Despite hope that nature was fighting back, it appeared that the global wipeout of species was accelerating, they said.
Speaking ahead of two next week on the state of British and European wildlife, Simon Stuart, from the International Union for the Conservation of Nature, admitted that the rate of extinction had not slowed.
Previously research has shown that world was currently in the midst of a "sixth great extinction" of species, which was being driven by natural habitat destruction, hunting, increasing number of alien predators, disease and climate change.
Some conservationists had hoped that rate of loss had been stemmed by the natural evolution of species.
But on Monday Mr Stuart, chairman of IUCN's species survival commission, admitted that point had now "almost certainly" been crossed.