~ by Archita Sharma (WWF-India)
The Central Indian Landscape has been a stronghold for several long-ranging wild mammals for several thousands of years. Its dense forests and rolling grasslands provide a rich holdout for tigers and several other carnivores, big and small, prowling, hunting, and stalking in the shadows of this wild heartland. But over the last century, Central India has lost a share of its forests to growing human settlements, railways, roads and rampant mining. So, a landscape that was once a contiguous expanse of dry deciduous forests is now fragmented, remaining with 34% forest cover, out of which only 8.5% is legally protected.
Long-ranging mammals need room to roam. In many mammalian carnivores, the juveniles move out of their mother’s territory to establish their own, with the males moving long distances to find new territories. When their dispersal routes to seek out prey, territory and mates are severed by roads, cities, villages, mining, and railways, natural populations can be isolated into small island populations. These small isolated populations face a higher risk of disease and extinction. Population genetic structure can help us determine the isolation or connectivity in natural populations.
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To put it simply, genetic structure captures the level of genetic variation existing in a population. If mammals from different forest patches are mating with each other, they will be genetically more similar and will consequently have a lower genetic structure. Similarly, isolated populations will be genetically more distinct and will have higher genetic structure. Using DNA as a tool, estimating structure and connectivity in mammals can help us understand fine scale impacts of fragmentation. While there are several such studies on tigers, very little is known about how fragmentation impacts other mammalian species.
A new study published in Diversity and Distribution answers how the same level of habitat fragmentation has differential impacts multiple mammalian species in the threatened Central Indian Landscape. Researchers from the National Centre for Biological Sciences (NCBS), Bengaluru, investigated the genetic structure and connectivity in four long-ranging mammalian species — jungle cat, leopard, sloth bear, and tiger. The four species have varying body size, diet and dispersal abilities, and are expected to have species-specific responses to fragmentation. The two major objectives of the study were (1) to understand how the genetic structure is partitioned between these four species and (2) to find out how different landscape features like roads, built-up areas, and human density impact connectivity and dispersal in the study species.
Researchers used a citizen science approach for collecting field samples for their study. Multiple teams of volunteers, mostly students and wildlife enthusiasts, walked on forest trails and roads and collected animal fecal matter or scat samples in the dry seasons between November 2012 and April 2017. The field team covered forested areas both inside and outside protected areas – Kanha Tiger Reserve, Pench Tiger Reserve, Bandhavgarh Tiger Reserve, Achanakmar Tiger Reserve, Nagzira-Navegaon Tiger Reserve, Satpura Tiger Reserve, Sitanadi-Udanti Tiger Reserve, Panna Tiger Reserve, and Tipeshwar Wildlife Sanctuary, in addition to nine territorial forest divisions. Scats from sampled individuals – 90 jungle cats, 82 leopards, 104 sloth bears, and 117 tigers revealed intriguing results.
In a first, DNA analyses reveal how the genetic structure of the four mammalian species depends on their distribution patterns in Central India. Jungle cats and sloth bears have a wide and continuous distribution in the landscape, followed by leopards, and then tigers. Continuous distribution is likely to result in more genetic exchange between populations and lower genetic structure. Patchy distribution increases genetic differentiation between populations, resulting in a higher genetic structure. Tigers seem to have the highest genetic structure between populations, followed by leopards, sloth bears, and jungle cats, an important finding. The authors speculate that the drastic decrease in tiger population over the last two centuries along with their current restricted distribution might have resulted in low genetic similarity between populations. Meanwhile, jungle cat populations are more genetically similar possibly due to much wider distribution, comparatively weaker habitat association, and lower dispersal ability.
Smaller wild cats like jungle cats have missed out on popularity because of their more charismatic and larger cat cousins like leopards and tigers, and not just from scientific literature, but funnily enough, from popular fiction like The Jungle Book as well. While leopard, tiger and sloth bear find representation as Bagheera, Shere Khan, and Baloo respectively, jungle cats do not make an appearance in this fictional account. Yet in reality, they are in fact, quite common in the Central Indian landscape. Jungle cats are superbly adapted to make use of both forests and agricultural lands. The research team collected several scat samples in and around agricultural fields. Quite popular among the locals of the landscape, jungle cat is known as ‘raan billa’ or ‘janglee bilaao’ in many parts of Central India. Studies to prioritize habitat areas used by smaller and less iconic mammals such as the jungle cat are rarely carried out. The present study not only addresses the genetic structure of jungle cats but also highlights the conservation value of non-protected areas and small habitat fragments embedded in human-modified landscapes for small mammals like the jungle cat.
Based on genetic signatures, the study also parsed out how different landscape variables – land-use change, human presence and density of linear features like roads, railways and canals impact movement and connectivity in the study species. It was found that land-use change was the best predictor of genetic connectivity in all four species. The spread of built-up areas offers resistance to movement in all four species. While density of roads, railways and canals have a high negative impact on the movement of jungle cats in particular. Taken together, humans have negatively impacted the movement and connectivity of all four wild mammals in the landscape.
Interestingly, roads have a variable impact on the four species. For instance, tigers avoid high-traffic roads but don’t mind smaller low-to-medium traffic roads. Nevertheless, low-to-medium traffic roads negatively impact dispersal in leopards. About 70% of roads crisscrossing central India are low to medium traffic roads. The research team encountered several jungle cats killed on such smaller roads while carrying out fieldwork. So, smaller roads may not impact tiger connectivity but bear a big cost to the dispersal of both jungle cats and leopards. Based on these findings, the authors suggest installing wildlife mitigation structures on not just national and state highways but smaller roads as well.
Left unchecked, fragmentation of habitats will pose the greatest threat to long-ranging wild mammals in India. For their long-term persistence, mammalian populations need to stay connected to each other. Along with delineating wildlife corridors for tiger movement, we should also conserve non-protected forests and small open habitat patches used by species lying outside protected areas like sloth bears and jungle cats. This is the first comparative study in mammals from India to demonstrate a relationship between distribution and genetic structure. The study also leaves us with a better conceptual understanding of how different wild mammals interact with their changing landscape. These findings can play a crucial role in tailoring new and effective conservation strategies in and around protected areas catering multi-species connectivity.
Original paper: Thatte, P., Chandramouli, A., Tyagi, A., Patel, K., Baro, P., Chhattani, H., & Ramakrishnan, U. (2020). Human footprint differentially impacts genetic connectivity of four wide-ranging mammals in a fragmented landscape. Diversity and Distributions, 26(3), 299–314. https://doi.org/10.1111/ddi.13022
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