1 | INTRODUCTION
Understanding the processes that shape species’ distributions and the composition of assemblages is within the centre of ecological research (Brown & Maurer, 1987; Cavender-Bares, Kozak, Fine, & Kembel, 2009; McGill, Enquist, Weiher, & Westoby, 2006; Ricklefs, 2004). A straightforward approach to a process-based understanding is to investigate functional traits that link the physiology of a species with the ambient environment in which the species occurs (Chown, Gaston, & Robinson, 2004; Violle et al., 2007). Ectothermic species must absorb thermal energy from their environment to be active and to maintain fundamental physiological processes, including growth and reproduction (Angilletta, 2009; Huey & Kingsolver, 1989). Therefore, ectotherms have evolved several behavioural (e.g. wing-whirring or basking; Corbet, 1980; May, 1979) and morphological adaptations to the climate in which they live (May, 1976; Angilletta, 2009).
Two of the most important morphological traits that influence the distribution of ectothermic organisms are probably their surface colour, particularly colour lightness (melanism), and body size. Fundamental physical principles link both traits to the heat gain and loss of an organism (Clusella-Trullas, van Wyk, & Spotila, 2007; Shelomi, 2012). On the one hand, melanisation of the cuticle determines the absorption of solar radiation and hence heat gain, a mechanism referred to as thermal melanism (Clusella-Trullas et al., 2007; Gates, 1980; Kalmus, 1941). On the other hand, since an increase in body size implies a reduction of the surface area to volume ratio, larger bodies are able to retain heat more efficiently than smaller bodies (Shelomi, 2012). Besides thermoregulation, greater melanisation increases resistance against pathogens, by enhancing the structural integrity of cells (Gloger’s rule, Rapoport, 1969; Wilson, Cotter, Reeson, & Pell, 2001) and a larger body size is advantageous under dry conditions, as a lower surface area to volume ratio reduces water loss (Kühsel, Brückner, Schmelzle, Heethoff, & Blüthgen, 2017; Remmert, 1981).
While the colour lightness and body size of a species should determine the climate in which it can live, the extent to which that species realises the potential environmental niche depends on its dispersal ability. The most important differences in species’ abilities and propensities to disperse are related to the stability of their respective habitats. In general, species restricted to spatially and temporally stable habitats have lower dispersal abilities than species adapted to less stable habitats (Southwood, 1977). Freshwaters provide an ideal model system to test the predictions of this “habitat-stability-dispersal hypothesis” (Southwood, 1977). In the northern hemisphere, lentic water bodies (e.g., ditches and lakes) are ephemeral and date back to the Pleistocene, whereas the locations of rivers and streams (lotic waters) that carry water throughout the year have remained largely unaltered since the Mesozoic (Bohle, 1995 and sources therein). Species adapted to lentic waters have therefore evolved a suite of adaptations (i.e., trait syndromes) to cope with climatic changes, including morphological adaptations that facilitate mobility (Arribas et al., 2012; Hof, Brändle, & Brandl, 2006; Marten, Brändle, & Brandl, 2006; Pinkert et al., 2018) as well as behavioural adaptations (Corbet, 1980).
Recent studies have shown that the ecological differences between species adapted to lentic and lotic habitats carry a phylogenetic signal (Letsch, Gottsberger, & Ware, 2016). Moreover, these differences have led to contrasting biogeographical and diversification patterns between the two groups (Abellán, Millán, & Ribera, 2009; Hof, Brändle, & Brandl, 2008). For instance, Dehling et al. (2010) showed that the richness of lotic animals decreases from southern to northern Europe, whereas the richness of lentic animals is highest in central Europe. A broadly similar pattern has been reported for the richness of lentic and lotic Odonata (dragonflies and damselflies) on a global scale (Kalkman et al., 2008). Thus, in contrast to almost all other Odonata, the two youngest families (Coenagrionidae and Libellulidae; Rehn, 2003) that constitute the majority of lentic species globally (Kalkman et al., 2008) are disproportionally diverse in temperate climates. This suggests stronger trait-environment relationships in odonates of lentic than lotic habitats due to the greater ability of the former to cope with climatic changes in the past. However, despite strong theoretical reasons for an impact of species’ dispersal ability on biogeographical patterns, to what extent dispersal can modify trait-environment relationships remains largely unexplored.
Analyses of the large-scale patterns of interspecific variation in physiological traits offer a powerful approach to elucidate the general processes that shape biodiversity patterns (Chown et al., 2004). These macrophysiological inferences are based on the assumption that the explanations for large-scale diversity patterns are found at lower levels of biological organisation, as functional traits influence the fundamental physiological rates of individuals and populations whereas the consequences thereof play an important role in determining a species’ fundamental niche (Gaston & Blackburn, 2000). However, almost all of the studies conducted so far on the interspecific variation of colour lightness and body size in ectothermic species are based on expert range maps generated by interpolating species occurrence records across suitable habitats (e.g., Pinkert, Brandl, & Zeuss, 2017; Zeuss, Brandl, Brändle, Rahbek, & Brunzel, 2014; Zeuss, Brunzel, & Brandl, 2017). Hence, previous evidence of colour- and size-based thermoregulation has three important limitations. First, although at geographical scales expert range maps are generally considered to allow robust estimations of the full environmental range of species, the underlying distribution information tends to overestimate species’ real distributional ranges (Hurlbert & Jetz, 2007; Merow, Wilson, & Jetz, 2017). Second, the inherent spatial structure of expert range maps has been shown to inadvertently generate spurious spatial patterns for the richness and mean trait values of assemblages (Hawkins et al., 2017). Third, distribution data with a coarse resolution generate “synthetic” assemblages of species that do not necessarily form local assemblages. For instance, expert range maps typically also include records of populations that may no longer exist (or never existed) and pool species from different habitat types. Therefore, whether the previously documented relationships of colour lightness and body size with climate also scale to the local assemblage level remains largely unexplored.
In this study, we investigated trait-environment relationships using spatially explicit survey data for local assemblages of dragon- and damselflies (Odonata) across Europe. Specifically, according to the thermal melanism hypothesis and Bergmann’s rule sensu lato, we expected 1) an increase in the colour lightness of local assemblages of odonates with increasing temperature and 2) an increase in the body size of local assemblages of odonates with decreasing temperature. In addition, given that adaptations to spatially and temporally less stable habitats allow lentic species to better cope with climatic changes (habitat-stability-dispersal hypothesis), we predicted that the slopes of these relationships would be stronger for lentic than for lotic assemblages.