Asked by: Brian Lisle, Huddersfield
You can estimate the total number of species in the world by graphing the decreasing number of new species discovered each year to predict the end point. Or you can extrapolate the number of new species found per hectare of rainforest, to the number of hectares that haven’t been studied. Or you can graph the body size of each new species found, on the assumption that larger species tend to be discovered sooner, and extrapolate that.
The different statistical models over the years have been gradually homing in on a figure of 8.7 million total species. Currently, 1.64 million have been named, so that’s 81 per cent left to find (the 86 per cent figure was based on 2011 totals). This only covers eukaryotes (animals, plants and fungi) though. A 2016 study estimated that bacteria could add almost another trillion species.