Data from: Coexistence through mutualist-dependent reversal of competitive hierarchies

Mechanisms that allow for the coexistence of two competing species that share a trophic level can be broadly divided into those that prevent competitive exclusion of one species within a local area, and those that allow for coexistence only at a regional level. While, the presence of aphid-tending ants can change the distribution of aphids among host-plants, the role of mutualistic ants has not been fully explored to understand coexistence of multiple aphid species in a community.The tansy plant (Tanacetum vulgare) hosts three common and specialised aphid species, with only one being tended by ants. Often, these aphids species will not coexist on the same plant, but will coexist across multiple plant hosts in a field. In this study, we aim to understand how interactions with mutualistic ants and predators affect the coexistence of multiple species of aphid herbivores on tansy. We show that the presence of ants drives community assembly at the level of individual plant, i.e. the local community, by favouring one ant-tended species, Metopeurum fuscoviride, while preying on the non-tended Macrosiphoniella tanacetaria and, to a lesser extent, Uroleucon tanaceti. Competitive hierarchies without ants were very different from those with ants. At the regional level, multiple tansy plants provide a habitat across which all aphid species can coexist at the larger spatial scale, while being competitively excluded at the local scale. In this case, ant mutualist-dependent reversal of the competitive hierarchy can drive community dynamics in a plant-aphid system.

能够使共享同一营养级（trophic level）的两种竞争物种实现共存的机制，大致可分为两类：一类是在局域范围内阻止某一物种被竞争排除（competitive exclusion），另一类则仅能在区域尺度上实现物种共存。尽管护蚜蚁（aphid-tending ants）可改变蚜虫在寄主植物间的分布，但针对互利共生蚂蚁（mutualistic ants）在群落内多种蚜虫物种共存机制中的作用，目前尚未得到充分探究。
菊蒿（Tanacetum vulgare）可寄主三种常见且专性的蚜虫物种，其中仅有一种会受到蚂蚁看护。通常情况下，这些蚜虫物种不会在同一植株上共存，但会在田间的多株寄主植物间实现共存。
本研究旨在探究与互利共生蚂蚁及天敌的互作，如何影响菊蒿上多种植食性蚜虫（aphid herbivores）物种的共存格局。研究结果显示，蚂蚁的存在会通过偏向性庇护一种受蚂蚁看护的蚜虫物种——黑尾长管蚜（Metopeurum fuscoviride），同时捕食未受蚂蚁保护的短尾菊蒿蚜（Macrosiphoniella tanacetaria），并在一定程度上捕食菊蒿长管蚜（Uroleucon tanaceti），从而驱动单株植株（即局域群落（local community））层面的群落组装。
无蚂蚁存在时的竞争等级（competitive hierarchies）与有蚂蚁时存在显著差异。在区域尺度上，多株菊蒿构成了一个可供所有蚜虫物种在更大空间尺度下共存的栖息生境，而在局域尺度上部分蚜虫物种则会被竞争排除。
在此情境下，依赖互利共生蚂蚁的竞争等级逆转，可驱动植物-蚜虫系统中的群落动态。