In simplified agricultural landscapes, the continuous availability of food and nesting resources that pollinators depend on is limited, threatening pollinator populations and the essential pollination services they provide. Winter oilseed rape (OSR) could provide a temporary fix since it produces large amounts of nectar and pollen during spring flowering, potentially compensating for the scarcity of floral resources in intensively farmed areas. However, it hasn’t been entirely clear what happens to pollinator communities in nearby semi-natural habitats over the rest of the season.
To investigate, a Safeguard researcher team from the University of Würzburg sampled bees across 40 semi-natural calcareous grasslands in Germany. They examined how OSR cover in the surrounding landscape affected bee abundance throughout the season, and whether grassland size, habitat quality, and bee life-history traits shaped those effects.
The results differ strikingly between bee groups. The abundance of most wild bees was positively related to landscape OSR cover during OSR flowering. The most likely explanation is spillover: wild bees foraging on flowering OSR moved into adjacent calcareous grasslands, which offer the abundant nesting sites that OSR fields cannot provide. For bumblebees, the pattern was different - their abundance increased with OSR cover only after flowering, and in large grasslands. Being less restricted in their nesting needs, bumblebees appear to build up populations in OSR-rich landscapes and then benefit from the continuous floral resources that large calcareous grasslands provide once the crop has finished flowering.
However, the benefits were not equal or lasting. The positive effect on wild bees was short-lived, tied to the flowering window itself, while for bumblebees, benefits were delayed.
The findings highlight that calcareous grasslands play an important role: they serve as critical nesting habitat during OSR flowering and as high-quality foraging habitat afterwards. Mass-flowering crops and semi-natural habitats are not interchangeable. Pollinator communities in calcareous grasslands can benefit from OSR in the surrounding landscape, but the conservation of these grasslands remains essential.
Read the full study here.