Demography of a small, isolated tiger (Panthera tigris tigris) population in a semi-arid region of western India

Abstract

BackgroundTiger populations have declined globally due to poaching, prey depletion, and habitat loss. The westernmost tiger population of Ranthambhore in India is typified by bottlenecks, small size, and isolation; problems that plague many large carnivore populations worldwide. Such populations are likely to have depressed demographic parameters and are vulnerable to extinction due to demographic and environmental stochasticity. We used a combination of techniques that included radio telemetry, camera traps, direct observations, and photo documentation to obtain 3492 observations on 97 individually known tigers in Ranthambhore between 2006 and 2014 to estimate demographic parameters. We estimated tiger density from systematic camera trap sampling using spatially explicit capture-recapture (SECR) framework and subsequently compared model inferred density with near actual density.ResultsSECR tiger density was same as actual density and recovered from 4.6 (SE 1.19) to 7.5 (SE 1.25) tigers/100km2 over the years. Male: female ratio was 0.76 (SE 0.07), and cub: adult tigress ratio at 0.48 (SE 0.12). Average litter size was estimated at 2.24 (SE 0.14). Male recruitment from cub to sub-adult stage (77.8%, SE 2.2) was higher than that of females (62.5%, SE 2.4). But male recruitment rate as breeding adults from the sub-adult stage (72.6%, SE 2.0) was lower than females (86.7%, SE 1.3). Annual survival rates, estimated by known-fate models, of cubs (85.4%, CI95% 80.3–90.5%) were lower than that of juvenile (97.0%, CI95% 95.4–98.7%) and sub-adult (96.4%, CI95% 94.0–98.9%) tigers. Adult male (84.8%, CI95% 80.6–89.2%) and female (88.7%, CI95% 85.3–92.2%) annual survival rates were similar. Human-caused mortality was 47% in cubs and 38% in adults. Mean dispersal age was 33.9 months (SE 0.8), males dispersed further (61 Km, SE 2) than females (12 Km, SE 1.3). Higher age of first reproduction (54.5 months, SE 3.7) with longer inter-birth intervals (29.6 months, SE 3.15) was likely to be an effect of high tiger density.ConclusionDemographic parameters of Ranthambhore tigers were similar to other tiger populations. With no signs of inbreeding depression there seems to be no eminent need for genetic rescue. The best long-term conservation strategy would be to establish and manage a metapopulation in the Ranthambhore landscape.