Influences of oilseed rape area and aggregation on pollinator abundance and reproductive success of a co-flowering wild plant

Mass-flowering crops such as oilseed rape (Brassica napus) are attractive for some pollinators. Consequently, flowering oilseed rape fields may compete for pollinators with co-flowering wild plants occurring in adjacent non-crop habitats. We evaluated the influence of flowering oilseed rape through both its area and its configuration in the landscape on pollinator abundance and on the reproductive success of a co-flowering wild plant functionally close to oilseed rape in surrounding grasslands. We monitored the production of seeds of a pollinator-generalist plant, Cardamine pratensis, and sampled pollinator communities in 22 permanent grasslands. To understand the landscape effect of oilseed rape, we quantified its area and aggregation in the landscape at 1 000 m around each grassland. We measured the flower abundance in the grasslands and C. pratensis conspecific plant abundance to consider local influences. At the landscape scale, C. pratensis seed set was unaffected by the aggregation of oilseed rape patches but it was reduced by increasing oilseed rape area in the landscape. At the local scale, the conspecific plant abundance enhanced seed set (i.e. facilitation effect), whereas flower abundance in the grassland reduced it (i.e. competition effect). Analyses revealed a significant positive correlation between seed set and honeybee abundance, but no correlation was found between seed set and solitary bee abundance. Oilseed rape aggregation and the area of semi-natural habitats in the landscape had a weak negative effect on honeybee abundance. Oilseed rape area had either a positive or a negative effect on solitary bee abundance depending on oilseed rape aggregation in the landscape. Flowering oilseed rape can either enhance or reduce the pollinator abundance and the reproductive success of wild plants depending on its quantity and spatial arrangement. Guidance for plant and pollinator conservation should therefore consider both landscape aspects.