Species’ responses to antagonists, such as fungal pathogens and insect herbivores, are part of a species’ niche and are thought to contribute to species coexistence. In theory, coexistence is supposed to be maintained by the effects of equalizing mechanisms that reduce fitness differences among species, and stabilizing mechanisms that are the result of niche differences. The presence of stabilizing niche differences is hallmarked by negative frequency-dependent per capita rates of increase. However, rigorous experimental testing of how insect herbivores and fungal pathogens contribute to negative frequency dependence among co-occurring plant species, in particular in the context of biological invasions, is scarce. We present an experimental study in a grassland in central Germany, in which we established native and exotic-dominated communities on 1 m²-plots, and reduced above- and belowground fungal pathogens and insect herbivores by applying pesticides. We manipulated the frequencies of six native and six exotic focal species, in order to assemble one dominant and five rare species per community. We quantified productivity and diversity for each community, and recorded data on per capita rates of increase and other fitness components for two native and two exotic focal species over three years. We found that native and exotic-dominated communities did not differ in their productivity and diversity. Reduction of fungi and insects increased productivity but the effect was similar for both species origins, suggesting that effects of enemy release are transitory or not existent. Negative frequency-dependence was common in our species, but could not be linked to interactions with antagonists. Although focal species responded to the reduction of antagonists, these effects were only present at low frequency, suggesting a greater importance of intraspecific competition, e.g. for resources, in explaining the observed stabilizing niche differences. Offsetting responses of the two exotic focal species to both antagonist groups highlight the significance of equalizing mechanisms among native and exotic species at the community level. Our study demonstrates the importance of both equalizing and stabilizing mechanisms in the context of biological invasions and differential contributions of antagonistic interactions to these mechanisms.

物种对诸如真菌病原体和昆虫食草动物之类的拮抗剂的反应是物种生态位的一部分，被认为有助于物种共存。从理论上讲，应该通过减少物种之间适应性差异的均衡机制以及由于生态位差异而产生的稳定机制来维持共存。稳定的生态位差异的存在以人均频率依赖性负增长率为特征。但是，对于昆虫食草动物和真菌病原体如何导致共生植物物种之间的频率依赖性为负的严格实验性测试是稀缺的，尤其是在生物入侵的情况下。我们在德国中部的草原进行实验研究，我们在1平方米的土地上建立了以当地人和外来人为主的社区，并通过施用农药减少了地上和地下的真菌病原体和昆虫食草动物。我们操纵了6种本土物种和6种外来物种的频率，以便每个社区聚集一种优势物种和五种稀有物种。我们对每个社区的生产力和多样性进行了量化，并记录了三年中两个本地和两个外来重点物种的人均增长率和其他适应度数据。我们发现，以本地人和外来人为主的社区在生产力和多样性上没有差异。减少真菌和昆虫可提高生产率，但对两个物种起源而言，其影响均相似，这表明敌人释放的影响是暂时的或不存在的。负频率依赖性在我们的物种中很常见，但不能与与拮抗剂的相互作用相关。尽管局灶种对拮抗剂的减少有反应，但这些作用仅以较低的频率出现，表明种内竞争（例如对资源的竞争）在解释观察到的稳定生态位差异方面具有更大的重要性。这两个外来物种对两个拮抗类群的抵消反应突出了在社区一级平衡本地和外来物种之间机制的重要性。我们的研究表明，在生物学入侵和拮抗作用对这些机制的不同贡献的背景下，平衡和稳定机制的重要性。尽管局灶种对拮抗剂的减少有反应，但这些作用仅以较低的频率出现，表明种内竞争（例如对资源的竞争）在解释观察到的稳定利基差异方面具有更大的重要性。这两个外来物种对两个拮抗类群的抵消反应突出了在社区一级平衡本地和外来物种之间机制的重要性。我们的研究表明，在生物学入侵和拮抗作用对这些机制的不同贡献的背景下，平衡和稳定机制的重要性。尽管局灶种对拮抗剂的减少有反应，但这些作用仅以较低的频率出现，表明种内竞争（例如对资源的竞争）在解释观察到的稳定生态位差异方面具有更大的重要性。这两个外来物种对两个拮抗类群的抵消反应突出了在社区一级平衡本地和外来物种之间机制的重要性。我们的研究表明，在生物学入侵和拮抗作用对这些机制的不同贡献的背景下，平衡和稳定机制的重要性。在解释观察到的稳定利基差异。这两个外来物种对两个拮抗类群的抵消反应突出了在社区一级平衡本地和外来物种之间机制的重要性。我们的研究表明，在生物学入侵和拮抗作用对这些机制的不同贡献的背景下，平衡和稳定机制的重要性。在解释观察到的稳定利基差异。这两个外来物种对两个拮抗类群的抵消反应突出了在社区一级平衡本地和外来物种之间机制的重要性。我们的研究表明，在生物入侵和拮抗作用对这些机制的不同贡献的背景下，平衡和稳定机制的重要性。