Decline of corals across the Caribbean

Toby Gardner, Isabelle Côté, Jenny Gill, Alastair Grant and Andrew Watkinson.

Centre for Ecology, Evolution and Conservation and

Tyndall Centre for Climatic Change Research,

University of East Anglia

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Coral reefs are one of the most spectacular and productive environment on earth, but they are also one of the most vulnerable as corals are very sensitive to nutrient enrichment, overfishing, increased sedimentation and physical impacts of people and boats. Our work has documented a massive region-wide decline of corals across the entire Carribean basin, with average hard coral cover being reduced by 80% in three decades.


Photograph copyright Karen Cheney

There has been widespread concern about degradation of coral reefs for some years, and a number of studies have documented the deterioration of reefs in single study locations, or across single states. Larger scale assessments of the state of coral reefs have been based on a qualitative or semi-quantitative synthesis of these individual studies.

We have collated data on hard coral cover at a total of 263 sites from 65 separate studies and carried out a meta-analysis to examine rates of decline across the whole Carribean basin. The average cover of hard corals has declined from about 50% to 10% over the past 30 years. This 80% decline over such a short period represents a greater rate of loss than any of the estimates of the rates of clearance of tropical rainforest, and is unprecedented in the past few millenia. The sharpest declines in coral cover occurred during the 1980's. Since then rates of loss have declined, but there is no evidence for significant recovery, with the possible exception of Jamaica.

The change from a healthy coral reef:
To a reef dominated by dead coral or algae is dramatic.

Photos copyright William F. Precht

The decline is probably the result of a combination of a number of processes, rather than a single Carribean-wide cause. Corals require very clean seawater, that is low in suspended solids and nutrients. Increased run-off of sediment from land can smother the delicate structures that the corals use to feed and reduce the intensity of light reaching the symbiotic argae that live within the coral's body. Increased nutrient concentrations in coastal waters, as a result of discharges of sewage effluents and agricultural fertilisers, can increase the growth of fleshy seaweeds, which are then able to outcompete the slower growing corals. This is exacerbated by overfishing, which removes the herbivorous fish that keep the faster growing algae in check while allowing the corals to continue growing. A mass mortality of the sea urchin, Diadema antillarum during the 1980s has also contributed towards this shift towards algal domination. Other contributors are white band disease of the coral Acropora and physical disturbance from hurricanes.

These results have been published in the journal Science:

Gardner, T.A., Côté, I. M., Gill, J.A., Grant, A. and Watkinson, A.R., 2003. Long-term region-wide declines in Caribbean corals. Science 301:958- (published electronically in Science Express, 17th July 2003). Abstract

Supplementary material supporting the Science paper, including a full list of study sites is online at: http://www.sciencemag.org/cgi/content/full/1086050/DC1 and a summary by Science of this and three other papers on coral death in the same issue is online here.

A follow up paper, examining the contribution of hurricanes to this decline, is scheduled to appear in the journal Ecology. Please contact us for more details of this.

Toby A. Gardner, Isabelle M. Côté, Jennifer A. Gill, Alastair Grant, and Andrew R. Watkinson,
2004. Hurricanes and Caribbean coral reefs: Immediate impacts, recovery trajectories and contribution to long-term coral decline. Ecology in press.

The work was carried out while Toby Gardner was working in Isabelle Côté's Tropical Marine Ecology Group, which is part of the Centre for Ecology, Evolution and Conservation at University of East Anglia. It was also supported by the Tyndall Centre - a UK national centre for the study of the impacts of climatic change, which has its headquarters at the University of East Anglia.

We are very grateful to a great number of coral researchers across the Carribean, who made available their unpublished survey data.

The following web sites give more detailed information on coral reefs:

http://www.coralreef.noaa.gov/
http://www.uvi.edu/coral.reefer/


If you are interested in undergraduate study in ecology, biology, environmental sciences or related fields, please look at the web pages describing the University of East Anglia's degree programmes in Biology, Ecology and Environmental Sciences. We also offer an MSc programme in Applied Ecology and conservation and PhD programmes in all aspects of Ecology, Evolutionary Biology and Conservation. For more information on research opportunities,please contact individual faculty in the research area that is of interest to you or email our science postgraduate office scipg@uea.ac.uk for an information pack.