The answer depends entirely on what you mean by “most animals,” and each interpretation points to a different place. If you are counting raw numbers of individual creatures, the sub-Arctic tops the list thanks to staggering populations of tiny soil-dwelling nematodes. If you are asking where animal biomass is greatest, the ocean wins by a wide margin. And if you care about species diversity, tropical forests and coral reefs consistently outperform everywhere else. The question sounds simple, but unpacking it reveals some genuinely surprising geography.
Abundance, Biomass, and Diversity Point to Different Places
When ecologists talk about “the most animals,” they are usually measuring one of three things. Abundance is a raw headcount: how many individual organisms live in a given area. Biomass is total weight, often expressed in gigatons of carbon. And species richness (or biodiversity) counts how many distinct species share a habitat. These three metrics almost never agree. A square meter of boreal soil can contain thousands of nematode worms but only a handful of species. A patch of tropical reef may hold hundreds of species but far fewer total individuals than that same patch of dirt. A comprehensive estimate of life on Earth found that animal biomass totals roughly 2 gigatons of carbon, and the bulk of it is marine.1PubMed Central. The biomass distribution on Earth So if you are simply asking “where is the most animal stuff,” you are looking at the ocean. But that answer glosses over the fact that different kinds of places dominate under different measurements.
The Sub-Arctic Wins the Headcount
The most abundant animals on Earth are not whales or ants or fish. They are nematodes, tiny roundworms that live in soil by the trillions. A global mapping effort using nearly 7,000 soil samples estimated that roughly 440 billion billion nematodes inhabit the planet’s surface soils, with a combined biomass of about 0.3 gigatons.2PubMed. Soil nematode abundance and functional group composition at a global scale What surprised researchers was where these worms concentrate. About 38% of all soil nematodes live in sub-Arctic regions, compared with 24% in temperate zones and 21% in the tropics.2PubMed. Soil nematode abundance and functional group composition at a global scale The boreal forests and tundra of Canada, Scandinavia, and Siberia are, by sheer headcount, the most animal-packed landscapes on Earth.
This seems counterintuitive. We tend to picture the tropics as teeming with life, and in many ways they are. But cold, wet, organic-rich soils create ideal conditions for nematodes, which fill every level of the soil food web, from bacteria-feeders to predators of other nematodes.3BioScience. A Worm’s World: Ecological Flexibility Pays Off for Free-Living Nematodes in Sediments and Soils A single handful of boreal forest soil can contain more individual animals than the entire visible fauna of a city park. Most people never see nematodes, so the sub-Arctic rarely gets credit for the astonishing density of animal life under its surface.
Tropical Forests Lead in Species Diversity
Switch the metric from headcount to species richness, and the tropics dominate. Lowland tropical rainforests consistently harbor more animal species per unit area than any other terrestrial habitat. Ecuador’s Yasuní National Park, in the western Amazon, holds apparent world records for richness of amphibians, reptiles, bats, and trees, placing it among the most biodiverse places on the planet.4PubMed Central. Global Conservation Significance of Ecuador’s Yasuní National Park A single hectare of Yasuní forest can contain more tree species than the entire continent of North America, and each of those tree species supports its own community of insects, birds, and other animals.
Ants are a good proxy for understanding why the tropics are so rich. A global synthesis found that ground-dwelling ant abundance is strongly concentrated in tropical and subtropical regions, with leaf-litter ants reaching their highest densities in forests.5PubMed Central. The abundance, biomass, and distribution of ants on Earth In the canopy of lowland tropical rainforests, ants are so extraordinarily abundant that researchers have suggested many arboreal species function partly as “herbivores,” feeding on plant and insect exudates rather than hunting prey, because there simply are not enough conventional food sources to support so many predators.6PubMed. Explaining the abundance of ants in lowland tropical rainforest canopies That is one of those findings that quietly reshapes how you think about tropical forests: the canopy is not just populated by ants, it is dominated by them.
How much biodiversity separates old-growth tropical forest from degraded or simplified habitats? Researchers sampling environmental DNA from rainwater in tropical forests found that old-growth plots yielded roughly 1.3 to 1.9 times more taxonomic diversity than nearby plantation plots, with 61 vertebrate taxa versus 32 and 276 insect taxa versus 153.7PubMed Central. A framework for modelling desert locust population dynamics and large-scale dispersal That gap is a reminder that “tropical” alone is not the magic ingredient. The structural complexity of old-growth forest, with its layered canopy, decaying wood, and epiphyte gardens, creates the niches that support all those species.
The Ocean Holds Most of Earth’s Animal Biomass
While tropical forests win on land, the ocean is where the majority of animal biomass resides. The same global census that estimated total animal biomass at about 2 gigatons of carbon concluded that animals are mainly marine.1PubMed Central. The biomass distribution on Earth This makes sense when you consider that the ocean covers about 70% of Earth’s surface and extends to depths of more than 10 kilometers. Its three-dimensional volume dwarfs the habitable space on land.
Copepods, tiny crustaceans that drift in open water, are among the most numerous multicellular animals in the sea. Along a transect in the eastern Atlantic, researchers found total copepod abundances of 148,000 to 197,000 individuals per square meter when counting from the surface down to 2,000 meters, with the highest concentrations packed into the upper 100 meters.8ScienceDirect (Progress in Oceanography). Copepod distribution and biodiversity patterns from the surface to the deep sea along a latitudinal transect in the eastern Atlantic Ocean (24°N to 21°S) That number represents just one transect in one ocean. Extrapolated globally, copepods alone likely number in the quadrillions. They form the base of the marine food web, converting phytoplankton into food for fish, whales, and seabirds.
The Deep Sea and Its Hidden Vertebrates
When people picture ocean life, they tend to think of coral reefs, kelp forests, or the sunlit surface waters. But a vast amount of animal life exists in the deep, dark middle layers of the ocean. Bristlemouth fish of the genus Cyclothone have been referred to as the most abundant vertebrates on Earth. These finger-length fish live primarily between 400 and 1,500 meters deep, never migrating to the surface, and they show up in enormous concentrations across much of the world’s oceans.9ScienceDirect (Progress in Oceanography). Carbon remineralization by small mesopelagic and bathypelagic Stomiiforms in the Northeast Atlantic Ocean Despite their mind-boggling numbers, bristlemouths are almost invisible to everyday experience. They are too small and too deep for commercial fishing, and they disintegrate quickly in nets, so their true population is still poorly constrained.
The deep ocean is a useful corrective to the assumption that the most animal-packed places on Earth should be obvious to the naked eye. Much of the planet’s animal abundance is hidden from us, whether underground in soil or suspended in water columns we never visit.
Coral Reefs and the Marine Biodiversity Peak
For marine species diversity, the answer is the Coral Triangle, a region spanning Indonesia, the Philippines, Papua New Guinea, and surrounding waters. It is home to about 4,350 marine fish species and is widely recognized as the global epicenter of marine biodiversity.10PubMed Central. Assessing species diversity of Coral Triangle artisanal fisheries: A DNA barcode reference library for the shore fishes retailed at Ambon harbor (Indonesia) That fish count alone exceeds the total for entire ocean basins elsewhere. Add in the corals themselves (which are animals), the mollusks, sea stars, sea cucumbers, and thousands of invertebrate species, and the Coral Triangle is in a league of its own.
Why this particular patch of ocean? Several factors converge. The region sits at the junction of the Pacific and Indian Oceans, creating complex current patterns that bring nutrients and larvae from many directions. Sea-level changes during ice ages repeatedly fragmented and reconnected habitats, driving speciation. And the warm, shallow waters support the coral growth that provides structural habitat for everything else. Coral reefs worldwide cover less than 1% of the ocean floor but support roughly a quarter of all marine species, and the Coral Triangle is where reef complexity reaches its peak.
Sandy beaches, by contrast, show a simpler pattern. A global survey of more than 200 sandy beaches found that species richness increases from temperate to tropical sites, though total abundance depends heavily on beach slope.11Geomorphology. Global patterns in sandy beach macrofauna: Species richness, abundance, biomass and body size Gentle-slope beaches at temperate latitudes can actually have more individual animals than tropical ones, a reminder that even within a single habitat type, abundance and diversity can pull in opposite directions.
Polar Seas and Seasonal Animal Explosions
The Southern Ocean surrounding Antarctica deserves its own mention. Antarctic krill, a shrimp-like crustacean up to 65 millimeters long, forms some of the largest animal aggregations on the planet. Their distribution is circumpolar, concentrated between Antarctica’s continental shelf break and the Southern Antarctic Circumpolar Current Front.12ScienceDirect. Biomass of Antarctic krill (Euphausia superba) in the eastern Indian sector of the Southern Ocean (80–150°E) in the 2018–19 austral summer A single survey of the eastern Indian sector estimated krill biomass at over 4 million tonnes in just that one slice of the Southern Ocean.12ScienceDirect. Biomass of Antarctic krill (Euphausia superba) in the eastern Indian sector of the Southern Ocean (80–150°E) in the 2018–19 austral summer Total circumpolar estimates put the figure far higher, likely hundreds of millions of tonnes.
Krill swarms can stretch for kilometers and turn the water pinkish-red. These aggregations are so dense that they are visible from space via satellite. During the austral summer, when phytoplankton blooms fuel explosive krill reproduction, the Southern Ocean briefly becomes one of the most animal-dense marine environments on Earth. The seasonal boom-and-bust nature of polar seas is part of why no single answer to “the most animals” holds year-round.
Caves and Underground Aggregations
Some of the most spectacular concentrations of animals are found underground. Cave-roosting bats routinely gather in colonies numbering in the millions. The Monfort Bat Cave Sanctuary on Mindanao Island in the Philippines is home to enormous aggregations of Geoffroy’s rousette fruit bats, which crowd together in densities that create measurable behavioral costs from overcrowding.13PubMed. The behavioural costs of overcrowding for gregarious cave-dwelling bats Bracken Cave in Texas shelters an estimated 15 to 20 million Mexican free-tailed bats each summer, making their nightly emergence one of the densest vertebrate spectacles anywhere on land.
These underground refuges function as extreme density hotspots. A cave mouth may occupy a few hundred square meters, but the animals packed inside can outnumber every visible animal in the surrounding landscape. Caves also harbor unique invertebrate communities, though these tend to be notable for their oddness and specialization rather than their abundance.
Cities as Animal Hotspots
Urban areas rarely come to mind in a conversation about animal abundance, but they probably should. Commensal species, the animals that have evolved to exploit human resources, reach extraordinary densities in cities. Rats are a prime example. Urban rats thrive by exploiting the food waste and shelter that accompany large human populations, and their numbers are linked to climate warming, urbanization, and human population density.14PubMed Central. Increasing rat numbers in cities are linked to climate warming, urbanization, and human population In some cities, rat populations are thought to approach or even exceed the human population.
Beyond rats, consider cockroaches, pigeons, sparrows, and the billions of arthropods living in building crevices, sewers, and subway tunnels. A dense city like New York or Mumbai supports a surprisingly rich animal community, though it is overwhelmingly made up of a few hyper-successful generalist species rather than the diverse assemblages of a tropical forest. The concentration of food energy in cities, garbage, pet food, restaurant waste, and landscaping, creates habitats that certain animals exploit with spectacular efficiency.
Why the Answer Keeps Shifting
Every few years, new survey technologies reshape our understanding of where animals concentrate. Environmental DNA sampling, where researchers collect traces of genetic material from water, soil, or even rainwater, is revealing animal communities that traditional field surveys miss entirely. Satellite-based biomass estimates have improved our picture of krill swarms and open-ocean life. And soil sampling at global scales has only recently become feasible, which is why nematodes were so long overlooked as Earth’s most abundant animals despite being right under our feet.
The honest picture is that the planet’s animal life is distributed across multiple, non-overlapping hotspots depending on which question you ask. Sub-Arctic soils for raw headcount. The open ocean for total biomass. Tropical forests and the Coral Triangle for species diversity. Polar seas and cave systems for jaw-dropping seasonal or spatial aggregations. No single pin on a map captures “the most animals,” because animal life has found a way to pack itself densely into nearly every environment Earth offers.
Extinction and What Could Be Lost
Deep-time records give some perspective on how fragile these concentrations are. Mass extinctions have repeatedly wiped out large fractions of animal diversity. During the Permian-Triassic event, an estimated 24 to 58% of marine genera disappeared, and the Triassic-Jurassic boundary saw losses of up to 42 to 66% of genera.15Nature Communications. DeepDive: estimating global biodiversity patterns through time using deep learning In each case, recovery took millions of years. The animal hotspots that exist today, whether the Coral Triangle, Yasuní, or the Southern Ocean, are not permanent features of the planet. They are products of current climate, ocean circulation, and evolutionary history, all of which are changing. The places with the most animals are also, in many cases, the places with the most to lose.