Article

Population Survey of Bornean White-Bearded Gibbon, Hylobates albibarbis,in Two Selective Logging Concessions in Central Kalimantan and West Kalimantan

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Abstract

We surveyed Bornean white-bearded gibbon, Hylobates albibarbis, population densities in lowland and hill dipterocarp forests within the Sari Bumi Kusama and Suka Jaya Makmur logging concessions. These surveys were conducted from 20 to 30 March 2018 in Sari Bumi Kusuma areas and 14 to 24 April 2018 in Suka Jaya Makmur. We used a fixed-point count method to detect and plot morning song bouts of gibbon groups at systematically placed listening posts. At each location in Sari Bumi Kusuma and Suka Jaya Makmur, 18 listening posts were established, and 2 observers noted and plotted gibbon morning great calls. Gibbon group density was higher at Suka Jaya Makmur than at Sari Bumi Kusuma; we estimated that there were 2.29 groups per square kilometre in the former area and 1.86 groups/km2 in the latter. Since Bornean white-bearded gibbons are strictly protected by the Government of Indonesia and listed as an Endangered species by the International Union for Conservation of Nature (IUCN), we need better management practices to protect the populations of this species within production forests (outside protected areas).

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... All of the 20 recognized species of gibbons give loud songs and in all but two, male and female components are organized into rather stereotyped duets which signal the presence of breeding groups in fixed territories. At the end of a day of listening, compass directions and times of calls from different LPs are used to map the locations of groups through "triangulation," and population density is estimated by dividing the number of mapped groups by the "listening area," the area within which all groups can be heard (e.g., Brockelman & Srikosamatara, 1993;Brockelman et al., 2009;Buckley et al., 2006;Cheyne et al., 2008;Estrada et al., 2004;Hamard et al., 2010;Nijman & Menken, 2005;Nongkaew et al., 2017;O'Brien et al., 2004;Phoonjampa & Brockelman, 2008;Rawson et al., 2009;Ray et al., 2015;Traeholt et al., 2005;Yanuar et al., 2019). Because gibbons do not duet every day, it is necessary to estimate a density correction factor consisting of the probability of a group singing per day, or over a period of days (i.e., its "availability" for detection) to prevent underestimation of density. ...
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Effective conservation demands more accurate and reliable methods of survey and monitoring of populations. Surveys of gibbon populations have relied mostly on mapping of groups in “listening areas” using acoustical point‐count data. Traditional methods of estimating density in have usually used counts of gibbon groups within fixed‐radius areas or areas bounded by terrain barriers to sound transmission, and have not accounted for possible decline in detectability with distance. In this study we sampled the eastern hoolock gibbon (Hoolock leucogenys) population in Htamanthi Wildlife Sanctuary (WS), Myanmar, using two methods: the traditional point‐count method with fixed‐radius listening areas, and a newer method using point‐transect Distance analysis from a sample point established in the center of each listening point array. The basic data were obtained by triangulating on singing groups from four LPs for 4 days, in 10 randomly selected sample areas within the sanctuary. The point transect method gave an average density of 3.13 groups km⁻², higher than the estimates of group density within fixed‐radius areas without correction for detectability. A new method of analysis of singing probability per day (p[1]) gave an estimate of 0.547. Htamanthi WS is an important conservation area containing an estimated 7000 (95% confidence interval: 5000–10,000) hoolock groups. Surveys at Htamanthi WS and locations in the Hukaung Valley suggest that the extensive evergreen forests in northern Myanmar have the capacity to support 2–4 (average about 3) groups of hoolock gibbons per km², but most forests in its range have yet to be surveyed. Research Highlights • Surveys of gibbon populations employing better statistical methods yield higher population estimates than older, more “traditional,” methods. • Htamanthi Wildlife Sanctuary in northern Myanmar is an important refuge for this Vulnerable ape species.
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