TIGERS do not segregate on regional lines
Shomita Mukherjee joined the Masters in Wildlife Science course at the Wildlife Institute of India (WII) in 1988. Since then she has standardised techniques to estimate large cat diet and studied some small cats in India, including the jungle cat, leopard cat, caracal, fishing cat and rusty-spotted cat. Her PhD work was conducted in Sariska Tiger Reserve on the jungle cat, caracal and golden jackal. Currently she uses non-invasive molecular tools to study small cats in India and has standardised molecular techniques for such surveys. A member of the IUCN Cat Specialist Group and the Species Survival Commission, her current work explores the link between ecology, evolution and genetic variation in small felids and how such information could contribute towards their conservation. She speaks to Akash Bisht about the debate surrounding inbreeding
What impact does inbreeding have on wild animals, especially tigers?
Inbreeding is a condition when genetically similar individuals in a population breed with each other and over time individuals of the population become even more similar to each other. The population suffers from a loss of genetic diversity. This can lead to adverse conditions that result from inbreeding (inbreeding depression) such as muscle degeneration (observed in humans), low sperm count and quality, inability to fight some infections (lowered immunity) etc. However, it need not always be the case that if related individuals breed, their offspring will always be at risk. The probability is that the risk will increase with inbreeding. In a small population, the chance of mating with related individuals increases and this leads to inbreeding effects. It's like having one box with a hundred marbles of different colours, say 10 red, 10 blue, 50 green and 30 yellow. Another box has only 10 red, 30 green and 10 yellow. Now if you close your eyes and pick two marbles from box one and 2 from box two, you are more likely to get different colours from the first box but two of the same colour (perhaps green) from the second box. Similarly, the chances of mixing with similar individuals also increase with a drop in population.
Would taking away genetically unrelated tigers from Ranthambore Tiger Reserve (RTR) lead to increased inbreeding among the tigers there?
Yes, that is true but it depends on how many tigers are there in RTR and what their relatedness profile is like. For instance, if you take away a million people from India, it may not make as much of a genetic difference as it would if you take away a similar number from a less populous country. The initial population size matters. However, if you selectively take away a million of a particular kind, then it would matter. So if you take away all the different males and leave only the related ones, then of course it would matter.
What implications would this have on the resident population of the park?
It could lead to inbreeding effects later on.
Population fluctuations and inbreeding have been a norm in RTR. Why have these factors become so relevant now?
We do not know to what extent the population in RTR is inbred and if at all it is, then no one, to my knowledge, has measured it. The question is why these fluctuations have happened. It's because of removal by poaching. Is that not reason enough to warrant action? Moreover, if we aggravate the situation by meaning to improve it, then we are acting in an illogical and ill-informed manner. Small pockets of isolated populations are no good for conservation in the long run. So, for instance, if we say that we are going to make our tiger population free of risk from poachers (which should be our aim), then we have to think of the long-term implications of inbreeding if the populations are small and the individuals within are related to a large extent. If we have not thought of that, it means we do not even think our tigers will survive long enough for the effects of inbreeding to show up.
Inbreeding is common among endangered species and fragmented populations. How can we maintain viable population of such species without inbreeding?
Inbreeding need not be 'common' in endangered species but there are higher chances of it happening due to their small population. If the population drops drastically due to, say, poaching and then if it continues unabated, the population will go extinct before inbreeding kicks in. If we have identified and solved that problem, then we have to assess the risks of inbreeding in the longer term. Inbreeding need not happen over very short time scales. It depends on the generation time of the species and the level of loss of genetic diversity. To tackle inbreeding, the only solution is to introduce variation by bringing in individuals from another area.
What is your take on the WII study done by Dr SP Goyal that suggests avoiding genetic mixing of tigers from various landscapes?
I have no clue why and on what basis he says they cannot be mixed. The barriers and fragmentation of habitat that we see today are all due to human-made changes. In fact, the tigers should be brought from other places. On the one hand, we rant about fragmented landscapes and talk of connecting these with corridors for greater genetic exchange, but then when we have a chance, we bring in related individuals from a very similar population. I suppose a hundred years ago forests were connected across Rajasthan, Madhya Pradesh and Maharashtra. Let's for a moment forget the names of states - those are political boundaries. The forests must have been contiguous and in that case tigers must have been exchanging genes across populations.
Can tiger populations of central India and western India be mixed? What would be the implications?
If the populations were naturally disjunct and deeply separated genetically due to physical or climatic barriers, then mixing them would be an issue. Tigers range over varied habitats with various climatic conditions and they also range very widely on an individual level (i.e., individuals disperse over hundreds of kilometres). In this case, it is human actions that have fragmented populations and the landscape, and so, given a chance, the tigers would have mixed naturally. The implications would be very good. Tigers do not segregate on regional lines the way we do. A tiger from Rajasthan will be most willing to mate with one from Karnataka, Maharashtra or West Bengal. In fact, Sariska Tiger Reserve (STR) could have been a model for taking care of fragmented populations. We had a great chance here to change things. We are starting from scratch here and nowhere in future are these individuals going to find constant connectivity to reach anywhere outside. We could have actually introduced lots of variation and then looked at the future to see the potential of this population versus one with very low variation.
Some experts believe that tigers from Madhya Pradesh and Maharashtra would not survive in STR owing to different climatic conditions. What do you think?
How do these tigers (or any other species) survive in zoos when brought from various other places? They are even exported to zoos abroad. Individual tigers are known to disperse over hundreds of kilometres and possibly cover varied climatic conditions in doing so. We are not introducing Siberian Tigers into STR. That would certainly be very problematic. But, climatic conditions in central India and STR are not that different for a species that occupies a very wide range. Summers are just as hot and winters are just as cold. Water is available everywhere in STR due to various check dams and water bodies. In fact, the transients from these areas that are spilling over into buffer zones could be brought to STR. For one, transients are still trying to establish territories and hence are often pushed into marginal habitats (with less forest cover) and so there would not be a problem for them to settle in STR. Secondly, by doing this you are not disturbing the existing social set-up of tigers in the areas from where the transients are being picked up for translocation.
The views expressed here are personal and not that of any organisation