Phenological imbalance in the supply and demand of floral resources: Half the pollen and nectar produced by the main autumn food source, Hedera helix, is uncollected by insects

Abstract

Declines in floral resources, pollen and nectar, are considered one cause of pollinator decline. However, the supply and demand of floral resources can vary temporally. In Britain, autumn has been suggested as a period of high floral resource availability due to the flowering of ivy (Hedera helix), a common native plant, combined with fewer insects active during this season. Here, we directly quantified the proportion of pollen and nectar produced by ivy, the primary autumn food source, which is uncollected by the flower-visiting insect community. We quantified the proportion of nectar produced but uncollected by comparing the mass of nectar sugar accumulated in insect-accessible versus inaccessible ivy flowers and by surveying the presence of wasted, crystalised, nectar on flowers. Pollen wastage was quantified by comparing pollen counts on anthers at the start of anthesis versus anthers dropped from ivy flowers. Approximately, half the floral resources, 59% nectar and 44% pollen, were uncollected by the flower-visiting insect community in autumn. As ivy flowers supply most of the available nectar and pollen in autumn, our results show that a large proportion of all floral resources are wasted in autumn. Our results are the first to show that a season can be characterised by a large surplus of floral resources relative to collection by flower-visiting insects. These results demonstrate the importance of considering seasonal variation in floral resources in the conservation of bees and other flower-visiting insects.