Humanity has shaped the environment since time immemorial. From cities to farms to highways, our influence has become so great that we have pushed planetary boundaries, increasing concerns about climate change and the increasing rate of extinction of the animals and plants with which we share the Earth.
But measuring exactly how much human activity has driven species extinct is far from easy. While projects to bring back lost species, promoted by companies such as Colossalprovide a glimmer of hope, they may be drops in a vast ocean of threatened biodiversity.
There have been five mass extinctions on Earth caused by natural disasters, and now some scientists say a sixth may already be happening, this time entirely due to humans. Are we really on the verge of another planetary collapse? Or could the situation be more complex than the alarmist headlines suggest?
There is no clear definition of mass extinction
There is more to the definition of mass extinction than most people think. Scientists generally agree that this is due to the loss of at least 75 percent of species appeared in less than two million years.geologically short period. However, from a human perspective, it may not take us millions of years to see that something is wrong.
This is where the debate begins. Although humans have not yet caused even an approximate 75 percent loss, proponents of a sixth mass extinction argue that during our relatively short presence on Earth we have caused destruction so dramatic that it would be comparable on a smaller scale.
But not everyone agrees. John Vince, a professor of ecology and evolutionary biology at the University of Arizona, wonders whether these claims stand up to rigorous scientific scrutiny.
“Some people, especially with the sixth mass extinction, have said things are above the background extinction rate. But because the background extinction rate is essentially an average, you will always fluctuate above and below it, regardless of extreme extinction events,” Vince says. “If you look at very short time frames, you can get huge swings that don't mean much in the long run.”
Time frames, Vince emphasizes, are key. Extinction measured over millions of years behaves differently than extinction measured over centuries. And which level of the evolutionary tree you examine—species, genus, or family—can completely change history.
“The genus will be older than the species, so actually losing it is much worse than just a random species,” he explains. “We're losing all of that evolutionary history. That's one of the reasons why we should focus on genera rather than just species.”
Read more: Understanding ocean recovery after mass extinction events may help us in the future
Extinction rates are falling again
Latest study takes advantage of this overall picture by examining the extinction of higher taxa over the past 500 years. Using the IUCN Red List, a global database of threatened species, Vince and Harvard graduate student Kristen Saban found that 102 genera, along with 10 families and two orders, were extinct, corresponding to about 900 species.
Most extinct genera were monotypic, meaning they contained only one species, and most were distributed on islands. But here's the most striking thing: by and large, less than half a percent of all registered births disappeared during this period. What's even more surprising is that the rate of loss peaked about a century ago and has been declining ever since.
This is in sharp contrast to the statements of scientists such as Ceballos et al.who argue that the current extinction threatens the existence of human civilization.
Vince also highlights the difficulty of measuring extinction. We don't actually know how many species there are on Earth.; estimates range from two million to three trillion. A widely cited study puts the figure at around eight million, although 80 percent of these species are still hypothetical.
And sometimes types of disappeared thoughts appear again, like Lord Howe stick insect, rediscovered in 2001. on a remote Australian island almost a century later. Many others likely disappeared before scientists could document them. All of this makes extinction data inherently incomplete, forcing researchers to interpret trends through a fog of uncertainty.
These could be the early stages of a mass extinction
Scientists arguing for a sixth mass extinction say such uncertainty masks a deeper crisis. They note that the Red List, although widely used, highly biased: Almost all birds and mammals have been studied, but only a small fraction of invertebrates (animals without backbones that make up more than 90 percent of known species). Taking into account the likely losses of invertebrates will lead to a sharp increase in extinction rates, they say.
Amphibiansin particular are under threat. One third or more of the 6,300 known frogs, salamanders and caecilians are threatened, especially in tropical regions with narrow ranges. According to many biologists, this concentration of risk suggests we may be in the early stages of a sixth mass extinction.
Human activity is central to this problem: no other species has altered Earth's ecosystems on such a scale. Despite increased conservation efforts, many species continue to disappear undetected. Some researchers see this as a natural extension of human dominance; others warn that it marks a biodiversity crisis unprecedented in Earth's history.
Read more: Permian extinction: life on Earth almost disappeared during the “great extinction”
Conservation matters no matter what.
Vince agrees that extinctions occur above background levels and that humans are responsible, but does not call it a mass extinction.
“We have to get the science right,” he says. “For it to be sixth, it needs to be compared to the other five.”
He prefers the term “extinction crisis,” which better captures the data and moral urgency. “It shouldn’t matter whether there are any consequences for humans at all, because none of these extinctions should have happened,” Vince says. “Every one of them matters. It's just wrong and we need to stop it.”
Accuracy, he argues, is essential for conservation science to remain credible. “People won't take us seriously if we say human life will be destroyed by the loss of some birds on the islands a hundred years ago,” he says. “It doesn't help. It doesn't mean you're against the idea of extinction, but you're trying to make science healthy.”
The study also offers hope: conservation can work. “The reason we think we’re seeing this decline,” he says, “is because of conservation.”
The decline in extinction rates over the past century may reflect the results of global efforts to protect species and their habitats. Although the debate about the sixth mass extinction continues, human actions still matter. Rigorous science combined with thoughtful conservation can make a significant difference to the future of life on Earth.
Read more: An oasis of life that protected plants during the Permian mass extinction
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