Today, the fossilized remains of these creatures are buried beneath a conspicuous layer of sediment or rock that geologists call the Cretaceous-Paleogene (K-Pg) boundary. The layer is typically enriched in the element iridium, released by the shattering of the Mount Everest–sized asteroid that smashed into modern-day Mexico one fateful day 66 million years ago and brought the Cretaceous period to a violent end. “It basically happened in the worst possible place,” says Witts, a lecturer at the University of Bristol. Research published earlier this year by him and his colleagues suggests that massive quantities of sulfur from bedrock at the impact site, in the present-day Yucatán peninsula, were blasted into the atmosphere, creating aerosols that blocked sunlight for several months, cooled the climate for decades, and fell down as acid rain—all contributing to the collapse of global ecosystems. The fossil record falls comparatively silent after the K-Pg boundary, as an estimated 76 percent of marine species were wiped out, for instance. And as life recovered over the next million years, only a subset of the lineages that previously roamed the Earth were among its ranks. Like their popular terrestrial counterparts, “the big marine reptiles—they never come back,” says Witts.
For the first time, we are trying to put our finger on a geologically superlative event while it’s happening.
—Jacquelyn Gill, University of Maine
The K-Pg extinction is the most recent of five events in Earth’s history that scientists consider mass extinctions, defined by paleontologists as events where more than 75 percent of species vanish within a geologically short period of time, typically less than two million years. The four previous mass extinctions were also thought to have involved climatic changes—due to large-scale volcanic eruptions, for example—and in one case obliterated all but 5 percent of species. (See illustration below.) In between these events were smaller extinction episodes and periods of relative stability, with new species often arising at rates that compensated for species losses.
Now, many scientists fear that the next ordeal of this scale is close—this time around, caused by our species, which sprang onto the scene within the last few hundred thousand years. Although we’re still far away from reaching the 75 percent mark, extinction rates are climbing, and many more species appear to be on the brink. Scientists point to the worldwide destruction of natural habitats and the exploitation of wild species, along with climate change, pollution, and ecological disruption caused by the spread of invasive organisms, as driving factors. Indeed, Witts says he reckons that the sheer speed of environmental change today is similar to that caused by the asteroid.
Whether current biodiversity loss—a crisis by any measure—meets the criteria for another mass extinction is hotly debated. Much of the debate hinges on accurately measuring the scale of modern-day and prehuman extinction, which is complicated by an incomplete understanding of present and past biodiversity. Some scientists also question whether diagnosing a mass extinction is even relevant.
“We’re in this really unusual position, where, for the first time, we are trying to put our finger on a geologically superlative event while it’s happening,” says Jacquelyn Gill, a paleoecologist at the University of Maine. However, when it comes to biodiversity loss, “just evoking the fact that our influences could even be on the scale of a comet or some of these other big events in the past—I think that should be giving us pause.”
THE BIG FIVE
Most scientists agree that five events in Earth’s history qualify as “mass extinctions”—defined as events where more than three-quarters of estimated species are wiped out. These ordeals were caused by natural phenomena, typically involving climatic changes, although the exact processes involved and the chain of events are often debated. Current trends suggest we’re now in another extinction crisis, although it’s unclear if whether that amounts to a sixth mass extinction.
The Ordovician-Silurian mass extinction event may have wiped out some 85 percent of species, including many of the invertebrates this period is known for. Some scientists hypothesize the extinction crisis was driven by changes in ocean chemistry or a cooling climate that caused sea levels to drop as glaciers formed.
The late Devonian period was characterized by many environmental changes, although it’s not clear what caused an estimated 70 to 80 percent drop in species numbers. Some scientists don’t consider this event a mass extinction event based on analyses suggesting the biodiversity trend was driven more by a decrease in speciation rates than by an increase in extinction rates.
Commonly referred to as the “Great Dying,” this extinction event is estimated to have wiped out more than 95 percent of marine species, as well as some terrestrial amphibians and reptiles. Many scientists think a key culprit was widespread volcanic activity.
Global warming triggered by heavy volcanic activity is hypothesized by some scientists to have caused the end-Triassic extinction event that obliterated up to 80 percent of Earth’s species. These included many ammonites and land-dwelling crocodile relatives.
The most famous and well-studied of mass extinction events, the so-called K-Pg extinction killed off the nonavian dinosaurs after an asteroid collided with Earth. Some researchers believe the crisis was exacerbated by heavy volcanic activity in what is now India.
Human activity—causing land use change, global warming, and pollution—are driving heightened extinction rates and population declines across many taxa. Some scientists argue we’re currently facing a sixth mass extinction, but others say it’s too early to make that call.
Estimating species extinctions
Extinction rates are often measured with a metric called millions of species-years (MSY), or the estimated number of extinctions per 10,000 species per 100 years, for any given taxa in question. To assess the current biodiversity crisis, researchers typically focus on investigating whether current extinction rates are above background rates over the past few million years, and if so, by how much. But these two metrics are tricky to estimate, Gill notes. Scientists don’t even know how many species exist or have existed in the past.
The fossil record—an incomplete archive of biodiversity, as not all organisms fossilize—has produced various estimates of background extinction rates, that is, the usual rate of species turnover in between mass extinction events. They usually fall somewhere between 0.1 and 2 extinctions per MSY for different taxa. At most, that’s two species disappearing in a century for every 10,000 species present. That’s lower than the average rate at which species diversify, meaning that most of the time, the planet is becoming more speciose, not less, says conservation scientist Stuart Pimm of Duke University’s Nicholas School of the Environment.
It’s almost impossible to make a credible scientific comparison between what is happening now and what happened in the past.
—Stuart Pimm, Duke University
Many estimates for today’s extinction rates are based on the Red List compiled by the International Union for Conservation of Nature (IUCN). Although it’s the most comprehensive database of the conservation statuses of wild species, only around 142,500 animal, plant, and fungal species have been assessed—just 6.5 percent of the roughly 2.2 million species known to science, which itself is a fraction of the species on Earth. There are, however, good records for vertebrates, such as mammals and birds, for which the IUCN has evaluated nearly all described species. In 2015, based on vertebrate data, ecologist and conservation biologist Gerardo Ceballos of the Universidad Nacional Autónoma de México and his colleagues estimated that the rate of species loss over the past century is up to 200 extinctions per MSY—“incredibly high compared to what happened in the last few million years,” Ceballos says. “For us, it was a clear indication that we have entered a sixth mass extinction.”
Ceballos says the IUCN-based figures are underestimates. The IUCN is notoriously cautious about listing species as extinct, fearful of committing the “Romeo error”—declaring something as dead even though it’s really still alive and causing it to lose vital conservation funding. But Ceballos can name a handful of species that haven’t been spotted in decades and that he considers extinct, but which the IUCN still lists as endangered. These include Mexico’s imperial woodpecker (Campephilus imperialis) and the Guadalupe storm petrel (Hydrobates macrodactylus), he says.
IUCN records for invertebrates are even less helpful, notes biologist Robert Cowie of the University of Hawai‘i at Mānoa. These organisms represent the overwhelming majority of biodiversity, but as of 2020, the IUCN has evaluated only around 1.5 percent of known invertebrate species. The organization has only documented 63 insect extinctions out of more than a million known insects—a figure so small that some scholars have concluded that overall extinction rates are extremely low and not of concern. But this ignores many invertebrate extinctions that have likely gone unnoticed, Cowie says. “When you’re talking about invertebrates, then you cannot take just those [IUCN] numbers.”
The sampling problem
Fortunately, there are other ways to gauge extinction rates. Many scientists have used the species-area curve—an ecological relationship between the size of an area and the number of species it contains—to predict the number of extinctions due to habitat loss. Because this method generates accurate predictions for well-known taxa, “we have confidence that our predictions of extinctions for invertebrates are likely to be true, even if so few are described,” writes Pimm to The Scientist in an email. As early as 1995, he and his colleagues used this curve to estimate that extinction rates of plants and animals were 100 to 1,000 times higher than background levels. That’s around 1,000 to 10,000 times faster than species are diversifying, a comparison Pimm considers to be more useful than differences between past and present extinction rates. Places rich in endemic species, such as islands, are hardest hit.
However, statistical ecologist Andrew Solow of the Woods Hole Oceanographic Institution notes that species-area estimates are imprecise. One complication is uncertainty about the exact ranges of different species—especially poorly understood and unknown ones—and whether or not they could seek refuge elsewhere if their habitat is destroyed. And extinction depends not just on habitat loss, but also on the population size of a given species and of the species it interacts with. In fact, scientists have argued that the method either underestimates or overestimates extinction rates. “This kind of calculation [can] give you an idea about what’s going on, but it’s not really giving a precise or really completely defensible estimate of species loss,” Solow says. “Extrapolation is always a problem.”
In 2015, Cowie and his colleagues tried a different approach. They consulted mollusk experts about the conservation status of 200 randomly selected land snail species. Those inquiries determined that 10 percent of the snails were most likely extinct, including some that the IUCN considered merely endangered. Based on this figure, the team estimated that up to 5,200 terrestrial mollusk species are now likely extinct, out of more than 73,000 known species. Extrapolating further, that would suggest that between 7.5 and 13 percent of all 2 million known species on Earth have gone extinct since around 1500, the date from which the IUCN records extinctions. That’s far more than the 882 species officially reported extinct by the IUCN in 2020, and corresponds to a rate of 150–260 extinctions per MSY.
Scientists are gradually getting a handle on other little-understood taxa. One 2019 study used IUCN data and extinction reports in the scientific literature for seed-bearing plants to estimate a current extinction rate of up to 500 times a background rate previously reported by Pimm and colleagues of up to 0.35 extinctions per MSY for plants. Mirroring a trend observed in animals, plant extinctions were dominated by species that were only recently discovered, by which time they were already rare.
In the marine realm, where biodiversity in general is poorly cataloged, strikingly few extinctions have been documented; Cowie counts only one documented extinction of an exclusively marine fish species, for instance. That’s probably partly because members of a species tend to be more widely dispersed in the ocean—hence harder to wipe out—and possibly because they’ve been somewhat shielded from human impacts, Cowie says. The relative invulnerability of marine life is a key difference from previous mass extinctions, in which ocean species—particularly clams, snails, and ammonites—were slammed, Witts adds. “When we start really seeing evidence for a lot of elevated losses in those groups, then we’re really in trouble.”
Nevertheless, most scientists agree that overall extinction rates are much higher now than at any time in the past several million years, and perhaps even higher than during previous mass extinctions. For some, like Ceballos, it’s clear that the current crisis is a mass extinction. But others disagree. “It’s almost impossible to make a credible scientific comparison between what is happening now and what happened in the past,” Pimm says, one reason being that while modern estimates are typically based on species, those that draw from the fossil record are based on genera or families. Comparing uncertain modern extinction rates—which are calculated based on data covering just a few hundred years, at most—with similarly shaky ones documented from an incomplete fossil record spanning thousands or millions of years is like comparing apples and oranges, agrees paleoecologist Felisa Smith of the University of New Mexico.
And technically, we haven’t hit the 75 percent mark yet, Pimm adds. One 2011 review concluded that, while we could meet that threshold in less than 540 years if all threatened birds and mammals became extinct within a century and extinction rates continue unabated, today’s crisis “does not yet qualify as a mass extinction in the paleontological sense of the Big Five.” But holding such a high standard for an event to qualify as a mass extinction doesn’t allow a diagnosis until it’s essentially too late, notes ecologist Edd Hammill of Utah State University. “Just because something does not meet the criteria of mass extinction does not necessarily mean, ‘Oh, well, it’s all fine then.’”
UNDERSTANDING TODAY’S CRISIS
Estimates for extinction rates today are often based on data from the International Union for Conservation of Nature’s (IUCN) Red List, which compiles the conservation status of threatened species. Yet the organization has assessed only a fraction of known species—themselves only a fraction of the total number of species on Earth. And even among assessed species (data shown below), many are considered data-deficient.
Beyond extinction rates
To Ceballos, focusing only on extinction rates downplays the current biodiversity crisis. Extinction is only the terminal stage of drawn-out population declines. In 2017, Ceballos and his colleagues examined IUCN data for 27,600 terrestrial vertebrate species and found alarming rates of population decline even in still-abundant species. In 177 species for which detailed data were available then, all had lost at least one-third of their historic range, and for more than 40 percent of species, their range had shrunk by more than 80 percent since 1900. Other groups have also documented staggering population declines of many oceanic and freshwater fish species.
Many invertebrates, too, have seen steep declines in recent decades. Headlines have warned of an impending “insect apocalypse.” Yet there is considerable variation across insect taxa, notes ecologist Aletta Bonn of the Helmholtz Center for Environmental Research in Germany. In recent research in which Bonn and her colleagues tracked dragonflies and damselflies in Germany over the past 35 years, they learned that warm-adapted species, as well as ones with longer wing lengths, had actually increased in their distribution. The scientists also observed a slight recovery in river-dwelling dragonfly species, which could be linked partly to recent efforts to restore river habitats. “With insects, because they have short population cycles, the right management can actually bring them back,” Bonn says.
It’s also important to consider the consequences of population declines, Ceballos adds. Extinction, whether at the global or local scale, leaves functional holes in ecological networks. That’s particularly concerning given that some ecologically vital species are among the ones most impacted by human activity. In 2020, Hammill, together with ecosystem ecologist Trisha Atwood of Utah State University and their colleagues, discovered that based on IUCN data for birds, mammals, and reptiles, the species at highest risk of extinction were not predators, as Atwood says is often assumed, but herbivores, especially large-bodied ones, which have disproportionate influences on soils, water tables, vegetation, and biogeochemical processes. It’s not the first time this pattern has been seen. Since around 125,000 years ago, extinctions of large-bodied animals have surged—concurrent with the rise and spread of Homo sapiens, who hunted these animals, some of Smith and colleagues’ research shows. “That unravels the ecosystem in ways that these previous mass extinctions wouldn’t necessarily,” Smith says. “I’m not saying it’s worse or better, I’m just saying that this focus on ‘Is this a mass extinction?’ seems shortsighted because the real focus should be on the damage we’re doing to Earth’s ecosystems.”
Researchers agree that such trends are catastrophic to many species, including humans, who depend on various animal, plant, and fungal species for food, pollination of crops, carbon storage, cultural purposes, and other reasons. Whether scientists call the current loss of biodiversity a mass extinction depends partly on whether they think it will spur enough societal action to stop biodiversity loss. “If [the term] galvanizes people to really care about this issue, I’m happy with it,” remarks theoretical ecologist Stephen Hubbell, a professor emeritus at the University of California, Los Angeles. But Gill wonders whether extinction rates necessarily need to be superlative for people to care about them. “I think we can make statements about the urgency of the extinction crisis without needing to evoke these large-scale geologic events,” she says. Either way, Smith adds, “we need as a species to have a serious conversation [about] what we want the planet to look like.”
Perhaps, Witts speculates, creatures living millions of years from now will find something that looks just as striking as the K-Pg boundary when they discover the geological legacy of humans. “It’ll probably be some horrible sort of black shale,” Witts says—full of the carbon humans have pumped into the air, along with traces of nuclear testing and metals extracted from the Earth to build the modern world. In the worst case, he continues, patterns of fossils might reveal mass die-offs rivaling the scale of the K-Pg extinction. “Whether [we’re] quite at the stage of a mass extinction yet, I don’t know. But we’re certainly on the way there.”