The hidden dynamics of low coral cover communities

Abstract

The coral reef crisis has resulted in many reefs stabilizing in a low coral cover state, and this condition is exemplified by the fringing reefs of St. John, US Virgin Islands, where coral cover has remained low for decades. To evaluate the demographic features maintaining this condition, a decade of coral abundance was examined in three domains: recruits, small colonies (≤ 40-mm diameter), and cover. While cover was stable from 2007 to 2016, other aspects of coral community structure changed in taxonomically unique ways differing among domains. Most changes occurred gradually, although the warm year of 2010 was associated with perturbations in growth, mortality, and density that differed among domains and taxa, and summed to a weak signal distinguishing communities before and after ~ 2011/2012. As a result, coral community structure was taxonomically shuffled within each domain, while low cover was maintained. These low coral cover communities may be characterized by functional redundancy among taxa and a weakening of interactions among them that traditionally have supported emergent properties like community calcification. With persistent failure of recruitment to augment coral cover, some corals may already occur at such low abundances that they cannot perform the ecological services with which they were once associated.