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）与随机过程可协同作用，驱动物种组成产生时空维度的变异。然而，学界对生态动力学究竟以基于生态位的选择还是随机过程为主导的条件，仍缺乏充分认知。
本研究依托温带早期演替草地的野外实验，结合β多样性（beta diversity）零模型（null models），探究了土壤养分供给对不同样地间物种组成变异中，基于生态位的选择与随机动力学相对重要性的影响。实验中人为调控土壤养分有效性，每个实验样地均播种由25个物种构成的种子混合组合，随后允许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地，其群落组成变异由随机迁入与灭绝事件主导。与之相反，养分富集会降低随机性过程的相对重要性，并施加确定性环境过滤作用：通过筛选少数对光照具有更强竞争能力的物种，使群落结构趋于均质化。
本研究证实，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的关键环境特征。进一步研究发现，富营养化草地的替代稳定态（alternative states），源于某一特定功能群物种组成的初始随机变异——该功能群在高养分供给条件下可成为优势类群。
本研究还探讨了沿土壤养分梯度，生态动力学从随机过程向生态位驱动过程转变的潜在机制。