Emergent zonation and geographic convergence of coral reefs

Environmental degradation is reducing the variability of living assemblages at multiple spatial scales, but there is no a priori reason to expect biotic homogenization to occur uniformly across scales. This paper explores the scale-dependent effects of recent perturbations on the biotic variability of lagoonal reefs in Panama and Belize. We used new, and previously published core data to compare temporal patterns of species dominance between depth zones and between geographic locations. After millennia of monotypic dominance, depth zonation emerged for different reasons in the two reef systems, increasing the between-habitat component of beta 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. Twenty-four reef cores were extracted at water depths above 2 in in Bahia Almirante, a coastal lagoon in northwestern Panama. The cores showed that finger corals of the genus Porites dominated for the last 2000-3000 yr. Porites remained dominant as the shallowest portions of the reefs grew to within 0.25 in of present sea level. At intermediate depths of 5-10 m, however, declining water quality in the Bahia enabled the lettuce coral Agaricia tenuifolia to replace Porites during the last several decades. In Belize, the staghorn coral Acropora cervicornis dominated the rhomboid shoals of the central shelf lagoon for millennia. As the shallowest portions of the rhomboid shoals approached sea level around 500 years ago, Porites spp. replaced Ac. cervicornis in an intrinsically driven successional sequence. At intermediate depths, white-band disease killed Ac. cervicornis in the late 1980s, precipitating a transition to dominance by Ag. tenuifolia. Phase shifts between coral taxa and the emergence of coral zonation in both locations were mediated by the sea urchin Echinometra viridis, which controlled algal growth. Opposite trends in biotic variability at the habitat and geographic levels highlight the complex, scale-deperident nature of the response of coral reefs to intense perturbations.