The effect of experimental pollinator decline on pollinator-mediated selection on floral traits

Human-mediated environmental change, by reducing mean fitness, is
hypothesized to strengthen selection on traits that mediate interactions
among species. For example, human-mediated declines in pollinator
populations are hypothesized to reduce mean seed production by increasing
the magnitude of pollen limitation and thus strengthen pollinator-mediated
selection on floral traits that increase pollinator attraction or pollen
transfer efficiency. To test this hypothesis, we measured two female
fitness components and six floral traits of Lobelia siphilitica plants
exposed to supplemental hand-pollination, ambient open-pollination, or
reduced open-pollination treatments. The reduced treatment simulated
pollinator decline, while the supplemental treatment was used to estimate
pollen limitation and pollinator-mediated selection. We found that plants
in the reduced pollination treatment were significantly pollen-limited,
resulting in pollinator-mediated selection for taller inflorescences and
more vibrant petals, both traits that could increase pollinator
attraction. This contrasts with plants in the ambient pollination
treatment, where reproduction was not pollen-limited and there was no
significant pollinator-mediated selection on any floral trait. Our results
support the hypothesis that human-mediated environmental change can
strengthen the selection on traits of interacting species and suggest that
these traits have the potential to evolve in response to changing
environments.