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Ecology

The Borneo rainforest is estimated to be around 140 million years old, making it one of the oldest rainforests in the world. The current dominant tree group, the dipterocarps, has dominated the Borneo lowland rain forests for millions of years. It is the centre of the evolution and distribution of many endemic species of plants and animals, and the rainforest is one of the few remaining natural habitats for the endangered Bornean orangutan. It is an important refuge for many endemic forest species, including the Borneo elephant, the eastern Sumatran rhinoceros, the Bornean clouded leopard, the Bornean rock frog, the hose's palm civet and the dayak fruit bat.

Peat swamp forests occupy the entire coastline of Borneo. The soil of the peat swamp is comparatively infertile, while it is known to be the home of various bird species such as the hook-billed bulbul, helmeted hornbill and rhinoceros hornbill. There are about 15,000 species of flowering plants with 3,000 species of trees (267 species are dipterocarps), 221 species of terrestrial mammals and 420 species of resident birds in Borneo. There are about 440 freshwater fish species in Borneo (about the same as Sumatra and Java combined). The Borneo river shark is known only from the Kinabatangan River. In 2010, the World Wide Fund for Nature (WWF) stated that 123 species have been discovered in Borneo since the " Heart of Borneo " agreement was signed in 2007.

The WWF has classified the island into seven distinct ecoregions. Most are lowland regions:

According to analysis of data from Global Forest Watch, the Indonesian portion of Borneo lost 10.7 million hectares (26 million acres) of tree cover between 2002 and 2019, of which 4 million hectares (9.9 million acres) was primary forest, compared with Malaysian Borneo's 4.4 million hectares (11 million acres) of tree cover loss and 1.9 million hectares (4.7 million acres) of primary forest cover loss. As of 2020, Indonesian Borneo accounts for 72% of the island's tree cover, Malaysian Borneo 27%, and Brunei 1%. Primary forest in Indonesia accounts for 44% of Borneo's overall tree cover.