Dryness, wetness and temporary flooding reduce floral resources of plant communities with adverse consequences for pollinator attraction

Climate change alters precipitation regimes world‐wide and is regarded as a major threat for pollinators and pollination services. Yet, not much is known on how wetter as well as drier conditions affect food resources for pollinators and pollinator attraction in a multi‐species community context. It is unclear how community shifts under changed hydrological conditions might affect pollinators. This mesocosm study addresses existing research gaps by investigating effects of drought, wetness and temporary flooding on floral resources and pollinator attraction within a plant community, initially including nine insect‐pollinated and six wind‐pollinated species. Floral resources were assessed over three growing seasons. I monitored community descriptors (percentage of herbs and herbaceous species richness, presence of flowering insect‐pollinated individuals), floral traits (flower height, size and weight, floral sugar content of one key species) and proxies for plant fitness (pollinator attraction, seed and flower weight of one key species). Wetness and especially dryness decreased the species richness and biomass proportion of insect‐pollinated herbaceous species and flowering herbaceous plant individuals occurred less frequently. Permanent wetness decreased the floral sugar content of the key species Trifolium pratense. Wet, temporary flooded and especially dry communities were visited by pollinators less often. Active pollinators spent less time within wet and flooded communities, while not at all visiting dry communities. Seed and flower weight of the obligate xenogamous T. pratense in flooded and dry communities decreased, indicating negative consequences for plant fitness caused by a lack of pollination. Synthesis. While dryness had negative effects for floral resources both in terms of community descriptors and floral traits, negative effects of wetness and temporary flooding were mostly caused by a decrease of insect‐pollinated herbaceous species. The study thus indicates that shifts in plant community composition are decisive for a predictive understanding of plant–pollinator interactions under environmental change, but these have been neglected in past research. Changing precipitation patterns will adversely affect floral resources and pollinator attraction in agriculturally used temperate grassland, which might have widespread negative consequences for pollination services and food security in the coming decades.