Leaping out from Bangkok's vast concrete sprawl is a kidney-shaped green space, home to hundreds of plant and bird species, and where cars are outnumbered by bicyles.

But residents and campaigners fear the unique ecosystem and character of the city's so-called "Green Lung" is under threat as developers lure locals with lucrative land deals.

Bang Krachao is an artificial island formed by a canal and a bend in the meandering Chao Praya river.

The area stands apart on Google Maps: a swathe of greenery in an otherwise concrete jungle of traffic-choked streets, towering condos and sprawling factories.

While the rest of Bangkok has developed at a dizzying pace in the last five decades -- often with little thought put into sustainable long-term planning -- Bang Krachao remains an oasis of calm.

Shaped like a kidney and covering 16 square kilometres its pathways are popular with weekend cyclists and daytrippers seeking respite from Bangkok's busy streets Lillian SUWANRUMPHA, AFP

Covering 16 square kilometres (six square miles), its pathways are popular with weekend cyclists and expat daytrippers seeking respite from helter-skelter of Bangkok's streets.

But the fight is now on to stop the concrete consuming Bangkok's last tropical sanctuary.

Bang Krachao's abundant space and proximity to the city centre have caught the eye of investors.

Soaring land prices are teasing residents into selling up.

"I feel bad to sell it but my aunt is ill. She needs the money to take care of her health," Supi Saengta, 62, who has lived in the area her whole life, but is now selling the family's 6400-square-metre plot of land, which could fetch 24 million baht ($685,000).

More buildings mean more roads -- a major change in an area where many residents still get around on a network of raised concrete footpaths that snake through the tropical foliage.

Eventually "these paths will be knocked down and replaced by big roads which block the waterways," said Jakkaphan Thruadmarakha, an environmental campaigner who was born in the area.

Bang Krachao's abundant space and proximity to Bangkok's centre have caught the eye of investors. Lillian SUWANRUMPHA, AFP

"We can already see that some of the canals are becoming stagnant and have problems with water drainage," he added, urging future development on the wedge of land to be sustainable.

- Green not grey -

Those battling to keep the Green Lung green have some powerful backers in their corner.

Thailand's revered late King Bhumibol Adulyadej is said to have declared his wish for Bang Krachao to be preserved for future generations.

His daughter, the popular Princess Sirindhorn, has made several visits to the area.

In the wake of Bhumibol's death last October, the current military government announced a plan to safeguard Bang Krachao's wild character.

The three-year scheme, which involves the Royal Forestry Department, Kasetsart University and the Thai oil firm PTT, aims to renovate public green spaces and ensure that at least 60 percent of the area remains free from development.

Thailand's revered late King Bhumibol Adulyadej is said to have wanted Bang Krachao to be preserved for future generations Lillian SUWANRUMPHA, AFP

The difficulty in a country like Thailand, where land is in short supply and corruption rampant, is developers and powerful businesses have long found ways to circumvent, or simply ignore, environmental protections.

"If we do nothing, the traditional way of life, with houses in farmland, with mangrove forest surrounding Bang Krachao, will disappear," warns Montathip Sommeechai, a lecturer in Kasetsart University's forestry faculty.

She says many of the environmental challenges facing the district have their roots in the changing lifestyles of those who live there.

Whereas most residents once made their living from farming, many now just tend to their gardens in their free time, so alternative uses for the land need to be found.

Montathip hopes that by encouraging organic agriculture, Bang Krachao could become an "urban food bank" for the Thai capital.

Ecotourism is also being touted as a part of a possible sustainable future.

And the sleek, eco-friendly Bangkok Treehouse hotel, which opened up on the eastern bank of Bang Krachao five years ago, is leading the way with solar powered rooms.

"An important thing is that the atmosphere here is like up-country," says 27-year-old manager Tanaporn Wittayasiripaiboon. "We tried to design the place to blend nature with a modern and unique style."

Amid Bangkok's relentless economic and demographic growth, for now Bang Krachao is a throwback to simpler times.

But it will take ingenuity and political will to keep the Green Lung breathing.