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.

厘清群落内物种存续的制约机制，乃是群落组装理论（community assembly theory）及其在生物多样性（biodiversity）保护与提升实践中的核心议题。“环境异质性假说（environmental heterogeneity hypothesis）”预言：生境资源变异性越高，物种共存的可能性便越强。就群落组装过程而言，环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——即环境过滤作用（environmental filters）——来影响物种相对丰度与生物多样性水平。我们在将本土草原物种播种至撂荒地前，通过调控影响高草草原（tallgrass prairie）植物多样性的关键资源——土壤深度与氮有效性（nitrogen availability）——以提升环境异质性水平。在群落组装的前15年中，我们对比了两组重复样地（每组处理设置4个重复）的养分有效性变异、地上年净初级生产力（aboveground annual net primary productivity, ANPP）与物种组成差异：一组为施加土壤异质性调控的样地，另一组为未进行资源调控的对照样地，以此检验“环境异质性假说”。该调控处理提升了环境异质性水平，该水平以硝态氮（NO3-N）有效性与ANPP的变异系数（coefficient of variation）进行衡量。然而，两类异质性处理下的植物多样性整体并无显著差异，且均随时间呈指数式衰减且衰减模式无显著差异。两类处理的物种丰富度（species richness）均随时间线性下降，但在初始播种8年后开展第二次繁殖体添加（propagule addition）的第2年，异质性更高的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），异质性更高的土壤中物种随时间的丢失速率更低（0.60 物种·年⁻¹）。两类处理的群落均随时间呈现出显著的趋同特征：所有处理的优势物种均发生更替，且克隆型C4草本须芒草（Andropogon gerardii）的盖度逐渐提升。我们认为，异质性对植物多样性的影响较弱，可归因于某一克隆物种的优势度不断提升——这使得土壤处理的尺度相较于植物个体尺寸偏小——此外还包括扩散限制，以及缺少在高频火制度（fire regime）下已知可提升植物多样性的关键驱动因素——放牧（grazing）。因此，若要引导群落组装以实现高生物多样性目标，相较于提升环境异质性，或许更应着眼于调控能够降低优势度、促进新物种迁入的过程。