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For decades, scientists have warned that we may be on the cusp of, or already entering, the sixth global mass extinction. In other words, this period of centuries or millennia may turn out to be the sixth time in Earth's history that 75% of all species have gone extinct.

The rapid rise of temperatures caused by greenhouse-gas emissions, combined with habitat destruction, is leading to an acceleration of extinctions around the globe, the argument goes. That can lead to a downward spiral of extinctions as the web of life deteriorates. 

These days, one of the scientific voices warning against jumping to that conclusion comes from Tucson. Prof. John Wiens, from the University of Arizona's Department of Ecology and Evolutionary Biology, has been arguing in a series of papers this year that previous arguments for the idea of a sixth mass extinction are flawed. 

Wiens, working with former U of A undergraduate Kristen Saban, attempts to show that previous cases made for the idea that we're in or entering a mass extinction depended on unreliable extrapolations. 

In part, the problem with these conclusions is that they didn't adequately take into account either the reasons for or the timing of extinctions over the last 500 years, Saban and Wiens argue.

Extinctions in that period occurred for different reasons — largely invasive species entering islands — than are the main threat now — mainly habitat loss. They found that 65% of extinctions in the last 500 years took place among species that evolved in island sites such as Hawaii. 

Also, the rate of extinction has not been accelerating for the biggest groups of species, and in many cases peaked in the 1800s and 1900s, they point out. 

Extinctions among "plants and arthropods significantly declined over the last 100 years after peaking in the early 1900s," Wiens said in an interview Monday. 

Arthropods are creatures like cockroaches, shrimp, scorpions and ticks that have segmented bodies and legs as well as an exoskeleton and other shared characteristics and are common around the world. 

Saban, now a Harvard University graduate student, and Wiens write in their October 2025 paper, published in the , "Most known biodiversity consists of mainland, terrestrial species (mostly arthropods and plants; approx 90%) that are most often threatened by habitat loss, whereas recent extinctions have been dominated by tetrapods and mollusks on islands and have most often been related to invasive species."

"Thus, these past extinctions should not necessarily be seen as a preview of future extinctions across all organisms."

Their October paper was preceded by two others this year in which Saban and Wiens poke at and prod the sixth-mass-extinction hypothesis. 

, titled Questioning the Sixth Mass Extinction, did just that. Wiens and Saban argued, among other things, that the pace of extinction does not lend itself to the likelihood that Earth will reach the 75% threshold from the current decline in biodiversity. 

"We are convinced that Earth is on the brink of major biodiversity loss," Wiens and Saban wrote. "But we are skeptical that the current biodiversity crisis is a mass extinction event."

Saban said via email: "The main takeaway that I would emphasize here is that as scientists, we need to be careful and rigorous both in conducting our research and conveying it to the public. This is especially important in light of growing public mistrust in science and as it pertains to hot-button themes such as mass extinctions or climate change."

That piece brought whose work they had cited — Robert Cowie, Philippe Bouchet and Benoit Fontaine. As it turns out, Cowie is a research professor at the University of Hawaii and doesn't take so kindly to people minimizing the significance of extinctions of island species. 

More specifically, the three of them argue that Saban and Wiens minimized the number of extinctions by dividing the number of known extinctions into the total estimated number of identified species. The better way to extrapolate an extinction rate, they argue, is to consider the number of known extinctions as a percentage of the number of species evaluated within that group. 

So, for example, about 300 mollusks are known to be extinct out of about 6,000 that have been evaluated, for an approximately 5% rate of extinction, Cowie said, but there are tens of thousands more mollusk species that have not been evaluated, and it's unclear what number of those have gone extinct. 

More broadly, they say, Wiens and Saban, by downplaying the likelihood of a sixth mass extinction, are providing justification for those who would prefer not to have any limits on their despoiling of nature for profit. 

"If you say 'Let’s play down the possibility that there’s an issue,' then that plays into the hands of the people who are destroying the as we speak," Cowie said in an interview. 

But Tierra Curry, co-director of the endangered species program at the Tucson-based , noted that Wiens and Saban "go out of their way to point out that we are in an extinction crisis."

Indeed, they say that one of the lessons of recent decades is that conservation works, and that avoiding a sixth mass extinction is a rather timid goal. We should be aiming to lose a lot fewer than the 75% of species that mark mass-extinction events, by common definition. 

Rather than trying to buttress a mass-extinction argument, they write, "we instead favor finding and ameliorating the largest threats to biodiversity and identifying, prioritizing, and protecting those species most at risk."

In fact, ln November, Wiens worked with the Center for Biological Diversity to petition the U.S. Fish and Wildlife Service to list in the Mule Mountains of Southern Arizona as an endangered species. 

"One of the reasons we think extinction rates are declining is because of conservation," Wiens said. "This seems to be working. We should keep doing this."