Hans Nicholas Jong, Jakarta – Large infrastructure projects have long dominated debate over the future of Indonesia's Batang Toru ecosystem, the main stronghold of the critically endangered Tapanuli orangutan.
But a new study suggests that while those projects have accelerated forest loss, the greatest direct threat to the ape's habitat now comes from a much less visible source: the cumulative impact of small-scale agriculture and logging.
Recently published in the journal Biological Conservation, the study combines satellite imagery, causal inference, and years of ethnographic fieldwork – including interviews with local communities – to assess the drivers of forest loss in Batang Toru.
Home to an estimated 716 Tapanuli orangutans (Pongo tapanuliensis), Batang Toru lost 7,659 hectares (18,925 acres) of forest – about 5% of its forest cover – between 2000 and 2023, the researchers found.
Forest loss accelerated markedly after 2012, increasing at a rate significantly higher than historical trends would have predicted. The shift coincided with the development of three major extractive projects in the landscape: the Martabe gold mine, the Batang Toru hydropower project, and the Sarulla geothermal project.
Using a counterfactual analysis, the researchers estimated that Batang Toru lost an additional 3,472 hectares (8,579 acres) of forest after the projects began, compared with what would have been expected had they never happened.
At the same time, however, the researchers found that small-scale agriculture and logging accounted for roughly 70% of direct forest loss in the landscape during the same period.
The findings suggest that while large-scale development projects remain an important conservation concern, efforts that focus only on dams or mines risk overlooking the largest ongoing pressures on the habitat of the world's rarest great ape.
Changing rural economies and the rise of PHAT logging
The study links much of Batang Toru's recent forest loss to changes in local livelihoods and commodity markets.
The researchers found that agricultural expansion accelerated after 2012 as farmers increasingly shifted away from traditional livelihoods toward commercial crops. Falling rubber prices, coupled with rising demand for bananas, encouraged many communities to convert land into small-scale banana plantations.
"In several villages, people previously relied on harvesting benzoin resin, but as prices declined, that became one of the triggers for forest clearing in the central part of the landscape," study co-author Lubabun Ni'am, a postdoctoral researcher at Heidelberg University in Germany, told Mongabay. "Among the main commodities we found were rubber and, over the past 10 to 15 years, bananas."
Unlike traditional mixed farming systems, the bananas are typically grown as small-scale monocultures for markets outside the region, including Jakarta, Lubabun said. The researchers stress that these trends are ultimately driven by economic pressures rather than by local communities' willingness to clear forests.
"We have to look at the local economy because one of the main drivers of forest clearing is economic pressure," said study co-author Koen Meyers, conservation director at the Sumatran Orangutan Society (SOS). "There must be alternative livelihood opportunities for local communities."
The study also identifies the emergence of a little-known logging mechanism known as Pemegang Hak Atas Tanah (PHAT), which translates into "landholder rights," as another major driver of forest loss, particularly in the eastern part of the Batang Toru ecosystem.
The mechanism was originally intended to allow holders of land rights to legally harvest naturally growing timber from their own land while ensuring the timber entered the government's administrative and royalty-payment systems.
In practice, however, the researchers found that the scheme was frequently abused.
"This [logging] has increased exponentially in the last five to six years," lead author Rachakonda Sreekar, from the University of Queensland in Australia, told Mongabay.
Drawing on years of fieldwork and interviews with local communities, the researchers documented cases in which village heads issued statements of land possession (SKTs) over land that remained under customary communal ownership. Logging entrepreneurs then partnered with local elites to obtain access to timber through the PHAT mechanism, they said.
According to Meyers, logging often extended well beyond the areas covered by PHAT documents.
"Very often, they harvest timber outside the designated PHAT area," he said. "We frequently see them logging outside the PHAT areas, even extending into Protection Forest."
The findings echo concerns that have recently prompted action by the Indonesian government.
In December 2025, the Ministry of Forestry suspended nationwide access to the Forest Product Management Information System (known by the Indonesian acronym SIPUHH) for natural timber harvested under the PHAT scheme, pending a comprehensive review of the mechanism. It cited concerns about suspected illegal logging and timber laundering for the suspension.
Rather than abolishing PHAT, the ministry has introduced a new evaluation process requiring applications to undergo stricter scrutiny of land tenure, logging locations, and environmental impacts before access can be restored.
Why the eastern block matters
The researchers say these findings are particularly worrying because much of the agricultural expansion and PHAT logging is concentrated in the eastern block of the Batang Toru ecosystem, an area that has historically received less conservation attention than the western block.
"Historically, the Eastern Block has not received much attention from conservation organizations," said study co-author Erwin Alamsyah Siregar, director of the Indonesian conservation group Yayasan Tangguh Hutan Khatulistiwa (TaHuKah). "Most have focused on the Western Block, perhaps because that is where the private sector is concentrated."
The Tapanuli orangutan population is made up of three subpopulations, geographically separated by the Batang Toru River, roads, and human settlements. Based on a recent population study, the western block supports the largest known subpopulation, estimated at around 514 orangutans, while the eastern block is thought to contain only about 159 individuals. The remainder occur in the southern block.
For Meyers, the concern is not simply the amount of forest being lost, but where it's occurring.
"The problem is that the orangutan population in the Eastern Block is already very small," he said. "Once the population drops below about 150 orangutans, it is no longer considered viable."
He warned that continued logging could sever the last remaining forest connections between orangutans in the northern and southern parts of the eastern block: "If the PHAT scheme continues and the forests are cleared, there will no longer be any connectivity between orangutans in the northern and southern parts of the Eastern Block."
Multiple threats, not competing explanations
Although agriculture and PHAT logging account for the largest share of direct forest loss in Batang Toru, the researchers caution against interpreting their findings as evidence that large extractive projects pose only a minor threat to the ecosystem.
Instead, they say the study highlights that the Tapanuli orangutan faces multiple threats operating at different scales, all of which require attention.
Their counterfactual analysis, Sreekar said, showed substantial additional forest loss following the development of the Martabe gold mine, Batang Toru hydropower project, and Sarulla geothermal energy project.
"It shows that Tapanuli orangutans face multiple threats, and that conservation strategies must tackle both large-scale development impacts and the persistent, cumulative effects of small-scale forest clearing," he said. "These are simply two different threats facing Tapanuli orangutans today, and both need to be urgently addressed to prevent extinction."
Among the three projects, the researchers found the strongest direct effects associated with the hydropower project, which has contributed to forest loss through land clearing for infrastructure as well as associated disturbances, including flooding.
The gold mine and geothermal project, by comparison, appear to have had a relatively smaller direct footprint within the Batang Toru Key Biodiversity Area.
Most of the Martabe mining operation, the study notes, occurs just outside the Key Biodiversity Area, limiting its direct contribution to habitat loss within the landscape.
However, the researchers stress that this should not be interpreted as evidence that the projects have negligible conservation impacts.
Road construction, new infrastructure, and an influx of workers associated with large developments can indirectly accelerate forest loss by improving access to previously remote forests and stimulating additional agricultural expansion and logging, they note.
"The extractive industries are still important," Sreekar said. "Their impacts may extend well beyond the areas they directly occupy."
The researchers add that understanding these indirect effects will require further study.
The paper also notes that both the Martabe gold mine and the Sarulla geothermal project have introduced biodiversity offset and conservation initiatives. However, it doesn't evaluate whether those measures have successfully compensated for habitat loss or achieved their intended conservation outcomes.
Protecting forests beyond protected areas
The study also argues that conserving the Tapanuli orangutan will require a broader approach to protecting forests, particularly those that fall outside Indonesia's formal protected area network.
Many of the forests most affected by agricultural expansion and PHAT logging are classified as "areas for other uses," or APL, meaning they're legally zoned for nonforest purposes and generally lack the protections afforded to conservation areas or protected forests.
Yet many of these APL landscapes remain heavily forested and support high conservation value (HCV) habitat, including habitat for the Tapanuli orangutan.
"The condition of the APL forests was actually better than many officially designated forest areas," said Erwin of TaHuKah. "That is precisely why these APL forests deserve protection."
The researchers say the findings highlight a broader challenge across Indonesia: zoning decisions don't always reflect ecological realities on the ground.
Historically, some forests in Batang Toru that now support important orangutan habitat were redesignated from protected forest to APL, leaving them vulnerable to being cleared despite retaining significant conservation value, according to the researchers.
As a result, the researchers argue, conservation strategies focused solely on national parks and other formally protected forests risk overlooking some of the most important remaining habitat for the species.
"Most conservation organizations do not focus on APL areas," Meyers said. "The question is: what tools do we actually have to manage APL areas that contain High Conservation Value forests, including habitat for orangutans and tigers?"
One possible solution, according to Erwin, is the creation of preservation areas, a new conservation designation introduced under Indonesia's revised conservation law in 2024.
Unlike conventional conservation areas, preservation areas can be established outside existing protected areas to safeguard important wildlife habitat or reduce disaster risks.
"They can be established to protect habitat for endangered species or to reduce disaster risks," Erwin said. "Both of these functions are highly relevant in the Eastern Block of Batang Toru."
However, the legal framework remains incomplete. The government has yet to issue the implementing regulations governing how preservation areas will be designated and managed.
According to Erwin, the provincial conservation department of North Sumatra, where Batang Toru is located, is currently preparing an indicative map identifying areas that could qualify as preservation areas.
"We are advocating for the Eastern Block to be included," he said.
Conserving orangutans means supporting communities
Beyond stronger legal protection, the researchers say safeguarding the Tapanuli orangutan will ultimately depend on addressing the economic pressures driving forest loss.
Rather than blaming local communities, they argue that conservation efforts should provide viable alternatives to continued forest conversion through secure land tenure, improved livelihoods, and incentives that reward long-term forest stewardship.
Sreekar said this means conservation cannot focus exclusively on stopping individual development projects. Instead, it must also tackle the gradual, cumulative pressures that continue to erode orangutan habitat across the landscape.
For Erwin, that also means investing more directly in local communities.
"Now, we must invest in solutions that allow both people and nature to thrive, such as securing legal land tenure for local communities to give them a long-term stake in forest protection, supported by financial incentives and community-led monitoring to deter illegal logging," he said.
The study focuses on the Batang Toru ecosystem, which remains the species' primary stronghold. Although researchers recently documented another cluster of Tapanuli orangutans in a peat swamp forest roughly 32 kilometers (20 miles) away, conservationists say protecting Batang Toru remains critical because it supports the overwhelming majority of the known population.
With the vast majority of the global Tapanuli orangutan population concentrated in the Batang Toru landscape, the researchers say addressing both large-scale development and the smaller, cumulative drivers of habitat loss will be essential if the world's rarest great ape is to survive.
Extreme weathers
The need to address these multiple threats has become even more urgent following another recent study, which suggests that landslides triggered by extreme rainfall associated with Cyclone Senyar in November 2025 likely killed about 7% of the estimated global population of Tapanuli orangutans.
The researchers behind the other paper identified more than 50,000 individual landslide scars and estimated that about 8,300 hectares (20,500 acres) of forest in the western block of Batang Toru were affected by the disaster.
The forest loss from this six-day event exceeded the total forest loss recorded across the entire ecosystem between 2000 and 2023, highlighting the unprecedented role of extreme weather, likely amplified by climate change, in altering landscapes, especially in areas already weakened by years of forest loss and fragmentation like Batang Toru.
"Therefore, it is even more urgent to strengthen the resilience of the Batang Toru landscape through forest restoration, protecting local communities and biodiversity from future climate disasters," the researchers noted.
Citations
Sreekar, R., Ardiantiono, Ni'am, L., Meyers, K., Lim, J. Y., Zeng, Y.,... Watson, J. E. M. (2026). Disentangling the impacts of agriculture, logging, and extractive industries on Tapanuli orangutan habitat: A geo-ethnographic assessment. Biological Conservation, 321, 111960. doi:10.1016/j.biocon.2026.111960
Meijaard, E., Wafiy, M., Ni'Mattulah, S., Dennis, R., Hadisiswoyo, P., Sheil, D.,... Wich, S. (2026). Extreme rainfall further endangers the world's rarest great ape. Current Biology, 36(12), 3176-3183.e2. doi:10.1016/j.cub.2026.05.029