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.