Reduced visitation to buzz-pollinated Cyanella hyacinthoides in the presence of other pollen sources in the hyperdiverse Cape Floristic Region

Many plant species have floral morphologies that restrict access to floral
resources, such as pollen or nectar, and only a subset of floral visitors
can perform the handling behaviours required to extract restricted
resources. Due to the time and energy required to extract resources from
morphologically complex flowers, these plant species potentially compete
for pollinators with co-flowering plants that have more easily accessible
resources. A widespread floral mechanism restricting access to pollen is
the presence of tubular anthers that open through small pores or slits
(poricidal anthers). Some bees have evolved the capacity to remove pollen
from poricidal anthers using vibrations, giving rise to the phenomenon of
buzz-pollination. These bee vibrations that are produced for pollen
extraction are presumably energetically costly, and to date, few studies
have investigated whether buzz-pollinated flowers may be at a disadvantage
when competing for pollinators’ attention with plant species that present
unrestricted pollen resources. Here, we studied Cyanella hyacinthoides
(Tecophilaeaceae), a geophyte with poricidal anthers in the hyperdiverse
Cape Floristic Region of South Africa, to assess how the composition and
relative abundance of flowers with easily accessible pollen affect bee
visitation to a buzz-pollinated plant. We found that the number of
pollinator species was not influenced by community composition. However,
visitation rates to C. hyacinthoides were reduced when the relative
abundances of flowers with more accessible resources were high. Visitation
rates were strongly associated with petal colour, showing that flower
colour is important in mediating these interactions. We conclude that
buzz-pollinated plants might be at a competitive disadvantage when many
easily accessible pollen sources are available, particularly when
competitor species share its floral signals.