研究認為陸地移動的同時，也會影響到生物多樣性

原文網址：http://www.geologypage.com/2017/05/continents-continue-moving-study-suggests-effects-biodiversity.html

研究認為陸地移動的同時，也會影響到生物多樣性

大陸漂移和板塊構造學說係指地球龐大的地殼會發生緩慢但永不止息的運動，此概念雖然於1912年就已經有人提出，但到1960年代才被普遍接受。板塊運動會改變地球的樣貌，在3億3500萬年前，許多陸塊曾結合成盤古超大陸，之後於1億7500萬年前左右開始分裂。

板塊構造學說提出之後，科學家開始猜想陸塊的變化會如何影響物種的形成與滅亡，也就是所謂的生物多樣性。1970年，美國加州大學的James Valentine和Eldridge Moores提出大陸分裂會創造出更多元的生態棲地，並促成對生物多樣性有利的氣候和環境條件。

在這週(2017年5月15日)的《美國國家科學院院報》(Proceedings of the National Academy of Sciences)中，兩位美國威斯康辛大學麥迪遜分校的地質學家，探討了幾個目前為止最齊全的地質與古生物資料庫，得出他們的前輩思考方向確實沒錯：海洋生物在大陸分裂時傾向變得更為多元，而在陸地聚合時，物種數目則趨向穩定――甚至有所下降。

他們的報告著重在沉積岩中的海洋生物化石，因為相較於陸生生物化石，牠們的種類更加眾多且易於研究。

地質科學教授Shanan Peters和博士後研究員Andrew Zaffos，跟美國加州大學柏克萊分校的Seth Finnegan合作，比對了自5億4100萬年前開始，從「寒武紀大爆發」時大量產生的多細胞動物，其多樣性跟陸地的破碎程度，隨著時間經過兩者之間的關係。

研究人員先創造一個可以代表陸地相對破碎程度的參數，再將此參數跟古生物資料庫的全球化石數據對比。

結果就跟最初預測的一樣，但有少許不同。在分裂期間以及之後，海洋生物的多樣性會上升。在聚合階段，多樣化程度會開始踩煞車，然後海洋生物多樣性會維持在高值。

Peters表示研究無法精確測出為何陸塊移動會影響生物多樣性，但板塊構造作用確實有直接和間接影響。

傳統生態學理論指出族群從原有族群孤立出來後，隨著牠們進入空缺出來的生態區位，以及從共同祖先分離之後繁殖越來越多世代，會開始分化形成新的物種。這是為什麼現今的島嶼有那麼多特殊物種的原因之一。

但Peters表示間接效應也具有相當深遠的影響，「人們通常不太會想到這一點，但是陸地重新排列會對地球的海洋洋流、大氣環流以及季節強弱程度產生重大影響。地球系統的運作中有很大一部份會牽扯到地殼本身，以及地殼在地質時間尺度下的移動方式。」

Zaffos表示大陸聚合在一起會降低生物多樣性的說法是有道理的，「海洋生物多樣性最豐富的地方位於大陸邊緣的淺海地區。在印度撞上亞洲之前，兩者之間有面積更大的大陸邊緣地區可供海洋生命居住。」陸地變得更加破碎也會產生更多的孤立動物族群，另外，因為水氣的來源――海洋跟陸地相對靠近，也容易形成更多不同的氣候體系。

在這篇涵蓋超過5億年的研究中，有許多因子讓情況變得更加複雜：陸地的聚合―分裂―聚合循環僅發生了一次半，但小行星撞擊和氣候變遷造成了數次大滅絕，也同樣影響了海洋物種的數目；而地質時代於近期呈現的生物多樣性增加，可能只是反映出越接近近代，化石保存情況就越良好。然而，Peters和Zaffos檢視了由Peters參與建立，大量彙整北美地質研究的Macrostrat資料庫後，Zaffos表示：「北美的沉積岩層紀錄提供了一個完美的檢測工具，讓我們可以控制跟岩石紀錄有關的取樣偏差可能帶來的影響。」

1970年的研究第一作者Valentine在看過這篇PNAS的論文草稿後表示：「我對結果感到十分開心。順道一提，這篇新研究真的相當傑出而讓我更加滿意，因為作者讓此概念擁有相當嚴謹的科學基礎，現在看來其含有的基礎理念不可能再被動搖了。」

諷刺的是，海洋化石的研究正是Alfred Wegener於20世紀早期，發展板塊構造理論時的主要發源之一。從光明面來看，板塊運動現今終於可以用來解釋過去5億年來，海洋動物的多樣性為何會如此改變。

當1970年代有人將板塊構造運動和生物多樣性搭上關聯時，Peters說：「大體上來說只能算是思想實驗。」「當時雖然對生物多樣性的歷史已有一般程度的瞭解，但卻沒有多少數據可以測試這種想法。到十幾年以前，我們才終於能彙整所需的數據，來進行稍微比較嚴謹的分析。」

Peters以地質學家的長遠眼光來看，表示從數百萬年前海洋生物多樣性的趨勢開始逐漸下降。「化石紀錄似乎指出過去數百萬年來，多樣性有下降的現象，而此趨勢可能會維持下去。印度已經跟亞洲碰撞，而非洲正迎頭撞上往歐洲，因此地中海勢必將會關閉。不論我們今日因為何種原因喪失更多物種，從地質時間角度來看，生物多樣性只會越來越難以挽回。」

As continents continue moving, study suggests effects on biodiversity

Continental drift and plate tectonics—the notion that large chunks of Earth’s crust slowly but inexorably shift positions—was proposed in 1912 but not accepted until the 1960s. These movements changed the face of the planet—pieces of the continents congealed into the “supercontinent” Pangaea about 335 million years ago and then separated about 175 million ago.

Scientists began to speculate about how these alterations would affect the formation and extinction of species and thus, what we call biodiversity. In 1970, James Valentine and Eldridge Moores of the University of California suggested that broken-up continents would create more ecological niches and promote favorable climate and environmental conditions that are conducive to biodiversity.

In the Proceedings of the National Academy of Sciences this week (May 15, 2017), two University of Wisconsin–Madison geoscientists have plumbed some of the broadest databases in geology and paleontology to show that their predecessors were on the right track: Marine species tend to become more numerous when the continents divide, and to stabilize—maybe even decline—when continents congeal.

Their report focused on fossilized marine species in sedimentary rock, which are more numerous and easier to study than species that lived on land.

Shanan Peters, a professor of geoscience, Andrew Zaffos, a postdoctoral researcher, and collaborator Seth Finnegan at the University of California, Berkeley, correlated the degree of continental fragmentation through time, starting 541 million years ago, with the diversity of multicellular life, which expanded during the “Cambrian explosion.”

The researchers created an index to show relative continental fragmentation and then compared that index to global fossil data in the Paleobiology Database.

The result was as originally predicted, with a few twists. During and after periods of fragmentation, marine diversity increases. During consolidation, the brakes seem to be put on diversification and marine biodiversity tends to plateau.

The study was unable to determine exactly why the movement of continents affected biodiversity, but plate tectonics has both direct and indirect effects, Peters says.

Conventional ecological theory says that an isolated population will diverge from the original population, forming new species as organisms enter empty niches and as an increasing number of generations separate them from their common ancestor. This is one reason why modern islands have so many unique species.

But the indirect effects could also be dramatic, Peters says. “People don’t think about it too much, but the arrangement of continents on Earth has a huge effect on ocean currents, atmospheric circulation, how strong the seasons are. A whole range of things about how Earth works is determined by the crust, and that crust moves on geological time scales.”

There is logic behind the idea that a consolidated continent would have lower diversity, says Zaffos. “The vast majority of marine diversity is on continental edges, in shallow seas. Before India slammed into Asia, there was more area of continental margin that could be occupied by marine life.” Fragmented continents also have more isolated animal populations and tend to have different climate regimes because the ocean, the source of water vapor, is closer.

There were plenty of complications in a study covering more than a half-billion years: The consolidation-fragmentation-consolidation cycle ran only one-and-a-half times; the asteroid impacts and climate changes that contributed to several mass extinctions also affected the number of marine species; and the increasing biodiversity in recent geologic times could be a reflection of better fossil preservation. However, Peters and Zaffos examined a database Peters spearheaded called Macrostrat that collates a vast number of geological studies of North America. “The North American sedimentary record provided a sanity check on our study, allowing us to control for potential rock record-related sampling effects,” says Zaffos.

“I was delighted,” says Valentine, first author of the 1970 study, who read a draft of the PNAS paper. “And by the way, the new study is a really fine paper, which adds satisfaction because those authors have put the concept on a very firm scientific basis and it seems unlikely that the basic idea can be successfully challenged now.”

Ironically, the study of marine fossils was a major springboard when Alfred Wegener developed the theory of plate tectonics early in the 20th century. In a delightful about-face, plate tectonics has now been used to explain changes in the diversity of marine animals over the last half-billion years.

When the linkage between tectonics and biodiversity was made in 1970, “it was largely a thought experiment,” says Peters. “There was some general information about the history of biodiversity, but there was very little data to test the idea. Only in the past decade or so have all the data come together in a way that makes a somewhat rigorous analysis possible.”

The trend in marine biodiversity started to fall a few million years ago, says Peters, who takes the long view of a geoscientist. “The fossil record of biodiversity seems to indicate that diversity has been decreasing for the past few million years, and that trend could continue. India has already collided with Asia, and Africa is impinging on Eurasia, so eventually the Mediterranean will close. If we lose a lot of species today, for whatever reason, on a geological time scale, it’s going to be harder to recover.”

原始論文：Andrew Zaffos el al., “Plate tectonic regulation of global marine animal diversity,” PNAS (2017). DOI: 10.1073/pnas.1702297114

引用自：University of Wisconsin-Madison. “As continents continue moving, study suggests effects on biodiversity.” May 18, 2017.