The destruction and degradation of the tropical rain forest, particularly lowland forest, in Borneo and Sumatra is the main reason orangutans are threatened with extinction.
This has been caused primarily by human activity (intense illegal logging, conversion of forest to palm oil plantations and timber estates, mining, clearing forest for settlements, and road construction) and also by large-scale fires facilitated by the El Nino weather phenomena. Additionally, the illegal animal trade has been a factor in the decline of wild orangutan populations. Finally, orangutans are occasionally hunted and eaten by some of the indigenous peoples of Borneo as well as migrant loggers and plantation workers who do not have dietary prohibitions against eating primate bushmeat.
At one time the world’s wild orangutan populations likely included upward of hundreds of thousands of individuals, but current estimates indicate far fewer wild orangutans now remain. During the past decade orangutan populations have probably decreased by 50% in the wild. Although past climate shifts may have been responsible for some of this decline, orangutans are primarily threatened by human activities and development that cause the loss and degradation of their forest habitats. Currently, the IUNC has classified the Bornean orangutan as Endangered and the Sumatran orangutan as Critically Endangered
The most recent estimates of orangutan population sizes and distributions can be found at the IUCN Red List of Threatened Species Website. The IUCN Red List assessments show that approximately 7,300 Sumatran orangutans (Pongo abelii) remain in the wild. The most recent estimates for Bornean orangutans (Pongo pygmaeus spp.) fall between 45,000 and 69,000 individuals. These estimates, however, were obtained between the years 2000 and 2003. Since that time there has been substantial orangutan habitat loss on both islands, it is probable that current numbers on both islands are actually below those given on the IUCN Red List.