Humans have been shaping their environment since time immemorial. From cities to farms to highways, our impact has grown so large that we’ve pushed planetary boundaries, fueling concerns about climate change and rising extinction rates among the animals and plants we share the Earth with.
But measuring exactly how far human activity has driven species to extinction is far from simple. While headline-grabbing projects to bring back lost species, like those promoted by companies such as Colossal, offer a glimmer of hope, they may be drops in a vast ocean of biodiversity under threat.
Earth has seen five mass extinctions caused by natural cataclysms and some scientists now argue that a sixth may already be underway, this time driven entirely by humans. Are we really on the brink of another planet-wide collapse? Or could the situation be more nuanced than the alarmist headlines suggest?
No Clear Definition on Mass Extinction
There’s more to defining a mass extinction than most people realize. Scientists broadly agree it involves the loss of at least 75 percent of species within less than two million years, a geologically short period. From a human perspective, though, we might not need millions of years to see that something is off.
That’s where the debate begins. Although humans haven’t yet caused anywhere near that 75 percent loss, proponents of a sixth mass extinction argue that within our relatively short presence on Earth, we’ve 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 hold up under careful scientific scrutiny.
“Some folks, particularly with regards to the sixth mass extinction, have said anything above the background extinction rate counts. But because the background extinction rate is basically a mean, you’re always going to fluctuate above and below it, independent of extreme extinction events,” Wiens says. “If you’re looking at very short timescales, you can get huge fluctuations that don’t mean much in the long run.”
Time frame, Wiens emphasizes, is key. Extinction measured across millions of years behaves differently than extinction measured across centuries. And what level of the evolutionary tree you examine — species, genus, or family — can change the story entirely.
“Genus will be older than species, so it’s actually much worse to lose those than just a random species,” he explains. “We’re losing all of this evolutionary history. That’s part of the reason why we should be focusing on genera and not just species.”
Read More: Understanding Ocean Rebound After Mass Extinction Events Could Help Us in the Future
Extinction Rates Have Been Declining Again
Wiens’s recent study takes that big-picture view, examining extinction over the past 500 years across higher taxa. 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, which translates to roughly 900 species.
Most of the extinct genera were monotypic, meaning they contained only one species, and most were restricted to islands. But here’s the striking part: In the grand scheme of things, less than half a percent of all recorded genera disappeared during this period. Even more surprising, the rate of loss peaked about a century ago and has been declining ever since.
This stands in sharp contrast to claims by scientists such as Ceballos et al., who argue that the current extinction event threatens the persistence of human civilization.
Wiens also highlights the difficulty of measuring extinction. We don’t actually know how many species exist on Earth; estimates range from two million to three trillion. A widely cited study puts the figure at around eight million, though 80 percent of those species are still hypothetical.
And sometimes species thought gone reappear, like the Lord Howe stick insect, rediscovered in 2001 on a remote Australian island after nearly a century. Many others likely disappeared before scientists could document them. All this makes extinction data inherently incomplete, forcing researchers to interpret trends through the fog of uncertainty.
These Might Be Early Stages of Mass Extinction
Scientists arguing in favor of a sixth mass extinction counter that such uncertainties mask a deeper crisis. They note that the Red List, while widely used, is heavily biased: nearly all birds and mammals have been assessed, but only a tiny fraction of invertebrates (animals without backbones that make up more than 90 percent of known species) have. Accounting for likely invertebrate losses, they say, would push extinction rates dramatically higher.
Amphibians, in particular, are under threat. One-third or more of the 6,300 known frogs, salamanders, and caecilians are at risk, especially in tropical regions with narrow ranges. To many biologists, that concentration of risk suggests we may be in the early stages of a sixth mass extinction.
Human activity is central to this concern: no other species has reshaped Earth’s ecosystems at such scale. Even as conservation expands, many species continue to vanish unnoticed. Some researchers see this as a natural extension of human dominance; others warn it marks a biodiversity crisis unprecedented in Earth’s history.
Read More: The Permian Extinction: Life on Earth Nearly Disappeared During the ‘Great Dying’
Conservation Matters No Matter What
Wiens agrees that extinctions are occurring above background levels and that humans are responsible but he stops short of calling it a mass extinction.
“We’ve got to get the science right,” he says. “For it to be the sixth, it’s got to compare to the other five.”
He prefers the term “extinction crisis,” which better reflects the data and moral urgency. “It shouldn’t matter whether there are consequences for humans at all, because none of those extinctions should have happened,” Wiens says. “Each one matters. It’s just the wrong thing to do, and we need to stop it.”
Precision, he argues, is essential for conservation science to stay credible. “People aren’t going to take us seriously if we claim that human life is going to be extinguished by the loss of some birds on islands a hundred years ago,” he says. “That doesn’t help. It doesn’t mean you’re against the idea of an extinction event but you’re trying to make the science healthy.”
The study also offers hope: conservation may be working. “The reason why we think we see that declining pattern,” he says, “is because of conservation.”
Lowering extinction rates over the last century could reflect the results of global efforts to protect species and habitats. While the debate over a sixth mass extinction continues, human action still matters. Careful science, paired with thoughtful conservation, can make a tangible difference for the future of life on Earth.
Read More: A Life Oasis Protected Plants During the Permian Mass Extinction Event
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