A multidimensional study on population size, deadwood relationship and allometric variation of Lucanus cervus through citizen science

The European stag beetle, Lucanus cervus, is the largest saproxylic beetle and a flagship species whose populations should be monitored according to the EU Habitats Directive. Different studies have addressed its abundance and phenology, its relationship with deadwood, or the allometry of males related to different reproductive strategies. While monitoring efforts are complicated by the limited phenological window of adults (active at dusk during a few weeks), different innovative strategies have been tested in the last few years, with a promising role of citizen science. We tested the possibility of characterising key dimensions of a L. cervus population of an Italian Natural Park by involving a network of citizen scientists in a two-week sampling campaign along standardised sampling transects, using a capture-mark-recapture (CMR) approach. We investigated its relationship with deadwood at different sites, estimated population size with a Bayesian analysis and characterised the allometric variation by assembling a dataset of morphological traits. With the participation of 41 volunteers, we collected and measured 651 individuals along four sampling transects. Our analyses uncovered a large L. cervus population (estimated to consist of almost 3400 individuals), characterising its relationship with deadwood availability in different areas, and the absence of the expected male allometry. Our results suggest that even a woodland composed by relatively young trees can host a large L. cervus populations if cut deadwood of suitable size is available. The engagement of volunteers, to which the results were presented, proved crucial to cost-efficiently study different key ecological aspects of an insect species with a short phenology.