In 1972, Catholic missionaries entered the Chaco forest of northern Paraguay and forced Oscar Pisoraja’s family, and their nomadic Ayoreo people, to leave with them. Many perished from thirst on the long march south. Settled near the village of Carmelo Peralta on the Paraguay River, dozens more died from illnesses. Still, the survivors kept up some traditions – hunting for armadillos; weaving satchels from the spiky caraguatáplant. “We felt part of this place,” says Pisoraja, now 51.

Today, his community – and other indigenous peoples across the Chaco, a tapestry of swamp, savanna and thorny forest across four countries that is South America’s largest ecosystem after the Amazon – are confronting a dramatic new change.

On 13 December, Paraguay’s president, Mario Abdo Benítez, visited Carmelo Peralta to launch construction work on a $103m (£76m) bridge that will cross the Paraguay River to Brazil. On the Paraguayan side, the bridge joins a $445m highway – already half-finished – carving a strip of asphalt for 340 miles (550km) east to west through the Chaco.

In 2024, when both are completed, the massive infrastructure project across the Chaco, the Bioceanic Corridor, will connect cattle ranchers and soya-bean farmers in Brazil and Paraguay with their lucrative Asian markets, via northern Argentina and Chile. Space is also being left for a parallel freight railway.

It’s the final nail in the coffin for the Chaco and all its peoples

“This is a historic day for our country,” said Abdo Benítez, likening the highway to a new Panama Canal.

“We’re going to integrate our two peoples,” Reinaldo Azambuja Silva, governor of the Brazilian state of Mato Grosso do Sul, told the ceremony in Carmelo Peralta. “This is the realisation of a dream.”

But campaigners say the Bioceanic Corridor is a nightmare, accelerating destruction of the Chaco – the fastest-vanishing forest on Earth – and piling deadly pressure on its native inhabitants, including some who shun the outside world. In 2019, an area of forest the size of a football pitch was destroyed in Paraguay’s Chaco every two minutes.

“It’s the final nail in the coffin for the Chaco and all its peoples,” says Miguel Lovera, director of Iniciativa Amotocodie, a Paraguayan conservation organisation.

Paraguay’s Chaco is home to more than a dozen indigenous peoples. But the Ayoreo are particularly exposed to the changes brought by the highway: many live along its path, and already face severe poverty and social marginalisation.

Leaders from the 11 Ayoreo communities near Carmelo Peralta say the highway poses many threats: from deadly road accidents and rapid social changes to increased illegal deforestation of traditional hunting and foraging grounds.

Yet they felt forced to approve the project, says Juan de la Cruz, an Ayoreo local government official. “Even if we said no, they would still build it,” he says.

The new road makes travelling to hospitals easier, its supporters say. Until roadworks began in 2019, the surrounding Alto Paraguay region – an area the size of Austria – had no asphalt road. Buses sometimes get stuck for weeks along muddy tracks, with stranded passengers having to be airlifted to safety.

But the highway is already killing indigenous people, according to locals. Recent traffic accidents and drownings in roadside drainage pools killed several of their community, including three Ayoreo girls, they say.

Local consultations with indigenous communities were rushed, argues Lovera. Dozens of lorries carrying quarried material for the highway are now hurtling through Ayoreo land every day, he says. “They’ve never seen this kind of traffic before. They were duped,” Lovera adds.

A host of damaging cultural upheavals are likely to follow. Leaders fear the coming tide of passing truckers and ranchers will spread drug use, prostitution and petty crime.

These effects have already been seen elsewhere in the Chaco among other indigenous communities since the building of the 480-mile north-south Trans-Chaco highway in the 1970s, which Paraguay is also widening and resurfacing.

“The building of a highway always brings negative things with it,” Pisoraja reflects.

The Bioceanic Corridor also threatens wildlife vital to the Ayoreo. More endangered animals will be flattened by speeding lorries, including slow-moving giant anteaters and the aguará guazú – a wolf-like canine.

Wildlife tunnels under the road “mitigate the problem but not entirely, and there aren’t enough of them”, says Luis Recalde, a conservationist.

Illegal hunting on Ayoreo territory has also intensified, says Enrique Pebi, president of the Union of Native Ayoreo of Paraguay. He contrasts the mass slaughter of giant armadillos, marsh deer, peccaries and jaguars by outsiders with the Ayoreo’s traditional consumption of some animals for subsistence. “They just use them for target practice,” he laments.

Most concerning, locals say, is evidence that the highway is speeding up deforestation. This makes it even harder for the Ayoreo to hunt, forage for honey, fruit and roots, and gather medicinal plants; practices that are key to their survival and culture.

We want to keep it as it is, our reserve. Everything’s there. We always say: it’s our only market

Ayoreo villages near Carmelo Peralta accepted fishing boats and tractors in exchange for allowing a 30-mile road through their territory, says Pebi. “The things they’ve given us will wear out in five or six years,” he says. “I don’t know how many hectares we’ve lost for ever.”

Meanwhile, amid a local property rush, Brazilian ranchers have bulldozed a track into Ayoreo land further along the Bioceanic Corridor, and started felling trees, leaders say, showing photos of clearances to the Guardian. The Ayoreo are now too afraid of being shot by foreign “invaders” – armed security guards on expanding nearby ranches – to forage alone, says Pebi.

“We want to keep it as it is, our reserve,” he adds. “To go hunting, to get honey, to get homemade remedies. Everything’s there. We always say: it’s our only market.”

More than 140,000 sq km (54,000 square miles), a fifth of the entire Chaco, has been felled since 1985. This accelerating deforestation has global consequences. The Chaco holds 14 times more carbon-dense biomass than previously thought, one recent study found. Smoke-blackened palms line the highway – testament to the uncontrolled, man-made blazes to clear land for cattle that have swept the Chaco.

Unlike the “fishbone” pattern of deforestation along highways in the Amazon, deforestation in the Chaco clears huge rectangles, Nasa observed.

Greater deforestation spurred by the new highway also threatens about 150 Ayoreo, in at least 10 small groups, living in voluntary isolation in the Chaco’s forests, say the leaders of settled Ayoreo communities. Excluding the Amazon, they are the only documented indigenous people in the Americas seeking to avoid contact with modern society.

For 60 miles, the highway passes near the Patrimonio Natural y Cultural Ayoreo Totobiegosode (PNCAT), a 5,500 sq km Ayoreo refuge. Reports sometimes circulate that Ayoreo people choosing to live in isolation have been killed by interlopers, but are difficult to officially confirm because Paraguay’s authorities do not monitor their numbers, location or wellbeing, says Lovera.

A 2020 Earthsight report found that Brazilian ranching firms were illegally deforesting chunks of the PNCAT reserve and that leather sourced from the area has been used in luxury cars made by European companies such as BMW. Earthsight also singled out a supplier of the Chortitzer Cooperative – a huge cattle, grains and dairy company owned by the Mennonite community of Loma Plata, where the Bioceanic Corridor’s first stage ends.

Florian Reimer, Chortitzer’s manager, told the Guardian that its associate had obtained environmental permits to raze parts of the forest. “We’re totally against illegal deforestation,” he insisted.

Loma Plata – and the nearby Mennonite colony of Filadelfia – offer a vision of what the rest of the Chaco may soon look like. An orderly grid of roads encloses thin lines of trees. Indigenous peoples, uprooted by deforestation and forced conversion elsewhere in the Chaco, live marginal existences on the outskirts of town.

Enlhet-speaking men, formerly far-ranging nomads, cluster on street corners waiting for a day’s labour. The women often engage in sex work. This carries no stigma in traditional Ayoreo culture, but violent assaults and murders of Ayoreo women by non-indigenous men have increased in recent years.

Chortitzer spends about $1.5m every year on health and educational projects benefiting about 3,500 local families, says Patrick Friesen, Chortitzer’s communications manager.. “Our objective is to try to live together.”

“When I started here in ’75, all this was forest,” says Augustino Lovero, an Enlhet employee of Chortitzer’s dairy plant. “The image of the Chaco is going to change. The asphalt will bring many people, with their factories.”

In Ayoreo settlements near Loma Plata, leaders were also sceptical of the highway’s promised benefits. Basui Picanerei says his village of Ebetogue still lacks reliable drinking water and land titles.

“The Bioceanic road brings a lot of danger for the Ayoreo,” says Mateo Sobode Chiqueno, an Ayoreo historian who has spent 40 years recording his people’s memories on to hundreds of cassette tapes.

Lovera urges Paraguay to give indigenous communities land far from the road’s impact zone – or risk “genocide, willing or unwilling”.

A Paraguay public works ministry spokesperson says the road project was diverted south to avoid several Ayoreo villages and the proposed bridge at Carmelo Peralta re-routed three kilometres north to avoid Ayoreo land.

Near Loma Plata, the Bioceanic Corridor links with the Trans-Chaco highway, and will plunge deeper into the western Chaco in 2022. “The Chaco connects us to the world,” a billboard proclaims.

The new highway “connects all the sufferings of many, and the good of a few, the businessmen,” counters De la Cruz, the Ayoreo leader from Carmelo Peralta.

“And us, we’ll be left at the roadside watching them pass by.”

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