Nectar bacteria, but not yeast, weaken a plant-pollinator mutualism

Mutualistic interactions are often subject to exploitation by species that are not directly involved in the mutualism. Understanding which organisms act as such ‘third-party’ species and how they do so is a major challenge in the current study of mutualistic interactions. Here, we show that even species that appear ecologically similar can have contrasting effects as third-party species. We experimentally compared the effects of nectar-inhabiting bacteria and yeasts on the strength of a mutualism between a hummingbird-pollinated shrub, Mimulus aurantiacus, and its pollinators. We found that the common bacterium Gluconobacter sp., but not the common yeast Metschnikowia reukaufii, reduced pollination success, seed set and nectar consumption by pollinators, thereby weakening the plant–pollinator mutualism. We also found that the bacteria reduced nectar pH and total sugar concentration more greatly than the yeasts did and that the bacteria decreased glucose concentration and increased fruct...互利共生相互作用常受到非直接参与该互利关系的物种的剥削。厘清哪些生物可充当此类‘第三方’物种，以及其作用机制，是当前互利共生相互作用研究领域的一大核心挑战。本研究证实，即便生态习性相近的物种，作为第三方物种时也可表现出截然相反的作用效果。我们通过实验对比了栖居花蜜的细菌与酵母菌对蜂鸟传粉灌木橙黄猴面花（Mimulus aurantiacus）及其传粉者之间互利关系强度的影响。研究发现，常见细菌葡糖杆菌属未定种（Gluconobacter sp.）而非常见酵母菌瑞氏梅奇酵母（Metschnikowia reukaufii），会降低传粉成功率、结籽率与传粉者的花蜜消耗量，进而削弱植物-传粉者的互利共生关系。此外我们还观测到，相较于酵母菌，细菌对花蜜pH值与总糖浓度的降幅更为显著，且细菌会降低葡萄糖浓度并提升果糖……