Data from: Water stress strengthens mutualism among ants, trees, and scale insects

Abiotic environmental variables strongly affect the outcomes of species interactions. For example, mutualistic interactions between species are often stronger when resources are limited. The effect might be indirect: water stress on plants can lead to carbon stress, which could alter carbon-mediated plant mutualisms. In mutualistic ant–plant symbioses, plants host ant colonies that defend them against herbivores. Here we show that the partners' investments in a widespread ant–plant symbiosis increase with water stress across 26 sites along a Mesoamerican precipitation gradient. At lower precipitation levels, Cordia alliodora trees invest more carbon in Azteca ants via phloem-feeding scale insects that provide the ants with sugars, and the ants provide better defense of the carbon-producing leaves. Under water stress, the trees have smaller carbon pools. A model of the carbon trade-offs for the mutualistic partners shows that the observed strategies can arise from the carbon costs of rare but extreme events of herbivory in the rainy season. Thus, water limitation, together with the risk of herbivory, increases the strength of a carbon-based mutualism.

非生物环境变量会显著影响物种相互作用的结果。例如，资源受限条件下，物种间的互利共生相互作用往往更强。此类影响可能存在间接路径：植物遭受水分胁迫可引发碳胁迫，进而改变以碳为媒介的植物互利共生关系。在蚁-植物互利共生体系中，植物为蚁群提供栖息场所，蚁群则保护植物免受植食者侵害。本文研究发现，在沿中美洲降水梯度分布的26个样地中，该广布性蚁-植物共生体系中双方的投入均随水分胁迫程度加剧而增加。在降水较少的区域，芳香破布木（Cordia alliodora）会通过取食韧皮部的介壳虫为阿兹特克蚁（Azteca）提供更多碳源——这类介壳虫可为蚂蚁提供糖类，而蚂蚁则会更好地保护合成碳源的叶片。当遭受水分胁迫时，树木的碳库规模更小。针对共生双方的碳权衡模型表明，本文观测到的策略可由雨季中罕见但极端的植食事件所带来的碳成本所解释。因此，水分限制与植食风险共同增强了以碳为基础的互利共生关系的强度。