Phenology and flower species availability define wild bee communities on river embankments

Linear landscape elements contribute to pollinator conservation, with enhancing floral resources as the most common restoration practice. However, theory predicts that floral resource availability is not always the limiting factor, as other, non-floral resources can become more limiting once enough flowers are available. We currently lack large-scale studies covering a vegetation richness gradient that could reveal the predicted saturation that should occur with floral resource limitation relief. Here, we present a comprehensive analysis of bee communities from 157 dike grasslands covering a flower richness gradient in the Netherlands in relation to vegetation composition and landscape characteristics. We show that wild bee abundance and species richness increased with more flowering plant species present but saturated at low flower species richness. This early saturation suggests that even modest conservation efforts could effectively alleviate floral resource limitation for pollinators, but that further floral resource increase could lead to diminishing returns. Nevertheless, red-listed species occurrence increased with flower species richness without saturating effects and correlated more strongly with landscape-level factors. Additionally, our results highlight the unique conservational value of river embankments for wild bees, as we encountered nearly half (156) of all Dutch bee species, with nearly 10% of individuals belonging to red-listed species.