Ness J H, Morris W F, Bronstein J L
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721, USA.
Ecology. 2006 Apr;87(4):912-21. doi: 10.1890/0012-9658(2006)87[912:iqaqom]2.0.co;2.
Generalized, facultative mutualisms are often characterized by great variation in the benefits provided by different partner species. This variation may be due to differences among species in the quality and quantity of their interactions, as well as their phenology. Many plant species produce extrafloral nectar, a carbohydrate-rich resource, to attract ant species that can act as "bodyguards" against a plant's natural enemies. Here, we explore differences in the quality and quantity of protective service that ants can provide a plant by contrasting the four most common ant visitors to Ferocactus wislizeni, an extrafloral nectary-bearing cactus in southern Arizona. The four species differ in abundance when tending plants, and in the frequency at which they visit plants. By adding surrogate herbivores (Manduca sexta caterpillars) to plants, we demonstrate that all four species recruit to and attack potential herbivores. However, their per capita effectiveness in deterring herbivores (measured as the inverse of the number of workers needed to remove half of the experimentally added caterpillars) differs. Using these among-species differences in quality (per capita effectiveness) and quantity (number of workers that visit a plant and frequency of visitation), we accurately predicted the variation in fruit production among plants with different histories of ant tending. We found that plant benefits (herbivore removal and maturation of buds and fruits) typically saturated at high levels of ant protection, although plants could be "well defended" via different combinations of interaction frequency, numbers of ant workers per interaction, and per capita effects. Our study documents variation among prospective mutualists, distinguishes the components of this variation, and integrates these components into a predictive measure of protection benefit to the plant. The method we used to average saturating benefits over time could prove useful for quantifying overall service in other mutualisms.
广义的兼性互利共生通常表现为不同伙伴物种所提供的益处存在很大差异。这种差异可能是由于物种间相互作用的质量和数量以及物候不同所致。许多植物物种会分泌花外蜜，这是一种富含碳水化合物的资源，用以吸引蚂蚁物种，这些蚂蚁可充当植物抵御天敌的“保镖”。在此，我们通过对比四种最常见的访问韦氏强刺球果仙人掌（一种生长在亚利桑那州南部、带有花外蜜腺的仙人掌）的蚂蚁，来探究蚂蚁为植物提供保护服务的质量和数量差异。这四个物种在照料植物时的数量以及访问植物的频率各不相同。通过在植物上添加替代食草动物（烟草天蛾幼虫），我们证明所有这四个物种都会招募并攻击潜在的食草动物。然而，它们在威慑食草动物方面的人均效力（以移除一半实验添加幼虫所需的工蚁数量的倒数来衡量）有所不同。利用这些物种间在质量（人均效力）和数量（访问植物的工蚁数量及访问频率）上的差异，我们准确预测了有着不同蚂蚁照料历史的植物之间果实产量的差异。我们发现，尽管植物可以通过相互作用频率、每次相互作用的工蚁数量以及人均效力的不同组合得到“良好保护”，但植物的益处（食草动物移除以及芽和果实的成熟）通常在高水平的蚂蚁保护下会趋于饱和。我们的研究记录了潜在互利共生者之间的差异，区分了这种差异的组成部分，并将这些组成部分整合为对植物保护益处的预测指标。我们用于对随时间饱和的益处进行平均的方法，可能对量化其他互利共生关系中的总体服务有用。
