Over the course of just eight years, the forests surrounding Indonesia’s Lake Poso, an ecological and evolutionary “gem” on the island of Sulawesi, have been whittled away, satellite data and imagery show, while flooding has intensified, and traditional livelihoods suffer.

Lake Poso is Indonesia’s third-largest lake, 32 kilometers long by 16 wide (20 by 10 miles), and lies in Poso district in the province of Central Sulawesi. The lake harbors many unique species found nowhere else on Earth, leading to its designation as an Alliance for Zero Extinction (AZE) site.

Satellite data from Global Forest Watch reveal ongoing encroachment into the Pamona Nature Reserve at the lake’s southeastern edge, with 681 hectares (1,683 acres) of humid primary forest lost within the AZE site between 2002 and 2023. This represents 48% of total tree cover loss during the period.

Data from the national statistics agency show forest cover Poso district decreased from 514,651 hectares (1.27 million acres) in 2014 to 358,828 hectares (886,680 acres) in 2021, a loss equivalent to the size of London in just eight years.

Sunardi Katili, head of the Central Sulawesi chapter of Walhi, Indonesia’s leading environmental NGO, identified three primary drivers of deforestation: nickel mining, oil palm plantations, and smallholder farm expansion.

“The demand for natural resources in the form of nickel for the electric car battery industry and stainless steel for household appliances has created high market demand,” Katili told Mosintuwu media journalist Pian Siruyu in May 2024.

Cultural significance and community impact

Lake Poso supports approximately 60,000 people in surrounding districts, and is located some 260 kilometers (162 miles), an eight-hour drive, from Palu, Central Sulawesi’s provincial capital. For Indigenous communities, the lake holds deep cultural significance.

“The indigenous people of Lake Poso refer to Lake Poso using the third person singular pronoun: he/she. This shows the respect and perspective that the lake is life,” Lian Gogali, founder of Institute Mosintuwu, a local organization that supports conflict survivors, told Mongabay in an email.

Traditional communities maintain rice fields, plantations and water buffalo grazing grounds along the shoreline, while local fishers preserve centuries-old fishing practices. However, these traditional ways of life are facing challenges. The region experienced 12 major floods in just the first five months of 2024, more than all the floods in the previous year.

Historical and hydroelectric context

From 1998 until 2000, Poso was the site of deadly sectarian violence between local Muslim and Christian communities. Hundreds of people were killed in the conflict and thousands more were forced to leave their homes, according to U.S.-based watchdog organization Human Rights Watch.

“At the time of the violent conflict, communities in Poso district fled their lands and farms (some sold them), [and] when they returned they were powerless to fight for their lands together because they were segregated based on religious or tribal identity,” Gogali said.

Now, Lake Poso faces pressure from a controversial 515-megawatt hydropower project operated by PT Poso Energy. According to interviews for a 2020 Mongabay article, after the period of sectarian conflict, many local officials welcomed the $700 million dam project in 2005, hoping it would bring vying religious communities together for progress on the island.

However, Freddy Kalengke, a local fisher quoted in The Jakarta Post,  said eel catches have drastically declined.

“We caught a lot of sugiri [the local word for the eels] with fish traps. We used to catch 20, 30 even 40 kilograms [44-88 pounds] each night,” Kalengke said. He added he “would count himself lucky these days if he could catch 5 kg [11 lbs] of eels in one night, a rare event now.”

The decline began in 2019 when Poso Energy built the dam, blocking the migratory patterns of the eels, according to Kalengke. The company also dredged the bottom of the lake and river to increase water flow into the hydropower plant dam.

Poso Energy’s environmental manager, Irma Suriani, told The Jakarta Post the company had compensated villagers for harvest losses and livestock deaths during trial periods and that the river improvement process was 85% complete. The company also told The Jakarta Post that it had installed fishways to help conserve the local eel population, though local fishers report significant declines in their catches since the dam’s construction.

“As a source of renewable energy, the hydropower plant produces fewer carbon emissions than a fossil-fuel powered plant with the same capacity” and will help the government achieve its net-zero carbon emission goals, Suriani told The Jakarta Post. Indonesia aims to reduce its emissions by 31.9% independently by 2030, with the energy sector expected to contribute a 15.5% reduction.

Poso Energy didn’t respond to Mongabay’s requests for comment on this story.

Forest loss threatens an ecological gem

Lake Poso was formed around 2 million years ago by tectonic activity and has evolved into what scientists consider a “natural laboratory” for studying evolution. Long-lived lakes are valuable research sites for scientists studying biodiversity, evolution and geological processes. However, unique, range-restricted species make places like Lake Poso susceptible to environmental disruption, according to researchers.

“I think the big issue with Poso is the almost complete lack of information,” Doug Haffner, an emeritus professor of limnology at the University of Windsor, Canada, who calls the lake a global “gem” and has studied it for more than 25 years, told Mongabay in 2020. “No one has really worked on Lake Poso. I think we are the only group with information on lake physics, chemistry, and biology, and that is a very limited data set for one of the most important lakes in Indonesia.”

Lake Poso hosts several threatened species found nowhere else. The lake’s exceptional biodiversity is particularly evident in its invertebrates, with high endemism among mollusks and shrimps. Notable among these unique species are the endangered Migmathelphusa olivacea crab and Sarasin’s goby (Mugilogobius sarasinorum), which exist only in these waters.

“There are not that many ancient systems around, and we recognize them as real harbors of biodiversity,” Haffner said.

The rapid forest loss around the lake threatens more than just the trees. Forests help to protect Lake Poso’s delicate ecosystem by acting as a natural filter. Their root systems prevent erosion and trap sediments that would otherwise flow into the lake, while the forest floor purifies water as it moves through the soil. Forest cover also helps maintain stable water temperatures through shade and regulates water flow into the lake — functions that become clear in their absence, as evidenced by the region’s increasing flood frequency.

A 2023 study by authors from the Indonesian government’s National Research and Innovation Agency recommended preserving forests along the shoreline to maintain water quality. While a 2024 study by researchers at the University of Bern, Switzerland, found that Lake Poso’s water chemistry has remained surprisingly stable despite development pressures, scientists warn this resilience may not last.

Community strength

The human cost of environmental degradation is evident in local testimonies. According to The Jakarta Post, nearly 100 hectares (250 acres) of rice fields were affected by the flooding from the hydroelectric plant in 2023, impacting as many as 114 farmers.

“The most heart-broken were the women in the village,” Dewa, a local resident, told the newspaper. “They were distraught about the future of the children. When the rice fields turned yellow [from flooding] and we couldn’t harvest, many started to cry as they saw their hopes drowned by the water.”

In response, community members and grassroots organizations are working to protect the lake. Women have emerged as key environmental defenders through the Alliance of Lake Poso Guardians, working to protect forests and maintain traditional farming practices — in the hopes of preserving both lake and livelihood.

Banner image of community member Ibu Selly protest against PT Poso Energy construction on Lake Poso. Photo courtesy of Mosintuwu / Ray Rarea.

Liz Kimbrough is a staff writer for Mongabay and holds a Ph.D. in Ecology and Evolutionary Biology from Tulane University, where she studied the microbiomes of trees. View more of her reporting here.

Indonesia’s Lake Poso, an evolutionary ‘gem,’ threatened by dam

Citations:

Kaban, S., Ditya, Y. C., Makmur, S., Fatah, K., Wulandari, T. N., Dwirastina, M., … Samuel, S. (2023). Water quality and trophic status to estimate fish production potential for sustainable fisheries in Lake Poso, Central Sulawesi. Polish Journal of Environmental Studies, 32(5), 4083-4093. doi:10.15244/pjoes/168102

Damanik, A., Janssen, D. J., Tournier, N., Stelbrink, B., Von Rintelen, T., Haffner, G. D., … Vogel, H. (2024). Perspectives from modern hydrology and hydrochemistry on a lacustrine biodiversity hotspot: Ancient Lake Poso, Central Sulawesi, Indonesia. Journal of Great Lakes Research, 50(3), 102254. doi:10.1016/j.jglr.2023.102254

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