Once ravaged by mining and logging, Costa Rica's Corcovado National Park now shelters 2.5% of Earth's biodiversity. But as tourism grows, its future hangs in the balance.

"I was stalked by a puma once when I was a park ranger," says Danny Herrera-Badilla, as we trek down a trail in the tangled jungles of Costa Rica's Corcovado National Park. Sunlight filters through the canopy. Overhead I can hear the chatter of tanagers, the clack-clack of toucans and the distant boom of a motmot. Off to my left, the Pacific Ocean flashes through the trees, and a stripy-tailed coati bustles busily through the undergrowth.

"Actually, it's not the cats you have to worry about," Herrera-Badilla adds, as we emerge beside an estuary. "The white-lipped peccaries are really dangerous. And the snakes. We have a lot of venomous snakes. And scorpions. Some spiders. Oh yes, and crocodiles."

He points across the river. A portly saltwater crocodile is basking on the far bank; its jaws fixed in a reptilian grin. "He's waiting for his breakfast," Herrera-Badilla says. "Which reminds me. I'm hungry." We sit down on a log and tuck into our packed snack of tacos and plantains, listening to scarlet macaws squabbling in the treetops. Apart from us, there isn't a soul around.

I've only been in Corcovado for two days, but that's long enough to understand what a wild place this is. Covering 424 sq km of the Osa Peninsula, a remote pocket of land in Costa Rica's far south-west, the park contains the largest area of primary tropical rainforest in Pacific Central America. Sandwiched between the Pacific Ocean to the west and the deep Golfo Dulce to the east, it's a vast wildlife refuge; more like an island than a peninsula. Some 500 tree species, 400 birds, 116 reptiles, 6,000 insects and 140 mammals are found here, as well as the endangered Baird's tapir and harpy eagle. It's estimated that Corcovado contains 2.5% of the world's biodiversity – an extraordinary statistic for somewhere smaller than the Isle of Man.

Founded in 1975, Corcovado celebrates its 50th anniversary in 2025, so I've travelled south from the capital of San José to find out what's worked over the last half-century – and to ponder what the next 50 years might hold in a world in which wild places like Corcovado are under threat. Primary forests are a rare and dwindling resource: according to the United Nations, they have reduced globally by 800,000 sq km since 1990.

The flight from San Jose takes just under two hours, our twin-engine plane gliding low over jungled hills, white beaches and blue bays before bumping down on the dusty airstrip in Puerto Jiménez, a beach town about 40km from Corcovado's eastern edge.

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The creation of the national park was a watershed moment for the Osa Peninsula. In the previous decades, this was Costa Rica's wild west frontier. Illegal logging and poaching was widespread; tracts of forest had been lost to slash-and-burn agriculture and clandestine gold mining along the rivers was commonplace. The national park ended these damaging practices and allowed areas of deforested land to regenerate. Between 1987 and 2017, Osa's forest cover actually increased by 11%, in contrast to many other rainforests across Central and South America.

The national park also brought something else with it: ecotourism. Corcovado is now Costa Rica's 10th most visited national park, receiving........

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