Spatial distribution and density of the Lar gibbon Hylobates lar and Siamang Symphalangus syndactylus in relation to canopy structure and disturbance in a lowland dipterocarp forest, Sumatra.

Abstract

Forest structure is an important factor in the persistence of arboreal primates, and understanding the relationship between primate populations and habitat characteristics is essential for their conservation. Indonesia is experiencing one of the highest rates of deforestation globally, and with such a large amount of historical logging having taken place, only ~3.8% of the remaining forest is classified as primary. Sumatran lowland forests are one of the most threatened habitat types, though few studies have been conducted on primate populations within this habitat. The aim of the study was to ascertain forest vegetation structure between four different land units (hills, plains, alluvial and hill-swamp), and identify which structural variables are important indicators of habitat suitability for the two sympatric gibbon species, the lar gibbon Hylobates lar, and the siamang Symphalangus syndactylus. The field study was conducted from 1st March to 1st August 2016 within the Sikundur region of the Gunung Leuser National Park, North Sumatra, where both large and small-scale logging practices have been undertaken. At ten forest survey locations, auditory sampling methods using fixed points counts were used to produce hylobatid density estimations, and 4-6 vegetation plots per site were undertaken to identify differences in forest structure. Lar gibbon densities ranged from 1.19 – 3.56 groups/km2 and were found to be correlated with median tree height and crown area (F(9)= 5.604, p = 0.031, R2 = 0.459), and negatively correlated with frequency of trees <20m (F(9)= 6.775, p = 0.035, R2 = 0.616), and with canopy connectivity <15% (F(9)= 22.45, p =0.001, R2 = 0.737). Siamang densities ranged from 0.40 – 2.11 groups/km2, and were correlated with frequency of trees between 10-20m in height (F(9)= 5.576, p = 0.046, R2 = 0.411), and with canopy connectivity between 50% -75% (F(9)= 5.930, p = 0.041, R2 = 0.426). These results indicate these hylobatid species exhibit a degree of tolerance and behavioural flexibility to habitat disturbance, though the preservation of tall trees and the provision of a significant amount of canopy connectivity is required for their continued presence in lowland forests. It is hoped that the results of this study will contribute to the preservation of Sumatran’s lowland forest, and the gibbon species that inhabit them.