The ecological evolution of reefs

Many groups of extinct and extant organisms have aggregated to form reefs for over 3.5 billion yr (Ga). Most of these communities, however, grew under ecological and environmental controls profoundly different from those that govern modern coral reefs. Not only has the global distribution of reefs varied considerably through geological time-determined largely by sea level, and latitudinal temperature/saturation gradients-but more importantly the trophic demands of reef-building organisms have changed, as has the degree of biological disturbance faced by sessile biota in shallow marine environments. Reefs differentiated into open surface and cryptic communities as soon as open frameworks developed in the Proterozoic, some 1.9 million yr ago (mya) and diverse and complex ecosystems were established by the early Cambrian (similar to 520 mya). Calcified heterotrophs were conspicuous in reefs during the Paleozoic and early Mesozoic, but considerable rigidity was imparted to these often otherwise fragile communities both by indirect microbial processes that induced the formation of carbonate and by rapid early cementation. While photosymbiosis was probably acquired by scleractinian corals early in their history (similar to 210 mya), this does not appear to have immediately conferred a superior reef-building ability. Large, modular corals and coralline algae showed notable powers of regeneration after partial mortality but poor ability to compete with macroalgae for limited substrate in the absence of intense herbivory; they did not rise to prominence in reef communities until the early to mid-Cenozoic. This may be related to the appearance at this time of major predator groups such as echinoids, limpets, and particularly fish that are capable of rapid algal denudation and excavation. The existence of this reciprocal relationship is corroborated by the observation that branching corals, which appear to flourish because and not in spite of breakage, show a particularly dramatic increase in diversity coincident with the increase in predation pressure from the late Mesozoic onwards.