Portfolio Effects and Synchrony in Coral Communities.

Theoretical and empirical work has demonstrated that population asynchrony can stabilize total community fluctuations through the portfolio effect. Stability and portfolio effects can arise as a statistical property of random fluctuations among populations in high diversity systems, or from predictable differences in biological responses to environmental fluctuations. Using a 9-year coral abundance time series from 15 sites at a remote oceanic atoll, we determine if coral communities exhibit synchronous or asynchronous population dynamics, and whether these dynamics are driven by consistent patterns of covariation among populations. From 2013 to 2017 total community abundance was stable and associated with population asynchrony, driven by random fluctuations among populations, rather than predictable patterns of covariation among taxonomic groups. During the second phase of the study (2018 to 2021) community abundance declined dramatically and systematically, likely driven by a coral predator outbreak, and was associated with population synchrony. Coral populations were spatially and temporally variable in both phases of the study, and there was little support for patterns of either positive or negative covariation among taxonomic pairs as predicted by putative life-history strategies. While the coral community demonstrated an inherent capacity for stability consistent with portfolio effects, this capacity was limited by the intensity and frequency of the disturbance regime.