• Round Island burrowing boa, extinct in 1975

Round Island burrowing boa, extinct in 1975 (Photo : National Geographic)

It's an impossibly cruel figure but it's true: humans have been the main cause of the loss of more than half of all the living species on Earth since the 1970s. If allowed to continue, a Sixth Mass Extinction might become possible and some scientists are saying this event has already begun.

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The numbers of mammals, birds and fish plunged an average of 58 percent between 1970 and 2012, reported the World Wildlife Federation (WWF) and the Zoological Society of London (ZSL) in The Living Planet report published Oct. 25.

Wildlife populations could decline by 67 percent compared to 1970 by the end of the decade if nothing is done, the report said.

An assessment of 14,152 populations of 3,706 species of mammals, birds, fish, amphibians and reptiles from around the world reveals a 58 percent plunge between 1970 and 2012 -- with no sign the average two percent drop in numbers each year will slow.

Overall terrestrial species, which are found in habitats ranging from grasslands to forests, have seen populations plummet by two-fifths since 1970.

Freshwater species are faring far worse, with declines of four-fifths between 1970 and 2012.

These figures prompted experts to warn nature was facing a global "mass extinction" for the first time since the destruction of the dinosaurs.

Human behavior continues to drive the decline, especially in freshwater habitats, said Ken Norris, director of science at ZSL. Human activities such as poaching, farming, fishing, wildlife trafficking, mining and pollution are pushing wildlife "to the edge," with these anthropogenic reasons being worsened by climate change.

"Importantly, however, these are declines -- they are not yet extinctions -- and this should be a wake-up call to marshal efforts to promote the recovery of these populations," said Norris.

For decades scientists have been warning that human actions are pushing life toward a Sixth Mass Extinction. Evidence in this year's Living Planet Report supports this, said Marco Lambertini, WWF director general.

"We are feeling the impact of a sick planet -- from social, economic and climate stability to energy, food and water security -- all increasingly suffering from environmental degradation.

Moving toward a more resilient planet, however, will require a huge shift toward more sustainable, renewable energy, and for people in rich nations to eat less animal protein and reduce waste, said the report.

Lambertini said there is evidence things are beginning to change, noting global carbon dioxide emissions have stabilized over the past two years. More needs to be done, however.

"Without action, the Earth will become much less hospitable for all of us," the report said.