Abstract
With many plant–pollinator interactions undergoing change as species’ distributions shift, we require a better understanding of how the addition of new interacting partners can affect plant reproduction. One such group of 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 behavior 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.
Highlights
Many plant–pollinator interactions are undergoing change due to multiple anthropogenic influences (González-Varo et al, 2013; Goulson et al, 2015)
A floral visitor bites a hole in the corolla (“primary robbing”) or utilizes an existing hole previously created by another robber (“secondary robbing”) to feed from nectar, which often results in no contact with the stigmas or anthers and no contribution to pollination (Inouye, 1983; Rojas- Nossa et al, 2016)
The pollination biology of this species is well known in the native range, yet to our knowledge, only one record exists of nectar robbing in populations across Europe, associated with robbing specialist Bombus wurflenii (Reinig & Rasmont, 1988), whereas in American populations we found that the plants are robbed at a high rate (Figure 1)
Summary
Many plant–pollinator interactions are undergoing change due to multiple anthropogenic influences (González-Varo et al, 2013; Goulson et al, 2015). Negative effects of robbers include damage to the reproductive organs, a reduction of the attractiveness of the floral display, and exhaustion of the nectar reward, all of which could potentially alter the foraging behavior of legitimate pollinators that are required for plant reproductive success (Irwin et al, 2010). In these populations, the bumblebees Bombus hortulanus and B. rubicundus, and some species of hummingbird and flower piercers (specialized robbers in the genus Diglossa) frequently feed on nectar from D. purpurea by robbing the flowers. It is possible that, through increased replenishment of nectar, robbing could reduce energetic resources available for fruit production, regardless of effects on bee visitation (Navarro, 2001) We measured both male and female components of reproduction for plants and quantified the visitation rates by naturally foraging bumblebees to each treatment. We recorded other measures of bumblebee visitation patterns on inflorescences, including time spent visiting flowers and proportion of an inflorescence's flowers that were visited in a foraging bout
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