Influences of Forest Structure on the Density and Habitat Preference of Two Sympatric Gibbons ( Symphalangus syndactylus and Hylobates lar )

Forest structure, defined as the three-dimensional vertical and horizontal distribution of canopy vegetation, has a great influence on the distribution patterns and abundance of forest primates. The complexity of this structural canopy produces a diverse range of microhabitats and distinct ecological niches, allowing ecologically similar species to coexist. Degradation of forests through anthropogenic factors significantly alters forest structure, and arboreal species such as gibbons are particularly vulnerable to these changes because of their reliance on canopy for survival. We investigated how forest structural variables influenced the density of two sympatric gibbon species (siamangs, Symphalangus syndactylus and lar gibbons, Hylobates lar ) in Sikundur, a historically disturbed tropical lowland forest in north Sumatra. We used auditory sampling to establish group density in 10 locations and assessed structural characteristics of forest within 4–6 vegetation plots in each location. Lar gibbon group densities were 0.53–3.10 groups/km 2 and siamang group densities were lower, with 0.0–1.0 groups/km 2 . The densities of both species were positively influenced by median height of first bole and the percentage of canopy connectivity. Lar gibbon group density was positively related to large (diameter at breast height 30–100 cm), tall (20–25 m) trees with a large crown area (100–300 m 2 ), while siamang group density showed no significant relationships with these variables. These findings show canopy connectivity and height to first bole are significant structural variables for the continued presence of both gibbon species although, ecologically similar species can be influenced by different levels of structural disturbance. This canopy structural variation between sympatric species existing in the same ecosystem allows for home range overlap, low interspecific competition, and coexistence.