Data from: Is biotic resistance enhanced by natural variation in diversity?

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 underlying processes by dissecting how biotic resistance to new invaders may be shaped by the same environmental influences that determine diversity and other community properties. In grasslands with heterogeneous soils, we added invaders and removed competitors to analyze the causes of invasion resistance. Abiotic resistance was measured using invader success in the absence of the resident community. Biotic resistance was measured as the reduction in invader success in the presence of the resident community. Invaders were most successful where biotic resistance was lowest and abiotic resistance was highest, confirming the dominant role of biotic resistance. Contrary to theory, though, biotic resistance was highest where both species richness and functional diversity were lowest. In the multivariate framework of a structural equation model, biotic resistance was independent of community diversity, and was highest where fertile soils led to high community biomass. Seed predation slightly augmented biotic resistance without qualitatively changing the results. Soil-related genotypic variation in the invader also did not affect the results. We conclude that in natural systems, diversity may be correlated with invasibility and yet have little effect on biotic resistance to invasion. More generally, the environmental causes of variation in diversity should be considered when examining the potential functional consequences of diversity.

将生物多样性与生态系统功能相关联的理论，因在更多样化的本土群落（native communities）中普遍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调控外来入侵种生物抗性（biotic resistance）的环境影响因素——这些因素同时也是决定群落多样性与其他群落属性的关键因子——来揭示其背后的潜在过程。在土壤异质性的草原生态系统中，我们通过引入入侵种（invader）并移除本地竞争者的方式，解析入侵抗性的成因。非生物抗性（abiotic resistance）通过入侵种在本地群落缺失条件下的入侵成功率来衡量；生物抗性则以入侵种在本地群落存在时的入侵成功率下降幅度为量化指标。入侵种在生物抗性最低、非生物抗性最高的生境中入侵成功率最高，这证实了生物抗性在入侵防御中的主导作用。但与理论预期相悖的是，生物抗性在物种丰富度和功能多样性均最低的群落中反而最高。在结构方程模型（structural equation model）的多变量分析框架下，生物抗性与群落多样性并无关联，且在肥沃土壤催生高群落生物量的生境中，生物抗性达到最高水平。种子捕食作用（seed predation）对生物抗性仅有小幅增强，并未从本质上改变研究结果。入侵种中与土壤相关的基因型变异也未对研究结果产生影响。我们的研究结论表明：在自然生态系统中，群落多样性或许与可入侵性（invasibility）存在相关性，但对入侵的生物抗性几乎没有影响。更广泛而言，在探究多样性潜在的生态功能效应时，应当考虑导致多样性差异的环境成因。