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Dam drove ‘collapse’ of rainforest bird populations in Thailand

  • A 165-square-kilometer (64-square-mile) reservoir in the lowland rainforest of Thailand has led to the “collapse” of the region’s bird populations, according to recent research.
  • Built in 1986, the Ratchaprapha dam altered the habitat and led to deforestation, resulting in the decline of many species and the local extinction of perhaps five.
  • The authors of the study say their findings highlight concerns about whether hydroelectric dams “are worth the environmental costs.”

The construction of a hydroelectric dam in Thailand in the 1980s devastated the area’s wild birds, leaving behind just a few of the many species that once inhabited the region, a study has found.

The journal Global Ecology and Conservation published the research Nov. 11.

The Chiew Larn reservoir. The flooding caused by the Ratchaprapha dam flooded the rainforests and created small islands. Image by Greg Irving.

Greg Irving, a graduate student in conservation ecology at King Mongkut’s University of Technology Thonburi in Bangkok and the study’s lead author, said the findings demonstrated the importance of determining, before a project’s approval, how damming a river would impact resident plants and animals.

Those considerations should include the “irreplaceable loss of lowlands and degradation of mainland habitat induced by edge effects,” Irving said in a statement.

The research found that bird habitat vanished as the Khlong Phrasaeng River swelled upstream of the Ratchaprapha dam to form the Chiew Larn reservoir in 1986. The rising waters swamped the surrounding lowland rainforests, leaving small islands; along the edges of the reservoir, people cut down trees and burned the forests, leaving birds with little refuge.

A straw-headed bulbul (Pycnonotus zeylanicus). Image by Wich’yanan Limparungpatthanakij.

Irving and his colleagues’ investigation surveyed bird populations on the islands and the reservoir’s shores. They then compared their species counts with the numbers of birds living beyond the dam in more intact parts of the Khlong Saeng-Khao Sok forest complex, mainland Thailand’s largest remnant of protected forest.

The team’s fieldwork revealed that the dam left behind a smaller variety of birds: of all the birds the scientists observed, the majority were from just eight or nine species known for their ability to cope with degraded forest habitats. The dam’s construction diminished the numbers of species dependent on healthy rainforests. It probably also wiped out five species completely, shortly after the Chiew Larn reservoir swelled to its current 165-square-kilometer (64-square-mile) size.

A great slaty woodpecker (Mulleripicus pulverulentus). Image by Wich’yanan Limparungpatthanakij.

After the dam was built, the area’s straw-headed bulbuls (Pycnonotus zeylonicus), listed as critically endangered by the IUCN, probably ended up in the pet trade, the authors report. As the waters crept up the trunk of a tree that housed a pair of Storm’s storks (Ciconia stormi) — the last breeding pair of this endangered species in Thailand — locals reportedly took the nest and the chicks inside. And the largest woodpecker in the world, the great slaty woodpecker (Mulleripicus pulverulentus), couldn’t find trees large enough to roost in, the team writes.

Their exploration of the forest on the islands and reservoir’s edges uncovered “remarkably similar” habitats flush with bamboo, instead of the variety of canopy-forming species typically found in vibrant rainforests. The researchers also found that the area affected by fires and felling extended to 61 square kilometers (24 square miles) beyond the reservoir’s shores.

A Storm’s stork (Ciconia stormi), a species that disappeared after the dam’s construction. Image by Wich’yanan Limparungpatthanakij.

This study highlights the challenges in trying to offset infrastructure development in one area with biodiversity habitat protection or restoration elsewhere, the authors say.

“Lowland rainforests across Asia are irreplaceable and nearly depleted,” Antony Lynam, a conservation biologist with the Wildlife Conservation Society’s Asia Program and one of the study’s co-authors, said in the statement. “[They] should be conserved and expanded wherever feasible. They cannot be offset.”

As these findings add to the growing evidence that hydroelectric power is the “least green” renewable source of energy, the authors write that “Serious consideration should be given to whether any given large hydropower project is truly worth the environmental cost.”

Banner image of a great slaty woodpecker (Mulleripicus pulverulentus) by Wich’yanan Limparungpatthanakij. 


BirdLife International. (2017). Ciconia stormi (amended version of 2016 assessment). The IUCN Red List of Threatened Species 2017: e.T22697685A110066434. Downloaded on 13 December 2018.

BirdLife International. (2018). Pycnonotus zeylanicus. The IUCN Red List of Threatened Species 2018: e.T22712603A132470468. Downloaded on 13 December 2018.

Irving, G. J., Round, P. D., Savini, T., Lynam, A. J., & Gale, G. A. (2018). Collapse of a tropical forest bird assemblage surrounding a hydroelectric reservoir. Global Ecology and Conservation, e00472.

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