The largest-scale investigation of temperature tolerances undertaken to date has been that of Addo-Bediako et al. (2000), who compiled data on both upper and lower lethal limits to test the climatic variability hypothesis underlying Rapo-port's Rule (or the increase in species ranges with increasing latitude—see Gaston et al. 1998). They found pronounced differences in the extent of variability in these limits. Latitudinal variation in upper lethal limits, though significant (a range of about 30°C), is much less pronounced than spatial variation in LLTs (a range of about 60°C) (Fig. 5.23). In addition, the extent of the variation at a given latitude differs considerably. While variation in upper lethal limits appears to be roughly similar across the globe, variation in lower lethal limits is more pronounced at high latitudes. This variation with latitude might be due mostly to a latitudinal increase in the variety of situations either promoting or reducing the risks of low-temperature injury encountered by insects. For example, at those latitudes where there is a significant accumulation of snow, species overwintering in subnival habitats are likely to encounter less extreme temperatures than those in more exposed habitats. Addo-Bediako et al. (2000) also found pronounced hemispheric asymmetry in LLTs with Southern Hemisphere species showing reduced cold hardiness, compared with their Northern Hemisphere counterparts, and this may well be due to hemispheric differences in absolute minimum temperatures recorded in continental areas (with the exception of continental Antarctica) (Fig. 5.24).
An investigation of CTmax and CTmin in dung beetles across a 2400-m altitudinal gradient in South Africa provided evidence for similar patterns at a regional scale (Gaston and Chown 1999a). Here, CTmax shows considerably less variation with altitude than does CTmin, a pattern similar to that found in the global study. Thus, thermal tolerance range increases with an increase in latitude or altitude, and in both cases this change is associated with an increase in climatic variability with latitude/altitude. Thus, at both spatial scales, interspecific variation in upper limits is much less marked than that in lower limits.
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