The future of tigers, with approximately 3,600 remaining in fragmented habitats worldwide, hinges not just on population expansion but also on “gene flow” or the ability of the species to migrate and breed outside their territorial gene pool, finds a new study.
Gene flow — both within tiger subspecies and between subspecies separated today by national boundaries — is vital in protecting the existing “reserve” of genetic diversity of this endangered species, say authors of a paper published in the latest edition of the Journal of Heredity . “Low genetic diversity can negatively impact the fitness of a species, and thus it’s survival,” they add.
When the gene flow is low, tiger populations would have to compensate with an “unattainable” growth rate to maintain current levels of genetic diversity, the study predicted from statistical simulations. In one simulation they calculated that if fragmentation led to the complete isolation of the estimated 402 tigers in the Western Ghats, the population would have to rise to 18,000 individuals to maintain the same genetic diversity in 150 years.
For the study, researchers used previously sequenced DNA of 125 individuals (from five existing tiger subspecies: Sumatran, Malayan, Bengal, Indochinese and Siberian) to predict the population size needed to maintain existing genetic diversity 150 years from now.
“The lower the genetic diversity, the greater the population numbers needed to ensure the species’ survival,” said co-author Uma Ramakrishnan, researcher at the National Centre for Biological Sciences and a co-author of the paper. “But yes, promoting gene flow is a much harder conservation strategy than attempts to increase population, because you have to look beyond park boundaries and at the natural dispersal of these animals, which can be over hundreds of kilometres.”
In another study published in PLoS ONE last year, Dr. Ramakrishnan and others had found that an individual tiger had migrated over a distance of up to 600 km in its lifetime in Central India, hazarding everything from highways to human habitation. Underscoring the need for “landscape-level connectivity” it added that “most wild tigers live in small, isolated protected areas within human dominated landscapes in the Indian subcontinent.”
Protecting genetic diversity is also “extremely important to avoid deleterious effects of inbreeding,” the new research paper says, citing the example of the Florida panther, a puma subspecies, which dwindled to 30 individuals after a dramatic decrease in its range in the 1990s. The demographic reduction led to inbreeding, which, in turn, shrunk the genetic diversity of the subspecies.
Shrinking the genetic diversity affected the population in a few notable ways — reduced reproductive capacity, increased cardiac defects and prevalence of infectious diseases.
While inbreeding has not yet affected wild tigers, their survival has never been more precarious: their habitat has shrunk to just seven per cent of historic estimates. Poaching has eliminated on average 104 tigers each year since 2000, and three of nine subspecies are now extinct — Bali, Javan, and Caspian.
The good news is that the Indian subcontinent, which harbours 60 per cent of the world’s wild tiger population, also has significant genetic diversity in the species. But once lost, the reserve of genetic diversity will not be regained “for millions of years, even with higher population sizes.” In fact “every tiger — including those in zoos, which presently outnumber those in the wild — is important as a potential reserve of the genetic diversity of the species,” the paper adds.
