Why are some plant-nectar robber interactions commensalisms?

Novel nectar robbing negatively affects reproduction in Digitalis purpurea

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.

Our Current Understanding of Commensalism

Commensalisms, interactions between two species in which one species benefits and the other experiences no net effect, are frequently mentioned in the ecological literature but are surprisingly little studied. Here we review and synthesize our limited understanding of commensalism. We then argue that commensalism is not a single type of interaction; rather, it is a suite of phenomena associated with distinct ecological processes and evolutionary consequences. For each form of commensalism we define, we present evidence for how, where, and why it occurs, including when it is evolutionarily persistent and when it is an occasional outcome of interactions that are usually mutualistic or antagonistic. We argue that commensalism should be of great interest in the study of species interactions due to its location at the center of the continuum between positive and negative outcomes. Finally, we offer a roadmap for future research.