A sixth mass extinction is not looming, study argues. But there’s still cause for concern

Humans have wiped out more than 100 species — with many more on the brink or experiencing large declines in population.

Some scientists have argued that we have entered a “sixth mass extinction” event akin to the one that wiped out dinosaurs 66 million years ago. But this time the culprit is biological annihilation caused by humans rather than a city-size asteroid.

A new study published Thursday in the journal PLOS Biology argues, however, that while the decline in biodiversity is real, insects, plants and animals are not disappearing at rates anywhere near approaching a mass extinction, a phenomenon typically defined by the loss of 75% of all species over an geological interval of time. Only five mass extinctions have occurred over the 4.5 billion years of Earth’s history.

Instead, the study argues, recent extinctions of plant and animal groups are rare and often confined to island habitats. What’s more, rates of extinction may be decelerating, in part due to intensifying conservation efforts, particularly for mammals and birds.

“One thing we emphasize, every single one of these extinctions is a tragedy and should never have happened and should not happen in the future,” said study author John Wiens, a professor of ecology and evolutionary biology at the University of Arizona.

Most extinctions are among birds and mammals

The analysis conducted by Wiens and coauthor Kristen Saban, a graduate student at Harvard University, was based on 163,022 plant and animal species assessed by the International Union for Conservation of Nature. It focused on genus-level extinctions since 1500.

Genus is a biological classification that groups different but related species. For example, the genus Canis includes wolves, dogs, coyotes and jackals. However, a genus can also be monotypic, containing only one single, unique species such as a narwhal, ginkgo tree or platypus.

Wiens said he and Saban decided to conduct a genus-level analysis because it likely represented more evolutionary history than a species-level analysis.

The study found that 102 genera have gone extinct in the past 500 years — 90 animals and 12 plants. In addition, it found extinctions in two broader categories in the scientific classification system of life: 10 families, which group related genera, and two orders, which group related families.

Genera that have gone extinct over the past 500 years included the dodo, (Raphus), sea cow (Hydrodamalis) and Cylindraspis, a group of recently extinct giant tortoises that lived on Mauritius and nearby islands. Other extinct branches of the tree of life included Hawaiian honeyeater birds of the family Mohoidae and the Dinornithormes order which grouped giant flightless birds such as New Zealand’s moas.

The study acknowledged some important limitations to the research. Most notably, there may be many extinct genera that were not included by the IUCN, an issue that could be especially problematic in insects, for which relatively few genera have been identified but which contain about half of all known species.

The majority of these extinctions were among mammals (21 genera) and birds (37 genera), the study noted. They represented a total of 179 species.

This rate, the paper argued, made extinctions rare — only representing 0.45% of the 22,760 genera assessed by the International Union for Conservation of Nature, according to the study.

The analysis also found that the majority of extinctions took place among genera that lived exclusively on islands. For example, most bird extinctions took place in the Mascarene Islands, the Hawaiian Islands and New Zealand, the study noted.

Island habitats are particularly vulnerable to invasive species, often brought by human settlers, Wiens said, and not necessarily representative of a broader extinction risk.

Surprisingly, the analysis also suggested that the genus-level extinction rates appear to have begun declining, with the fastest extinction rates occurring in the decades 1870s, 1890s and 1900s.

“We found instead that extinctions of genera are very rare across plants and animals, that they were mostly of genera found only on islands, and that these extinctions actually slowed down over the last 100 years instead of rapidly accelerating,” Wiens said in a statement.

The research is at odds with a 2023 study, based on 5,400 genera of vertebrate animals that found that genus-level extinctions were “rapidly accelerating,” arguing that “we are in the sixth mass extinction event.”

However, that research focused 5,400 species of vertebrate animal and excluded fish, insects and plant life, taking into account only on a small fraction of life on this planet, Wiens said.

The authors of that study, Gerardo Ceballos, a senior researcher at the Institute of Ecology at the National Autonomous University of Mexico, and Paul Ehrlich, Bing Professor Emeritus of Population Studies at Stanford University, said they adopted the sixth mass extinction thesis because their analysis of current biodiversity data indicated that Earth is losing species and genera at much higher rates than at any other point in the last million years.

“In other words, the thousands of species that were lost in the previous century would have taken thousands of years to disappear in regular times. The trends are universal, affecting all organisms, including vertebrates, invertebrates, plants, fungi, and microbes,” the pair told CNN via email.

“The concept of the sixth mass extinction and the biodiversity crisis are scientifically interconnected,” they added.

Biodiversity crisis vs. mass extinction

While widespread consensus exists about the broader loss of biodiversity, there is great debate about the exact rate at which it is happening and the scale, said Sadiah Qureshi, a historian of science at the UK’s University of Manchester and the author of the new book “Vanished: An Unnatural History of Extinction.” Qureshi, who was not involved in the research, said many geologists do not believe that the current crisis meets the threshold of past mass extinctions in the geological record.

“While claims about the Sixth Mass extinction might work as a call to action, apocalyptic claims about loss are just as likely to make people feel as if there is nothing they can do,” she said via email. “We must remember that we can still make a meaningful difference and so it is important to maintain hope.”

The current biodiversity crisis and a sixth mass extinction are separate concepts that should be disconnected, according to Conrad Labandeira, a senior scientist and curator of fossil arthropods at the Smithsonian National Museum of Natural History in Washington, DC. He was not involved in the latest research but has studied trends in insect extinction, noting that many insect genera have survived mass extinctions unscathed.

“The current biodiversity crisis exists … whereas the sixth mass extinction is interpretative,” Labandeira said. “There still should be a call to action, emphasizing for the preservation of natural ecosystems as a mechanism for preserving modern biodiversity, including those taxa that are endangered.”

It’s difficult to detect and document extinctions, particularly among poorly known groups such as invertebrates, plants and fungi, which are less studied than birds and mammals, said Stuart Butchart, chief scientist at conservation charity BirdLife International.

“Confirming extinctions is hugely challenging, because it requires confidence that the last individual of a species has died: this is harder for species for which we understand their distribution, habitats, ecology and behaviour less well,” said Butchart, who is also an honorary research fellow at the University of Cambridge’s department of zoology.

He called the question of whether a sixth mass extinction was looming a distraction, saying the current extinction rates are of grave concern and happening at a scale that threatens human livelihood and well-being.

“Mass extinctions occur extremely rapidly in geological time, but still take between tens of thousands to several million years,” he added.

“On human timescales, it is therefore extremely difficult to know whether the last few decades or centuries comprise the start of another mass extinction event.”

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