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.