Species-habitat networks reveal key habitats for landscape-level wild bee conservation.

Most agricultural landscapes are composed of a variety of habitats. A landscape perspective is needed to understand biodiversity decline, but many studies focus on single habitat types. In addition, the use of local resources by species within and across habitats implies that species and their habitats are linked in species-habitat networks. However, studies on these networks are scarce. Here, we used grid-based sampling to assess wild bees at 224 sampling locations across all major habitat types, that is, arable land, grassland, forest and orchard, in 14 differently composed agricultural landscapes of Southern Germany. We assigned wild bees to habitat types based on the dominant habitat cover surrounding their sampling location to establish species-habitat networks and assessed how these networks differed in modularity and robustness to habitat loss. Orchards harbored more wild bees than expected based on their proportional cover in the landscape, indicating a preference for this extensively managed but threatened habitat by wild bees. Orchards also supported the highest species richness and proportion of oligolectic wild bees, while forests harbored the lowest richness and more social species. Landscape diversity affected both structure and robustness of bee-habitat networks in response to the simulated loss of habitats. Networks in more diverse landscapes had higher modularity but tended to be less robust, showing that greater landscape diversity and modularity do not necessarily buffer against the effects of habitat loss. However, this effect appeared to be mainly driven by increases in network size, as standardized modularity and robustness (z-scores) were not affected by landscape diversity. We could show that species-habitat networks are a powerful tool to inform ecologists and policy makers about the importance of key habitats and landscape diversity for species conservation. Key habitats for wild bee conservation include extensively managed habitats like traditional orchards. Nevertheless, all habitat types support a similar proportion of endangered species, emphasizing the importance of a diverse landscape. Conserving wild bees requires a variety of complementary habitats at the landscape scale and must consider the management of traditional and intensively managed habitats alike. Policy measures targeting landscape diversity are urgently needed.