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Small mammals stranded by hydropower dams die out surprisingly fast: Study

Chiew Larn reservoir, Thailand

Chiew Larn reservoir is surrounded by forested hills and limestone karst cliffs that once enclosed the Khlong Saeng river valley. Image courtesy of Jonathan Moore

  • Forest fragmentation has long been known to impact species survival: small, isolated populations with access to limited resources are at greater risk of extinction.
  • In 1987, the Chiew Larn reservoir was formed in southern Thailand as part of a hydropower scheme, creating more than 100 forested islands inhabited by newly stranded animals.
  • A new study documents the alarmingly quick collapse of the reservoir archipelago’s small mammal communities, resulting in the loss of nearly every species and dominance by one invasive rodent.
  • Tropical biologists warn the study reflects the global trend of fragmentation in tropical forests, which is ravaging both species diversity and ecosystem resilience.

The Chiew Larn reservoir in southern Thailand looks like it’s been part of the landscape for millennia. Ancient limestone karst cliffs rise from its waters, and emerald-hued forested islands dot its surface. But the 165-square-kilometer (64-square-mile) reservoir is entirely man-made, formed in 1987 when the government of Thailand built a hydropower dam and flooded the mountainous Khlong Saeng River Valley.

As the water levels rose, forested peaks were transformed into more than 100 islands, inhabited by newly stranded animals that once roamed the entire valley. By this means, the reservoir not only created the conditions to generate power, it also created an impromptu wild laboratory in which biologists could study the impacts of fragmentation on forest fauna.

Now, researchers tracking species diversity on the islands over the past three decades have documented the loss of nearly all small mammal species. Isolated from mainland forests and other islands, each small mammal assemblage declined rapidly, culminating in the dominance of a single invasive species: the Malayan field rat (Rattus tiomanicus). The team published their results recently in the journal Current Biology.

Malayan field rat
A Malayan field rat (Rattus tiomanicus) in a small mammal trap. By 2020, this species accounted for 97% of small mammal captures. Image courtesy of Jonathan Moore

The complete collapse of a mammal community within such a short time frame is “very rare” and is stark evidence of how forest fragmentation quickens the pace of extinction, according to Jonathan Moore, lead author of the study and Ph.D. candidate at the University of East Anglia in the U.K.

“You only have a few decades before you’re losing the majority of species,” he told Mongabay. “When you flood forests, the forested islands created have very little conservation value over the long term.”

Researchers first surveyed small mammals on a dozen islands in the Chiew Larn reservoir between 1992 and 1994, five to seven years after the valley was flooded. They set live small mammal traps on the ground, within understory vegetation, and on tree trunks, and marked each mammal they captured with a tiny ear tag to avoid double counting, then released them. Teams then returned and repeated the surveys on the same islands two decades later, and again in 2020, 33 years after the islands were formed.

During the first surveys, they found 12 small mammal species and evidence that some were beginning to outcompete others. For instance, Malayan field rats, a species native to southern Thailand but nonetheless considered invasive due to its ability to thrive in human-disturbed environments, were already abundant on smaller islands. By 2012 and 2013, species diversity had dwindled to just six species; and in 2020, Moore and his colleagues recorded only three, with Malayan field rats accounting for 97% of all captures. Control surveys in forests surrounding the reservoir revealed no such dominance.

Map of Chiew Larn reservoir
Map of Chiew Larn reservoir in southern Thailand’s Surat Thani province showing the islands (those surveyed for small mammals in red). Figure from Moore et al. 2022

The researchers had initially predicted it would take 40 years for the mammals to disappear from the reservoir. But the latest surveys show it has happened already, just 33 years after fragmentation. “We saw the community collapse much faster than the worst-case model was predicting,” Moore said. “It’s even faster than expected.”

The fast pace of extinction was due to the double whammy of fragmentation and the spread of the invasive Malayan field rat throughout the island archipelago, according to the study. Native mammals already coping with the stresses of habitat fragmentation, such as small population size, limited food resources, and habitat degradation, had little chance to recover once the highly competitive rats gained a foothold.

“The characteristics of the Malayan field rat make them hyperdominant,” Moore said. “They can breed really fast … they’re really good swimmers, so they can swim between islands if food resources are diminishing, whereas [other] species, especially squirrels and tree shrews, can’t actually traverse the water.” The rats are also aggressive and forage on a wide range of foods, lending them a competitive advantage in degraded habitats.

Researchers in Thailand
Researchers check live mammal traps on a forested island in Surat Thani province, Thailand. Image courtesy of Jonathan Moore

Luke Gibson, a tropical biologist at the Southern University of Science and Technology in Shenzhen, China, and a co-author of the study, said the results reflect the current global trend toward species uniformity in tropical ecosystems. He cautioned that as forests are carved up by humans and specialist species die out, fragments dominated by a small number of disturbance-tolerant species will lack resilience and crucial ecosystem services, such as seed dispersal via mammals.

In addition to outcompeting other small mammals, members of the Rattus genus are also known to impact bird, reptile, invertebrate and plant diversity on islands worldwide, by preying directly on them or their eggs and seeds. Booming rodent populations also pose risks to nearby human populations due to their role as disease vectors and their crop-raiding habits.

David Luther, a conservation biologist at George Mason University, who was not involved in the new study, said the findings are broadly in line with similar investigations of impacted island fauna in other parts of the world. He also noted that a similar loss of diversity tends to follow fragmentation of purely terrestrial habitats, such as the Atlantic Rainforest and the Brazilian Amazon. However, when fragments are connected by land rather than water, species diversity has a chance to recover via habitat corridors, he said: “It can be much easier for species to recolonize these locations if second-growth vegetation is allowed to regrow.”

Mammal trap
A live mammal trap set up on a tree branch, the traps were baited with a mix of bananas, oats and peanut butter. Image courtesy of Jonathan Moore

Ultimately, the researchers recommend retaining and restoring large tracts of tropical forest to avoid the detrimental impacts of fragmentation in the first place. They say preserving extensive swaths of forest will also maintain predator populations and other key ecological processes that keep invasive species in check.

However, despite studies that indicate hydropower infrastructure poses greater risks to tropical forest biodiversity than solar or wind power, dams continue to be built in rainforests around the world. “This is particularly problematic, because these same areas are also key habitats for biodiversity, especially for large predators which require large undisturbed forest areas to survive,” Gibson said. He added that as new reservoirs are created, it will be vitally important to protect their surrounding forests to ensure that displaced species continue to survive.

The new study suggests that mitigating the impacts on small mammal communities stranded on islands within reservoirs might be harder. Extinction, it seems, will eventually come calling.

Banner image: Chiew Larn reservoir is surrounded by forested hills and limestone karst cliffs that once enclosed the Khlong Saeng river valley. Image courtesy of Jonathan Moore


Moore, J. H., Palmeirim, A. F., Peres, C. A., Ngoprasert, D., & Gibson, L. (2022). Invasive rat drives complete collapse of native small mammal communities in insular forest fragments. Current Biology, 32(13), 2997-3004. doi:10.1016/j.cub.2022.05.053

Palmeirim, A. F. & Gibson, L. (2021). Impacts of hydropower on the habitat of jaguars and tigers. Communications Biology4, 1358. doi:10.1038/s42003-021-02878-5

Gibson, L., Wilman, E. N., & Laurance, W. F. (2017). How green is ‘green’ energy? Trends in Ecology & Evolution32(12), 922-935. doi:10.1016/j.tree.2017.09.007

Carolyn Cowan is a staff writer for Mongabay. Follow her on Twitter @CarolynCowan11

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