The air is heavy with pollution, making the sun feel even hotter. Patricia Heather-Hayes, aka Trish, is watching a lioness through her binoculars as it dozes under an acacia tree in Nairobi national park. A dynamic woman in her sixties, she knows all the big cats by name: “That one’s Athi. She has three cubs.” Trish is also keeping an eye on some giraffes, antelopes and zebras, a little farther away, as they move across the savannah. The skyscrapers on the horizon make an unusual backdrop. But what she also notices, each time she tours the park, are the plastic bags, empty bottles and other forms of food packaging that have caught in the bushes or litter the side of the tracks.

This protected nature reserve, which extends over 117 sq km, is the only one in the world to be home to wild animals while being part of a capital city. But Nairobi is one of the fastest-growing metropolises in Africa, increasingly endangering the park in its midst.

“We find more and more waste, carried by the wind from homes nearby or dropped by visitors,” says this lawyer, who works for a local firm. “The other day I saw a snake die. It was trapped in a soda can and couldn’t get out.” Once a month she leads a big clean-up operation with other members of the Friends of Nairobi National Park (Fonnap), of which she is deputy-head. Armed with long barbecue tongs – to avoid getting out of the Jeep – Heather-Hayes deftly manoeuvres, avoiding the ruts but picking up all the rubbish. That day she filled five 50-litre bin liners.

Nairobi national park was founded in 1946 by British settlers and is the oldest one in Kenya. It attracts 120,000 visitors a year, eager to see four of the big five: lions, leopards, rhinoceroses and buffalo. “We don’t have any elephants, because the park is too small with insufficient woodland,” says Hudson Okum, a guide working for the Kenya Wildlife Service (KWS), the state-owned body that manages national parks. In all the park boasts 80 species of mammal – including 39 lions and 90 rhinos – 450 bird species, 40 different amphibians and reptiles and 500 tree species.

The reserve hugs the southern perimeter of the city, with a fence running round three of its four edges, north, east and west. Only to the south does it connect to the Athi-Kapiti plains, a huge semi-arid area covering 2,200 sq km. On this side the wildlife is free to come and go as it pleases.

“In the dry season they can find water in the park, thanks to manmade ponds, then fan out southward into the plains during the rainy season,” Okum explains.

But this annual migration is threatened by the encroaching city (population 3 million), as new infrastructure and factories go up, not to mention countless homes, some planned, others not. “In the 1990s you might see as many as 10,000 gnus in the park; nowadays there’s only ever a few hundred,” says Muraya Githinji, the park’s deputy head. “There are few herbivores because there’s less room and some of their migratory routes have been cut.”

In 2012 the then-president Mwai Kibaki announced plans for a 30km southern bypass in the north-western corner of the park. The aim of the Chinese-sponsored project is to reduce congestion in the city, clogged by cars and trucks. “Originally part of the bypass was supposed to run along the edge of the park. But this area was divided up and sold to property developers under dubious conditions,” alleges Dr Paula Kahumbu, head of Wildlife Direct. “Rather than demolish these illegal structures the government opted to cut through the park itself. But only parliament is empowered to change the borders of a reserve.”

Backed by two other non-government organisations, Kahumbu took the case to court in May 2013 and had work on the disputed section of the road suspended. Work continues on the rest of the bypass until the government can publish new plans. Cranes busy themselves beside massive heaps of gravel, while hundreds of trucks thunder back and forth on the track beside the park, stirring up clouds of dust.

Meanwhile, a projected railway line along the park’s north-eastern perimeter may further encroach on it. The line is designed to modernise goods transport in the region, linking the port at Mombasa to Nairobi, and Uganda, Rwanda and South Sudan beyond. “This scheme may also run through the park, with negotiations under way for the final route,” Githinji says. In November, according to the Kenyan daily The Star, the transport minister asked his opposite number at the environment to hand over not 25 hectares of land, as originally planned, but 100 hectares. “Land has such a high economic value here that the park, which doesn’t earn much, carries little weight,” says David Marechal, who spent 13 years raising lion cubs at the park’s orphanage.

The plains to the south are increasingly fragmented and degraded. “In the late 1970s the Masai people started selling their land, attracting settlers looking for plots cheaper than in Nairobi. They put up fences all over the place, preventing the movement of wildlife,” says Nickson Parmisa, deputy-head of the Kitengela sector, south-east of the reserve. From a hilltop he points out the human activities abutting the park, on the Athi-Kapiti plains: six cement works, quarries, flower farms, tourist lodges and even the Rongai urban area, with its roads, dwellings, corrugated-iron stalls and “Land for Sale” signs.

Coexistence between wildlife and humans is not always peaceful, particularly for the pastoral communities which bring their herds to the edge of the reserve. “In recent years lions have attacked livestock and Masai farmers have retorted by killing wild animals,” says Heather-Hayes. In partnership with KWS, Fonnap has funded the installation of lamps round about 30 bomas, the local version of a corral. “The lamps flash which scares off predators,” says Rik Banerjee, 18, a student taking part in the project. There have been no attacks since.

Parmisa is pinning his hopes on the first proper zoning plan to be drawn up by the communities located south of the park. It was endorsed by the ministry of lands in 2010. “The plan marks out areas for building and others which must be left untouched to allow wildlife to migrate,” he explains. For the time being the plan has not been enforced. “Will it ever be?” Githinji asks. “The communities are divided and there is a big temptation to carry on selling off land. If nothing is done the park may be completely cut off.”