Chapter 7 and Chown and Klok 2003«), and this change is not independent of the metabolic and developmental responses of individuals to their environments (Chown and Gaston 1999).
At the interspecific level, there have been many investigations of MCA (Chown and Gaston 1999), of which the most comprehensive latitudinal study was undertaken by Addo-Bediako et al. (2002). This investigation was based on a large compilation of insect metabolic rates, and the effects of body mass, experimental temperature, wing status (flying species tend to have higher metabolic rates than flightless ones—Reinhold 1999), and respirometry method were taken into account statistically prior to the assessment of the effects of environmental temperature (mean annual temperature). A weak, but significant effect of environmental temperature on metabolic rate was found such that species from higher latitudes tend to have higher metabolic rates than those from lower latitudes, as predicted by MCA (Fig. 1.5). Although Addo-Bediako et al. (2002) highlighted several potential problems with their analysis (such as the use of mean annual temperature to characterize an insect's thermal environment), they argued that their conclusions are robust, reflecting not only MCA in insects, but also the fact that metabolic rate variation is heritable, given that much variation in it is partitioned above the species level.
This broad-scale investigation also included an analysis of the slope of rate-temperature (R-T) curves across the Northern and Southern Hemispheres. Although data are limited for the latter, it appears that the slope of the R-T curve increases towards colder regions in the Northern Hemisphere, but remains unchanged in the south. Thus, reduced sensitivity of metabolic rate to temperature is likely to be characteristic of southern, cold-climate species, where climates are likely to be permanently cool and often cloudy, and opportunities for effective behavioural thermoregulation limited. By contrast, in northern, cold-climate species, where hot, sunny periods may be more frequent (Danks 1999), thus facilitating behavioural thermoregulation, greater R-T sensitivity would be expected. Hence, the metabolic benefits of R-T sensitivity that would accrue to species experiencing regularly sunny conditions in the Northern Hemisphere, are likely to be outweighed by the costs associated with a permanently depressed metabolism under cloudy conditions in the Southern Hemisphere. Such hemisphere-associated asymmetry in responses is characteristic of other physiological variables (Chapters 5 and 7) and may well contribute to large-scale differences in the biodiversity of the south and the north.
For those organisms that are able to occupy stressful environments, reductions in metabolic rate (for an organism of a particular size at a particular temperature compared to similar species or populations) are thought to be important adaptations to these conditions (Hoffmann and Parsons 1991). In the case of food and water stress a reduction in metabolic rate should mean a reduction both in ATP use and, in insects which exchange gases predominantly by convection, a reduction in water loss. Indeed, many authors have claimed that the reduced metabolic rates they find in insects they have investigated are a response either to dry conditions or to low food availability (see Chown and Gaston 1999 for review). Although the importance of reduced metabolic rates as a means to conserve water (Chapter 4), or as a general stress response is controversial, it is clear that, especially during dormancy, insects lower metabolic rates to overcome both a reduced energy intake and lack of water (Lighton 1991a; Davis et al. 2000; Chown 2002; Chown and Davis 2003). Moreover, in a few species, depression of metabolic rates appears to be actively promoted by means of behavioural aggregation, which promotes hypoxia, and consequently reduces metabolic rates (Tanaka et al. 1988; Van Nerum and Buelens 1997).
The controversy surrounding the idea of low metabolic rates as an adaptation to dry conditions in species inhabiting xeric environments is a consequence of three rather different issues. First, it is often stated that the contribution of respiratory transpiration to overall water loss is generally small, and therefore modification of metabolic rates is unlikely to contribute significantly to water conservation. Although this argument is now widely used, the data in support of it are contradictory (see especially Chown and Davis 2003) and the argument remains difficult to resolve (Chapter 4). Second, xeric environments are often characterized as much by patchy (in space and time) food resources as by patchy water availability. Therefore, it is difficult to determine which is likely to be the more important in selecting for low metabolic rates. Although responses to desiccation and starvation can be rather different in some species (Gibbs and Matzkin 2001; Gibbs 2002a), these two environmental factors might also act in concert to lower metabolic rates (Chown 2002). Third, because low temperature environments might select for higher metabolic rates and arid environments for low metabolic rates, and because temperature and water availability often show negative covariation (hot environments are often dry), it might be difficult to distinguish which of these factors is responsible for variation in metabolic rates (Davis et al. 2000).
These problems could be addressed in three ways. First, work could be conducted only on those animals for which a fully resolved phylogeny is available, along with information on both habitat temperature and water availability (or any other variables thought to be of significance). However, if covariation in the abiotic factors and metabolic rate is in the same direction across the phylogeny this method is unlikely to result in adequate resolution of the problem. Second, closely related species could be selected from environments that are largely similar with regard to most of their abiotic variables, but differ with regard to the variable of interest (although the problem of underlying causal variables may remain). Third, wide-ranging studies on a large variety of insects from many different habitats could be undertaken and their relationship to abiotic factors could be explored using general linear modelling techniques to obtain some idea of the most important abiotic variables affecting metabolic rate. Such a macrophysiological approach is likely to be confounded by problems associated with inadequate investigations of acclimation, as well as by relatively weak power to detect trends owing to variation associated with other factors. Nonetheless, it can provide a useful assessment of the factors likely to be influencing metabolic rate (Chown and Davis 2003). Owing to the paucity of these kinds of investigations, it seems reasonable to conclude that both temperature and water availability have considerable effects on metabolic rate, but their adaptive significance remains to be fully explored across a range of species.
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