Demographics and Population Dynamics Project the Future of Hard Coral Assemblages in Little Cayman

Abstract

Individual hard coral colonies from four representative reef sites around Little Cayman were surveyed yearly between 2010 and 2015, a period of non-disturbance between two elevated seawater temperature anomalies. Photographic censuses produced 7069 annual transitions that were used to describe the demographics (size class frequencies, abundance, area cover) and population dynamics under non-disturbance environmental conditions. Agariciids, Porites asteroides, and Siderastrea radians have replaced acroporids as the predominant massive corals. Recruitment rates were generally low (<1 colony per m 2 ), except for a fourfold recruitment pulse of S. radians that occurred in 2011. On average, 42% of coral recruits survived their first year but only 10% lived longer than four years. Temporal comparisons allowed correction factors to be calculated for in-situ methods that overestimate recruitment of colonies ≤2 cm in diameter and overlook larger colonies. Size class transitions included growth (~33%), stasis (~33%), partial mortality (10% -33%), and whole colony mortality, which decreased with increasing colony size (typically <10% for colonies with surface areas >30 cm 2 ). Transition matrices indicated that Little Cayman assemblages have declining hard coral populations (λ < 1) but as stable size class distributions progress toward higher proportions of colonies with >150 cm 2 surface areas, live area cover may remain relatively stable. Projection models indicated that downward population trends would be exacerbated even by mild disturbance (5% -10% mortality) scenarios. The fate of hard corals on Little Cayman's reefs was determined to be heavily dependent on the health and transitions of agariciid colonies. Conservation strategies that currently focus on restoration of Caribbean acroporids should be expanded to include agariciids, which were previously considered "weeds".