DOI: 10.5061/dryad.2fqz612p3
title: Morphological trait database of saproxylic beetles

description: The extinction of species is a non-random process, and understanding why
 some species are more likely to go extinct than others is critical for
 conservation efforts. Functional trait-based approaches offer a promising
 tool to achieve this goal. In forests, deadwood-dependent (saproxylic)
 beetles comprise a major part of threatened species, but analyses of their
 extinction risk have been hindered by the availability of suitable
 morphological traits. To better understand the mechanisms underlying
 extinction in insects, we investigated the relationships between
 morphological features and the extinction risk of saproxylic beetles.
 Specifically, we hypothesised that species darker in colour, with a larger
 and rounder body, a lower mobility, lower sensory perception, and more
 robust mandibles are at higher risk. We first developed a protocol for
 morphological trait measurements and present a database of 37 traits for
 1157 European saproxylic beetle species. Based on 13 selected, independent
 traits characterising aspects of colour, body shape, locomotion, sensory
 perception and foraging, we used a proportional-odds multiple linear
 mixed-effects model to model the German Red List categories of 744 species
 as an ordinal index of extinction risk. Six out of 13 traits correlated
 significantly with extinction risk. Larger species, as well as species
 with a broad and round body, had a higher extinction risk than small, slim
 and flattened species. Species with short wings had a higher extinction
 risk than those with long wings. On the contrary, extinction risk
 increased with decreasing wing load and with higher mandibular aspect
 ratio (shorter and more robust mandibles). Our study provides new insights
 into how morphological traits, beyond the widely used body size, determine
 the extinction risk of saproxylic beetles. Moreover, our approach shows
 that the morphological characteristics of beetles can be comprehensively
 represented by a selection of 13 traits. We recommend them as a starting
 point for functional analyses in the rapidly growing field of ecological
 and conservation studies of deadwood.
description: Thirty-two morphological traits were measured directly and five
 additional traits (wing load, wing aspect, mandibular aspect ratio, total
 hairiness and body roundness) were calculated based on these measured
 values. All traits were measured on the right side of the beetles’ bodies,
 with the left side measured only when body parts were damaged or missing.
 The morphological measurements of the beetles were made using a Leica M205
 C (Leica Microsystems CMS GmbH., Switzerland) stereomicroscope with a
 magnification range of 7.8× to 160× and an optical resolution of 0.952 µm.
 The line, segmented-line and area measurement tools of the Leica LAS Core
 (version LAS V4.1.0)<i> </i>measurement software were used in
 combination with the Leica DFC295 digital colour camera (Leica
 Microsystems CMS GmbH., Switzerland). Beetle mass was measured using
 Sartorius M5P microbalances (Sartorius AG, Germany) with a resolution of
 0.001 mg. The colour lightness of the beetle species was estimated using
 the workflow of a computer-assisted digital image analysis system
 (Schweiger & Beierkuhnlein, 2016; Zeuss et al., 2014). Digital
 photos published in the book “Käfer Mitteleuropas” (Dries, 2016) served as
 the reference. The beetle species in the book were photographed under
 standardised conditions in dorsal view against a white background.
 Subsequently, the white background of the red, green and blue (RGB)
 photographs was removed and the mean of the RGB colour channel across all
 pixels of each image was calculated. Possible colour lightness values thus
 ranged from 0 for completely blackish to 255 for completely whitish
 species. <strong>List of morphological
 trait:</strong> Colour lightness<br>
 Mass<br> Body length<br> Body width<br> Body
 height<br> Body roundness<br> Head length<br> Head
 width<br> Pronotum length<br> Pronotum width<br> Elytra
 length<br> Elytra width<br> Wing length<br> Wing
 width<br> Wing area<br> Wing aspect<br> Wing
 load<br> Antenna length<br> Mandible length<br> Proximal
 mandible width<br> Mandibular aspect ratio<br> Eye
 length<br> Eye area<br> Front femur length<br> Front
 femur width<br> Front tibia length<br> Mid femur
 length<br> Mid femur width<br> Mid tibia length<br> Hind
 femur length<br> Hind femur width<br> Hind tibia
 length<br> Hairiness head dorsal<br> Hairiness pronotum
 dorsal<br> Hairiness abdomen dorsal<br> Hairiness abdomen
 ventral<br> Hairiness total