Solitary bee abundance and species richness in dynamic agricultural landscapes

• Accounting for the cumulative effects of field cover is relevant for studying bee communities. • The introduction of temporary grasslands in rotations positively affects solitary bees. We investigated the influence of the landscape structure on solitary bee abundance and species richness in an agricultural area of western France. We focused on the role of semi-natural habitats, oilseed rape (OSR) and other crops. Our originality was to consider not only the spatial heterogeneity of the crop field mosaic but also its temporal heterogeneity through the crop rotations. Solitary bees were caught with colored pan traps in 15 margins of OSR fields and 35 margins of non-OSR fields. We found that solitary bee abundance and species richness were higher in margins of OSR fields than in margins of non-OSR fields, showing that early spring-flying species widely use this mass flowering crop. However the high number of rare species in margins of non-OSR fields (21 species recorded exclusively in these margins) highlighted the importance of these margins for the conservation of solitary bee diversity. The influence of the landscape context on solitary bees showed contrasted results according to the type of margin and the spatial scale. At the finest spatial scale, abundance in margins of OSR fields increased with increasing proportion of non-flowering crops (cereals and temporary grasslands) in the current year. At large spatial scales, solitary bee abundance in margins of non-OSR fields was positively affected by the proportion of long-term grasslands. Moreover, the proportion of fields only sown with cereals during the last 5 years negatively affected abundance and species richness at the large scales whereas the proportion of mixed fields (at least 1 year of grassland in the rotation) had a positive effect on species richness. We showed that accounting for the cumulative effects of field cover and management through the crop rotations is relevant for studying solitary bee communities. The introduction of less intensive covers, such as temporary grasslands, in cereal rotations positively influences these communities.