Pre–Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift
Pre–Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift
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DOI：
10.1126/science.abo3594
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发表时间：
2023-08
期刊：
影响因子：
56.9
通讯作者：
F. O’Keefe;R. Dunn;Elic M. Weitzel;M. Waters;Lisandro Martínez;W. Binder;J. Southon;J. Cohen
中科院分区：
文献类型：
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作者：
F. O’Keefe;R. Dunn;Elic M. Weitzel;M. Waters;Lisandro Martínez;W. Binder;J. Southon;J. Cohen
更新世巨型动物灭绝的原因很难确定，部分原因是化石记录的时空分辨率差，阻碍了物种消失与考古和环境数据的一致。我们在加州Rancho La Brea的大型动物中获得了172个新的放射性碳年代，时间跨度为15.6万年至1万年（ka）。在新仙女木期开始前的12.9 ka，有7种灭绝的巨型动物消失了。与高分辨率区域数据集的比较表明，这些消失与b ølling- allerll ød （14.69 ~ 12.89 ka）期间干旱化和植被变化引起的生态状态转变相吻合。时间序列模型表明，大规模火灾是物种灭绝的主要原因，而这种状态转变的催化剂可能是人类对干旱、变暖和越来越容易发生火灾的生态系统的影响。众所周知，在世界大部分地区，许多大型脊椎动物物种在更新世晚期灭绝。尽管气候变化和人类影响都与这些物种的灭绝有关，但导致这些物种灭绝的原因仍存在争议。O’keefe等人利用在La Brea沥青坑捕获动物所创造的大量化石记录，结合附近的岩心样本，发现了火灾和与火灾有关的生态系统的增加与大型哺乳动物灭绝之间的明确关系。作者认为，火灾的增加可能是由于气候变化引起的变暖和干燥，以及人类对该系统的影响越来越大。-Sacha Vignieri大约1万年前，加州的生态系统因气候变化而变得脆弱，大火导致了大型动物的灭绝。在更新世末期，地球上大多数大型哺乳动物（巨型动物）灭绝了。这些灭绝发生在全球不同的时间，导致陆地生态系统的剧烈重组。尽管对物种灭绝的因果关系进行了数十年的研究，但晚第四纪气候变化和不断扩大的人类影响的相对重要性一直难以理清，因为化石记录的年代学分辨率较低，无法将这些快速发生、紧密相关的现象联系起来。加州南部的Rancho La Brea （RLB）地区为研究最新更新世巨型动物种群和群落组成的十年尺度变化提供了一个独特的机会。在这个地方，自然形成的沥青渗漏包裹并保存了数百个，有时是数千个，来自更新世最后5万年的众多巨型动物物种的个体的骨头。几乎所有这些骨标本都保存了原始胶原蛋白，这使得放射性碳定年分析成为可能。结果对7个已灭绝物种和1个现存物种的172个标本进行了放射性碳测年，这些物种分别是：剑齿虎（Smilodon fatalis）、巨斑剑齿虎（Aenocyon dirus）、美洲豹（Panthera atrox）、古野牛（Bison antiquus）、西方马（Equus occidentalis）、哈兰副齿龙（parylodon harlani）、骆驼（Camelops hesternus）和美洲犬（Canis latranans），时间跨度为15.6万年至1万年。我们使用高分辨率的捕获年表来分析不同分类群在这段时间内的种群动态和局部消失的时间。为了研究晚第四纪环境变化和人类活动在驱动观测模式中的潜在作用，我们将我们的人口结构和巨型动物灭绝分析与已确定的区域和大陆古气候代用物、植被记录和模拟的人口增长进行了比较。我们使用时间序列模型来研究生态系统变化的动态，并评估这些不同现象之间的因果关系。利用几种方法对灭绝时间进行建模，确定了除土狼外的所有分类群在12.9 ka之前从RLB灭绝，早于新仙女木期的开始，远早于北美巨型动物的大陆灭绝。除了骆驼和树懒外，其他类群的消失都是同步的，它们的消失要早几百年，与b ølling - allerd期间的干旱化和树木消失相一致。剑齿虎（Smilodon）、Aenocyon、Panthera、Equus和Bison古野牛（Bison antiquus）同时消失，同时发生了以植物群落重组和前所未有的火灾活动为特征的区域生态状态转变。时间序列模型强烈暗示人类是状态转变和由此导致的物种灭绝的主要原因。结论：我们的数据记录了在新仙女木期开始之前，南加州从冰川后的巨型动物林地到人类介导的灌木林生态系统的转变。这一转变始于两千年来景观的逐渐开放和干燥，结束于一个突然的（300年）政权转变，其特征是巨型动物的完全灭绝和前所未有的火灾活动。这种状态的转变似乎是由人类在生态系统中引发的火灾引发的，这些生态系统受到快速变暖、特大干旱和数千年来大型食草动物从景观中消失的趋势的压力。这一事件与今天地中海生态系统中发生的过程相似。加州拉布雷亚牧场记录的生态事件序列。左上：沥青坑周围的环境潮湿凉爽，有大量的树木和大型哺乳动物。左下：随着人类对食草动物的压力增加，冰期后的变暖和干燥开始了。右上：气候和人类影响之间的协同作用使以前所未有的火灾活动为特征的突然生态状态转变成为可能。右下：树篱生态系统建立；巨型动物已经灭绝，只有土狼的陷阱还在焦油坑里继续。插图由c. townsend提供，由洛杉矶县自然历史博物馆提供
The cause, or causes, of the Pleistocene megafaunal extinctions have been difficult to establish, in part because poor spatiotemporal resolution in the fossil record hinders alignment of species disappearances with archeological and environmental data. We obtained 172 new radiocarbon dates on megafauna from Rancho La Brea in California spanning 15.6 to 10.0 thousand calendar years before present (ka). Seven species of extinct megafauna disappeared by 12.9 ka, before the onset of the Younger Dryas. Comparison with high-resolution regional datasets revealed that these disappearances coincided with an ecological state shift that followed aridification and vegetation changes during the Bølling-Allerød (14.69 to 12.89 ka). Time-series modeling implicates large-scale fires as the primary cause of the extirpations, and the catalyst of this state shift may have been mounting human impacts in a drying, warming, and increasingly fire-prone ecosystem. Description Editor’s summary It is well known that many large vertebrate species went extinct during the late Pleistocene in most regions of the world. What caused these extinctions remains debated, although both climate change and human impacts have been implicated. O’Keefe et al. used the extensive fossil record created by the entrapment of animals in the La Brea tar pits in conjunction with nearby core samples and found a clear relationship between an increase in fire—and fire-related ecosystems—and large mammal extinction. The authors argue that this increase in fire may have resulted from climate change–induced warming and drying in conjunction with increasing impacts of humans in the system. —Sacha Vignieri Around 10,000 years ago, megafaunal losses in California were driven by fires in an ecosystem made vulnerable by climate change. INTRODUCTION At the end of the Pleistocene, most of Earth’s large mammals (megafauna) became extinct. These extinctions occurred at different times globally, resulting in a drastic reorganization of terrestrial ecosystems. Despite decades of research on extinction causality, the relative importance of late-Quaternary climate changes and spreading human impacts have been difficult to disentangle because poor chronological resolution in the fossil record has precluded alignment of these rapidly occurring, tightly linked phenomena. RATIONALE The Rancho La Brea (RLB) locality in Southern California provides a unique opportunity to investigate decadal-scale changes in megafaunal populations and community composition across the latest Pleistocene. At this site, naturally occurring asphalt seeps entrapped and preserved the bones of hundreds, and in some cases thousands, of individuals from numerous megafaunal species across the last 50,000 years of the Pleistocene. Nearly all of these osteological specimens preserve original collagen, which permits precise radiocarbon dating analysis. RESULTS We obtained radiocarbon dates on 172 specimens from seven extinct and one extant species: Smilodon fatalis, Aenocyon dirus, Panthera atrox, Bison antiquus, Equus occidentalis, Paramylodon harlani, Camelops hesternus, and Canis latrans, spanning 15.6 to 10.0 thousand calendar years before present (ka). We used the resulting high-resolution chronology of entrapment at RLB to analyze population dynamics across this time interval and the timing of local disappearance for different taxa. To investigate the potential roles of late-Quaternary environmental change and human activities in driving the observed patterns, we compared our analyses of population structure and megafaunal extirpation against well-resolved regional and continental paleoclimatic proxies, vegetation records, and modeled human demographic growth. We used time-series modeling to investigate the dynamics of ecosystem change and evaluate causal relationships among these different phenomena. Modeling of extinction timing using several methods established that all taxa except coyotes were extirpated from RLB by 12.9 ka, before the onset of the Younger Dryas and well before the continental extinction of North American megafauna. The disappearance of all taxa was synchronous except for camels and sloths, which disappeared a few hundred years earlier in concert with aridification and tree loss during the Bølling–Allerød. The simultaneous disappearance of Smilodon, Aenocyon, Panthera, Equus, and Bison antiquus coincided with a regional ecological state shift characterized by floral community reorganization and unprecedented fire activity. Time-series modeling strongly implicates humans as the primary cause of the state shift and resulting extinctions. CONCLUSION Our data document a transition from a postglacial megafaunal woodland to a human-mediated chaparral ecosystem in Southern California before the onset of the Younger Dryas. This transition began with gradual opening and drying of the landscape over two millennia, and terminated in an abrupt (300-year) regime shift characterized by the complete extirpation of megafauna and unprecedented fire activity. This state shift appears to have been triggered by human-ignited fires in an ecosystem stressed by rapid warming, a megadrought, and a millennial-scale trend toward the loss of large herbivores from the landscape. This event parallels processes occurring in Mediterranean ecosystems today. Sequence of ecological events as recorded at Rancho La Brea, California. Top left: conditions around the tar pits were moist and cool, with abundant trees and megafaunal mammals. Bottom left: the onset of postglacial warming and drying begins as human pressure on herbivores increases. Top right: the synergy between climatic and human impacts enables a sudden ecological state transition characterized by unprecedented fire activity. Bottom right: a chapparal ecosystem is established; megafauna are extinct, and only coyote entrapment continues at the tar pits. ILLUSTRATIONS BY C. TOWNSEND, COURTESY OF THE NATURAL HISTORY MUSEUMS OF LOS ANGELES COUNTY