Inspired by a mountain lion isolated from potential mates, the world's largest wildlife bridge is being built in Los Angeles to allow animals to roam freely.

There had been rumours for a few months in the Hollywood Hills that a 90lb (41kg) beast was skulking through one of Los Angeles's most heavily trafficked parks at night. But it wasn't until a camera captured a photo of a mountain lion in 2012 that scientists could confirm the lore.

P-22, P as in puma (another word for mountain lion), was a 1.5-year-old male the National Park Service determined had journeyed by himself nearly 30 miles (48km), crossing two major freeways, to end up in Griffith Park, in central LA just outside of Hollywood, where he took up permanent residence.

Nearly overnight, P-22 became Los Angeles's newest celebrity – seen as a mascot for some locals. But P-22's story was also one of isolation, for the mountain lion was miles away from others of his species and would likely stay mateless for life.

When Beth Pratt, California regional executive director at the non-profit National Wildlife Federation, first read about P-22, she immediately called Jeff Sikich, a wildlife biologist for the National Park Service.

"He gave me a tour of Griffith Park. And then he talked about not just P-22 but the entire mountain lion population in the Santa Monica Mountains. And P-22 was just one example. The worst example," says Pratt.

Pratt learned from Sikich that while P-22 was rare in that he had survived leaving the western Santa Monica Mountains to cross into the eastern flank of the mountain range, he was not alone in his seclusion. Most of the mountain lions living in the western range were also stuck and inbreeding, because of how the city's transportation infrastructure had evolved.

In fact, road development in Los Angeles has cut off many species from their normal roaming territory, from bobcats to birds. Solitary mountain lions, which seek out "home ranges" of up to 250 sq miles (648 sq km), are among the most affected.

"These roads were just literally dooming this population to extinction because they were trapped and isolated and were inbreeding themselves out of existence, which science was starting to show," says Pratt. "And it was that day [at Griffith] I literally was like, 'Oh my God. We have to do something.'"

• P-22, Hollywood's celebrity mountain lion, ends his reign

Thirteen years later, Los Angeles is gearing up to open the largest wildlife crossing in the world. Set to open in 2026, the 165ft (51m)-longbridge will mirror the desert terrain of the nearby valley and be dotted with rocks and low shrubs to reconnect the Santa Monica Mountain range. It will allow mountain lions and other species, such as coyotes, bobcats and deer, to roam more freely.

Wildlife crossings are used globally as a way to let animals safely cross roads and highways, from migrating red crabs in Australia's Christmas Island to "ecoducts" in the Netherlands used by wild boar, badgers and foxes. But Los Angeles's crossing stands out not just for its magnitude, but its location: in the US's second largest city, often considered the birthplace of the country's modern highway system.

What Pratt didn't know when she first learned of P-22 back in 2012 is that there was already an effort underway in Los Angeles to fix the problem of separated wildlife corridors.

Scientist and researcher Paul Edelman had for decades been putting together an effort to build a bridge between mountain ranges to let animals like pumas range more freely and lower the risk of being run over.

His efforts began in 1989 when he was a consultant for the non-profit The Nature Conservancy and was given a grant by the Santa Monica Mountains Conservancy, a state authority, to study what it would take to connect Southern California's three mountain ranges.

"I was just put in a fortunate position to where I was told, 'Okay, here's some money, and you get a year to study this and produce something.' And so I got to devote my working life to studying it and it just very soon became crystal clear what we needed to do," says Edelman, who is now deputy director of........

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