Habitat, spatial and temporal drivers of diversity patterns in a wild bee assemblage

Across Europe conservation actions have been implemented to mitigate the decline of pollinators in agricultural landscapes. However, recent concerns have appeared about their efficiency to promote pollinator diversity. To increase the efficiency of these interventions, one must acquire a better knowledge of the target species diversity patterns and its sources of variations at different spatial and temporal scales. This study sets out to identify the main sources of variation in wild bee assemblages at a regional scale (450 km²) in mass-flowering crops and semi-natural habitats. During three consecutive sampling years, we monitored bee diversity and its temporal and spatial turnovers. We show that an intensive agricultural landscape in western France can hold nearly 200 wild bee species at a regional scale, i.e. 20 % of the whole bee fauna known in mainland France. Wild bee diversity was 3–4 times lower in oleaginous crops than in semi-natural habitats, with a substantial number of these being social and gregarious species. Spatial and seasonal species turnover in semi-natural habitats explained 28.6 and 34.3 %, respectively, of regional species richness. Given the importance of the spatial component of the bee diversity turnover, we suggest wild bee conservation efforts should be carried out at relevant spatial scales. The spatial turnover was estimated to be steeper within 50 km² scales. This provides an order of magnitude for the spatial extent of relevant conservation units within which one may concentrate conservation efforts in order to optimise the number of species promoted per surface area.