Site-specific impacts of a major hurricane on alpha and beta diversity in tropical forest seedling communities

Large scale disturbances are known to impact the alpha and beta diversity of communities. However, whether these disturbances increase or decrease diversity is often debated. The goal of this study was to quantify how the diversity of the seedling community was impacted within and across elevation in the El Yunque forest of Puerto Rico following a major hurricane. We tested two alternative hypotheses, that hurricanes are relatively more homogenizing or non-homogenizing forces, by quantifying changes in alpha and beta diversity of the seedling community post-hurricane. This approach highlights whether ecological mechanisms associated with community homogenization (species-specific survival, successional processes, and reduced environmental heterogeneity) or non-homogenization (resource release, increased environmental heterogeneity, and stochastic processes) structure the seedling community post-hurricane. We compared species richness, Fisher’s α, Simpson’s evenness, and multiple aspects of beta diversity within and among 25 seedling plots at 300, 400, and 500 m in elevation pre- and post-hurricane. We found that species richness, diversity, and evenness were higher post-hurricane, but abundance decreased 19%. Increases in alpha diversity suggest that hurricanes are non-homogenizing forces potentially linked with increases in light levels promoting colonization of early-successional species and resource release for other light-demanding species. The beta diversity results varied in their support for hurricanes as homogenizing depending upon the spatial scale of the analysis, potentially due to a combination of mechanisms including species-specific survival and site-specific differences. To fully grasp how the seedling community responds and recovers from disturbance, additional long-term monitoring will be needed to allow insight into the future of species richness, abundance, and spatial and temporal changes in community composition.

大规模扰动会对群落的α多样性（alpha diversity）与β多样性（beta diversity）产生影响，这已是学界共识。然而，这类扰动究竟会提升还是降低群落多样性，历来存在诸多争议。本研究以波多黎各埃尔云克（El Yunque）森林为研究对象，旨在量化强飓风过后，不同海拔梯度及其内部的幼苗群落多样性所受的影响。本研究通过量化飓风过后幼苗群落的α与β多样性变化，验证了两个对立假说：飓风属于相对更具均质化效应的作用力，抑或是非均质化作用力。该研究方法旨在揭示，飓风过后塑造幼苗群落结构的生态学机制，究竟偏向于群落均质化相关路径（如物种特异性存活、演替过程与环境异质性降低），还是非均质化相关路径（如资源释放、环境异质性提升与随机过程）。我们对比了海拔300m、400m与500m处的25个幼苗样地在飓风前后的物种丰富度、费希尔α多样性指数（Fisher’s α）、辛普森均匀度指数（Simpson’s evenness）以及β多样性的多个维度变化。研究结果显示，飓风过后的物种丰富度、多样性与均匀度均有所提升，但群落个体丰度下降了19%。α多样性的提升表明，飓风属于非均质化作用力，这可能与光照水平升高促进先锋物种定植，以及为其他喜光物种释放资源有关。β多样性的分析结果则因研究空间尺度而异，其对飓风均质化效应的支持程度存在差异，这可能是物种特异性存活与生境特异性差异等多种机制共同作用的结果。若要全面理解幼苗群落对扰动的响应与恢复过程，尚需开展长期监测，以深入解析物种丰富度、个体丰度以及群落组成的时空动态未来走向。