Abstract
The first comprehensive survey of gibbons (Hylobates spp.) across Indonesian Borneo was carried out over 3 years to (1) determine whether densities of gibbon species are cor- related with vegetation characteristics, and if so, whether the same characteristics are correlated with density across all forest types; and (2) determine population densities in the survey areas and identify threats to the areas. To achieve this, a total of 8 forest blocks were surveyed, involving 53 independent survey locations and repeat surveys in 3 forest blocks. Our data show that gibbons are ubiquitous where there is forest; however, the quality of forest affects population density, for- est block size affects longevity of populations, and populations are susceptible to the 'compression effect', i.e. populations occupy smaller fragments at unsustainably high densities. We show the effects of forest disturbance (logging, fire, fragmentation) on gibbon distribution and density and highlight issues for long-term conservation. We discuss the use of minimum cross-sectional area, habitat variables and presence of top foods to determine population density and to identify a threshold below which gibbons cannot persist. We discuss the conservation issues facing all Bornean gibbons, including natural hybrids (H. muelleri × H. albibarbis). The answers to these research questions will help mitigate threats to gibbons and their habitat, as well as identify key habitat for gibbon populations within and outside the protected area network.
Highlights
Studies around the world have found that primate densities are influenced by the quality of their habitat
The first comprehensive survey of gibbons (Hylobates spp.) across Indonesian Borneo was carried out over 3 years to (1) determine whether densities of gibbon species are correlated with vegetation characteristics, and if so, whether the same characteristics are correlated with density across all forest types; and (2) determine population densities in the survey areas and identify threats to the areas
We show the effects of forest disturbance on gibbon distribution and density and highlight issues for long-term conservation
Summary
Studies around the world have found that primate densities are influenced by the quality of their habitat. No additional equipment is necessary to generate detection histories over several seasons, and modelling can provide information on the probability of detection of gibbons in different areas This method does not yet have a provision for calculating the proportion of lone, non-singing gibbons in the population, so accurate density estimates are not possible. As yet there are no case studies using this method for gibbons, but this method is consistently used in camera-trap studies where fixed points (cameras) are used to survey the density of independently distributed, home-range occupying large mammals This method could potentially remove some of the issues with surveying gibbons in more mountainous terrain, but the modelling and analysis will require experts. We hope that the answers to these research questions will help inform Indonesia’s management of protected areas in order to mitigate threats to gibbons and their habitat, as well as identify key habitat characteristics that are important to the survival of gibbon populations within and outside the protected area network
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