Shola Forests of the Western Ghats: How Millions of Years of Isolation Built a Sky Island of Endemic Species

A Shola forest patch rarely exceeds a few hectares. It sits in a valley crease between rolling montane grasslands in the upper Western Ghats, places like Munnar, Kodaikanal, and the Nilgiris, tucked low enough to catch moisture but surrounded on all sides by open terrain that most of its species cannot cross. The grassland is not a degraded forest. It is a distinct, ancient habitat maintained by frost, fire, and the specific hydrology of high-altitude plateaus. The forest patches and the grassland exist in a locked relationship: neither is the other's future state.

This is the sky island structure. Each Shola patch is an island of closed-canopy forest in a sea of open montane grassland, and the sea is just as impassable to most forest species as open ocean. Two patches separated by 500 metres of grassland might as well be separated by a strait. Gene flow stops. Populations diverge. Given enough time, and the Shola system has had somewhere between 26 and 65 million years, placing its origins in the Eocene, divergence produces species that exist in one patch, or one range of patches, and nowhere else on the planet.

The Western Ghats, recognised as one of the world's eight hottest biodiversity hotspots by Conservation International, contains roughly 5,000 species of flowering plants, of which about 1,700 are endemic. The Shola forests carry a disproportionate share of that endemism. The Nilgiri tahr (Nilgiris tahr, Nilgiritragus hylocrius) is found only in the upper Ghats. The lion-tailed macaque, one of the most endangered primates in India, depends on these forests. The Shola flycatcher (Ficedula nigrorufa) breeds only in Shola interiors. The Nilgiri pipit nests only in the adjacent montane grasslands.

The biological fact that makes this striking: species that have been isolated long enough do not just look different, they become reproductively incompatible with their nearest relatives. A study published in the journal Molecular Phylogenetics and Evolution found that several amphibian lineages in the Western Ghats show divergence times consistent with Gondwana fragmentation, meaning some of what lives in these forests has been on a separate evolutionary path since before the Indian subcontinent collided with Asia. The Shola patches are not refuges where species retreated. They are laboratories where species were made.

The grassland surrounding each Shola patch is not passive. It is actively maintained. At elevations above 1,800 metres in the Nilgiris and the Palani Hills, winter temperatures drop low enough to kill tree seedlings that try to colonise the open slopes. Periodic fire, historically set by indigenous communities and by lightning, clears any woody encroachment that frost misses. The Toda community of the Nilgiris has managed these grasslands through controlled burning for centuries, a practice that ecologists now recognise as functionally integral to maintaining the Shola-grassland mosaic.

Remove the fire, and woody invasives, particularly Lantana camara and Acacia mearnsii, both introduced species, move in. They are not stopped by frost. They colonise the grassland, reduce the open buffer between forest patches, and paradoxically threaten the very forest by altering the moisture and light conditions at the forest edge. The grassland's openness is what keeps the islands isolated enough to protect their endemic species. Closing that gap does not connect the islands, it destroys what made them islands in the first place.

Tea and eucalyptus plantations replaced vast areas of Shola and montane grassland through the colonial period and continued expanding after independence. The Nilgiris lost an estimated 40 percent of their natural Shola cover to plantation agriculture by the late twentieth century, according to assessments by the Ashoka Trust for Research in Ecology and the Environment (ATREE). What remained was further fragmented.

Climate change adds a different pressure. The montane species in these forests are cold-adapted. They cannot move to higher elevations because there are no higher elevations, the peaks of the Ghats are already their ceiling. A 2020 study in the journal Global Change Biology modelled climate trajectories for endemic species in the Western Ghats and found that warming of 2°C above pre-industrial levels would eliminate climatically suitable habitat for a significant proportion of montane endemics. The sky islands cannot move. Their species evolved precisely because they could not.

The Shola forests are legally protected in patches, the Mukurthi National Park, the Eravikulam National Park, and the Kudremukh National Park together hold some of the largest intact blocks, but protection of the forest without protection of the adjacent grassland misses the point of the system entirely.

What the Shola forests have spent millions of years building is not just species richness. It is the record of what life does when it is left alone long enough: it stops being general and becomes exact. Every endemic creature here is a precise answer to a precise set of conditions that exist nowhere else. The conditions are narrowing. The answers, once lost, cannot be reconstructed from the question.