Increases in salinity variability harm both oysters and their predators, offsetting predicted effects on population dynamics

Projecting the effects of future climate conditions for predator-prey systems can be challenging, because species' environmental tolerances can differ and both environmental reaction norms and predator functional responses are nonlinear. We addressed this issue in the context of Eastern oysters (Crassostrea virginica) in the Gulf of Mexico, USA. Oysters are an ecologically and economically important estuarine species threatened by a variety of stressors including prolonged exposure to extremely low or high salinity. In this region, a major oyster predator, the southern oyster drill (Stramonita haemostoma) thrives at high salinity and is impaired by low salinity. As estuarine salinity becomes more variable (e.g. more regional droughts leading to prolonged high-salinity conditions), one might expect drill predation to become more intense, as when high salinity led to a predator-driven oyster population collapse in Apalachicola Bay, FL in 2012. To test that expectation, we simulated the dynamics of a two-species integral projection model with salinity-dependent demography for both species, including size-structured and salinity-dependent predator feeding behaviour, based on laboratory experiments. We forced the model with simulated salinity time series that matched the climatology and autocorrelation structure of historical salinity in Apalachicola Bay, but with increased standard deviation, reflecting the range of increased variability in regional precipitation over future decades predicted by global climate models. Surprisingly, the model predicted that the expected range of increased salinity variability had little effect on oyster abundance or the probability of quasi-extinction. A sensitivity analysis revealed that this was because the negative effects of salinity variability on oysters were mostly balanced out by the inhibition of drill predation by low-salinity anomalies. Additionally, the negative effects of increasing salinity variability could be counteracted by increasing drill mortality (such as by manual culling). This analysis illustrates the importance of accounting for environmentally dependent species interactions when forecasting climate-driven changes to population dynamics.