Data from: Non-linear effects of phylogenetic distance on early-stage establishment of experimentally introduced plants in grassland communities

1. The phylogenetic distance of an introduced plant species to a resident native community may play a role in determining its establishment success. While Darwin’s naturalization hypothesis predicts a positive relationship, the preadaptation hypothesis predicts a negative relationship. Rigorous tests of this now so-called Darwin’s naturalization conundrum require not only information on establishment successes but also of failures, which is frequently not available. Such essential information, however, can be provided by experimental introductions. 2. Here, we analysed three datasets from two field experiments in Germany and Switzerland. In the Swiss experiment, alien and native grassland species were introduced as seeds only with and without disturbance (tilling). In the German experiment, alien and native grassland species were introduced both as seeds and as seedlings with and without disturbance (tilling), and with and without fungicide application. For the seedling introduction experiment, there was an additional herbivore-exclusion treatment. 3. Phylogenetic distance affected establishment in the three datasets differently, with success peaking at intermediate distances for the seed datasets, but decreasing with increasing distances in the seedling dataset. Disturbance favored seedling survival, most likely by weakening the resident community. 4. Synthesis: By analyzing experimental introductions, we show that the relationship between phylogenetic distance and establishment, at least for seedling emergence, may actually be non-linear with an optimum at intermediate distances. Therefore, Darwin´s naturalization hypothesis and the preadaptation hypothesis need not be in conflict. Rather, the mechanisms underlying them can operate simultaneously or alternately depending on the life stage and on the environmental conditions of the resident community.

1. 外来植物物种与本地土著群落之间的系统发育距离（phylogenetic distance）可能会影响其定植成功的概率。尽管达尔文归化假说（Darwin’s naturalization hypothesis）预测二者呈正相关关系，但预适应假说（preadaptation hypothesis）则预测二者呈负相关关系。如今被称为“达尔文归化难题”（Darwin’s naturalization conundrum）的这一科学问题，其严格检验不仅需要定植成功的相关数据，也需要定植失败的记录，但这类关键信息往往难以通过常规观测获取。不过，实验引种能够提供这类至关重要的研究数据。
2. 本研究对来自德国与瑞士的两项野外实验的三套数据集进行了分析。在瑞士的实验中，仅通过种子途径引入外来与本地草地物种，设置了有干扰（翻耕，tilling）与无干扰两组处理。在德国的实验中，则同时通过种子与幼苗两种途径引入外来与本地草地物种，设置了有/无干扰（翻耕）、有/无杀菌剂施用（fungicide application）四组处理；其中幼苗引入实验还额外增设了植食动物排除处理（herbivore-exclusion treatment）。
3. 系统发育距离对三套数据集的定植过程影响存在显著差异：种子数据集的定植成功率在中等距离处达到峰值，而幼苗数据集的定植成功率则随距离增加持续下降。干扰处理可显著提升幼苗存活率，其核心机制大概率是削弱了本地土著群落的竞争压力。
4. 总结：通过分析实验引种数据，本研究表明，系统发育距离与定植成功率之间的关联（至少就幼苗出苗（seedling emergence）而言）可能并非线性的，而是在中等距离处存在最优值。因此，达尔文归化假说与预适应假说并非必然对立，二者背后的调控机制可根据物种的生活史阶段（life stage）以及本地群落的环境条件，同时或交替发挥作用。