Sympatric Gibbons in Historically Logged Forest in North Sumatra, Indonesia

Abstract

The complex structural canopy of tropical forests is extremely important for the survival and continued presence of arboreal primates. The destruction and degradation of tropical rainforest on the Indonesian island of Sumatra is causing significant declines in the endemic gibbon species residing in these shrinking habitats. This chapter compares recent density estimates of the lar gibbon (Hylobates lar) and the siamang (Symphalangus syndactylus) in a historically logged area of lowland forest, Sikundur, north Sumatra, to range-wide densities of both species and the ecologically similar agile gibbon (Hylobates agilis) across the island. Density estimates for Sumatran gibbon species are largely influenced by altitude and habitat preference. Siamang densities in Sikundur were similar to previously obtained range-wide densities, whereas lar gibbon densities were lower than their reported natural density range. Sikundur’s degraded forest, consisting of reduced tree heights and low tree connectivity, has potentially impeded the ability of the lar gibbon to attain higher densities. However, the presence of these small apes in this degraded lowland forest, albeit at lower densities, demonstrates that these areas can still be important habitats for gibbons, and emphasises the importance of ongoing regeneration of previously degraded forest for the future survival of these species.