Terrain Roughness And Human Disturbance Affects Tiger Dispersal In The Western Ghats And Central India
- Topographical roughness of the landscape and human disturbance are correlated with tiger gene flow in the Western Ghats and Central India, but gene flow was associated with smooth, flat terrain in the former landscape while it was correlated with steep topography and rough ridges in the latter.
- Past human disturbances have been relatively lower in the smoother terrain in the Western Ghats whereas extensive human activities in Central India have left mostly rough terrain as a refuge for tigers.
- Preserving wilderness and taking into account the adaptation of tigers to landscapes features are necessary for designing effective corridors.
Roughness of the terrain is an essential factor driving tiger gene flow in the northern Western Ghats as well as in Central India, finds a new study. Additionally, in both landscapes, the terrain used by tigers for dispersal depends on the level of human disturbance; tigers tend to avoid areas with a high human footprint.
The Western Ghats boasts the largest contiguous tiger population in the world. Understanding how landscape structure between different reserves within the region affects tiger dispersal is useful for conservation planning. But landscape-wide studies that have explored these effects are scarce.
Listed as Endangered by the International Union for Conservation of Nature, tigers in India are mostly found inhabiting six different forested landscapes—Shivalik Hills and the Gangetic plains, Central India, Eastern Ghats, Western Ghats, North-eastern Hills and Brahmaputra plains, and Sundarbans—with different topographical features and levels of human disturbance, the study states. To shed light on how variations in topography such as ruggedness, for example, affects tiger dispersal, studies are needed in each type of landscape.
Within these landscapes that are often also inhabited by humans, tiger populations are scattered into small, fragmented networks of subpopulations, which are usually too small to persist in isolation in the long-term, explain the researchers. Tigers, mostly found in reserves within these landscapes, move from one population to another to find mates and prey.
Maintaining connectivity between core populations through landscape management of corridors is, therefore, essential to ensure genetic diversity and to prevent inbreeding depression—a phenomenon where the offspring of related individuals have lower survival and fertility.
“Tiger habitat is already extremely fragmented,” said ecologist Jean-Philippe Puyravaud from Sigur Nature Trust and co-author of the study. “The largest tiger population in the world (400) is found in and near the Nilgiri Biosphere Reserve. This population may appear to be large, but in actual fact, it is in great danger of losing its genetic diversity. Since all other tiger populations are smaller, genetic drift is a reality in the smaller, isolated populations. For this reason, all efforts should be made to reconnect reserves whenever possible.”
Correlating landscape features with tiger gene flow
In an earlier 2017 study from Central India covering Madhya Pradesh and parts of the neighbouring states of Rajasthan, Jharkhand and Maharashtra, Anuradha Reddy, senior scientist at the Centre for Cellular and Molecular Biology (CCMB) in Hyderabad and her colleagues found that tiger gene flow is mainly influenced by the roughness of the terrain.
Tiger dispersal may not always result in gene flow, said Puyravaud. Gene flow is an important measure, he explained, that measures the connection between populations. It provides proof “that two areas have a single population.”
This time, Reddy, Puyravaud along with coauthors Samuel Cushman and Harika Segu, conducted a similar study in the northern Western Ghats. By looking at gene flow data they tried to find out how landscape features between tiger populations such as topographic roughness, relative slope position and human disturbances affect and limit the connectivity of these populations.
Human disturbance to tiger habitats can take many forms and is not only from village settlements, deforestation, and agriculture but also from “people who trek and disturb tigers even slightly,” including jeep safaris as well as festivals among other activities, elaborated Puyravaud.
The genetic data of 115 tigers was gleaned mostly from tiger faeces collected from protected areas within the study area, which covered 30,000 square kilometres from Bhadra Tiger Reserve in the north to the Nilgiri Biosphere Reserve, the latter of which straddles the states of Karnataka, Tamil Nadu and Kerala.
The team optimized a landscape resistance model for the Western Ghats by correlating the genetic distance between individual tigers to landscape features in between. They then compared these findings with those from Central India to assess how and why limiting landscape features differ between the two regions. The researchers also examined how differences in land use over the past five decades have affected the landscape.
Rough terrain in Central India versus smooth in the Western Ghats
The results showed that like in Central India, topographical roughness was the main feature associated with tiger gene flow in the Western Ghats. In both regions human disturbance controlled how tigers respond to topography. Unlike in Central India where tiger gene flow was associated with rough terrain and ridge tops, gene flow in the Western Ghats was correlated with smooth terrain.
The ruggedness of the landscape is a key feature, Puyravaud said. “Tigers mate in valleys in southern India and on ridges in Central India.” This, he reasons, is likely related to how humans occupied and used the landscapes in the past.
“In southern India, large reserves were protected by Maharajas in good habitats (flat terrain that had escaped agriculture)” explained Puyravaud, but in Central India “this did not happen the same way,” and all of the better lands were used for agriculture leaving only rough terrain without a human footprint.
As a result, Puyravaud said, tigers have preserved their original habits in the South, “whereas in Central India, human pressure is such that they have adapted” to the “more rugged environment and now,” they tend to “mate – disperse along ridges,” which have become a refuge from human disturbances.
“The work of these authors highlights the critical need for landscape connectivity to maintain viable wildlife populations and the need to understand which landscape features enhance or inhibit movement of animals,” said Ruth DeFries, professor of ecology and sustainable development at Columbia University, who was not involved in the study. “Such work can provide input into places where corridors might be most effective.”
Indeed, these findings have implications for the designing of effective corridors for population connectivity. Although “tigers are excellent dispersers and can cross even agricultural landscapes (villages, cities and modern highways can’t be crossed), “they avoid humans and the slightest human presence is a disturbance,” revealed Puyravaud.
Consequently, it is “tremendously important to preserve some wilderness” in the core areas of reserves, Puyravaud stressed, pointing out that “most core areas are also used by humans.” He suggested that “with proper organisation of landscape development, population connectivity may be preserved for the rare and elusive dispersing tigers, without excessive risk for people or cost to the community.”
Reddy, P.A., Puyravaud, J.-P., Cushman, S.A., Segu, H. (2019). Spatial variation in the response of tiger gene flow to landscape features and limiting factors, Animal Conservation, doi.org/10.1111/acv.12488.