Population Density and Abundance of Ebony Leaf Monkeys (Trachypithecus auratus) in West Bali National Park, Indonesia

Abstract

Most of the information available on the conservation status of the ebony leaf monkey (Trachypithecus auratus), a species categorized as "Vulnerable" on The IUCN Red List, comes from studies conducted in Java. However, these findings may not be representative of other islands of the Indonesian archipelago, such as Bali. In order to estimate the density and abundance of the ebony leaf monkey population in Prapat Agung Peninsula, located in the northern part of the West Bali National Park, Indonesia, we used repeated line transect distance sampling, a standard method for census surveys of wild animal populations, including primates. The estimated group density, individual density, group size, and total population size were 0.95 group/km2, 7.11 individuals/km2, 7.49 individuals/group, and 422 individuals, respectively. The comparison of these values with those obtained from a previous study conducted 10 years ago in the same area and with the same method showed a marked decrease in population density and abundance as well as changes in the spatial distribution of ebony leaf monkeys. Our data suggest that such trends may be at least partially explained by anthropogenic disturbances, including illegal logging activities and habitat fragmentation. Given these alarming signs, and to better assess trends in the Balinese ebony leaf monkey populations change over time, we urge for the replication of the same survey design in the same study area, at least every five years. Such a survey effort is crucial not only to better understand the socio-ecology of ebony leaf monkeys, but also to determine conservation priorities and devise management plans related to the protection of the populations of this vulnerable primate species in Indonesia.