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.