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Morphological trait database of saproxylic beetles

Citation

Hagge, Jonas et al. (2021), Morphological trait database of saproxylic beetles, Dryad, Dataset, https://doi.org/10.5061/dryad.2fqz612p3

Abstract

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.

Methods

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) 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.

List of morphological trait:

Colour lightness
Mass
Body length
Body width
Body height
Body roundness
Head length
Head width
Pronotum length
Pronotum width
Elytra length
Elytra width
Wing length
Wing width
Wing area
Wing aspect
Wing load
Antenna length
Mandible length
Proximal mandible width
Mandibular aspect ratio
Eye length
Eye area
Front femur length
Front femur width
Front tibia length
Mid femur length
Mid femur width
Mid tibia length
Hind femur length
Hind femur width
Hind tibia length
Hairiness head dorsal
Hairiness pronotum dorsal
Hairiness abdomen dorsal
Hairiness abdomen ventral
Hairiness total

Funding

BiodivERsA, Award: BIOESSHEALTH

Belmont Forum, Award: BIOESSHEALTH

BMBF, Award: BIOESSHEALTH

BiodivERsA, Award: BIOESSHEALTH

Belmont Forum, Award: BIOESSHEALTH

BMBF, Award: BIOESSHEALTH