Habitat Characteristics and Their Effects on the Density of Groups of Western Hoolock Gibbon (Hoolock hoolock) in Namdapha National Park, Arunachal Pradesh, India

Abstract

Understanding the relationship between a species and its habitat is crucial for conservation action planning. The Endangered western hoolock gibbon (Hoolock hoolock) has a fragmented distribution in northeast India, Bangladesh and parts of western Myanmar. Namdapha National Park in Arunachal Pradesh contains the largest population of western hoolock gibbons in India. We carried out an auditory sampling survey and habitat analysis of western hoolock gibbons in the park from September to December 2012 in three potentially suitable forest types: tropical broad-leaved forest, tropical wet evergreen forest, and wet temperate forest. A new method of analysis of singing frequency revealed that only 21–23% of groups sing per day. Group detectability declined sharply beyond a listening radius of 600 m. Auditory sampling across 15 listening areas revealed an estimated mean density of 3.65 groups km−2. We found no significant differences in density among forest types. The habitat study revealed a total of 122 species of trees (girth at breast height ≥30 cm) in the three forest types, representing 73 genera in 41 families, with the highest number of tree species in wet evergreen forest (93) followed by tropical broad-leaved forest (52) and wet temperate forest (40). None of the vegetation traits we measured (mean canopy cover, girth, density, and total basal area of all trees) and no habitat disturbance factors correlated significantly with gibbon density. This lack of correlation may have been due to the prevailing anthropogenic effects that adversely affected gibbons and their habitat, such as forest degradation, road widening, and hunting, overriding the relatively smaller natural variation in vegetation. This study adds to our knowledge of the habitat requirements of hoolock gibbons and indicates that Namdapha National Park is more important to conservation of the western hoolock than previously thought.