Forecasting the impacts of Marine Heatwaves on habitat-forming invertebrate species

Extreme Climate Events (ECEs) challenge our ability to predict how climate change will alter ecological communities. Many ECEs, such as Marine Heatwaves (MHWs), are discrete, pulse-like events against a background of warming climatological means. MHWs can ultimately result in lethal stress and mass mortality events of marine organisms. Variability in MHW extremeness (across parameters of magnitude, duration, and frequency) makes forecasting individual organism response, and by extension population and ecosystem-level responses, difficult. We developed a mechanistic framework for predicting individual mortality risk and resulting population dynamics attributable to thermal stress experienced during MHW events. This framework uses thermal tolerance landscapes and dynamic survival models to estimate probability of death within a population during a MHW event. Focusing on habitat-forming species, and in particular the eastern Oyster (Crassostrea virginica), we demonstrate that extended, low-magnitude MHW events can be more common and as impactful as short, extreme MHW events. Our physiological approach yields substantial  improvements over statistical oceanographic models and standard  physiological thresholds of stress for predicting in situ impacts of MHW events on critical ecosystem-forming marine species. MHWs have been attributed to population and ecosystem tipping, and our approach provides one potential mechanistic lens of understanding how the extremeness of MHWs resonate across ecological scales.