TEMPERATURE TOLERANCES, GEOGRAPHIC RANGE DYNAMICS, AND EXTINCTION: COMPLEX, MULTILEVEL RESPONSES TO CLIMATE CHANGE

An underappreciated fact about the temperature structure of the oceans is that some sea-surface temperatures are far more common than others. Species and clades have evolved accordingly, so that many widespread bivalve species today evidently track the most widespread temperatures rather than tolerate a broad range of temperatures. However, such a strategy is risky over the long term, because evolutionary responses to changing ocean climates and circulation patterns are evidently slow or constrained, and cross-level effects – species vs. clade – can be complex. Widespread but narrow-temperature species are extinction-resistant to local or regional environmental perturbations, and can modulate geographic ranges in response to steepening or shallowing of thermal gradients, but will have no advantage against radical, global perturbations to oceanic temperature structure, as at the K-Pg boundary. This effect may explain the shift in the hierarchical level of extinction selectivity seen there: geographic range at both the species and clade level are effective buffers against extinction during “normal” times (>95% of geologic time), but only clade-level geographic range is effective for marine bivalves during the end-Cretaceous --clades can be widespread even if each species is narrow-ranging. As expected from this pattern, regional extinctions in temperate faunas (mostly via emigration) are significantly correlated with Pliocene-Recent temperature drops (using the PRISM model for Pliocene climate, e.g. Dowsett et al. 2012, Nature Climate Change). However, those extinctions do not match the expected net drop in regional diversity given present-day diversity-temperature relationships, owing to regional variation in immigration and origination. Biotic responses to impending climate change are difficult to predict. Species’ thermal maxima may be hard to shift upward evolutionarily, implying extinction in the face of warming, but the PETM, the sharpest warming (and acidification) event of the past 65 Myr, had little lasting effect on bivalve diversity or biogeography. More important for the immediate future may be the collision between temperature-driven spatial shifts and anthropogenic disruption of suitable migration targets and corridors by pollution, over-exploitation, and eutrophication.