SAN FRANCISCO, July 10 (Xinhua) -- Biologists at Stanford University in the first global evaluation of the kind found that more than 30 percent of all vertebrates have declining populations.

Two vertebrate species go extinct every year on average, but few people notice, perhaps because the rate seems relatively slow -- not a clear and present threat to the natural systems we depend on, noted the authors of the new study published Monday in Proceedings of the National Academy of Sciences.

A 2015 study co-authored by Paul Ehrlich, professor emeritus of biology at Stanford, and colleagues showed that Earth has entered an era of mass extinction unparalleled since the dinosaurs died out 66 million years ago. The specter 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).

The new analysis looks beyond species extinctions to provide a clear picture of dwindling populations and ranges. The researchers mapped the ranges of 27,600 species of birds, amphibians, mammals and reptiles -- a sample representing nearly half of known terrestrial vertebrate species -- and analyzed population losses in a sample of 177 well-studied mammal species between 1990 and 2015.

Using range reduction as a proxy for population loss, the study finds more than 30 percent of vertebrate species are declining in population size and range. Of the 177 mammals for which the researchers had detailed data, all have lost 30 percent or more of their geographic ranges and more than 40 percent have lost more than 80 percent of their ranges.

Tropical regions have had the greatest number of decreasing species while temperate regions have seen similar or higher proportions of decreasing species.

Particularly hard hit have been the mammals of south and southeast Asia, where all the large-bodied species of mammals analyzed have lost more than 80 percent of their geographic ranges.

The study's maps suggest that as much as 50 percent of the number of animal individuals that once shared Earth have disappeared, as have billions of animal populations, amounting to what the authors call "a massive erosion of the greatest biological diversity in the history of Earth."

"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 new 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."

"This is the case of a biological annihilation occurring globally, even if the species these populations belong to are still present somewhere on Earth," co-author Rodolfo Dirzo, a Stanford professor of biology, was quoted as saying in a news release.