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

Abstract

Forest structure, defined as the three-dimensional vertical and horizontal distribution of canopy vegetation, has 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 co-exist. Degradation of forests through anthropogenic factors significantly alters forest structure, and arboreal species such as gibbons are particularly vulnerable to these changes due to 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/km2 and siamang group densities were lower, with 0.0-1.0 groups/km2. 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 (DBH 30-100 cm), tall (20-25 m) trees with a large crown area (100-300m2), 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

Effects of anthropogenic disturbance on group densities of Thomas' langurs (Presbytis thomasi) within a lowland tropical forest, north Sumatra

“…Unlike the two gibbon species ( Hylobates lar and Symphalangus syndactylus ) residing in the survey location (Hankinson et al, 2021) there was no clear relationship determined between forest structural variables and Thomas' langur group densities in any of the 10 survey locations. This may be due to two possible reasons.…”

Assessment of climate change impacts on one of the rarest apes on Earth, the Cao Vit Gibbon Nomascus nasutus

“…to affect the density of other gibbon species and may be important to consider at the site level (Hankinson et al 2021). Also, future studies that include models of other key interacting species in the community, such as fruit trees or macaques, would further inform conservation efforts along the Sino-Vietnamese border.…”

Population densities ofHylobates agilisin forests with different disturbance histories in Ulu Muda Forest Reserve, Malaysia

“…Mean percentage of canopy cover in the vegetation plots, and mean percentage of forest cover in 2000 and proportion of forest lost between 2000 and 2018 from the remotely sensed data were included in the best models for group density, which is consistent with the findings of many other studies indicating the importance of intact forest canopies for other small apes species (e.g., Brockelman & Ali, 1987; Fleagle, 2013; Gittins, 1983; Whittaker, 2009). The density of H. lar (but not S. syndactylus ) also had a positive relationship with size (DBH) and height of trees (Hankinson et al, 2021). Conversely, in the present study, we did not detect an effect of DBH or tree height.…”