Global wildlife populations have fallen by 58% since 1970, a report says.
The Living Planet assessment, by the Zoological Society of London (ZSL) and WWF, suggests that if the trend continues that decline could reach two-thirds among vertebrates by 2020.
The figures suggest that animals living in lakes, rivers and wetlands are suffering the biggest losses.
Human activity, including habitat loss, wildlife trade, pollution and climate change contributed to the declines.
Dr Mike Barrett. head of science and policy at WWF, said: "It's pretty clear under 'business as usual' we will see continued declines in these wildlife populations. But I think now we've reached a point where there isn't really any excuse to let this carry on.
"We know what the causes are and we know the scale of the impact that humans are having on nature and on wildlife populations - it really is now down to us to act."
However the methodology of the report has been criticised.
The Living Planet Report is published every two years and aims to provide an assessment of the state of the world's wildlife.
This analysis looked at data collected on 3,700 different species of birds, fish, mammals, amphibians and reptiles - about 6% of the total number of vertebrate species in the world.
The researchers then analysed how the population sizes had changed over time since 1970.
The last report, published in 2014, estimated that the world's wildlife populations had halved over the last 40 years.
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