Loss of old pastures sparked global increase in grassland fires – study

The age of mammals, a mural by Rudolph F. Zallinger. Photo: © 2016 Peabody Museum of Natural History, Yale University

The loss of large North American herbivores, including ancient horses, has led to a dramatic increase in grassland fires across the continent, researchers have found.

A similar trend around the loss of large grazing animals has also been observed on other continents.

50,000 to 6,000 years ago, many of the world’s largest animals, including iconic grassland grazers like the woolly mammoth, giant bison, and a range of ancient horse species, were extinct.

The loss of these grazing species has triggered a dramatic increase in fire activity in the world’s grasslands, according to a new study led by Yale published in the journal Science.

Working with the Utah Museum of Natural History, scientists at Yale University have compiled lists of large extinct mammals and their approximate dates of extinction on four continents.

Data showed that South America lost the most grazers (83% of all species), followed by North America, at 68%. These losses were significantly higher than in Australia (44%) and Africa (22%).

They then compared these results with the records of fire activity revealed in the 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 large herbivores were extinguished.

Continents that lost more grazers (South America, then North America) experienced a larger increase in the extent of fires, while continents that experienced lower extinction rates (Australia and Africa) saw little change in prairie fire activity.

“These extinctions resulted in a cascade of consequences,” said Allison Karp, postdoctoral associate in the Department of Ecology and Evolutionary Biology at Yale and corresponding author of the article. “Studying these effects helps us understand how herbivores are shaping global ecology today.”

Widespread extinctions of megaherbivores have had major impacts on ecosystems, ranging from the collapse of predators to the loss of fruit trees that once depended on herbivores for their dispersal.

But Karp and senior author Carla Staver, associate professor of ecology and evolutionary biology at Yale School of Arts and Sciences, questioned whether there was an increase in fire activity in ecosystems as well. of the world, in particular due to an accumulation of dry grass, leaves or wood caused by the disappearance of giant herbivores.

They found that in the prairies grass-fed fires were on the increase.

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

Grassland ecosystems around the world have been transformed after the loss of grazing tolerant grasses due to the disappearance of herbivores and increased fires. New grazers, including livestock, eventually adapted to the new ecosystems.

That’s why scientists should consider the role of grazing cattle and wild grazers in fire mitigation and climate change, the authors said.

“This work really highlights how important grazers can be in shaping fire activity,” Staver said. “We need to pay close attention to these interactions if we are to accurately predict the future of fires. “

Report: Bill Hathaway

Global response of fire activity to late Quaternary grazer extinctions
Allison T. Karp, J. Tyler Faith, Jennifer R. Marlon, and A. Carla Staver.
Science, 25 November 2021, Vol 374, Number 6571, pp. 1145-1148, DOI: 10.1126 / science.abj1580

The summary can be read here.


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