Scientists warn of threat of sixth mass extinction

The biodiversity crisis “is as serious as climate change”, but not as well known to the general public, regrets Gerardo Ceballos, professor at the National Autonomous University of Mexico, and co-author of this study published in the journal PNAS.

But there is “urgency”, because what is at stake is “the future of humanity”, he told AFP.

Numerous studies already exist on the disappearance of species, but the specificity of this one is that it has looked at the extinction of entire genera.

“The loss of entire branches from the tree of life”

In the classification of living beings, the genus is between the rank of the species and that of the family. For example, the dog is a species belonging to the genus Canis, itself in the Canidae family.

“I think this is the first time that we have tried to evaluate the extinction rate at a level higher than that of the species,” Robert Cowie, a biologist at the University of Hawaii, commented for AFP who did not participate in the study. “This demonstrates the loss of entire branches of the tree of life,” a representation of life first developed by Charles Darwin.

The study shows that “we are not just cutting down twigs, but that we are using a chainsaw to get rid of large branches,” added Anthony Barnosky, professor emeritus at the University of California at Berkeley.

73 extinct genera

The researchers relied in particular on the lists of extinct species from the International Union for Conservation of Nature (IUCN). They focused on vertebrate species (excluding fish), for which more data are available.

Of about 5,400 genera (comprising 34,600 species), they concluded that 73 of them had become extinct in the last 500 years — most in the last two centuries. Firstly birds, followed by mammals, amphibians and reptiles.

“Based on the extinction rate over the last million years, you would expect two genera to go extinct, but we lost 73,” said Gerardo Ceballos.

According to the study, the extinction of these 73 genera should have taken 18,000 years, not 500.

These estimates remain uncertain, with many species not even known, and fossil records incomplete. But according to the researcher, they are probably underestimated.

A possible “collapse of civilization”

The cause of these extinctions? Human activities, which destroy habitats for crops, infrastructure and other needs, but also overexploitation (overfishing, hunting, animal trafficking, etc.).

However, the loss of a genus can have consequences on the functioning of an entire ecosystem. With a possible “collapse of civilization” in the long term, argues Gerardo Ceballos.

“If you have a wall made of bricks, and each brick is a kind, removing a brick is not going to cause the wall to collapse,” he compares. “But if you take away many more, then the wall falls. »

According to him, there is no doubt, this is a sixth mass extinction. Whether it has already begun, however, remains a matter of debate, although all experts agree that the current rate of extinction is alarming.

An arbitrary value of 75% of species lost in a short period of time is widely used to define a mass extinction

Still possible to act

The last mass extinction was 66 million years ago, when an asteroid impact caused the disappearance of the dinosaurs.

“An arbitrary value of 75% of species lost in a short period of time is widely used to define a mass extinction,” explains Robert Cowie. According to this threshold, the sixth mass extinction has “not yet occurred.”

But if “species continue to go extinct at the current rate (or faster), then this will happen,” he says. We can say that we are at the beginning of a potential sixth mass extinction.”

“The window for action is closing quickly,” warns Gerardo Ceballos. But we still have time to save many genres.”

The priority is to stop the destruction of natural habitats, and to restore those lost, insists the researcher. Governments, businesses and people need to know what is happening, and what the consequences are. »