Demystifying the Sundarban tiger: novel application of conventional population estimation methods in a unique ecosystem

Abstract

Conserving large populations with unique adaptations is essential for minimizing extinction risks. Sundarban mangroves (>10,000 km2) are the only mangrove inhabited by tigers. Baseline information about this tiger population is lacking due to its man-eating reputation and logistic difficulties of sampling. Herein, we adapt photographic capture-mark-recapture (CMR) and distance sampling to estimate tiger and prey densities. We placed baited camera stations in a typical mangrove in 2010 and 2012. We used telemetry based tiger home-range radius (5.73 km, SE 0.72 km) to estimate effective trapping area (ETA). An effort of 407 and 1073 trap nights were exerted to photocapture 10 and 22 unique tigers in 2010 and 2012. We accounted for use of bait by modelling behaviour and heterogeneity effects in program MARK and secr package in program R. Using traditional CMR, tiger number was estimated at 11 (SE 2) and density at 4.07 (SE range 3.09–5.17) in 2010 while in 2012, tiger number was 24 (SE 3) and density 4.63 (SE range 3.92–5.40) tigers/100 km2. With likelihood based spatially explicit CMR, tiger densities were estimated at 4.08 (SE 1.51) in 2010 and 5.81 (SE 1.24) tigers/100 km2 in 2012. Using distance sampling along water channels, we estimated chital density at 5.24/km2, SE 1.23 which could potentially support 4.68 tigers/100 km2 [95 % CI (3.92, 5.57)]. Our estimates suggest that Sundarban tiger population is one of the largest in the world and therefore merits high conservation status.