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Intensifying rice farming is threatening the ecosystem and fishing community of one of South-East Asia鈥檚 most important wetland areas.

鈥淭he floodplain is undergoing a fairly rapid shift away from natural habitats that supports bird populations when they鈥檙e flooded to rice cultivation,鈥� says Simon Mahood at the Wildlife Conservation Society in Cambodia.

Cambodia鈥檚 largest lake, Tonl茅 Sap, gathers a massive volume of water from various tributaries whose changing ebb and flow means a huge shift in water level over the course of a year. It is known for the floating villages of the people who live on the lake and mostly depend on fishing for subsistence 鈥� some houses sit on posts high enough to keep them above seasonal high waters while others float on the surface.

These vast wetlands support many threatened species, while fishing has supported people in the region for millennia.

But farmers have been converting flooded forests and grasslands around the lake and in upstream areas to rice farms for years. Mahood and his colleagues wanted to see if they could track the rate of change and the impacts of these ecosystem shifts.

They chose 1000 random points on the landscape around Tonl茅 Sap spread over about 14,000 square kilometres and used a combination of satellite images and ground surveys to determine if they were rice farms or more natural ecosystems.

They found that agricultural intensification in the area has reduced grassland and scrubland by about a third over the past 15 years based on satellite images going back to 2003. This in turn has caused carbon stored in the landscape to decline by 12 per cent based on standard assessments which give values for rice versus scrub, the researchers say.

Mahood says that as more land is converted to rice farms upstream from the lake, the water reaching the lake is reduced. Lower seasonal water levels then make it easier for farmers to clear more of the natural flooded forest and grasslands to convert into yet more rice farms.

Flooded forests are important sanctuaries for spawning fish, giving their young a place to fatten up before heading into other parts of the wetlands. If these areas are converted to rice paddies, harmful chemicals from fertiliser can get into the water. The habitat loss also means there are fewer of the fish that many lake residents depend on for food and livelihood, which pushes them towards more rice farming.

鈥淚f people aren鈥檛 catching fish, they鈥檙e going to do something else,鈥� Mahood says.

He adds that the fish are also important parts of a working ecosystem that feeds waterbirds in the region like the Bengal florican, which is listed as critically endangered by the International Union for Conservation of Nature.