Ecology. Michael Begon

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).

      2.4.3 Distributions and extreme conditions

      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).

      APPLICATION 2.5 Tropical species at particular risk from climate change

      Khaliq et al. (2014) found that most of the endotherm species in their dataset (Figure 2.20b, c) currently experience maximum ambient temperatures that are within their tolerance limits during most months of the year, and note that in a warming climate many should still be able to find suitable temperature conditions within their current range. However, vulnerability to higher future ambient temperatures increases from the poles towards tropical regions, even though increases of temperature predicted for temperate and polar regions exceed those in the tropics. This is because species in tropical regions tend to live closer to their upper temperature limits and even small increases in ambient temperatures may challenge their survival. Moreover, projections of declining precipitation in tropical areas worsen the prospects for tropical species, because water availability is crucial for endotherms to compensate thermal stress. These results highlight the threats from global climate change in tropical zones, which harbour the greatest amount of biodiversity worldwide.

      2.4.4 Distributions and the interaction of temperature with other factors

      Although organisms respond to each condition in their environment, the effects of conditions may be determined largely by the responses of other community members. Temperature does not act on just one species: it also acts on its competitors, prey, parasites and so on. This, as we saw in Section 2.2, was the difference between a fundamental niche (where an organism could live) and a realised niche (where it actually lives). For example, an organism will suffer if its food is another species that cannot tolerate an environmental condition. This is illustrated by the distribution of the rush moth (Coleophora alticolella) in England. The moth lays its eggs on the flowers of the rush Juncus squarrosus and the caterpillars feed on the developing seeds. Above 600 m, the moths and their caterpillars are little affected by the low temperatures, but the rush, although it grows, fails to ripen its seeds. This, in turn, limits the distribution of the moth, because caterpillars that hatch in the colder elevations will starve as a result of insufficient food (Randall, 1982).

      disease

      The effects of conditions on disease may also be important. Conditions may favour the spread of infection (winds carrying fungal spores), or favour the growth of the parasite, or weaken the defenses of the host. For example, during an epidemic of southern