Data from: Does natural variation in diversity affect biotic resistance?

1. Theories linking diversity to ecosystem function have been challenged by the widespread observation of more exotic species in more diverse native communities. Few studies have addressed the key underlying process by dissecting how community diversity is shaped by the same environmental gradients that determine biotic and abiotic resistance to new invaders. 2. In grasslands on highly heterogeneous soils, we used addition of a recent invader, competitor removal, and structural equation modelling (SEM) to analyse soil influences on community diversity, biotic and abiotic resistance, and invader success. 3. Biotic resistance, measured by reduction in invader success in the presence of the resident community, was negatively correlated with species richness and functional diversity. However, in the multivariate SEM framework, biotic resistance was independent of all forms of diversity, and was positively affected by soil fertility via community biomass. Abiotic resistance, measured by invader success in the absence of the resident community, peaked on infertile soils with low biomass and high community diversity. Net invader success was determined by biotic resistance, consistent with this invader's better performance on infertile soils in unmanipulated conditions. 4. Seed predation added slightly to biotic resistance without qualitatively changing the results. Soil-related genotypic variation in the invader also did not affect the results. 5. Synthesis In natural systems, diversity may be correlated with invasibility and yet have no effect on either biotic or abiotic resistance to invasion. More generally, the environmental causes of variation in diversity should not be overlooked when considering the potential functional consequences of diversity.

1. 将生物多样性与生态系统功能相联系的理论，因在更多样化的本地群落中广泛观测到更多外来物种（exotic species）的现象而受到挑战。目前鲜有研究通过解析环境梯度如何同时塑造群落多样性，以及决定本地群落对新型入侵物种的生物与非生物抵抗能力这一核心底层过程，来回应这一质疑。
2. 在土壤高度异质性的草原生态系统中，我们通过引入新近入侵物种、移除本地竞争者以及结构方程模型（structural equation modelling, SEM）等实验手段，分析了土壤因子对群落多样性、生物与非生物抵抗性以及入侵成功度的影响。
3. 以本地群落存在时入侵成功度的降幅为指标衡量的生物抵抗性，与物种丰富度和功能多样性呈负相关关系。然而在多元结构方程模型框架下，生物抵抗性与各类多样性指标均无关联，而是通过群落生物量受到土壤肥力的正向调控。以本地群落缺失时入侵成功度为指标衡量的非生物抵抗性，则在低生物量、高群落多样性的贫瘠土壤中达到峰值。最终净入侵成功度由生物抵抗性决定，这与该入侵物种在未受人为干扰的贫瘠土壤中表现更佳的观测结果相符。
4. 种子捕食（seed predation）对生物抵抗性仅有小幅提升，并未从本质上改变研究结论；入侵物种的土壤相关基因型变异（genotypic variation）同样未对实验结果产生显著影响。
5. 总结：在自然生态系统中，群落多样性或与可入侵性（invasibility）呈相关关系，但却不会对生物或非生物入侵抵抗性产生任何实质影响。更广泛而言，在探讨生物多样性的潜在功能效应时，不应忽视造成群落多样性变异的环境成因。