Am Nat. 2019 Feb;193(2):200-212. doi: 10.1086/701056. Epub 2018 Dec 26.
Coexistence requires that stabilizing niche differences, which cause species to limit themselves more than others, outweigh relative fitness differences, which cause competitive exclusion. Interactions with shared mutualists, which can differentially affect host fitness and change in magnitude with host frequency, can satisfy these conditions for coexistence, yet empirical tests of mutualist effects on relative fitness and stabilizing niche differences are largely lacking within the framework of coexistence theory. Here, we show that N-fixing rhizobial mutualists mediate coexistence in four naturally co-occurring congeneric legume (Trifolium) species. Using experimental greenhouse communities, we quantified relative fitness and stabilizing niche differences for each species in the presence of rhizobia originating from conspecific or congeneric hosts. Rhizobia stabilized coexistence by increasing the self-limitation of Trifolium species grown with rhizobia isolated from conspecifics, thus allowing congeners to increase when rare. Greenhouse-measured invasion growth rates predicted natural, unmanipulated coexistence dynamics of Trifolium species over 2 years at our field sites. Our results demonstrate that interactions with shared mutualists can stabilize the coexistence of closely related species.
共存需要稳定的生态位差异，这些差异导致物种自我限制的程度超过其他物种，超过相对适合度差异，这些差异导致竞争排斥。与共享互利共生体的相互作用可以不同程度地影响宿主的适合度，并随着宿主频率的变化而变化，这些相互作用可以满足共存的条件，但在共存理论的框架内，互利共生体对相对适合度和稳定生态位差异的影响的实证检验在很大程度上仍然缺乏。在这里，我们表明，固氮根瘤菌互利共生体在四种自然共存的同属豆科植物（三叶草）物种中介导共存。使用实验温室群落，我们在存在来自同种或同属宿主的根瘤菌的情况下，量化了每种物种的相对适合度和稳定的生态位差异。根瘤菌通过增加与同种根瘤菌分离的根瘤菌生长的三叶草物种的自我限制，从而允许同属物种在稀有时增加，从而稳定了共存。温室测量的入侵增长率预测了我们野外地点 2 年来三叶草物种自然、未受干扰的共存动态。我们的结果表明，与共享互利共生体的相互作用可以稳定密切相关物种的共存。
