Loss of Caribbean corals
The loss of live coral is an acknowledged symptom of coral reef degradation. The first large-scale measurement of coral loss was made for the Caribbean basin. In this region, there has been an 80% decline in coral cover since the mid 1970s. There is a debate regarding the ultimate cause of coral loss which centers mainly on the roles played by overexploitation of herbivorous fish and direct coral mortality. Although low coral cover was once thought to be an alternative and stable ecosystem state, there is now evidence that reefs can return to a coral-dominated condition.
Pattern of loss
The earliest measurements of Caribbean coral cover taken by scientists date from the middle of the 20th century. A compilation of records since then shows that Caribbean coral cover has declined since 1977, from an average across the region of approximately 50% to only 10% by 2001 (Figure 1). This represents an 80% decline in 25 years, which ranks among the fastest rates of loss for any ecosystem in the world. The low coral cover has been maintained since 2001, and there is no evidence of further losses at the regional scale.
A large portion of the regional decline in coral cover can be attributed to the near-disappearance of two coral species, Acropora palmata and Acropora cervicornis (Figure 2). These large, branching corals were formerly extremely abundant on shallow-water reefs, but outbreaks of white-band disease, beginning in the late 1970s, have significantly reduced populations of these corals across the region.
No one knows what Caribbean coral reefs looked like centuries ago, and hence whether periods of low coral cover have naturally occurred. However, it is possible to get a glimpse of the past by examining of cores taken from ancient reefs. While such paleoecological reconstructions do not reveal the extent of coral cover, they do show which species were present. Cores taken from reefs in Belize, Jamaica and Panama suggest that Caribbean coral assemblages have been dominated by acroporid corals for centuries, if not millennia. The present state of Caribbean coral reefs therefore appears to be a novel condition.
Causes of coral loss
There are many factors implicated in the loss of corals in the Caribbean. Some, such as pollution and sedimentation arising from deforestation and coastal development, occur at local scales (Table 1). Others, such as disease outbreaks and thermally induced coral bleaching, can affect large parts of the region at once.
|Table 1 – Natural and anthropogenic causes of coral loss in the Caribbean and their geographic scale of action. Causes in parentheses may be worsened by anthropogenic activity. |
| Natural|| Anthropogenic |
|Local scale ||Storms|
|Regional scale || (Disease)|
|Coral bleaching |
There is currently an unresolved debate surrounding the ultimate cause of coral loss. Three main ideas have been proposed. First, coral loss is the result of overfishing. Centuries ago, abundant herbivorous fish kept macroalgae on Caribbean coral reefs in check. As these fish were depleted by fishing, their ecological role was assumed by sea urchins. The mass mortality of the abundant herbivorous sea urchin Diadema antillarum in the early 1980s left few herbivores on Caribbean reefs, releasing algae to overgrow and kill corals. Second, coral loss is due to pollution. Nutrient enrichment from development and agricultural run-off has favored the growth of algae, and caused the subsequent mortality of corals. The fact that coral loss has occurred on remote reefs, far away from human influences, suggests that this cause may not be a general one. Finally, coral loss is due to factors that cause direct coral mortality. These factors include diseases, coral bleaching and storms, all of which have increased in incidence in the past four decades. Although the debate has been polarized into these three distinct positions, it is likely that coral loss at any given location has been caused by one or more of these factors
Alternative stable states?
The ubiquity of reefs with low coral cover is generating concern that two alternative stable states may exist for these ecosystems: one dominated by coral and the other dominated by algae. This possibility is worrisome because it is very difficult to make a system shift from one stable state to another. Thus, reefs with low coral cover could remain in that condition in perpetuity. Fortunately, there is now evidence, from various parts of the Caribbean, that reefs that experienced extensive coral loss in the 1980s are now recovering. The recovery of corals on many of these reefs has been associated with the resurgence of Diadema antillarum populations. There is therefore some hope that Caribbean coral reefs can return to their former coral-rich state.
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