By Eric Hamilton
10000年前左右，冰層最近一次大量消融時地球的植被發生了快速的變化。對於全球花粉化石的調查結果發現地球植被目前的變化速度至少和當時一樣快。
為了研究過去的植物生態系與它們的變化過程，研究人員前往湖泊或者其他適合的環境來鑽取岩芯，這些層層堆疊的沉積物裡通常含有數千年來不斷堆積下來的花粉顆粒。接下來，透過鑑定並計算裡面不同種類的花粉顆粒有多少，研究人員可以重建當地的植被組成。最後，利用花粉豐度隨著時間的變化，就能估計出植被的變化速率。圖片繪自Milan Teunissen Van Manen
當時的植物競相駐足先前被冰封的地表，同時為了適應升高華氏10度左右的全球氣候，使得植物群集的組成發生了快速的演替。而在3000到4000年前的某個時候，地球的植物群集再次加速變化，時至今日已經媲美甚至超過了10000年前的速度。
目前可以清楚看到人類造成的影響已經主宰了生態系。5月20日發表在⟪科學⟫(Science)的研究指出，這類影響可以追溯到最早期的文明，以及農業、森林砍伐和人類影響地景的各種方式興起的時候。
這項成果也提出隨著當下的氣候變遷加入這段歷史悠久的變化過程，未來數十年生態系的變化速度會繼續加快。此外，研究顯示生物多樣性在近代的變化趨勢，代表了生態系將要開始發生長時間的加速轉變。對於其他全世界生物多樣性在過去一世紀加速變化的近期報告來說，本研究也為它們提供了歷史脈絡。
這篇新研究由一組國際科學團隊領導，他們分析的資料取自一個新的古生態學資料庫――「林鼠」(Neotoma Paleoecology Database)，此開放取用的工具蒐集並遴選了數百位科學家研究過去的生態系而得到的數據。「林鼠」的負責人為威斯康辛大學麥迪遜分校的地理學教授Jack Williams，他也是這項新研究的主持人之一。
為了瞭解從大約18000年前，末次冰期即將結束以來植物生態系的變化過程以及速度，研究作者分析「林鼠」裡面1100多筆的花粉化石紀錄，範圍涵蓋了除南極洲以外的所有大陸。
研究人員在阿拉斯加的聖保羅島採集岩芯樣品。照片來源：Jack Williams
「在冰河期結束的時候，生態系發生了徹底的轉變，規模可達整個生物群系(biome)，」Williams表示，他也負責遴選「林鼠」北美花粉資料庫所羅列的數據。「過去數千年生態系再次歷經了此等變化。如此長遠的變化開始時間可能比我們過往認為的還要更早。」
花粉化石可以極為敏銳地測出過去植物群集的組成。當湖泊周圍的花粉落入水中，它們會由老至新層層往上堆疊。科學家取出沉積物岩芯並且詳細鑑定裡面的花粉種類之後，就能重建數千年來植物生態系的組成如何改變。
但是每根沉積物岩芯大概都只能提供一個地點的資訊，因此要真正地從全球尺度來分析植被的變化，就需要蒐集許多花粉紀錄並加以遴選。而「林鼠」彙整的數千個資料點便能幫助科學家看出全球的變化趨勢。挪威卑爾根大學與威斯康辛大學麥迪遜分校的研究人員，以及「林鼠」在世界各地的資料管理員聯合起來進行了這項新的分析。
團隊利用最新的統計工具來分析這些花粉紀錄，使他們可以更完整地分析植物群集過去18000年以來變化的速度有多快。
他們發現不同大陸的變化速度在8000到16000年前各自迎來第一次高峰。這項差異的成因可能是每個大陸的氣候變遷模式與進程各有不同，這又牽扯到冰河後退、大氣二氧化碳濃度上升、地球軌道變化以及海洋大氣環流變化。
接著生態系便處於穩定的狀態。但是到了4000年前左右，變化速度又開始飛升並且持續至今。大部分的植物生態系的變化速度，至少和冰河期引發變動時的巔峰速度一樣快。
「這項發現令人驚訝，因為過去數千年氣候並沒有發生什麼太大的事件，但是生態系的變化卻是末次冰期以來我們看到的變化中最劇烈的之一，甚至比其他的都還嚴重。」
雖然這項分析著重在找出生態系的變化，而沒有正式去判別背後的成因，但是生態系在近代的變化可以對應至全世界集約農業興起，以及城市與文明最初出現的時候。
Williams 表示分析結果中一項有趣的特徵是：就算各大陸土地利用、農業發展與都市化的過程都不一樣，但是全世界的生態系都在非常久以前就開始加速變化。
科學家創造「人類世」(Anthropocene)這個詞彙來稱呼當前的地質時代，代表這段時間人類為世上最具影響力的事物。「其中一項尚未解答的問題是：人類世開始於什麼時候？」Williams表示。「這項成果顯示3000到4000年前，人類就已經對世界造成了巨大的衝擊，並且持續至今。」
科學家表示這項成果的涵義之一卻也令人難過。生態系在過去因為氣候變遷造成的轉變與土地利用造成的轉變大部分並未重疊；但現在土地利用變得更加密集的同時，全世界也因為溫室氣體累積而加速暖化。隨著人類造成的直接衝擊與人為引發的氣候變遷聯合起來使植物群集發生變化，未來生態系的轉變速度很可能會再次創下新高。
在研究生態系的科學中，一項重要的問題是生態系隨時間的變化速度有多快。變化速度相當快的生態系，代表物種組成在非常短的時間內有重大改變。圖片繪自Milan Teunissen Van Manen
Earth’s vegetation is changing faster today than it has over the last 18,000 years
A global survey of fossil pollen has discovered that the planet’s vegetation is changing at least as quickly today as it did when the last ice sheets retreated around 10,000 years ago.
Beginning some 3,000-to-4,000 years ago, Earth’s plant communities began changing at an accelerating pace. Today, this pace rivals or exceeds the rapid turnover that took place as plants raced to colonize formerly frozen landscapes and adapt to a global climate that warmed by about 10 degrees Fahrenheit.
The research, published May 20 in Science, suggests that humanity’s dominant influence on ecosystems that is so visible today has its origin in the earliest civilizations and the rise of agriculture, deforestation and other ways our species has influenced the landscape.
This work also suggests ecosystem rates of change will continue to accelerate over the coming decades, as modern climate change further adds to this long history of flux. And by showing that recent biodiversity trends are the start of a longer-term acceleration in ecosystem transformations, the new study provides context for other recent reports that global biodiversity changes have accelerated over the last century.
An international collaboration of scientists led the new analysis, which was powered by an innovative database for paleoecological data. The Neotoma Paleoecology Database is an open-access tool that gathers and curates data on past ecosystems from hundreds of scientists. Neotoma is chaired by University of Wisconsin–Madison professor of geography Jack Williams, who helped lead the new research.
The study authors analyzed more than 1,100 fossil pollen records from Neotoma, spanning all continents except Antarctica, to understand how plant ecosystems have changed since the end of the last ice age about 18,000 years ago, and how quickly this change occurred.
“At the end of the ice age, we had complete, biome-scale ecosystem conversions,” says Williams, who also curates Neotoma’s North American pollen database. “And over the past few thousand years, we’re at that scale again. It has changed that much. And these changes began earlier than we might have thought before.”
Fossil pollen provides an extremely sensitive measure of past plant communities. As pollen from surrounding plants falls into lakes, it settles in layers from oldest at the bottom to newest at the top. Scientists can extract sediment cores and conduct the painstaking work of identifying pollen and reconstructing plant ecosystems over thousands of years.
Yet each sediment core only provides information about one place on Earth, so true global-scale analyses of past vegetation change require the amassing and curating of many such records. Neotoma has gathered thousands of such datapoints to help scientists uncover global trends. Researchers from the University of Bergen in Norway, UW–Madison, and Neotoma data stewards from around the world collaborated to perform the new analysis.
Using these pollen records, the team applied new statistical methods to better analyze how quickly plant communities have changed in the last 18,000 years.
They discovered that the rate of change initially peaked between 8,000 and 16,000 years ago, depending on the continent. These continental differences are likely caused by different timing and patterns of climate change linked to retreating glaciers, rising carbon dioxide concentrations in the atmosphere, changes in Earth’s orbit, and changes in ocean and atmospheric circulation.
Ecosystems then stabilized until about 4,000 years ago. Then, the rate of change began a meteoric rise that continues today, when most plant ecosystems are changing at least as fast as they did at the peak of ice-age-induced flux.
“That was a surprising finding, because over the last few thousand years, not a whole lot was happening climatically, but the rates of ecosystem change were as big or bigger than anything we’ve seen from the last ice age to the present,” says Williams.
Although this analysis of pollen records was focused on detecting ecosystem changes, rather than formally determining causes, these recent ecosystem changes correlate with the beginning of intensive agriculture and the earliest cities and civilizations around the world.
Williams says that one intriguing feature of these analyses is that the early rise is so early worldwide, even though each continent had different trajectories of land use, agricultural development and urbanization.
Scientists have coined the term the Anthropocene to describe the modern geological period, when humans are the dominant influence on the world. “And one of the questions has been, when did the Anthropocene begin?” says Williams. “This work suggests that 3,000 to 4,000 years ago, humans were already having an enormous impact on the world (and) that continues today.”
A sobering implication from this work, say the scientists, is that in the past, the periods of ecosystem transformations driven by climate change and those driven by land use were largely separate. But now, intensified land use continues, and the world is warming at an increasing rate due to the accumulation of greenhouse gases. As plant communities respond to the combination of direct human impacts and human-induced climate change, future rates of ecosystem transformation may break new records yet again.
原始論文：Ondřej Mottl, Suzette G. A. Flantua, Kuber P. Bhatta, Vivian A. Felde, Thomas Giesecke, Simon Goring, Eric C. Grimm, Simon Haberle, Henry Hooghiemstra, Sarah Ivory, Petr Kuneš, Steffen Wolters, Alistair W. R. Seddon, John W. Williams. Global acceleration in rates of vegetation change over the last 18,000 years. Science, 2021 DOI: 10.1126/science.abg1685
引用自：University of Wisconsin-Madison. "Earth's vegetation is changing faster today than it has over the last 18,000 years."
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