Extinction Risk and Diversification Are Linked in a Plant Biodiversity Hotspot

It is widely recognized that we are entering an extinction event on a scale approaching the mass extinctions seen in the fossil record. Present-day rates of extinction are estimated to be several orders of magnitude greater than background rates and are projected to increase further if current trends continue. In vertebrates, species traits, such as body size, fecundity, and geographic range, are important predictors of vulnerability. Although plants are the basis for life on Earth, our knowledge of plant extinctions and vulnerabilities is lagging. Here, we disentangle the underlying drivers of extinction risk in plants, focusing on the Cape of South Africa, a global biodiversity hotspot. By comparing Red List data for the British and South African floras, we demonstrate that the taxonomic distribution of extinction risk differs significantly between regions, inconsistent with a simple, trait-based model of extinction. Using a comprehensive phylogenetic tree for the Cape, we reveal a phylogenetic signal in the distribution of plant extinction risks but show that the most threatened species cluster within short branches at the tips of the phylogeny—opposite to trends in mammals. From analyzing the distribution of threatened species across 11 exemplar clades, we suggest that mode of speciation best explains the unusual phylogenetic structure of extinction risks in plants of the Cape. Our results demonstrate that explanations for elevated extinction risk in plants of the Cape flora differ dramatically from those recognized for vertebrates. In the Cape, extinction risk is higher for young and fast-evolving plant lineages and cannot be explained by correlations with simple biological traits. Critically, we find that the most vulnerable plant species are nonetheless marching towards extinction at a more rapid pace but, surprisingly, independently from anthropogenic effects. Our results have important implications for conservation priorities and cast doubts on the utility of current Red List criteria for plants in regions such as the Cape, where speciation has been rapid, if our aim is to maximize the preservation of the tree-of-life.学界公认，人类正进入一场规模可与化石记录中所见大规模灭绝事件相媲美的灭绝事件。当前物种灭绝速率估计较背景灭绝速率高出数个数量级，若当前趋势持续，该速率还将进一步攀升。在脊椎动物中，体型、繁殖力与地理分布范围等物种性状是评估其灭绝脆弱性的重要预测因子。尽管植物是地球生命的根基，但人类对植物灭绝事件及其脆弱性的认知仍相对滞后。本研究以全球生物多样性热点（biodiversity hotspot）之一的南非开普地区为研究对象，厘清了植物灭绝风险的潜在驱动机制。通过对比英国与南非植物区系的红色名录（Red List）数据，本研究证实不同区域间灭绝风险的分类学分布存在显著差异，这与基于单一性状的简单灭绝风险模型并不相符。借助针对开普地区的全面系统发育树（phylogenetic tree），我们揭示了植物灭绝风险分布存在系统发育信号，但同时发现受威胁程度最高的物种聚集于系统发育树末端的短分支处——这与哺乳动物的灭绝风险演化趋势截然相反。通过分析11个典型演化支（clade）内受威胁物种的分布情况，我们认为物种形成方式能够最佳解释开普地区植物灭绝风险的异常系统发育结构。本研究结果表明，开普植物区系中灭绝风险升高的解释机制，与学界已明确的脊椎动物灭绝风险解释机制存在显著差异。在开普地区，年轻且演化速率较快的植物支系具有更高的灭绝风险，且该现象无法通过与简单生物性状的相关性来解释。至关重要的是，我们发现尽管最脆弱的植物物种正以更快的速度走向灭绝，但令人意外的是，这一过程与人为活动影响并无关联。倘若我们的目标是最大化保护生命之树（tree-of-life）的完整性，那么本研究结果对保护优先级的制定具有重要参考价值，同时也对开普这类物种形成速率较快的区域所采用的现行植物红色名录标准的实用性提出了质疑。