How many living things have we never met?

The Catalogue of Life lists about 2.44 million species of eukaryotes in its August 2026 release. The World Register of Marine Species passed 250,000 accepted living marine species in May 2026, and is edited continuously as taxonomy is revised.

New species are described by the thousand every year, and the rate is not slowing. A 2025 analysis of the Catalogue of Life found the fastest years all since 2015, with a peak of 17,044 new species in 2020. Costello, May and Stork report more taxonomists describing species than ever before, their number growing faster than the rate of description itself. Others doubt that counting authors on taxonomic papers measures effort, and most of the taxonomists who answered a poll for the 2011 analysis thought the number of experts in their own field was falling.

A widely cited 2011 analysis estimated roughly 8.7 million eukaryotic species, of which about 2.2 million are marine, implying that around 86% of species on Earth and 91% of those in the ocean had not yet been described.

It became the default figure, and it is challenged from both sides. Costello, May and Stork put the total lower, at 5 million give or take 3, with 1.5 million named. Estimates built on DNA run far higher, to around 20 million insects alone once species that look identical are counted separately. At the centre of every one of these ranges, most species are still unnamed.

The methods disagree by an order of magnitude, and Mora and colleagues set them out side by side. Host-specificity ratios gave Erwin 30 million arthropods in the tropics. Known-to-unknown ratios gave Hodkinson and Casson 1.84 to 2.57 million insects worldwide. Asking taxonomists gave a range of 3 to 100 million species. Extrapolating from well-studied groups assumes that poorly studied groups behave similarly, and there is no guarantee they do.

Fungi are a stark case. The Catalogue of Life’s August 2026 release lists about 174,000 described species. A 2017 review of the evidence put the real total at 2.2 to 3.8 million, against 120,000 species accepted at the time: about 3 to 5 per cent named. An analysis of fungal DNA from more than 200 studies has since put it higher, at about 6.3 million, which its authors call conservative.

One analysis of microbial scaling laws put global diversity at as many as a trillion species. A census of 1.7 billion ribosomal RNA sequences from 492 studies arrived at 2.2 to 4.3 million prokaryotic operational taxonomic units instead, and says in its abstract that it refutes the trillion. A formal comment by John Wiens argued that bacteria living only in particular insects and other animals put the total in the hundreds of millions or billions instead. The census authors replied that thousands of animal samples were in their data.

More than five orders of magnitude separate the census from the trillion, although both count the same unit: sequences grouped at 97 per cent similarity. That gap is method, a scaling law carried from single communities to the whole planet against a statistical census of what sequencing has found. Beneath both lies a harder question, where to draw a species boundary in organisms that pass genes between lineages, and the census authors allow that finer genetic resolution may reveal far more kinds than their unit does. That boundary is a convention, not a discovery.

Whether extinction is outrunning description is disputed. Mora and colleagues expect the slow advance of description to leave species extinct before anyone knew they existed. Costello, May and Stork answer that the despair rests on overestimates of how many species there are, on a belief that the expertise to describe them is shrinking, and on alarmist extinction rates. Their own range for extinction is 0.01 to 1 per cent per decade, at most 5, and they call it poorly quantified.

The number of lineages that will disappear without ever being recorded cannot be measured, because measuring it would require the record that does not exist.

Existing catalogues are biased towards species that are large, abundant and widely spread, so what is missing should be the opposite: small ranges and small bodies, in hotspots and in less explored places such as the deep sea and the soil. For fungi, a 2017 review adds little-explored habitats such as lichens, and specimens already in collections that nobody has yet studied.

If extrapolation from taxonomic hierarchy is sound, the total for animals, plants, fungi and protists settles in single-digit millions, and the cataloguing project is finishable. At the description rates and costs Mora and colleagues assumed, finishing it would take about 1,200 years, 303,000 taxonomists and US$364 billion.

If the scaling laws behind the trillion hold, the total is so large that a complete catalogue is not a meaningful goal. On this view the right question is not how many species exist but what kinds of life exist, and how they are distributed.