Borneo is the third-largest island in the world and the largest in Asia. At the geographic centre of Maritime Southeast Asia, in relation to major Indonesian islands, it is located north of Java, west of Sulawesi, and east of Sumatra.
The Borneo rainforest is estimated to be around 140 million years old, making it one of the oldest rainforests in the world. 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 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, 221 species of terrestrial mammals and 420 species of resident birds in Borneo. There are about 440 freshwater fish species in Borneo, The Borneo river shark is known only from the Kinabatangan River. In 2010, the World Wide Fund for Nature stated that 123 species have been discovered in Borneo.
The WWF has classified the island into seven distinct ecoregions. Most are lowland regions:
The highest elevations of Mount Kinabalu are home to the Kinabalu mountain alpine meadow, an alpine shrubland notable for its numerous endemic species, including many orchids.
According to analysis of data from Global Forest Watch, the Indonesian portion of Borneo lost 10.7 million hectares of tree cover between 2002 and 2019, of which 4 million hectares was primary forest, compared with Malaysian Borneo's 4.4 million hectares of tree cover loss and 1.9 million hectares 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.