Drivers of high density and implication of sterilization to control long-tailed macaque populations in Bali, Indonesia

While declining globally, the long-tailed macaque (Macaca fascicularis) is paradoxically
conspicuous in anthropogenic habitats. In some areas, populations can even be found in high density, a scenario often associated with conflict with humans calling for sustainable management measures. Reproduction control via sterilization is increasingly used as a management practice to control the size of locally “overabundant” populations. Despite a growing interest in this practice, empirical data are critically lacking to rigorously examine its efficiency. Using the Balinese macaque as a study model, our first objective was to demonstrate which factors drive high densities by comparing demographics and ecological conditions of 10 study groups. We found that group size increased as a function of the proportion of human food consumed by the macaques, and group density inflated with habitat anthropization (LMs, p < 0.05). Our second objective was to provide an overview of a 5-year sterilization program conducted with one large population (density: 47 ind/ha) in Ubud Monkey Forest. Prior to the intervention, a stochastic matrix population model indicated that 47% of the reproductive females needed to be sterilized to achieve a null population growth. Over 5 years, we sterilized 187 females, i.e. 54% of the reproductive females. Demographic projections for the next 25 years indicated that regular and equivalent sterilization efforts must be undertaken every 4 years to maintain a stability of the population size. This study provides crucial support to the management decisions and demonstrates the strict requirements of sterilization to control population size.