Extinct passenger pigeon (picture from 2014): Only in the museum

Photo: Susan Walsh/ AP

Humanity threatens the environment in which it lives. Current status: the sixth mass extinction in Earth's history, as researchers call it. It is triggered by humans and the changes they leave behind on the planet. Two scientists now show that the crisis could possibly go even deeper. Because not only species are becoming extinct – but entire genera, they write in the journal "Proceedings of the National Academy of Sciences".

Gerardo Ceballos of the National Autonomous University of Mexico and Paul Ehrlich of the Stanford School of Humanities and Sciences call the disappearance of entire species "mutilation of the tree of life". Thanks to improved information on the conservation status of species collected in databases, they were able to assess extinction at the genus level. The researchers examined 5400 genera of terrestrial vertebrates, which comprise 34,600 species.

The result: 73 genera of terrestrial vertebrates have become extinct since 1500 AD. Birds suffered the greatest losses, with 44 extinct genera, followed by mammals, amphibians and reptiles. Based on the historical extinction rate in mammals, the current rate of extinction of vertebrate genera exceeds that of the last million years by 35 times, the authors write. Without human influence, the earth would probably have lost only two species during this time. But things turned out differently: In just five centuries, humans triggered a wave of species extinction that would otherwise have occurred in 18,000 years, it continues.

This is also serious because the extinction of genera outweighs the extinction of species. According to Ceballos, other species from the same genus can take over parts of the role in the ecosystem, and evolutionary potential would be preserved. He explains it figuratively: If a single branch falls away, neighboring branches could intertwine and fill the gap. However, if whole branches fall off, a hole remains in the canopy. It could take tens of millions of years before it is closed. Humanity cannot wait so long for its life support systems to recover, Ceballos said, according to a statement, because the stability of our civilization depends to a large extent on the services provided by the Earth's biodiversity.

What the disappearance of a genus can mean for humans is made clear by the researchers in the spread of Lyme disease. The disease is transmitted, for example, by white-footed mice, which used to compete with passenger pigeons for food. With the disappearance of pigeons and the decline of predators such as wolves and cougars, mouse populations have increased – and with them the cases of Lyme disease in humans.

The loss of species could also exacerbate the climate crisis. "Climate disturbances accelerate extinction, and extinction interacts with climate, because the type of plants, animals and microbes on the planet is one of the most important determinants of what kind of climate we have," Ehrlich said, according to the statement. The scientists are calling for immediate countermeasures – and greater public awareness of the crisis of species extinction.