19 August 2010, by Tom Marshall
Woolly mammoths and other big mammals may have died out because declining grasslands and spreading forests reduced their food supply.
Some researchers have argued that hunting and competition for land from humans killed off creatures like mammoths, woolly rhinos and cave lions. But a new study by scientists at Bristol and Durham Universities and at Lund University in Sweden, published in Quaternary Science Reviews, suggests otherwise.
'The change from productive grasslands across large areas of northern Eurasia, Alaska and Yukon to less productive tundra-like habitats had a huge effect on many species, particularly on the large herbivores like the woolly rhinoceros and woolly mammoth,' says Professor Brian Huntley of the School of Biological Sciences at Durham.
'Mammoths and other mega-mammals found it increasingly difficult to find food. We believe this loss of food supplies was the major contributing factor to the extinction of these mega-mammals,' he adds.
The researchers reviewed evidence on the climate and vegetation of the northern hemisphere during and at the height of the last Ice Age, 21,000 years ago. In particular they examined the output of models of the Earth's vegetation cover, driven in turn by climate models, as well as records of atmospheric carbon dioxide levels over the period.
They used the records of ancient pollen found in sediment cores from peat bogs and lake beds to check this output matched reality, looking at a vast area including Eurasia and the area around the Bering land bridge, which connected Alaska and the Canadian Yukon to Siberia.
'The big question is whether there was anything except chance driving which species went extinct and which didn't.'
Professor Brian Huntley, Durham University
The conclusion was that over much of the Earth, grasslands' production of biomass fell precipitously as the climate became warmer and wetter, and forests expanded. These changes went alongside increasing levels of carbon dioxide in the atmosphere
This would have made grazing much harder for herbivores like mammoths, with knock-on effects throughout the food chain. Forests produce plenty of biomass, but not much of it is good eating for animals that have evolved to graze. This could have been an important factor in the extinction of many large mammals in northern Eurasia and North America.
Woolly mammoths retreated from northern Europe to Siberia 11,500 years ago after being found in large numbers throughout many parts of Europe, including the UK, for most of the previous 100,000 years or more. Their population dwindled until only a few remnant populations were found on islands off northern Siberia, and these animals seem to have become smaller and smaller over the period in response to worsening conditions.
Carnivores like cave lions may have found it harder to hunt in forested conditions than on grassy plains, and their plant-eating prey would have become rarer as the land available for grazing shrank.
Our ancestors were spreading across the globe around the same time, and have been blamed for hunting mammoths and other large mammals to extinction. But Huntley says the team's research has provided a more plausible explanation for these extinctions.
'We now have evidence of how earlier climatic fluctuations drove changes in the abundance of animal species, and a mechanism that can explain how climate change would have affected herbivore populations,' he says.
The question remains of why some animals died out while others survived. Species like the musk ox, the reindeer and the Saiga antelope were part of the same community of grassland-dwelling big animals, but have survived to the present day – although some, like the musk ox, have such little genetic variation in their modern populations that scientists think they must have been reduced to a tiny number of individuals before managing to claw their way back from the brink of extinction.
'The big question is whether there was anything except chance driving which species went extinct and which didn't,' Huntley comments. 'Why did mammoths and giant deer die out while musk oxen and reindeer survived?'
The research shows how comparatively fast climate change has led to mass extinctions in the past, with the largest animals often the most vulnerable because they need so much food. 'This is a model for what may happen as a result of rapid climate change over the next century linked to human activity,' Huntley says. 'It is food for thought in these times of global warming and human-induced climate change.'