Number of species

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."

Source : Global species richness estimates have not converged for Trends in Ecology & Evolution.

The paper cited above 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."

See Scaling laws predict global microbial diversity PNAS 113 (21) 5970-5975;

Current estimates are that, at best, only a tiny fraction of species on Earth have so far been properly catalogued and described.

A set of 2017 experiments at Stanford University, US, found that "Nearly all microbes inside us unknown to science"

Also see Definition of speciesplugin-autotooltip__plain plugin-autotooltip_bigDefinition of species

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"The species problem is the set of questions that arises when biologists attempt to define what a species is. Such a definition is called a species concept; there are at least 26 recognized species concepts. A species concept that works well for sexually reproducing organisms such as birds may be useless for species that reproduce asexually, such as bacteria."

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