The last report, published in 2014, estimated that the world's wildlife populations had halved over the last 40 years.
This assessment suggests that the trend has continued: since 1970, populations have declined by an average of 58%.
Dr Barrett said some groups of animals had fared worse than others.
"We do see particularly strong declines in the freshwater environment - for freshwater species alone, the decline stands at 81% since 1970. This is related to the way water is used and taken out of fresh water systems, and also the fragmentation of freshwater systems through dam building, for example."