Mass extinctions during the past 500 million years rapidly removed branches from the tree of life and required millions of years for evolution to generate functional replacements for the extinct organisms. We are in the sixth mass extinction event. Unlike the previous five, this one is caused by a single species, Homo sapiens. Beyond any doubt, it is more severe than previously assessed and is rapidly accelerating. A duo of scientists from Stanford University and the Universidad Nacional Autonoma de Mexico now shows — by examining 5,400 vertebrate genera (excluding fishes) comprising 34,600 species — that 73 genera became since 1500 CE in what they call a ‘mutilation of the tree of life.’
Simple schematic representation of the mutilation of the Tree of life because of generic extinctions and extinction risks. The bottom half of the tree depicted as dead branches shows examples of the extinct genera, and the upper half shows examples of genera at risk of extinction. Extinct genera: (I) lower row left – Delcourt’s giant gecko (Hoplodactylus, left), of which the only specimens known were found in a museum without a label, but probably they were found in New Zealand; and saddle-backed Rodrigues giant tortoise (Cylindraspis, right) from Rodrigues Island in the Indian Ocean. Lower row right – Yunnan Lake newt (Cynops, left) from China; and the gastric brooding frogs (Rheobatrachus, right) from rainforests in Queensland, Australia. (II) Second bottom-up row left – thylacine (Thylacinus, left), the largest carnivorous marsupial, last known from Tasmania; and Yangtze River dolphin or baijii (Lipotes, right) from China, one of very few freshwater dolphins. Second bottom-up row right – elephant birds (Aepyornis, left), the largest birds surviving to modern times, represent also both an extinct genus and family (Aepyornithidae) endemic to Madagascar; and Moho birds (genus Moho, right) represent also both an extinct genus and family (Mohidae) from Hawaii. Endangered genera: (III) Third bottom-up row left – King cobra (Ophiophagus, left) from Asia; and gavial (Gavialis, right) from India and Nepal. Third bottom-up row right – Alpine newt (Ichthyosaura, left) from Europe; and Mahogany frog (Abavorana, right) from the Malay Peninsula. (IV) Upper row left: volcano rabbit (Romerolagus, left) known from few mountains close to Mexico City, and elephant (Loxodonta, right) from Africa. Upper row right – ‘i’iwi or scarlet honeycreeper (Drepanis, left) from Hawaii; and kakapo (Strigops, right) a flightless parrot from New Zealand. Image credit: Marco Antonio Pineda.
Over the last century the pace of many human activities has so accelerated, and human overpopulation grown so severe, to have created a dramatic global environmental transformation.
Most natural ecosystems have been highly modified or have disappeared altogether, and the abundance of wildlife has been greatly reduced.
In well-studied major taxonomic groups, thousands of species and myriad populations have vanished.
The precise number of recent extinctions is impossible to know, but current animal species extinction rates are estimated to be hundreds or thousands of times higher than the background rates that prevailed for millions of years prior to the agricultural revolution.
Information on species’ conservation statuses from the IUCN, Birdlife International, and other databases has improved in recent years, which allowed Dr. Gerardo Ceballos from the Universidad Nacional Autonoma de Mexico and Stanford University’s Professor Paul Ehrlich to assess extinction at the genus level.
Drawing from those sources, the researchers examined 5,400 genera of land-dwelling vertebrate animals, encompassing 34,600 species.
Seventy-three genera of land-dwelling vertebrates, the authors found, have gone extinct since 1500 CE.
Birds suffered the heaviest losses with 44 genus extinctions, followed in order by mammals, amphibians, and reptiles.
“As scientists, we have to be careful not to be alarmists, but the gravity of the findings in this case called for more powerful language than usual,” Dr. Ceballos said.
“We would be unethical not to explain the magnitude of the problem, since we and other scientists are alarmed.”
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, 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. The scientists 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,” Professor Ehrlich said.
To prevent further extinctions and resulting societal crises, Dr. Ceballos and Professor Ehrlich are calling for immediate political, economic, and social action on unprecedented scales.
“Increased conservation efforts should prioritize the tropics, since tropical regions have the highest concentration of both genus extinctions and genera with only one remaining species,” they said.
They 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,” they said.
“The idea that you can continue those things and save biodiversity is insane. It’s like sitting on a limb and sawing it off at the same time.”
Their paper was published today in the Proceedings of the National Academy of Sciences.
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Gerardo Ceballos & Paul R. Ehrlich. 2023. Mutilation of the tree of life via mass extinction of animal genera. PNAS 120 (39): e2306987120; doi: 10.1073/pnas.2306987120
