The Birds and the Bees: Using Selective Exclusion to Identify Effective Pollinators of African Tree Aloes

Abstract

Bees are frequent visitors to plants that, on the basis of floral traits, would be expected to be specialized for bird pollination. We conducted bird exclusion experiments to establish the contribution that bees make to the female component of pollination success of five “ornithophilous” Aloe species in the succulent‐rich subtropical thicket vegetation in southeastern South Africa. Experiments showed that all five aloes are genetically self‐incompatible and require cross‐pollination for seed set. Seed set in flowers exposed to both bird and bee visitors was generally severalfold higher than in flowers exposed to bee visitors only. The magnitude of this effect varied greatly among species and was affected by floral traits and bee behavior. In three aloes where the relatively short corolla tubes are blocked by exserted filaments, bees made a negligible contribution to seed production. However, in two aloes with long tubular flowers and included filaments, bees often crawled into the perianth tube to obtain nectar and contributed to seed production. Pollen‐collecting honeybees contribute the least to pollination and may even impact negatively on aloe reproductive success by acting as pollen thieves.