The study conducted by researchers from Stanford University and the National Autonomous University of Mexico and published in Proceedings of the National Academy of Sciences (PNAS) claims that the demise of billions of populations of both rare and common species "means the sixth mass extinction event is already well underway and is more severe than perceived."
"The situation has become so bad it would not be ethical not to use strong language," said the researcher who led the study, Gerardo Ceballos of the Universidad Nacional Autónoma de México.
The study noted that habitat loss, overexploitation, invasive organisms, pollution, toxification, and climate disruption have led to "catastrophic declines in both the numbers and sizes of populations of both common and rare vertebrate species."
There have been five "mass extinctions" in the past 500 million years of Earth's history. The last one was some 66 million years ago, when 76 percent of all species were lost, including the dinosaurs, due to volcanic activity, climate change and the impact of asteroids.
The study used a sample of 27,600 terrestrial vertebrate species and a more detailed analysis of 177 mammal species that faced population extinction between 1900 and 2015, which showed a high degree of population decay in vertebrates, even in species of low concern.
"But few people notice, perhaps because the rate seems relatively slow - not a clear and present threat to the natural systems we depend on," the researchers note.
The risk of extinction hangs over about 41 percent of all amphibian species and 26 percent of all mammals, according to the International Union for Conservation of Nature (IUCN), which maintains a list of threatened and extinct species.