Tigers on an island8 min read . Updated: 13 Jan 2018, 10:56 AM IST
Protecting forest corridors is essential to ensure the genetic diversity of tigers
On the night of 29 December, the large male tiger Bajirao was run over by a speeding vehicle on National Highway 6 near Bajargaon in Maharashtra. Bajirao, a favourite with tourists, was the dominant male tiger from Bor Tiger Reserve, India’s smallest tiger reserve, with an estimated population of 10-12 tigers.
Incidentally, Bor’s “critical tiger habitat" of 138 sq. km is smaller than Bajirao’s home range—a 150 sq. km area that spans swathes of forest and human-dominated land in the districts of Wardha and Nagpur. According to tourist safari guides, Bajirao was an infrequent visitor to Bor. That night, the tiger seems to have run out of luck outside the protective cover of the reserve. It probably got blinded by the high beams of passing vehicles.
Around 35% of India’s wild tigers may share Bajirao’s fate as they live dangerously outside protected reserves. Unplanned development is turning protected areas like Bor into isolated islands. Three studies on tiger genetics published in 2017 draw alarming connections between the demolition of forest corridors and its effect on the reproductive progression of the big cat. If current trends of deforestation continue, then inbreeding, diseases and the lack of genetic variations will push the species towards extinction.
What affects tiger connectivity?
Prachi Thatte is a wildlife biologist and the principal author of Maintaining Tiger Connectivity And Minimizing Extinction Into The Next Century: Insights From Landscape Genetics And Spatially-explicit Simulations, a new research paper published by the journal Biological Conservation. The study examined the population connectivity of tigers across nine tiger reserves in central India, including Kanha, Bandhavgarh, Satpura, Tadoba-Andhari and Bor. “Our research suggests that there is a high probability, as much as 56%, for tigers to go locally extinct from small reserves like Bor if the current population gets isolated from other reserves. Tigers need to remain connected with other subpopulations to maintain a healthy genetic variability. As the median number of tigers within most reserves are low for a viable population interlinking, forest corridors are vital for the future of tigers," she says.
Thatte’s study began in 2012 and has involved five years of rigorous work. The first two years were spent collecting tiger scat samples across Maharashtra, Madhya Pradesh and Chhattisgarh. This was followed by standard laboratory work and analysis at the National Centre for Biological Sciences (NCBS), a part of the Tata Institute of Fundamental Research (TIFR), in Bengaluru.
Of the 580 scat samples Thatte managed to collect after long treks through prime tiger forests, 289 showed up as positive tiger poo. “Every day we can process 12 samples and it takes two days to know the species. Then we go for individual identification. It takes another week for 10 samples," explains Thatte.
“It’s a painstaking research as the DNA that we get from faecal samples is poor in quality. It’s also low in quantity due to weathering. DNA is only found in the outer layer and once the cells leave the body, they perish. It’s like a huge torn-up book which we try to investigate," says Uma Ramakrishnan, associate professor at the NCBS and an award-winning molecular ecologist. She is a co-author of the paper.
Towards the end of her postdoctoral research at Stanford University, Ramakrishnan often wondered if there was only one type of tiger in India. It seemed odd to her that India, with 60% of wild tigers in the world, should have very little genetic variation. When she returned to India in 2004, this was a vital factor behind the setting up of her lab in NCBS, which specializes in studying population genetics and the evolutionary history of mammals.
From the 289 positive identifications, Thatte could narrow down to genetic data of 116 individual tigers from nine tiger reserves. This data was simulated through the permutation and combination of 86 different scenarios. These factored in the various future impacts of land use change, human settlements and road-rail networks.
The empirical data points to an increasing human footprint on the landscape, from traffic intensity to dense human settlements, and dwindling genetic variety. Current infrastructure development does not incorporate conservation goals while developing project plans. The widening of NH-7 in the Kanha-Pench landscape, for example, has been without adequate wildlife corridors. This undermines the ongoing tiger conservation work in this important biodiverse landscape. The fact that genetic diversity has reduced over time, points to the urgent need to restore and protect corridors between tiger reserves. “The future is bleak for the species if forest corridors are not interlinked. This is hard scientific proof one can use while defending a case in court," says Ramakrishnan.
The tiger genome
In August, Meghana Natesh, another researcher working under Ramakrishnan, published a collaborative genome-wide data study of tigers in India. The study, Conservation Priorities For Endangered Indian Tigers Through A Genomic Lens, is a first of its kind. It revealed the existence of three genetically distinct clusters of tigers in India.
“The genome not only defines an organism but also encapsulates its evolutionary journey. It holds many secrets about the species’ history as well as its future. In my lab we do it the hard way, diving deep into the world of genetics, investigating and asking the difficult question," says Ramakrishnan, who is also the co-author of Natesh’s research paper.
She analysed 38 individual tiger samples from 17 different protected areas to divide the Indian tiger into three genetic baskets—the northwestern cluster, the southern cluster and the central Indian cluster. The tiger genome is 2.4 billion base pairs long. While previous studies looked at 10-12 regions in the genome, Natesh and Ramakrishnan analysed data from over 10,000 regions. They looked at the genetic variation of the entire tiger genome using single-nucleotide polymorphism, the most common genetic variant in DNA. Genetic variation is the raw material for evolution and survival of future generation of tigers. They need to remain connected.
“Our analyses reveals that tigers from the northwestern cluster (Ranthambhore Tiger Reserve) are genetically isolated and the genetic diversity in the reserve is much lower in comparison to all other clusters which are, as of now, well connected with each other. The loss of genetic connectivity is something to worry about because the long-term health of Ranthambhore’s tigers will now depend on how they are able to connect with neighbouring tiger populations from central India ," Natesh says.
Ranthambhore currently boasts a population of around 65 tigers, including cubs. However, the 392 sq. km tiger reserve doesn’t have the capacity to sustain such tiger numbers. Being solitary and territorial in nature, the young tigers of Ranthambhore will soon venture out of the protected area. There are countless examples of such stray tigers either falling victim to poachers or getting crushed under superfast trains on the Delhi-Mumbai railroad.
Tigers of central India
Tigers usually aren’t like Bajirao, bold enough to move into human-dominated landscapes. Most choose to be far from human activity, moving through rough terrain along high forest ridges to avoid any contact.
This was cited by scientist P. Anuradha Reddy after observing genetic data from 309 tigers over an area of 697,000 sq. km in central India. In her research paper, Tiger Abundance And Gene Flow In Central India Are Driven By Disparate Combinations Of Topography And Land Cover, published in June, she showed how dispersing tigers use the entire central Indian landscape to roam.
Reddy works at the Laboratory for the Conservation of Endangered Species at the Centre for Cellular and Molecular Biology in Hyderabad. According to her, growing urbanization and an increase in mining, rail and road links, irrigation and thermal projects are further fragmenting the iconic tiger landscape of central India. These developments now stand as barriers that limit the gene flow of tigers. “The study tries to explain how tigers move through this highly fragmented landscape and understand the genetic-carrying capacity of different protected areas," she says. Reddy hopes that policy makers and park managers prioritize core areas and corridors, using empirical data.
Subhoranjan Sen, field director, Pench Tiger Reserve in Madhya Pradesh, has been keeping a tab on Kanha-Pench and other corridors around Pench. He gives the example of a sub-adult male tiger from the Pench Tiger Reserve. “He was photographed in Pench in 2008. Thereafter he disappeared only to appear in 2011 in the Mukki range of Kanha Tiger Reserve. He established a territory here and reigned as the dominant male for over three years, siring several litters of cubs. This tiger used the Kanha-Pench corridor, which is now threatened by a plan to broaden the national highway that runs through the Pench Tiger Reserve," he says. Sen also gives another example of a male born in Nagzira, who made his way to Pench in Maharashtra and then to Pench in Madhya Pradesh between 2015 and 2016.
Park managers and tiger conservationists acknowledge that the need of the hour is to keep tigers connected for future healthy genetic legacy. But what concerns Sen and other park managers is that while the tiger numbers have been increasing over the years, the forested areas that harbour them have been decreasing.
“In October 2016, one of the sub-adult males from Alikatta in the centre of Pench turned up in camera trap images in the Matkuli range in Satpura Tiger Reserve. This is the first photographic evidence of the viability of the highly neglected Pench-Satpura corridor. This corridor passes through mostly degraded forests and is threatened by coal mines, roads and other issues. In February 2016, another sub-adult from Pench was found in a crop field adjacent to the corridor just south of Chhindwara," he says. “Further, four tigers have completed the migration from Pench in Madhya Pradesh to the Bhandar reserved forests to the west of Nagzira in Maharashtra. Until now it was believed that tigers migrating across long distances were exclusively males. As a pleasant surprise, two of the four tigers that successfully negotiated the over 160km stretch were females," adds Sen.
It is projected that half of India’s human population will live in cities by 2030. Urban areas, both old and new, will need space to grow. If this space is nibbled out of forest corridors, then this will snap the genetic umbilical cord and jeopardize the future generations of India’s tigers.