Coral reefs in the face of ecological threats of the 21st century

Abstract

To date, more than a quarter of tropical coral reefs of the World Ocean are believed to be totally destroyed. Given the present rate of reef degradation, this value may double in the nearest 30 years. For a significant part of the coastal population of the planet, the destruction of coral ecosystems implies the loss of a major food resource, natural protection from storms, and significant (if not the only) revenues from exploitation of reefs, especially in the tourist industry. Finally, the disappearance of populated low-laying coral islands may threaten the local communities by depriving them of living space. Global negative effects include temperature anomalies of the sea surface waters and an increase in CO2 concentration in the atmosphere, which leads to acidification of the World Ocean. The local negative effects include an increase in sedimentation and eutrophication, cyclone and storm passages, coral diseases, chemical pollutions, mechanical destruction of corals by humans, and anthropogenic depletion of functional groups of fish and invertebrates. The entire range of responses of coral ecosystems to these stressful factors is reviewed both at the level of separate taxa and at the level of the ecosystem in general. An analysis of literature data suggests with high probability that, in the middle of this century, more than 50% of tropical coral communities will suffer a collapse stage, especially in areas with a high human population density. According to the most optimistic scenario, the integrated effect of the reviewed negative factors will result in the survival of coral ecosystems in some areas. However, after global transformations, these ecosystems will be dominated by the most resistant taxa, mainly massive and encrusting growth forms of long-lived species with low growth rates and high competitive ability. Among such taxa, Poritidae show the highest adaptive capability. According to the most pessimistic scenario, scleractinian communities will be replaced by alternative communities of macroalgae or noncalcareous anthozoans everywhere.