Population status of the Bornean orang-utan (Pongo pygmaeus) in the Sebangau peat swamp forest, Central Kalimantan, Indonesia H. Morrogh-Bernard a, *, S. Husson a , S.E. Page b , J.O. Rieley c a 38 Ashton Road, Siddington, Cirencester, Gloucestershire GL7 6HP, UK b Department of Geography, University of Leicester, Leicester LE1 7RH, UK c School of Geography, University of Nottingham, Nottingham NG7 2RD, UK Received 5 June 2002; received in revised form 5 June 2002; accepted 6 June 2002 Abstract A survey of the density and population size of Bornean orang-utan (Pongopygmaeus) was carried out in 1995 and 1996 in an area of peat swamp forest in the Sungai (River) Sebangau catchment, Central Kalimantan, Indonesia. Densities were calculated for four forest sub-types by counts of orang-utan sleeping platforms (nests) along line transects. Densities were found to be highest in the tall interior and mixed swamp forest sub-types. Low pole forest supported the lowest density. Habitat disturbance caused by log- ging was shown to affect orang-utan density within mixed swamp forest. The orang-utan population for a larger peat covered landscape unit (9200 km 2 ), including the Sebangau catchment, was estimated to be between 5671 (  955) and 8951 (  1509) indi- viduals, based upon the area of each forest type, the level of disturbance in each area and corrected to prevent overestimates. This study identifies the presence of a very large, self-sustaining orang-utan population in this region and emphasises the urgent requirement for greater protection of Kalimantan’s peat swamp forests in the light of recent and rapid habitat degradation. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Orang-utan, Peat swamp forest, Kalimantan, Population estimate, Nest count survey, Conservation 1. Introduction The orang-utan (Pongo spp.) was once widespread in Asia but is now found only on the islands of Borneo and Sumatra where it exists as two distinct species (Pongo pygmaeus and Pongo abelii), re-classified from two sub-species in 1999 (Groves, 1999). The Bornean orang-utan (Pongo pygmaeus) is believed to occur as three sub-species, P. p. pygmaeus, P. p. wurmbii and P. p. morio (Groves, 1999), although Warren et al. (2001) suggest four sub-populations that may or may not be distinct enough to be classified as sub-species. Whatever the case, current knowledge about the distribution, population size and conservation status of all sub-spe- cies/sub-populations is incomplete. In 1995, the popu- lation of the Bornean orang-utan was estimated to be between 10,200 and 15,500 individuals (Rijksen et al., 1995). The uncertainty surrounding these estimates arises from an incomplete knowledge of species distribution, combined with evidence that suggests population numbers are undergoing continuous decline (Soemarna et al., 1995; Rijksen and Meijaard, 1999). The orang-utan is known to inhabit primary and secondary forest and is typically found in lowland dipterocarp, freshwater and peat swamp forests. It has also been recorded in hill forests up to about 1500 m although at much lower densities than in other habitats (MacKinnon, 1974; Rijksen, 1978; Payne, 1988; Payne and Andau, 1989; Rijksen and Meijaard, 1999). All of these habitats are reducing in extent as a result of degradation and loss, principally from timber extraction (legal and illegal), forest fires and forest clearance for agriculture and settlement (Wilson and Wilson, 1975; EIA, 1998; Rijksen and Meijaard, 1999; Yeager, 1999). Reduction in habitat area, hunting of orang-utan for bushmeat and the capture of young animals for the pet trade, have greatly increased the vulnerability of the Bornean orang-utan in recent decades leading to its classification as an endangered species by the World Conservation Union (IUCN, 2000). It is also included 0006-3207/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(02)00186-6 Biological Conservation 110 (2003) 141–152 www.elsevier.com/locate/biocon * Corresponding author. E-mailaddress: hmorroghbernard@yahoo.com (H. Morrogh-Bernard).