大滅絕造成生物多樣性低落與恐龍統治

原文網址：https://news.ncsu.edu/2017/10/mass-extinctions-led-to-low-species-diversity-dinosaur-rule/

大滅絕造成生物多樣性低落與恐龍統治

Mick Kulikowski

大滅絕是指全世界有超過50%物種滅絕的事件。地球曾經發生過五次大滅絕事件，其中兩次造成僅有少數所謂的「頑強」物種生還，牠們的子嗣在地球從巨變復原的期間往全世界擴散。這項發現或許可以闡明當今物種以極快速率滅絕會如何改變未來的生物族群結構。

北卡羅來納州立大學與北卡羅來納自然科學博物館的獨立博士後研究員David J. Button，和他的同僚仔細觀察2.6億年前至1.75億年前將近900種脊椎生物的化石紀錄，時間橫跨了二疊紀晚期、整個三疊紀至侏儸紀早期。在這段期間發生了兩次大滅絕事件，Button表示兩次滅絕事件之後都產生了類似模式，因此可以推測其他滅絕事件――包括現今正在發生的生物多樣性流失――或許也會造成相同結果。

Button表示：「大滅絕不只會減少動物多樣性，也會影響動物與生態系的分布狀態，也就是生物地理。隨著物種因為滅絕事件消失，牠們的生態棲位也會跟著空缺下來。在滅絕事件過後，空出來的生態棲位會被倖存和新演化出來的『頑強』物種佔據。這些為數不多的生存專家在一時之間迅速擴張並主宰了地球，使得全球生態系變成多樣性低落的『災後動物相』(disaster fauna)。」

水龍獸(Lystrosaurus)是這些生存專家之一，其為一種草食性的哺乳動物近親，體型從狗到豬的大小不等，並且具有獠牙來幫助牠們挖掘植物。

Button表示大約發生在2.52億年前的二疊紀晚期事件使得新的動物族群演化出來，包括最初的恐龍、鱷魚和哺乳類與蜥蜴的近親。而發生在2.01億年前左右的三疊紀晚期事件則殲滅了許多當時生存的主要族群，為恐龍之後接管地球打下了基礎。

「二疊紀晚期的事件造成了90%的海洋生物和70%的陸生脊椎動物滅亡，原因可能是過度活躍的火山活動造成的氣候變遷。當時火山噴濺出大量的玄武岩質岩漿並釋放氣體至大氣當中，使得二氧化碳濃度急遽升高，造成嚴重的暖化與沙漠化。」Button表示，「三疊紀晚期的事件也跟火山活動有所關聯。」

共同作者，伯明罕大學的古生物學教授Richard Butler表示：「大滅絕是種能從基本層面重塑生態系的全球災難。我們的分析提供了重要資料，顯示這類毀滅事件對動物分布的影響和改變有多麼深遠。」

另一位共同作者，阿根廷自然科學博物館的研究人員Martín Ezcurra表示：「化石紀錄讓我們有機會測試演化理論在長時間尺度下的運作情形，這在侷限於現生動植物的演化研究上是無法辦到的。」

Button表示辨認大滅絕事件前後化石紀錄呈現的模式，有助於研究人員預測現今的生物多樣性流失導致的後果。

他補充：「深入瞭解這些過往發生的危難，可以告訴保育人員應該採取何種措施來避免現今的動物遭受類似命運。」

這篇發表於《自然通訊》(Nature Communications)的論文另一位共同作者是里茲大學的Graeme T. Lloyd。本研究的資金來源為新居禮夫人人才培育計畫(編號630123)、歐洲研究委員會創新計劃(編號637483)和青年探索研究獎(編號140101879)。

Mass extinctions led to low species diversity, dinosaur rule

Two of the earth’s five mass extinction events – times when more than half of the world’s species died – resulted in the survival of a low number of so-called “weedy” species that spread their sameness across the world as the Earth recovered from these dramatic upheavals. The findings could shed light on modern high extinction rates and how biological communities may change in the future.

David J. Button, an NC State and North Carolina Museum of Natural Sciences postdoctoral research scholar, and colleagues examined fossil records of almost 900 vertebrate species dating back between 260 and 175 million years ago – from the late Permian through the Triassic and early Jurassic periods. Two mass extinction events occurred during this time. Button says that similar patterns arising after two mass extinctions implies that other extinction events may have the same results – including current biodiversity loss.

“Mass extinctions not only reduced animal diversity, but also affected the distribution of animals and ecosystems, or biogeography,” Button said. “As species are removed by extinction, their ecological niches are left vacant. Following the extinction event, these niches are occupied by surviving and newly evolving ‘weedy’ species. These few generalists spread out and dominated for a time, leading to a low-diversity global ‘disaster fauna.’”

One of these generalists was the Lystrosaurus, a plant-eating early mammal relative that ranged from dog- to pig-sized. It had tusks to help it dig up plant matter.

The late-Permian event – occurring around 252 million years ago – allowed new groups to evolve, including the earliest dinosaurs, crocodiles and relatives of mammals and lizards, Button said. The late-Triassic event, which occurred around 201 million years ago, wiped out many major groups, setting the stage for dinosaurs to take over.

“The late-Permian event caused about 90 percent of sea life and 70 percent of land-living vertebrates to become extinct, probably as a result of climate change from hyperactive volcanism – when volcanoes spewed basalt lava and released gases into the atmosphere causing large increases in carbon dioxide and severe warming resulting in desertification,” Button said. “The late-Triassic event is also associated with volcanism.”

“Mass extinctions were global disasters that fundamentally reshaped ecosystems,” said Richard Butler, professor of palaeobiology at the University of Birmingham and a co-author of the study. “Our new analyses provide crucial data that show just how profoundly these cataclysmic events changed and influenced animal distribution.”

“The fossil record has the potential to test evolutionary hypotheses in long time spans, which is not possible if evolutionary research is limited to living plant and animals,” said Martín Ezcurra, a researcher at the Museo Argentino de Ciencias Naturales who co-authored the paper.

Identifying patterns across mass extinction events in the fossil record can help researchers make predictions about the consequences of current biodiversity loss, Button said.

“Further understanding of these ancient crises will help to inform conservation efforts to prevent modern animals from suffering a similar fate,” he added.

Graeme T. Lloyd of the University of Leeds also co-authored the paper, which is published in Nature Communications. The study was funded by a Marie Curie Actions grant (630123), a European Research Council Starting Grant (637483) and a Discovery Early Career Researcher (DE140101879) award.

原始論文：David J. Button, Graeme T. Lloyd, Martín D. Ezcurra, Richard J. Butler. Mass extinctions drove increased global faunal cosmopolitanism on the supercontinent Pangaea. Nature Communications, 2017; 8 (1) DOI: 10.1038/s41467-017-00827-7

引用自：North Carolina State University. "Mass extinctions led to low species diversity, dinosaur rule."