Abstract
With many plant-pollinator interactions undergoing change, we require a
better understanding of how the addition of new interacting partners,
such as antagonists, can affect plant reproduction. One such group of
antagonistic floral visitors, nectar robbers, can deplete plants of
nectar rewards without contributing to pollination. The addition of
nectar robbing to the floral visitor assemblage could therefore have
costs to the plant´s reproductive output. We focus on a recent plant
colonist, Digitalis purpurea, a plant that in its native range is rarely
robbed, but experiences intense nectar robbing in areas it has been
introduced to. Here, we test the costs to reproduction following
experimental nectar robbing. To identify any changes in the behaviour of
the principal pollinators in response to nectar robbing, we measured
visitation rates, visit duration, proportion of flowers visited and rate
of rejection of inflorescences. To find the effects of robbing on
fitness, we used proxies for female and male components of reproductive
output, by measuring the seeds produced per fruit and the pollen export
respectively. Nectar robbing significantly reduced the rate of
visitation and lengths of visits by bumblebees. Additionally, bumblebees
visited a lower proportion of flowers on an inflorescence that had
robbed flowers. We found that flowers in the robbed treatment produced
significantly fewer seeds per fruit on average but did not export fewer
pollen grains. Our finding that robbing leads to reduced seed production
could be due to fewer and shorter visits to flowers leading to less
effective pollination. We discuss the potential consequences of new
pollinator environments, such as exposure to nectar robbing, for plant
reproduction.