Stasis, biological disturbance, and community structure of a Holocene coral reef

Disturbances have drastically altered Caribbean coral reefs over the past two decades. Acropora cervicornis (staghorn coral), which predominated at intermediate depths (5-25 m) from the 1950s through the 1970s, has virtually disappeared from most reef environments. Other coral species have declined as well, and the cover of macroalgae has increased. In apparent contrast, fossil reef sequences suggest that the species composition and zonation of coral assemblages did not change during the Pleistocene and Holocene. One interpretation of these observations is that coral species persisted on Caribbean reefs for hundreds of thousands of years as components of tightly integrated communities, and that a rare or unique combination of disturbances led to the synchronous decline of A. cervicornis and other corals throughout the region. The hypotheses of (1) community integration and (2) a unique, recent community transition, were tested by ecological and paleoecological observations in the shelf lagoon of the Belizean Barrier Reef. The reef growing along the flanks of Channel Cay, a lagoonal shoal, was monitored by point counts along transects over a ten-year period (1986-95). This reef was covered primarily by A. cervicornis at 3-15 m depth until the late 1980s. After 1986, A. cervicornis experienced a mass mortality from White Band Disease, an epizootic of presumed bacterial origin. The cover of A. cervicornis dropped from ∼70% in 1986 to nearly 0% in 1993. Agaricia spp. (lettuce corals) responded opportunistically to the availability of free space in the form of A. cervicornis skeletal rubble. Agaricia, which had been a minor constituent of the sessile biota (10% cover in 1986), replaced A. cervicornis as the most common occupant of space on the reef (56% cover in 1995). The percent cover of other coral species and macroalgae remained low throughout the ten-year period. Similar changes were observed on other reefs over an area of at least 250 km2. The Acropora-to-Agaricia transition left a clear signature in the sedimentary record. Trenches dug into the reef at Channel Cay revealed the accretion of a layer of Agaricia rubble with a mean thickness of 22 cm in the decade after 1986. Due to the unconsolidated, uncompacted nature of the reef sediments, evidence of previous Acropora-to-Agaricia transitions should have been visible in the fossil record as vertical accumulations of A. cervicornis branches interrupted by layers of imbricated Agaricia rubble. Coring studies at Channel Cay revealed that no other Agaricia layers were deposited during at least the past 3800 years; the recent transition was unique on a time scale of millennia. This result supports the contention that excursions from the Acropora-dominated situation are unusual in the history of Channel Cay and nearby reefs. However, the dynamics of the transition do not support the community integration hypothesis for the Channel Cay reef, indicating instead that different coral taxa in this assemblage responded differently, or not at all, to a large-scale biotic disturbance. The community transition also underscores the potential for biological factors in general, and disease in particular, to alter the composition of ecological communities and their sedimentary remains.