Data from: Resource availability determines the importance of niche-based vs. stochastic community assembly in grasslands

Niche-based selection and stochastic processes can operate simultaneously to generate spatial and temporal variation in species composition. Yet, the conditions under which ecological dynamics are dominated by niche-based vs. stochastic processes are poorly understood. Using a field experiment in early-successional temperate grassland and null models of beta diversity, this study investigates the effects of soil nutrient supply on the relative importance of niche-based selection vs. stochastic dynamics for variation in species composition among sites. Nutrient availability was manipulated experimentally, individual seed mixtures with 25 species were sown in each experimental plot, and then stochastic and deterministic niche-based assembly processes were allowed to happen. We found that compositional variation among grassland plots with low nutrient supply was driven by stochastic immigration and extinctions. In contrast, nutrient enrichment reduced the importance of stochasticity and imposed a deterministic environmental filter that homogenized communities through the selection of few species with greater competitive ability for light. This demonstrates that soil nutrient availability is a critical environmental feature that dictates the degree to which terrestrial plant communities are controlled by niche-based selection vs. stochastic assembly processes. Our study shows further that alternative states of eutrophic grasslands emerge from initial stochastic variation in the composition of a particular functional group of species that can become dominant at high nutrient supply. We discuss potential mechanisms underlying the shift from stochastic to niche-driven dynamics along soil nutrient gradients.基于生态位的选择（niche-based selection）与随机过程（stochastic processes）可共同作用，驱动物种组成产生时空变异。然而，当前学界对生态动态究竟以基于生态位的选择还是随机过程为主导的具体条件，仍缺乏深入认知。本研究依托温带早期演替草地的野外控制实验，结合β多样性零模型（null models of beta diversity），探究了土壤养分供给对样点间物种组成变异中，基于生态位的选择与随机动力学（stochastic dynamics）相对重要性的调控效应。实验中人为调控养分有效性：每个样方均播种包含25个物种的单一种子混合体系，随后任由随机与确定性生态位组装过程自然发生。研究结果显示，养分供给水平较低的草地样方中，物种组成变异由随机的迁入与灭绝过程主导。与之相反，养分富集降低了随机性过程的相对权重，并施加了确定性环境过滤作用：通过筛选出少数对光照竞争能力更强的物种，使得群落结构趋于均质化。这表明土壤养分有效性是决定陆地植物群落受基于生态位的选择还是随机组装过程主导程度的关键环境因子。本研究进一步揭示，富营养化草地的多种稳定状态，源自某一可在高养分供给条件下成为优势类群的特定功能群物种组成的初始随机变异。本研究还探讨了沿土壤养分梯度，生态动态从随机过程主导转向生态位驱动的潜在机制。