Using non-invasively collected genetic data to estimate density and population size of tigers
A new study, involving Chester Zoo’s Conservation Scientist Dr Simon Tollington, demonstrates that using non-invasively collected genetic data is a good approach for monitoring tiger populations in landscapes where using camera trapping might be challenging.
Simon Tollington co-supervised the lead scientist, Dr Abdul Aziz, during his PhD research at the Durrell Institute of Conservation and Ecology (DICE) at the University of Kent.
Currently in press in Global Ecology and Conservation, the study assessed the effectiveness of alternative non-invasively collected genetic data to estimate the density and population size of tigers in the Bangladesh Sundarbans.
Tiger habitats have been decreasing drastically and now occupy only seven percent of their ancestral range according to a 2007 study by Dinerstein et al. The Indian and Bangladesh Sundarbans, which represents 10.236 km2 of one of the most biologically diverse mangrove forest in the world, has been identified as a key region for long-term conservation of the iconic big cat and developing a reliable monitoring approach for the species is therefore critical.
Dr Simon Tollington tells us more:
The Sundarbans of Bangladesh remain a stronghold for tigers and it is important that we are able to monitor trends in absolute population size so that conservation management strategies can be implemented and evaluated. Analysing DNA from faeces not only allows us to identify the number of individuals but can potentially provide information on movement, diet and population fragmentation.
Estimates of tiger population size in the region have been previously performed using secondary signs such as tracks and by using camera traps. However, DNA technology allows conservationists to incorporate non-invasive genetic techniques to survey tiger populations. This study represents the first estimate of tiger population in the Sundarbans using this approach.
Using standard transect techniques, a survey team of trained field staff covered more than 1,000km searching for tiger scats and hair. The 440 samples of tiger faeces and hair collected were taken to the Durrell Institute of Conservation and Ecology, University of Kent, for DNA extraction, species authentication, and individual identification using microsatellites. Once the number of individual tigers was calculated from the collected faeces a density estimate for the area and total population size could be estimated.
The study concluded that there are approximately 2.85 tigers/100 km2 across the sampling area which is comparable to the 2.17 tigers/100 km2 estimated by Dey et al. in a recent camera trap survey conducted in the same region. This estimate suggests a total population size of tigers in the Bangladesh Sundarbans of around 121 individuals.
The team wrote in the newly published paper: “we have demonstrated the utility of non-invasive genetic sampling to assess the tiger population of the Bangladesh Sundarbans.”
Camera trap surveys in this region are known to be quite challenging as the tidal waters wash up the forest land, leaving only few recognisable tiger signs to be used for camera placement.
“Given these challenges with camera trapping, we have demonstrated that a non-invasive genetic sampling approach could overcome these constraints with considerable success.” explains Dr Aziz, now an Associate Professor in the Department of Zoology at Jahangirnagar University in Bangladesh.
T.K. Dey, M.J. Kabir, M.M. Ahsan, M.M. Islam, M.M.R. Chowdhury, S. Hassan, M. Roy, Q. Qureshi, D. Naha, U. Kumar, Y.V. Jhala. 2015. First Phase Tiger Status Report of Bangladesh Sundarbans. Bangladesh Forest Department, Ministry of Environment and Forests, Government of Bangladesh.
E. Dinerstein, C. Loucks, E. Wikramanayake, J. Ginsberg, E. Sanderson, J. Seidensticker, J. Forrest, G. Bryja, A. Heydlauff, S. Klenzendorf, P. Leimgruber, J. Mills, T.G. O'Brien, M. Shrestha, R. Simons, M. Songer. 2008. The fate of wild tigers. Bioscience, 57, 508.
Using non-invasively collected genetic data to estimate density and population size of tigers in the Bangladesh Sundarbans
Authors: Aziz, M.A., Tollington, S., Barlow, A., Greenwood, C., Goodrich, J.M., Smith, O., Shamsuddoha, M., Islam, M.A. and Groombridge, J.J. 2017. Using non-invasively collected genetic data to estimate density and population size of tigers in the Bangladesh Sundarbans. Global Ecology and Conservation.
Abstract: Population density is a key parameter to monitor endangered carnivores in the wild. The photographic capture-recapture method has been widely used for decades to monitor tigers, Panthera tigris, however the application of this method in the Sundarbans tiger landscape is challenging due to logistical difficulties. Therefore, we carried out molecular analyses of DNA contained in non-invasively collected genetic samples to assess the tiger population in the Bangladesh Sundarbans within a spatially explicit capture-recapture (SECR) framework. By surveying four representative sample areas totalling 1994 km2 of the Bangladesh Sundarbans, we collected 440 suspected tiger scat and hair samples. Genetic screening of these samples provided 233 authenticated tiger samples, which we attempted to amplify at 10 highly polymorphic microsatellite loci. Of these, 105 samples were successfully amplified, representing 45 unique genotype profiles. The capture analyses of these unique genotypes within the SECR model provided a density estimate of 2.85 ± SE 0.44 tigers/100 km2 (95% CI: 1.99e3.71 tigers/100 km2) for the area sampled, and an estimate of 121 tigers (95% CI: 84e158 tigers) for the total area of the Bangladesh Sundarbans. We demonstrate that this non-invasive genetic surveillance can be an additional approach for monitoring tiger populations in a landscape where camera trapping is challenging.