Mammal diversity is dwindling to such a degree that it will take 3 to 5 million years to restore current levels of biodiversity and 5 to 7 million years to return biodiversity to pre-human levels. And that's only if extinction rates fall back to natural levels within 50 years, a study published in the Proceedings of the National Academy of Sciences has found.
"Although we once lived in a world of giants: giant beavers, giant armadillos, giant deer, etc., we now live in a world that is becoming increasingly impoverished of large wild mammalian species," Jens-Christian Svenning, an ecologist at Aarhus University and study co-author, said in a statement.
"The few remaining giants, such as rhinos and elephants, are in danger of being wiped out very rapidly."
This is the sixth mass extinction in 450 million years and the only one not caused by a natural disaster but rather the destructive activities of one species (in this case, us). In the first five mass extinctions, biodiversity very slowly crept back to previous levels with evolution filling the gaps with a wave of new species. Now, researchers at Aarhus University have used advanced evolutionary simulations to predict how long it would take to return to the levels of biodiversity we see today if pollution, poaching, and habitat destruction (and, therefore, extinction levels) return to natural levels in 50 years or less.
To do so, the team regenerated 2.5 billion years of evolutionary history using an extensive database of mammals, both alive and extinct. Their models accounted for the fact that many species alive today (including the black rhino) are facing extremely high rates of extinction – the Asian elephant, for example, has less than a 33 percent chance of surviving into the next century. Using data on the evolutionary relationships and body sizes of all the different mammalian species, the team worked out the amount of time that would be lost from past and potential future extinctions, and how long it would take to recover.
The return to natural extinction levels within half a century is a best-case scenario. Even then, the researchers found it would take 3 to 5 million years to restore mammalian diversity to current levels, considering the loss we expect to see in the near-future. To return mammalian diversity to pre-modern human levels would take 5 to 7 million years.
Not all species (and their extinction) are equal, the researchers add. While there are only around 500 pygmy sloths left in the world, they are one of the youngest species of mammal evolutionarily speaking and their extinction could be countered by the remaining 5,418 mammal species in less than two years. In contrast, the aardvark is the sole remaining species in its order. It's biodiversity implications would be far greater.
"Large mammals, or megafauna, such as giant sloths and saber-toothed tigers, which became extinct about 10,000 years ago, were highly evolutionarily distinct. Since they had few close relatives, their extinctions meant that entire branches of Earth's evolutionary tree were chopped off," said Matt Davis, a palaeontologist and the lead author of the study.
"There are hundreds of species of shrew, so they can weather a few extinctions. There were only four species of saber-toothed tiger; they all went extinct."
But it's not all bleak. The team hopes this research can be used to identify and prioritize evolutionarily distinct species. "It is much easier to save biodiversity now than to re-evolve it later," said Davis.