Estimates of Orangutan Distribution and Status in Sumatra

Abstract

At the Population and Habitat Viability Analysis (PHVA) Workshop for orangutans, estimates of habitat and population numbers for Sumatran orangutans (Pongo pygmaeus abelii) were derived from field biologists who studied, or are presently studying, orangutans at Ketambe in Gunung Leuser National Park. For Sumatra, the exact boundaries of orangutan distribution are not known, but there are several distinct populations, including the lesser-known Singkil population and the Sembabala-Dolok Sembelin population. The Greater Leuser orangutan population, which extends beyond the national park boundaries, is thought to cover approximately 11,710 km² and has two distinct populations. Using a correction or “safety” factor to derive population estimates, the western population is thought to number 5,700 individuals, and is the most important orangutan population in Sumatra; the eastern population is thought to number 3,500 individuals. Within the restricted boundaries of Gunung Leuser National Park, the area covered by the western population is 5,570 km² and the corrected population size is about 3,450 individuals; the area covered by the eastern population is 2,957 km² and the corrected population size is about 2,400 individuals. The total number of orangutans in the park probably is about 5,850 individuals. The Greater Leuser populations were judged to be among the strongest in Southeast Asia, in terms of numbers and potential for protection. Gunung Leuser National Park was considered to be the most vital habitat to the long-term survival of the Sumatran orangutan.