Mammals cannot evolve fast enough to escape current extinction crisis

The rate at which humans are eradicating plant and animal species is so rapid that evolution, nature's defense mechanism, is unable to keep up. According to a study team lead by Aarhus, in the next fifty years, so many mammal species will become extinct due to present conservation efforts that it will take the natural world three to five million years to recover.

Over the last 450 million years, there have been five upheavals in which the environment altered so drastically that most plant and animal species on Earth became extinct. New species have gradually emerged via evolution to fill in the voids left by each major extinction.

There is now a sixth mass extinction, however unlike previous extinctions, this one is man-made rather than the result of natural calamities. According to calculations made by a group of scientists from the Universities of Gothenburg and Aarhus, evolution cannot keep up with the rate of extinctions.

The estimate, which was just published in PNAS, indicates that even if mammals diversified at their usual rates, it will still take them 5 to 7 million years to restore biodiversity to the level it was before modern humans developed, and 3-5 million years merely to achieve present levels.

Certain animals are more unique than others.

The researchers made advantage of their vast mammal database, which contains hundreds of extinct species in addition to those that existed only a few centuries ago when Homo sapiens swept throughout the world. This made it possible for the researchers to examine how our species affects other mammals in its entirety.

All species, though, are not equally significant. Some prehistoric species had few near relatives and belonged to unique evolutionary lineages. Examples of these animals are the odd South American Macrauchenia (picture a lama with an elephant trunk) and the Australian marsupial lion Thylacoleo, which resembled a leopard. Whole limbs of the evolutionary tree of life vanished along with these creatures as they went extinct. These species were gone, but with them went their special ecological roles and the millions of years of evolutionary history they represented.

"Big animals, or megafauna, were very different from one another in terms of evolution. Examples of these include sabre-toothed tigers and enormous sloths, which became extinct around 10,000 years ago. According to palaeontologist Matt Davis of Aarhus University, who oversaw the work, "their extinctions meant that entire branches of Earth's evolutionary tree were chopped off because they had few close relatives." Also, he says:

Shrews can withstand a few extinctions because there are hundreds of species of them. Sabre-toothed tigers were limited to four species, and all of them became extinct.

lengthy replacement wait times rhinoceros

Not only is it difficult to recover 2.5 billion years of evolutionary history, but the extinction rate of modern animals is also rising. There is a significant chance that critically endangered animals, like the black rhino, may go extinct in the next fifty years. Less than 33 percent of Asian elephants, one of the two remaining species of a once-dominant mammalian group that also includes mastodons and mammoths, are expected to live through this century.

When calculating the lost evolutionary history, the researchers took these anticipated extinctions into account and posed the question, Can current animals naturally restore this lost biodiversity?

The researchers were able to calculate the amount of evolutionary time that would be lost from past and potential future extinctions as well as the length of time it would take for recovery by using powerful computers, sophisticated evolutionary simulations, and extensive data about the evolutionary relationships and body sizes of extinct and current mammals.

The researchers projected a best-case future in which species extinction and habitat destruction by humans had halted, bringing extinction rates down to the low background levels recorded in fossils. It will take mammals three to five million years to diversify sufficiently to replace the branches of the evolutionary tree that they are predicted to lose during the next fifty years, even under this extremely optimistic scenario. Over five million years will pass before the remnants of the massive Ice Age species can reproduce.

Giving conservation work top priority

"Although there used to be a lot of giants in our world—big armadillos, huge deer, giant beavers, etc.—the number of enormous wild mammalian species in our globe is steadily declining. The study is part of a larger research program on megafauna, led by Professor Jens-Christian Svenning of Aarhus University. "The few remaining giants, such as rhinos and elephants, are in danger of being wiped out very rapidly," Svenning adds.

Still, the study team's news isn't all terrible. Their information and techniques may be utilized to swiftly identify threatened, evolutionarily unique species, allowing us to concentrate conservation efforts on preventing the most catastrophic extinctions.

According to Matt Davis, "it is much easier to save biodiversity now than to re-evolve it later."