Scientists say we're witnessing the planet's sixth mass extinction - and 'biological annihilation' is the latest sign

When kids learn about extinction in school, they're told about creatures that have disappeared from the planet, and those that are endangered.

But rarely are they told that we are currently witnessing a mass extinction event - an incredibly rare phenomenon in which the majority of species on the globe die off.

This has happened five times in the history of Earth so far. The one happening now will be the sixth.

According to a study recently published in the journal Proceedings of the National Academy of Sciences, populations of animals all over the planet are declining so rapidly that the researchers say a process of "biological annihilation" is now ongoing.

"As much as 50% of the number of animal individuals that once shared Earth with us are already gone, as are billions of populations," the ecologists and biologists behind the study wrote.

It's not just species facing total collapse that we should worry about, they said - local population extinctions (when a species disappears from a specific region but not the whole planet) are a "prelude to species extinctions." In other words, declines in populations of animals that aren't yet categorized as endangered are indicators of a mass extinction.

This trend is further evidence that the Earth "is already well into a sixth mass extinction episode," the authors of the study wrote.

Global population collapse

For this study, researchers looked at 27,600 vertebrate species - almost half of all the ones we know of - and found that more than 30% of them are in decline. Entire local populations of these animals are becoming extinct in certain areas, even though the overall numbers for many species haven't yet triggered alarms.

The extinction of local populations is reason for alarm, the authors wrote, because the disappearance of essential species from local ecosystems will cause cascading effects that ripple through the entire system. When larger populations of animals disappear, the smaller populations left behind are also much more vulnerable and closer to extinction, as Ed Yong pointed out in The Atlantic.

For example, if forest elephants disappear from all but one national park in Africa, the ones that used to disperse seeds and create pathways for smaller animals and plant life in other parts of the continent would no longer facilitate those processes. And those changes the ecosystem could in turn wipe out plants that rely on elephants for seed dispersal, and the animals that consume those plants.

The collapse of local populations of pollinators like honeybees could have an even more devastating effect, since fruit-bearing plants rely on those pollinators. If those plants don't survive the bee population losses, that affects the many animals, including humans that rely on those plants for food. (The team behind the study didn't focus on insects, but note that the same population declines are happening to outside the realm of terrestrial vertebrates as well.)

"[B]y losing populations (and species) of vertebrates, we are losing intricate ecological networks involving animals, plants, and microorganisms," the authors wrote. "This suggests that, even if there was not ample sign that the crisis extends far beyond that group of animals, today's planetary defaunation of vertebrates will itself promote cascading catastrophic effects on ecosystems, worsening the annihilation of nature."

The ongoing cause of this is, of course, humans.

As people have spread across the globe, we've built on top of animal habitats, spread invasive species, hunted populations down to nothing, fished more than 90% of large predators out of the sea, released toxic pollutants, and harvested entire forests.

Plus, since the start of the industrial age, human activity has been releasing greenhouse gases into the atmosphere. These have not been the biggest driver of species decline so far, but in the future they could push this mass extinction event into something that can't be reversed, as David Wallace Wells noted in a recent feature in New York Magazine:

"The Earth has experienced five mass extinctions before the one we are living through now, each so complete a slate-wiping of the evolutionary record it functioned as a resetting of the planetary clock, and many climate scientists will tell you they are the best analog for the ecological future we are diving headlong into. Unless you are a teenager, you probably read in your high-school textbooks that these extinctions were the result of asteroids. In fact, all but the one that killed the dinosaurs were caused by climate change produced by greenhouse gas."

REUTERS/Ricardo Moraes

Can the sixth extinction be stopped?

The authors behind the new study suggested their work shows how far along the sixth extinction already is. They said their seemingly "alarmist" language is essential in the face of such an alarming situation.

Experts who study mass extinctions agree about the gravity of the present moment. But some disagree that the extinction event is already underway.

Smithsonian paleontologist Doug Erwin told Peter Brannen for a story in The Atlantic that if we were in the midst of such an event, it would be already too late to act. And the data doesn't indicate that's the case yet, though it does suggest we could be about to begin a mass extinction.

That distinction is purely academic for most of us, however, since experts like Erwin and the authors of this new study agree that the world's sixth mass extinction will proceed unchecked unless humans take immediate action. Curbing the trend will require both intensive local efforts to conserve species and habitats, and global efforts to prevent the climate from tipping too far into a danger zone.

As the authors of the study wrote:

"[W]e emphasize that the sixth mass extinction is already here and the window for effective action is very short, probably two or three decades at most. All signs point to ever more powerful assaults on biodiversity in the next two decades, painting a dismal picture of the future of life, including human life."

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