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Abstract Wild bees are the most important group of pollinators among insects. In agroecosystems, they promote successful pollination and thus ensure crop yields. In order to enhance species diversity and abundance of wild bees in farmland, it is essential to study the impact of the surrounding landscape as well as local habitat factors on the occurrence of wild bees, and identify and implement biodiversity enhancing measures. We established a national-wide monitoring program for wild bees in Austria’s main agricultural regions for the first time (project BINATS 2). This was achieved through a stratified random sampling design comprising 1,000 transects across 100 test areas. In this paper, we investigate the direct and indirect effects of local habitat and landscape factors on the richness of wild bee species, their abundance and selected functional traits (ground nesting, above-ground nesting, solitary, social and polylectic bees, Fabaceae specialists, bumblebees). Additonally, we analyse whether these effects vary between the early and late seasons using structural equation modeling (SEM). Our results show that wild bee species richness and abundance are directly positively related to flower cover and semi-natural open landscapes. Grasslands and insect pollinated crops contribute to increased local flower cover, and thereby indirectly enhance wild bee species richness and abundance. Although insect pollinated crops have a stronger positive effect on flower cover in spring compared to summer, the effect on wild bee species richness and abundance does not vary significantly throughout the season. Solitary and above-ground nesting bees are also positively influenced by the percentage of organic farming in the different test areas, while bumblebees – beside above-ground nesting wild bees – are specifically enhanced by plant diversity. In addition, bumblebees are the only group directly positively affected by insect pollinated crops. These results demonstrate the strong and complex dependency of wild bees on flowering fodder plants and semi-natural open landscapes. This highlights the risks of their decline within agricultural landscapes caused by a reduction of various landscape structures and diverse floral richness present over long periods of time. We also confirm that organic farming is a crucial factor for promoting certain wild bee traits.
Ockermüller et al. (Mon,) studied this question.