Getty images.

There are a lot of things we don’t know for certain about the planet we call home: How many species exist today? How exactly did past extinctions play out? How bad will climate change be in the near future?

One thing we do know, and that a new study in the peer-reviewed journal Proceedings of the National Academy of Sciences documents in painstaking detail, is that we are living through a massive die-off of wild animals—and we are uncomfortably close to causing the planet’s sixth mass extinction.

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In this study, rather than focus solely on full-blown extinctions, which only two or so vertebrates a year experience, the scientists draw attention to the dwindling of population sizes and ranges, which they consider a “prelude” to global extinction and a warning sign ignored at the peril of the planet.

“The massive loss of populations and species reflects our lack of empathy to all the wild species that have been our companions since our origins,” said the study’s lead author, Gerardo Ceballos of the National Autonomous University of Mexico. “It is a prelude to the disappearance of many more species and the decline of natural systems that make civilization possible.”

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In the study, the scientists reviewed 177 well-studied mammal species and found that all of them had lost at least 30% of their territory between 1900 and 2015 and that more than 40% of them lost at least 80% of their geographic range during that time.

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The scientists also considered a larger sample of some 27,600 species of land-based vertebrates, finding that a third of them are in decline.

According to the authors:

Using a sample of 27,600 terrestrial vertebrate species, and a more detailed analysis of 177 mammal species, we show the extremely high degree of population decay in vertebrates, even in common “species of low concern.” Dwindling population sizes and range shrinkages amount to a massive anthropogenic erosion of biodiversity and of the ecosystem services essential to civilization. This “biological annihilation” underlines the seriousness for humanity of Earth’s ongoing sixth mass extinction event.

Decreasing land vertebrates, as exemplified with these four species, include taxa with different conservation status (e.g., low concern, critically endangered), current geographic range (e.g., large, very restricted), and abundance (e.g., common, rare). The data on conservation status, current geographic range, and abundance are from IUCN (28). Barn swallow image courtesy of Daniel Garza Galindo (photographer).

Previous studies have shown that the average rate of vertebrate species loss over the last century is up to 100 times higher than the background rate. According to the International Union for Conservation of Nature (IUCN), about 26 percent of all mammals face the specter of extinction. Some of our the most treasured species, such as rhinos, elephants, giraffes, tigers, and polar bears, face the most perilous future as their populations dwindle rapidly from the hundreds of thousands into the tens of thousands and even just thousands. Even as some 400,000 wild elephants remain, at present rates they could be functionally extinct within a couple decades.

The percentage of species of land mammals from five major continents/subcontinents and the entire globe undergoing different degrees (in percentage) of decline in the period ∼1900–2015. Considering the sampled species globally, 56% of them have lost more than 60% of their range, a pattern that is generally consistent in Africa, Asia, Australia, and Europe, whereas in South America and North America, 35–40% of the species have experienced range contractions of only 20% or less.

The study found that while population extinction in land vertebrates is widespread across the globe, it is especially prominent in tropical regions with an abundance of species. However, when the population extinctions are evaluated as a percentage of total species, temperate regions—with lower species diversity—show higher proportions of population lss.

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According to the study, “habitat loss, overexploitation, invasive organisms, pollution, toxification, and more recently climate disruption, as well as the interactions among these factors” are all contributing to the unfolding disaster.

So we know for sure we are in a major biological disaster—but is it a mass extinction? It could very well be the beginning of one. Or even the middle. Like with climate change, it’s hard to know for certain how bad the situation is before it’s too late.

The study’s authors offer their concluding reasoning on this issue:

Earth’s sixth mass extinction episode has proceeded further than most assume. The massive loss of populations is already damaging the services ecosystems provide to civilization. When considering this frightening assault on the foundations of human civilization, one must never forget that Earth’s capacity to support life, including human life, has been shaped by life itself.