Functional trait variation of an emblematic saproxylic beetle along rural-to-urban gradients across Europe

Urban areas can support diverse communities of plants and animals. Yet, urbanisation can affect functionally important species traits, potentially impacting population dynamics. The saproxylic beetle Elater ferrugineus L. is associated with large trees and is often used as an indicator of species-rich saproxylic communities. It is an important target for conservation and it is listed as a near-threatened species on the European Red List. Few studies have quantified the impact of urbanisation on the ecology and intraspecific variation in functional traits of arthropods, other than pollinators. We studied how the local abundance of E. ferrugineus and functionally important response traits (e.g., width of the pronotum, length of the elytron and wing, wing area, body mass and wingload) changed along urbanisation gradients in eight European cities using pheromone traps installed on large solitary trees. We analysed the effects of the surrounding built-up area and tree cover on our response variables while accounting for potential confounding effects due to tree size and the availability of microhabitats. Urbanisation had a strong negative effect on the local abundance of E. ferrugineus, while the amount of tree cover had a positive influence. We found no significant impact of urbanisation on the functional traits of this species, except for a significantly higher wingload in city centres. Our results provide a better understanding of the ecological processes impacting this saproxylic beetle and underpin the importance of future research on urbanisation's impact on arthropods.