Genetic diversity key to tiger survival

NOT JUST NUMBERS Anjali Vaidya elaborates on a study that highlights the risks India's tigers face due to genetic isolation, while also suggesting way

Genetic diversity key to tiger survival

Tiger numbers have started rising in recent years, which is good news for an endangered species reduced to a few thousand worldwide. India is part of a 13-country effort to double the animal’s population by 2022, in part through increased monitoring and protection. However, for any species so diminished, protecting what we have left is not enough. Most of the world’s remaining tigers live in tiny, guarded fragments, each of which is precarious on its own. Even in the absence of major threats such as poaching, isolation can leave animals at serious risk. Local shifts in the environment such as drought or the onset of disease can easily sweep these remnants away.

How isolated are India’s tigers? A recent study in Scientific Reports by researchers at the National Centre for Biological Sciences (NCBS), Bengaluru, sought to partially answer this question. India’s remaining tigers number over 2,000, representing more than half of the world’s tiger population. But these tigers are spread thin, across increasingly patchy habitat, and many are restricted to protected but disconnected reserves. Using DNA from 38 tigers across 17 tiger reserves, the NCBS study sketched an outline for an India-wide genetic map of the tiger, using Single Nucleotide Polymorphisms (SNPs) to make genetic comparisons between
individuals.

What sets the current study apart from previous similar work is the use of SNPs, which allows researchers to look at thousands of points of difference between individuals, spanning the genome. Genetic variation grows naturally within species, over time.

Differences then arise between populations as they migrate and grow progressively more isolated from each other. With high enough resolution, a map of genetic differences can tell us not just about the present state of a species but also how it got there, and where it might be going.

The NCBS study was a preliminary sweep, highlighting areas at risk due to genetic isolation, which can then be studied in more depth. Mapping genetic diversity and relatedness between tiger populations, the study also demonstrated the areas where the tigers once travelled freely in this fragmented landscape. With the help of wildlife corridors, tigers might one day travel these paths again.

Genetically isolated

The genes of a species hold its history. Tigers once roamed the length and breadth of India, as well as much of the rest of Asia. Historical records chronicling the animal’s slow fall are bound to be incomplete, but the tiger genome adds to the animal’s story. We will never know for sure how many tigers once lived.

We know that at least 80,000 were killed between the latter decades of the 19th and early 20th centuries, their skins proffered to collect bounty payments in British India, but many were also killed for sport. One Indian prince killed 1,100 tigers within his lifetime — half of India’s tiger population today. The drop in tiger numbers in the 19th century was precipitous enough that it shows up in the tiger genome as a bottleneck, roughly two centuries back: wiping out much of the species’ genetic variation, even as tens of thousands were killed due to hunting and habitat loss.

When a species comes close to extinction and then recovers, as the tiger is beginning to do, its struggles are still far from over. Genetic variation can take many thousands of years to return to previous levels. The African cheetah suffered a severe bottleneck roughly 10,000 years ago, and still has low genetic variation as a result. And in the long run, low genetic variation can mean low resilience for a species. Even if the population is doing well at the moment, it lacks the genetic resources to adapt to a changing environment – rather like an individual with a narrow skill set in a fluctuating job market.

If the winds change, as they always do, being perfectly adapted to yesterday’s environment may turn into a liability.

Thus, the current study’s finding that the tigers of Ranthambore Tiger Reserve in Rajasthan have low genetic variability, in addition to being genetically isolated from tigers in Central and Southern India, is a potential cause for concern. Ranthambore Tiger Reserve is a success story in many respects: its tiger population has grown to such an extent in the last decade that the reserve has started exporting tigers to other protected areas, such as Sariska Tiger Reserve, in order to reduce overpopulation. But a relatively homogeneous gene pool may spell trouble in the long run — both for Ranthambore Tiger Reserve, and reserves like Sariska Tiger Reserve repopulated exclusively with
Ranthambore Tiger Reserve’s tigers.

Conservation strategies

Uma Ramakrishnan, professor at NCBS and one of the study’s authors, suggests that more research is needed before devising possible conservation strategies here, such as introducing tigers to Ranthambore from elsewhere in order to broaden the population’s genetic diversity. “We should be cautious before recommending large scale movements of animals,” she says. Adding too many tigers not closely related to the present population may dilute genetic adaptations to the local environment. But action is still required, since small isolated populations with low genetic variation are not just ill-adapted to change, but also at risk for inbreeding depression, characterised by low fertility and susceptibility to disease.

Central India (including the North East) contained the most genetic variation of the three broad genetic clusters identified by the study, with the other two being Ranthambore and South India. However, this storehouse of variation is also under threat, given that tiger reserves tend to be small and physically disconnected, dividing the gene pool into a collection of Noah’s Arks under variable levels of protection. Threats to reserves in Central India include development projects, such as the future widening of National Highway 7 in Kanha and Pench Tiger Reserves, and the forthcoming submergence of Panna Tiger Reserve, all in Madhya Pradesh.

One way to encourage the flow of tigers (and therefore genes) across an unpredictable, human-filled landscape is to build wildlife corridors. Uma’s group is presently coming out with an additional study analysing how to place wildlife corridors between tiger populations in Central India in order to maximise gene flow. Such corridors would re-connect populations which are genetically close, and were therefore most likely connected in the recent past.

Due to the animal’s rarity, tiger DNA can be hard to come by. However, the study’s authors acknowledge that data from 38 individuals can only provide an initial step, pointing towards areas which could benefit from future work. The next step is to gather more genetic data, and use that data to construct conservation strategies that won’t just preserve the tiger for future generations, but also allow it to flourish.

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