前所未有的大型哺乳類滅絕浪潮跟古代人類有所關聯

原文網址：https://news.unl.edu/newsrooms/today/article/unprecedented-wave-of-large-mammal-extinctions-linked-to-ancient-humans/

前所未有的大型哺乳類滅絕浪潮跟古代人類有所關聯

by Scott Schrage 

根據一篇發表在期刊《科學》(Science)的新研究，智人、尼安德塔人和其他近代的人類近親開始獵殺大型哺乳類而使牠們體型縮小(透過消滅牠們)的時間比過去認為的至少早了90,000年。

體型使大象相形見絀的真猛瑪象、跟大象差不多大的地懶，以及各種劍齒虎是260萬年前至12,000年前，漫步在地球的巨型哺乳類名單中的明星動物。過往研究提出35,000年前左右，澳洲的大型哺乳類開始比較小的種類以更快的速度消失――此現象稱為「體型偏頗的滅絕事件」(size-biased extinction)。

透過地質紀錄和年代更加久遠的化石中陸續得出的新數據，這篇新研究認為至少125,000年前，非洲就已經開始發生體型偏頗的滅絕事件。研究指出當時非洲的哺乳類體型已經比其他大陸的小了50%，儘管較大的陸塊一般來說可以讓更大的哺乳類生活其上。

但是，當人類從非洲遷徙出去，研究人員發現其他體型偏頗的滅絕事件也開始出現，而且地點和時間順序跟已知的人類遷移模式正好如出一轍。隨著時間經過，其他大陸上哺乳類的平均體型越來越接近非洲大陸，之後更是遠低於此。活過這段時期的哺乳類體型通常都遠遠小於走向滅絕的哺乳類。

根據這篇由新墨西哥大學的Felisa Smith主持的研究，此次發生在近代的體型偏頗滅絕事件，不論規模和尺度都超越了過去6600萬年來記錄到的其他所有類似事件。

「在人類影響成為重要因素之後，哺乳類的巨大身形才開始讓牠們更易絕種。」內布拉斯加大學林肯分校的Kate Lyons表示。她與Smith和史丹佛大學及加州大學聖地牙哥分校的研究人員共同撰寫了此篇研究。「人類學紀錄顯示智人大約是在20萬年前正式成為一個物種，因此在我們誕生成一個物種的不久之後這個事件便發生了。它看起來就像是我們一手造成的。」

「從生活史的觀點來看確實有合理之處。如果你捉到一隻兔子，你可以餵飽你的家人一個晚上；但如果你能獵殺一隻大型哺乳類，就可以餵飽整個村莊。」

相較之下，研究團隊發現沒有什麼證據可以支持過去6600萬年來的體型偏頗滅絕事件，是因為氣候變遷而造成的說法。作者表示大型哺乳類和小型哺乳類在這段時間受到溫度變化的影響程度似乎是相等的。

讓地球改頭換面

團隊也更進一步地去探討哺乳類的滅絕可能會對全世界的生物多樣性造成什麼影響。為此他們提出了這個問題：如果現今被列為易危物種或是瀕危物種的哺乳類，在未來200年內全數滅絕會發生什麼事情？

Lyons表示在此情況下，剩下的哺乳類中體型最大的會是家牛；而平均重量則會驟降至不到10公斤，大約是約克夏㹴犬的大小。

「如果這種趨勢繼續下去，所有當今處境危險的(哺乳類)都消失了，那麼能量流和物種組成都會整個改頭換面。」新墨西哥大學的生物學教授Smith表示。「事實上，世界各地的哺乳類體型都會倒退回4000萬年前的哺乳類大小。」

Lyons表示對於全世界的生態系來說，這種重組過程會帶來相當深遠的影響。大型哺乳類通常是草食動物。牠們會攝取大量的植物，使得植物中的養分以更高的效率被帶到生態系各處。她說如果大型哺乳類持續消失，則剩下來的哺乳類並不足以取代如此重要的生態腳色。

「大型哺乳類提供給生態系的功能類型跟小型哺乳類提供的差異相當大。」Lyons表示。「生態系在未來的樣貌會變得十分、十分不同。上一次哺乳類群集長成這樣且平均體型如此矮小，是在恐龍滅絕之後。」

「我們現今的所作所為可能會在非常短的時間之內，就完全抹除哺乳類歷時4000萬至4500萬年在體型方面的演化歷程。」

與Smith 和Lyons共同著作此篇研究的為史丹佛大學的Jon Payne，以及加州大學聖地牙哥分校的Rosemary Elliott Smith。團隊的補助來自於國家科學基金會。

Unprecedented wave of large-mammal extinctions linked to ancient humans

Homo sapiens, Neanderthals and other recent human relatives may have begun hunting large mammal species down to size — by way of extinction — at least 90,000 years earlier than previously thought, says a new study published in the journal Science.

Elephant-dwarfing wooly mammoths, elephant-sized ground sloths and various saber-toothed cats highlighted the array of massive mammals roaming Earth between 2.6 million and 12,000 years ago. Prior research suggested that such large mammals began disappearing faster than their smaller counterparts — a phenomenon known as size-biased extinction — in Australia around 35,000 years ago.

With the help of emerging data from older fossil and geologic records, the new study estimated that this size-biased extinction started at least 125,000 years ago in Africa. By that point, the average African mammal was already 50 percent smaller than those on other continents, the study reported, despite the fact that larger landmasses can typically support larger mammals.

But as humans migrated out of Africa, other size-biased extinctions began occurring in regions and on timelines that coincide with known human migration patterns, the researchers found. Over time, the average body size of mammals on those other continents approached and then fell well below Africa’s. Mammals that survived during the span were generally far smaller than those that went extinct.

The magnitude and scale of the recent size-biased extinction surpassed any other recorded during the last 66 million years, according to the study, which was led by the University of New Mexico’s Felisa Smith.

“It wasn’t until human impacts started becoming a factor that large body sizes made mammals more vulnerable to extinction,” said the University of Nebraska-Lincoln’s Kate Lyons, who authored the study with Smith and colleagues from Stanford University and the University of California, San Diego. “The anthropological record indicates that Homo sapiens are identified as a species around 200,000 years ago, so this occurred not very long after the birth of us as a species. It just seems to be something that we do.

“From a life-history standpoint, it makes some sense. If you kill a rabbit, you’re going to feed your family for a night. If you can kill a large mammal, you’re going to feed your village.”

By contrast, the research team found little support for the idea that climate change drove size-biased extinctions during the last 66 million years. Large and small mammals seemed equally vulnerable to temperature shifts throughout that span, the authors reported.

Off the face of the Earth

The team also looked ahead to examine how potential mammal extinctions could affect the world’s biodiversity. To do so, it posed a question: What would happen if the mammals currently listed as vulnerable or endangered were to go extinct within the next 200 years?

In that scenario, Lyons said, the largest remaining mammal would be the domestic cow. The average body mass would plummet to less than six pounds — roughly the size of a Yorkshire terrier.

“If this trend continues, and all the currently threatened (mammals) are lost, then energy flow and taxonomic composition will be entirely restructured,” said Smith, professor of biology at New Mexico. “In fact, mammalian body size around the globe will revert to what the world looked like 40 million years ago.”

Lyons said that restructuring could have “profound implications” for the world’s ecosystems. Large mammals tend to be herbivores, devouring large quantities of vegetation and effectively transporting the associated nutrients around an ecosystem. If they continue to disappear, she said, the remaining mammals would prove poor stand-ins for important ecological roles.

“The kinds of ecosystem services that are provided by large mammals are very different than what you get from small mammals,” Lyons said. “Ecosystems are going to be very, very different in the future. The last time mammal communities looked like that and had a mean body size that small was after the extinction of the dinosaurs.

“What we’re doing is potentially erasing 40 to 45 million years of mammal body-size evolution in a very short period of time.”

Smith and Lyons authored the study with Jon Payne of Stanford University and Rosemary Elliott Smith from the University of California, San Diego. The team received support from the National Science Foundation.

原始論文：Felisa A. Smith, Rosemary E. Elliott Smith, S. Kathleen Lyons, Jonathan L. Payne. Body size downgrading of mammals over the late Quaternary. Science, 2018; DOI: 10.1126/science.aao5987

引用自：University of Nebraska-Lincoln. "Unprecedented wave of large-mammal extinctions linked to prehistoric humans."