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The end of the mammoths gave rise to a tinderbox world that was more likely to catch fire.
Researchers studying the extinction of megaherbivores during the past 50,000 years found that fires increased as grasses and dead leaves built up, with implications for modern climate change.
The loss of some of the world's largest-ever land animals saw the distant past gripped by wildfires.
A team of American researchers found that the extinction of animals such as the giant ground sloth, the mammoth and giant bison led to more fires over 10,000 years ago. The extinction of the herbivores is believed to have left more grass and dead leaves available to provide the fuel for the flames.
Their findings add fuel to suggestions that grazing animals should be introduced and supported to control modern forest fires, which are likely to become more common as the world warms.
Dr Allison Karp, the paper's lead author, says, 'These extinctions led to a cascade of consequences. Studying these effects helps us understand how herbivores shape global ecology today.'
The findings of the researchers, led by Yale University, were published in the journal Science.
The Pleistocene Epoch was a time of climate instability, with variations in the Earth's orbit affecting the amount of the Sun's energy reaching the surface. Glaciers formed and retreated across the world on numerous occasions as the climate varied over millions of years.
Towards the end of the Pleistocene, the last Ice Age came to its conclusion as glaciers retreated towards the poles, opening up vast new areas of land that had previously been buried.
This changing climate took its toll on the megaherbivores that were roaming the cold world. From around 50,000 to 4,000 years ago, many species were put under increasing pressure and did not survive this transition.
Dr Adrian Lister, who researches the extinction of these giant animals at the Museum and who was not involved with the paper, says, 'Megafaunal extinctions, as the name implies, affected almost entirely the largest, grazing mammals and some of the predators that depended on them.
'These species could be ecosystem engineers which affected the habitat by their feeding on and trampling of grasses, damaging trees, and fertilising the ground with their dung. Their extinction, whether caused by climate change, human activity, or a combination of the two, is likely to have had wider-reaching effects on the environment.
'Moreover, this effect would have varied globally as extinctions were not uniform across the continents. They were highest in the Americas, moderate in Eurasia, and low in Africa.'
Without herbivores to hold them back, grasslands and other vegetation could spread, with previous research showing that fire would have then taken over as the main limit on plants' distribution.
However, whether this would have led to an increase in fires was yet to be demonstrated. The researchers in this paper used charcoal samples and known megaherbivore traits to see just how the world was affected.
The researchers estimated the changes across four continents – North and South America, Africa and Australasia. Europe and Asia were excluded due to difficulties in obtaining reliable data for analysis.
South America lost 83% of its grazing herbivores, leaving just five remaining species. North America was also badly affected, while Africa was the least. The majority of the remaining species, particularly in the Americas, were ruminants which use multiple stomachs to obtain as much energy from plants as possible.
As these extinctions mounted, grassland fires increased significantly in the Americas as fewer animals were consuming the plants. Meanwhile, in Australasia and Africa, the levels of fire stayed the same (or even fell) thanks to the lower levels of extinction.
Beyond grasslands, fires increased across a range of other habitats in all four continents. While Australasia showed the strongest increase, North America had a limited difference between grass and non-grass environments. This has been attributed to rising temperatures at the time which made fires in general more likely.
As for the browsers, animals which eat fruits, shoots and leaves, their extinction did not have as severe an impact on woodlands as expected. While researchers are not exactly sure of the cause, they believe that some woodlands may be less vulnerable to fire as they exist in very moist environments, such as the rainforest.
This differing effect, however, does appear to vindicate our early ancestors of being totally responsible for the higher level of fires. While they are still likely to have hunted the megaherbivores and started fires, the effect would be the same for all environments if humans were the main cause.
The researchers hope that their research will help mitigate modern climate change by ensuring more is done to support grazing animals, which can reduce the risk of serious fires.
Co-author Professor Carla Staver says, 'This work really highlights how important grazers may be for shaping fire activity. We need to pay close attention to these interactions if we want to accurately predict the future of fires.'