About 8.7 million species of living things are estimated to exist on Earth, of which roughly 2.2 million have been formally described and catalogued by science. The widely cited Mora et al. (2011) analysis in PLOS Biology placed the figure at 8.7 million (±1.3 million), split into about 7.8 million animals and 611,000 fungi, with plants and protists adding modest totals. Only about 1.5 million of those had been named at the time of that study. A 2023 reassessment put described eukaryotic species closer to 2.1 to 2.2 million, meaning humanity has documented perhaps one-quarter of the planetary total. The count excludes bacteria and archaea, where the number is effectively unknowable.
Why the Number Keeps Changing: The Mechanism Behind the Estimate
Counting every living thing on Earth is not a census problem. It is a statistical extrapolation problem, because no one can sample the whole planet. Every credible figure rests on the same three-step logic: take a known group, measure how many new species each unit of effort discovers, then project that discovery rate onto the unknown remainder.
The mechanism is called the taxonomic accumulation curve. When researchers survey a forest plot, they tally species that are new to science. If the discovery rate flattens, the group is nearing complete description. If it stays linear, most species remain unnamed. Mora's team applied this to 1.2 million described species and found that discovery rates for most groups were still climbing, not flattening.
The second mechanism is scaling. Larger organisms are better studied and occupy larger ranges, so they are more completely catalogued. Smaller organisms, especially in the deep sea and tropical soils, are almost entirely unstudied. This asymmetry is why the estimate skews so heavily toward animals.
In plain terms: scientists know how complete the easy categories are, then infer how incomplete the hard ones must be.
A key caveat: these figures describe eukaryotes, organisms with cells that contain a nucleus. The 8.7 million estimate is a eukaryote-only number. It does not count bacteria, archaea, or viruses, and those domains dwarf everything else in raw abundance and genetic diversity.
A Quantified Comparison: Described vs. Estimated vs. Undiscovered
The gap between what exists and what is known varies enormously by group. This table reconciles the major published figures, so the numbers do not contradict each other: Mora's 8.7 million is the total estimate, while the 2.1 to 2.2 million figure is the described count as of the 2020s.
| Group | Described species (approx.) | Estimated total | Share described |
|---|---|---|---|
| Animals | 1.4 million | 7.8 million | ~18% |
| Fungi | 150,000 | 611,000 | ~25% |
| Plants | 390,000 | 390,000–400,000 | ~98% |
| Protists | 80,000 | 200,000–400,000 | ~25% |
| Bacteria and archaea | ~30,000 | 10 million to 1 trillion (speculative) | <1% |
The plants row is the striking one. Flowering plants are nearly fully described, which is why botanists treat the plant count as close to final while zoologists treat the animal count as a lower bound. Fungi are the opposite: only about a quarter of estimated fungal species are named, and fungal diversity is where several recent revisions have pushed estimates upward.
The bacterial row should be read as a hypothesis, not a measurement. A 2016 study by Locey and Lennon in PNAS produced a statistical estimate of roughly 1 trillion microbial species, using scaling laws borrowed from macroecology. That number is not consensus, and many microbiologists consider it an upper bound rather than a working figure.
The Time Course of Discovery: A Timeline of the Species Count
The species count is not a fixed constant. It has grown steadily since Linnaeus, and the rate of growth has itself changed. This timeline traces the major milestones that shaped today's estimate.
| Period | Milestone | Count or significance |
|---|---|---|
| 1758 | Linnaeus publishes Systema Naturae, 10th edition | ~4,000 species formally named |
| 1800 | Early global cataloguing | ~50,000 species |
| 1900 | Taxonomic expansion | ~400,000 species |
| 1960 | Modern taxonomy begins | ~1,000,000 species |
| 2011 | Mora et al. estimate | 1.2 million described, 8.7 million estimated |
| 2019 | Global Assessment (IPBES) | ~1 million species threatened with extinction |
| 2023 | Reassessment of described eukaryotes | ~2.1–2.2 million described |
Three things stand out. First, description accelerated for two centuries, then slowed as the easy species were already named. Second, the estimated total has not risen much since 2011, but descriptions continue to accumulate. Third, the discovery rate for new species has stayed roughly linear at around 15,000 to 18,000 new species per year in recent decades, which is consistent with a large unnamed backlog.
The 2019 IPBES Global Assessment added a different dimension: roughly 1 million of those species are threatened with extinction, many within decades. That figure is a policy estimate, not a census, but it reframes the count from curiosity to conservation urgency.
Where the Hidden Species Actually Live: Deep Sea, Soils, and Insects
The undiscovered majority is not evenly distributed. It clusters in three environments that are expensive to sample and rarely visited.
The deep sea covers about 65 percent of Earth's surface and, below roughly 3,000 feet, is almost entirely unpictured. Estimates suggest that 60 to 90 percent of deep-sea species remain undescribed. Each remotely operated vehicle expedition typically returns specimens that are new to science.
Tropical soils and leaf litter hold enormous fungal and invertebrate diversity. A single hectare of tropical forest can contain more insect species than all of Europe. The organisms are small, hard to extract, and require specialists to identify.
Insects are the largest known group by far: about 1 million described species, with estimates of total insect diversity ranging from 5 million to 10 million or more. Beetles alone account for roughly 400,000 described species. This is the single biggest unknown in the 8.7 million figure.
A non-obvious edge case: the deep sea and deep subsurface communities challenge the very definition of a species. Microbes that swap genes freely, and organisms that live at the limits of what counts as life, make the species concept itself fuzzy at the smallest scales.
Corrected Misconceptions About the Count
Several widely repeated claims about Earth's biodiversity do not survive scrutiny. Correcting them sharpens the number.
First, "we have only explored 5 percent of the ocean" is a rough figure often applied to species, not area. It conflates mapping the seafloor with cataloguing its biology. The species gap is real, but the 5 percent number refers to seafloor mapped by sonar, not life described.
Second, "there are 8.7 million species" is often stated as fact. It is a statistical estimate with a stated margin of error of about ±1.3 million, and it excludes microbes entirely. Treating it as a hard number overstates precision.
Third, the claim that "most species are insects" is true by count, but the claim that "most species are already known" is false. The described-to-estimated ratio of roughly 1 in 4 across eukaryotes is the actionable number.
Fourth, bacteria are not a small footnote. If the Locey and Lennon scaling is even partly right, microbial diversity exceeds all eukaryotic diversity by orders of magnitude. The species count as usually reported is a count of visible and near-visible life, not of all life.
In plain terms: the headline number is real, but it is a statistical projection about non-microbial life, and it carries a margin of error larger than the population of most countries.
What This Means in Practice: Actions and Limits
The most useful response to an incomplete species count is not to wait for a complete one. Conservation and monitoring decisions already rest on partial data, and they are made every year.
Positive actions that follow from the estimate:
- Support taxonomic capacity, not just remote sensing. Describing species requires trained systematists, and that workforce has been shrinking for decades, which slows the accumulation curve precisely where it is steepest.
- Protect undersampled habitats such as deep-sea vents, tropical soils, and old-growth forests before they are catalogued. Undescribed species can be lost before they are named.
- Treat inventory data as infrastructure. DNA barcoding and environmental DNA sampling are cheaper than morphological description and can extend the sampling reach by orders of magnitude.
- Use the described-to-estimated ratio when setting survey budgets. A region with 20 percent of its species described is not ready for a "complete" biodiversity assessment.
The limitations are equally concrete. No sampling method reaches every organism, and the species concept erodes at microbial scales. Estimates carry confidence intervals that are rarely reported alongside the headline figure. And discovery rates are influenced by funding and expertise, not only by what actually exists, so a rising count can reflect more taxonomists rather than more species.
The honest summary is a number with two components: roughly 2.2 million described species and a projected total near 8.7 million for eukaryotes, with microbial diversity unquantified and potentially astronomical. The count of different living things on Earth is best understood as a work in progress with a known order of magnitude and an unknown precise value.