Data from: Environmental heterogeneity has a weak effect on diversity during community assembly in tallgrass prairie

Understanding what constrains the persistence of species in communities is at the heart of community assembly theory and its application to conserving and enhancing biodiversity. The “environmental heterogeneity hypothesis” predicts greater species coexistence in habitats with greater resource variability. In the context of community assembly, environmental heterogeneity may influence the variety and strength of abiotic conditions and competitive interactions (environmental filters) to affect the relative abundance of species and biodiversity. We manipulated key resources that influence plant diversity in tallgrass prairie (i.e., soil depth and nitrogen availability) to increase environmental heterogeneity prior to sowing native prairie species into a former agricultural field. We compared variability in nutrient availability, aboveground annual net primary productivity (ANPP), and the composition of species between replicate plots containing soil heterogeneity manipulations and plots with no resource manipulations (n = 4 per treatment) during the first 15 yr of community assembly as a test of the “environmental heterogeneity hypothesis.” The manipulations increased environmental heterogeneity, measured as the coefficient of variation in NO3-N availability and ANPP. Plant diversity, however, was similar and decayed exponentially and indiscriminately over time between the heterogeneity treatments. Species richness declined linearly over time in both heterogeneity treatments, but richness was higher in the more heterogeneous soil 2 yr following a second propagule addition 8 yr after the initial sowing. As a result, there was a lower rate of species loss over time in the more heterogeneous soil (0.60 species yr−1) relative to the control soil (0.96 species yr−1). Communities in each treatment exhibited strong convergence over time resulting from a shift in dominant species across all treatments and a gradual increase in the clonal C4 grass, Andropogon gerardii. We attribute the weak effect of heterogeneity on diversity to increasing dominance of a clonal species, which decreased the scale of soil treatments relative to plant size, dispersal limitation, and absence of a key driver (grazing) known to increase plant diversity under a frequent fire regime. Thus, steering community assembly to attain high biodiversity may depend more on manipulating processes that reduce dominance and facilitate the arrival of new species than promoting environmental heterogeneity.

阐明群落内物种存续的制约因素，是群落构建理论（community assembly theory）及其在生物多样性保护与提升实践中的核心议题。生境异质性假说（environmental heterogeneity hypothesis）预测：资源变异性更高的生境中，物种共存能力更强。在群落构建的框架下，生境异质性可通过调控非生物条件（abiotic conditions）与种间竞争相互作用的类型与强度（即环境过滤（environmental filters）），进而影响物种相对多度与生物多样性水平。本研究在将本土高草草原（tallgrass prairie）物种播种至废弃农田前，通过调控影响高草草原植物多样性的关键资源——即土壤深度与氮有效性（nitrogen availability）——以提升生境异质性。在群落构建的前15年期间，本研究对设置了土壤异质性调控的重复样地与无资源调控的对照样地（每组处理设4个重复，n=4）的养分有效性、地上年净初级生产力（aboveground annual net primary productivity，ANPP）变异度及物种组成进行对比，以此检验生境异质性假说。调控处理提升了生境异质性，该异质性以硝态氮（NO3-N）有效性与ANPP的变异系数（coefficient of variation）作为衡量指标。但不同异质性处理组的植物多样性整体水平相近，且随时间推移均呈无差异的指数衰减趋势。两类异质性处理组的物种丰富度均随时间呈线性下降趋势，但在初始播种8年后进行第二次繁殖体（propagule）添加的2年后，高异质性土壤样地的物种丰富度更高。因此，相较于对照土壤（0.96 物种·年⁻¹），高异质性土壤样地的物种随时间流失速率更低（0.60 物种·年⁻¹）。所有处理组的群落均随时间呈现显著的趋同现象，这源于所有处理组的优势物种发生更替，且克隆型C4草本须芒草（Andropogon gerardii）的盖度逐渐升高。本研究将异质性对多样性的微弱影响归因于克隆型物种的优势度不断提升：这一现象使得土壤处理的尺度相对于植物个体尺寸被缩小，同时还存在扩散限制（dispersal limitation），且缺失了在频繁火制度（frequent fire regime）下已知可提升植物多样性的关键驱动因子——放牧（grazing）。因此，若要通过调控群落构建以实现高生物多样性目标，相较于提升生境异质性，或许更应聚焦于调控那些能够降低优势度、促进新物种迁入的过程。