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

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.