How many species share the planet?

Since Linnaeus formalized the naming of species in 1758, taxonomists have described roughly 2.2 million of them, at a current rate of about 18,000 per year. How far along are we? The honest answer: unknown, and not by a little. Serious modern estimates for animals, plants, and fungi cluster around several million; for microbes, published estimates disagree by factors of thousands. Biology’s most basic census question, how many kinds of living things exist, has no reliable answer.

Why it matters

The species count is the denominator of conservation. Extinction rates, biodiversity targets, and claims about how much of life we are losing all lean on a total we do not have; species are certainly vanishing before ever being described, meaning the library is burning partly uncatalogued. The count also measures something profound: how thoroughly life has explored its possibilities, and how much of Earth’s biology remains unexamined for medicine, materials, and basic understanding. Most antibiotics came from soil organisms; an unknown majority of soil organisms have never been grown or named.

What has been tried

For visible life, the methods are clever extrapolations. The most cited, published in 2011, noticed that the numbers of phyla, classes, orders, families, and genera follow a consistent scaling pattern, and projected it downward to predict about 8.7 million eukaryotic species, implying more than eighty percent undescribed. Other approaches count beetles on single tree species and scale up, or use expert surveys per group, and land between two and twelve million. Fungi illustrate the gap: roughly 160,000 described, with environmental DNA suggesting millions. For bacteria and archaea the wheels come off entirely. Sequencing rather than culturing revealed staggering diversity, and one scaling analysis proposed Earth may host a trillion microbial species, while critics counter with estimates thousands of times smaller, partly because microbes lack an agreed species definition at all.

Where the edge is

The multicellular estimate might be settled to within a factor of two by better sampling of insects, fungi, and the deep sea. The microbial number is open at a more fundamental level, awaiting both technology and an agreed definition of what is being counted. Meanwhile the describers are in a race: several estimates suggest cataloguing all eukaryotes at current rates would take centuries, longer than many of the species have.

What would count as an answer

For plants, animals, and fungi: converging estimates from independent methods, tight enough to state a total with defensible error bars. For microbes: first a definition, then a census. The question was posed by a man who thought the total was in the tens of thousands.