The Threatened Atlantic Elkhorn Coral, Acropora palmata: Population Dynamics and Their Policy Implications.

Fossil data from multiple locations indicates that Atlantic elkhorn coral, Acropora palmata, formed shallow reefs throughout the Caribbean Sea since the Pleistocene. Beginning in the 1980s A. palmata has declined to a small fraction of its formerly vast extent throughout the region. In 2006, elkhorn coral was the first coral, along with its sister species, staghorn coral (Acropora cervicornis), to be included on the U.S. Endangered Species List. We used size-based matrix modeling to parameterize annual A. palmata population dynamics in Florida, over the course of one severe hurricane year (2005) and six calm years (2004, and 2006-2010), incorporating environmental stochasticity as inter-annual variability. We predicted that benthic cover would remain at current levels (4%) for the foreseeable future (until 2030) and beyond (until 2100), suggesting a lack of resilience following the 2005 hurricanes. Standard metrics for the quantification of number and size of individuals are essential to endangered species management. These usually straightforward tasks can be challenging for clonal, colonial organisms. Acropora palmata presents a particular challenge due to its plastic morphology and frequent fission. We quantifiedthree-dimensional colony surface area (CSA), the most ecologically relevant measure of size, for 14 prototypically arborescent A. palmata colonies using three-dimensional digital imaging software. To relate CSA to simple field metrics, we compared loglikelihood values and determined that planar projection was the best predictor. The, tight, linear relationship between planar projection and CSA enables ecological rates, such as reef accretion and gamete production, to be calculated from field data. Finally, we expanded the matrix population model to compare population dynamics in several locations across the Caribbean. The general trend for Acropora palmata is further reductions in population size by 2030. The most striking difference we quantified was between Jamaica, where population size is projected to increase, and all other locations, where population size is projected to remain stable or decline. Density of a key herbivore, the sea urchin Diadema antillarum, was an order of magnitude greater in Jamaica than in any other location. These increases are occurring 30 years after a devastating die-off suggesting that herbivory by urchins may facilitate A. palmata recovery