Multiple invaders commonly co‐occur in native ecosystems and in some cases have been shown to facilitate each other thus exacerbating impacts on native species, while in other cases one invader may reduce the impact of another due to competition. We therefore aimed at identifying mechanisms driving alien species interactions. We conducted a common garden experiment investigating all pairwise combinations of 20 alien annual plant species in Germany. We first tested whether competition or facilitation occurred more often. Secondly, we determined whether individual traits, hierarchical or absolute trait distances, multivariate trait or phylogenetic distance explained alien plant interactions best. Thirdly, we assessed whether accounting for trait plasticity explains plant performance better than species‐level trait averages. While the magnitude of interspecific competition compared to intraspecific competition was on average larger across the 190 alien species combinations, interspecific facilitation still occurred in 24% of cases. Interactions could be better explained by hierarchical trait distances which reflect competitive ability, compared to phylogenetic and multivariate trait distance (reflecting niche differences). This finding supports criticisms about the applicability of testing limiting similarity versus environmental filtering and the community phylogenetic approach. Specifically, when growing taller and having a lower specific leaf area than the co‐occurring alien neighbour, biomass and seed number of individuals in mixture increased compared to growing alone. Effects of seed mass, root:shoot ratio and flowering time depended on the performance measure. In contrast to recent suggestions, accounting for trait plasticity did not necessarily explain plant performance better than models using species‐level trait averages. These results advance our fundamental understanding of trait‐interaction relationships, strengthening recent findings on the importance of competitive hierarchy in shaping interactions and community structure. In particular, applying these concepts to the novel case of alien‐alien interactions, is crucial given the continuing accumulation of alien species around the globe.

中文翻译：

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越强越好，特征等级制度正在推动外来物种相互作用

多种入侵者通常在原生生态系统中同时发生，并且在某些情况下已被证明可以相互促进，从而加剧了对原生物种的影响，而在其他情况下，一个入侵者可能由于竞争而减少另一入侵者的影响。因此，我们旨在确定驱动外来物种相互作用的机制。我们进行了一项常见的花园实验，研究了德国20种外来一年生植物物种的所有成对组合。我们首先测试了竞争或便利化是否发生得更频繁。其次，我们确定个体性状，等级性或绝对性状距离，多元性状或系统发生距离是否最好地解释了外来植物相互作用。第三，我们评估了性状可塑性的计算是否比物种水平的性状平均值更好地解释了植物的性能。在190个外来物种组合中，种间竞争相对于种间竞争的强度平均更大，而种间促进仍发生在24％的案例中。与系统发育和多元特征距离相比（反映利基差异），可以通过反映竞争能力的等级特征距离更好地解释相互作用。这一发现支持了关于限制相似性测试与环境过滤以及社区系统发生方法的适用性的批评。具体来说，当与同伴的外来邻居长得更高并且比叶面积更低时，与单独生长相比，混合个体的生物量和种子数量会增加。种子质量，根：枝比率和开花时间的影响取决于性能指标。与最近的建议相反，性状可塑性的解释并不一定比使用物种水平性状平均值的模型更好地解释植物的性能。这些结果提高了我们对特质-互动关系的基本理解，加强了关于竞争层次在塑造互动和社区结构中的重要性的最新发现。考虑到全球外来物种的不断积累，将这些概念应用到外星人与外星人互动的新颖案例中尤其重要。加强关于竞争等级制度在塑造互动和社区结构中的重要性的最新发现。考虑到全球外来物种的不断积累，将这些概念应用到外星人与外星人互动的新颖案例中尤其重要。加强关于竞争等级制度在塑造互动和社区结构中的重要性的最新发现。考虑到全球外来物种的不断积累，将这些概念应用到外星人与外星人互动的新颖案例中尤其重要。