Extinction. This ominous word has one meaning. The death of a species. And it is a word that we seem to hear so often these days, especially in the news. But the opposite is possible. Today (July 29), which is Global Tiger Day (also called International Tiger Day), the world and India can celebrate the recovery of at least one endangered species. 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?
Genetics and connectivity
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. Small and isolated populations face a high probability of extinction. 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. Also, 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. On their own, most tiger populations do not have a high chance of survival. So why are we not seeing extinctions happening more often? Is this because tiger populations are connected to each other? We know that most tiger reserves in India are small and embedded in human-dominated landscapes. So, does the landscape between tiger reserves (agricultural fields, reserve forests, built-up areas and roads) allow tigers to move through them?
One way to answer this question 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. 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. For example, in our research we analysed tiger genetic samples in the central Indian tiger landscape and investigated genetic sharing between populations. Our results were surprising. 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. Using this understanding of connectivity, we were able to simulate scenarios for the future where we asked (given specific land-use change in the next 100 years), how our tiger populations might be affected? Would there be more extinction in the future? Or would they stay connected?
Our results showed that extinction could be avoided if corridors were safeguarded. What was striking was that fencing tiger reserves and isolating them resulted in high extinction. We used these models to also predict the impact of impending development projects in central India — widening of certain highways, for instance, would make them barriers, thereby increasing extinction substantially. These results along with other studies were used in court to petition for (and win a mitigation measure) — having an underpass to allow wildlife movement and connectivity. 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.
In Similipal and Rajasthan
But what about tiger populations that are already isolated? People have always wondered why black tigers were found only in the Similipal tiger reserve in Odisha. Our recent work on pseudo-melanistic or black tigers found in Odisha has demonstrated the genetic effects of isolation. 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.
After we found the causal genetic variant, we travelled through Similipal and collected tiger excreta/scat. We looked for this specific genetic variant in tiger DNA and found that it was common only in Similipal, where 60% of the tigers carried at least one copy. Other analyses have suggested that the tigers in Similipal form a small and isolated population. All our results 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. While we do not see adverse effects of inbreeding as yet, individuals are related and carry potentially disadvantageous genetic variants, which might affect the survival and the reproduction of tigers in Ranthambore in future. In short, we are seeing the genetic effects of isolation and small population size in wild tigers at some locations.
Strategies for the future
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. 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.
Uma Ramakrishnan is a professor at the National Centre for Biological Sciences, Tata Institute of Fundamental Research, Bengaluru. She is also a member of the Biodiversity Collaborative