Landscape-level oilseed rape cover shapes seasonal patterns of wild bee abundance in conservation areas

In simplified agricultural landscapes, the continuous availability of food and nesting resources for pollinators, often associated with semi-natural habitats, is limited. This threatens pollinator populations and essential pollination services. Winter oilseed rape (OSR) can temporarily compensate for this lack of floral resources in spring by providing large amounts of nectar and pollen. While flowering OSR attracts pollinators, knowledge about its effects on pollinator populations in adjacent semi-natural habitats over the season remains vague. We therefore asked how OSR cover affects bee abundance in high-value semi-natural habitats throughout the season, and how habitat area, quality, and bee life-history traits modulate these effects. To answer this, we sampled bees in 40 semi-natural calcareous grasslands in Germany. Wild bee abundance (excluding bumblebees) in grasslands was positively related to landscape OSR cover during OSR flowering. Bumblebee abundance showed a positive relationship with OSR cover only after flowering, and only in large grasslands. Wild bees foraging on flowering OSR likely spilled over into calcareous grasslands which offer abundant nesting sites. Bumblebees, less restricted in nesting needs, built up populations in OSR-rich landscapes and benefited from continuous floral resources in large grasslands after OSR flowering. Although OSR positively affected wild bees, this effect was short-lived for non- Bombus bees and delayed for bumblebees. Our findings emphasize the ecological value of calcareous grasslands as high-quality foraging habitats after OSR flowering and crucial nesting sites during OSR flowering. We conclude that pollinator communities on calcareous grasslands can benefit from mass-flowering crops in the surrounding agricultural landscapes, but the conservation of these grasslands remains essential.