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.共享同一营养级的两种竞争物种得以共存的机制，大致可分为两类：一类可在局域范围内阻止某一物种被竞争排除，另一类则仅能在区域尺度上实现物种共存。尽管护蚜蚁的存在能够改变蚜虫在寄主植物间的分布格局，但学界尚未充分阐明互惠型护蚜蚁在群落内多种蚜虫物种共存机制中的作用。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庇护。这类蚜虫通常不会在同一植株上共存，但会在田间的多株寄主植物间形成共存格局。本研究旨在探明互惠型护蚜蚁与天敌的相互作用，如何影响菊蒿上多种植食性蚜虫物种的共存机制。研究结果表明，蚂蚁的存在会通过优先庇护受其照料的物种黑腹菊长管蚜（Metopeurum fuscoviride），同时捕食非庇护型的菊小长管蚜（Macrosiphoniella tanacetaria）以及程度较轻的菊乌蚜（Uroleucon tanaceti），从而调控单株植株（即局域群落）层面的群落组装过程。无蚂蚁存在时的竞争等级与有蚂蚁存在时存在显著差异。在区域尺度上，多株菊蒿共同构成的生境可支持所有蚜虫物种在更大的空间范围内实现共存，而在局域尺度上这些蚜虫则会发生竞争排除。在此情形下，依赖互惠型蚂蚁的竞争等级逆转，可驱动植物-蚜虫系统中的群落动态变化。