Abstract
Climate change is altering phenology and distributions of many species and further changes are projected. Can species physiologically adapt to climate warming? We analyse thermal tolerances of a large number of terrestrial ectotherm (n = 697), endotherm (n = 227) and plant (n = 1816) species worldwide, and show that tolerance to heat is largely conserved across lineages, while tolerance to cold varies between and within species. This pattern, previously documented for ectotherms, is apparent for this group and for endotherms and plants, challenging the longstanding view that physiological tolerances of species change continuously across climatic gradients. An alternative view is proposed in which the thermal component of climatic niches would overlap across species more than expected. We argue that hard physiological boundaries exist that constrain evolution of tolerances of terrestrial organisms to high temperatures. In contrast, evolution of tolerances to cold should be more frequent. One consequence of conservatism of upper thermal tolerances is that estimated niches for cold-adapted species will tend to underestimate their upper thermal limits, thereby potentially inflating assessments of risk from climate change. In contrast, species whose climatic preferences are close to their upper thermal limits will unlikely evolve physiological tolerances to increased heat, thereby being predictably more affected by warming.
Highlights
There is a general expectation that climate, both at present and historically, governs the broadest outlines of species distributions
If tolerances to high temperatures are highly conserved across clades, natural selection might not be expected to be acting as strongly on this trait, resulting in a weak relationship. We examined these expectations by plotting physiological metrics of critical tolerance (CTmax and critical minimum temperatures (CTmin) for ectotherms and upper critical temperatures (UCT) and lower critical temperatures (LCT) for endotherms) against metrics of ambient temperature averaged across species ranges
Similar patterns were found among individuals of the same species for a small number of South American lizard species, suggesting that selection could act at intraspecific level for lower thermal tolerances in cold environments, while reduced opportunities might exist for selection of upper tolerances under warm conditions
Summary
There is a general expectation that climate, both at present and historically, governs the broadest outlines of species distributions. Investigations of differential variation in traits underpinning thermal biology are starting to provide important insights into the extent to which fundamental niches might evolve, and what the consequences thereof are for species distribution modelling and for predicting species responses to climate change, especially in environments as different as those found in marine and terrestrial systems (e.g. Sunday et al 2012).
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