On May 22—the International Day for Biological Diversity—conservationists, scientists, and governments alike reflect on the staggering variety of life on Earth and the urgent need to protect it. Biodiversity, broadly defined, refers to the variety and variability of life at all levels: ecosystems, species, and genetic diversity within species. It underpins everything from food systems and medicines to climate regulation and clean water.
But despite its importance, quantifying biodiversity is notoriously difficult. There is no single way to measure it, and attempts to rank countries often spark passionate debate. Some favor absolute species counts. Others prioritize how densely biodiversity is packed into an area, or how many species are found nowhere else on Earth. Each method yields a different list—and none tells the whole story.
The importance of endemism
One especially meaningful metric is endemism—the proportion of species in a given area that are found nowhere else. Endemic species often evolve in isolation, shaped by unique geographies and ecological niches. Their presence indicates not just richness but irreplaceability. Losing them means global extinction, not just local disappearance.
A recent analysis ranked countries based on endemism across four groups—amphibians, birds, and mammals—revealing a pattern that comes as little surprise to biogeographers: islands and island continents dominate. Madagascar tops the list, with 81% of its amphibian, bird, and mammal species found nowhere else. Australia (65%) and the Philippines (53%) follow closely, trailed by Indonesia, Papua New Guinea, and Brazil.
Other notable entries include New Zealand, Cuba, the Solomon Islands, and even the United States—thanks in part to Hawaii, which hosts a wealth of endemic birds and insects. Mexico, China, Sri Lanka, and India also make strong showings, reinforcing their reputations as biodiversity hotspots. But it’s not just size that matters; isolation and habitat variety also play major roles.
Total species richness
Another way to assess biodiversity is by tallying the number of known species in a country. Using data on six major groups—amphibians, birds, fish, mammals, reptiles, and vascular plants—a global ranking emerges. Brazil tops the list, home to an average of 13% of Earth’s species across those six taxa. Indonesia (10.4%) and China (9.8%) follow, with Colombia and Peru rounding out the top five.
These rankings highlight the immense biodiversity of the tropics, especially South America and Southeast Asia. Ecuador, Mexico, India, and Australia all score highly as well, confirming their biological wealth.
The United States, often underestimated in these lists, appears in tenth place, driven by its sheer size and range of ecosystems—from Arctic tundra to subtropical swamps.
Richness per unit of area
Yet sheer numbers can be misleading. A large country like Russia has vast natural areas but relatively low species density. To correct for this, it’s possible to rank countries by species richness relative to land or sea area—a proxy for how tightly biodiversity is packed.
This approach yields a different leaderboard. Small nations like Singapore, Brunei, and Eswatini top the chart. So do Rwanda, Burundi, and Bhutan—countries with compact but ecologically complex terrain. In marine biodiversity rankings, island territories in the Caribbean and Pacific fare well, though their classification under other nations’ EEZs complicates comparisons.
When restricted to larger countries (those classified as “megadiverse”), Costa Rica leads in land-based species richness per unit area. Panama, Taiwan, French Guiana, and Ecuador also feature prominently. These nations punch above their weight, often combining tropical latitude, mountainous terrain, and high rainfall—all recipes for biological abundance.
Data caveats and political boundaries
Of course, these rankings have their limitations. Comprehensive biological surveys are expensive and unevenly distributed. Countries like Singapore benefit from meticulous cataloging, while others—particularly in Africa and parts of Asia—may be undercounted due to limited resources or security challenges.
More fundamentally, using political borders to define biodiversity zones is problematic. Nature doesn’t recognize human-drawn lines. The Congo Basin, for example, spans six countries, each of which appears middling in endemism rankings. But if the basin were counted as one biological unit, its endemic species would rival those of Madagascar or Indonesia.
Likewise, many islands rich in biodiversity are politically subsumed into larger nations. French Guiana’s forests, for instance, are a biodiversity goldmine, but its marine life is attributed to France. Such quirks skew calculations of biodiversity per unit of EEZ.
What the rankings tell us (and don’t)
These various approaches offer snapshots—not definitive judgments—of the living wealth held within each country’s borders. They help highlight trends: the centrality of the tropics, the significance of islands, and the overlap between biodiversity and cultural diversity. But they also underscore the complexity of measuring life on Earth.
Ultimately, no matter the metric, the takeaway is clear: biodiversity is not evenly spread, and its most concentrated forms are often the most vulnerable. Conservation efforts must be tailored not just to protecting large numbers of species, but to safeguarding the unique ecosystems that allow endemic species to evolve and persist.
As the global community continues to refine biodiversity data and invest in ecological stewardship, these rankings serve as both a celebration and a warning. They remind us that biodiversity is the product of millions of years of evolution—and that its preservation depends on the choices we make now.
