Spatio-temporal complementarity of floral resources sustains wild bee pollinators in agricultural landscapes

Targeted conservation and promotion of wild bees in agroecosystems requires understanding of relationships between different groups of bees and available floral resources across land-use types during the season and at the landscape scale. Here, we quantified floral resource amount and diversity across habitat types at different times during the season at the scale of entire landscapes (500 m radius) across 20 different agricultural landscapes. Moreover, we examined whether floral resource metrics obtained from these high-resolution floral resource maps are more suitable to assess and predict abundance and species richness of different bee pollinator groups, including rare species and important crop pollinators, sampled in these agricultural landscapes compared to traditional land-cover metrics. Floral resource availability shifted from flower-rich woody vegetation early in the season to herbaceous vegetation such as grasslands and crops later in the season, which was associated with a ten-fold decline in overall floral resource availability. Forest edges had highest per-area floral contributions in spring, whereas floral diversity of grasslands, in particular if extensively managed, was continuously high. Total wild bee species richness, as well as rare species richness and abundance of important crop pollinators, increased with floral resource availability and/or diversity contributed by forest edges and floral diversity of permanent grasslands. Rare bee richness was also positively related to floral resource amount provided by crops. Total bee richness and important crop pollinator abundance, but not rare bee richness, were positively related to overall floral resource amount, but not floral diversity, in the landscape. Floral resource maps based on floral resource supply by major habitat types early or late in the season predicted wild bee species richness (R2 =0.61) better than traditional descriptors of landscape composition such as proportion of semi-natural habitat. The pronounced temporal shifts in floral resource availability for pollinators from woody towards herbaceous vegetation during the season highlights the importance of taking a landscape-scale perspective on pollinator conservation. Our findings indicate that both rare bees and important crop pollinators benefit from complementary floral resources of forest edges and grasslands in agroecosystems. This reveals a potential synergy between the conservation of endangered species and the landscape scale management to promote pollination services. Our study also highlights that floral resource maps are useful tools in supporting more targeted pollinator conservation and pollination service management at the landscape level.