A new study by a team of scientists from WCS and other groups reveals that Cambodia’s iconic Tonle Sap Great Lake is losing massive amounts of habitat threatening its very existence.

Publishing their results in the journal Wetlands Ecology and Management, the authors conducted satellite-based land-cover analysis and discovered that that over the past two decades, nearly one third of the Tonle Sap Biosphere Reserve’s natural habitats have vanished, and that half of the Tonle Sap floodplain is now under rice cultivation.

Using a point-based land-cover assessment method, the authors quantified changes in floodplain vegetation between 1993 and 2018. They found that grassland cover declined from 3,160 to 519 square kilometers, while in the last decade scrubland cover declined from 8,660 to 6,776 square kilometers. The extent of natural habitats in the Tonle Sap floodplain declined from 74 percent in 1993 to 52 percent in 2018, as a result of intensification and expansion of rice cultivation. Recent El Nino conditions have facilitated clearing of scrubland for agriculture and accelerated the destruction, because the hot, dry conditions make it easier to burn the vegetation.

The author’s warn that a dangerous feedback loop is being created and could lead to a death spiral for Tonle Sap. Lower-than-average rainfall, construction of hydropower dams on the Mekong River and its tributaries, and water diversion for agriculture have led to lower water levels in the lake. These conditions result in a hotter, drier climate causing even lower water levels, which in turn create better conditions for clearing natural habitats in the floodplain for water-hungry dry-season rice cultivation.

Said Simon Mahood, Technical Advisor for WCS Cambodia and lead author of the study: “Habitats in the Tonle Sap Biosphere Reserve are changing fast, with catastrophic implications for communities and biodiversity. We need a coordinated shift in policy and practice in the agriculture sector to reduce the impact of rice cultivation on natural habitats, through adoption of Sustainable Rice Platform principles that safeguard natural habitat and reduce the environmental impact of rice cultivation.”

The Tonle Sap Great Lake is the world’s largest flood-pulse ecosystem, protected under the Tonle Sap Biosphere Reserve. Each year in the wet season, water flowing back up the Tonle Sap River from the Mekong River, and inflow from the catchment of the lake, causes it to expand up to five times its dry season area and volume.

The seasonally flooded forest, scrubland and grassland of the floodplain is vital habitat for a range of globally threatened species, including greater adjutant, Bengal florican and hairy-nosed otter. When flooded, the vegetation acts like a coral reef and nurtures an incredible abundance of fish. These habitats sustain one of the world’s largest inland fisheries, which provides food to millions of Cambodian people. Because dry-season rice has a simple structure during the flood-season, it supports lower fish populations than grassland or scrubland.

The authors say that, although important for economic development, expansion of irrigated rice cultivation has caused a significant reduction in fish populations, carbon stocks, and threatened species. The scale of these changes in vegetation are of a greater magnitude than those predicted from hydropower dams on the Mekong and climate change. Agricultural intensification is a serious threat to the integrity of the Tonle Sap ecosystem, which has received relatively little attention to date.

Colin Poole, Regional Director of WCS Greater Mekong and a co-author of the study, said: “The Tonle Sap Lake and Floodplain deserves more attention from policy makers. Local communities and threatened species need the support of government and NGOs to help them adapt to these rapid changes in the Tonle Sap Biosphere Reserve.”

WCS’s work in the Tonle Sap Lake and Floodplain is generously funded by Margaret A. Cargill Philanthropies, Agence Française de Développement, Critical Ecosystem Partnership Fund, and the USAID Greening Prey Lang project.