In the context of International Tiger Day (July 29), India should not lose sight of the fact that other factors are important to ensure the survival of this big cat.
In the context of International Tiger Day (July 29), India should not lose sight of the fact that other factors are important to ensure the survival of this big cat.
become extinct. This ominous word has a meaning. death of a species. And this is a word that we get to hear very often these days, especially in the news. But the opposite is possible. Today (July 29), which is Global Tiger Day (also known as International Tiger Day), The world and India can celebrate the recovery of at least one endangered species. India is now reporting an increase in tiger numbers, and a recent assessment by the International Union for Conservation of Nature shows that tiger numbers have increased by 40% since 2005. This is a cause for celebration. But is an increase in tiger numbers enough to stop their extinction?
genetics and connectivity
Decades of research in ecology and evolution shows that numbers are key to avoiding extinction. Populations with fewer than 100 breeding individuals have a high probability of extinction. Also, for populations to survive, they must be part of a larger landscape with other populations that are connected. Small and isolated populations face a high probability of extinction. This is because small populations are subject to coincidence/random events. These emergent events may cause them to lose beneficial genetic variants, while increasing the frequency of other, deleterious genetic variants. This process is called genetic drift. Also, individuals in smaller populations are more likely to interbreed, leading to inbreeding. This uncovers several slightly deleterious genetic variants present in all genomes. When expressed together, these deleterious genetic variants cause intrauterine depression, and reduce the survival and reproduction of living individuals.
A closer look at the distribution of tigers in their range reveals that most tiger ‘populations’ are less than 100. In itself, most of the tiger population does not have a high chance of survival. So why aren’t we seeing extinctions more often? Is it because tiger populations are intertwined? We know that most tiger reserves in India are small and embedded in a human-dominated landscape. So, does the landscape between tiger reserves (agricultural areas, reserve forests, built-up areas and roads) allow tigers to pass through them?
research findings
One way to answer this question is to use movement data obtained from radio-collared tigers, which are often difficult to come by rare and endangered species. Alternatively, tigers can be genetically sampled using their feces/dung, hair and other biological samples from various tiger reserves and analyzed in a laboratory. The genetic variants in tiger DNA can be identified and analyzed and compared across tiger reserves. Groups of tiger reserves that show shared genetic variation are well connected – the hypothesis is that intervening landscapes facilitate connectivity or movement.
On the other hand, sets of tiger reserves with low genetic variation must have barriers or landscapes that impede movement and connectivity. For example, in our research we analyzed tiger genetic samples in the Central Indian tiger landscape and examined genetic sharing among populations. Our results were surprising. Most land-use types were not too poor for tiger connectivity, including agricultural areas. However, the presence of built up areas and high-traffic roads significantly hindered the movement of tigers. Using this understanding of connectivity, we were able to simulate future scenarios where we asked (given typical land-use change over the next 100 years), how might our tiger population be affected? Will there be more extinctions in the future? Or will they stay connected?
Our results showed that extinction can be avoided if alligators are protected. Shockingly, the fencing and isolation of tiger reserves resulted in high extinctions. We also used these models to predict the impact of impending development projects in central India – the widening of some highways, for example, would block them, leading to substantial extinctions. These results, along with other studies, were used to petition court (and win a mitigation measure) – having an underpass to allow movement and connectivity of wildlife. In short, as long as we manage landscapes outside tiger reserves to allow movement of tigers, and hunt and protect tigers inside tiger reserves, tigers are sure to survive in landscapes like central India .
Similipal and in Rajasthan
But what about tiger populations that are already isolated? People have always wondered why black tigers are found only in Similipal Tiger Reserve of Odisha. Our recent work on the pseudo-melanistic or black tigers found in Odisha has demonstrated the genetic effects of isolation. The genome sequences of the zoo’s tigers contained pseudo-melanistic cubs, which showed that a single spelling mistake (or mutation) in a specific gene causes these tigers to look that way.
Mystery of ‘Black Tigers’ of Similipala | in focus podcast
When we found the causal genetic variant, we traveled to Similipal and collected the feces/scat of the tigers. We looked for this specific genetic variant in tiger DNA and found it was common only in Similipal, where 60% of tigers had at least one copy. Other analyzes have suggested that tigers form a small and isolated population in Similipal. All of our results point to genetic drift, or random events that lead to this genetic variant that makes pseudomelanistic coat color common only in similipals.
On the other side of India, in Rajasthan, genome sequences of wild tigers show that individuals in the Ranthambore Tiger Reserve show interbreeding. Although we do not yet see adverse effects of inbreeding, the individuals are related and have potentially harmful genetic variants, which may affect the survival and breeding of tigers in Ranthambore in the future. In short, we are seeing genetic effects of isolation and small population sizes in wild tigers in some places.
strategies for the future
While we celebrate the recovery of tiger populations just by looking at numbers, we should not lose sight of other factors that are critical to their continued survival, such as connectivity. Special attention needs to be given to 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 better understand tigers in terms of their conservation. The future of tigers will require a ‘dialogue’ between such data and management strategies to ensure their survival. India is fortunate to have so many wild tigers and we must work together to save them.
Uma Ramakrishnan is Professor at the National Center for Biological Sciences, Tata Institute of Fundamental Research, Bangalore. She is also a member of the Biodiversity Cooperation