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

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 (1km x
1km) in Germany, i.e. 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 analyzed 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 community weighted 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 (V. 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
semi-natural 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.