Lost in fragmentation: Occurrence of Capped Langur Trachypithecus pileatus in Upper Brahmaputra Valley of Assam

Historical deforestation of lowland tropical forests in the Upper Brahmaputra Valley of the Indian state of Assam has led to habitat fragmentation, negatively impacting the ecology of these natural ecosystems. Local populations of arboreal species, such as Capped Langurs (Trachypithecus pileatus) , have become confined to isolated fragments and are at risk of disappearing entirely due to ongoing anthropogenic activities. This study aimed to assess changes in the occupancy of Capped Langurs across 40 rainforest fragments between February 2019 and January 2020 and to identify the ecological and landscape-level factors influencing their persistence. Using generalised linear models, we examine the attributes at both patch and landscape levels that influence the occurrence of Capped Langurs in these fragments. Of the 40 surveyed sites, we found Capped Langurs only in 11 fragments. We discovered that the diversity of food tree species in these fragments had the most significant impact, explaining the presence of Capped Langur troops. Our findings highlight the urgency of targeted habitat restoration, emphasising the need to enhance and protect food and roost tree diversity in these degraded fragments. This will ensure a rich composition of native food and roosting trees to support Capped Langurs in these rainforest fragments.