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

Abstract

River embankments cover extensive areas in estuaries worldwide, yet their potential for pollinator conservation remains largely unexplored. Even more so than road verges, these embankments may offer widespread habitat for pollinators due to their size and sun-facing warm slopes. Here we present a comprehensive analysis of bee communities from 157 dike grasslands along the lower branches of the Meuse and Rhine rivers in the Netherlands in relation to vegetation composition and landscape characteristics. We encountered nearly half (156) of all Dutch bee species. Almost 10% of all individuals belonged to red-listed species, especially in early spring when we observed rich ground nesting bee communities with their associated kleptoparasites. Wild bee abundance and diversity 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. In contrast to the overall pattern, red-listed species occurrence increased with flower diversity without saturating effects and correlated stronger with landscape level factors. Together, these results highlight the unique conservational value of river embankments for wild bees and illustrate the unused potential at many dike grasslands worldwide.