Human-caused mass extinction is 'maiming entire branches of the tree of life'

The passenger pigeon or Carolina pigeon (Ectopistes migratorius) completely disappeared at the beginning of the 20th century due to massive hunting.

Oliver Thansan
Oliver Thansan
18 September 2023 Monday 11:23
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Human-caused mass extinction is 'maiming entire branches of the tree of life'

The passenger pigeon or Carolina pigeon (Ectopistes migratorius) completely disappeared at the beginning of the 20th century due to massive hunting. The Tasmanian tiger (Thylacinus cynocephalus) is considered extinct since the 1930s also due to hunting and the introduction of other species into its habitat. The baiji, Yangtze River dolphin or Chinese river dolphin (Lipotes vexillifer) has been included in the list of species 'possibly extinct in the natural environment', due to overfishing and the construction of dams that alter its habitat.

These are three examples of human activity triggering the extinction of species, a problem that is reaching proportions comparable to five major extinctions known on Earth in the last 600 million years.

Passenger pigeon, Tasmanian tiger and baiji are three specific cases but the problem may be even worse than estimated until now because they are disappearing (we are making disappear) not only isolated species, but genera in their entirety, "entire branches of the tree of life [understood as a representation of the genealogy of life and its evolutionary ramification]", according to a new study led by experts from Stanford University (United States) and the National Autonomous University of Mexico whose results have been published this week in Proceedings of the National Academy of Sciences (PNAS).

Gerardo Ceballos, principal investigator at the Institute of Ecology at the National Autonomous University of Mexico, and Paul Ehrlich, professor emeritus of Population Studies at the Stanford School of Humanities and Sciences, co-authors of the study, qualify the situation with a title -from the article scientific - that leaves no room for doubt: "Mutilation of the tree of life through mass extinction of animal genera."

"In the long term, we are having a huge impact on the evolution of life on the planet," Ceballos said. "But also, in this century, what we are doing to the tree of life will cause a lot of suffering to humanity," the Mexican scientist says in a joint note released by his university and Stanford University.

"What we are losing are our only known living companions in the entire universe," highlights Ehrlich, who is also a senior fellow, emeritus, at the Stanford Woods Institute for the Environment.

Information on the conservation status of species from the International Union for Conservation of Nature, Birdlife International and other databases has improved in recent years, allowing Ceballos and Ehrlich to assess extinction at the genus level. From these sources, these experts have examined 5,400 genera of terrestrial vertebrate animals, covering 34,600 species.

Ceballos and Ehrlich discovered that seventy-three genera of terrestrial vertebrates have become extinct from the 16th century until now. Birds suffered the greatest losses with 44 extinct genera, followed in order by mammals, amphibians and reptiles.

Based on the historical rate of genus extinction among mammals—estimated for the authors by Anthony Barnosky, professor emeritus of integrative biology at UC Berkeley—the current rate of extinction of vertebrate genera is 35 times that of the last million years. This means that, without human influence, the Earth would likely have lost only two genders during that time. In five centuries, human actions have triggered a wave of genus extinctions that would otherwise have taken 18,000 years to accumulate, what the article calls a “biological annihilation.”

“As scientists, we must be careful not to be alarmist,” Ceballos acknowledged, but the seriousness of the findings in this case, he explained, required stronger language than usual. "It would be unethical not to explain the magnitude of the problem, as we and other scientists are alarmed."

When one species dies, Ceballos explained, other species of its genus are often able to fulfill at least part of its role in the ecosystem. And because those species carry much of the genetic material of their extinct cousins, they also retain much of their evolutionary potential. Represented in terms of the tree of life, if a single “twig” (a species) falls, nearby twigs can branch out relatively quickly, filling the space much as the original twig would have done. In this case, the diversity of species on the planet remains more or less stable.

But when entire “branches” (genera) fall off, a huge hole is left in the canopy: a loss of biodiversity that can take tens of millions of years to “grow back” through the evolutionary process of speciation. Humanity cannot wait that long for its life-support systems to recover, Ceballos said, given how much the stability of our civilization depends on the services provided by Earth's biodiversity.

Take the increasing prevalence of Lyme disease: White-footed mice, the primary carriers of the disease, used to compete with passenger pigeons for food, such as acorns. With the disappearance of pigeons and the decline of predators such as wolves and mountain lions, mouse populations have increased, and with them, human cases of Lyme disease.

This example involves the disappearance of a single gender. A mass extinction of genera could mean a proportional explosion of disasters for humanity.

It also means a loss of knowledge. Ceballos and Ehrlich point to the gastric brooding frog, also the last member of an extinct genus. The females would swallow their own fertilized eggs and raise tadpoles in their stomachs, while "quenching" the stomach acid. These frogs could have provided a model for studying human diseases such as acid reflux, which can increase the risk of esophageal cancer, but they are now gone.

The loss of genres could also exacerbate the worsening climate crisis. "Climate disruption is accelerating extinction, and extinction is interacting with climate, because the nature of the plants, animals and microbes on the planet is one of the big determinants of the type of climate we have," Ehrlich said.

To prevent further extinctions and the resulting social crises, Ceballos and Ehrlich call for immediate political, economic and social action on unprecedented scales.

They noted that greater conservation efforts should prioritize the tropics, as tropical regions have the highest concentration of both extinct genera and genera with a single species remaining. The pair also called for greater public awareness of the extinction crisis, especially given how deeply it intersects with the more publicized climate crisis.

"The size and growth of the human population, the increasing scale of its consumption, and the fact that consumption is highly unequal are important parts of the problem," the authors said.

"The idea that you can continue doing these things and save biodiversity is crazy," Ehrlich added. "It's like sitting on a branch and cutting it at the same time."

Scientific article mentioned:

Mutilation of the Tree of Life via Mass Extinction of Animal Genera. Published Sept. 18 in the journal Proceedings of the National Academy of Sciences

DOI: 10.1073/pnas.2306987120