Fresh research has thrown the blame on mankind for most of the extinctions of major land mammals in the last 132,000 years.
They say there is a much stronger correlation between the spread of modern humans and extinctions than there is between climate change and extinctions.
The scientists from Aarhus University in Denmark said the extinction pattern across North America was not consistent with climate as the main driver because species with small geographical ranges were not disproportionately affected.
“The period of extinction in North America occurred approximately 11,500 to 10,000 years before present, well within our period of measured climate change, and broadly occurred when climate change was severe, but also consistent with the arrival of modern humans,” the researchers say.
They said their results confirmed earlier observations that extinctions could be severe even in relatively climatically stable regions in North America, where the vegetation changed little.
Christopher Sandom, Søren Faurby, Brody Sandel and Jens-Christian Svenning carried out what they said was the first global analysis of the extinction of large mammals, and their conclusion was clear – humans were to blame.
Their findings are sure to ignite the debate around the cause of the extinction of the horse and other major land mammals in North America, with most scientists in the field believing climate change was the vital driving factor.
The latest findings, published in the journal, Proceedings of the Royal Society B, involved a detailed analysis covering the last 132,000 years, but excluded the last 1000 years.
“Our results strongly underline the fact that human expansion throughout the world has meant an enormous loss of large animals,” Faurby said.
For almost 50 years, scientists have been discussing what led to the mass extinction of large animals, also known as megafauna, during and immediately after the last Ice Age.
One of two leading theories states that the large animals became extinct as a result of climate change.
Significant climate changes were recorded, especially towards the end of the last ice age – just as there had been during previous ice ages – and this meant that many species no longer had the potential to find suitable habitats and they died out as a result.
However, because the last ice age was just one in a long series of ice ages, it was puzzling that a corresponding extinction of large animals did not take place during the earlier ones.
The other theory concerning the extinction of the animals is “overkill”.
Modern man spread from Africa to all parts of the world during the course of a little more than the last 100,000 years.
In simple terms, the overkill hypothesis states that modern man exterminated many of the large animal species on arrival in the new continents.
This was either because their populations could not withstand human hunting, or for indirect reasons such as the loss of their prey, which were also hunted by humans.
In their study, the researchers produced the first global analysis and relatively fine-grained mapping of all the large mammals (with a body weight of at least 10kg) that existed from 132,000 to 1000 years ago – the period during which the extinctions took place.
They were thus able to study the geographical variation in the percentage of large species that became extinct on a much finer scale than previously achieved.
The researchers found that a total of 177 species of large mammals disappeared during this period – a massive loss. Africa lost a relatively low 18 species and Europe 19, while Asia lost 38 species. Australia and the surrounding area lost 26, while North America lost 43, including the horse, and South America lost a total of 62 species of large mammals.
The extinction of the large animals took place in virtually all climate zones and affected cold-adapted species such as woolly mammoths, temperate species such as forest elephants and giant deer, and tropical species such as giant cape buffalo and some giant sloths.
It was observed on virtually every continent, although a particularly large number of animals became extinct in North and South America, where species including sabre-toothed cats, mastodons, giant sloths and giant armadillos disappeared, and in Australia, which lost animals such as giant kangaroos, giant wombats and marsupial lions.
There were also fairly large losses in Europe and Asia, including a number of elephants, rhinoceroses and giant deer.
The results show that the correlation between climate change – that is, the variation in temperature and precipitation between glacial and interglacial periods – and the loss of megafauna is weak, and can only be seen in one sub-region, namely Eurasia.
Globally, human palaeobiogeography alone accounted for 64 percent of the variation in extinction, while temperature anomaly, in combination with precipitation velocity in the best climate-only model, had much weaker explanatory power – around 20 percent.
“The significant loss of megafauna all over the world can therefore not be explained by climate change,” said Sandom, “even though it has definitely played a role as a driving force in changing the distribution of some species of animals.
“Reindeer and polar foxes were found in Central Europe during the ice age, for example, but they withdrew northwards as the climate became warmer.”
On the other hand, the results show a very strong correlation between the extinction and the history of human expansion.
Svenning, a professor at the university, commented: “We consistently find very large rates of extinction in areas where there had been no contact between wildlife and primitive human races, and which were suddenly confronted by fully developed modern humans.
“In general, at least 30 percent of the large species of animals disappeared from all such areas.”
The researchers’ geographical analysis pointed strongly to humans as the cause of the loss of most of the large animals.
The results also drew a straight line from the prehistoric extinction of large animals via the historical regional or global extermination due to hunting – American bison, European bison, quagga, Eurasian wild horse or tarpan, and many others – to the current critical situation for a considerable number of large animals, such as the rhinoceros, as a result of poaching and hunting.
C. Sandom, S. Faurby, B. Sandel, J.-C. Svenning. Global late Quaternary megafauna extinctions linked to humans, not climate change. Proceedings of the Royal Society B: Biological Sciences, 2014; 281 (1787): 20133254 DOI: 10.1098/rspb.2013.3254
The full study can be read here.