Biotic homogenization of coral assemblages along the Florida reef tract

We tested the hypothesis that coral assemblages along the Florida reef tract have recently become more biologically homogeneous than they were in the past. We used a database that consisted of a probabilistic, 2 stage, stratified-random survey design to assess the condition of stony corals every summer from 2005 to 2010. At each of the 1176 sites in 9 putative subregions, examined over the 6 yr period, all coral colonies >4 cm were identified to species and their diameters were measured within replicated 10 m2 belt transects. This approach provided detailed spatial information on the species composition and size-frequency distributions of coral assemblages. Coral assemblages showed a nested relationship, decreasing in diversity and abundance north of 25° 40’ N. A comparison with previous studies suggested that major declines in recent decades in the primary reef-building corals Acropora palmata, Acropora cervicornis, and Montastraea spp. have homogenized assemblages across depths and reef zones. Of the 9 putative subregions, 6 were found to be redundant on the basis of coral composition. Florida’s reefs currently support a simpler coral assemblage than they did in the past, dominated by a small number of eurytopic, generalist species. The assemblages may be more stable now than during the previous several decades, but there has been a fundamental change in their composition and function. Through loss of the dominant reef-building coral species, the reefs of the Florida reef tract have lost the capacity to construct reef framework.