Abiotic, present-day and historical effects on species, functional and phylogenetic diversity in dry grasslands of different age

Many grasslands have disappeared over the last century as a result of anthropogenic land use intensification, while new patches are emerging through abandonment of arable fields. Here, we compared species (SD), functional (FD) and phylogenetic (PD) (alpha) diversity among 272 dry grassland patches of two age-classes: old and new, with the new patches being dry grasslands established on previous intensively managed fields during the last 30 years. We first compared SD, FD and PD, between patches of different age. Then, we performed generalized linear models to determine the influence of abiotic, present-day and historical landscape configuration variables on SD, FD and PD. By measuring abiotic variables, we explained the effect of environmental filtering on species diversity, whereas the present-day and historical landscape configuration variables were included to describe how the spatial and temporal configuration of the patches influence patterns of species. Finally, we applied partial regressions to explore the relative importance of abiotic, present-day and historical variables in explaining the diversity metrics and how this varies between patches of different ages. We found higher SD in the old compared to the new patches, but no changes in FD and PD. SD was mostly affected by abiotic and present-day landscape configuration variables in the new and the old patches, respectively. In the new patches, historical variables explained variation in the FD, while present-day variables explained the PD. In the old patches, historical variables accounted for most of the variation in both FD and PD. Our evidence suggests that the relative importance of assembly processes has changed over time, showing that environmental filtering and changes in the landscape configuration prevented the establishment of species in the new patches. However, the loss of species (i.e. SD) is not necessarily linked to a loss of functions and evolutionary potential.

近一个世纪以来，受人为土地利用集约化影响，大量草原生态系统已然消失；与此同时，随着耕地撂荒，新生的草原斑块不断涌现。
本研究针对272个旱生草原斑块展开对比分析，这些斑块分为新旧两个年龄组：其中新生斑块为近30年间在先前经集约化管理的耕地上发育形成的旱生草原。我们比较了不同年龄组斑块的物种多样性（species diversity, SD）、功能多样性（functional diversity, FD）与系统发育多样性（phylogenetic diversity, PD）的α多样性（alpha diversity）水平。
首先，我们对比了不同年龄组斑块间的SD、FD与PD；随后，通过构建广义线性模型，明确非生物变量、当代景观格局变量与历史景观格局变量对SD、FD及PD的影响效应。
通过测定非生物变量，我们解析了环境过滤作用对物种多样性的影响；而纳入当代与历史景观格局变量，则旨在阐明斑块的空间与时间配置如何塑造物种分布格局。
最后，我们采用偏回归分析，探究非生物、当代及历史变量对多样性指标的相对解释能力，以及该能力在不同年龄组斑块间的差异。
研究结果显示，老龄斑块的SD显著高于新生斑块，但FD与PD未表现出显著组间差异。
分别来看，新生斑块的SD主要受非生物变量影响，而老龄斑块的SD则主要受当代景观格局变量调控。在新生斑块中，历史变量可解释FD的变异，当代变量则主导了PD的变异。
在老龄斑块中，历史变量则主导了FD与PD的绝大多数变异。
本研究结果表明，群落构建过程的相对重要性随时间发生了显著变化：环境过滤作用与景观格局变化共同阻碍了新生斑块的物种定植。
但值得注意的是，物种多样性（即SD）的下降并不必然伴随功能多样性与进化潜力的丧失。