When Tigers Vanish From India's Wildlife Reserves, the Prey and Vegetation Collapse Too

By 2004, every tiger had been poached out of Sariska Tiger Reserve in Rajasthan. What followed was not silence, it was a slow, measurable unravelling. Sambar and chital populations, freed from predator pressure, expanded into areas they had previously avoided. Overgrazing stripped ground cover from sections of the reserve that had held dense understorey for decades. The dhok forest, already stressed by encroachment at the edges, lost regeneration in its interior. Sariska became the first Indian reserve to document, in real time, what ecologists call a trophic cascade: the chain reaction that moves through a food web when its controlling predator disappears.
The cascade does not work the way most people picture it. The obvious loss is the predator itself. The less obvious loss is the behaviour the predator enforced. Prey animals under active predation do not simply eat less, they eat differently. They avoid open ground. They move constantly. They do not linger in one patch of vegetation long enough to exhaust it. Remove the tiger, and the sambar stops moving. The grass does not grow back.
Research from Nagarhole and Kanha tiger reserves has tracked ungulate behaviour across areas with high tiger density versus areas where tigers are sparse or absent. The difference in grazing pressure is not marginal. In low-predation zones, herbivores concentrate around water sources and forest edges and stay there. In high-predation zones, the same species distribute across a wider area, their movement patterns shaped by the constant, low-level awareness that a predator might be present.
This is the ecology of fear, a term formalised by researchers studying wolf reintroduction in Yellowstone but documented independently in India's tiger reserves. The predator does not need to make a kill to affect the habitat. Its presence alone changes where prey animals graze, how long they stay, and which plant species get relief from browsing. When the predator is gone, that spatial discipline collapses. Certain palatable plant species get hammered. Others, less preferred by deer and nilgai, take over. The composition of the vegetation shifts.

In parts of Sariska post-collapse, invasive lantana spread into patches that had been kept open by diverse grazing patterns. Lantana is toxic to most native herbivores and suppresses the germination of native species beneath it. A single predator's absence had created conditions that a weed then locked in.
The most counterintuitive finding from India's tiger reserves is that vegetation structure, the height, density, and species mix of plant cover, is partly a record of predator history. Forest sections that have had continuous tiger presence for decades show measurable differences in understorey diversity compared to sections that experienced even brief predator gaps.

This is not a small effect. A 2015 study published in Biological Conservation, drawing on data from multiple Indian tiger reserves, found that areas with higher tiger density correlated with significantly greater tree sapling recruitment, the next generation of forest. The mechanism is indirect: tigers suppress deer numbers and movement, deer suppress saplings by browsing, so tigers protect saplings by keeping deer moving. The tiger never touches a sapling. But the sapling's survival depends on the tiger being somewhere in the territory.
Soil is part of this too. Where herbivores concentrate and overgraze, soil compaction increases, water infiltration drops, and erosion follows. The rivers and streams running through these reserves carry more sediment. Aquatic species dependent on clear, slow water lose habitat. The cascade does not stop at the treeline.

Tigers are not the only apex predator whose removal reshapes Indian habitats. In the Deccan plateau and parts of Madhya Pradesh and Maharashtra, the Indian wolf historically kept nilgai and blackbuck populations in check across scrub and grassland. As wolf numbers declined through the 20th century, through persecution, habitat loss, and prey depletion, nilgai populations expanded dramatically. Nilgai are large, difficult to deter, and not protected under hunting law in several states precisely because farmers lobbied for that status after crop raids became economically devastating.
The irony is precise: the removal of the predator that kept nilgai populations in balance created the conditions for nilgai to become a pest, which then created pressure to cull them, which further disrupted the scrubland food web. Wildlife managers in Maharashtra have documented this loop directly. The wolf's absence is felt not in wolf territory but in wheat fields fifty kilometres away.
Panna Tiger Reserve in Madhya Pradesh lost all its tigers to poaching by 2009. The National Tiger Conservation Authority reintroduced tigers from Kanha and Bandhavgarh between 2009 and 2011. What followed has been studied closely. Prey populations, which had begun to show the spatial compression typical of predator-free zones, redistributed within a few years of the reintroductions. Vegetation monitoring in Panna showed early signs of recovery in heavily grazed patches within a decade.
Recovery is not fast and it is not guaranteed. Sariska received reintroduced tigers starting in 2008, but lantana infestation and continued human pressure at the reserve boundary have slowed forest recovery considerably. The cascade runs in both directions: predator loss degrades the habitat, but predator return does not automatically reverse the damage on the same timeline. Some of what breaks in a predator-free period takes far longer to repair than it took to destroy.
Project Tiger, launched in 1973, was built on the intuition that protecting tigers meant protecting everything beneath them in the food web. The data from five decades of reserves, Ranthambore, Jim Corbett, Tadoba, Nagarhole, Sundarbans, has made that intuition precise. Tiger density correlates with prey diversity, vegetation health, and watershed integrity across every reserve where long-term data exists.
The tiger is not a symbol of India's wildlife. It is a load-bearing structure in the habitats it occupies. When Sariska's tigers disappeared, the reserve did not become a tiger-free version of what it was. It became something else entirely, and the thing it became was written in the grass, the lantana, the compacted soil, and the silted streams, none of which had ever seen a tiger up close.