Quantifying the Extent of North American Mammal Extinction Relative to the Pre-Anthropogenic Baseline

Earth has experienced five major extinction events in the past 450 million years. Many scientists suggest we are now witnessing a sixth, driven by human impacts. However, it has been difficult to quantify the real extent of the current extinction episode, either for a given taxonomic group at the continental scale or for the worldwide biota, largely because comparisons of pre-anthropogenic and anthropogenic biodiversity baselines have been unavailable. Here, we compute those baselines for mammals of temperate North America, using a sampling-standardized rich fossil record to reconstruct species-area relationships for a series of time slices ranging from 30 million to 500 years ago. We show that shortly after humans first arrived in North America, mammalian diversity dropped to become at least 15%–42% too low compared to the “normal” diversity baseline that had existed for millions of years. While the Holocene reduction in North American mammal diversity has long been recognized qualitatively, our results provide a quantitative measure that clarifies how significant the diversity reduction actually was. If mass extinctions are defined as loss of at least 75% of species on a global scale, our data suggest that North American mammals had already progressed one-fifth to more than halfway (depending on biogeographic province) towards that benchmark, even before industrialized society began to affect them. Data currently are not available to make similar quantitative estimates for other continents, but qualitative declines in Holocene mammal diversity are also widely recognized in South America, Eurasia, and Australia. Extending our methodology to mammals in these areas, as well as to other taxa where possible, would provide a reasonable way to assess the magnitude of global extinction, the biodiversity impact of extinctions of currently threatened species, and the efficacy of conservation efforts into the future.

在过去4.5亿年间，地球曾经历过五次大规模灭绝事件。诸多学者认为，当前人类活动正驱动着第六次大规模灭绝事件的发生。然而，当前学界难以量化此次灭绝事件的真实规模——无论是针对大陆尺度下的特定分类群（taxonomic group），还是全球范围的生物群落（biota），这主要是因为此前缺乏前人类世（pre-anthropogenic）与人类世（anthropogenic）生物多样性基准线的对比数据。本研究基于经过采样标准化的丰富化石记录，重建了距今3000万年至500年间多个时间切片的物种-面积关系，以此计算温带北美哺乳动物的上述基准线。研究结果显示，在人类首次抵达北美后不久，哺乳动物多样性便相较于数百万年来维持的"正常"多样性基准线下降了至少15%至42%。尽管北美哺乳动物多样性在全新世（Holocene）的下降早已被定性认知，但本研究的结果首次提供了量化指标，明确了此次多样性下降的实际严重程度。若将大规模灭绝定义为全球范围内至少75%的物种消失，那么我们的数据表明，在工业化社会开始产生影响之前，北美哺乳动物的物种损失比例已经达到了该基准线的五分之一至一半以上（具体数值因生物地理区（biogeographic province）而异）。目前尚无足够数据对其他大陆开展类似的量化评估，但南美、欧亚大陆与澳大利亚的全新世哺乳动物多样性出现定性下降的结论，也已得到学界广泛认可。若将本研究的方法推广至上述区域的哺乳动物类群，以及在可行范围内推广至其他生物类群，将为评估全球灭绝规模、当前受威胁物种灭绝事件对生物多样性的影响，以及未来保护工作的成效提供可靠途径。