The rainforest is filled with Darwinian menace. That menace plagued the Spanish when they arrived in 1522, and Costa Rica was a listless and oddly egalitarian Spanish colony for 300 years. Even the Spanish governor had to farm his own land. The rainforest was an obstacle to William Walker, an American mercenary who set out to annex Central America to the Southern Confederacy in 1856. He had already conquered Nicaragua, declared himself president and reinstated slavery, but in the Battle of Rivas, Walker’s army was defeated by a Costa Rican militia.

This battle for the country’s soul is complex and ongoing. More than a quarter of the world’s biodiversity was lost in the last 35 years, and this is one of the most biodiverse places on earth.

Subsequent invasions have been subtler. The hills around the town of Jaco and its famous beach are scoured by heavy machinery, clearing the trees for more gated communities, and hotels and condominiums are being built along the white sand. Billboards in English advertise new developments, and construction cranes hover near the beach like carrion birds. All of these foreign-owned developments are bringing much-needed employment, and all of them are reducing habitat and refining the economic stratification that began 150 years ago.

This battle for the country’s soul is complex and ongoing – and increasingly critical. Costa Rica has done more than any country to preserve its rainforest. But more than a quarter of the world’s biodiversity was lost in the last 35 years, and the Costa Rican rainforest is one of the most biodiverse places on earth.

From the Península de Osa, we took a Nature Air Twin Otter – the company slogan is “the world’s first carbon-neutral airline” – to the central valley outside San José, then drove southwest to the Pacific coast. At the Río Grande de Tárcoles, we parked and walked over the bridge to stare down at 25 fat American crocodiles sunning themselves in the shallow muddy water, waiting for ecotourists to throw more Tostitos their way.

“Why are they all under the bridge?” my son asked.

“To eat the people who fall off,” my daughter told him.

“People fall off the bridge?”

“Obviously.”

Farther along the coast, past the condo developments, is the Pacific Rain Forest Aerial Tram, which gives canopy tours of the rainforest and offers zipline rides. My daughter had been lobbying for the zipline experience for a month, something she had tried once before, and it appealed to her increasing appetite for adventure. We took the eight-passenger tram to the top of the mountain while a guide pointed out bulbous black termite nests, green iguanas and a gargantuan king vulture. A monkey chattered in the distance and my son lip-synced to it. He has taken up lip-syncing to ambient sounds – monkeys, thunder, airport public service announcements – a peculiar talent.

Childhood disappears the way the rainforest does, in increments. The loss of innocence is a theme that haunts Christianity, literature and societies in equal measure, and childhood is the central metaphor. Rousseau, William Blake and other Romantics equated “primitive” societies with childhood and, therefore, with innocence. It was the proximity to nature that blessed them. Costa Rica sometimes bills itself as the original Eden, but innocence is ephemeral. Eventually, we grow up to view the world as a predatory ecosystem of Wal-Marts and hucksters. Or perhaps it’s just me. So I’m holding on to my daughter’s childhood just as she’s dead set on abandoning it.

We were outfitted with harnesses and helmets to come down a network of ziplines that were 40 metres above the forest floor – the longest, a daunting 316 metres. The first line left from a platform that was situated on the side of the mountain, and the drop was gradual; looking down, I saw the ground slip away. But the next one left from a small platform 40 metres up on a tree. Justine stood poised at the edge, and my stomach sank as she peered down at the 12-storey drop. We could hear the previous person still on the line, the noise humming along the wires in a mechanical drone.

Justine got the clear sign and turned to smile, then stepped into the abyss, zipping away from me.

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