We used a trait-based approach to disentangle Darwin’s conundrum along environmental gradients and tested the contribution of earlier established invaders to biotic resistance of resident communities in boreal lakes. For this, we used a linear mixed model to test if lake surface area, water temperature, the proportion of earlier established invaders and functional distances to resident species predict the establishment of introduced species. We found that in colder and smaller lakes, introduced fishes that were ecologically distant to resident species had higher establishment success, supporting Darwin’s naturalisation hypothesis. However, in warmer and larger lakes, the establishment was higher for species ecologically closer to residents, supporting the adaptation hypothesis. Therefore, the support for Darwin’s hypotheses depends on how environmental gradients affect biotic interactions. Contrary to our expectation, we found that earlier established invaders seem to favour the establishment of further introduced species and did not contribute to the biotic resistance of the resident community. Our study revealed that using environmental gradients is essential to disentangle Darwin’s conundrum for introduced species in boreal lakes and contribute to explain why the results of previous studies are conflicting.

中文翻译：

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水温和湖泊大小解释了达尔文在北方湖泊中建立鱼类的难题

我们使用基于特征的方法沿着环境梯度解开达尔文的难题，并测试了较早建立的入侵者对北方湖泊居民社区生物抗性的贡献。为此，我们使用线性混合模型来测试湖表面积、水温、较早建立的入侵者的比例和与驻留物种的功能距离是否预测引入物种的建立。我们发现在较冷和较小的湖泊中，在生态上远离居民物种的引入鱼类具有更高的建立成功率，支持达尔文的归化假设。然而，在温暖和更大的湖泊中，生态上更接近居民的物种的建立更高，支持适应假设。所以，对达尔文假设的支持取决于环境梯度如何影响生物相互作用。与我们的预期相反，我们发现较早建立的入侵者似乎有利于进一步引入物种的建立，并且对居民社区的生物抗性没有贡献。我们的研究表明，使用环境梯度对于解开达尔文关于北方湖泊引入物种的难题至关重要，并有助于解释为什么以前的研究结果相互矛盾。