Previous macrophysiological studies suggested that temperature-driven colour lightness and body size variations strongly influence biogeographical patterns in ectotherms. However, these trait-environment relationships scale to local assemblages and the extent to which they can be modified by dispersal remains largely unexplored. We test whether the predictions of the thermal melanism hypothesis and the Bergmann’s rule hold for local assemblages. We also assess whether these trait-environment relationships are more important for species adapted to less stable (lentic) habitats, due to their greater dispersal propensity compared to those adapted to stable (lotic) habitats.

We quantified the colour lightness and body volume of 99 European dragon- and damselflies (Odonata) and combined these trait information with survey data for 518 local assemblages across Europe. Based on this continent-wide yet spatially explicit dataset, we tested for effects temperature and precipitation on the colour lightness and body volume of local assemblages and assessed differences in their relative importance and strength between lentic and lotic assemblages, while accounting for spatial and phylogenetic autocorrelation.

The colour lightness of assemblages of odonates increased and body size decreased with increasing temperature. Trait-environment relationships in the average and phylogenetic predicted component were equally important for assemblages of both habitat types but were stronger in lentic assemblages when accounting for phylogenetic autocorrelation.

Our results show that the mechanism underlying colour lightness and body size variations scale to local assemblages, indicating their general importance. These mechanisms were of equal evolutionary significance for lentic and lotic species, but higher dispersal ability seems to enable lentic species to cope better with historical climatic changes. The documented differences between lentic and lotic assemblages also highlight the importance of integrating interactions of thermal adaptations with proxies of the dispersal ability of species into trait-based models, for improving our understanding of climate-driven biological responses.

We compiled from literature, extensive Information on water body location and type (i.e. habitat; lentic or lotic) and the composition of local assemblages of Odonata (dragonflies and damselflies) across Europe. This included a distribution data of 5,703 records of 99 species occurring in 518 local assemblages across 28 European countries.

We calculated the average colour lightness (colour lightness) and body volume (body volume) of species using scanned (24-bits, sRGB, 1200 dpi resolution) drawings of the dorsal body surfaces of European Odonata. We cropped out the body (head, abdomen, and thorax) using functions of Adobe Photoshop CS2. Based on these images the average colour of the pixels of an image across the red, green and blue channels was calculated as an estimate of the colour lightness of a species (8-bit grey values ranging from 0: absolute black to 255: pure white). We also scaled the images with the magnification factor provided by its source and used that to calculate body volume in cm³ (π × [½ length of pixel row]² × pixel edge length) as an estimate of the body size of a species based on the assumption that odonates generally have a cylindrical body form. The calculations were performed using functions of the R-package png. We then averaged the colour lightness and body volume values across the species (sp_rich) of each local assemblage.

We extracted 19 commonly used bioclimatic variables from Chelsa-climate.org based on geographical coordinates (lat, lng) of the assemblages (with a buffer radius of approximately 1 km) with a resolution of 2.5 arcminutes. After a principal component analysis of all 19 bioclimatic variables (e.g. PC_t1), we considered only annual mean temperature and annual precipitation (bio1 and bio12, respectively) since they contributed strongly to the overall trends in temperature and precipitation.