Many scientists say it’s abundantly clear that Earth is entering its sixth mass-extinction event, meaning three-quarters of all species could disappear in the coming centuries.

That’s terrifying, especially since humans are contributing to this shift.

But that’s not even the full picture of the “biological annihilation” people are inflicting on the natural world, according to a study published Monday in the Proceedings of the National Academy of Sciences. Gerardo Ceballos, an ecology professor at the Universidad Nacional Autónoma de México, and his co-authors, including well-known Stanford University biologist Paul Ehrlich, cite striking new evidence that populations of species we thought were common are suffering in unseen ways.

“What is at stake is really the state of humanity,” Ceballos told CNN.

Their key findings: Nearly one-third of the 27,600 land-based mammal, bird, amphibian and reptile species studied are shrinking in terms of their numbers and territorial range. The researchers called that an “extremely high degree of population decay.”

The scientists also looked at a well-studied group of 177 mammal species and found that all of them had lost at least 30% of their territory between 1900 and 2015; more than 40% of those species “experienced severe population declines,” meaning they lost at least 80% of their geographic range during that time.

Looking at the extinction crisis not only in terms of species that are on the brink but also those whose populations and ranges are shrinking helps show that “Earth’s sixth mass extinction is more severe” than previously thought, the authors write. They say a major extinction event is “ongoing.”

“It’s the most comprehensive study of this sort to date that I’m aware of,” said Anthony Barnosky, executive director of the Jasper Ridge Biological Preserve at Stanford University, who was not involved in the study. Its value, Barnosky said, is that it makes visible a phenomenon typically unseen by scientists and the public: that even populations of relatively common species are crashing.