Stability of Reef-Coral Assemblages in the Quaternary

Abstract

At small spatial and temporal scales reefs are non-equilibrial, dynamic, disturbance-dominated ecosystems. At larger scales, however, the community structure of coral reefs appears stable; coral assemblages from the same environments exhibit striking similarities in species composition and dominance on time scales of decades to hundreds of thousands years. Thus, community membership over time is commonly more stable and persistent than that expected by chance alone. In both the Caribbean and Indo-Pacific, patterns of zonation observed on modern reefs are faithfully recorded in fossil reef sequences preserved through successive Pleistocene high-stands of sea level. Many paleoecologists, however, view the changes in both sea level and sea-surface temperature (SST) recorded during Pleistocene glaciations as major disturbance events requiring community reassembly de novo after each event. Why did reef communities respond in a repetitive fashion to the frequent and supposedly large environmental fluctuations of the Ice Ages? Two major hypotheses have been developed to explain the observed stability of reef assemblages through the Quaternary. The first invokes interspecific interaction or interdependence as an emergent property, which stabilizes community composition for long periods even in the face of environmental change. The second recognizes that the persistence of communities includes or implies persistently stable environments and faunal tracking of environments even when conditions vary. The null model for persistence-stability is that similar community types should recur whenever and wherever similar environmental conditions exist, so long as the same general species pool is available for recruitment. Analyzing reef facies preserved within a sequence-stratigraphic framework allows us to test the null model on Quaternary reefs. We propose here that reassembly was unnecessary, because reef communities were able to track even the most rapid changes in sea level, producing recurrent biofacies largely through asexual and sexual recruitment from local populations. Analysis of climate change and accompanying tropical SSTs associated with glacial cycles shows they were not sufficient to cause coral populations or the coral reefs they build to disappear and then to reorganize anew. We are, therefore, unable to reject the null model. Incremental faunal tracking of suitable habitats through time and in regional space is the likely mechanism conferring persistence-stability in these coral assemblages.