Kansas Biological Survey and Department of Ecology & Evolutionary Biology, University of Kansas, Lawrence, Kansas, 66047, USA.
Department of Plant Biology and Center for Ecology, Southern Illinois University, Carbondale, Illinois, 62901, USA.
Ecol Appl. 2020 Jan;30(1):e02014. doi: 10.1002/eap.2014. Epub 2019 Nov 4.
The "environmental heterogeneity hypothesis" predicts that variability in resources promotes species coexistence, but few experiments support this hypothesis in plant communities. A previous 15-yr test of this hypothesis in a prairie restoration experiment demonstrated a weak effect of manipulated soil resource heterogeneity on plant diversity. This response was attributed to a transient increase in richness following a post-restoration supplemental propagule addition, occasionally higher diversity under nutrient enrichment, and reduced cover of a dominant species in a subset of soil treatments. Here, we report community dynamics under continuous propagule addition in the same experiment, corresponding to 16-20 yr of restoration, in response to altered availability and heterogeneity of soil resources. We also quantified traits of newly added species to determine if heterogeneity increases the amount and variety of niches available for new species to exploit. The heterogeneous treatment contained a factorial combination of altered nutrient availability and soil depth; control plots had no manipulations. Total diversity and richness were higher in the heterogeneous treatment during this 5-yr study due to higher cover, diversity, and richness of previously established forbs, particularly in the N-enriched subplots. All new species added to the experiment exhibited unique trait spaces, but there was no evidence that heterogeneous plots contained a greater variety of new species representing a wider range of trait spaces relative to the control treatment. The richness and cover of new species was higher in N-enriched soil, but the magnitude of this response was small. Communities assembling under long-term N addition were dominated by different species among subplots receiving added N, leading to greater dispersion of communities among the heterogeneous relative to control plots. Contrary to the deterministic mechanism by which heterogeneity was expected to increase diversity (greater variability in resources for new species to exploit), higher diversity in the heterogeneous plots resulted from destabilization of formerly grass-dominated communities in N-enriched subplots. While we do not advocate increasing available soil N at large scales, we conclude that the positive effect of environmental heterogeneity on diversity can take decades to materialize and depend on development of stochastic processes in communities with strong establishment limitation.
“环境异质性假说”预测资源变异性会促进物种共存，但很少有实验支持植物群落中的这一假说。先前在草原恢复实验中对这一假说进行的 15 年测试表明，人为改变土壤资源异质性对植物多样性的影响较弱。这种反应归因于恢复后补充繁殖体添加后的丰富度短暂增加，在养分富集下偶尔多样性更高，以及在部分土壤处理中优势物种的覆盖减少。在这里，我们报告了同一实验中在连续繁殖体添加下的群落动态，对应于恢复的 16-20 年，以响应土壤资源可利用性和异质性的改变。我们还量化了新添加物种的特征，以确定异质性是否增加了新物种可利用的生态位数量和种类。异质性处理包含了改变养分有效性和土壤深度的因子组合；对照样地没有进行任何处理。在这项为期 5 年的研究中，由于先前建立的草本植物的覆盖度、多样性和丰富度更高，特别是在 N 富集的亚区，异质性处理中的总多样性和丰富度更高。添加到实验中的所有新物种都表现出独特的特征空间，但没有证据表明异质性处理相对于对照处理包含更多不同物种的新物种，代表更广泛的特征空间。新物种的丰富度和覆盖度在 N 富集的土壤中更高，但这种反应的幅度很小。在长期 N 添加下组装的群落，在接受添加 N 的亚区中由不同的物种主导，导致异质性相对于对照处理的群落分散程度更大。与预期异质性会增加多样性的确定性机制（新物种可利用的资源变异性更大）相反，异质性样地的多样性更高是由于 N 富集亚区中以前以草为主的群落不稳定。虽然我们不主张在大范围内增加可利用的土壤 N，但我们的结论是，环境异质性对多样性的积极影响可能需要几十年才能实现，并取决于具有强烈建立限制的群落中随机过程的发展。
