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.基于生态位的筛选（niche-based selection）与随机过程（stochastic processes）可共同作用，驱动物种组成产生空间与时间维度的变异。然而，目前学界对生态动态究竟由基于生态位的筛选还是随机过程主导的相关条件仍知之甚少。本研究依托早期演替温带草原（early-successional temperate grassland）野外原位实验与β多样性零模型（null models of beta diversity），探讨了土壤养分供给对不同样点间物种组成变异的两种驱动过程——基于生态位的筛选与随机动态——的相对重要性的影响。实验中人为调控养分有效性，每个实验样方均播种由25个物种组成的单一种子混合体系，随后允许随机与确定性的基于生态位的群落组装过程自然发生。研究结果显示，低养分供给的草原样方间的物种组成变异，由随机的物种迁入与灭绝过程驱动。与之相反，养分富集降低了随机过程的相对重要性，并施加了确定性的环境过滤（environmental filter）：通过筛选出少数光竞争能力更强的物种，实现群落均质化。该结果表明，土壤养分有效性是决定陆地植物群落由基于生态位的筛选还是随机组装过程主导程度的关键环境特征。本研究进一步发现，富营养化草原（eutrophic grasslands）的交替稳定态，源自某类可在高养分供给条件下成为优势类群的特定功能群（functional group）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从随机动态向生态位驱动动态转变的潜在机制。