There is a need to facilitate active ‘gene flow’ into the Rajasthan tiger population from Central Indian tiger landscapes
Ranthambore National Park’s tiger population has seen dramatic fluctuation in the last half a century. Before Project Tiger came into force in 1973, Ranthambore was on the verge of losing all of its tigers due to a combination of hunting, poaching, and habitat destruction.
Since then, from just 14 tigers in the early 1970s, the count went up to 40 tigers in the 1980s, then fell again to 16 tigers in 2006. After that, the number has been going up consistently — climbing to about 40 again in 2016.
A study during this last recovery phase has found that on average, a tigress in Ranthambore can have more than one cub every two years, and said that the tiger population can grow rapidly if anthropogenic pressures are offset. And, living up to this expectation, the Ranthambore population went up to 69 tigers by 2022.
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Ecologically, this is as good as it can get for tigers in a semi-arid habitat and in a tiger reserve the size of Ranthambore.
Tigers physically isolated
No two tigers have similar stripes. So, tiger scientists estimate the number of individuals in a population of this elusive species using their ‘selfies’ taken by camera traps. And, photographed in great numbers and loved immensely by naturalists, the tigers of Ranthambore have scripted a success story for Project Tiger.
However, beneath this hard-won success lurks a dark threat not visible to the eye. While camera traps capture tigers in their visible glory, genetic studies are revealing threats.
From a genetic perspective, the size of 69 tigers makes for a small population. Multiple genetic studies on Ranthambore tigers have established that, due to geographical and genetic isolation, there is inbreeding in the population.
As recently as the 15th century, Ranthambore tigers were part of the larger forested landscape and genetically connected to tigers in the Terai region. In the ensuing centuries, forested areas under agriculture caused fragmentation in the larger northwestern tiger landscape.
A 2012 genetic study identified first-generation migrants from Ranthambore in Kuno and Madhav protected areas but was concerned that Ranthambore is rapidly losing its geographical connectivity with the central Indian tiger landscape.
While tigers don’t change their stripes, there is great churning in their populations. A healthy tiger population has a high turnover of tigers — about a third to half of the tigers in the population are new tigers every year, ensuring the vitality of the tiger population. However, even such populations can be adversely impacted by fragmentation and from shrinking of their habitat — as has happened to Ranthambore.
The deformity in eyes of tigress T99 from Ranthambore Tiger Reserve may not be a cause for alarm but is a reason for further genetic investigation into possible inbreeding depression in the population. Image: Supplementary Information with the 2021 genetic study / Kaushalkumar Patel
No conclusive evidence yet for inbreeding depression
Inbreeding occurs in a ‘small isolated population’ because closely related individuals mate with each other. A 2021 study found that Ranthambore tigers are nearly two times more inbred than Central Indian tigers. Another genetic study shows that most tigers in Ranthambore are descended from a single tigress named T16, also popularly known as ‘Machali’ to naturalists.
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This inbreeding can potentially cause ‘inbreeding depression’ in the population, which is a reduction in the fitness of a population. This, in turn, can lead to reduced survival, physical deformities, and infertility. So, could the recorded deformity in the eyes of tigress T99 of Ranthambore be evidence for inbreeding depression?
Professor Uma Ramakrishnan at the National Centre for Biological Sciences, senior author of the 2021 study said there is no need to be alarmed. “We cannot be sure of this unless we test it, maybe through looking at other such cases, or continued data on reproductive rates, other possible deformities, etc. But this is the right time to think about these things,” she said.
The process of natural selection counters inbreeding depression: over time, individuals who are unfit due to inbreeding will die off or fail to reproduce. This process is referred to as ‘genetic purging’ — the gene pool in the population is getting ‘cleaned’ through the removal of harmful genetic variations. Very interestingly, this 2021 study observed that Ranthambore tigers had fewer ‘bad genes’, and this could be because of ‘purging’.
Unfortunately, the level of inbreeding is so high that these fewer ‘bad genes’ are found at high frequencies. Thereby, the study alerts that the observed level of inbreeding is empirically associated with a high risk of extinction and the Ranthambore population is not genetically viable in the short- and long-term.
So, there is a need to facilitate ‘gene flow’ into the Ranthambore population. This involves the introduction of new genes from an external population to mask ‘bad genes’ and increase genetic diversity.
About the ‘gene flow’ into Rajasthan metapopulation
The study recommended ‘genetic rescue’ to offset any possible impacts of inbreeding depression in the population. And, genetic rescue is understood as increasing genetic diversity through translocation — by introducing a tiger from a suitably different landscape to bring about a genetic mix into the Ranthambore tiger population.
Genetic rescue as mitigation comes with risks of inducing ‘outbreeding depression’ because such assisted ‘gene flow’ could also introduce other damaging alleles. Anubhab Khan, a postdoc at the University of Copenhagen and first author of 2021 study said, “Genetic rescue needs to begin soon. However, it is important to verify that there are fitness consequences to inbreeding,” and Uma Ramakrishnan added, “We should of course think deeply about a strategy for rescue: how many individuals, and which populations as sources. We can use simulations to explore rescue scenarios.”
The study also recommended maximising connectivity as the best strategy to minimise extinction. “This would require increasing tiger numbers in Rajasthan’s tiger reserves since large populations are sources of dispersing individuals. And, to have adequate prey density and structural connectivity between all Rajasthan tiger reserves,” Uma Ramakrishnan said.
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An earlier study in 2017 that also did not find evidence for inbreeding depression suggests that the Ranthambore tigers be managed as a metapopulation — connected to other tiger populations in the northwestern tiger landscape.
According to this study, facilitating migration between satellite populations, and thereby ‘gene flow’ in the metapopulation is the best long-term conservation strategy. There seems to be no imminent need for genetic rescue, the study said.
The need for ‘genetic rescue’ has to be weighed in light of the challenges and the realistic timeline for establishing the northwestern/Rajasthan tiger metapopulation and for the onset of natural migrations between the Central Indian metapopulation and the Rajasthan metapopulation. Map for illustration purposes only. Source: NTCA for Tiger Reserve boundary and Corridor. Rajasthan Forest Department for other forest areas. Map: Narendra Patil
However, Ranthambore is the only source population in the northwestern / Rajasthan tiger landscape and all tigers in Ramgarh Vishdhari TR, Mukundara Hills TR, Kuno NP, and Madhav NP are tigers from this small founding population. So, can the natural dispersal within the future Rajasthan metapopulation bring about the required ‘gene flow’? Or is there a need for ‘genetic rescue’ by introducing tigers from different landscapes into the envisaged metapopulation of Rajasthan tigers?
The need for active ‘genetic rescue’ and its urgency have to be weighed in light of the on-ground challenges of establishing the Rajasthan metapopulation and realistic timeline for the onset of natural ‘gene flow’ to occur. The unpalatable truth is that the reality contrasts very drastically from what is desirable.
The natural dispersals of tigers between satellite populations in tiger reserves of Rajasthan, and the immigration of tigers from central Indian into Rajasthan metapopulation may take a very long time to realise.
The fragmentation is severe and the intensity of anthropogenic pressures from infrastructure is high. There is a persistent tendency on the part of governments to cut project costs by either not having mitigation measures against fragmentation or by resorting to inadequate measures. Both legal and illegal mining and the political clout this lobby wields are set against the ideal conservation needs.
Read more: A roar in captivity
Spelling out one possible strategy for implementing ‘genetic rescue’, Uma Ramakrishnan suggested, “One option might be to introduce both Ranthambore tigers and Central Indian tigers into a new area like Ramgarh Vishdhari TR and Mukundara Hills TR. They would mate and form a new gene pool that can then disperse into other Rajasthan populations.”
Active mitigation may be required
Time may not be the best healer for all damages. It may be unrealistic to wait for Rajasthan tigers to regain their genetic vitality through the very iffy and long-term prospect of restoring the corridors connecting them to the larger Central Indian tiger population. Mitigation weighed in by pragmatic limitations and urgency defined by genetic insights needs to be taken up.
Considering the significance of these genetic findings, even if there is no reason for panic, there is definitely an urgent need for scientists and managers to work together to investigate the potential inbreeding depression and to also work out a strategy for the genetic rescue of the Ranthambore tiger population.
According to a news report from two years back, the Rajasthan Forest Department had begun talks with the National Tiger Conservation Authority (NTCA) to “proactively address” the findings from the genetic study on inbreeding in Ranthambore. When the author of this article spoke to the Principal Chief Conservator of Forest and Chief Wildlife Warden of the Rajasthan Forest Department it was learned that the Forest Department was still awaiting recommendations from NTCA!
Response from NTCA to the author’s queries were awaited at the time of publishing the article.
Narendra Patil is a freelance writer. He writes on wildlife, ecology, nature conservation
Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth
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