THE DEMOGRAPHICS OF A 15-YEAR DECLINE IN COVER OF THE CARIBBEAN REEF CORAL MONTASTRAEA ANNULARIS

Abstract

On Caribbean reefs, a striking trend of the last 25 years has been the decline in cover of the framework-building coral Montastraea annularis, a species that has dominated reefs throughout the region for millennia. Clearly, such losses are important ecologically, but to evaluate their significance fully, they need to be placed in the context of the proximal causes and balanced against the potential for gains in cover through growth and recruitment. In this study, a population of M. annularis in St. John, U.S. Virgin Islands, was censused annually from 1988 to 2003 to quantify coral cover and construct a size-based demographic model. The model was developed to explore the mechanisms of change in coral cover and to ascertain likely trajectories for future population growth. Over the study period, the cover of M. annularis declined from 41% in 1988, to 12% by 1999 (a 72% decline) but remained unchanged statistically for the last five years of the study. Between 1988 and 2003, colony abundances declined by 57% (from 47 colonies/m2 to 20 colonies/m2), and the losses were driven mostly by the death and fission of medium to large colonies (i.e., ≥151 cm2). By 2003, the population had proportionally more small colonies (70% were ≤50 cm2) and fewer large colonies (3% were >250 cm2) than in 1988 (60% and 6%, respectively), and the changes in population structure had accelerated ≥14% in terms of the rate of change in population size and the time necessary to attain equilibrium of colony size structure. Importantly, this analysis revealed an ongoing and imminent population decline coincident with the recent period of apparently stable coral cover. Fifty-year projections indicate the strong likelihood of extirpation of M. annularis at this particular site in St. John (in contrast to a continuation of constant low cover) and suggest that the 1988 population structure cannot be restored by recruitment. It is unlikely that the population decline will reverse until there is an amelioration of the conditions that kill individual colonies.