Data, species taxonomy and ecological information
The specimen’s data for this study have been collected from two previous barcoding studies on central European beetles (Insecta: Coleoptera) (Hendrich et al., 2016; Rulik et al., 2017) performed in the framework of German Barcoding initiatives (i.e., German Barcode of Life project: https://www.bolgermany.de; Bavarian Barcoding project: http://www.faunabavarica.de/). These projects aim at building a reference library of DNA barcodes for all available organisms in Germany collecting, where possible, ten specimens per species, from locations as distinct as possible throughout the Germany and the neighboring countries in order to capture genetic variability (Figure 1).
In order to avoid an underestimation of intraspecific genetic differences and to maximize the amount of complete available data from BOLD, (http://www.barcodinglife.org), we excluded from the analysis redundant genetic information represented by identical syntopic haplotypes with equal geographical coordinates and incomplete ecological or geographical information on the specimens. From these, we retained only those species for which were available comprehensive ecological information (see below).
Species taxonomy used as backbone for this study is derived from Klausnitzer & Köhler (1998) and subsequent works (Köhler, 2000, 2011a, b; Bleich et al., 2016) reflecting the current species taxonomy applied in German coleopterist’s community (http://www.coleokat.de/de/fhl/ (status: 2016)). Eventual inconsistencies of current classification with the source of ecological data (Koch 1989, 1991, 1992) were adopted by F.K. in his curated data base with help of the numerical identifier for each species (Lohse & Lucht, 1999). For our meta-analysis, we selected among available data to examine the effect of four major ecological variables (body size, biotype preference, habitat preference, and feeding habits) in the context of geographical distance and genetic (mtDNA) differentiation.
Mitochondrial DNA (mtDNA) has been widely used in phylogenetic studies of animals because it evolves much more rapidly than most nuclear DNA, resulting in the accumulation of differences between closely related species (Brown et al., 1979; Moore, 1995). The rapid pace of sequence evolution in mtDNA, and in particular in COI , results in differences between populations that have only been separated for brief periods of time, making it a relevant way to infer divergence at population level (Avise et al., 1987). COI has been found to consistently differentiate species and for which large libraries of sequences in constant growth have become available linked to voucher specimens (Hebert et al., 2003a, b; Hebert & Gregory, 2005).
For the autecological information on the species (Table S1), which we regard here as a proxy for the entire species ecology, we used data on feeding habits, habitat preference, biotope preferences, and body size (derived from a data base curated by F.K. based mainly on Koch, 1989, 1991, 1992 and many further, more detailed publications not mentioned here). Feeding habits included eight different, more generic classes: coprophagous, polyphagous, mycetophagous, necrophagous, phytophagous, saprophagous, xylophagous, and zoophagous (Figure 3B). While the same number of classes was used for the habitat preference: soil; eurytop, rotting matters, nests, mushrooms, vegetation, water, and dead wood (Figure 3A), the biotope preferences were represented by four different categories: no biotope preference, wetlands, open-land biotypes, and forests (Figure 3C). Five size classes were arbitrary defined: 0-2 mm, 2-5 mm, 5-10 mm, 10-20 mm, 20-50 mm. These reflect roughly eco-functional groupings in the food web due to body size. However, therefore, the ecological classes are not equally represented among the sampled species (Figure 3D).