Survey of Hylobates agilis albibarbis in a logged peat-swamp forest: Sabangau catchment, Central Kalimantan

Abstract

Few data are available on gibbon populations in peat-swamp forest. In order to assess the importance of this habitat for gibbon conservation, a population of Hylobates agilis albibarbis was surveyed in the Sabangau peat-swamp forest, Central Kalimantan, Indonesia. This is an area of about 5,500 km(2) of selectively logged peat-swamp forest, which was formally gazetted as a national park during 2005. The study was conducted during June and July 2004 using auditory sampling methods. Five sample areas were selected and each was surveyed for four consecutive days by three teams of researchers at designated listening posts. Researchers recorded compass bearings of, and estimated distances to, singing groups. Nineteen groups were located. Population density is estimated to be 2.16 (+/-0.46) groups/km(2). Sightings occurring either at the listening posts or that were obtained by tracking in on calling groups yielded a mean group size of 3.4 individuals, hence individual gibbon density is estimated to be 7.4 (+/-1.59) individuals/km(2). The density estimates fall at the mid-range of those calculated for other gibbon populations, thus suggesting that peat-swamp forest is an important habitat for gibbon conservation in Borneo. A tentative extrapolation of results suggests a potential gibbon population size of 19,000 individuals within the mixed-swamp forest habitat sub-type in the Sabangau. This represents one of the largest remaining continuous populations of Bornean agile gibbons. The designation of the Sabangau forest as a national park will hopefully address the problem of illegal logging and hunting in the region. Further studies should note any difference in gibbon density post protection.

Population mapping of gibbons in Kalimantan, Indonesia: correlates of gibbon density and vegetation across the species’ range

“…Gibbon density was estimated using fixed-point counts, as described by Brockelman & Ali (1987) and following the protocol described by Buckley et al (2006), Cheyne et al (2008) and Hamard et al (2010). therland 2000).…”

Density and population estimate of gibbons (Hylobates albibarbis) in the Sabangau catchment, Central Kalimantan, Indonesia

“…In addition, encounters with gibbon groups prior to habituation were used (data from November 2004-June 2005). We obtained a mean group size of 4.05, whereas Buckley et al (2006) reported a mean group size of 3.4, although they recognise that individuals may have been missed, as the gibbons were unhabituated at the time of their study. Lone animals (whose singing is not very predictable) were not included in the quadrangulation analysis, but lone animals sighted by gibbon behaviour teams were included in density calculations.…”

“…Average home-range size for the MSF is calculated as 53 ha [compared with 47 ha reported by Buckley et al (2006)]. This difference in home-range size is not significant and can be attributed to the fact that they found seven more groups in the study area.…”

“…The LPF is poor-quality habitat, and a low to zero density is to be expected. Habituation of gibbons is vital to obtain accurate group sizes (3.4 for Buckley et al (2006) and 4.05 for this study). Whereas auditory sampling is useful for obtaining group locations and number of groups in a study area, without accurate group sizes, population numbers are not as reliable.…”

“…The Sabangau catchment comprises three different forest types: mixed swamp forest (MSF), low pole forest (LPF) and tall interior forest (TIF). Only the MSF has been surveyed for gibbons, by Buckley et al (2006), who estimated the gibbon population in this forest type at about 19,000. The MSF occupies only 40% of the total area of the Sabangau forest (Husson et al 2007).…”