Changes in future land use, increased fragmentation of tiger habitat and the inevitable loss in genetic diversity puts tiger populations at small and isolated reserves at high risk of extinction. Unplanned development can increase the probability of extinction by over 50%, a recent study has found.
The study, Maintaining Tiger Connectivity and Minimising Extinction into the Next Century: Insights from Landscape Genetics and Spatially-Explicit Simulations, published in the February 2018 issue of the Biological Conservation journal, examined the population connectivity of tigers across nine reserves, and used genetic data to infer the impact of changing landscapes on the species and simulate their extinction probability in different scenarios.
The study, conducted by researchers from the National Center for Biological Sciences, TIFR, Mumbai; the NGO Wildlife Conservation Trust; the Foundation for Ecological Research, Advocacy and Learning, Puducherry; and the School of Public and Community Health Sciences, University of Montana, in the United States, projected that unplanned development would result in a 35% decrease in genetic variability (lower heterozygosity) and a 56% increase in the average extinction probability of tigers within these protected areas.
Increasing tiger numbers in such a scenario will decrease the extinction probability by just 12 percentage points (from 56% to 44%), the study found.
Like other large carnivores, tigers require vast swathes of jungle to hunt and thrive, and are particularly vulnerable to habitat fragmentation. India has nearly 65% of the world’s tigers, and the significant recent strides in conservation notwithstanding, several studies have highlighted the genetic risks that the species faces in its increasingly isolated populations, and consequent inbreeding.
For the latest study, the researches collected genetic information from faecal samples from wild tigers, and used these for landscape genetic simulations (simulating genetic data alongside changes in specific landscape conditions such as infrastructure, roads) to model 86 different scenarios. The simulations incorporated the impact of land use changes in the future on the inferred population connectivity at the time, and the probability of extinction — thus linking basic science to policy on changing land use, and planned infrastructure development.
“This is to see how changes in landscapes are going to change tiger connectivity. How far do tigers move? There was quite an interesting 2013 study that showed tigers were moving long distances. The longest was between Kanha and Nagarjunasagar-Srisailam Tiger Reserve, where it was 650 km as the crow flies,” said Aditya Joshi of the Wildlife Conservation Trust, one of the authors of the new study. [‘Connectivity of Tiger (Panthera tigris) Populations in the Human-Influenced Forest Mosaic of Central India’, Ramakrishnan et al., PLoS ONE 8(11): e77980; Open access, http://bit.ly/2rLybTb%5D
“Then we got curious… connectivity exists. We looked at development work, the rate at which landscape is changing. We then looked at genetic data and correlated that with landscape features — roads, urbanisation, the rate at which some roads were becoming four-laned,” said Joshi, who was also a co-author of the 2013 study.
The 2018 study showed that restoring and protecting corridors between protected areas could lead to the “least decline in genetic variation” — around 20%. While small, isolated protected areas had the “highest risk of extinction”, the addition of a buffer reduced the extinction probability by 23%. Adding a buffer zone around small populations that are currently connected, reduced the extinction probability by as much as 70%.
Another scenario that the study looked at was the impact of mines and the associated change in land use. Increase in mined area and increase in the built-up area can lead to “15 times higher extinction probability” in small and medium protected forests such as the Bandhavgarh Tiger Reserve (Madhya Pradesh), the Sanjay National Park (Madhya Pradesh-Chhattisgarh) and the Umred Karhandla Wildlife Sanctuary (Maharashtra), all of which are close to coal fields, the study found. The scenario factors in the government’s target of increasing the contribution of the mining sector to the GDP from 2% to 5%, and the amendment of the Mines and Minerals (Development and Regulation) Act in 2016, which expedited environmental clearances.
A key threat to tiger populations is the increase in road traffic, which is estimated to grow at 13% per annum for the next two decades. The busy Mumbai-Kolkata National Highway 6 (old numbering), the widening of which is being considered, cuts through the Nagzira Tiger Reserve and Nawegaon National Park in the Bhandara and Gondia districts of Maharashtra. There has already been an increase in agricultural activities and construction along the highway, making it harder for tigers to move across. Should NH6 become a barrier for tiger movement between the corridors it bisects, the chances of extinction of the animal in that area will increase from 19 times to 65 times, the study shows.
“Roads are the biggest game changers,” Joshi said. “In the west there has been a lot of research on road ecology. But it is a very new science in India,” he added.
The study has suggested the installation of mitigation structures — under- and over-passes for wildlife to pass — before roads are built or widened.