發現之前不為人知的巨型海洋生物滅絕事件

原文網址：http://www.media.uzh.ch/en/Press-Releases/2017/marine-Megafauna.html

發現之前不為人知的巨型海洋生物滅絕事件

蘇黎世大學的研究人員表示在超過200萬年以前，有三分之一的大型海洋動物，像是鯊魚、鯨魚、海鳥和海龜就此消失。這起先前不為人知的滅絕事件不僅對地球歷史中的生物多樣性有巨量影響，同時也對生態系的功能造成衝擊。

陸上巨型動物群中有一大部分，譬如劍齒虎和猛瑪象，在冰河期期間就此滅亡可謂眾所皆知的事件。蘇黎世大學和柏林自然博物館的研究人員顯示在更早以前，海洋也發生了類似的滅絕事件。

發現新的滅絕事件

這組國際團隊調查了上新世至更新世(西元前530萬年至9700年左右)的海洋巨型動物群的化石。主要作者Catalina Pimiento解釋：「我們可以顯示出在200萬到300萬年前海洋巨型動物群有三分之一就這樣消失了。因此傳承至人類的海洋巨型動物群集實際上已經有所改變，並以多樣性降低的狀態運作著。」她在任職於蘇黎世大學古生物研究所和博物館時進行了此篇研究。

新發現的滅絕事件中影響最深的為海洋哺乳類，其多樣性下降了55%。另外滅絕的則有多達43%的海龜，以及35%的海鳥和9%的鯊魚。另一方面，在接下來的更新世則有新的物種發展出來，當時海洋巨型動物群有四分之一在上新世時還未出現，包括北極熊Ursus、海燕Oceanodroma以及企鵝 Megadyptes。然而總體來說，生物多樣性之後就未曾達到先前的水準。

對功能多樣性造成的影響

為了得到滅絕事件造成的後果，研究團隊將焦點放在淺層的海岸棚區，探討整個功能體(functional entities)的喪失對海岸生態系的影響。功能體中的動物族群雖然未必相關，但牠們在生態系中具備的功能有類似的特性。他們發現在上新世時，海岸水域中有七種功能體就此消失。

即使失去7個功能體和三分之一的物種總數相對來說並不嚴重，仍會使整體功能多樣性大量受損：生態系的功能多樣性總共喪失了17%，另外21%則有所改變。先前普遍存在的掠食者消失，同時新的競爭者崛起，使得海洋動物被迫適應這些變化。此外，研究人員也發現在滅絕事件發生的時間，由於海平面高度產生劇烈變化，導致海岸棲地嚴重縮小。

大型溫血海洋動物特別脆弱

研究人員提出富有生產力的海岸棲地急遽消失，加上其他海洋環境因子，像是海流的變化，皆為這起滅絕事件的重要成因。Pimiento博士解釋：「我們的模型呈現出溫血動物特別容易邁入滅絕。舉例來說，像是某些種類的海牛和鬚鯨、還有巨型鯊魚『巨齒鯊』(Carcharocles megalodon)都在此時消失。這項研究顯示在地質史上不久之前的過去，海洋巨型動物群比之前推測的還要容易受到全球環境變遷影響。」研究人員也指出這可以比喻成現今的情況：像鯨魚或海豹之類的大型海洋生物特別容易受到今日的人為活動影響。

Previously Unknown Extinction of Marine Megafauna Discovered

Over two million years ago, a third of the largest marine animals like sharks, whales, sea birds and sea turtles disappeared. This previously unknown extinction event not only had a considerable impact on the earth’s historical biodiversity but also on the functioning of ecosystems. This has been demonstrated by researchers at the University of Zurich.

The disappearance of a large part of the terrestrial megafauna such as saber-toothed cat and the mammoth during the ice age is well known. Now, researchers at the University of Zurich and the Naturkunde Museum in Berlin have shown that a similar extinction event had taken place earlier, in the oceans.

New extinction event discovered

The international team investigated fossils of marine megafauna from the Pliocene and the Pleistocene epochs (5.3 million to around 9,700 years BC). “We were able to show that around a third of marine megafauna disappeared about three to two million years ago. Therefore, the marine megafaunal communities that humans inherited were already altered and functioning at a diminished diversity”, explains lead author Dr. Catalina Pimiento, who conducted the study at the Paleontological Institute and Museum of the University of Zurich.

Above all, the newly discovered extinction event affected marine mammals, which lost 55 per cent of their diversity. As many as 43 per cent of sea turtle species were lost, along with 35 per cent of sea birds and 9 per cent of sharks. On the other hand, the following new forms of life were to develop during the subsequent Pleistocene epoch: Around a quarter of animal species, including the polar bear Ursus, the storm petrel Oceanodroma or the penguin Megadyptes, had not existed during the Pliocene. Overall, however, earlier levels of diversity could not be reached again.

Effects on functional diversity

In order to determine the consequences of this extinction, the research team concentrated on shallow coastal shelf zones, investigating the effects that the loss of entire functional entities had on coastal ecosystems. Functional entities are groups of animals not necessarily related, but that share similar characteristics in terms of the function they play on ecosystems. The finding: Seven functional entities were lost in coastal waters during the Pliocene.

Even though the loss of seven functional entities, and one third of the species is relatively modest, this led to an important erosion of functional diversity: 17 per cent of the total diversity of ecological functions in the ecosystem disappeared and 21 per cent changed. Previously common predators vanished, while new competitors emerged and marine animals were forced to adjust. In addition, the researchers found that at the time of the extinction, coastal habitats were significantly reduced due to violent sea levels fluctuations.

Large warm-blooded marine animals are more vulnerable

The researchers propose that the sudden loss of the productive coastal habitats, together with oceanographic factors such as altered sea currents, greatly contributed to these extinctions. “Our models have demonstrated that warm-blooded animals in particular were more likely to become extinct. For example, species of sea cows and baleen whales, as well as the giant shark Carcharocles megalodon disappeared”, explains Dr. Pimiento. “This study shows that marine megafauna were far more vulnerable to global environmental changes in the recent geological past than had previously been assumed”. The researcher also points to a present-day parallel: Nowadays, large marine species such as whales or seals are also highly vulnerable to human influences.

原始論文：Catalina Pimiento, John N. Griffin, Christopher F. Clements, Daniele Silvestro, Sara Varela, Mark D. Uhen, Carlos Jaramillo. The Pliocene marine megafauna extinction and its impact on functional diversity. Nature Ecology & Evolution, 2017; DOI: 10.1038/s41559-017-0223-6

引用自：University of Zurich. "Previously unknown extinction of marine megafauna discovered."