“We demonstrate that after more than six decades, estimates of global species richness have failed to converge, remain highly uncertain, and in many cases, are logically inconsistent.”
Authors Caley, Fisher and Mengersen writing in a 2014
paper for Trends in Ecology & Evolution.
The paper looked at the estimates for total land and marine animal species, which varied from 0.5 million to 10 million species.
A 2015 paper in the journal Proceedings of the National Academy of Sciences (using mathematical scaling law methods to predict likely microbial species numbers) found that :
“Using this scaling law combined with the lognormal model of biodiversity, we predict that Earth is home to as many as 1 trillion (1012) microbial species.”
The current estimates show that, at best, only a tiny fraction [what fraction?] of species on Earth have so far been properly catalogued and described.
A set of 2017 experiments at Stanford found that "Nearly all microbes inside us unknown to science"