Seasonal and Habitat-Specific Density of Sambar (Rusa unicolor) in Achanakmar Tiger Reserve, Chhattisgarh, Central India

Abstract

The sambar (Rusa unicolor) is the largest Indian cervid and a principal prey species of the tiger
(Panthera tigris), yet it is listed as Vulnerable on the IUCN Red List because of widespread
population declines. Reliable, season- and habitat-specific density estimates are needed to
evaluate prey availability in tiger reserves. We estimated the density and group density of
sambar in Achanakmar Tiger Reserve (ATR), Bilaspur, Chhattisgarh, using line transect
distance sampling over one complete annual cycle (April 2025 to March 2026). Twenty-five 2-
km transects (five in each of five habitats: mixed forest, sal forest, bamboo forest, teak
plantation and grassland) were walked in three seasons (post-monsoon, winter and summer).
Detection functions were fitted with a uniform key function with cosine adjustment, which was
selected for all seasons. Density increased from 2.18 ± 0.59 individuals/km² in the post-
monsoon season to 2.87 ± 0.38 in winter and 3.01 ± 0.48 in summer; group density followed
the same pattern (1.59 ± 0.19, 1.82 ± 0.21 and 1.86 ± 0.17 groups/km²). Mean group size was
small (1.37–1.61 individuals), and effective strip width ranged from 28.33 m (winter) to 34.50
m (post-monsoon). Habitat strongly influenced density: grasslands supported the highest annual
mean density (4.21 individuals/km²), followed by mixed forest (3.44), sal forest (2.53), teak
plantation (2.21) and bamboo forest (1.07). Summer density exceeded post-monsoon density in
every habitat. High densities were recorded in the grassland and mixed forest of the
Achanakmar, Lamni, Surahi and Chapparwa areas, while the Koechi, Kota and Lormi buffer
areas held very low densities. Our results show that sambar in ATR occur at moderate densities
with pronounced seasonal and habitat-level heterogeneity, and they emphasise the importance
of protecting grassland–mixed forest mosaics and water sources for sustaining the tiger's prey
base.