae not only occupy crucial settlement space for new corals, but also resist consumption by herbivores through physical and chemical defences. The processes that underpin this shift in dominance, also known as a macroalgal phaseshift, are poorly understood but are often associated with reductions in herbivores, primarily parrotfish and urchins which continually graze algae preventing widespread overgrowth; increased nutrient concentrations from sources such as untreated sewage and agricultural run-off; as well as an increased prevalence of coral bleaching events and disease epidemics. The major drivers behind such changes are almost certainly the combined impacts of both human and natural disturbances. This combination has resulted in more than 80% loss of live coral cover in the Caribbean over the last 30 years. Ever increasing human pressures, such as overfishing, coastal development and pollution, have reduced the resilience of many reefs to such an extent that the effects of random natural disturbances such as hurricanes can, and have been, catastrophic. For example, in 1998, the effects of Hurricane Mitch resulted in 50-70% loss of coral cover in many parts of Honduras and the effects still persist to this day. The availability of new substrate through direct physical damage, combined with reductions in parrotfish and urchin populations through overfishing and disease, enabled rapid colonisation of algae preventing coral settlement and growth. Despite localised recovery in some areas, there are few signs of wide-scale restoration.

With the predicted increase in frequency and intensity of tropical storms and hurricanes in the Caribbean region, coupled with persistent and escalating human pressures, there is an increasing requirement for research into minimising some of the long-term impacts associated with these disturbances. Recovery of parrotfish, which provide the crucial ecological function as consumers of algae, can potentially aid recolonisation of corals and hence assist the possible reversal of macroalgal phase-shifts. The coral reefs of the Mesoamerican Reef, as in many developing nations, provide vital ecological and economic functions through both direct and indirect goods and services providing food, recreation and tourism, coastal protection as well as aesthetic and cultural values. Understanding the nature and complexity of coral reefs is a major challenge for ecologists worldwide, and conservation of keystone species, such as parrotfish, is an essential step in this process. Effective management can enhance coral reef conservation and maintain the vital goods and services provided to society.