Ecological determinants of occupancy of golden langur Trachypithecus geei and its population characteristics in India

Habitat loss and fragmentation significantly contribute to the population decline of endangered species and pose conservation challenges. The canopy-living primates are highly susceptible to such habitat loss or fragmentation. Golden langur is one such primate with limited occupancy. In the present study, we aimed to identify factors influencing the occurrence of the endangered golden langur which is confined to a narrow region of north-western Assam, India, and its population characteristics using an occupancy framework. Ten teams of 4–5 people walked 1772.11 km in 1363 grid cells of 50 ha in size. We constructed detection histories of the species for each grid from three replications. We obtained 740 detections of golden langurs and estimated the probability of occupancy to be 0.57 ± 0.04SE in Manas Biosphere Reserve (BR) and 0.69 ± 0.08SE in the southern fragments. The moist deciduous forests positively and rainfall negatively influenced the occupancy probability in Manas BR, while forest cover and canopy height positively influenced the occupancy of golden langurs in the southern fragments. The encounter rate of golden langur in Manas BR was 0.37 groups/km and 0.67 groups/km in the southern fragments. The mean group size of the multi-male multi-female groups was 11± 4 SD (10.71± 4.72 SD in Manas BR and 10.60 ± 3.81 SD in the southern fragments). The mean group size and composition did not vary between the two areas indicating the ability of the species to adapt to changing habitat conditions. Fragment size did not show any relation between the percent grid cells with the langur occurrence, encounter rate, and mean group size. The unique langur group count was multiplied by the mean group size and calculated the minimum population size of golden langurs in India to be 7396 langurs. Since golden langur is highly folivorous, their persistence depends on the availability of suitable habitats, thus, the management of moist deciduous forests and contiguity of the canopy is crucial to ensure their survival.