Change and stasis in marine hybrid zones in response to climate warming

Aim We test the prediction that hybrid zones between warm- and cold-adapted species will move towards the territory formerly occupied by the cold-adapted species in response to a warming climate. We use multiple tests of this prediction to distinguish amongst potential mechanistic hypotheses of responses to climate change. Location We sampled 97 locations on the Atlantic coast of Spain and France and the English Channel that span three hybrid zones formed between two species of marine mussels (Mytilus galloprovincialis and M. edulis). Methods Mussels were sampled in 2005-07 and analysed at a nuclear gene (Glu5') that is diagnostically differentiated between the subject species. Results were compared to those of studies made in the same region over the past two decades. Historical change in sea surface temperature (SST) was analysed using National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation Daily SST. Species distribution models (random forest and maximum entropy) of the current distribution of mussels were constructed and validated by hindcasting the historical distributions of these species. Validated models were used in combination with forecasts of SST to predict changes in mussel distribution to 2050 and 2100. Results We show that over the past two decades two of the hybrid zones in France have not changed in either position or shape. The third hybrid zone, however, has shifted in the predicted direction, c. 100 km eastward into the warming English Channel. Species distribution modelling strongly implicates changes in winter cold SST as driving this change in the position of one of the hybrid zones. Forecasts of future SST indicate that rapid changes in distribution will occur over the next century. Main conclusions Hybrid zones can be used to conduct repeated tests of ecological responses to climate change and can be valuable in sorting among prospective mechanistic hypotheses that underlie that change. Winter temperatures, but not seasonal high temperature, appear to control the distribution of both species. Species distribution modelling indicates that the collapse of these hybrid zones is imminent, with the rapid expansion of the subtropical species in response to continuing SST warming.