GLOBAL CHANGE AND BIOTIC HOMOGENIZATION OF CORAL REEFS

Caribbean reefs have been ecologically volatile over the last 30 yr. Two of the dominant, framework-building corals, Acropora palmata (elkhorn coral) and Ac. cervicornis (staghorn coral), were killed en masse by a regional outbreak of white-band disease (WBD) beginning on the late 1970s and extending though the early 1990s. The apparently contrasting stability of coral assemblages on a scale of tens to hundreds of thousands of years in the fossil record is either the result of time-averaging (damping of past decadal-scale volatility), or it represents a long record of persistence (decadal-scale persistence summing to long-term stability). Cores extracted from lagoonal reefs in Belize showed that Ac. cervicornis dominated continuously for at least the last 3-4 kyr. Beginning in the late 1980s, WBD virtually eliminated Ac. cervicornis, and Agaricia tenuifolia (lettuce coral) increased opportunistically to become the dominant coral species at intermediate depths. Likewise, Ag. tenuifolia recently became dominant at intermediate depths in a coastal lagoon on the Caribbean side of Panama. As in Belize, the shift in Panama was unprecedented on a millennial scale; however, its cause (altered patterns of land use) and the identity of the previous dominant (the finger coral Porites furcata), were different. After millennia of monotypic dominance, depth zonation emerged for different reasons in the two reef systems, increasing the between-habitat component of diversity in both taxonomic and functional terms. The increase in between-habitat diversity caused a decline in geographic-scale variability as the two systems converged on a single, historically novel pattern of depth zonation. Opposite trends in biotic variability at the habitat and geographic levels highlight the complex, unprecedented, and scale-dependent nature of the response of coral reefs to intense perturbations over the last several decades.