According to a Danish study, following the mass extinction event triggered by humans, it will take between 3 and 5 million years for biodiversity to return to its current levels.
In the course of its slow but inexorable advance, nature has developed a series of evolutionary mechanisms to regulate the density of animal and plant species. The appearance of a bizarre, hairless primate, Homo sapiens, capable at once of wiping out thousands of species and of trying to bring back creatures that are already extinct, has, however, upset the whole game. We have driven animals and plants to the brink of extinction at an unsustainable rate, never seen before, effectively rendering the selective mechanism of evolution useless.
In the history of the planet, five major catastrophes have occurred, nicknamed the "Big Five", which caused the collapse of biodiversity. Currently, according to many scientists, one of these epochal events is under way: Earth's sixth mass extinction. Until a few tens of thousands of years ago the planet was populated by colossal creatures, such as woolly rhinoceroses, aurochs, mammoths, cave bears and giant ground sloths. Within a relatively short period of time, coinciding with the advent of our species, these animals became extinct. Since Homo sapiens first appeared on the planet, according to a recent study, it has caused the extinction of 83 per cent of wild mammal species and half of all plants. The extinction rate has increased exponentially since the industrial revolution; from 1500 to the present day, more than 300 species of terrestrial vertebrates have become extinct and many others are on the verge of extinction. According to American biologist Edward Osborne Wilson, we lose between 11,000 and 58,000 species every year — roughly one species every twenty minutes.
A group of researchers from the Danish universities of Aarhus and Gothenburg has calculated that, given the current rate of extinction of mammals, it will take between three and five million years for nature to recover. Five million years — a span of time so vast that our minds are not truly capable of grasping it. To try to help us do so, we can recall that our species only appeared around 300,000 years ago. Following the previous mass extinctions, which occurred over the last 450 million years as a result of major environmental upheavals, evolution slowly and meticulously filled the gaps left by extinct species with new ones.
This time, however, extinctions are occurring at too rapid a pace, not allowing evolution to keep up. The researchers estimated that, if mammals were to diversify at a normal rate, it would still take between five and seven million years for biodiversity to return to the levels reached before the evolution of modern humans (in terms of species diversity and population size, it is of course not possible for the same biodiversity to be recreated). It will take "a little less time" — between three and five million years — for it to return to its current state instead.
Read also: How bison will save us from climate change. The vision of Pleistocene Park
Read also: Wild, the hope of rewilding for a better world
The study, published in the journal Pnas, was carried out by analysing a large database of mammals, which includes not only existing species but also the hundreds of species that lived in the recent past and became extinct due to human impact. Using advanced simulations and data on the evolutionary relationships and body sizes of existing and extinct mammals, the researchers attempted to understand whether and when surviving mammals will be able to "regenerate" lost biodiversity. The researchers hypothesised an optimistic scenario in which humans have stopped destroying habitats, significantly reducing extinction rates, but even in this rosy future it will take the aforementioned 3–5 million years to regenerate the branches of the evolutionary tree that could be severed within the next fifty years.
Not all species, the researchers highlighted, carry the same importance from an evolutionary standpoint. "Some large extinct mammals, such as giant ground sloths and sabre-toothed tigers, which went extinct around 10,000 years ago, were highly evolutionarily distinct and, since they had few close relatives, their extinctions resulted in the severing of entire branches of Earth's evolutionary tree," explained palaeontologist Matt Davis of the University of Aarhus, who led the study. "There are hundreds of species of shrew, so some extinctions might be recovered from, but there were only four species of sabre-toothed tiger, and they all went extinct."
Read also: What is megafauna and why is it going extinct as it did thousands of years ago
Read also: 10 things you (perhaps) don't know about tapirs
The permanent disappearance of these unique creatures has also deprived the planet of the peculiar ecological functions, shaped over millions of years of evolutionary history, that they performed. Consider mammoths and other prehistoric pachyderms: these animals, once widespread across many areas of the planet, had a major impact on the environment by altering its vegetation and forcing ecosystems to adapt. Today large mammals are gravely threatened and their populations, at best, have been halved. Within the next fifty years we could lose the black rhinoceros, while Asian elephants, a reflection of the grandeur of the ancient fauna, have less than a 33 per cent chance of surviving beyond this century.
At one time, colossal size represented a winning evolutionary strategy, but the advent of humans has upended this paradigm and driven these ancient giants — with their all too slow reproductive rates — to extinction. "We used to live in a world of giants, whereas now we live in a world increasingly impoverished of large wild mammal species," said Professor Jens-Christian Svenning of Aarhus University, who leads a megafauna research programme. "The few giants that remain, such as rhinoceroses and elephants, risk being wiped out very rapidly." The study could, however, help prevent the disappearance of unique and precious species by allowing conservation efforts to be focused on avoiding the most evolutionarily significant extinctions. "It is much easier to save biodiversity now, rather than waiting for it to evolve again in the future," concluded Matt Davis.