A leopard. Photo: IrmaB/Pixabay
Leopards are what ecologists call ‘generalists’. These big cats can thrive in a wide range of habitats – from lush, undisturbed evergreen forests to sugarcane fields and other human-dominated landscapes – and on a wide variety of prey (spotted deer, goats, dogs, all are fair game). No wonder then that these highly adaptable big cats are found across most of India. However, between 75-90% of the country’s leopard population could have declined over the last 120-200 years, suggests a study that analysed leopard genes.
Animal genes hold many stories. Fragments of DNA left behind in animal poop – such as those in the often similar-looking tiger and leopard scats – can confirm which species the poop belongs to. Genetic material in animal scats can also tell us how well animal populations are doing and even how they fared in the past, by providing estimates of animal population sizes.
That’s how numerous genetic studies since the 2000s for instance, have inferred the drastic decline of Eurasian otters (caused by hunting, habitat loss and water pollution) across many parts of Europe over the last century. In the most recent such study published in 2016, scientists in France found evidence of this decline in the microsatellites – short, specific repeats of DNA at different regions (or loci) of the genetic material – of 144 otters. They used a ‘bottleneck test’ to reveal genetic ‘bottlenecks’ or sharp declines in population sizes, as implied by low genetic variation; declining populations usually have lower genetic variation when compared to stable ones.
To find out if India’s leopards have undergone such population declines, a team led by scientist Samrat Mondol from the Wildlife Institute of India studied genetic variation in these big cats. They collected 778 leopard scats from both inside and outside protected areas across the Terai landscape of north India (spanning Bihar, Uttar Pradesh and Uttarakhand).
To include the Western Ghats and central India in their analyses, the team used 143 leopard samples obtained from an older study led by Mondol in 2015 in their analyses; these were samples collected from 12 states including Andhra Pradesh, Jharkhand and Karnataka. From these samples, the team identified 199 individual leopards in total; they used 13 microsatellite loci to individually identify each leopard. They had to discard four samples from northeast India because they were too few for a meaningful analysis.
Four distinct leopard sub-populations in India
These methods revealed four distinct genetic sub-populations of leopards in India excluding the northeast – in the Western Ghats, Deccan Plateau, Shivaliks (including leopards in the Himalaya) and the Terai. However, analysing population changes over time for each of these sub-populations using three separate methods (that incorporated several genetic data into predictive models) revealed signs of population bottlenecks – or declines in leopard populations – in all four sub-populations.
The Western Ghats had lost approximately 75% of its leopard population, while the Deccan Plateau and Shivaliks had lost 90%. The Terai had also lost around 88% of its leopards when compared to historical populations. The north Indian populations (Terai and Shivaliks) showed the most recent decline (around 120 years ago), while the Western Ghats showed an older decline at around 200 years ago.
Many facts suggest that this decline is mostly human-induced, wrote Mondol in an email to Mongabay-India.
“This is the time when the Indian subcontinent saw major land-use changes (habitat conversion, agricultural expansion etc.),” he wrote.
Bounty hunting during the British rule in India when a large number of tiger and leopards were persecuted must also have added to the population decline; in recent times, poaching and conflict have also claimed a lot of leopard lives, he added.
Ecological estimates to add support
To compare these results showing a genetic population decline with ecological estimates of leopard extinction probability, the team analysed factors influencing leopard distribution across the country in three landscapes (Western Ghats, central India and north India) as well.
Their models incorporated factors including leopard presence, peoples’ tolerance to the big cats and presence of protected areas to study this, and found that leopards are still distributed widely across India. Cultivated lands, barren areas and deciduous forests were associated with a higher leopard presence. Calculations of extinction probabilities (on a scale of 0 to 1) showed higher values in the north India (0.37) and Deccan (0.21) sub-populations when compared to the Western Ghats (0.17).
In their study published in PeerJ, the authors write that these results suggest that “leopards demand similar conservation attention like tigers in India”, and call for a census exercise for these spotted big cats too.
A decline worth re-investigating in detail
However, scientists who study population genetics including demographic declines caution that these steep declines could be worth re-investigating. Molecular ecologist Göran Spong, who has also studied leopard genetic variation and associated population sizes in the past, commented that using genetic data at the level of markers [such as microsatellites] to make inferences about demographic processes is “always tricky”.
Limitations of genetic data such as long lag times – which means that there is often a time delay in the reduction of genetic diversity because the processes that affect it (such as mutations in DNA) are slower than demographic changes – call for “careful interpretation”, wrote the Senior Lecturer at the Swedish University of Agricultural Sciences, in an email to Mongabay-India.
“In the current paper, the relatively low number of individuals, further partitioned into even smaller clusters, worry me slightly. That the leopard population has indeed decreased (perhaps by a lot) in India seems rather likely when considering the demographic changes in the human population, and the authors’ genetic data support this. But I suspect their results would vary quite a bit depending on the set of assumptions they use (for example population substructure, mutational model, and so on). In fact, we ran a few bottleneck tests on the severely bottlenecked Swedish lynx population (down to less than 30 individuals, today 2,500), without seeing any signs of a bottleneck.”
Trishna Dutta from the University of Goettingen, whose study of gene flow in leopard populations in central India in 2013 did not find signs of a genetic bottleneck in this population said that she was “indeed surprised by this steep a decline, as it went undetected earlier”. In an email to Mongabay-India, she wrote: “If this sharp a decline is indeed true, it warrants further investigation, preferably through the use of museum specimens across different time scales, as mentioned by the authors in their study.”