巨獸消失，野火遍起

 原文網址：https://news.yale.edu/2021/11/25/loss-ancient-grazers-triggered-global-rise-fires

By Bill Hathaway

距今50000年至6000年前，全世界許多體型龐大的動物都滅亡了，包括生活在草原、極為代表性的食草者(grazer)：長毛象、巨型野牛、古馬……等。由耶魯大學主持，發表在期刊《科學》(Science)的新研究指出，這些食草動物消失引發了全世界草原的野火活動劇烈增加。

這幅Rudolph F. Zallinger的壁畫描繪了猛瑪象生活的年代。(圖片來源：耶魯大學皮博迪自然史博物館，2016)

耶魯大學的科學家與猶他自然史博物館合作之下整理出一份清單，列出四座不同大陸當中分別有哪些絕種的大型哺乳類以及牠們大概是在什麼時候滅亡。資料顯示南美洲食草者消失的比例最高(佔全部種類的83%)，接下來是北美洲(68%)，遠遠高出澳洲(44%)與非洲(22%)。

他們接著將這項發現與湖泊沉積物得出的野火活動紀錄互相比較。他們利用全球各地410個地點的木炭紀錄，建立了各大洲區域性野火活動的歷史紀錄，結果發現大型食草者消失之後野火活動的頻率也增加了。有較多食草者消失的大陸(南美，其次是北美)野火增加的程度也越高，而絕種比率較低的大陸(澳洲與非洲)草原的野火活動則沒有出現太大的變化。

「這些動物絕種造成了一連串的後果，」論文通訊作者Allison Karp表示。她是耶魯大學生態與環境生物學系的博士後研究員。「研究這些效應可以幫助我們瞭解草食動物如何塑造出目前全世界的生態樣貌。」

許多地方的巨型草食動物絕種對於生態系有重大影響：從掠食者跟著衰亡，到過往仰賴草食動物來散播果實的樹木消失都包括在內。但是Karp和這篇文章的資深作者，耶魯大學文理學院生態學與演化生物學的副教授Carla Staver，猜想巨型草食動物絕種是否也會讓全世界生態系的野火活動增加――具體理由為巨型草食動物消失會讓乾草、葉子和木頭累積在環境當中。

不過Karp和Staver注意到古代的食葉動物(browser)，也就是乳齒象、雙門齒獸、大地懶這類啃食林地中灌木與樹木的動物，在同一期間也有許多種類滅絕了，但是牠們消失對於林地的野火活動造成的影響比較輕微。

草食動物消失以及野火增加造成耐啃食的草類消失，使得全世界的草原生態系跟著轉變。最後出現可以適應新型生態系的新食草者，包括家畜在內。

作者表示這是為什麼科學家在減少野火以及研究氣候變遷時，應該要考慮啃食草類的家畜與野生動物所扮演的腳色。「這項研究確實凸顯了食草者對於野火活動可能具有非常重要的影響，」Staver表示。「如果想要精確預測未來的野火活動，我們需要更加關注這些交互作用才行。」

 

Loss of ancient grazers triggered a global rise in fires

From 50,000 years to 6,000 years ago, many of the world’s largest animals, including such iconic grassland grazers as the woolly mammoth, giant bison, and ancient horses, went extinct. The loss of these grazing species triggered a dramatic increase in fire activity in the world’s grasslands, according to a new Yale-led study published Nov. 26 in the journal Science.

In collaboration with the Utah Natural History Museum, Yale scientists compiled lists of extinct large mammals and their approximate dates of extinctions across four continents. The data showed that South America lost the most grazers (83% of all species), followed by North America (68%). These losses were significantly higher than in Australia (44%) and Africa (22%).

They then compared these findings with records of fire activity as revealed in lake sediments. Using charcoal records from 410 global sites, which provided a historical record of regional fire activity across continents, they found that fire activity increased after the megagrazer extinctions. Continents that lost more grazers (South America, then North America) saw larger increases in fire extent, whereas continents that saw lower rates of extinction (Australia and Africa) saw little change in grassland fire activity.

“These extinctions led to a cascade of consequences,” said Allison Karp, a postdoctoral associate in Yale’s Department of Ecology & Evolutionary Biology and corresponding author of the paper. “Studying these effects helps us understand how herbivores shape global ecology today.”

Widespread megaherbivore extinctions had major impacts on ecosystems — ranging from predator collapse to loss of fruit-bearing trees that once depended on herbivores for dispersal. But Karp and senior author Carla Staver, associate professor of ecology and evolutionary biology in Yale’s Faculty of Arts and Sciences, wondered if there was also an increase in fire activity in the world’s ecosystems, specifically due to a buildup of dry grass, leaves, or wood caused by the loss of giant herbivores. They found that, in grasslands, grass-fueled fires increased.

However, Karp and Staver note that many ancient browser species — such as mastodons, diprotodons, and giant sloths, which foraged on shrubs and trees in wooded areas — also went extinct during the same period but that their losses had less impact on fires in wooded areas.

Grassland ecosystems across the world were transformed after the loss of grazing-tolerant grasses due to the loss of herbivores and increase in fires. New grazers, including livestock, eventually adapted to the new ecosystems.

That’s why scientists should consider the role of grazing livestock and wild grazers in fire mitigation and climate change, the authors said. “This work really highlights how important grazers may be for shaping fire activity,” Staver said. “We need to pay close attention to these interactions if we want to accurately predict the future of fires.”

原始論文：Allison T. Karp, J. Tyler Faith, Jennifer R. Marlon, A. Carla Staver. Global response of fire activity to late Quaternary grazer extinctions. Science, 2021; 374 (6571): 1145 DOI: 10.1126/science.abj1580

引用自：Yale University. "Loss of ancient grazers triggered a global rise in fires."