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Vertebrate Declines and the Sixth Mass Extinction
We are in the midst of a mass extinction, many scientists have warned — this one driven not by a catastrophic natural event, but by humans. The unnatural loss of biodiversity is accelerating, and if it continues, the planet will lose vast ecosystems and the necessities they provide, including fresh water, pollination, and pest and disease contro
On Monday, there was more bad news: We are racing faster and closer toward the point of collapsethan scientists previously thought, according to research published in the Proceedings of the National Academy of Sciences. The extinction rate among terrestrial vertebrate species is significantly higher than prior estimates, and the critical window for preventing mass losses will close much sooner than formerly assumed — in 10 to 15 years.
“We’re eroding the capabilities of the planet to maintain human life and life in general,” said Gerardo Ceballos, an ecologist at the National Autonomous University of Mexico and lead author of the new study.
The current rate of extinctions vastly exceeds those that would occur naturally, Dr. Ceballos and his colleagues found. Scientists know of 543 species lost over the last 100 years, a tally that would normally take 10,000 years to accrue.
“In other words, every year over the last century we lost the same number of species typically lost in 100 years,” Dr. Ceballos said.
If nothing changes, about 500 more terrestrial vertebrate species are likely to go extinct over the next two decades alone, bringing total losses equivalent to those that would have taken place naturally over 16,000 years.
To determine how many species are on the brink of extinction, Dr. Ceballos and co-authors Paul Ehrlich, a conservation biologist at Stanford University, and Peter Raven, an environmentalist at the Missouri Botanical Garden, turned to population data for 29,400 terrestrial vertebrate species compiled by the International Union for Conservation of Nature.
Of those species, 515 — or 1.7 percent — are critically endangered, they found, with fewer than 1,000 individuals remaining. About half of these species comprise fewer than 250 individuals.
The researchers also examined species with populations between 1,000 and 5,000. When the scientists added those 388 species to their original analysis, they found an 84 percent geographic overlap — largely in the tropics — with species in the critically endangered group.
The loss of some will likely trigger a domino effect that sends others into a downward spiral, ultimately threatening entire ecosystems, the authors report. Dr. Ceballos compared this process to removing bricks from the wall of a house.
“If you take one brick out, nothing happens — maybe it just becomes noisier and more humid inside,” he said. “But if you take too many out, eventually your house will collapse.”
Conservationists, therefore, should consider all species with populations under 5,000 individuals to be in danger of extinction, Dr. Ceballos and his colleagues concluded.
“This is a substantial increase in what we have typically thought of as endangered,” said Daniel Blumstein, an ecologist at the University of California, Los Angeles, who was not involved in the research.
The new study also emphasizes the importance of protecting individual populations of animals, not just a species itself. Based on an analysis of the current and historical ranges of critically endangered species, the researchers calculated that more than 237,000 individual populations have disappeared since 1900.
In a previous study, Dr. Ceballos and Dr. Ehrlich similarly found that 32 percent of 27,600 vertebrate species’ populations are declining around the world.
As populations disappear from geographic areas, the species’ function there also disappears. The loss of honeybees in the United States, for example, would deal an economic blow of more than $15 billion, but the species itself would still survive elsewhere around the world.
“The population declines of common species — top predators, large-bodied herbivores like the rhino, pollinators and others — have large effects on the way ecosystems function even when they are far from extinction,” said Rebecca Shaw, chief scientist at the World Wildlife Fund, who was not involved in the research.
“Ceballos and his colleagues are telling us with scientific certainty that the survival of these species is linked to our own survival,” she added.
Dr. Ehrlich emphasized that the study’s overall findings were almost certainly a gross underestimate of the true scope of the extinction problem. Their analysis did not take plants or aquatic or invertebrate species into account, and it included only approximately 5 percent of terrestrial vertebrates for which scientists have population data.
The findings are “in fact what one would expect in the gathering biodiversity crisis,” said Thomas Lovejoy, an ecologist at George Mason University, who was not involved in the research. The paper “should be considered a major wake-up call while there is still time to make a difference.”
That so few people are aware of the impending crisis, Dr. Lovejoy added, is a cause of the crisis itself.
Many who are aware may simply feel the loss is not consequential. “People say, ‘What the hell of a difference does it make to me?’” Dr. Ehrlich said.
But often the role of a particular plant or animal in an ecosystem has become apparent only after the species in question is gone.
Passenger pigeons, for example, once numbered in the billions. Their voracious appetite for seeds limited population growth of other seed-eating species, including white-footed mice — the natural reservoir for the bacterium that causes Lyme disease.
After the passenger pigeon’s extinction, white-footed mice populations exploded, and the risks to human health increased. The impacts of the passenger pigeon’s extinction, researchers wrote in Science, “are still being felt a century after the last passenger pigeon died.”
As humans continue to encroach on nature and wildlife, Dr. Ceballos and his colleagues warn of a cascading series of impacts — including more frequent occurrences of new diseases and pandemics. The coronavirus that launched the pandemic originated in a wild animal, most scientists believe.
“The vaccine for Covid-19 was natural habitat,” Dr. Ceballos said. “The pandemic is a great example of how badly we’ve treated nature.”
With enough species losses, ecosystems will eventually fail, destabilizing economies and governments and triggering famine and refugee crises. But there are steps that can be taken now, Dr. Ceballos said.
Habitat loss and wildlife trade are currently responsible for the brunt of the problem, whereas climate change has yet to unleash “the full tsunami” of its impacts, Dr. Ceballos said.
To offset the most urgent wave of extinctions, he and his colleagues call for an immediate end to illegal wildlife trade.
“There’s no way this can be continued, wiping out species and putting the whole of humanity in danger,” Dr. Ceballos said. “We can solve this immediate problem.”
They also call for a halt to deforestation and a complete reform of the legal wildlife trade — one that prioritizes sustainability over profits.
“The most fundamental problem is reducing the scale of the human enterprise, especially its consumptive demands on the biosphere,” Dr. Ehrlich said.
Making these changes will require electing leaders who prioritize the environment, redistributing resources and slowing human population growth. To help organize these efforts, Dr. Ceballos and Dr. Ehrlich launched a new global initiative called Stop Extinction.
The initiative aims to provide a framework for creating new national agreements, as well as tools for educating and activating the public about the unfolding extinction crisis.
“All of us need to understand that what we do in the next five to 10 years will define the future of humanity,” Dr. Ceballos said.
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