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

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/km in the low suitable habitat and 5.55 groups/km in the high 2 2 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.

Automated summary

This study aimed to update the 2010 survey results, investigate the population size and density of Javan gibbons in the Dieng Mountains, and examine their relationship with habitat characteristics. The results showed a likely increase in the gibbon population, with high group densities in different habitat types and an estimated overall population size of 1092. This suggests some success in conservation efforts over the past decade, but further effort is needed to ensure the long-term future of this threatened species.