Alien Phytogeographic Regions of Southern Africa: Numerical Classification, Possible Drivers, and Regional Threats

The distributions of naturalised alien plant species that have invaded natural or semi-natural habitat are often geographically restricted by the environmental conditions in their new range, implying that alien species with similar environmental requirements and tolerances may form assemblages and characterise particular areas. The aim of this study was to use objective numerical techniques to reveal any possible alien phytogeographic regions (i.e. geographic areas with characteristic alien plant assemblages) in southern Africa. Quarter degree resolution presence records of naturalised alien plant species of South Africa, Lesotho, Swaziland, Namibia and Botswana were analysed through a divisive hierarchical classification technique, and the output was plotted on maps for further interpretation. The analyses revealed two main alien phytogeographic regions that could be subdivided into eight lower level phytogeographic regions. Along with knowledge of the environmental requirements of the characteristic species and supported by further statistical analyses, we hypothesised on the main drivers of alien phytogeographic regions, and suggest that environmental features such as climate and associated biomes were most important, followed by human activities that modify climatic and vegetation features, such as irrigation and agriculture. Most of the characteristic species are not currently well-known as invasive plant species, but many may have potential to become troublesome in the future. Considering the possibility of biotic homogenization, these findings have implications for predicting the characteristics of the plant assemblages of the future. However, the relatively low quality of the dataset necessitates further more in-depth studies with improved data before the findings could be directly beneficial for management.

已入侵自然或半自然生境的归化外来植物物种（naturalised alien plant species），其分布往往受新生境环境条件的地理限制，这意味着具有相似环境需求与耐受能力的外来物种可形成植物组合群，并成为特定区域的特征类群。本研究旨在通过客观数值方法，揭示南部非洲地区可能存在的外来植物地理区（alien phytogeographic regions）——即具有特征性外来植物组合群的地理区域。研究团队针对南非、莱索托、斯威士兰、纳米比亚及博茨瓦纳的归化外来植物物种四分度分辨率物种存在记录（quarter degree resolution presence records），采用分裂层级分类技术（divisive hierarchical classification technique）进行分析，并将分析结果绘制成地图以供进一步解读。分析结果显示存在两个主要外来植物地理区，且可进一步细分为8个低级外来植物地理区。结合特征物种的环境需求知识，并辅以进一步的统计分析支持，本研究针对外来植物地理区的主要驱动因子提出假说，认为气候及相关生物群系（biomes）等环境特征是最为关键的驱动因素，其次是改变气候与植被特征的人类活动（如灌溉与农业活动）。目前多数特征物种尚未被认定为入侵植物类群，但其中许多物种未来有可能演变为有害入侵物种。考虑到生物同质化（biotic homogenization）的可能性，本研究结果对预测未来植物组合群的特征具有指导意义。但本数据集的质量相对较低，因此在研究结果能够直接应用于管理实践之前，还需要采用优化后的数据集开展更深入的后续研究。