2.2 | Trait data
Following the most common approach used in the analysis of morphological
traits based on digital images (Zeuss et al., 2014, 2017; Pinkert et
al., 2017), we calculated the average colour lightness and body volume
of species using drawings of European Odonata (Dijkstra & Lewington,
2006). To prepare images for the analysis, the body (head, abdomen, and
thorax) in scanned drawings of species’ dorsal body surfaces (24-bits,
sRGB, 1200 dpi resolution) was cropped out and saved to separate files
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). In addition, these images were scaled with the magnification
factor provided in Dijkstra and Lewington (2006) and used 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(Urbanek,
2013).
Body volume instead of linear size measures, such as wing length, head
width and body length, was used because as a three-dimensional measure
it allows for a more realistic estimate of the body mass of a species
(Kühsel et al., 2017). Note that previous studies showed that the colour
lightness and body volume estimates are correlated between drawings from
different sources and between males and females (Pinkert et al., 2017,
Zeuss et al., 2017). Subsequently, the average colour lightness and body
volume were calculated across the species of each local assemblage.
Previous studies found that the colour lightness and body size, as well
as the habitat preference of European odonates, carries a phylogenetic
signal (Zeuss et al. 2014; Letsch et al., 2016; Pinkert et al., 2017).
Based on a recent phylogeny of the European odonates (Pinkert et al.,
2018), we, therefore, partitioned the total variance of the colour
lightness and body volume into a phylogenetic component and a
species-specific component using Lynch’s comparative method (Lynch,
1991), implemented in the R-package ape (Paradis et al., 2004).
The phylogenetic component represents the variation in colour lightness
and body volume predicted by the phylogenetic relationships of the
species, whereas the species-specific component is the difference of the
observed trait estimate from the phylogenetically predicted part. The
advantage of this method is that it allows assessing the effect of
phylogenetic signals in traits (i.e., P - component) that is often
neglected as a source of bias, in addition to the model results that
have been corrected for phylogenetic autocorrelation (i.e., S -
component).