Simulated Nectar Robbing Does Not Affect Pollinator-Mediated Selection on Floral Traits of Impatiens capensis

Abstract

Premise of research. Floral traits are thought to evolve primarily in response to pollinators, but other environmental factors can also affect selection on these traits. One such factor is nectar robbing. Robbers, which damage flowers and remove nectar but generally do not pollinate, could have direct effects on selection on floral traits and/or indirect effects on pollinator-mediated selection on these traits.Methodology. To test whether nectar robbing has indirect effects on pollinator-mediated selection on floral traits, we experimentally robbed Impatiens capensis flowers. For robbed and unrobbed flowers, we estimated directional selection differentials on five traits via pollen deposited per flower and seeds per fruit. We tested whether traits that were under selection via seeds per fruit were also under selection via pollen deposited per flower, as expected if selection is pollinator mediated. We then tested whether selection via both pollen deposited per flower and seeds per fruit differed between robbed and unrobbed flowers, as expected if robbing has indirect effects on pollinator-mediated selection.Pivotal results. There was significant selection via seeds per fruit for a larger landing pad, a larger corolla tube opening, and a longer nectar spur. However, these traits were not under selection via pollen deposited per flower. Selection on floral traits did not differ between robbed and unrobbed flowers, either via pollen deposited per flower or via seeds per fruit.Conclusions. We did not find evidence that nectar robbing has indirect effects on pollinator-mediated selection. Instead, there was no evidence of pollinator-mediated selection on floral traits of I. capensis in either the robbed or the unrobbed treatment. However, there was significant selection on three floral traits that was not caused by pollinators or robbing, suggesting that floral traits of I. capensis could evolve in response to selection by other unidentified environmental factors.