Tropical rainforests have the highest biodiversity of any biome on Earth. These forests cover roughly 6% of the planet’s land surface yet harbor more than half of all terrestrial vertebrate species. No other biome comes close to matching that concentration of life, whether measured by the sheer number of species, the variety of ecological roles they fill, or the proportion found nowhere else on the planet.
What the Numbers Look Like
Tropical forests are home to over 50% of the world’s land-dwelling vertebrates, including mammals, birds, amphibians, and likely reptiles. Up to 29% of all global vertebrate species are endemic to tropical forests, meaning they exist only there and nowhere else. Within this broader category, humid tropical forests (what most people picture when they think “rainforest”) dominate, sheltering more than 90% of all tropical forest vertebrates.
The Amazon basin alone illustrates the scale. It contains around 40,000 plant species and more than a thousand bird species, from hummingbirds and macaws to the bizarre, leaf-eating hoatzin. These numbers for a single rainforest region rival or exceed the total species counts of entire continents at higher latitudes. The Neotropics, the region spanning Central and South America, holds nearly half of all tropical forest vertebrates worldwide.
Why Tropical Rainforests Dominate
The pattern of biodiversity peaking near the equator and declining toward the poles is one of the oldest observations in ecology, known as the latitudinal diversity gradient. Several reinforcing factors explain why tropical rainforests sit at the top.
Year-round energy and warmth. Tropical rainforests receive intense, consistent solar energy throughout the year. Average temperatures stay above 18°C with heavy rainfall exceeding 0.6 meters annually. This combination fuels enormous plant growth, which in turn supports complex food webs with many layers of habitat, from the forest floor to the canopy and above it.
Climate stability over deep time. While temperate and polar regions have been repeatedly disrupted by ice ages, droughts, and dramatic temperature swings, equatorial regions have maintained relatively stable tropical climates since the Cretaceous period, over 60 million years ago. This stability gave species long, uninterrupted stretches of time to diversify without being wiped out by catastrophic climate shifts. Researchers describe the tropics as both a “cradle” that fosters the origin of new species and a “museum” that preserves biodiversity over geological time.
High speciation, low extinction. Warm, stable conditions promote consistently high rates of speciation, the process by which new species emerge. Modeling studies show that tropical regions maintained the highest species richness and the highest net diversification rates from the Cretaceous onward, while mid and high latitudes experienced only patchy bursts of speciation. Species in the tropics also tend to evolve narrower environmental tolerances, which means populations separated by a river, a mountain ridge, or even a few kilometers of dense forest can diverge into distinct species more readily.
How Scientists Compare Biodiversity
Biodiversity is not a single number. Ecologists use several approaches to measure it, and the choice of method matters. The simplest is species richness: a raw count of how many species live in a given area. By this measure, tropical rainforests win decisively at virtually every scale.
More sophisticated indices weigh not just the number of species but how evenly individuals are distributed among them. A forest with 100 species where one species makes up 90% of all individuals is less functionally diverse than one where 100 species are roughly equally abundant. These indices have known limitations. They can be sensitive to sample size, may undercount rare species, and don’t always capture differences between communities that have similar evenness but different total species counts. Still, across methods, the conclusion holds: tropical rainforests consistently rank highest in overall biodiversity.
Other High-Biodiversity Biomes
Tropical rainforests lead the pack, but a few other biomes stand out for specific types of diversity. Coral reefs are sometimes called the “rainforests of the sea” because they support an outsized share of marine species relative to the small area they cover. Tropical dry forests and mangrove forests, while less species-rich than humid rainforests, still contribute meaningfully to global totals.
The Mediterranean biome deserves special mention for plant endemism. The Mediterranean Basin qualifies as a global biodiversity hotspot largely because of its extraordinary number of plant species found nowhere else. Islands and mountain ranges within the region acted as refuges during past ice ages, allowing isolated plant populations to diverge into unique species. Mountain belts on Mediterranean islands harbor the highest rates of endemism, though total species richness drops with elevation.
One surprising twist involves soil microorganisms. Unlike large animals and plants, bacteria and fungi in the soil do not always follow the same tropical-equals-most-diverse pattern. Studies have found relatively low levels of bacterial and fungal richness in tropical forest soils compared to some temperate regions. Dominant tropical plant species may actually suppress microbial diversity by preventing other organisms from establishing. So while tropical rainforests win for visible life, the underground picture is more complicated.
What Qualifies as a Biodiversity Hotspot
The formal concept of a “biodiversity hotspot” was established to focus conservation resources on the areas where protection would save the most species. An area must meet two criteria. First, it must contain at least 1,500 endemic plant species, representing 0.5% of all known plant species worldwide. Second, it must have already lost 70% or more of its original vegetation. Vertebrate endemism is tracked but does not independently qualify a region for hotspot status.
This framework highlights a painful reality: the most biodiverse places are also among the most threatened. Many tropical rainforest regions meet both criteria easily, combining staggering species counts with rapid habitat destruction.
Protecting the Most Biodiverse Places
The Kunming-Montreal Global Biodiversity Framework, adopted by nearly 200 countries, set a target of protecting at least 30% of the world’s land and 30% of marine and coastal areas by 2030. The agreement specifically calls for prioritizing areas of particular importance for biodiversity, including regions high in species richness, those with threatened species, and threatened biomes and habitats. Tropical rainforests check every one of those boxes.
With more than 20% of tropical forest endemic vertebrates already at risk of extinction, the gap between the biodiversity these forests contain and the protection they receive remains one of the central challenges in conservation. The forests that took tens of millions of years to accumulate their extraordinary diversity can lose it in decades.