Extrinsic control of species replacement on a Holocene reef in Belize: the role of coral disease

Well-preserved, Holocene coral reefs provide the opportunity to discriminate between models of intrinsically driven succession and extrinsically driven species replacement, especially when paleontological patterns can be combined with ecological observations of the underlying mechanisms. Rhomboid shoals in the central shelf lagoon of the Belizean Barrier Reef experienced a recent and dramatic change in community composition. Agaricia tenuifolia replaced Acropora cervicornis as the dominant coral species at 3-15 m depth along the flanks of the reefs. We tested the hypothesis that shallowing upward caused this shift in dominance. A core extracted from 0.5 m water depth on one of the shoals, Channel Gay, revealed a shallowing-upward shift in dominance from Acropora to Porites divaricata. This successional sequence was quite different from the Acropora-to-Agaricia transition observed in four cores from 6-11 m water depth. Ecological observations showed that Agaricin became the dominant at greater than or equal to 3 m depth after Acropora populations were decimated by a regional outbreak of white-band disease. The Acropora-to-Agaricia transition was clearly a case of extrinsically driven species replacement rather than an intrinsically driven, successional, shallowing-upward sequence.