Humans have shaped their environment since time immemorial. From cities to farms to highways, our impact has become so great that we have pushed planetary boundaries, fueling concerns about climate change and increasing extinction rates of the animals and plants with which we share the Earth.
But measuring exactly the extent to which human activity has driven species to extinction is far from simple. While headline-grabbing projects aimed at bringing back lost species, like those promoted by companies like Colossal, offer a glimmer of hope, they may well be just drops of water in a vast ocean of threatened biodiversity.
Earth has experienced five mass extinctions caused by natural cataclysms, and some scientists now say a sixth may already be underway, this time entirely man-made. Are we really on the verge of another planet-wide collapse? Or could the situation be more nuanced than the alarmist headlines suggest?
No clear definition of mass extinction
The definition of a mass extinction is more complex than most people think. Scientists largely agree that this involves the loss of at least 75 percent of species in less than two million years, a geologically short period. From a human perspective, however, it may not take us millions of years to notice that something is wrong.
This is where the debate begins. Although humans have not yet caused a loss close to 75%, proponents of a sixth mass extinction argue that our relatively short presence on Earth has triggered a disruption so dramatic that it could be comparable on a smaller scale.
But not everyone agrees. John Wiens, a professor of ecology and evolutionary biology at the University of Arizona, questions whether these claims stand up to careful scientific scrutiny.
“Some people, especially when it comes to the sixth mass extinction, have said something that exceeds the background extinction rate. But because the background extinction rate is basically an average, you’re always going to fluctuate above and below it, regardless of extreme extinction events,” says Wiens. “If you look at very short time frames, you can get huge swings that don’t mean much in the long run.”
Timing, Wiens emphasizes, is essential. Extinction measured over millions of years behaves differently from extinction measured over centuries. And the level of the evolutionary tree you look at – species, genus or family – can completely change the story.
“The genera will be older than the species, so losing them is much worse than losing a random species,” he explains. “We’re losing all of this evolutionary history. That’s part of the reason we should focus on genera and not just species.”
Learn more: Understanding ocean rebound after mass extinction events could help us in the future
Extinction rates have decreased again
Wiens’ recent study takes this global view, examining the extinction of higher taxa over the past 500 years. Using the IUCN Red List, a global database of threatened species, Wiens and Harvard graduate student Kristen Saban found that 102 genera have gone extinct, along with 10 families and two orders, corresponding to about 900 species.
Most extinct genera were monotypic, meaning they contained only one species, and most were restricted to islands. But here’s what’s striking: overall, less than half a percent of all recorded genres disappeared during this period. Even more surprising, the loss rate peaked about a century ago and has been declining ever since.
This stands in stark contrast to the claims of scientists such as Ceballos et al., who say that current extinction threatens the persistence of human civilization.
Wiens also highlights the difficulty of measuring extinction. We don’t really know how many species exist on Earth; estimates range from two million to three trillion. One widely cited study puts the figure at around eight million, although 80 percent of these species are still hypothetical.
And sometimes, species thought to be extinct reappear, like the Lord Howe stick insect, rediscovered in 2001 on an isolated Australian island after almost a century. Many more likely disappeared before scientists could document them. All of this makes extinction data inherently incomplete, forcing researchers to interpret trends through the fog of uncertainty.
These may be the first stages of a mass extinction
Scientists arguing for a sixth mass extinction counter that such uncertainties mask a deeper crisis. They note that the Red List, while widely used, is heavily biased: Almost all birds and mammals have been assessed, but only a tiny fraction of invertebrates (the backboneless animals that make up more than 90 percent of known species) have been. They say accounting for likely invertebrate losses would significantly increase extinction rates.
Amphibians, in particular, are under threat. A third or more of the 6,300 known frogs, salamanders and caecilians are endangered, especially in narrow-range tropical regions. For many biologists, this concentration of risk suggests that we could be in the early stages of a sixth mass extinction.
Human activity is at the heart of this concern: no other species has reshaped terrestrial ecosystems on such a scale. Even as conservation grows, many species continue to disappear unnoticed. Some researchers see this as a natural extension of human domination; others warn that this is a biodiversity crisis unprecedented in Earth’s history.
Learn more: The Permian extinction: life on Earth almost disappeared during the “Great Dying”
Conservation is important no matter what
Wiens acknowledges that extinctions are occurring beyond background levels and that humans are responsible, but he stops short of calling it a mass extinction.
“We need to have the right science,” he says. “For it to be the sixth, it has to be comparable to the other five.”
He prefers the term “extinction crisis,” which better reflects the data and moral urgency. “It doesn’t matter if there are consequences for humans, because none of these extinctions should have happened,” Wiens says. “Everyone counts. This is simply not the right thing to do and we need to stop it.”
Accuracy, he says, is essential for conservation science to remain credible. “People won’t take us seriously if we claim that human life is going to disappear because of the loss of some birds on the islands a hundred years ago,” he says. “That doesn’t help. That doesn’t mean you’re against the idea of an extinction event, but that you’re trying to make science sound.”
The study also offers hope: conservation could work. “The reason we think we’re seeing this pattern of decline,” he says, “is because of conservation. »
Declining extinction rates over the past century may reflect the results of global efforts to protect species and their habitats. As the debate over a sixth mass extinction continues, human action remains important. Careful science, combined with thoughtful conservation, can make a tangible difference to the future of life on Earth.
Learn more: An oasis of life protected plants during the Permian mass extinction
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