4 | DISCUSSION
Our results demonstrate that the spatial variation in colour lightness and body size (i.e., volume) of assemblages of odonates across Europe is mainly driven by temperature. In line with the predictions of the thermal melanism hypothesis and Bergman’s rule sensu lato , our results showed that the analysed assemblages in warmer regions were consistently composed of, on average, lighter coloured and smaller species of dragon- and damselflies compared to assemblages in cooler regions. Our continent-wide yet spatially explicit assessment of these relationships reconciles previous macroecological (Pinkert et al., 2017; Zeuss et al., 2014, 2017) and experimental (e.g. May, 1991; Samejima & Tsubaki, 2010; reviewed in Clusella-Trullas et al., 2007) evidence indicating the general importance of mechanistic links of colour lightness and body size with the physiology and distribution of ectotherm species. In addition to the overall importance of colour- and size-based thermoregulation, our comparison of the trait-environment relationships of lentic and lotic assemblages of odonates revealed that the strength and relative importance of the climatic drivers of colour lightness and body size vary considerably between species with high and low dispersal/recolonisation ability.
Our study clearly showed that traits involved in thermoregulation influence the composition of dragon- and damselfly assemblages across Europe. According to the thermal melanism hypothesis, darker ectotherms are at an advantage in cool regions because of colour-based heat gain, and lighter ectotherms in warm regions because they reflect more solar radiation. In support of this hypothesis, we found that the colour lightness of Odonata assemblages increased with increasing temperature. The results of our analyses based on survey data together with the similar geographical patterns in colour lightness reported for assemblages of other ectothermic organisms at large geographical scales (Clusella-Trullas, Terblanche, Blackburn, & Chown, 2008; Heidrich et al., 2018; Schweiger & Beierkuhnlein, 2015; Stelbrink et al., 2019; Xing et al., 2018; Zeuss et al., 2014), confirm that thermal melanism is a mechanism of fundamental importance in ectothermic organisms across regions and scales. Furthermore, consistent with the predictions of Bergmann’s rule sensu lato , we found that the average body size of assemblages of odonates decreased with increasing temperature. Even though a recent macroecological study by Zeuss et al. (2017) found support for Bergmann’s rule in European odonates, its support in insects is generally equivocal (Shelomi, 2012), especially in studies conducted at small spatial and taxonomic scales. These contradictions in the results obtained at different scales have recently motivated debate about the reliability of large-scale assemblage-level studies, as it has been demonstrated that the type of distribution information on which most macroecological studies are based can purely by chance result in geographical patterns of species’ traits (Hawkins et al., 2017). Despite temperature explained a comparatively low variance in body size (c.f. Zeuss et al., 2017), our findings support Bergmann’s rule sensu lato in European odonates. Our support for both the thermal melanism hypothesis and Bergmann’s rule using spatially explicit survey data for European odonates show that the findings of studies based on expert range maps are robust to pseudo-replications of co-occurrences and the inherent geographical structures of species distributions (Hawkins et al., 2017).
Moreover, we also documented clear differences between species adapted to lentic and lotic habitats regarding the strength of the slopes of the considered trait-environment relationships and the relative importance of climatic drivers. Contrary to our third prediction, most of the relationships of average colour lightness and body size with temperature were equally strong between lentic and lotic assemblages. However, decomposing variations in colour lightness and body size showed that this is the result of similar responses of the phylogenetically predicted part of the traits of lentic and lotic species to climate, whereas relationships of the species-specific part of the traits were mostly stronger in lentic assemblages. Several studies have suggested that lentic species are stronger dispersers (e.g. Grewe, Hof, Dehling, Brandl, & Brändle, 2013; Hof et al., 2006; Marten et al., 2006) due to the negative relationship between habitat persistence and dispersal propensity (Southwood, 1962). Species adapted to lentic habitats are assumed to be closer to an equilibrium with ambient temperature (Dehling et al., 2010; Pinkert et al., 2018) and hence should dominate in recently recolonised regions (e.g., formerly glaciated northern parts of Europe; Pinkert et al., 2018). Accordingly, colour- and size-based thermoregulation together with high dispersal ability may have been hypothesised to cause contrasting biogeographical patterns between species adapted to lentic and lotic habitats over historical and evolutionary time scales (Hof et al., 2008; Pinkert et al., 2018). In fact, the distributional success and high diversity of lentic species in temperate regions seem to result not only from higher dispersal/recolonisation ability but also from an adaptive colour and body size evolution by lentic lineages. Our results suggest that adaptive colour and body-size are of similar importance for lentic and lotic species over evolutionary time scales, but that historical responses modified trait-environment relationships, with lentic species responding stronger to recent climatic changes than lotic species.
In light of previous zoogeographical and phylogeographical studies on dragon- and damselflies (Abellán et al., 2009; Kalkman et al., 2008; Pinkert et al., 2018; Sternberg, 1998), the documented differences in the trait-environment relationships of lentic and lotic species suggest that thermal melanism favours the colonisation of lineages of odonates in temperate climates. It has long been hypothesised that odonates are of tropical evolutionary origin and that only a few lineages acquired the ability to colonise and persist in temperate regions (e.g., Tillyard, 1916 p. 295). In a recent study, we found that the phylogenetic diversity of European Odonata assemblages decreased from the south-west to the north-east of the continent and that this pattern was mainly driven by the contemporary temperature (Pinkert et al., 2018). Latitudinal gradients of decreasing family or genus richness have been shown for odonates at the global scale (a simple proxy for the diversity of lineages; Kalkman et al., 2008). Furthermore, recent studies have documented a strong phylogenetic signal in the colour lightness of odonates and butterfly assemblages as well as differences in the importance of thermal melanism between butterfly families and associated these differences with a lower importance colour-based thermoregulation in tropical lineages (Zeuss et al., 2014, Pinkert et al., 2017, Stelbrink et al., 2019). Therefore, our finding that phylogenetically predicted part of the variation in colour lightness and body size is strongly driven by temperature suggested that colour- and size based thermoregulation might have played a central role in the adaptation to colder climates, whereas most Odonata lineages retained their initial tropical niche (see also Pinkert & Zeuss, 2018). Besides the differences in the strengths of the relationships of colour lightness and body size with temperature, our results show that the relative importance of temperature versus precipitation in shaping the geographical patterns of these traits differs between lentic and lotic assemblages. Although both annual mean temperature and annual precipitation consistently drove overall geographical patterns in the colour lightness and body volume of assemblages of odonates, lotic, but not lentic species seem to have an additional advantage of a higher size-based desiccation tolerance (Entling, Schmidt-Entling, Bacher, Brandl, & Nentwig, 2010), that also constrain their ability to thermoregulate via this trait. Specifically, we found that lotic assemblages in regions of lower precipitation were on average smaller than those in humid regions, which points to body size as an adaptation to water loss through the body surface (Kühsel et al., 2017). Furthermore, we showed that species adapted to lotic habitats were significantly larger in regions that are both warm and wet. This finding supports the predictions of Gloger’s rule (Wilson et al., 2001), which have been generally strongly supported by several large-scale studies (Pinkert et al., 2017; Stelbrink et al., 2019; Zeuss et al., 2014). Although studies have shown that melanisation impacts desiccation resistance (Parkash, Rajpurohit & Ramniwas, 2008; Parkash, Sharma & Kalra, 2009), we are cautious about interpreting a potential colour-based protection against water loss for two reasons; firstly, the environmental gradient of the study sites did not include extreme humid or dry regions and secondly, in our study annual precipitation was not an important driver of the variations in colour lightness European Odonata assemblages.