Disease threat: Genetic diversity loss
Small and isolated populations are inherently vulnerable to the effects of stochastic events such as disease outbreaks or climatic catastrophes. In addition, in a small and geographically isolated population the genetic variation necessary to permit adaptive evolution can gradually fade over a number of generations and deleterious genetic mutations can accumulate, resulting in a slowly progressive reduction in fitness. In contrast however, inbreeding (mating between closely related individuals) can result in a more rapid reduction in fitness, a phenomenon called inbreeding depression. The magnitude of the effects of inbreeding depression varies across taxa, but can affect birth weight, survival, reproduction, and resistance to disease, predation and environmental stress (Keller and Waller 2002). Genetic diversity loss, and the likelihood of inbreeding are both increased following genetic bottlenecks, (severe population contraction events). Even if population sizes recover following a bottleneck, the consequences of genetic diversity loss may affect longer term population recovery and survival (Alsaad et al. 2011).
Host species: Tigers.
Pathogenesis: Loss of genetic diversity occurs in small, isolated populations for the reasons outlined above.
Diagnosis: Complex – requires genetic analysis.
Vaccination / mitigation: Corridors between good tiger habitats to facilitate safe dispersal are important components of tiger population survival (eg: Thapa et al, 2017), in addition to protecting existing populations and maximising genetic diversity in captive populations for potential future re-introductions. Translocation of tigers between relatively disconnected subpopulations from which natural dispersal is limited may become more important in the future.
Free-ranging tiger occurrence:
The Amur tiger, Panthera tigris altaica, has the lowest genetic diversity of all the tiger sub-species (Rusello et al. 2004). There is now evidence of a recent bottle-neck consistent with severe population decline in the 1940s, contributing to genetic diversity loss and the likelihood of inbreeding depression (Alsaad et al. 2011). Alsaad et al. 2011 also reported an effective population size of less than 14 in an actual population of around 500, associated with this loss of genetic diversity. Amur tiger populations exist as two main sub populations in the Russian Far East: the largest in the Sikhote-Alin region, and a smaller population in south-west Primorye, with a likely dispersal barrier between them associated with the development corridor between Ussurisk and Vladivostock (Sorokin et al. 2016). In addition, there is a small, but growing Amur tiger population in north-east China, Jilin province, expanding west from the Hunchun National Nature Reserve (Wang et al. 2018). This population has the potential to connect across the border to the south-west Primorye population (Ning et al. 2019), although barriers to dispersal need to be addressed both across the border and within China.
The Bengal tiger, P. tigris tigris, has much higher genetic diversity than the Amur tiger (Mondol et al. 2009), but genetic diversity is considerably lower than that found in samples from Bengal tigers hunted prior to 1950 (Mondol et al. 2013). In central India, the importance of forest corridors has been highlighted, in addition to the threats that developments such as roads, railways, and mining pose to maintaining these corridors (Sharma et al. 2013). Further north, however, in the Terai Arc Landscape in the foothills of the Himalayas, genetic diversity is already being compromised by anthropogenic developments, despite prey abundance (Singh et al. 2017).
In the mangrove habitat of the Sundarbans, tigers are considered the same sub species as Bengal tigers, despite obvious morphological differences. Genetic diversity appears to be relatively low, but they should be considered an evolutionarily separate unit (ESU) to protect their genetics and their unique morphological adaptations to their habitat (Singh et al. 2015).
Distribution: All tiger range states potentially.
Alsaad, S., Soriguer, R.C., Chelomina, G., Sushitsky. Y.P. & Fickel, J. (2011) Siberian tiger’s recent population bottleneck in the Russian Far East revealed by microsatellite markers. Mammalian Biology 76: 722–726.
Keller, L.F. and Waller, D.M. Inbreeding effects in wild populations. Trends in Ecology and Evolution, 17: 230-241.
Mondol, S., Karanth, K.U., Kumar, N.S., Gopalaswamy, A.M., Andheria, A. & Ramakrishnan, U. (2009). Evaluation of non-invasive genetic sampling methods for estimating tiger population size. Biological Conservation, 142: 2350–2360.
Mondol, S., Bruford, M.W. & Ramakrishnan, U. (2013) Demographic loss, genetic structure and the conservation implications for Indian tigers. Proc R Soc B, 280: 20130496. doi:10.1098/rspb.2013.0496.
Ning, Y., Kostyria, A.V., Ma, J., Chayka, M.I., Guskov, V.Y., Qi, J., Sheremetyeva, I.N., Wang, M. & Jiang, G. (2019). Dispersal of Amur tiger from spatial distribution and genetics within the eastern Changbai mountain of China. Ecology and Evolution, 9: 2415–2424.
Russello, M.A., Gladyshev, E., Dale Miquelle, D. & Caccone, A. (2004) Potential genetic consequences of a recent bottleneck in the Amur tiger of the Russian far east. Conservation Genetics, 5: 707–713.
Sharma, S., Dutta, T., Maldonado, J.E., Wood, T.C., Panwar, H.S. & John Seidensticker, J. (2013). Spatial genetic analysis reveals high connectivity of tiger (Panthera tigris) populations in the Satpura–Maikal landscape of Central India. Ecology and Evolution, 3: 48–60.
Singh, S.K., Mishra, S., Aspi, J., Kvist, L., Nigam, P., Pandey, P., Sharma, R. & Goyal, S.P. (2015). Tigers of Sundarbans in India: Is the Population a Separate Conservation Unit? PLoS ONE, 10(4): e0118846. doi:10.1371/ journal.pone.0118846.
Singh, S.K., Aspi, J., Kvist, L., Sharma, R., Pandey, P., Mishra, S., Singh, R., Agrawal, M. & Goyal, P. (2017). Fine-scale population genetic structure of the Bengal tiger (Panthera tigris tigris) in a human-dominated western Terai Arc Landscape, India. PLoS ONE, 12(4): e0174371. doi:10.1371/journal.pone.0174371.
Sorokin, P.A., Rozhnov, V.V., Krasnenko, A.U., Lukarevsky, V.S., Naidenko, S.V. & Hernandez-Blanco, J.A. (2016). Genetic structure of the Amur tiger (Panthera tigris altaica) population: Are tigers in Sikhote-Alin and southwest Primorye truly isolated? Integrative Zoology, 11: 25–32
Thapa, K. et al (2017). Tigers in the Terai: Strong evidence for meta-population dynamics contributing to tiger recovery in the Terai arc landscape. PLoS ONE, 12(6): p.e0177548
Wang, D., Hu, Y., Ma, T., Nie, Y, Xie, Y. & Wei, F. (2016). Non-invasive genetics provides insights into the population size and genetic diversity of an Amur tiger population in China. Integrative Zoology, 11: 16–24 doi: 10.1111/1749-4877.12176.