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isotherms

There are very many examples of plant and animal distributions that are strikingly correlated with some aspect of environmental temperature (e.g. Figure 2.2a) and this kind of pattern may still hold even at gross taxonomic and systematic levels (Figure 2.18). At a finer scale, the distributions of many species closely match maps of some aspect of temperature. For example, the northern cool range boundary of wild madder plants (Rubia peregrina) is closely correlated with the position of the January 4.5°C isotherm (an isotherm is a line on a map joining places that experience the same temperature).

Figure 2.18 The treelines (high‐altitude limits of forest cover) of the world’s mountains seem to follow a common isotherm. This is 6.7 ± 0.8ºC), with very similar mean ground temperatures during the growing season across a wide range of latitudes from subarctic through equatorial areas to temperate southern hemisphere regions (the growing period differs according to latitude). The species composition of the forests is, of course, vastly different in the different regions.

Source: From Körner & Paulsen (2004).

However, such relationships need to be interpreted with some caution: they can be extremely valuable in predicting where we might and might not find a particular species (e.g. Figure 2.5); they may suggest that some feature related to temperature is important in the life of the organisms; but they do not prove that temperature causes the limits to a species’ distribution. For one thing, the temperatures measured for constructing isotherms for a map are only rarely those that the organisms experience. In nature an organism may choose to lie in the sun or hide in the shade and, even in a single day, may experience a baking midday sun and a freezing night. Moreover, temperature varies from place to place on a far finer scale than will usually concern a geographer, but it is the conditions in these ‘microclimates’ that will be crucial in determining what is habitable for a particular species. For example, the prostrate shrub Dryas octopetala is restricted to altitudes exceeding 650 m in north Wales, UK, where it is close to its southern limit. But to the north, in Sutherland in Scotland, where it is generally colder, it is found right down to sea level.

On the other hand, Payne et al. (2016) were able to demonstrate a strong correlation between the warm boundary isotherms of nine well‐studied fish species and their optimum temperatures for activity, somatic growth and reproductive growth (Figure 2.19): this is good evidence of a causal link.

Figure 2.19 Warm boundary limits of nine Australian fish species are correlated with species‐specific optimum fish performance. Optimum temperature (T_opt) is shown for maximum activity, somatic growth or reproductive growth (gonadosomatic index, GSI, a measure of gonad mass relative to total body mass) measured in the wild (four species provided both activity and reproductive growth data, giving 13 points in total). The species‐specific warm equatorward range boundary is the average temperature of the warmest month at the range limit.

Source: From Payne et al. (2016).