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For many species, distributions are accounted for not so much by average temperatures as by occasional extremes, especially occasional lethal temperatures that preclude its existence. For instance, injury by frost is probably the single most important factor limiting plant distribution. To take one example: the saguaro cactus (Carnegiea gigantea) is liable to be killed when temperatures remain below freezing for 36 h, but if there is a daily thaw it is under no threat. In Arizona, the northern and eastern edges of its distribution correspond to a line joining places where on occasional days it fails to thaw. Thus, the saguaro is absent where there are occasionally lethal conditions – an individual need only be killed once.

you only die once

Similarly, there is scarcely any crop that is grown on a large commercial scale in the climatic conditions of its wild ancestors, and it is well known that crop failures are often caused by extreme events, especially frosts and drought. For instance, the climatic limit to the geographic range for the production of coffee (Coffea arabica and C. robusta) is defined by the 13°C isotherm for the coldest month of the year. Much of the world’s crop is produced in the highland microclimates of the São Paulo and Paraná districts of Brazil. Here, the average minimum temperature is 20°C, but occasionally cold winds and just a few hours of temperature close to freezing are sufficient to kill or severely damage the trees (and influence world coffee prices).

global variation in thermal tolerances

Species at higher latitudes experience greater seasonal temperature variation and are expected to be able to withstand greater temperature extremes. This general pattern has been reported for ectotherms (Figure 2.20a) (Sunday et al., 2011) but does it also apply to endotherms, which maintain a high and constant temperature and are thus decoupled to a degree from the influence of ambient conditions? The climate variability hypothesis has been tested in a meta‐analysis of physiological studies that allowed the thermoneutral zones (see Section 2.3.3) to be estimated for hundreds of bird and mammal species (Khaliq et al., 2014). Figure 2.20b and c plots the thermoneutral zones estimated for individual bird and mammal species in relation to both latitude and climatic variability (the latter estimated as the annual range between the monthly average of daily maximum temperatures of the warmest month and minimum temperatures of the coldest month). Note how temperature variability is generally lower in tropical regions. The climate variability hypothesis was supported in the case of birds but not mammals, a difference that may be related to their different lifestyles, with mammals often able to create their own preferred microclimates in burrows and dens.

Figure 2.20 Geographic variation in thermal tolerances. (a) Terrestrial ectotherms, including arthropods, reptiles and amphibians, as estimated by the range between their upper and lower lethal or critical temperature limits, and (b) birds and (c) mammals as illustrated by the breadths of their thermoneutral zones, plotted in relation to latitude and, in the case of (b) and (c) climatic variability (grey bars) at the capture sites of the individuals used in published physiological experiments. Major bird and mammal orders are highlighted by different colours. Climatic variability is the annual range between maximum and minimum temperatures.

Source: (a) After Sunday et al. (2011). (b, c) After Khaliq et al. (2014).