Functional groups of wild bees respond differently to faba bean Vicia faba L. cultivation at landscape scale

Abstract

1. Concerns about insect declines are growing and the provisioning of ecosystem services like pollination may be threatened. To safeguard biodiversity, greening measures were introduced within the reform of the EU's Common Agricultural Policy. One measure commonly applied by farmers is the cultivation of nitrogen fixing crops. Although underlying studies are largely missing, this measure is criticized as providing no significant biodiversity benefit. 2. Using a landscape-scale approach, we selected 30 paired study landscapes (1 km × 1 km) in Germany, that is, 15 study landscapes with faba bean (FB) fields (Vicia faba L.) and 15 without any grain legumes. Flower-visiting wild bees were recorded with transect walks at the field margins of different crops using a stratified sampling approach. We analysed the effect of FB cultivation and landscape composition on the abundance and species richness of wild bees as well as on the functional composition of the bee communities. 3. Bumblebee densities (Bombus spp. Latreille) were more than twice as high in FB compared to control landscapes after the flowering of the beans. Non-Bombus wild bee densities, however, were not affected by FB cultivation, but were enhanced by increasing amounts of semi-natural habitats (SNH). 4. After the beans' blooming had ceased, FB landscapes had a higher proportion of wild bees collecting pollen from Fabaceae than control landscapes. The communityweighted means for bee size, measured as intertegular distance, were not affected by FB cultivation, but we found smaller species and species with shorter tongues with an increasing percentage of SNH. 5. Synthesis and applications. The cultivation of faba bean Vicia faba L. strongly increased bumblebee densities throughout the season. This indicates that also on-field greening measures can support biodiversity. Nevertheless, since only functional groups adapted to faba bean benefit, measures to promote seminatural habitats in agricultural landscapes need to be implemented. We conclude that the combination of on-and off-field measures is essential to maintain farmland biodiversity and the Common Agricultural Policy should furthermore promote both.

Identity of mass-flowering crops moderates functional trait composition of pollinator communities

“…Faba bean cultivation affected the proportion of social bees and of faba bean foragers in SNH, depending on the sampling time. While we observed higher proportions in control than in bean landscapes when faba beans were in full bloom, the effect was reversed afterwards as also shown in Beyer et al (2020). While a higher proportion of social bees might result from an increase of social bees or a decrease of solitary bees or both, those effects were probably driven by bumblebees, the most dominant group of social bees in our study landscapes.…”

“…We selected 30 paired 1 km 9 1 km study landscapes in three different regions in Germany (see Beyer et al 2020) (Fig. 1).…”

“…Landscape selection was based on a similar landscape composition of the study landscapes within a pair. For further details see Beyer et al (2020).…”

“…We showed that faba bean cultivation enhances bumblebee densities in SNH, which is in accordance with our hypothesis. Accordingly, bumblebee densities were enhanced in faba bean landscapes, when sampling within field margins (Beyer et al 2020). Bumblebees are known to preferentially forage on flowers, that offer high quality, protein-rich pollen, like Fabaceae (Somme et al 2015) and are, next to honeybees, the most common flower visitors to faba 1).…”

“…While SNH flower cover was the most important determinant for non-Bombus wild bee densities recorded in the respective SNH, the cover of SNH had no effect on wild bees. However, when investigating wild bees outside SNH, such as field margins or crop fields, SNH cover has been shown to be an important driver of wild bee densities (Nayak et al 2015;Beyer et al 2020). Wild bees have been shown to shift between different crop and non-crop habitats in agricultural landscapes according to their flowering phenology and changing resource availability Fig.…”