Morphological trait database of saproxylic beetles

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.