Active management fosters species richness of wild bees in limestone quarries

Abstract

Open and unreclaimed quarries often host species-rich plant and animal communities. However, the specific factors that determine biodiversity in such anthropogenic habitats are largely unknown. This applies in particular to diverse groups of insects. We therefore studied bee assemblages of 16 limestone quarries, eight active and eight abandoned ones, in a Central European landscape with a long history of quarrying. We analysed the influence of macroclimate, landscape quality and habitat quality on overall species richness as well as on the number of threatened and oligolectic species. Our study revealed strong differences in habitat quality and composition of bee assemblages between active and abandoned quarries. Active quarries were larger, had more different pollen sources and a higher cover of early but a lower cover of later successional stages than abandoned quarries. As a result, species richness of bees was higher in active compared to abandoned quarries. Additionally, active quarries were characterized by a unique bee assemblage consisting of several indicator species. Availability of pollen sources and nesting habitats predicted bee species richness in the quarries. Overall, habitat quality and composition of bee assemblages were strongly dependent on quarry management. Active mining continuously created early-successional stages rich in pollen sources and sunlit nesting habitats, favouring species richness of bees in general and of threatened and oligolectic species in particular. Successional speed is generally low in quarries. Nevertheless, in abandoned quarries, later successional stages increasingly dominated due to ongoing succession and, hence, habitat quality and species richness of bees decreased. Considering this, we propose to expand existing quarries rather than build new ones. From a conservation point of view, this approach has several advantages: (i) existing quarries are often already refuges for colonies of rare species; (ii) many species of conservation concern have a low mobility and are therefore unable to occupy remote habitats and (iii) in the impoverished modern-day landscapes, source population that may colonize new quarries are often lacking.