contained them in the first place, for ecological reasons. Admin- istratively, 78% of the species’ present range lies within Aceh, and the remaining 22% in North Sumatra (Map 4). A total of 13 districts in Aceh, and eight in North Sumatra, contain forests where wild Sumatran orangutans still occur.
Te orangutan’s distribution on the island is not contiguous, due to both natural and man-made features. Te main natu- ral geographic barriers to orangutans are rivers, which they can only cross if narrow enough to be bridged by fallen trees or
through canopy connections, and high mountain ranges, which these lowland forest animals tend not to cross. Human activities have resulted in forests becoming fragmented, e.g. by roads and plantations, which likewise fragment the orangutan populations they contain into ever smaller and more isolated forest patches.
Approximately 78% of the area where the remaining wild Suma- tran orangutans occur lies within the Leuser Ecosystem (Wich et al. 2008), a 2.6 million hectare conservation area (Map 5). Tis vast area encompasses the smaller Gunung Leuser Nation-