Effects of thermal stress on key processes driving coral-population dynamics

A primary goal of ecology is to understand population dynamics by quantifying key processes that regulate population densities through time. Change to any one process may alter the population trajectory, its genetic constitution, and its size-frequency distribution. This study sought to quantify which of the 5 processes-(1) recruitment, (2) post-settlement mortality, (3) growth, (4) partial mortality and (5) total colony mortality-most influenced corymbose Acropora coral-population dynamics on reefs in southern Japan. Here we tracked coral reef colonies at a windward site and a leeward site from 1996 to 2001 through a thermal anomaly (+1.8 degrees C in 1998). We were particularly interested in examining which of those processes had the largest effect on population dynamics. To predict size-frequency distributions, we constructed a system of nonlinear differential equations that included those 5 processes. Recruitment at the windward site remained consistent through time, whereas the leeward site had more variable recruitment rates at all depths, and showed a considerable decrease after the 1998 thermal anomaly that remained suppressed for many years. At all depths, the proportion of colonies transitioning to a larger size class was higher during the thermal anomaly than in other years. Partial mortality was generally highest on 20 to 30 cm length colonies, but was not a good indicator of thermal stress. Total colony mortality was most evident for the smallest and largest corymbose Acropora colonies during the thermal anomaly, effectively narrowing the size-frequency distributions. Sensitivity analyses revealed that total colony mortality had the largest effect on coral-population growth rate (lambda), followed by post-settlement mortality and recruitment. Quantifying the population processes that lead to state changes is a prerequisite for understanding reef dynamics in a rapidly warming ocean.