The world is embarking on its sixth mass extinction with animals disappearing about 100 times faster than they used to, scientists warn, and humans could be among the first victims of the next extinction event.

Not since the age of the dinosaurs ended 66 million years ago has the planet been losing species at this rapid a rate, a study led by experts at Stanford University, Princeton University and the University of California, Berkeley said.

The study "shows without any significant doubt that we are now entering the sixth great mass extinction event," co-author and Stanford University professor of biology Paul Ehrlich said.

And the study, which was published in the journal Science Advances on Friday and described by its authors as "conservative", said humans were likely to be among the species lost.

"If it is allowed to continue, life would take many millions of years to recover and our species itself would likely disappear early on," lead author Gerardo Ceballos of the Universidad Autonoma de Mexico said.

The analysis was based on documented extinctions of vertebrates, or animals with internal skeletons such as frogs, reptiles and tigers, from fossil records and other historical data.

The modern rate of species loss was compared to the "natural rates of species disappearance before human activity dominated".

It can be difficult to estimate this rate, also known as the background rate, since humans do not know exactly what happened throughout the course of Earth's 4.5 billion year history.

For the study, researchers used a past extinction rate that was twice as high as widely-used estimates.

If the past rate was two mammal extinctions per 10,000 species per 100 years then the "average rate of vertebrate species loss over the last century is up to 114 times higher than it would be without human activity, even when relying on the most conservative estimates of species extinction," the study said.

"We emphasise that our calculations very likely underestimate the severity of the extinction crisis because our aim was to place a realistic lower bound on humanity's impact on biodiversity."

The causes of species loss range from climate change to pollution to deforestation and more.

According to the International Union for Conservation of Nature, about 41 per cent of all amphibian species and 26 per cent of all mammals are threatened with extinction.

"There are examples of species all over the world that are essentially the walking dead," Professor Ehrlich said.

The study called for "rapid, greatly intensified efforts to conserve already-threatened species, and to alleviate pressures on their populations - notably habitat loss, over-exploitation for economic gain and climate change".

AFP