Group density estimates for lar gibbons (Hylobates lar) and siamangs (Symphalangus syndactylus) and the quantitative analysis of the vegetative characteristics of the Sikundur field site.

Abstract

Southeast Asia’s tropical forests encompass 20-25% of the world’s plant and animal species. However, at the moment, this region is in the midst of a biodiversity crisis since it is experiencing a higher rate of forest loss than any other region. Indonesia has one of the highest deforestation rates globally, it experienced a total loss of 0.84 Mha of primary forest cover in 2012, with 51% of this loss occurring in lowland forests. Habitat loss and degradation challenge the survival and persistence of all forest vertebrates and create critical problems for arboreal animals, such as gibbons and siamangs. Gibbons and siamangs!are arboreal primates, dependent on closed continuous canopies for feeding and travelling. Apart from information from the Sabangau Catchment in central Kalimantan, there is very limited data available for these primate populations across Indonesia. This study aims to identify which forest structure components are important indicators of habitat suitability for lar gibbons and siamangs. Lar gibbon and siamang group densities were determined at a field site in Sumatra, Indonesia, and it was investigated whether there was a difference across three disturbed land unit types (Alluvial, Plains, Hills) in gibbon and siamang densities and vegetation characteristics Primate densities were calculated using auditory sampling methods at three sets of listening posts, while forest structure was analysed for 30 500m long line transects and 15 25 x 25m plots. Primate group densities were calculated using spatially explicit capturerecapture models. The results showed that the Alluvial land unit had the highest primate densities, as well as larger trees and greater availabilities of various branch sizes than the other land units. Furthermore, the Plains showed lower frequencies of tall trees >25m and had the lowest gibbon and siamang densities. These first analyses at this site support the prediction that the most suitable habitat for the highly arboreal gibbons and siamang contains mature large trees with a variation in branch supports.