Vardi, T.

Population dynamics of the threatened Atlantic Elkhorn Coral

2011

University of Caliornia, San Diego

Ph.D. Dissertation

"Fossil data from multiple locations indicates that Atlantic Elkhorn Coral, Acropora palmata, formed and occupied shallow reefs throughout the Caribbean Sea since the Pleistocene. Since the 1980s A. palmata has declined to a small fraction of its formerly vast extent throughout the region. The demise has been recognized, and in 2006 it was included on the U.S. Endangered Species List. With a growth rate almost ten times as fast as other corals, up to 10 cm/year, and a relatively fragile branching architecture, its population expands primarily via fragmentation and regrowth rather than sexual recruitment. This life history strategy confers an adaptive advantage to life in the shallows, where high wave-energy can inflict frequent damage to the two cell layers that comprise coral tissue. However, during hurricanes, this strategy is put to the test, and populations must exhibit great resilience in order to persist. This challenge was a normal part of A. palmata life for at least 125,000 years. In the present ocean, however, we show that resilience in the face of storms has been compromised. In the Florida Keys, where hurricane frequency is high, we used size-based matrix modeling to parameterize annual Acropora palmata population dynamics over the course of one severe hurricane year (2005) and six calm years (2004, and 2006 to 2010). Using the most likely scenario of severe hurricane recurrence rate (20 years), and incoporating environmental stochasticity as inter-annual variability, we predicted that percent cover of benthos would remain at the currently depressed level (4%) for the foreseeable future (until 2030) and beyond (until 2100). This suggests a lack of resilience from the hurricanes effecting the Florida Keys in 2005. Standard metrics for the quantification of number and size (or life stage) of individuals are essential to endangered species management and the study of population biology. These usually straightforward tasks can be challenging for clonal, colonial organisms. A. palmata presents a particular challenge due to its plastic morphology, frequent fission, and rapid regrowth from fragments. We quantified surface area, the most ecologically relevant measure of size for a coral, for fourteen prototypically arborescent A. palmata colonies using three-dimensional digital imaging software. To relate surface area to simple field metrics, we compared loglikelihood values, and determined that planar projection was the best predictor of three-dimensional colony surface area, significantly outperforming linear field metrics (p < 0.001, R2 = 0.995). Finally we expanded the matrix population model developed for the Florida Keys, to compare population dynamics in several locations, Curacao, Jamaica, Navassa, Puerto Rico, and the Virgin Gorda (British Virgin Islands), across the Caribbean Basin. Althoug rates of change in abundance and percent cover during the 2004 to 2011 study period were equivocal, the underlying dynamics are not. The general trend for Acropora palmata is further reductions in population size (lambda <1) and extent by 2030 if no serious conservation action is taken. During storm intervals, population size was predicted to decrease regardless of location. This would seem like an expected result, were it not for fragmentation. Moreover, this is in contrast to an expansion in population size during medium intensity storms, as predicted by an A. palmata population model parameterized during the early 1990s. This could signal a continual, gradual decline in resilience. 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. Further, Jamaica is the only location where both abundance and percent cover increased during the study period. 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 occuring thirty years after a devastating hurricane. They suggest that herbivory by urchins may facilitate Acropora palmata recovery, and provide hope that resilience has not yet been completely eliminated in Acropora palmata population dynamics."

CRCP Project ID 20114; Project Title: Quantitative Population Modeling of Acropora spp.; Principal Investigator: Jennifer Moore