Scleractinian corals as facilitators for other invertebrates on a Caribbean reef

There is increasing evidence that facilitative effects of various organisms can play important roles in community organization. However, on tropical coral reefs, where scleractinian corals have long been recognized as important foundation species creating habitat and resources that are utilized by a diversity of taxa, such relationships have rarely been studied and never within the contemporary theoretical context of facilitation. In the present study, we surveyed coral reefs on the south coast of St. John, US Virgin Islands, with the goal of quantifying the relationship between 'coral traits' (3 distinctive characteristics of scleractinian communities) and the abundance and diversity of benthic invertebrates associated with the reefs. We defined coral traits as coral diversity, percentage cover of live coral, and the topographic complexity created by coral skeletons, and statistically evaluated their roles in accounting for the abundance and diversity of conspicuous invertebrates at 25 sites. The analysis yielded contrasting results in terms of the putative facilitative roles of scleractinian corals. Coral traits were significantly and positively related to the diversity of reef-associated invertebrates, but were not related to invertebrate abundance. Topographic complexity (but not coral cover) had relatively strong explanatory ability in accounting for the variation in invertebrate diversity, although a substantial fraction of the variance in invertebrate diversity (45%) remained unexplained. While these results are correlative, they demonstrate that a statistical majority of the variation in the diversity of conspicuous invertebrates on Caribbean reefs can be explained by the role of coral skeletons in creating topographic relief with diverse morphologies, although processes independent of coral traits also play important roles. In an era of globally declining coral cover, these findings suggest that the progressive loss of coral skeletons from tropical reefs will lead to substantial losses of invertebrate diversity that might initially be obscured by conserved abundances.