Data from: Why are some plant—nectar robber interactions commensalisms?

Many plants that bear hidden or recessed floral nectar experience nectar robbing, the removal of nectar by a floral visitor through holes pierced in the corolla. Although robbing can reduce plant reproductive success, many studies fail to find such effects. We outline three mechanistic hypotheses that can explain when interactions between plants and nectar-robbers should be commensilistic rather than antagonistic: the non-discrimination (pollinators do not avoid robbed flowers), visitor prevalence (robber visitation is rare relative to pollinator visitation), and pollen saturation (stigmas receive sufficient pollen to fertilize all ovules with one or very few pollinator visits) hypotheses. We then explore these mechanisms in the North American subalpine, bumble bee-pollinated and nectar-robbed plant Corydalis caseana (Fumariaceae). We first confirmed that the effects of nectar robbing on female reproductive success were neutral in C. caseana. We then tested the three mechanisms underlying neutral effects using a combination of observational studies and experiments. We found evidence for all three mechanisms. First, consistent with the non-discrimination hypothesis, pollinators failed to discriminate against experimentally robbed flowers or inflorescences even though naturally robbed flowers offered significantly lower nectar rewards than unrobbed flowers. Second, C. caseana was more commonly visited by pollinators than by nectar robbers, in accordance with the visitor prevalence hypothesis. Third, stigmas of virgin (unvisited) flowers as well as those visited once by pollinators were saturated with pollen, with all stigmas bearing pollen loads several orders of magnitude higher than the number of ovules per fruit, consistent with the pollen saturation hypothesis. Our investigation of the mechanisms driving the commensal outcome of nectar robbing in this system deepens our understanding of the ecology of nectar robbing and contributes to a more general understanding of the variation in the outcomes of interactions between species.

许多具有隐蔽或深藏型花蜜的植物会遭遇盗蜜（nectar robbing）——即访花者通过在花冠上打孔取走花蜜的行为。尽管盗蜜会降低植物的繁殖成功率，但诸多研究却未能观测到此类负面影响。我们提出三项机制性假说，用以解释植物与盗蜜者之间的相互作用何时会呈现偏利共生（commensalistic）而非拮抗（antagonistic）效应：其一为非歧视假说（传粉者不会回避已遭盗蜜的花朵）；其二为访者相对丰度假说（盗蜜者的访花频率远低于传粉者）；其三为花粉饱和假说（柱头接收的花粉足以通过一次或极少几次传粉授精所有胚珠）。随后，我们在北美亚高山带的熊蜂传粉、同时遭盗蜜的卡氏紫堇（Corydalis caseana，紫堇科Fumariaceae）中对上述机制展开了探究。我们首先证实，盗蜜对卡氏紫堇的雌性繁殖成功率并无显著影响。随后，我们结合观测研究与控制实验，对介导这一中性效应的三项机制进行了检验。我们的研究结果为三项假说均提供了支持证据：其一，符合非歧视假说：尽管自然状态下遭盗蜜的花朵其花蜜报酬显著低于未遭盗蜜的花朵，但传粉者并未表现出对人工盗蜜后的花朵或花序的回避行为。其二，符合访者相对丰度假说：卡氏紫堇的访花者以传粉者为主，盗蜜者的访花频率相对较低。其三，符合花粉饱和假说：未受访的处女花柱头以及仅被传粉者访问过一次的柱头均已被花粉饱和，所有柱头的花粉负载量均较每果所含胚珠数量高出数个数量级。我们对该系统中盗蜜产生偏利共生效应的机制研究，深化了学界对盗蜜生态学的认知，并为理解物种间相互作用结果的多样性提供了更为普适的理论视角。