Soil is the single most biodiverse habitat on Earth, likely home to roughly 59% of all species on the planet. That answer surprises most people, who tend to picture tropical rainforests or coral reefs when they think of life packed into one place. Those ecosystems are spectacularly rich, too, but the question of “what has the most biodiversity” turns out to depend heavily on how you frame it: which organisms you count, what scale you examine, and whether you’re asking about a habitat, a biome, a geographic region, or an entire branch of the tree of life.
Soil as Earth’s Richest Habitat
A 2023 review of the biodiversity literature estimated that soil harbors about 59% of all species on Earth, roughly double what previous estimates suggested.1PubMed Central. Enumerating soil biodiversity That figure includes representatives from the simplest organisms (bacteria, archaea) to the most complex (mammals that burrow, nest, or depend on soil-dwelling prey). Some groups are almost entirely subterranean: about 99% of enchytraeid worm species live in soil, along with roughly 90% of all fungal species and more than 85% of plant species when you count their root systems and the soil organisms they depend on.1PubMed Central. Enumerating soil biodiversity
What makes soil so extraordinarily rich is its three-dimensional structure. A single gram can contain hundreds of thousands of microbial taxa.2PubMed Central. High Microbial Diversity Promotes Soil Ecosystem Functioning Pore spaces of different sizes, moisture gradients, pH patches, and organic matter at various stages of decomposition create a patchwork of microhabitats at scales invisible to the naked eye. A pinch of forest soil holds more microbial species than you’d find among all the birds in a national park. That microbial diversity also has functional consequences: studies show that higher microbial diversity in soil supports more robust ecosystem processes like nutrient cycling and decomposition.2PubMed Central. High Microbial Diversity Promotes Soil Ecosystem Functioning
Tropical Forests and the Vertebrate Crown
If your mental image of biodiversity involves colorful frogs, toucans, and jaguars rather than soil nematodes, tropical forests still dominate. Using range maps and habitat data, researchers have shown that tropical forests hold about 62% of all terrestrial vertebrate species, more than double the number found in any other land biome.3PubMed Central. What on Earth Has the Most Biodiversity? Within that category, humid tropical forests (rainforests) are the real powerhouses, sheltering more than 90% of tropical forest vertebrates. And among the world’s tropical regions, the Neotropics, stretching from southern Mexico through Central America and deep into South America, harbor close to half of all tropical forest vertebrate species.3PubMed Central. What on Earth Has the Most Biodiversity?
These numbers are for vertebrates alone. Tropical rainforests also support staggering numbers of insects, plants, fungi, and microbes. A single hectare of Amazonian forest can contain more tree species than the entire British Isles. The pattern extends vertically: the canopy, understory, and leaf litter each host distinct communities of organisms that barely overlap. That vertical layering, combined with year-round warmth and moisture, is a big part of why these forests are so packed with life.
Yet the most species-rich pockets of tropical rainforest are vanishing. A 2024 study found that high-integrity tropical rainforests, those with minimal human disturbance, are increasingly rare for threatened and declining vertebrates, making their preservation a critical conservation priority.4PubMed Central. Global rarity of high-integrity tropical rainforests for threatened and declining terrestrial vertebrates
Coral Reefs Pack the Highest Density in the Sea
Coral reefs cover less than 1% of the ocean floor but are the most diverse ecosystems in the sea, with the highest density of biodiversity of any habitat on the planet.5Ecosystem Health. Coral Reef Ecosystems and Human Health: Biodiversity Counts! That last point is worth pausing on: reefs may not contain the most total species in absolute number (that title goes to soil), but the number of species per unit area is unmatched anywhere on Earth. A single reef system can support thousands of fish species, hundreds of coral species, and countless invertebrates, algae, and microorganisms, all crammed into a structure you could walk across in minutes.
Reefs thrive in warm, shallow, sunlit waters, and their three-dimensional calcium carbonate architecture creates innumerable hiding spots, feeding surfaces, and micro-environments. The relationship between the coral animals themselves and the photosynthetic algae living inside their tissues generates the primary productivity that fuels the whole system. When that relationship breaks down from heat stress, the result is coral bleaching, which can collapse the entire web of species that depends on the reef structure.
Insects and the Sheer Weight of Numbers
When it comes to named animal species, insects are overwhelmingly dominant. They make up nearly three-quarters of all described animal species on Earth.6PubMed Central. Ancient origin of high taxonomic richness among insects Current estimates place the total number of insect species at roughly 5.5 million, with beetles alone accounting for about 1.5 million species and the broader group of terrestrial arthropods reaching around 7 million.7PubMed. How Many Species of Insects and Other Terrestrial Arthropods Are There on Earth?
Those numbers dwarf every other animal group. All mammals, birds, reptiles, amphibians, and fish combined represent a small fraction of insect diversity. The reasons include insects’ small body size (which allows more species to partition the same physical space), their short generation times (which accelerate evolutionary divergence), and their co-evolution with flowering plants over the past hundred million years. Every time a new plant lineage evolves, it opens ecological niches for insects that feed on it, pollinate it, or parasitize it. That feedback loop has been running for a long time.
The Fungal and Microbial Kingdoms We Barely Know
Fungi represent one of the least explored frontiers of biodiversity. For decades, the standard estimate was about 1.5 million fungal species worldwide. More recent work suggests the true number is somewhere between 2.2 and 3.8 million.8PubMed Central. Fungal Diversity Revisited: 2.2 to 3.8 Million Species Only a fraction of those have been formally described. Most fungi are microscopic and live in soil, inside plant tissues, or within decaying organic matter, which makes them easy to overlook. Some are symbionts that help plants absorb nutrients, others decompose dead material, and a substantial number are parasites. The sheer variety of ecological roles fungi fill, combined with how many remain unnamed, makes them one of the biggest gaps in our understanding of life on Earth.
Bacteria and archaea add another order of magnitude. Microbial diversity is so vast that major collaborative efforts like the Earth Microbiome Project have been devoted just to cataloging which microbial communities live where.9Nature. A communal catalogue reveals Earth’s multiscale microbial diversity Standardized sequencing across hundreds of samples has revealed that microbial communities vary dramatically by habitat: the microbes in a hot spring share little overlap with those in ocean sediment or in a patch of temperate grassland soil. Each environment hosts its own distinct community, and many of the genetic sequences recovered have no close match to anything in existing databases. In other words, researchers keep finding organisms that don’t resemble anything we’ve cataloged before.
Viruses add yet another layer. In the ocean alone, viral communities are tremendously diverse and play active roles in shaping microbial populations. A global survey of ocean viruses found distinct biogeographic patterns, with viral communities passively transported by ocean currents and locally shaped by the environmental conditions that structure their host populations.10PubMed. Patterns and ecological drivers of ocean viral communities Whether viruses count as “biodiversity” depends on whether you consider them alive, a question biologists love to argue about. But their genetic diversity is immense, and their ecological influence on the organisms we do consider alive is hard to overstate.
The Deep Sea and Other Extreme Habitats
The deep ocean floor is the largest biome on Earth by area, and it is among the least explored. A major survey across the Clarion-Clipperton Zone in the northeast Pacific identified over 50,000 megafaunal specimens from more than 400 distinct forms across 13 animal phyla, spanning 5,000 kilometers of abyssal seabed.11Nature Ecology & Evolution. Carbonate compensation depth drives abyssal biogeography in the northeast Pacific That’s just the creatures large enough to see. The microbial picture is even richer. Sampling from the abyssal plains of the southeastern Atlantic at depths around 5,000 meters revealed great diversity among microbial eukaryotes, with a high percentage of sequences having no close matches in genetic databases.12PubMed Central. Large-scale patterns in biodiversity of microbial eukaryotes from the abyssal sea floor The deep sea keeps producing organisms entirely new to science precisely because so little of it has been sampled.
Hydrothermal vents are a particularly striking example. Sediments from the Auka vent field in the Gulf of California yielded 325 reconstructed microbial genomes representing 54 different phyla, more than 30% of all currently recognized microbial phyla, from a single site.13The ISME Journal. Microbial communities of Auka hydrothermal sediments shed light on vent biogeography and the evolutionary history of thermophily These superheated, chemically extreme environments support communities powered by chemical energy rather than sunlight, running on an entirely different energy economy from the surface world. The organisms living there include lineages that are deeply ancient and distantly related to anything in more familiar ecosystems.
Nematodes and Parasites as Hidden Players
Nematodes, or roundworms, are among the most species-rich groups of multicellular animals on Earth, and a large proportion of them live as parasites.14PubMed Central. Large-scale genetic investigation of nematode diversity and their phylogenetic patterns in New Zealand’s marine animals They inhabit soil, freshwater, marine sediments, and the bodies of virtually every other animal group. A single mammal species can host dozens of nematode species, each adapted to a different organ or tissue. Parasitism in general is an enormously successful life strategy: by some estimates, parasitic species outnumber free-living ones. Every large organism walking, swimming, or flying around is itself a habitat for smaller organisms, and those host-associated communities add a hidden dimension to global biodiversity that’s easy to miss when you’re counting species by ecosystem.
Fish illustrate this vividly. A study of microbial communities across 101 marine fish species found that the strongest predictor of microbial community composition was the body site sampled, meaning different parts of the same fish host fundamentally different microbial communities, shaped by anatomy and physiology.15Nature Communications. Host biology, ecology and the environment influence microbial biomass and diversity in 101 marine fish species Gills, guts, and skin each select for distinct microbial neighbors. Every individual animal is a small ecosystem in its own right.
Why the Tropics Win, Over and Over
A consistent pattern across nearly every group of organisms is the latitudinal diversity gradient: species numbers increase as you move from the poles toward the equator.16PubMed Central. Explanations for latitudinal diversity gradients must invoke rate variation This is one of ecology’s oldest recognized patterns and one of the hardest to fully explain. Hypotheses range from higher energy input in the tropics (more sunlight, more warmth, more productivity) to faster mutation rates, to longer evolutionary stability (tropical regions have been warm and wet for longer than temperate and polar regions have been cold), to simply more area at tropical latitudes. The honest answer is that no single explanation accounts for the pattern. Recent work argues that any adequate explanation must involve differences in the rates at which species form and go extinct, not just static environmental conditions.16PubMed Central. Explanations for latitudinal diversity gradients must invoke rate variation
Mountain ranges within the tropics add another wrinkle. The Andes, for example, act as both evolutionary “cradles” and “museums.” A study of flowering plants in the northern Neotropics found that young endemic species cluster in the high-elevation Andean regions (the cradles), while older endemic lineages persist in the lowlands of the Caribbean, Orinoco, and Amazon basins (the museums).17PubMed Central. Unveiling evolutionary cradles and museums of flowering plants in a neotropical biodiversity hotspot Temperature differences across elevation gradients appear to drive this separation. Interestingly, a study of Andean hummingbirds found that old and young endemic species don’t always cluster in the same valleys the way the classic hypothesis predicts, suggesting the “cradle-museum” framework is more nuanced than a simple overlay of stability and novelty.18PubMed Central. Biodiversity cradles and museums segregating within hotspots of endemism The upshot is that tropical mountains are species factories, but the specific mechanisms are still being worked out.
How Much of Life Remains Undiscovered
Despite 250 years of taxonomic work and more than 1.2 million species formally cataloged, one influential estimate suggests that about 86% of existing species on land and 91% of species in the ocean have not yet been described.19PLOS Biology. How Many Species Are There on Earth and in the Ocean? Those percentages are striking: for every species we’ve named, there may be six or seven we haven’t. The gap is widest for small organisms, particularly insects, nematodes, fungi, and microbes, groups where new species turn up with every targeted survey and where a single understudied habitat can yield hundreds of undescribed forms.
This means that debates about “what has the most biodiversity” are necessarily debates about what we’ve measured well versus what we’ve barely looked at. Tropical forests and coral reefs are famous for biodiversity partly because they’ve been studied intensively for centuries. Soil biodiversity and deep-sea diversity are getting attention more recently, and every new study shifts the picture. The honest summary is that the most biodiverse place on Earth might be somewhere we haven’t adequately sampled yet.
Why Counting Species Isn’t the Whole Story
Biodiversity isn’t only about how many species exist in a place. Conservationists also care about functional diversity (how many different ecological roles are filled) and phylogenetic diversity (how much evolutionary history is represented). A community of 50 closely related beetle species and a community containing a beetle, a fern, a fungus, a bacterium, and a fish might have the same species count, but the second group captures far more of life’s evolutionary and functional range.
These dimensions don’t always line up the way you’d hope. A study testing whether protecting the most phylogenetically diverse set of species would also capture the most functional diversity found that, on average, maximizing phylogenetic diversity captured about 18% more functional diversity than a random selection would. But in more than a third of comparisons, the phylogenetically diverse sets actually contained less functional diversity than random picks.20PubMed Central. Prioritizing phylogenetic diversity captures functional diversity unreliably That’s a sobering finding for conservation planning. It means there’s no simple shortcut: protecting evolutionary uniqueness doesn’t automatically protect the range of ecological functions an ecosystem performs.
What Threatens the Most Biodiverse Places
Land-use change, principally the conversion of forests and other wild habitats to agriculture and urban areas, is currently the primary driver of species extinctions worldwide.21PubMed. Future habitat loss and extinctions driven by land-use change in biodiversity hotspots under four scenarios of climate-change mitigation Biodiversity hotspots, the regions that concentrate the most species in the smallest areas, are disproportionately affected because they tend to be in the tropics and subtropics, where human population growth and agricultural expansion are fastest.
Climate change is compounding the problem. Modeling of projected habitat shifts in tropical hotspots has found that warming-induced extinction rates could in some scenarios exceed those caused by deforestation alone.22PubMed. Global warming and extinctions of endemic species from biodiversity hotspots Species endemic to small geographic ranges, like mountaintop amphibians or island-dwelling plants, are especially vulnerable because they have nowhere to move when conditions shift. The cruel irony is that the places with the most to lose are often the ones with the least capacity for large-scale conservation investment.
Past Worlds That Were Just as Rich
It’s tempting to assume that today’s ecosystems are the pinnacle of biological complexity and that the further back you go in geological time, the simpler life must have been. Recent paleontological work challenges that assumption. Fossil evidence increasingly suggests that some deep-time ecosystems, including those of the Cretaceous period, may have been as complex and diverse as anything alive today.23CORDIS | European Commission. 375 Million Years of the Diversification of Life on Land: Shifting the Paradigm? Mass extinctions disrupted global ecosystems catastrophically, but they also opened ecological space for new groups to diversify, eventually rebuilding richness through different lineages. The dinosaur extinction, for instance, cleared the way for mammals and birds to radiate into the roles they fill today. Earth’s biodiversity is not a steady upward climb toward a modern peak; it’s a series of expansions, collapses, and rebuilds that have produced extraordinary richness at multiple points in the planet’s history.