Forest stucture and group density of Thomas' langur monkey, Presbytis thomasi.

Abstract

Tropical forests contain a substantial portion of global biodiversity, and provide a wide range of ecosystem services. Anthropogenic activities such as logging and agriculture alter the physical structure of forests and thus impact arboreal primates through altered availability of food and suitable sleeping sites and reduced ability to move through the forest canopy. There are many studies detailing the negative impacts of anthropogenic disturbance on primate populations in tropical forests, however few of these adequately explain the mechanisms behind these impacts. This study investigated the structure of disturbed forest at Sikundur, North Sumatra, Indonesia and the group density of an arboreal primate, Presbytis thomasi, in order to identify links between forest structure and primate density. Quantitative data was collected on forest structure from line transects and plots and compared between three land units: alluvial, hills and plains. Group densities of Presbytis thomasi were estimated using line transects and vocal arrays. Prior to this study, vocal arrays had not been applied to P. thomasi, however this method is more effective, especially in disturbed forest where visibility is poor, and primates are unhabituated. Top height and the proportion of large, emergent trees was consistently low throughout the study site due to selective logging in the 1970s and 80s. However, alluvial forest has a significantly higher diameter at breast height, and lower height-DBH ratio than hills and plains, indicating that these trees are more mature. Illegal logging remains a regular occurrence in this area, but appears to be less in alluvial forest, which is less accessible than plains and hills. Plains forest was observed to be the most frequently exploited by local villagers. Group density of P. thomasi was highest in hills forest, which has a significantly higher bole height. Habitat preferences of P. thomasi may be linked to density of suitable sleeping trees and levels of human traffic within the forest.