Effect of human disturbance on the group size and population structure of geladas (Theropithecus gelada) in and surrounding area of Borena Sayint National Park, Ethiopia

Abstract

Little is known about the effect of anthropogenic habitat disturbances on the group size and population structure of geladas (Theropithecus gelada), even though such data are crucial to implementing successful conservation actions and management plans. In this study, we compared the group size and composition of gelada populations inhabiting protected (Borena Sayint National Park) and unprotected habitats that experience different levels of human disturbances in Wollo, Ethiopia. We collected gelada population data using the total count method to determine the population number and to examine the influence of anthropogenic disturbances on the group sizes and social systems (band, all-male unit (AMU), or one-male unit (OMU)) of geladas. Overall, we counted a total of 2786 individuals of geladas belonging to 59 groups in both protected and unprotected habitats. The mean band size of geladas in the protected habitat was 81.6 individuals, while it was 40.1 in the unprotected habitat. In the protected habitat, the mean number of adult males per group (including both bands and OMUs) was 5.7, while it was 3.1 in the unprotected habitat. In the protected site, the mean number of immatures per group was 21.5, whereas it was 10.5 in the unprotected habitat. Likewise, the mean number of adult females per group in the protected habitat was nearly twice that of the unprotected site. However, the proportion numbers of age/sex compositions were not significantly different between the two habitats. In the protected habitat, the overall average adult male to adult female sex ratio was 1:5.1, while it was 1:4.8 in the unprotected area. The band size of geladas in the protected habitat was significantly and positively correlated with the altitude. Our study shows that geladas can withstand anthropogenically disturbed habitats by adjusting their band sizes to the optimal level. Such flexibility in group size is an adaptation strategy of geladas for maximizing foraging efficiency in response to broad grassland habitat losses and degradations. Although geladas are flexible in their population ecology, it is crucial to implement appropriate conservation measures to ensure their long-term persistence in the human-modified landscapes of the Ethiopian Highlands.