MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser Vol. 411: 73–87, 2010 doi: 10.3354/meps08640 Published July 29 INTRODUCTION Coral-population dynamics are influenced by popu- lation processes that act on individual colonies at all life history stages from differential recruitment and survival (Birkeland 1977, Vermeij & Sandin 2008) through to senescence (Rinkevich & Loya 1986, Bak & Meesters 1999). Still, we know little about population processes and how they vary spatially, seasonally and under different environmental conditions. We under- stand even less about differential weightings and sen- sitivities of populations to slight process changes and the repercussions of those changes on subsequent pop- ulation trajectories. In a time of rapid climate change (IPCC 2007), it is pertinent to quantify population pro- cesses to determine which of them are most sensitive to environmental shifts and examine those processes within the context of population vulnerability to phase shifts (Scheffer & Carpenter 2003, van Nes & Scheffer 2007, Guttal & Jayaprakash 2008). This study investigated 5 coral-population processes: (1) recruitment (newly settled juveniles), (2) postsettle- ment mortality (death of newly settled recruits), (3) colony growth (transition into a larger size class), © Inter-Research 2010 · www.int-res.com *Email: lynnette.roth@gmail.com Effects of thermal stress on key processes driving coral-population dynamics Lynnette Roth 1, * , Semen Koksal 2 , Robert van Woesik 1 1 Department of Biological Sciences, and 2 Department of Mathematical Sciences; Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, USA ABSTRACT: A primary goal of ecology is to understand population dynamics by quantifying key pro- cesses that regulate population densities through time. Change to any one process may alter the pop- ulation 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) par- tial 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°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 consider- able 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 small- est 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 (λ), 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. KEY WORDS: Sensitivity · Size-frequency distribution · Modeling · Bleaching · Temperature Resale or republication not permitted without written consent of the publisher