Scientists warn of loss of entire branches of the "tree of life"

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.

Many studies already exist on species extinctions, but the specificity of this one is to have looked at the extinction of entire genera.

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 family Canidae.

"I think this is the first time that we have tried to assess the rate of extinction at a higher level than that of the species," Robert Cowie, a biologist at the University of Hawaii who was not involved in the study, told AFP. "This demonstrates the loss of entire branches of the tree of life," a depiction of life first developed by Charles Darwin.

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

73 extinct genera

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

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

To understand whether this rate is higher than normal, the researchers then compared this result to the estimated extinction rate from fossil records over the very long term.

An Indri Indri lemur in the Andasibe Nature Reserve, Madagascar, September 17, 2008 © ROBERTO SCHMIDT / AFP

"Based on the extinction rate of the last million years, you would expect two genera to become extinct, but we have lost 73," Ceballos said.

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.

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

However, the loss of a gender can have consequences on the functioning of an entire ecosystem. With a possible "collapse of civilization", argues Gerardo Ceballos.

"If you have a wall made of bricks, and every brick is a kind, removing a brick is not going to cause the wall to collapse," he said. "But if you remove many more, then the wall falls."

"Still time" to act

According to him 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.

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

"An arbitrary value of 75% of species lost in a short period of time is widely used to define a mass extinction," says 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 it will happen," he says. "We can say that we are at the beginning of a potential sixth mass extinction."

Its particularity? That it is triggered by a species, the human, also having the power to remedy it.

"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, who hopes for a rapid awareness: "Governments, companies and people need to know what is happening, and what the consequences are."

© 2023 AFP