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Figure 1.6 Best fit polynomial regression lines (±95%) showing the relationship between latitude and absolute maximum (top line) and absolute minimum (bottom line) temperatures across the New World (negative latitudes are north of the equator).

Source: Gaston and Chown (1999). Oikos 86, 584-590, Blackwell Publishing.

Figure 1.7 The relationship between elevation and (a) critical thermal minimum (CTmin), and (b) critical thermal maximum (CTmax) for dung beetles collected at six localities across a 2500 m elevational range in southern Africa.

Source: Gaston and Chown (1999). Oikos 86, 584-590, Blackwell Publishing.

Impetus for large-scale, geographic studies has also come from renewed interest in the utility of this approach for understanding physiological diversity. Indeed, it provides the only means to test several important hypotheses concerning physiological variation (such as interspecific metabolic cold adaptation; Addo-Bediako et al. 2002), and a useful way to address several others, such as the influence of climatic variability on physiological traits (Lovegrove 2000). In the latter case, geographically extensive studies have demonstrated that broad-scale environmental variation can influence insect physiological traits in ways that have previously gone unrecognized. Perhaps the most significant examples in this regard are the likely influence that climatic variability has had on hemispheric variation in thermal tolerances (Sinclair et al. 2003c) and rate-temperature relationships (Addo-Bediako et al. 2002; Chown et al. 2002a).

Large-scale studies have been given additional momentum by the recent development of nutrient supply network models as a means to account for the apparent prevalence of three-quarter scaling of metabolic rate (and other physiological rates) in all organisms (West et al. 1997). The extent to which the 'quarter-power' scaling assumed by these models applies to all organisms has been questioned (Dodds et al. 2001), and their utility for explaining global variation in species richness (Allen et al. 2002) has also come under considerable scrutiny. However, the models have rekindled interest in and excitement about the mechanisms underlying the scaling of physiological traits, and the broader ecological implications of physiological trait variation.

These examples make it clear that macro-physiology, or the investigation of variation in physiological traits over large geographic and temporal scales, and the ecological implications of this variation (Chown et al. 2004), are now an essential component of physiological investigations.

1.4 Growing integration

By the standards of only a few decades ago, the variety of approaches that can now be used to explore ecological success and its evolution, and consequently the mechanisms underlying patterns in biodiversity, is remarkable. These include functional genomics (Bettencourt and Feder 2002; Anderson et al. 2003; Feder and Mitchell-Olds 2003), genetic engineering (Feder 1999), comparative biochemistry (Mangum and Hochachka 1998; Storey 2002), laboratory selection (Gibbs 1999), evolutionary physiology (Feder et al. 2000a), and macrophysiology (Chown et al. 2004). They also include integrated investigations in which the need to simultaneously explore several different characteristics at a variety of levels is recognized, irrespective of whether these levels include behavioural, morphological, or life history traits (Chai and Srygley 1990; Kingsolver 1996; Jackson et al. 2002; Ricklefs and Wikelski 2002; Huey et al. 2003). Moreover, the diversity of approaches and opportunities for their integration seem set to continue their expansion as sequences of entire genomes become available for increasingly large numbers of species (Levine and Tjian 2003), and transcription profiling becomes more common. This situation is very different to the one which characterized much of twentieth-century physiological ecology, when physiology and ecology largely developed along separate paths (Spicer and Gaston 1999).

The benefits of integration of studies across the ecological and genetic hierarchies (Brooks and Wiley 1988) are likely to be profound. Such work will not only provide comprehensive understanding of the evolution of wild organisms in situ, but will also facilitate prediction of the outcomes of unintentional large-scale experiments such as climate change, biotic homogenization, and habitat destruction (Feder and Mitchell-Olds 2003). This is, perhaps, best illustrated by work on the adaptation of Drosophila to environmental extremes (Hoffmann et al. 2003b; Lerman et al. 2003), and the likely consequences of these responses for Drosophila diversity (Hoffmann et al. 2001b; Hoffmann et al. 2003a). Here, the distinction between classical model organisms and wild species is being blurred, as Feder and Mitchell-Olds (2003) argued it should be. Moreover, this integrated approach is genuinely able to address some of the major concerns precipitated by human manipulation of the environment, such as how different groups of species

(e.g. generalists versus specialists) might respond to a changing environment (Hill et al. 2002), and how dispersal ability itself is likely to evolve (Zera and Denno 1997; Davis and Shaw 2001; Thomas et al. 2001).

1.5 This book

Given the growing integration of insect physiological ecology, the objectives of this book are threefold. First, to examine interactions between insects and their environments from a physiological perspective that integrates information across a range of approaches and scales. This will be done first by describing physiological responses from the molecular level through to that of the individual. Clearly, the degree to which this can be achieved varies with the trait in question. For example, it is relatively straightforward to develop this approach for thermal tolerance because of the immense amount of work at the sub-individual level in Drosophila, where the genes underlying thermal responses are known and their locations on the chromosomes have been identified (Bettencourt and Feder 2002; Anderson et al. 2003). However, in the case of other traits, such as interactions between gas exchange and metabolism (Chown and Gaston 1999), molecular level work lags far behind the level of understanding available at the tissue, organ, and whole-individual levels.

The second objective is to demonstrate that evolved physiological responses at the individual level are translated into coherent patterns of variation at larger, even global scales. Such variation cannot be detected using the small-scale, mechanistic approaches that are the cornerstone of much physiology. However, modern statistical and computer-based visualization methods (such as Geographic Information Systems—see Liebhold et al. 1993), make this large-scale approach relatively straightforward. Crucially, the value of applying this technique depends on the quality and spatial extent of the available data, a theme that is explored towards the end of the book. Because we are regularly concerned with taxonomic variation in traits, we provide two modern phylogenies as a context within which the animals and traits we discuss can be viewed—one for the arthropods (Fig. 1.8), and one for the insect orders (Fig. 1.9).

The third objective is realized as something of an epiphenomenon. This is to draw attention to the ways in which methods and measurement techniques might have an influence on the conclusions drawn by a particular study. It is becoming increasingly apparent that what is done in a given experiment to a large extent determines the outcome. To some, this might seem obvious as the rationale for experimentation. However, these effects can be subtle, and can reflect investigations of different traits when this was not the initial intention. The best example of this physiological uncertainty principle is the measurement of heat shock, where ostensibly similar methods provide information on different traits.

We begin in Chapter 2 with nutritional physiology and ecology. All organisms require a source of energy, which can then be partitioned between the multitude of tasks that allow them to respond to environmental challenges and to strive to contribute their genes to the next generation. Here, we also examine development and growth because of its dependence on food quantity and quality, but take cognizance of the fact that even under ideal nutritional circumstances, insects can manipulate growth and development rates in response to other environmental cues (Nylin and Gotthard 1998; Margraf et al. 2003). Chapter 3 concerns metabolism and gas exchange. In general, insects make use of aerobic metabolism to catabolize substrates, and as a consequence, they have developed a sophisticated system for gas exchange that also has attendant problems of water loss and the likelihood of enhanced oxidative damage. Nonetheless, insects are also adept at living under hypoxic conditions, and metabolism under these circumstances is explored. Variation in the costs of living owing to variation within individuals depending on activity, between individuals as a consequence of size, and between populations and species is also explored.

Because water availability and temperature are two of the most significant environmental variables influencing the distribution and abundance of insects (Rogers and Randolph 1991; Chown and Gaston 1999), Chapters 4,5, and 6 explore the ways in which insects respond to these abiotic variables.

Insect Phylogeny

Figure 1.8 Composite phylogeny for the major arthropod taxa.

Note: Several of the clades are unstable, and relationships between some major taxa have yet to be resolved. Source: Compiled from Giribet eta/. (2001, 2002).

Figure 1.8 Composite phylogeny for the major arthropod taxa.

Note: Several of the clades are unstable, and relationships between some major taxa have yet to be resolved. Source: Compiled from Giribet eta/. (2001, 2002).

Archaeognatha Thysanura Ephemeroptera Odonata Plecoptera Embioptera Orthoptera Phasmatodea Mantophasmatodea Grylloblattodea Dermaptera Isoptera Mantodea Blattaria Zoraptera Thysanoptera Hemiptera Homoptera Psocoptera Phthiraptera Coleoptera Neuroptera Megaloptera Raphidioptera Hymenoptera Trichoptera Lepidoptera Strepsiptera Diptera Siphonaptera Mecoptera

Figure 1.9 Composite phylogeny for the insect orders, following Dudley's (2000) compilation.

Note: The Mantophasmatodea is placed between the clade including the Phasmatodea and the one including the Grylloblattodea, based on the interpretations provided by Klaas et al. (2002), although they note that the phylogenetic placement of this new order has not been resolved.

Insect Phylogeny

Figure 1.9 Composite phylogeny for the insect orders, following Dudley's (2000) compilation.

Note: The Mantophasmatodea is placed between the clade including the Phasmatodea and the one including the Grylloblattodea, based on the interpretations provided by Klaas et al. (2002), although they note that the phylogenetic placement of this new order has not been resolved.

Chapter 4 concerns not only the routes of water loss from insects, but also the ways in which water is obtained, particularly in species that have different feeding habits and which occupy different environments. Moreover, because osmoregulation is inseparable from water balance, the mechanistic basis of osmoregulation is also examined. This chapter is concluded with a discussion of the considerable insights that have been provided from laboratory selection and comparative studies of Drosophila (Gibbs et al. 2003a). Chapter 5 deals with responses of insects to temperature. Because the mechanisms underlying responses to high and low temperatures, but particularly heat and cold shock, are similar in several respects, responses to high and low temperatures are considered together. In Chapter 6, the focus is on behavioural and physiological thermoregulation and its consequences. Because this field has been repeatedly reviewed, rather than focusing on the well-known examples, we draw attention particularly to recent advances in understanding of thermoregulation. We also discuss the ecological implications of thermoregulation, especially in terms of competition and predation.

In Chapter 7 we synthesize information on the spatial extent of the physiological information that is available on insects. We highlight the fact that large-scale studies, in combination with mechanistic work, suggest that responses to temperature in insects might be different to those in other ectotherms. We also show that understanding of the evolution of body size, a trait that is of considerable ecological and physiological significance (Peters 1983), cannot be separated from investigations of several physiological traits. Moreover, these traits not only act in concert to determine body size, but body size in turn constrains these physiological variables. We conclude by drawing attention to the likely responses of insects to global climate change, and the role of an integrated insect physiological ecology in providing a basis for understanding and predicting these responses.

Before moving on, we offer a word of caution. Those readers who are enthusiastic about hormonal regulation, muscle physiology, neuronal functioning, sensory perception, sclerotization, the physics of flight, and the intricacies of insect clocks and diapause, will not find these topics discussed here. We make reference to them where appropriate, and acknowledge that insects would not be capable of interacting with the environment without systems that regulate internal function and coordinate external responses. However, our primary objectives are concerned with other issues. We urge disappointed readers to examine the excellent reviews of such topics provided by Nijhout (1994), Chapman (1998), Field and Matheson (1998), Sugumaran (1998), Dudley (2000), Denlinger et al. (2001), Nation (2002), Denlinger (2002), and Merzendorfer and Zimoch (2003), and then to come back to this book.

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