Conservation news

Study confirms what scientists have been saying for decades: the sixth mass extinction is real and caused by us

The Panamanian golden toad is classified as
The Panamanian golden toad is classified as “Extinct in the Wild” on the IUCN Red List. Photo by Rhett A. Butler.

Humans are wiping species off the plant at a rate at least 100 times faster than historical levels, providing further evidence that we’re in the midst of a sixth great extinction, concludes a new study based on “extremely conservative” assumptions on past and current extinction rates.

The research, published last week in the journal Science Advances by an international team of prominent scientists, differs from previous extinction estimates in its methodology. Instead of projections based on mathematical models that anticipate species losses based on the extent of habitat destruction (species-area curves), the new paper compiles a list of species that have actually gone extinct since 1500 and compares it to a historical baseline extinction rate that is 100 to 1,000 percent higher than conventionally assumed.

Numbers of species used in the Table 2 calculations of ‘highly conservative’ and ‘conservative’ modern extinction rates based on the IUCN Red List. For the highly conservative rates, only species verified as ‘extinct’ (EX) were included; for the conservative extinction rates, species in the categories ‘extinct in the wild’ (EW) and ‘possibly extinct’ (PE) were also included. Image and caption courtesy of Ceballos et al (2015)

Even using this conservative approach results in a conclusion that many scientists have been warning of for decades: humanity is driving mass extinction among species that share the planet. This extinction event is unfolding at a rate that rivals past mass extinctions caused by extraterrestrial collisions, atmospheric poisoning, and hyperactive volcanism.

“Our analysis shows that current extinction rates vastly exceed natural average background rates, even when (i) the background rate is considered to be double previous estimates and when (ii) data on modern vertebrate extinctions are treated in the most conservative plausible way,” write the authors. “We emphasize that our calculations very likely underestimate the severity of the extinction crisis because our aim was to place a realistic ‘lower bound’ on humanity’s impact on biodiversity. Therefore, although biologists cannot say precisely how many species there are, or exactly how many have gone extinct in any time interval, we can confidently conclude that modern extinction rates are exceptionally high, that they are increasing, and that they suggest a mass extinction under way—the sixth of its kind in Earth’s 4.5 billion years of history.”

The authors add that the situation is actually far worse if loss of populations are considered.

“We focus exclusively on species, ignoring the extirpation of populations—the units relevant to ecological functioning and the delivery of ecosystem services,” they write. “Population extinction cannot be reliably assessed from the fossil record, precluding any analysis along the lines of that presented here. Also, although it is clear that there are high rates of population extinction, existing data are much less reliable and far harder to obtain than those for species, which will remain true for the foreseeable future.”

Cumulative vertebrate species recorded as extinct or extinct in the wild by the IUCN (2012). Graphs show the percentage of the number of species evaluated among mammals (5513; 100% of those described), birds (10,425; 100%), reptiles (4414; 44%), amphibians (6414; 88%), fishes (12,457; 38%), and all vertebrates combined (39,223; 59%). Dashed black curve represents the number of extinctions expected under a constant standard background rate of 2 E/MSY. Conservative estimate. Image and caption courtesy of Ceballos et al (2015)

Number of years that would have been required for the observed vertebrate species extinctions in the last 114 years to occur under a background rate of 2 E/MSY. Red markers, highly conservative scenario; blue markers, conservative scenario. Note that for all vertebrates, the observed extinctions would have taken between 800 to 10,000 years to disappear, assuming 2 E/MSY. Different classes of vertebrates all show qualitatively similar trends. Image and caption courtesy of Ceballos et al (2015)

Biodiversity faces a multitude of threats from human activity, most of which are tied to mankind’s expanding ecological footprint in the form of conversion of natural habitats for food production and living space; resource extraction and consumption; and waste, which is driving climate change and polluting the world’s oceans and rivers. Secondary threats come from the introduction of alien invasive species and other alterations to ecosystems and communities. Rising human population and consumption are making these issues increasingly intractable.

The authors conclude that while the trend can still be reversed, the time for action is short and fleeting.

“Our analysis emphasizes that our global society has started to destroy species of other organisms at an accelerating rate, initiating a mass extinction episode unparalleled for 65 million years. If the currently elevated extinction pace is allowed to continue, humans will soon (in as little as three human lifetimes) be deprived of many biodiversity benefits. On human time scales, this loss would be effectively permanent because in the aftermath of past mass extinctions, the living world took hundreds of thousands to millions of years to rediversify,” they write.

“Avoiding a true sixth mass extinction will require rapid, greatly intensified efforts to conserve already threatened species and to alleviate pressures on their populations—notably habitat loss, overexploitation for economic gain, and climate change. All of these are related to human population size and growth, which increases consumption (especially among the rich), and economic inequity. However, the window of opportunity is rapidly closing.”

The Micronesian kingfisher (Todirhamphus cinnamominus) is classified as “Extinct in the Wild” omn the IUCN Red List. Photo by Rhett A. Butler.

The overall conclusion that humans are the driving force behind the sixth mass extinction is consistent with a large body of research. For example, just three weeks prior to the paper’s publication, a paper in Biodiversity and Conservation reached a similar conclusion also based on IUCN data. The author, Malcolm L. McCallum of the University of Illinois Springfield, argued that species losses since 1980 have been faster that the Cretaceous–Paleogene extinction event that wiped out dinosaurs 65 million years ago.

“Vertebrate extinction moved forward 24–85 times faster since 1500 than during the Cretaceous mass extinction,” McCallum wrote. “The magnitude of extinction has exploded since 1980, with losses about 71–297 times larger than during the Cretaceous–Paleogene event.”

“If species identified by the IUCN/SSC as critically endangered through vulnerable, and those that are data deficient are assumed extinct by geological standards, then vertebrate extinction approaches 8900–18,500 times the magnitude during that mass extinction. These extreme values and the great speed with which vertebrate biodiversity is being decimated are comparable to the devastation of previous extinction events. If recent levels of extinction were to continue, the magnitude is sufficient to drive these groups extinct in less than a century.”

Americans drove the passenger pigeon to extinction more than a century ago.

While McCallum’s outlook is more stark than that of the Science Advances paper, it also concludes by urging immediate action to protect biodiversity.

“The outcomes of this study combined with previously published research provide for a clear conclusion. Multiple, unrelated investigators have approached the question of a sixth mass extinction using multiple tools. Their message has been consistent,” he writes. “Whether 1000 times background or 9000 times the extinction rate at K–Pg, it is blatantly obvious that biodiversity losses must be reined in.”

NOTE: This article was updated at 8:00 pm Pacific to include reference to, and quotes from, McCallum’s paper published earlier in Biodiversity and Conservation.