From the swampy lowlands at 150 meters to the wind-scoured summit of Mount Kerihun at 1,790 meters, the park stacks eight distinct forest types along its altitude gradient. Towering dipterocarps dominate the lowland canopy, their buttress roots spreading wide across the thin tropical soil. Climb higher and the dipterocarps give way to oaks and chestnuts, the air thickening with moisture as hill forest transitions into sub-montane cloud forest. At least 97 species of orchid and 49 species of palm have been cataloged here, though botanists suspect these numbers barely scratch the surface. The park holds 67 threatened plant species and 81 threatened animal species under the IUCN Red List, and initial surveys have documented over 300 bird species, 25 of them endemic to Borneo.

Betung Kerihun shelters the critically endangered Bornean orangutan alongside seven other primate species, from the acrobatic Mueller's Bornean gibbon to the tiny, wide-eyed Horsfield's tarsier. But protection on paper has not translated into safety on the ground. WWF data from 2002 found that roughly 31,000 trees had been illegally felled inside the park. More alarming still, reports documented 10 to 15 orangutans being trafficked every month from West and Central Kalimantan forests to supply markets in Jakarta and Denpasar. For an animal that reproduces slowly, losing one infant to the pet trade can ripple through a local population for decades. The park's vast size, which should be its greatest advantage, becomes a vulnerability when two dozen rangers must cover an area larger than the entire island of Bali.

Long before anyone drew park boundaries, Dayak communities lived along these rivers. Groups including the Iban, Tamambaloh, Kantu, Kayan Mendalam, and Punan Hovongan have inhabited the upper Kapuas watershed for centuries, practicing shifting cultivation and harvesting rattan, resins, and other non-timber forest products. Their relationship with the landscape is intimate and practical: they hunt, fish, and farm in patterns refined over generations. Conservation programs now work with these communities to develop sustainable livelihoods, recognizing that a park this large cannot be protected by rangers alone. The forest's future depends on the people who know it best.

Betung Kerihun shares its northern boundary with Malaysia's Lanjak Entimau Wildlife Sanctuary, a 2,000-square-kilometer reserve that creates a combined protected area spanning the international border. Together, the two reserves have been proposed as a UNESCO World Heritage Site under the name Transborder Rainforest Heritage of Borneo. The concept is elegantly simple: orangutans and hornbills do not carry passports, and a forest fragmented by national boundaries cannot function as a single ecosystem. Whether the nomination succeeds may depend on how well two governments can cooperate on a challenge that neither can solve alone, protecting one of the last great rainforests on an island where the chainsaws have not yet fallen silent.