A young lion looks towards the Nairobi skyline at the Nairobi national park on August 10, 2015. With all the attention on the cruel slaughter of Cecil the Lion by a US trophy hunter in Zimbabwe, the first international campaign to protect the imposing "king of beasts" kicked off in Kenya on World Lion Day to draw attention to the silent extermination of the big cats around the world, iconised by the African lion and Asian tiger. According to National Geographic, some 200,000 lions roamed across Africa a century ago. Today, there are less than 30,000. AFP PHOTO / TONY KARUMBA (Photo credit should read TONY KARUMBA/AFP/Getty Images)

Over the last half-billion years, scientists say there have been five mass extinction events on Earth in which a wide diversity of species on this planet suddenly died off. Now, there’s growing evidence that a sixth mass extinction is unfolding, according to scientists who track species around the globe. In a new study, researchers say the current mass extinction is even “more severe than perceived” and amounts to “biological annihilation” affecting thousands of species.
In the study, published Monday in the journal Proceedings of the National Academy of Sciences, researchers from Stanford University and the National Autonomous University of Mexico unveiled a granular look at population trends among 27,600 species of birds, amphibians, mammals, and reptiles — half of the world’s known terrestrial vertebrates — including detailed analysis of 177 species of mammals. 
The results are grim: researchers found an “extremely high degree of population decay” among vertebrates, even in species considered at low …