Since early August, Borneo wildfires have torn through fragile rainforests in Borneo and Sumatra, spreading toxic haze across Indonesia and affecting communities throughout Kalimantan, the Indonesian region of the world’s third-largest island.
The fires are being driven by El Niño-related shifts that have brought severe drought and parched conditions to the island’s peatlands, which are among the planet’s most effective carbon sinks and critical habitat for wildlife found nowhere else.
Sebangau National Park in Central Kalimantan hosts an estimated 6,000 to 9,000 critically endangered Bornean orangutans, one of the largest remaining populations. According to the nonprofit Borneo Nature Foundation, a fire crossed the park boundary for the first time in early September.
“Our teams pushed it back, and it was largely confined to sedge and open peat, but some reforestation sites are impacted,” said Dr. Susan Cheyne, the foundation’s science director and vice chair for the International Union for the Conservation of Nature’s special project group for small apes. “Our priority is to keep fire out of the forest, because that protects every orangutan in it at once.” She said capturing and relocating apes is a last resort as volunteer crews work to keep flames away from habitat and nearby villages.
Sebangau is also a refuge for some of the world’s rarest primates, including the largest known population of Bornean white-bearded gibbons, an ape Cheyne has studied for more than two decades.
Gibbons are famed for their morning songs. Males sing solo at dawn, with females joining later for duets.
Cheyne said that during fires in 2006 gibbons sang less often and for shorter periods on smoky days, a concern because song is essential for territory defense and pair bonds. She noted that a similar pattern has since been documented in wild Bornean orangutans exposed to wildfire smoke.
Gibbons rarely descend to the ground, and survivors are often stranded in small, isolated forest patches with limited food. Other Southeast Asia endemics, including red langurs and long-tailed and southern pig-tailed macaques, also face shrinking numbers with each blaze. Little is known about fire impacts on nocturnal slow lorises and western tarsiers, Cheyne added.
The park’s vast peat swamps consist of partially decomposed plant matter that has accumulated over millennia. In parts of Sebangau, the peat dome reaches about 12 meters deep.
Peatlands are naturally waterlogged and typically do not burn. But in the 1990s and early 2000s, illegal loggers dug hundreds of canals to float timber out. Those channels continue to drain the swamps, drying the peat and turning it into fuel.
“In a long dry season like this one, a dried-out peat swamp is a tinderbox,” Cheyne said.
She explained that peat fires are especially difficult to fight because they burn downward and laterally through the subsurface, smoldering underground for weeks after surface flames fade. These so-called zombie fires can be nearly invisible from above, sometimes showing only a thin wisp of smoke while peat burns a meter below, igniting tree roots and toppling trees. The fires also produce dense, toxic smoke.
Since 2015, the foundation and local partners have built more than 1,000 dams to block drainage canals, planted nearly a million trees, drilled hundreds of water boreholes and expanded support for up to eight fire teams.
“That preparation is why we are holding,” Cheyne said. Still, conditions remain fragile. In the short term, she said, keeping crews fed, equipped and safe costs about 100 dollars per team per day. Priorities include equipment such as thermal drones to detect underground peat fires and rewetting peat to prevent reignition. The foundation is seeking support from international organizations, advocacy groups and individual donors.
Borneo wildfires strain primate habitats as rains lag
Hotspots in Kalimantan have declined since the start of the month, according to the foundation, but significant rain is not expected until late October or November.
“We are not out of this yet,” Cheyne said.
She added that the foundation’s co-founder, Simon Husson, recently observed a troop of monkeys moving through trees along miles of scorched riverbank during a river journey.
“That is the picture,” she said. “Life hanging on right at the edge.”
