Setting the limit: cold rather than hot temperatures limit intertidal distribution of a coastal foundation species.

Long-lasting restoration success of foundation species requires understanding their responses to climate change. For species with broad distributions, lower latitudes may serve as a proxy for future warming at higher latitudes. Such space-for-time substitutions are a powerful tool for developing climate change predictions for species distributed along steep elevational gradients. To understand climate resilience of a key coastal foundation species, we examined the upper elevational limit of the native Olympia oyster (Ostrea lurida) along its entire range at 26 sites spanning 21° latitude, from British Columbia to Baja California. Counter to our expectations, high air temperatures did not affect variation in the upper limit of Olympia oysters. Indeed, Olympia oysters extended high into the intertidal zone at the warmer southern sites, and shading did not influence the upper limit. Our models indicated instead that extreme low temperatures set the upper limit for Olympia oysters at higher latitudes. In contrast, neither the Pacific oyster (Magallana gigas), a co-occurring global invader, nor barnacles exhibited clear latitudinal patterns. These findings suggest that Olympia oysters and restoration projects aimed at supporting their recovery will be resilient to increased temperatures projected by climate change models. Our results also illustrate the importance of testing the assumption that species on steep elevational gradients are living close to their upper thermal limits and will be negatively impacted by warming; for this foundation species, the assumption was false. Latitudinal studies enhance understanding of species response to climate stressors and are key to the design of climate-resilient conservation strategies.