Context

India is now reporting increased tiger numbers, and a recent International Union for Conservation of Nature assessment suggests that tiger numbers have increased by 40% since 2005. This is cause for celebration. But is the rise in tiger numbers enough to prevent their extinction?

Relations between distribution and genetic variation

• Decades of research in ecology and evolution suggest that numbers are critical to avoid extinction. 
• Populations that are smaller than 100 breeding individuals have a high probability of extinction.
• At the same time, for populations to persist, they should be part of larger landscapes with other such populations that are connected.
• This is because small populations are subject to chance/random events.
• These chance events may cause them to lose advantageous genetic variants, while other, detrimental genetic variants might increase in frequency.
• This process is called genetic drift.
• Individuals in small populations are more likely to be related, leading to inbreeding.
• This exposes the many slightly disadvantageous genetic variants that are present in all genomes.
• When expressed together, these detrimental genetic variants cause inbreeding depression, and reduced survival and reproduction of inbred individuals.
• A closer look at the distribution of tigers across their range shows that most tiger ‘populations’ are smaller than 100.
• This raises a question why are we not seeing extinctions happening more often? Is this because tiger populations are connected to each other?

Research findings about movement of tigers

• One way to answer the question about not so frequent extinction is to use movement data sourced from radio-collared tigers, often difficult to come by for a rare and endangered species.
• Alternatively, tigers can be genetically sampled using their excreta/scat, hair and other biological samples from different tiger reserves and analysed in a laboratory.
• Genetic variants in tiger DNA can be identified and analysed and compared across tiger reserves.
• Genetic variation in landscape with connectivity: Sets of tiger reserves that show shared genetic variation are well connected — the inference is that the intervening landscapes facilitate connectivity or movement.
• On the flip side, sets of tiger reserves that share less genetic variation must have barriers or landscapes that impede movement and connectivity.
• Most land-use types were not too bad for tiger connectivity, including agricultural fields.
• However, the presence of built-up areas and high traffic roads greatly impeded tiger movement.
• Results showed that extinction could be avoided if corridors were safeguarded.
• In summary, as long as we manage landscapes outside tiger reserves to allow tiger movement, and protect prey and tigers inside tiger reserves, tigers are sure to survive in landscapes such as central India.

Genetic changes in isolated tiger population

• Black tigers were found only in the Similipal tiger reserve in Odisha.
• Genome sequences of a litter of zoo tigers that included pseudo-melanistic cubs revealed that a single spelling mistake (or mutation) in a specific gene causes these tigers to look this way.
• Pseudo-melanistic or black tigers found in Odisha has demonstrated the genetic effects of isolation.
• Results of the research pointed to genetic drift, or random events that have lead to this genetic variant that causes pseudomelanistic coat colour becoming common only in Similipal.
• On the other side of India, in Rajasthan, genome sequences from wild tigers reveal that individuals in the Ranthambore tiger reserve show inbreeding.
• In short, we are seeing the genetic effects of isolation and small population size in wild tigers at some locations.

Way forward

• Focus on connectivity: While we celebrate the recovery of tiger populations only by looking at numbers, we must not lose sight of other factors that are critical to their continued survival, such as connectivity.
• Special attention is needed for populations that are becoming isolated and facing the genetic consequences of such isolation.
• The future of such populations may depend on genetic rescue or even the introduction of novel genetic variants.

Conclusion

We are fortunate that novel genome sequencing technology provides an opportunity to understand tigers much better in the context of their conservation. The future of tigers will require a ‘dialogue’ between such data and management strategies in order to ensure their survival. India is lucky to have so many wild tigers and we must work together to save them.

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Back2Basics: Pseudo-melanism

• Tigers have a distinctive dark stripe pattern on a light background of white or golden.
• A rare pattern variant, distinguished by stripes that are broadened and fused together, is also observed in both wild and captive populations.
• This is known as pseudo-melanism, which is different from true melanism, a condition characterised by unusually high deposition of melanin, a dark pigment.
• While truly melanistic tigers are yet to be recorded, pseudo-melanistic ones have been camera-trapped repeatedly, and only, in Simlipal, a 2,750-km tiger reserve in Odisha, since 2007

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