METHODOLOGY
Study Area: The study was conducted in Chitwan National Park (27°16.56’- 27°42.14’N, 83°50.23’-84°46.25’E), the first national park situated in south central Nepal covering 953 sq. km. It is a designated Ramsar site and UNESCO world heritage site. The park landscape consists of alluvial plain and mountain range of the outer Himalaya known as Churiya hills. Seasonal and perennial rivers originate in the Churiya and cascade down to the lowlands before being drained by three major river systems. The East Rapti River runs along the northern boundary of park, Reu river runs along the southern boundary of the park, both the rivers mix before being drained to the Ganges by the Narayani river that forms the western boundary of the park. In the east, the park is contagious with the boundary of Parsa National Park . The study area lies in the Indomalaya biogeographic realm and consists of tropical moist forest vegetation dominated by deciduous forest of Shorea robusta (70% ) followed by grassland (10%), riverine forest (7%), mixed forest (7%) and wetlands (4%). The successional gradient of the park is formed of ten grasslands and three forest associations (Lehmkuhl J.F.1999). The area has hot and humid summer monsoon from mid‑May to late‑September when 90% of the 2,400mm of rain falls. The monsoon causes dramatic floods which alter the courses and character of the rivers and thus plays important role in shaping park’s landscape particularly creating and maintaining different successional stages of habitats. Temperatures reach a maximum of 38°C during wet summer and drop to a minimum of 6°C in dry winter. The matrix of different habitat conditions and climate makes this area a hotspot of biodiversity harbouring the largest population of rhinos, tigers and sloth bears and many other threatened flora and fauna in Nepal. The core park area is surrounded by a 750 sq.km buffer zone, which consists of forests, settlements and agriculture lands. The park and the local people jointly initiate community development activities and manage natural resources in the buffer zone. Thirty-to-fifty percent of the park revenue particularly from tourism activities is provided to these buffer zone communities for community development and natural resource management activities. The buffer zone community is socially mixed of indigenous fishing community and immigrated hill communities relying mostly on subsistence agriculture.
Survey Design: We overlaid grids of 4*4 km over the map of study area (Figure 1). With a random start we surveyed in grids at a systematic spacing of 4 km. This checkerboard sampling design minimizes autocorrelation between sampling grids, facilitates concentration of survey efforts, even coverage of large and hostile study area and is suitable for studying medium to large mammals with relatively ease. It has been used to study elephants (Thapa et al. 2019), tigers (Thapa & Kelly, 2017) and antelope (Krishna et al. 2008). This resulted in a total of 45 grids which covered 720 sq.km (43% coverage of park area). Our grid size is comparable to the home range of sloth bears which is estimated as 9 and 14 sq.km for male and female sloth bears respectively (Joshi et.al 1995). Studies on sloth bears in India have shown that the spatial replicates produce occupancy estimates similar to the temporal replicates and thus are useful for distribution assessments of species when field resources are limited or logistic challenges preclude traditional survey approaches that yield temporally replicated data (Srivathsa et al.2017). We used four spatial replicates of one km length in each grid.
Field Methods: We carried out sign surveys within the selected 45 grids with a sampling effort of four km in each grid. Two observers walked in the grid and recorded both direct sightings and indirect signs such as scat and footprint found within the visible width of the search trail. We divided this search trails or replicates into equal distanced segments of 200 m. A segment was recorded as occupied ‘1’ if we made a direct visual sighting or if we detected the footprint or scat of sloth bears. Sloth bears are often found to defecate in open areas along the road, over the bridges and on the ‘machans’ wildlife view towers and other exposed surfaces. Absence of black bears, sun bears or other species of similar size and habit in the study area enabled us in unambiguous identification of the sloth bear signs in the field. To standardize the detection process, avoid biases that may arise from misidentification and decay of signs and adhere to the closure assumption in occupancy studies, we only included fresh sloth bear signs – direct sighting, footprints, and scat along sample trails ( Putman R.J. 1984; Morin et al.2016; Rota et al. 2009; Laing et al. 2003). Field surveys were carried out between March and June of 2020. Within the segments, we collected sloth bear presence/absence data and associated ecological, environmental and anthropogenic variables. When the signs and covariates were detected in the segment it was recorded as ‘1’ and ‘0’ otherwise. If the sampling could not be done because of logistic reasons, outside park jurisdiction or under intense human use, the segment was treated as a missing observation.