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Over the past 450 million years, life on Earth has been devastated by at least five mass extinctions, which are typically defined as catastrophes that wipe out more than 75 percent of species in a short amount of time. Many scientists have proposed that we are entering a Sixth Mass Extinction, this time driven by human activity, though debates still rage over the validity and consequences of this claim.

Now, a team led by Robert Cowie, research professor at the University of Hawaii’s Pacific Biosciences Research Center, argues that “the Sixth Mass Extinction has begun on land and in freshwater seems increasingly likely,” according to a recent article published in Biological Reviews. 

“We consider that the Sixth Mass Extinction has probably started and present arguments to counter those who would deny this,” said the team, which also included biologists Philippe Bouchet and Benoît Fontaine of the Muséum National d’Histoire Naturelle in Paris, France, in the article.

“Denying it is simply flying in the face of the mountain of data that is rapidly accumulating, and there is no longer room for skepticism, wondering whether it really is happening,” added the authors.

Cowie and his colleagues refer to a multitude of studies cataloging the extinction of species across clades, but the article is primarily built around their research into mollusks, an invertebrate family that includes snails, clams, and slugs. This focus counteracts the disproportionate attention that vertebrates, such as birds and mammals, receive in the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, among other conservation efforts.

“The IUCN information on the extinction of birds and mammals is probably quite accurate,” said Cowie in a call. “However, they have not evaluated any but a tiny, tiny fraction of invertebrates such as insects and snails and spiders and crustaceans, which constitute 95 percent of animal diversity.”

As a result, invertebrates provide “more of a random sample of biodiversity,” he noted. 

Past studies have used IUCN data to refute the notion that we are entering a Sixth Mass Extinction. Cowie and his colleagues pushed back on this assumption by compiling extinction rates of land snails and slugs. Extrapolating from those data, the team concluded that anywhere from 7.5 and 13 percent of species may have become extinct since the year 1500, a figure that is in line with many other estimates that suggest catastrophic biodiversity losses due to human pressures.

“The bottom line is that all these estimates that people have made indicate a much higher rate of extinction now than in the past,” Cowie said.

This extinction crisis is far more pronounced on land than in the oceans, according to the article, though many marine species are also threatened as the result of human activity. Extinctions are also generally occurring much more rapidly in island ecosystems, such as Hawaii, compared to continental biomes.

In addition to raising alarms about a possible mass extinction, Cowie and his colleagues address a range of counterarguments that they say downplay the severity of human pressures on world species, or even suggest that humans should harness these ecological changes for our own benefit.

The team argues that this kind of “laissez-faire attitude to the current extinction crisis is morally wrong,” according to the article, and advocates for more urgent measures to address the loss of species due to human activity.

“I feel obligated to express opinions about what we feel should be done given this crisis situation,” Cowie said. “I’m not just going to present the data and say be done with it. I’m going to say what we should do to resolve this issue, because it’s an important issue.”

Ultimately, the team acknowledges that conservation efforts can feel futile in the face of this massive problem, and suggests that more energy should be spent on efforts to gather specimens of disappearing species before they are lost forever.

“We don’t think there’s a positive ending; we think it’s kind of a disaster,” Cowie concluded. “We feel that the most important thing we can do for the future is to preserve as many of these species as possible in museums, so that in 200, 300, or 500 years from now, people will still be able to say this is what the Earth once had. I strongly believe that.”