Thermal tolerance and the global redistribution of animals

The redistribution of life on Earth has emerged as one of the most significant biological responses to anthropogenic climate warming(1-3). Despite being one of the most long-standing puzzles in ecology(4), we still have little understanding of how temperature sets geographic range boundaries(5). Here we show that marine and terrestrial ectotherms differ in the degree to which they fill their potential latitudinal ranges, as predicted from their thermal tolerance limits. Marine ectotherms more fully occupy the extent of latitudes tolerable within their thermal tolerance limits, and are consequently predicted to expand at their poleward range boundaries and contract at their equatorward boundaries with climate warming. In contrast, terrestrial ectotherms are excluded from the warmest regions of their latitudinal range; thus, the equatorward, or 'trailing' range boundaries, may not shift consistently towards the poles with climate warming. Using global observations of climate-induced range shifts, we test this prediction and show that in the ocean, shifts at both range boundaries have been equally responsive, whereas on land, equatorward range boundaries have lagged in response to climate warming. These results indicate that marine species' ranges conform more closely to their limits of thermal tolerance, and thus range shifts will be more predictable and coherent. However, on land, warmer range boundaries are not at equilibrium with heat tolerance. Understanding the relative contribution of factors other than temperature in controlling equatorward range limits is critical for predicting distribution changes, with implications for population and community viability.