While enormous wildfires affecting the world’s grasslands may feel very much a recent phenomenon fuelled by the human-driven climate crisis, new research suggests we have been here before, although for a very different reason.
A study has found that as they went extinct, the loss of ancient grazing megafauna such as the woolly mammoth, the giant bison and ancient horses, may have played a significant role in increasing the level of fires.
Over a period from 50,000 to 6,000 years ago, many of the planet’s largest land animals went extinct – many of which were huge grazing species that had significant impacts on the environments through which they moved in search of food.
The study, led by academics at Yale University, indicates the loss of these grazing species “triggered a dramatic increase in fire activity in the world’s grasslands”.
In order to understand the level of losses of grazing species across the world, the 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, with 83 per cent of all species wiped out in this pre-historic period, followed by North America (68%). These losses were significantly higher than in Australia (44%) and Africa (22%).
Using this data, they compared the losses of species 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.
They found that on the continents which had lost the most megagrazers – South America, then North America – there were subsequently larger increases in fire extent.
Meanwhile in Australia and Africa, the continents that saw lower rates of extinction, they recorded little change in grassland fire activity.
The loss of the megagrazers had broad implications for how ecosystems functioned, the researchers said.
“These extinctions led to a cascade of consequences,” said Allison Karp, a postdoctoral associate in Yale’s Department of Ecology & Evolutionary Biology, one of the authors of the paper.
“Studying these effects helps us understand how herbivores shape global ecology today.”
The team said such widespread megaherbivore extinctions had impacts ranging from predator collapse to loss of fruit-bearing trees that once depended on herbivores for dispersal.
It was while studying these impacts that Dr Karp and senior author Carla Staver, associate professor of ecology and evolutionary biology in Yale’s Faculty of Arts and Sciences, also wondered if there was also an increase in fire activity, which they hypothesised could occur 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 noted 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, the researchers said.
New grazing species, including livestock, eventually adapted to the new ecosystems.
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,” Dr Staver said.
“We need to pay close attention to these interactions if we want to accurately predict the future of fires.”