Study finds human-driven mass extinction is eliminating entire branches of the tree of life – “We’re losing our only known living companions in the entire universe”

Number of generic extinctions per century among in different classes of vertebrates. The low number of reptiles and amphibia, which underestimate the magnitude of extinction pattern, is probably the result of the lack of information in earlier centuries, where very few species had been described. The dotted line represents the background extinction rate. Graphic: Ceballos and Ehrlich, 2023 / PNAS — Number of generic extinctions per century among in different classes of vertebrates. The low number of reptiles and amphibia, which underestimate the magnitude of extinction pattern, is probably the result of the lack of information in earlier centuries, where very few species had been described. The dotted line represents the background extinction rate. Graphic: Ceballos and Ehrlich, 2023 / *PNAS*

By Sean Cummings
18 September 2023

(Stanford News) – The passenger pigeon. The Tasmanian tiger. The Baiji, or Yangtze River dolphin. These rank among the best-known recent victims of what many scientists have declared the sixth mass extinction, as human actions are wiping out vertebrate animal species hundreds of times faster than they would otherwise disappear.

Yet, a recent analysis from Stanford University and the National Autonomous University of Mexico, published this week in the Proceedings of the National Academy of Sciences, shows the crisis may run even deeper. Each of the three species above was also the last member of its genus, the higher category into which taxonomists sort species. And they aren’t alone.

Up to now, public and scientific interest has focused on extinctions of species. But in their new study, Gerardo Ceballos, senior researcher at the Institute of Ecology at the National Autonomous University of Mexico, and Paul Ehrlich, Bing Professor of Population Studies, Emeritus, in the Stanford School of Humanities and Sciences, have found that entire genera (the plural of “genus”) are vanishing as well, in what they call a “mutilation of the tree of life.”

“In the long term, we’re putting a big dent in the evolution of life on the planet,” Ceballos said. “But also, in this century, what we’re doing to the tree of life will cause a lot of suffering for humanity.”

“What we’re losing are our only known living companions in the entire universe,” said Ehrlich, who is also a senior fellow, emeritus, by courtesy, at the Stanford Woods Institute for the Environment.

Illustration of Thylacinus cynocephalus from John Gould’s The Mammals of Australia. This genus, also known as the Tasmanian tiger, was hunted to extinction by humans. Graphic: Henry Constantine Richter and John Gould / Public domain — Illustration of *Thylacinus cynocephalus* from John Gould’s *The Mammals of Australia*. This genus, also known as the Tasmanian tiger, was hunted to extinction by humans. Graphic: Henry Constantine Richter and John Gould / Public domain

A ‘biological annihilation’

Information on species’ conservation statuses from the International Union for the Conservation of Nature, Birdlife International, and other databases has improved in recent years, which allowed Ceballos and Ehrlich to assess extinction at the genus level. Drawing from those sources, the duo examined 5,400 genera of land-dwelling vertebrate animals, encompassing 34,600 species.

Seventy-three genera of land-dwelling vertebrates, Ceballos and Ehrlich found, have gone extinct since 1500 AD. Birds suffered the heaviest losses with 44 genus extinctions, followed in order by mammals, amphibians, and reptiles.

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

“As scientists, we have to be careful not to be alarmist,” Ceballos acknowledged – but the gravity of the findings in this case, he explained, called for more powerful language than usual. “We would be unethical not to explain the magnitude of the problem, since we and other scientists are alarmed.”

Patterns of distribution of extinct and extant land vertebrate genera. Most genera and monotypic ones are concentrated in tropical and subtropical regions of all continents. Patterns of distribution of extinction and extinction risk are different, showing some temperate regions such as Eastern US as concentration hotspots. (A) Total genera; (B) Monotypic genera; (C) Extinct genera and species; (D) Endangered (CR, EN, VU) genera. Graphic: Ceballos and Ehrlich, 2023 / PNAS — Patterns of distribution of extinct and extant land vertebrate genera. Most genera and monotypic ones are concentrated in tropical and subtropical regions of all continents. Patterns of distribution of extinction and extinction risk are different, showing some temperate regions such as Eastern US as concentration hotspots. (A) Total genera; (B) Monotypic genera; (C) Extinct genera and species; (D) Endangered (CR, EN, VU) genera. Graphic: Ceballos and Ehrlich, 2023 / *PNAS*

Next-level loss, next-level consequences

On many levels, genus extinctions hit harder than species extinctions.

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

But when entire “branches” (genera) fall off, it leaves a huge hole in the canopy – a loss of biodiversity that can take tens of millions of years to “regrow” 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 hinges on the services Earth’s biodiversity provides.

Take the increasing prevalence of Lyme disease: white-footed mice, the primary carriers of the disease, used to compete with passenger pigeons for foods, like acorns. With the pigeons gone and predators like wolves and cougars on the decline, mouse populations have boomed – and with them, human cases of Lyme disease.

This example involves the disappearance of just one genus. 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 final member of an extinct genus. Females would swallow their own fertilized eggs and raise tadpoles in their stomachs, while “turning off” their stomach acid. These frogs might have provided a model for studying human diseases like acid reflux, which can raise the risk of esophageal cancer – but now they’re gone.

Loss of genera could also exacerbate the worsening climate crisis. “Climate disruption is accelerating extinction, and extinction is interacting with the climate, because the nature of the plants, animals, and microbes on the planet is one of the big determinants of what kind of climate we have,” Ehrlich pointed out.

Number of years that would have taken for vertebrate genera to become extinct under the background extinction rate prevailing in the last million years. The number of years for all extinct vertebrate genera is 18,000 y. Reptiles and amphibians have fewer extinct genera, so their values are much smaller that mammals and birds. Graphic: Ceballos and Ehrlich, 2023 / PNAS — Number of years that would have taken for vertebrate genera to become extinct under the background extinction rate prevailing in the last million years. The number of years for all extinct vertebrate genera is 18,000 y. Reptiles and amphibians have fewer extinct genera, so their values are much smaller that mammals and birds. Graphic: Ceballos and Ehrlich, 2023 / *PNAS*

A crucial, and still absent, response

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

Increased conservation efforts should prioritize the tropics, they noted, since tropical regions have the highest concentration of both genus extinctions and genera with only one remaining species. The pair also called for increased 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 the consumption is very inequitable are all major parts of the problem,” the authors said.

“The idea that you can continue those things and save biodiversity is insane,” Ehrlich added. “It’s like sitting on a limb and sawing it off at the same time.”

Contact

Taylor Kubota, Stanford News Service: tkubota@stanford.edu

Study finds human-driven mass extinction is eliminating entire branches of the tree of life

Mutilation of the tree of life via mass extinction of animal genera

ABSTRACT: Mass extinctions during the past 500 million y rapidly removed branches from the phylogenetic tree of life and required millions of years for evolution to generate functional replacements for the extinct (EX) organisms. Here we show, by examining 5,400 vertebrate genera (excluding fishes) comprising 34,600 species, that 73 genera became EX since 1500 AD. Beyond any doubt, the human-driven sixth mass extinction is more severe than previously assessed and is rapidly accelerating. The current generic extinction rates are 35 times higher than expected background rates prevailing in the last million years under the absence of human impacts. The genera lost in the last five centuries would have taken some 18,000 y to vanish in the absence of human beings. Current generic extinction rates will likely greatly accelerate in the next few decades due to drivers accompanying the growth and consumption of the human enterprise such as habitat destruction, illegal trade, and climate disruption. If all now-endangered genera were to vanish by 2,100, extinction rates would be 354 (average) or 511 (for mammals) times higher than background rates, meaning that genera lost in three centuries would have taken 106,000 and 153,000 y to become EX in the absence of humans. Such mutilation of the tree of life and the resulting loss of ecosystem services provided by biodiversity to humanity is a serious threat to the stability of civilization. Immediate political, economic, and social efforts of an unprecedented scale are essential if we are to prevent these extinctions and their societal impacts.

SIGNIFICANCE: We are in the sixth mass extinction event. Unlike the previous five, this one is caused by the overgrowth of a single species, Homo sapiens. Although the episode is often viewed as an unusually fast (in evolutionary time) loss of species, it is much more threatening, because beyond that loss, it is causing rapid mutilation of the tree of life, where entire branches (collections of species, genera, families, and so on) and the functions they perform are being lost. It is changing the trajectory of evolution globally and destroying the conditions that make human life possible. It is an irreversible threat to the persistence of civilization and the livability of future environments for H. sapiens. Instant corrective actions are required.

Mutilation of the tree of life via mass extinction of animal genera