Data from: Linking Darwin's naturalisation hypothesis and Elton's diversity‐invasibility hypothesis in experimental grassland communities

1. Darwin's naturalisation hypothesis posing that phylogenetic distance of alien species to native residents predicts invasion success, and Elton's diversity‐invasibility hypothesis posing that diversity of native communities confers resistance to invasion, are both rooted in ideas of species coexistence. Because the two hypotheses are inherently linked, the mechanisms underlying them may interact in driving the invasion success. Even so, these links and interactions have not been explicitly disentangled in one experimental study before. 2. To disentangle the links between the two hypotheses, we used 36 native grassland herbs to create greenhouse mesocosms with 90 grassland communities of different diversities, and introduced each of five herbaceous alien species as seeds and seedlings. We used phylogeny and four functional traits (plant height, specific leaf area, leaf size, seed mass) to calculate different measures of phylogenetic and functional distance and diversity. Specifically, we tested how the alien‐native distance (phylogenetic or functional) and the native diversity (phylogenetic or functional) affected each other in their effects on germination, seedling survival, growth and reproduction of the aliens. 3. Overall, our results supported both hypotheses. Multivariate functional distance based on four traits jointly had stronger positive effects than phylogenetic distance and the univariate ones based on each trait separately. Moreover, the aliens were more successful if they were more competitive by being taller and having larger leaves with a lower SLA than the native residents. Univariate functional diversity based on each trait separately had stronger negative effects than phylogenetic and multivariate functional diversity. Most importantly, we found that the effects of alien‐native phylogenetic and multivariate functional distance became stronger as diversity increased. Our analyses with single traits also showed that the strength of the effects of both alien‐native hierarchical functional distances (indicative of competitive inequalities) and absolute functional distances (indicative of niche differences) increased at higher diversities, where competition is more severe. 4. SynthesisOur study explicitly demonstrates for the first time how the mechanisms underlying the two classical invasion hypotheses interact in driving invasion success in grassland communities. This may help to explain some of the puzzling results of studies testing either of the two hypotheses.1. 达尔文自然归化假说（Darwin's naturalisation hypothesis）提出，外来物种与本地土著物种的系统发育距离（phylogenetic distance）可预测其入侵成功概率；埃尔顿多样性-入侵性假说（Elton's diversity-invasibility hypothesis）则认为，本地群落的多样性可赋予群落抵御入侵的能力。二者均植根于物种共存理论。由于这两个假说内在关联紧密，其背后的调控机制可能在驱动入侵成功的过程中产生相互作用。尽管如此，此前尚无一项实验研究能够明确解析二者之间的关联与相互作用。 2. 为厘清这两个假说间的关联，本研究选取36种本土草本植物构建温室中型实验生态系统（greenhouse mesocosms），设置了90个不同多样性梯度的草地群落，并以种子和幼苗两种形式引入5种草本外来物种。研究基于系统发育关系与4种功能性状（植物高度、比叶面积（specific leaf area）、叶面积、种子质量），计算得到不同的系统发育与功能距离、多样性指标。具体而言，我们检验了外来-本地距离（系统发育或功能层面）与本地多样性（系统发育或功能层面）在影响外来物种萌发、幼苗存活、生长与繁殖的过程中是否存在交互效应。 3. 总体而言，本研究结果同时支持了上述两个假说。基于4种性状的多变量功能距离相比系统发育距离以及基于单个性状的单变量功能距离，展现出更强的正向调控效应。此外，若外来物种相较于本地土著物种具有更高株高、更大叶面积以及更低比叶面积，从而具备更强竞争能力，则其入侵成功率更高。基于单个性状的单变量功能多样性相比系统发育多样性与多变量功能多样性，展现出更强的负向调控效应。最为关键的是，研究发现随着本地多样性提升，外来-本地系统发育距离与多变量功能距离的效应强度均会增强。针对单个性状的分析同样显示，在竞争更为激烈的高多样性群落中，外来-本地层级功能距离（反映竞争不平等性）与绝对功能距离（反映生态位差异）的效应强度均会提升。 4. 总结 本研究首次明确阐释了这两个经典入侵假说背后的调控机制如何在草地群落中共同作用以驱动入侵成功。这一发现或有助于解释此前仅检验其中单一假说的相关研究中出现的诸多令人困惑的结果。