There’s a ridge in the Tropical Andes where, in the span of a single morning’s walk, you can pass through five distinct ecosystems. You start in cloud forest so thick with moisture that the bark of every tree is carpeted in moss and the air smells of earth and rain. Climb for an hour and the trees thin, giving way to high-altitude grassland, the páramo, where strange wax palms stand isolated against an open sky. Drop down the other side and the vegetation shifts again, the air warming, the canopy closing in, the sounds changing entirely.
This is the geography that made the Tropical Andes the most biodiverse place on Earth. Not a single forest or valley, but a mountain system so varied in altitude, rainfall, and microclimate that it functions like dozens of different ecosystems stacked on top of each other with each one a separate evolutionary world, separated from its neighbours by just a few hundred metres of slope.
The numbers that come out of this landscape are difficult to process. Over 30,000 species of vascular plants. Nearly 2,000 bird species and roughly a quarter of every bird species on the planet, concentrated in one mountain range. More than 1,500 species of amphibians, the majority found nowhere else. New species of frogs and orchids still being formally described every year, decades after scientists first started systematically cataloguing the place.
No other region on Earth comes close to this density of life. And understanding why requires understanding what makes a mountain range into a biodiversity engine.
The Elevation Escape: Nature’s Most Effective Survival Strategy
Picture the last ice age, 20,000 years ago. Global temperatures have dropped by around 5 to 6 degrees. Across the tropics, rainfall is failing and forests are contracting. Species that spent millions of years adapting to specific climates are suddenly finding those climates gone. In most places, the options were brutal: migrate thousands of kilometres, adapt fast enough, or disappear.
In the Tropical Andes, there was a third option. Move up the slope.
The elevation gradient in the Andes is steep enough that a species facing the wrong temperature doesn’t need to travel far to find a better one. A few hundred metres of altitude change is the equivalent of moving significantly closer to or further from the equator in terms of temperature. A bird, an orchid, a tree frog , any of them could track their preferred climate by shifting altitude rather than longitude. The mountain gave them a vertical escape route that flat terrain simply doesn’t offer.
Conservation International, which maintains the global biodiversity hotspot framework used by scientists worldwide, recognises the Tropical Andes as the single most biodiverse hotspot on the planet precisely because of this mechanism. The elevation escape didn’t just help species survive climate swings, it actively created new ones. When populations on different slopes got separated by a ridge or a valley, they began to diverge. Over thousands of generations, separate varieties emerged. The mountain range became a speciation machine, simultaneously sheltering existing life and generating new forms of it.
This is why the endemism in the Tropical Andes is so extraordinary. Endemism means species found in one place and nowhere else on Earth. The Andes has it at a scale no other region matches, roughly 20,000 plant species found only here, around 600 bird species that exist in this range and nowhere else, and hundreds of mammals and amphibians with similarly restricted ranges. These aren’t species that simply prefer the Andes. They evolved here, in the specific conditions this mountain system produced, and they have nowhere else to go.
What the Trade Winds Have to Do With It
Geography explains the vertical dimension of Andean biodiversity. But there’s a second factor that explains why the forests stayed intact long enough to become what they are: moisture.
The Tropical Andes sit at a remarkable intersection of atmospheric systems. Warm, moisture-laden air moves west from the Atlantic Ocean across the Amazon basin and rises as it hits the eastern slopes of the Andes. As it rises, it cools and releases that moisture as rainfall. The result is some of the wettest terrain on Earth with cloud forests so saturated that water drips from leaves even when it isn’t raining, and rivers that run clear and fast year-round.
This moisture kept the forests wet even during the ice ages when rainfall was collapsing across the rest of the tropics. Research using records preserved in cave mineral deposits which are essentially natural climate archives shows that the eastern Andes maintained precipitation levels close to modern norms throughout glacial cycles while surrounding regions dried out significantly. The Atlantic trade winds kept delivering moisture. The mountains kept catching it. The forests held on.
That combination of stable moisture from the east and vertical escape routes built into the terrain is what turned the Tropical Andes into the planet’s most resilient biodiversity refuge across millions of years of climate instability. Every time conditions worsened elsewhere, this was where life came through.
The Scale of What’s Here
Numbers help, but they need context to land properly.
The Tropical Andes biodiversity hotspot as defined by Conservation International covers a stretch of western South America running from Venezuela and Colombia in the north through Ecuador, Peru, and Bolivia to northwestern Argentina in the south. The original extent of the hotspot was around 1.26 million square kilometres. Roughly 25% of that original vegetation remains intact today; around 314,500 square kilometres of primary forest, páramo, and cloud forest that still function as the complex ecosystems they always were.
Within the hotspot overall, the species counts are staggering. Over 30,000 plant species, compared to roughly 17,000 in all of Europe. Around 1,700 bird species, more than are found in the entirety of North America. More than 1,000 species of amphibians, in a group that has been declining globally for decades due to climate change, disease, and habitat loss. Somewhere between 400 and 500 mammal species, including the spectacled bear, the only bear species native to South America and the mountain tapir, one of the rarest large mammals on Earth.
The density of this life relative to area is the point. The Tropical Andes covers roughly 1% of Earth’s land surface. The proportion of global biodiversity packed into that 1% puts every other region on the planet in perspective.
The Ucayali: Where the Andes Meets the Amazon
If the Tropical Andes is the engine of South American biodiversity, the Ucayali region of Peru is one of its most important outputs.
The Ucayali sits at the transition zone between the high Andes and the deep Amazon which is the point where the mountain ecosystem spills down into the largest tropical rainforest on Earth. This transition zone is where the speciation that happened in the Andean elevations gets exported into the broader Amazon basin, carried by rivers, birds, wind, and slow ecological spread over thousands of years.
The result is a place of extraordinary biological density. The Ucayali region sits within a complex that hosts more than 30,000 species of vascular plants and nearly 2,000 bird species. New species of frogs and insects are still being formally described from this region regularly. It is, by most scientific measures, one of the most biodiverse corners of the most biodiverse region on Earth.
It’s also facing the chainsaw directly.
Peru lost around 3.4 million hectares of forest between 2000 and 2020, with deforestation rates in the Amazon-facing regions accelerating sharply in recent years. The pressure comes from agriculture, cattle ranching, illegal logging, and the slow advance of infrastructure into areas that were once too remote to clear economically. The forests that survived ice ages and thermal events across millions of years are being removed at a pace measured in hours per hectare.
At Fund The Planet, we’ve stood at the edge of that clearing in the Ucayali. The line between intact primary forest and bare cleared land is sharp and recent; ancient trees that stood through the last ice age removed in an afternoon. The communities living alongside that line understand what’s being lost in a way that statistics can’t capture. They’ve been the stewards of this landscape for generations. The pressure they’re now facing is industrial, not local.
Why “Most Biodiverse” Is More Than a Superlative
Calling the Tropical Andes the most biodiverse place on Earth could sound like an interesting fact to file away and forget. The reason it matters goes deeper than the ranking.
The Tropical Andes produced much of the biodiversity that now populates the broader South American continent. When the ice ages ended and the Amazon began to recover, the species that repopulated it spread from Andean source populations. The genetic signatures of birds, plants, and amphibians across the Amazon basin point back to Andean origins. The mountain range functioned as a biodiversity factory across geological time — producing new species, sheltering existing ones, and seeding surrounding regions during warmer periods.
Losing the Tropical Andes, or continuing to fragment it past the point where that factory can function, means more than losing the species currently present. It means losing the mechanism that replenished South American biodiversity across millions of years of climate instability. The insurance policy the planet has relied on, repeatedly, becomes unavailable.
Scientists studying biodiversity hotspots consistently flag this dynamic: the most species-rich places aren’t just repositories. They’re sources. Protecting them isn’t just about keeping what’s there now, it’s also about maintaining the capacity to restore what’s lost elsewhere if conditions change again. That’s the scientific case for the Tropical Andes. The most biodiverse place on Earth earned that title over 120 million years of survival, generation, and resilience. What happens to it in the next few decades will shape what the South American continent looks like ecologically for a very long time.
The 25% That Remains
Here’s the thing about the numbers: 25% of the original Tropical Andes hotspot remaining intact sounds like a lot has already been lost. It has. But that 25% still represents hundreds of thousands of square kilometres of functioning primary ecosystem such as mountain forest, cloud forest, páramo grassland, transition zones that still does everything a refugium is supposed to do.
It can still shelter species through climate shifts. It can still generate new varieties. It can still seed surrounding regions. The elevation escape mechanism that made the Andes the planet’s most effective biodiversity refuge still works, in the areas where the forest still stands.
The question is whether enough of it remains connected and intact to keep functioning that way as the climate warms and land pressure increases. Connectivity matters as much as total area. A fragmented forest, carved into patches separated by cleared land, loses the ability to let species move in response to changing conditions which is exactly the movement that kept Andean species alive through past climate crises.
Protecting what remains, and maintaining the corridors that connect it, is where the leverage is. In a region that has proven across millions of years that it knows how to generate and sustain life, the most important thing humans can do is stay out of the way — and in the places where we haven’t, start reversing that.
Read more: Rainforest Refugia: What They Are and Why They Kept Life Alive
Read more: Which Rainforests Survived the Last Ice Age — And What Happened After
Frequently Asked Questions
What is the most biodiverse place on Earth? The Tropical Andes is recognised by Conservation International and the global scientific community as the most biodiverse region on Earth. It hosts more species per square kilometre than any other place on the planet, including over 30,000 plant species and around a quarter of all bird species.
Why is the Tropical Andes so biodiverse? Two main reasons. First, the steep elevation gradient creates dozens of distinct climate zones stacked vertically, allowing species to track changing conditions by moving up or down the slope rather than migrating thousands of kilometres. Second, persistent moisture from Atlantic trade winds kept the forests wet and functional even during ice ages when most other tropical forests contracted severely.
What are endemic species and why does the Andes have so many? An endemic species is one found only in a specific geographic area and nowhere else on Earth. The Andes produces high endemism because its complex terrain isolates populations on different slopes and valleys, allowing them to evolve separately over thousands of generations. Around 20,000 of the hotspot’s plant species are found nowhere else, as are roughly 600 of its bird species.
How much of the Tropical Andes is still intact? Around 25% of the original hotspot extent — approximately 314,500 square kilometres — remains as intact primary ecosystem. The rest has been cleared, degraded, or converted primarily for agriculture, cattle ranching, and mining over the past century.
Where is the Ucayali region and why does it matter? The Ucayali is a region of Peru sitting at the transition between the Andes and the Amazon. It’s one of the most biodiverse corners of the hotspot, where Andean species diversity flows into the broader Amazon basin. It’s also facing significant deforestation pressure from agriculture and logging.
