Lots of people know about the risk of losing endangered animals like pandas and tigers, but they're just the tip of the iceberg, according to a new study showing a major global extinction is currently underway.
The paper, published in Science Advances, is by no means the first to come to this conclusion. But the researchers here—from the National Autonomous University of Mexico, Stanford, Berkeley, Princeton, and the University of Florida—wanted to test the theory of a mass extinction event using very strict criteria. Even using conservative estimates, the researchers found that the rate of extinction in the last 115 years is as high as 50 times what it would be under normal circumstances.
"If we do nothing, in the next 50 years it will be a completely different world, something that humanity has never experienced," Gerardo Ceballos, lead author of the study and a senior ecology researcher at the National Autonomous University of Mexico, told me over the phone.
Ceballos and his colleagues found vertebrate species have been disappearing at an alarming rate for the last 500 years, roughly since humans started to have a significant impact on the environment. Since 1500, at least 338 vertebrate species have gone extinct, and if species continue to disappear at this rate, the planet's biodiversity could be significantly and permanently altered within three generations, the researchers warned.
In the history of the Earth, there have been five mass extinctions—periods when a large number of species die off within a short period of time. The most recent event, the Cretaceous-Tertiary mass extinction, wiped out the dinosaurs 66 million years ago.
Researchers have been trying to determine for years whether or not a sixth mass extinction is upon us already. Critics say scientists may be overestimating the rate at which current species are dying out, and underestimating the historic rate of extinction. So Ceballos and his colleagues wanted to conduct an analysis that used only very conservative estimations.
Extinction is a natural part of evolution. New species emerge, other species die out; it's always happened, and it will continue to happen as long as there's life on Earth. But the natural rate of extinction—called background extinction—is typically pretty low. Most studies on extinction rates peg it between 0.1 and 1 extinction per 10,000 species per 100 years (a measure called E/MSY). In other words: for every 10,000 species on Earth, up to one of them will disappear every 100 years.
But these previous studies have faced criticism for relying on those numbers, so Ceballos and his colleagues undertook an extensive review of fossil records to estimate a new background extinction just for vertebrates. (According to the study, there's not enough data to make decent estimates for non-vertebrate species). Their review found a background rate of 1.8 extinctions per 10,000 species per 100 years (1.8 E/MSY).
Next, the researchers collected data on all of the vertebrate species that are confirmed extinct, are considered extinct in the wild, or are believed to be extinct, according to the International Union of Conservation of Nature, from 1500 to today. Other studies in the past have used a wide range of estimations for how many species have gone extinct, Ceballos said, like looking at how much a habitat has decreased and extrapolating the number of species that would have disappeared. But, again, these researchers wanted to keep their numbers conservative, so they limited their analysis to the IUCN list.
When the researchers compared the background extinction rate to the actual extinction rate, even with these more conservative estimates, the gulf between the two was staggering. Since 1900, with a natural extinction rate of 2 E/MSY, about nine species would be expected to have disappeared. In reality, 477 vertebrate species are believed to have gone extinct.
Even when the researchers only included species that were confirmed extinct, 198 species, the die-off was 22 times the background rate.
"Let's say the extinction rate had been 2.2 or 2.3 E/MSY, that would not be a mass extinction. It's just a little higher than normal," Ceballos said. "But what we're finding is many, many times more. That's why we're certain that it's a mass extinction."
The study goes on to assert that the timing of this mass extinction makes it most likely human-caused: it's been happening as human culture has advanced and humans have had a greater impact on the world around us.
But why should humans really care? So we don't have the Dodo bird or the Yemen gazelle. who cares? But Ceballos explained that diversity is kind of like nature's insurance policy, and the more diversity we have, the healthier we all are.
He pointed to a case a few years ago in Panama, when there was an outbreak of hantavirus: a virus carried by rodents that can spread to humans and make them very sick. In protected wildlife areas, where there were many diverse species of rodents, the incidents of hantavirus were really low. It didn't spread as quickly because only some of the species were susceptible, Ceballos said. But in areas that had been impacted by forestry, and where only a handful of rodent species remained, the incidents of the virus were much higher.
"Losing one species in particular, maybe one mouse or one squirrel, would not have a huge impact," Ceballos explained. "But the problem now is we're losing so many species, the resiliency of the ecosystem collapses."
Ceballos said it's not too late for humans to right our wrongs and prevent every other species from vanishing around us. If we double down on conservation efforts and start reeling back the pressures on species—like habitat loss and climate change—Ceballos and his colleagues say we can stop the mass extinction in its tracks.
We could also do nothing, at our own peril, he said. There have been mass extinctions before, one wiping out 90 percent of life on Earth, and life went on.
"We know for sure that life will prevail," Ceballos said. "What is at stake here is whether humanity will be able to survive."