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. 达尔文归化假说（Darwin's naturalisation hypothesis）提出，外来物种与本地物种的系统发育距离可预测其入侵成功；埃尔顿多样性可入侵性假说（Elton's diversity‐invasibility hypothesis）则认为，本地群落的多样性可赋予其抵御入侵的能力。二者均根植于物种共存理论。由于这两个假说内在存在关联，其背后的作用机制可能共同驱动入侵成功。然而，此前尚无一项实验研究明确解析二者的关联与交互作用。
2. 为解析这两个假说间的关联，我们以36种本地草原草本植物为材料，构建了90个不同多样性水平的温室中宇宙群落（mesocosm），并分别以5种草本外来物种的种子与幼苗作为接种材料。我们利用系统发育关系与4种功能性状——株高、比叶面积（specific leaf area, SLA）、叶面积、种子质量——计算得到不同的系统发育距离、功能距离及多样性指标。具体而言，我们检验了外来种-本地种距离（系统发育或功能）与本地多样性（系统发育或功能）如何通过交互作用，影响外来种的萌发、幼苗存活、生长与繁殖。
3. 整体而言，本研究结果支持上述两个假说。基于4种性状的多变量功能距离，其对入侵成功的正向效应显著强于系统发育距离，以及基于单个性状的单变量功能距离。此外，若外来种相较于本地种具备更强的竞争力——如株高更高、叶片更大且比叶面积更低——则其入侵成功率更高。基于单个性状的单变量功能多样性，其对入侵的负向效应显著强于系统发育多样性与多变量功能多样性。最为关键的是，我们发现随着本地多样性的提升，外来种-本地种的系统发育距离与多变量功能距离的效应强度会增强。针对单个性状的分析同样显示，在多样性更高、竞争更为激烈的群落中，外来种-本地种的层级功能距离（反映竞争不平等性）与绝对功能距离（反映生态位分化）的效应强度均会提升。
4. 综合讨论 本研究首次明确阐释了这两个经典入侵假说背后的作用机制如何共同调控草原群落的入侵成功过程。该结果或有助于解释此前单独检验这两个假说时得到的一些令人困惑的研究结果。