Increasing rates of invasions in ecosystems worldwide necessitate experiments to determine the role of biotic interactions in the success and impact of multiple alien species. Here, we examined competitive and facilitative interactions among various combinations of three widespread and often co‐occurring invaders: the zebra mussel Dreissena polymorpha, and the macrophytes Elodea canadensis and Elodea nuttallii. Using a mesocosm‐based, factorial experimental design, we assessed the effect of interspecific competition on macrophyte growth rates in the absence and presence at varying biomass of D. polymorpha. Growth rates (wet g/day) of E. canadensis and E. nuttallii were similar when grown in isolation. When grown together, in the absence of D. polymorpha, E. canadensis growth was not significantly reduced in the presence of E. nuttallii and vice versa. In the presence of D. polymorpha (26.0 ± 1 mm), monocultural growth of E. canadensis was largely unaffected, while E. nuttallii growth was strongly enhanced. Low (2.64 g) and medium (3.96 g) mussel biomass led to negative interspecific effects between E. canadensis and E. nuttallii; at high (5.28 g) mussel biomass, the effect of interspecific competition was negated. Overall, D. polymorpha alleviated competitive interactions between the two invasive macrophytes when all three species co‐occurred, and substantially enhanced growth of E. nuttallii with increasing mussel biomass, thereby suggesting a possible influence on the relative dominance of these macrophytes in the field. Our study demonstrates how facilitations can cause shifts in dominance among closely related invaders. The consequences of such facilitations for the structure and function of communities remain to be explored generally.

中文翻译：

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我的朋友：侵入性淡水贻贝促进侵入性大型植物的生长并调节它们的竞争相互作用

世界范围内生态系统入侵率的增加需要通过实验来确定生物相互作用在多种外来物种的成功和影响中的作用。在这里，我们研究了三种广泛存在且经常同时发生的入侵者的各种组合之间的竞争性和促进性相互作用：斑马贻贝 Dreissena polymorpha，以及大型植物 Elodea canadensis 和 Elodea nuttallii。使用基于中宇宙的析因实验设计，我们评估了种间竞争对大型植物生长速率的影响，在不同生物量的 D. polymorpha 生物量不存在和存在的情况下。E. canadensis 和 E. nuttallii 的生长速率（湿克/天）在单独生长时相似。当一起生长时，在没有多形线虫的情况下，加拿大桉树的生长在 E. polymorpha 的存在下没有显着降低。nuttallii，反之亦然。在 D. polymorpha (26.0 ± 1 mm) 的存在下，E. canadensis 的单一栽培生长基本上不受影响，而 E. nuttallii 的生长强烈增强。低 (2.64 g) 和中等 (3.96 g) 贻贝生物量导致 E. canadensis 和 E. nuttallii 之间的负面种间效应；在高 (5.28 g) 贻贝生物量下，种间竞争的影响被抵消。总体而言，当所有三个物种同时发生时，D. polymorpha 减轻了两种侵入性大型植物之间的竞争性相互作用，并且随着贻贝生物量的增加显着增强了 E. nuttallii 的生长，从而表明可能对这些大型植物在田间的相对优势产生影响。我们的研究证明了便利如何导致密切相关的入侵者之间的支配地位发生变化。