The 'Human-Caused' Sixth Mass Extinction Is 'Likely Accelerating,' Study Suggests

Land-based vertebrates—animals with a backbone—are likely going extinct at higher rates than previously thought, a team of researchers have suggested.

According to a study published in the journal Proceedings of the National Academy of Sciences, the scientists say human pressures—such as the wildlife trade or habitat destruction—have contributed to the extinction of hundreds of species, while pushing many more to the brink.

Among the authors of the latest paper is Stanford University biologist Paul Ehrlich who was involved in a 2015 study that declared the Earth was currently experiencing its sixth mass extinction. Now, Erlich and colleagues warn that this "human-caused" mass extinction is "likely accelerating," leading to a degradation of vital ecosystem services and the collapse of biodiversity in ecosystems.

"The ongoing sixth mass extinction may be the most serious environmental threat to the persistence of civilization, because it is irreversible. Thousands of populations of critically endangered vertebrate animal species have been lost in a century, indicating that the sixth mass extinction is human-caused and accelerating," the authors wrote in the study.

"The acceleration of the extinction crisis is certain because of the still fast growth in human numbers and consumption rates. In addition, species are links in ecosystems, and, as they fall out, the species they interact with are likely to go also. In the regions where disappearing species are concentrated, regional biodiversity collapses are likely occurring," they said.

In their research, the scientists sought to shed new light on the extinction crisis by examining 29,400 species of terrestrial vertebrates in order to determine which ones were on the brink of disappearing. For the purposes of their study, the authors considered a species to be on the brink if the global population numbered less than 1,000 individuals.

They found that 515 species of terrestrial vertebrates—1.7 percent of the species analyzed—met this criterion, indicating that they could disappear within the next two decades.

To put this into perspective, scientists think that at least 543 terrestrial vertebrates went extinct over the course of the entire twentieth century

Of the 515 species that are currently on the brink of extinction, the researchers found that around half of these were represented by fewer than 250 individuals. These highly endangered animals are mostly found in tropical and subtropical regions that are significantly impacted by humans.

The researchers also examined data on 77 mammal and bird species that have been on the brink of extinction in the last century, finding that around 94 percent of their localized populations had disappeared. Assuming that similar trends apply to all species on the brink, the authors estimate that 237,000 distinct populations of the 515 species they identified in the study have gone extinct since 1900.

When distinct populations of these species disappear, there may be a knock-on effect on other animals as the specific ecosystem functions they once provided are lost. This interconnectedness is highlighted by the fact that 84 percent of species with populations under 5,000 live in the same areas as those with populations under 1,000, indicating that their fates may be closely linked.

According to the researchers, human pressures on the Earth's biosphere—the worldwide sum of all the planet's ecosystems—have been increasing, threatening thousands of species around the world. These pressures include population growth, habitat destruction, the wildlife trade, pollution and climate change.

Diverse ecosystems ranging from coral reefs to jungles depend on a complex web of long-evolved relationships between species to keep them healthy. But when key species are lost, the functioning of these ecosystems may suffer, degrading their ability to provide services that are important to humans, such as pollinating crops, or acting as defenses against natural disasters or disease.

"We conclude the human-caused sixth mass extinction is likely accelerating for several reasons," the authors wrote in the study. "First, many of the species that have been driven to the brink will likely become extinct soon. Second, the distribution of those species highly coincides with hundreds of other endangered species, surviving in regions with high human impacts, suggesting ongoing regional biodiversity collapses."

baby Grauer's gorilla
A baby Grauer's gorilla, a critically endangered species, rests in its mother's arms in the forest of Kahuzi-Biega National Park in northeastern Democratic Republic of Congo, on September 30, 2019. ALEXIS HUGUET/AFP via Getty Images

"Third, close ecological interactions of species on the brink tend to move other species toward annihilation when they disappear—extinction breeds extinctions. Finally, human pressures on the biosphere are growing rapidly," they said.

One example of how an increase in human pressures on the biosphere can lead to devastating consequences is the wildlife trade. It's an ongoing threat to endangered species, as well as human health, given the propensity for diseases to "spill over" from animals to people.

"When humanity exterminates populations and species of other creatures, it is sawing off the limb on which it is sitting, destroying working parts of our own life-support system," Ehrlich said in a statement. "The conservation of endangered species should be elevated to a national and global emergency for governments and institutions, equal to climate disruption to which it is linked."

"Our results reemphasize the extreme urgency of taking massive global actions to save humanity's crucial life-support systems," the authors wrote in the study.

Among several recommendations to help slow the sixth mass extinction, the authors called for a global ban on the wildlife trade and for all species with populations under 5,000 to be listed as critically endangered on the International Union for Conservation of Nature's Red List of Threatened Species.

"It's up to us to decide what kind of a world we want to leave to coming generations—a sustainable one, or a desolate one in which the civilization we have built disintegrates rather than builds on past successes," Peter Raven, president emeritus of the Missouri Botanical Garden and co-author of the study, said in a statement.