In a couple of hundred years the largest animal walking on land may well be a cow, new modelling predicts.
A paper published in the journal Science uncovers for the first time a startling correlation between human migration and the extinction of large animals.
The link between the loss of big creatures and the spread of Homo sapiens and other hominin species was well established by the time humans left Africa around 125,000 years ago, researchers led by biologist Felisa Smith of the University of New Mexico, US, found.
With no indication that the trend is abating, the team predicts that all currently endangered large terrestrial species will pass within the next couple of centuries, leaving cows, at an average weight of 900 kilograms, the biggest things left.
To make their finding, the scientists used two data sets. The first was a global record of all terrestrial species known, and classified according to body mass and diet, for the late Quaternary period, which started around a million years ago. The second was a similar record for all known species in the Cenozoic period, which started 66 million years ago and is known colloquially as the Age of Mammals.
The data for the Quaternary was further divided into five sections – from 125- to 70-thousand years ago, coinciding with hominin migration out of Africa; 70- to 20-thousand years ago, covering the period in which hominins settled in Europe and Asia and reached Australia; 20- to 10-thousand years ago, during which humans reached the Americas; 10-thousand years ago until the present; and, lastly, a projection for the next 200 years.
The data covering the Cenozoic was divided into million year increments, and used as a control.
Smith and her colleagues found “a striking and significantly size-biased pattern of mammalian extinction” linked to hominin migration. Over all, species that went extinct were larger by two to three orders of magnitude than those that survived. The pattern was consistent in every period and on every continent from the time humans or other hominins arrived.
The probability difference between large and small animal extinction decreased in recent times, but the scientists suggest this because urbanisation and other threats increased extinction risk for smaller creatures rather than diminishing it for the remaining large ones.
During the pre-hominin Cenozoic, the results were starkly different. Extinctions still occurred, of course, but there was no association between body mass and extinction risk. In a related finding, the data set revealed that during the same period there was no observable link between size, climate change and extinction.
“Neither small nor large mammals were more vulnerable to extinction during times of high climate variability,” the researchers report.
Smith colleagues also found that the association between size, the presence of hominins and extinction was evident much earlier than expected.
On a general measure, they noted, body size distributions are tied to land mass. Therefore, largest average body mass fauna should be found on the largest continent, Eurasia, decreasing sequentially in Africa, North and South America and finally Australia.
Looking at data for 125,000 years ago they found that the prediction was “largely met”, with the notable exception of Africa, wherein mean body mass was less than 50% of that recorded in Eurasia or the Americas. The African result, the scientists suggest, “reflects the long prehistory of hominin-mammal interactions”.
An important consequence of the finding, they add, is that the loss of large animal species has not been solely because of the activities of Homo sapiens, but is also attributable to other, now extinct, human relatives.
“As Neanderthals, Denisovans, and humans spread across the globe over the late Quaternary, a highly size-biased extinction followed,” they note.
Future extinctions, they predict, will continue the trend towards loss of body size.
“Thus,” they conclude, “the largest mammal on earth in a few hundred years may well be a domestic cow.”