Wild Bee Conservation: Habitat Heterogeneity and Ecological Improvement in Agricultural Land With Open Grassland.

Wild bee populations are declining due to habitat loss and degradation. Flower-rich heterogenous grasslands have the potential to compensate for the loss and provide valuable habitat for wild bees. To investigate the effects of different levels of habitat heterogeneity and of ecological habitat improvement to increase foraging resources on wild bee communities, we comparatively studied two Central European grassland types in Austria (AT) and Luxembourg (LU) using pan traps. We collected and identified > 5000 individuals representing considerable fractions of wild bee fauna of these countries and characterised habitat heterogeneity (number of habitat elements, ecotone lengths) using GIS data and flower-colour abundance (white, yellow, blue) using drone imaging. Our findings suggest that macro-scale heterogeneity alone may not drive wild bee assemblages in LU, pointing to the potential overriding importance of unmeasured microhabitat variables, such as nesting substrates and taxonomic floral composition. In AT, however, wild bee abundance increased with habitat heterogeneity as measured by the length of ecotones with trees and bushes. Ecological improvement of habitat was important in LU, in that formerly intensively farmed land with sown wildflower fallows hosted higher richness and abundance than extensively used grassland. In AT, in contrast, higher species richness and abundance occurred in extensively used grassland than in formerly intensively farmed land with wildflower strips; here, overall increased floral resource abundance on ecologically improved sites seemed to have less influence, and flower species diversity, the quality of floral resources or any combination of flower properties, may have been more important than just its quantity as assessed by flower-colour abundance. In conclusion, we emphasise the relevance of regional habitat heterogeneity to wild bees and the opportunities by ecological improvement of intensively farmed land. Conservation strategies should be tailored to regional landscape contexts, prioritising extensive grassland protection in Austria and targeted wildflower fallow establishment in Luxembourg.