Island—Bergstrom and Chown 1999). These global trends seem set to continue, accompanied by continuing increases in CO2, tropospheric ozone, and trace gases of other kinds, and an ongoing increase in the frequency, persistence, and intensity of El Nino events (Watson 2002).
The likely and realized responses of insects to changes in temperature, water availability, elevated CO2 levels and their interactions are the subject of a rapidly growing literature (Bazzaz 1990; Cammell and Knight 1992; Hoffmann and Parsons 1997; Cannon 1998; Coviella and Trumble 1999; Hill et al. 1999, 2002; Parmesan et al. 1999; Thomas et al. 2001; Bale et al. 2002; Erasmus et al. 2002). Indeed, it is now widely accepted that the signs of climate change are coherent and obvious for a range of both plant and animal taxa, and that these changes will continue (Walther et al. 2002; Parmesan and Yohe 2003; Root et al. 2003). For example, butterflies are expected to expand their northern range margins in the Northern Hemisphere, although this depends crucially on habitat availability, and the mobility of the species of concern. Highly mobile habitat generalists are likely to show much higher rates of change than less mobile specialists, which in some cases might be restricted by lack of habitat availability due to human land use (Hill et al. 1999, 2002). In two species of bush crickets, expansion of their range is being facilitated by a greater frequency of long-winged forms which can rapidly colonize new habitats (Thomas et al. 2001).
In the case of interactions between elevated temperature and CO2 levels, plant nutritional status, and herbivore responses, it is widely expected that elevated CO2 levels will result in a higher plant C: N ratio, prolonging herbivore larval growth rates, potentially reducing final adult size, and exposing larvae to higher predation rates (Buse et al. 1999; Coviella and Trumble 1999). Moreover, insects are expected to consume larger amounts of plant tissue and nutrient deficiencies are also likely to cascade up through higher trophic levels (Coviella and Trumble 1999). However, these generalizations belie complexities, such as those associated with the covariation of other gases, like O3, and with the trophic or functional group of the insects concerned (leaf chewers, bark borers, or litter dwellers) (Karnosky et al. 2003).
Rather than provide a review of the realized and expected effects of climate change, which can be obtained from a host of recent works, we draw attention to several important areas in which physiological ecology might have a significant role to play in promoting an understanding of the responses of insects to current environmental change.
1. As in other aspects of physiological ecology (Chown et al. 2002a), the range of investigations of responses to change (apart from simulation models) is taxonomically biased relative to the distribution of species among higher taxa. Investigations on Lepidoptera and aphids are particularly common, while those on other taxa tend to be rarer (Coviella and Trumble 1999). Likewise, crop pests are much more likely to enjoy attention than wild species, even though there are considerable interactions between the two groups.
2. There is much support for the idea that the responses of insects in the Southern and Northern Hemispheres might differ considerably (Chown et al. 2002a; Sinclair et al. 2003c), and that the latitudinal richness patterns of the two hemispheres are dissimilar (Blackburn and Gaston 1996; Gaston 1996). However, these differences, and their implications for species responses to climate change, remain poorly explored. Indeed, several reviews make statements regarding the general responses of insects to climate change, when in reality only Northern Hemisphere species are concerned. Few studies are addressing the likely and realized responses of Southern Hemisphere insects to climate change, as is clear from recent reviews of empirical work (Parmesan and Yohe 2003; Root et al. 2003).
3. The responses of insects to elevated temperature and elevated CO2 levels are more commonly investigated than responses to other variables. Nonetheless, not only is rainfall expected to decline in many areas, but extreme drought and extreme rainfall events are expected to increase in frequency (Watson 2002). Drought is known to have a substantial influence on a variety of insect species, particularly when the droughts are severe (Kindvall 1995; Pollard et al. 1997; Hawkins and Holyoak 1998), and prolonged hypoxia associated with submergence cannot be tolerated by most insect species. Inadequate understanding of the relationship between water availability and insect responses has been highlighted previously (Tauber et al. 1998). These responses can be quite subtle, though important. For example, the water status of host plants determines host quality, and therefore oviposition preference and larval performance, and so ultimately the population dynamics of the insect concerned (Scriber 2002). 4. While the differential responses of generalists versus specialists to climate change are becoming increasingly well known in butterflies in Europe (Hill et al. 2002), the relative importance of abiotic variables and biotic interactions to the responses of other species remain poorly known. In particular, determining whether there is a fast-slow life history dichotomy (Ricklefs and Wikelski 2002) which characterizes indigenous versus invasive insect species (Moller 1996) is of particular significance in the context of interactions between ongoing climate change and invasion (Sala et al. 2000).
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