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The narrative that Earth is experiencing a sixth mass extinction event in fundamentally flawed, and is pushed through an incorrect analysis of the data at hand, a new study proposes.
Species go extinct frequently, but a mass extinction event is characterized by a loss of of 75% of all species on the planet. By contrast, a new examination of the same data used to propose the idea of a sixth mass extinction instead shows that over the last 500 years, a mere 0.45% of all genera have gone extinct.
But how could two examinations of the same data produce such shockingly different conclusions?
The authors of a seminal paper in 2023 concluded that genus-level extinction rates were “rapidly accelerating”. In the tree of life, a genus is a group of related species, such as Panthera, which includes lions, leopards, and tigers. Genera are grouped into families, such as Felidae, and families into orders, such as Carnivora.
The authors, Gerardo Ceballos and Paul Ehrlich, proposed that by examining extinction rates at the genus-level or higher, they would get a better understanding of the biodiversity loss experienced since the dawn of civilization. By examining species assessed by the International Union for the Conservation of Nature (IUCN), the authors concluded that 73 genera had gone extinct in the last 500 years, far faster than what they proposed would be the “background extinction rate,” or the rate at which species would naturally die off without human influence.
Their calculations suggest that it would have taken 18,000 years for those 73 genera to die off, not 500. Next, the authors considered the various ecosystem services that many of the departed genera, like the passenger pigeon, provided, and concluded that if genera extinctions continue at their current rate, it risked “destroying the conditions that make human life possible”.
That’s a very strong statement for a scientist to make, reserved and data-dependent as they are.
Enter John Wiens and his colleague Kristen Saban at the Department of Ecology and Evolutionary Biology, University of Arizona. An emotive type, who comes off as immediately likable, Wien took some time over the last year to do what he believes science should do—a bit of self-correction.
“We did a major criticism of the sixth mass extinct idea in the spring,” he said, adding that it struck him immediately as unsubstantiated, “especially given the fossil evidence”.
“Someone says something ridiculous and people jump on it and disagree, even if it takes a couple years,” Dr. Wiens told WaL. “We can’t make outrageous statements if we expect people to believe in conservation and believe in science in general”.
Short on context
The Ceballos and Ehrlich paper reported that 73 genera had gone extinct, but in Wien and Saban’s reanalysis, the number is actually higher—103. That total, though, entombs the devil in the details.
Of that devilish total, less than half of one percent of all the genera on Earth, the majority were monotypic genera, in other words they contained one species. That was really how 78% of the variation between classes of animals that went extinct was explained. The largest number of genera-level extinctions were seen in mammals, birds, and turtles, because they have very small genera, in other words they contained fewer species.
Additionally, 76% of the total were endemic to islands, and that percentage was even higher among birds and mammals. An invasive species, such as a predator like the black rat, can cause extinction of ground nesting birds, for example, in extremely short time, and Wien doesn’t believe the fate of these island endemics should be extrapolated to make conclusions about continental taxa.
Another big finding was that while the Ehrlich and Ceballos paper claimed the rate of extinctions had accelerated by 35-times over the last 500 years, they calculated that century by century. Additionally, they calculated it cumulatively, with each genera-level extinction representing an “acceleration” of the overall rate of extinction.
When measured decade by decade, and by measuring the rate in terms of extinctions per decade rather than cumulatively, any semblance of a straight, sustained increase is lost. Not only that, but Wien explains that it buried a hugely positive, almost celebratory fact.
“Part of the reason [we wrote the paper], they show that the extinction of these genera are rapidly increasing and we show it’s actually significantly declining; the reason why is conservation is working, particularly for these mammals and birds,” he said. “It’s not all doom and gloom”.
A question of ethics
Though it could hardly be considered balanced on the topic of the environment, The Guardian newspaper actually has a news feed entitled “The Age of Extinction.” Euro News too has a news feed in its “Green” category entitled “Extinction”. These are at the end of the day, however, news outlets whose success is measured in story clicks. But for scientists, there’s supposed to be a standard of reporting what the data is saying, and not exaggerating to stretching out findings.
“I mean it’s good intentions right?” Dr. Wien said regarding Ceballos and Ehrlich. “They’re conservationists and they want people to care about this. I guess one way to make people care is to say ‘it’s going to destroy your life, it’s going to destroy your civilization,’ I just think it’s a poor example”.
“A paper like this should be the reason people trust science because we scientists self-police. Right?”
Based on their results, Wien, felt he had to do more to set the record straight, and followed the criticism up with the new analysis.
“We do not think that these genus-level extinctions are ‘destroying the conditions that make human life possible’ and will presumably not cause the ‘collapse of civilization’. Indeed, Ceballos and Ehrlich provided no evidence linking their results on genus-level extinctions to global ecosystem services or the collapse of civilization”.
“We… acknowledge that there could be many dark extinctions, with many distinct clades going extinct before being described. However, it seems unlikely that past extinctions of taxa that were never known scientifically will lead to the future collapse of human civilization. Moreover, given current skepticism toward conservation and science in general, we think it is especially important to be rigorous and accurate about current threats to biodiversity and their consequences for humanity, without overstatement or exaggeration,” they wrote. WaL
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PICTURED ABOVE: An adult male passenger pigeon specimen at the Smithsonian Institute. PC: James St. John CC 2.0.
