ACROPORA PALMATA, KEYSTONE SPECIES OR JUST ANOTHER WEED?

Dramatic reef decline over the past three decades has prompted debate on the roles of human stress versus natural cyclicity over a timeframe longer than our anthropocentric scale of measurement. At the center of this has been the disease-related collapse of shallow-water acroporids on Caribbean reefs. Jeremy Jackson first commented on the long-term stability of reef communities throughout the Quaternary, despite wide shifts in climate and sea level. Statements about the “unprecedented” nature of recent events have followed with increasing frequency and fervor. Central to this presumption are the following: 1) we can resolve events in the fossil record at a temporal scale similar to recent monitoring, and 2) spatial persistence in the fossil record reflects temporal continuity. A simple calculation of uninterrupted carbonate production by a stand of Acropora shows that what is left in the record is far short of what should be there. Thus, the emphasis changes from “are there gaps?” to “how do those gaps relate to long-term community stasis under natural conditions - and recent declines in acroporids?”

A. palmata has been the dominant shallow-water Caribbean species throughout the Quaternary. Yet, the spatially persistent branching-coral record in the Holocene shows clear temporal gaps, some of which are coincident across the region. One started roughly 6300 years ago and was accompanied by regional backstepping. A second started 3000 years ago and is marked solely by a 600-yr shift to massive-coral dominance. From a scientific perspective, if we start with the sound observation that a Caribbean reef crest absent branching acroporids has been a temporal aberration, then the focus shifts to the metabolic, reproductive and genetic characteristics of the species, how these relate to its periodic decline, and how this plays into longer-term community dynamics. From a management perspective, the recognition of past gaps may be largely moot. The recent role of anthropogenic stressors is well documented. Whatever the pattern of species recovery may have been in the past, this “reassembly” occurred in the absence of the numerous anthropogenic stresses faced by reefs today. Effective management strategies will depend on good answers to the above scientific questions – many of those answers remain hidden in the fossil record.