Population of the Javan Gibbon (Hylobates moloch) in the Dieng Mountains, Indonesia: An updated estimation from a new approach

Abstract

The Javan gibbon (Hylobates moloch) is endemic to the island of Java and its distribution is restricted from the western tip of Java to the Dieng Mountains in Central Java. Unlike the other known habitats that hold a large population of Javan gibbons, the Dieng Mountains have not been protected and experience various threats. This study, which was conducted in 2018 and 2021, aimed to provide an update of the current density and population size of Javan gibbons in Dieng after the most recent study in 2010 and to investigate their relationships with habitat characteristics (vegetation and elevation). The triangulation method and a new acoustic spatial capture-recapture method were used to estimate group density. A new approach for extrapolation, based on the habitat suitability model, was also developed to calculate population size. The results show that the Javan gibbon population in the Dieng Mountains has most likely increased. The mean group density in each habitat type was high: 2.15 groups/km2 in the low suitable habitat and 5.55 groups/km2 in the high suitable habitat. The mean group size (3.95 groups/km2, n = 20) was higher than those reported in previous studies. The overall population size was estimated to be 1092 gibbons. This population increase might indicate the success of conservation efforts during the last decade. However, more effort should be made to ensure the long-term future of this threatened species. Although the density significantly differed between habitat suitability types, it was not influenced by the vegetation structure or elevation. A combination of multiple variables will probably have a greater effect on density variation.

The Javan Gibbon (Hylobates moloch) Habitat Changes and Fragmentation in the Dieng Mountains, Indonesia

“…The time points of the satellite imageries were selected on the basis of the three survey years of the javan gibbon population conducted in Dieng mountains (Nijman & van Balen, 1998;Setiawan et al, 2012;Widyastuti et al, 2023Widyastuti et al, ), which were 1994Widyastuti et al, -1995Widyastuti et al, , 2009Widyastuti et al, -2010Widyastuti et al, , and 2021. Henceforth, these representing years were simplified to be 1994, 2009, and 2021.…”

“…This is identified based on the LULC maps and large roads that have been dissecting the forest. It then overlaid by the habitat suitability model for javan gibbon from previous study on the forest blocks layer (Widyastuti, 2021;Widyastuti et al, 2023) to identify the suitable forest blocks for the javan gibbon. The connectivity between the suitable blocks were characterized based on the relative distance to each other and the presence of existing corridor, which is plantation forests.…”

“…Dieng mountains remains the largest rain forest in Central Java and hold the second largest populations of javan gibbon after Gunung Halimun-Salak NP. Contrastingly, among the other largest habitat it is not protected under conservation area (Nijman & Van Balen, 1998;Nijman & Setiawan, 2001;Nijman, 2004;Setiawan et al, 2012;Widyastuti et al, 2023). Moreover, the landscape consisted of heterogenous mosaic land-uses and receive a greater risk to forest conversion and fragmentations (Nijman, 2004).…”