Coral reefs already are impacted by climate change and will be even more so in future. Among the most serious threats are positive thermal excursions that, coupled with UV stress, can lead to coral bleaching and death. Climate change models suggest that such mortality events may become annual occurrences across the tropics anytime between 2050-2100. In the Persian Gulf, the world’s hottest sea with coral reef development, the temperature regime is equivalent to predictions for the tropical ocean in 2100, and the recurrence frequency of mass coral mortality is among the highest anywhere. From 2009-2012, four subsequent annual coral mortality events provided a glimpse at coral population dynamics in a future, warmer, ocean. Changes over a 20-year period are documented, punctuated by the novel multi-year events. In contrast, the Chagos are one of the world’s least-impacted coral ecosystems but also here, due to increasing disturbance frequency and severity, dynamics reminiscent of the Gulf is beginning to show.
Results/Conclusions
This talk uses stage-based models to 1) quantify observed changes in coral population dynamics in relation to disturbance frequency 2) simulate future coral trajectories under variable disturbance and connectivity assumptions. The structure of data and models is explored and explained. Increasing disturbance-frequencies primarily skew size-distributions towards the smaller classes, which opens space for potential competitors (corals, algae or other organisms) and reduces fecundity exponentially. Also, the local species pool often suffers and patterns of dominance change. Shift in size classes, community composition and competition can rapidly lead to changes in stability patterns with the emergence of alternate states by acting alone or in concert. Depending on the steepness of thresholds and determining mechanisms, alternate states can be reached via bifurcations and be relatively stable, or gradually and be reversible. Models tend to be most sensitive to shifts in survivability, disturbance frequency, and levels of connectivity, where strong connectivity has the potential to mitigate more-frequent disturbance and lower survival. Some potential pitfalls in the construction and interpretation of such models are discussed. Models are used to interpret examples of coral dynamics from all oceans.