Roughly 1.2 to 1.5 million animal species have been formally described and given scientific names, but that catalog is far from complete. The most widely cited global estimate, published in 2011, predicted about 8.7 million eukaryotic species on Earth, with animals making up the overwhelming majority of that total. If that projection holds, we have named fewer than one in five animal species that actually exist. The gap between what we have cataloged and what is out there is driven by everything from the sheer vastness of tropical forests and deep oceans to the limits of how many trained taxonomists are working at any given time.
The Best Global Estimate and What It Covers
The 8.7 million figure comes from a study that used the hierarchical structure of taxonomy itself as a prediction tool. By analyzing the consistent pattern in how species are grouped into genera, families, orders, and so on, researchers extrapolated from well-known groups to poorly known ones. Their estimate was roughly 8.7 million eukaryotic species, give or take about 1.3 million, with around 2.2 million of those in the oceans.1PubMed Central. How many species are there on Earth and in the ocean? “Eukaryotic” includes plants, fungi, and protists alongside animals, but animals dominate the count. Insects alone are thought to number around 5.5 million species, and terrestrial arthropods as a whole roughly 7 million.2PubMed Central. New approaches narrow global species estimates for beetles, insects, and terrestrial arthropods Once you add vertebrates, marine invertebrates, worms, and the rest, the animal kingdom is thought to account for well over 7 million of that 8.7 million total.
It is worth noting that these estimates carry real uncertainty. The insect number, for example, sits in a range of about 2.6 to 7.8 million depending on the assumptions used.2PubMed Central. New approaches narrow global species estimates for beetles, insects, and terrestrial arthropods These are not just minor adjustments; at the low end, total animal diversity would be considerably smaller than at the high end. Still, most recent analyses converge on a few key points: the number of described species is a small fraction of the true total, insects and their relatives make up the bulk of animal life, and the ocean remains profoundly undersampled.
Why Insects and Arthropods Dominate
When people picture animal diversity, they tend to think of mammals, birds, reptiles, and fish. In reality, those groups are a sliver. Beetles alone may represent about 1.5 million species globally.3PubMed. How Many Species of Insects and Other Terrestrial Arthropods Are There on Earth? The famous quip attributed to J.B.S. Haldane about God having “an inordinate fondness for beetles” was not far off, at least numerically. Insects of all kinds dwarf every other animal group by species count, and the broader category of terrestrial arthropods, which adds spiders, mites, centipedes, and crustaceans living on land, pushes the number higher still.
The reason arthropods are so species-rich has to do with their biology. They are small, they reproduce quickly, and they can specialize on narrow food sources and microhabitats. A single tropical tree canopy can harbor dozens of beetle species found nowhere else. Early estimates of global insect diversity were built on exactly this kind of observation: an entomologist named Terry Erwin fogged tropical tree canopies with insecticide and counted the species that fell out, then used ratios of host-specialist beetles to all beetles to extrapolate upward.4Biological Journal of the Linnean Society. How many species of arthropods? Erwin’s estimate revised Erwin’s original figure was 30 million arthropod species, which later researchers revised downward using more refined data. Current estimates have converged closer to 7 million terrestrial arthropods, which is still an almost incomprehensible number.
The Ocean Paradox
The oceans cover about 70 percent of Earth’s surface, maintain relatively stable temperatures, and harbor representatives of more major animal body plans than land does. And yet only about 16 percent of all named species on Earth are marine.5PubMed. Marine Biodiversity, Biogeography, Deep-Sea Gradients, and Conservation That mismatch tells us more about where scientists have looked than about where species actually live. Terrestrial habitats, particularly forests, are easier to access and have been surveyed for centuries longer. Marine environments, especially the deep sea, remain spectacularly underexplored.
A study of deep-sea Mediterranean biodiversity estimated that about 66 percent of the species living in those habitats had not yet been described.6PLOS ONE. Deep-Sea Biodiversity in the Mediterranean Sea: The Known, the Unknown, and the Unknowable If that proportion holds for other deep-sea basins, the oceans may harbor far more animal species than the 16 percent figure implies. The deep-sea floor alone is thought to rival tropical forests in invertebrate diversity, though the species tend to be tiny and sparsely distributed, making them difficult to collect and describe.
Cryptic Species and the DNA Revolution
Some of the “missing” species are not in remote jungles or deep-sea trenches. They are sitting in museum drawers, lumped in with other specimens because they look nearly identical to the human eye. DNA barcoding, which uses short standardized gene sequences to distinguish species, has revealed that many organisms previously treated as a single species are actually complexes of several distinct ones. These are called cryptic species, and they appear to be common across the animal kingdom.
One striking example involved a well-known tropical butterfly, Astraptes fulgerator, which had been treated as a single species for over a century. DNA barcoding combined with decades of ecological observation showed it was actually at least ten separate species.7PubMed Central. Ten species in one: DNA barcoding reveals cryptic species in the neotropical skipper butterfly Astraptes fulgerator Similar findings have turned up in amphipods living in desert springs, where barcoding exposed extraordinary levels of hidden diversity within what had been considered a single widespread genus.8PubMed. DNA barcoding reveals extraordinary cryptic diversity in an amphipod genus: implications for desert spring conservation Large-scale barcoding of tropical insects reared through careful inventories has found cryptic species so frequently that the phenomenon appears to be the rule rather than the exception, and further studies are expected to substantially increase current estimates of insect richness.9PubMed Central. Nuclear genomes distinguish cryptic species suggested by their DNA barcodes and ecology
This means global species counts are likely to rise not just as scientists explore new places but as they reexamine familiar animals with modern tools. Every group that has been subject to thorough DNA analysis has yielded hidden species, and most groups have not yet been analyzed at that scale.
Environmental DNA and the New Toolkit
Traditional animal surveys require physically catching or observing an animal. Environmental DNA, or eDNA, takes a fundamentally different approach. Animals shed DNA into their surroundings constantly through skin cells, mucus, feces, and urine. By filtering a sample of water or soil and sequencing the DNA fragments in it, researchers can detect which species are present without ever seeing one.
Studies have shown that eDNA from small water samples of lakes, ponds, and streams can detect a wide range of rare and threatened freshwater animals, from amphibians and fish to mammals and crustaceans.10PubMed. Monitoring endangered freshwater biodiversity using environmental DNA When combined with high-throughput sequencing, entire faunas of amphibians and fish can be identified from pond water alone. Compared to traditional survey methods, eDNA metabarcoding has shown better detection probability overall for key aquatic vertebrate groups.11PubMed. Next-generation monitoring of aquatic biodiversity using environmental DNA metabarcoding
The implications for species counting are significant. eDNA is already turning up species in locations where they were not known to occur, and it can survey habitats that are difficult to sample physically, like deep lakes or cave systems. As costs continue to drop and reference databases grow, eDNA is likely to accelerate the pace of species detection, though formally describing and naming those species still requires traditional taxonomic work.
Soil and Mites, the Overlooked Frontier
Most people think of the tropics or coral reefs as the great frontiers of animal discovery, but the soil beneath your feet is arguably even less explored. Soil arthropods contribute a large proportion of total biodiversity on Earth, yet most remain undescribed.12PubMed Central. Towards a Canary Islands barcode database for soil biodiversity: revealing cryptic and unrecorded mite species diversity within insular soils Mites are the most abundant and species-rich arthropods in soil, and the uncertainty around their numbers is staggering. Published estimates of mite species richness span more than two orders of magnitude, from roughly 400,000 to 114 million.13PubMed. DNA barcodes expose unexpected diversity in Canadian mites
That two-orders-of-magnitude range is not a sign that scientists are bad at counting. It reflects how little fieldwork has been done on mites and how different assumptions about host specificity, habitat range, and cryptic diversity lead to wildly different extrapolations. A single handful of forest soil can contain hundreds of mite species, many new to science. Until DNA barcoding efforts catch up with this diversity, global animal totals will carry enormous uncertainty from this group alone.
The Parasite Question
Parasites are animals, and there may be an extraordinary number of them we have not yet described. At present, parasites of animals make up only about 5 percent of named species.14Integrative and Comparative Biology. Parasite Rates of Discovery, Global Species Richness and Host Specificity But many researchers suspect the true proportion is far higher. If every animal species hosts at least one parasite unique to it, as some local-scale studies have suggested, then parasites could account for roughly half of all animal species. The catch is that host specificity varies enormously. Well-studied hosts tend to be vertebrates, which are larger and more widespread, and they harbor more parasites per species than the small invertebrates that make up most of animal life. Extrapolating from vertebrate-parasite ratios to the entire animal kingdom probably overestimates parasite diversity, but even a conservative adjustment would add millions of species to global counts.14Integrative and Comparative Biology. Parasite Rates of Discovery, Global Species Richness and Host Specificity
Parasites also exemplify the discovery problem in miniature. Many are tiny, live inside their hosts, and go through complex life cycles involving multiple host species. Describing a new parasitic worm often requires killing and dissecting a host animal, which makes surveys slow and ethically constrained, particularly when the hosts are endangered.
The Discovery Rate Is Actually Speeding Up
Despite the size of the gap between described and estimated species, the pace of new descriptions has not plateaued. In fact, the largest numbers of new species described per year have all come within the past roughly two decades, between 2000 and 2020.15PubMed Central. The past and future of known biodiversity: Rates, patterns, and projections of new species over time The groups growing fastest in descriptions are the ones that are already the largest: animals, arthropods, insects, and beetles. This acceleration is driven partly by molecular tools like DNA barcoding, partly by greater international collaboration, and partly by targeted surveys in previously inaccessible regions.
Still, at current rates, it would take centuries to describe everything. And the race against extinction makes the timeline feel even more urgent.
Going Extinct Before Being Found
The fact that species are disappearing while the catalog is still deeply incomplete creates a grim arithmetic. Intensive fieldwork keeps finding species so rare and threatened that some become extinct within years of discovery. In Brazil’s Atlantic coastal forests, recent bird extinctions suggest that some species vanished before anyone described them scientifically.16PubMed. Species, extinct before we know them? Quantitative modeling has tried to estimate how large this invisible extinction toll is. Depending on the group and region, the proportion of undiscovered extinct species over all extinctions ranged from about 15 to 59 percent, meaning recent extinctions may be up to twice as large as the recorded number.17PubMed. Estimating how many undescribed species have gone extinct
This is not just a bookkeeping issue. Conservation planning depends on knowing what exists and where it lives. A species that disappears without ever being documented cannot be assessed as endangered, cannot have its habitat legally protected, and leaves no record for future researchers to learn from.
Why Scientists Disagree About the Number
Part of the difficulty in answering “how many types of animals are there” is that researchers do not fully agree on what counts as a type. There are roughly 26 published definitions of what a species is, and they do not always yield the same groupings.18Biological Conservation. Implications of different species concepts for conserving biodiversity Under a strict genetic species concept, populations with modest DNA divergence can be split into separate species. Under a broader biological species concept that emphasizes interbreeding, some of those same populations would be lumped together. The choice of species concept has practical consequences: splitting populations into more species can unlock more conservation funding and legal protection, while lumping can downplay the uniqueness of local populations.
This is not an abstract philosophical disagreement. When taxonomists revise a group and split one species into several, the global species count goes up overnight without a single new animal being discovered. When they merge two species that turn out to interbreed freely, the count drops. These revisions happen constantly, and they can shift the numbers for well-known groups by 10 to 20 percent.
Where Scientists Are and Are Not Looking
Research attention is not distributed evenly across the animal kingdom. Studies of animal biodiversity are heavily weighted toward vertebrates and temperate regions, even though invertebrates make up the vast majority of species and tropical regions harbor the most diversity.19PubMed Central. Scientific research on animal biodiversity is systematically biased towards vertebrates and temperate regions Birds and mammals receive far more attention per species than insects, spiders, or snails.20Scientific Reports. Taxonomic bias in biodiversity data and societal preferences This imbalance is sometimes called taxonomic bias, and it is driven by a mix of charisma, funding priorities, and simple ease of study.21BioScience. Measuring What We Don’t Know: Biodiversity Catalogs Reveal Bias in Taxonomic Effort
The practical upshot is that our knowledge of total animal diversity is lopsided. We have a reasonably good handle on how many bird and mammal species exist, perhaps within 5 to 10 percent of the true number. For insects, mites, nematodes, and many marine invertebrates, we may not even be within an order of magnitude. This skew also affects conservation. Species that are never described cannot be protected, and the groups most likely to go unnoticed are exactly the ones with the highest undescribed diversity.
New amphibian species illustrate how geography plays into the discovery gap. Of the hundreds of amphibian species described between 2005 and 2009, about 79 percent turned up in areas already flagged as data-deficient, places where surveys had been sparse.22Basic and Applied Ecology. Overcoming the Linnean shortfall: Data deficiency and biological survey priorities Even for a relatively well-studied group like amphibians, simply going to the right places with the right tools is enough to find species that no one has ever recorded.
Meiofauna and the Microscopic Frontier
Below the scale of insects and mites are the meiofauna: animals tiny enough to live between grains of sand or in the water film coating soil particles. This group includes nematodes, tardigrades, rotifers, gastrotrichs, and miniature crustaceans. They are genuine animals with nervous systems, muscles, and digestive tracts, but they are small enough that a drop of seawater may contain dozens of species. Meiofauna are ubiquitous in virtually every habitat on Earth, from polar ice to hydrothermal vents, yet they receive a fraction of the study effort devoted to larger organisms.
Nematodes alone may be among the most species-rich animal groups on the planet. Some older estimates placed nematode diversity at tens of millions of species, though more recent work tends toward lower but still enormous figures. The honest answer is that nobody knows within a factor of ten. Until these microscopic groups are surveyed with the same intensity as vertebrates and flowering plants, any estimate of total animal diversity is built on a foundation that is still largely guesswork in its lower levels.