The world is experiencing a "biological annihilation" of its animal species due to "human-caused global environmental problems" in recent decades, a new scientific study has found.
The demise of billions of animals from thousands of species both rare and common means the sixth mass extinction event is already well underway and is "more severe than perceived," the study published in a peer-reviewed journal, Proceedings of the National Academy of Sciences (PNAS), said.
"In the last few decades, habitat loss, overexploitation, invasive organisms, pollution, toxification, and more recently climate disruption, as well as the interactions among these factors, have led to the catastrophic declines in both the numbers and sizes of populations of both common and rare vertebrate species," it said.
There have been five "mass extinctions" in the past 500 million years of earth's history, during which 75 per cent of species disappeared.
The last one was some 66 million years ago, when 76 per cent of all species were lost, including the dinosaurs, due to volcanic activity, climate change and asteroid impact.
The study was conducted by researchers from Stanford University and the National Autonomous University of Mexico.
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 extremely high degree of population decay in vertebrates, even in species of low concern.
Of the 27,600 species that comprises nearly half of known vertebrate species, a third or 8,851 have decreased in population size and geographic range, even though they are not currently considered endangered, the study found.
In the 177 mammals, all have lost 30 per cent or more of their geographic ranges, while more than 40 per cent have experienced severe population decline with more than 80 per cent range shrinkage.
"Several species of mammals that were relatively safe one or two decades ago are now endangered," the study said, citing examples of massive declines in the population of cheetahs, orangutans, lions, pangolin and giraffes.
The study said a strong focus on species extinctions leads to "a common misimpression" that earth's habitat is not immediately threatened, just slowly entering an episode of major biodiversity loss.
