About 60 percent of the world's fish, birds, mammals, amphibians and reptiles have been wiped out over the past four decades, largely because of human activity, researchers say.
The Living Planet Report 2016, released on Thursday, blamed deforestation, pollution, overfishing, and the illegal wildlife trade for "pushing species populations to the edge".
According to the report by the World Wildlife Fund (WWF), in a collaboration with other international environmental advocacy groups, the five main drivers of wildlife decline are habitat loss, overconsumption, pollution, invasive species and disease.
"For the first time since the demise of the dinosaurs 65 million years ago, we face a global mass extinction of wildlife," said Mike Barrett, director of science and policy at WWF's UK branch.
"We ignore the decline of other species at our peril - for they are the barometer that reveals our impact on the world that sustains us."
The findings are based on long-term monitoring of some 3,700 vertebrate species spread across more than 14,000 distinct populations.
Victims include gorillas and orangutans, rhinos and elephants, tigers and snow leopards, but also faceless species such as corals - a crucial cornerstone not only of marine life but also coastal human communities.
Factory fishing has emptied the seas of 40 percent of sea life, and nine out of 10 fisheries in the world are either overfished or full-fished today.
Swathes of coral reef around the globe have already turned white, killed by warming waters, pollution and disease.
On the current trend, the assessment predicts that by 2020 populations of vertebrate species could have fallen by 67 percent from 1970 levels unless action is taken to reverse the damaging effects of human activity.
'Global mass extinction'
The report detailed the strain that agriculture places on freshwater systems.
"Human behaviour continues to drive the decline of wildlife populations globally, with particular impact on freshwater habitats," said Ken Norris, director of science at Zoological Society of London, an advocacy group that took part in producing the report.
"This should be a wake-up call to marshal efforts to promote the recovery of these populations," he added.
Freshwater environments such as lakes, rivers and wetlands have fared the worst, with an 81 percent decline in average population size between 1970 and 2012 for 881 species monitored.
Freshwater covers less than 1 percent of the Earth's surface, but is home to nearly 10 percent of all the planet's known species.
Marine and land vertebrates have suffered at about the same rate - with populations dropping 36 and 38 percent respectively over the same period - but for different reasons.
On land, the big threats are loss of land to agriculture and cities, followed by rampant hunting, mostly for food but also for commerce - much of it in endangered species.
African elephants, slaughtered mainly for their tusks, have dropped in number by more than a quarter since 2006.
Experts now agree that Earth has entered only the sixth "mass extinction event" - when species vanish at least 1,000 times faster than usual - in the past half-billion years.
"Wildlife is disappearing within our lifetimes at an unprecedented rate," said Marco Lambertini, director general of WWF International.
"Biodiversity forms the foundation of healthy forests, rivers and oceans. Take away the species and these ecosystems collapse, along with clean air, water, food and climate services they provide us."
Deon Nel, WWF conservation director, told Al Jazeera that in addition to 30 percent of the world's soil being degraded over the past 40 years, "we have lost 50 percent of tropical forests ... which are critical in stabilising the climate and absorbing carbon".
