Physical effects of habitat-forming species override latitudinal trends in temperature.

Latitudinal and elevational temperature gradients (LTG and ETG) play central roles in biogeographical theory, underpinning predictions of large-scale patterns in organismal thermal stress, species' ranges and distributional responses to climate change. Yet an enormous fraction of Earth's taxa live exclusively in habitats where foundation species modify temperatures. We examine little-explored implications of this widespread trend using a classic model system for understanding heat stresses - rocky intertidal shores. Through integrated field measurements and laboratory trials, we demonstrate that thermal buffering by centimetre-thick mussel and seaweed beds eliminates differences in stress-inducing high temperatures and associated mortality risk that would otherwise arise over 14° of latitude and ~ 1 m of shore elevation. These results reveal the extent to which physical effects of habitat-formers can overwhelm broad-scale thermal trends, suggesting a need to re-evaluate climate change predictions for many species. Notably, inhabitant populations may exhibit deceptive resilience to warming until refuge-forming taxa become imperiled.