Netravati Peak
Netravati Peak in Kudremukh National Park, Karnataka, where the shola-grassland mosaic ecosystem can be seen (Photo: Sesahdri K S)

Laboratory of Evolution

It was a summer afternoon in early 2006 when I first stepped into a shola forest. I was on a day-long trek up to the top of the Bandaje Arbi waterfall in Kudremukh National Park, Karnataka, as part of a large-mammal survey. As we climbed higher, the incline got steeper. The trees appeared shorter, and a crisp chillness surrounded us. Soon we caught a glimpse of the sky, and the gush of flowing water became louder. Another hour later the trail opened into a vista of grasslands. Navigating through the tall grass, we followed the sound of the stream. Fresh gaur dung suggested one must be nearby. Just as we reached the stream, the sun began to descend over the horizon. We spent a few minutes enjoying the fresh, cold water and then retraced our steps before nightfall.

Since that first memorable experience, I have walked through several shola forests along the length of the Western Ghats during my career. I’ve worked in the Nilgiri and the Agasthyamalai ranges, and though I have visited Kodaikanal, I haven’t yet ventured deep into its mysterious sholas, rich with life. The sholas are an ever-evolving cornucopia of nature’s secrets; I learn something new every time I walk through them.

Irrespective of where in the Western Ghats they are found, the shola-grassland mosaics are home to a large proportion of species that are found nowhere else on earth. Some of these endemic species are not only restricted to the shola-grassland mosaics, they are also further restricted to specific patches within them. Scientists are fascinated by such narrow and specific distribution patterns. Perhaps we have to go back in time to see why and how we got to where we are.

The shola forest-grassland mosaic comprises evergreen forest trees, often stunted, and luxuriant grass. Seen here is the shola along the path up to Tadiandamol, at the Karnataka–Kerala border. During the summer months, the grasslands begin to dry up and sustain myriad life, ranging from insects to birds such as harriers, which migrate from Europe and Central Asia to forage exclusively among grasslands (Photo: Seshadri K S)

What shapes these shola forests?

Humankind’s written and oral history can only take us back a few thousand years. But rocks can help us trace it back to millions of years ago. By studying rocks and sediments, we can sort the chronology of events on Earth into a geological timescale.

The Earth’s climate was incredibly variable, with several cycles of cooling and heating. Carbon deposits from swamps in shola forests indicate that 16,000–20,000 years ago, the climate was arid, resulting in a profusion of vegetation dominated by grasslands. Subsequently, the climate began to get wetter, and around 11,000 years ago, the forests are thought to have recolonised the vegetation. The monsoon began to weaken around 6,000 years ago, and the grasses are thought to have resurged, resulting in the present-day grassland-forest matrix.

While climate did shape these forests on a geological timescale, studies show that elevation and topography may also be strong influencers in the persistence of shola forest mosaics. Along the length of the Western Ghats, topological features such as valleys, and climatic factors, like precipitation during dry months and annual temperature, likely determine the presence and extent of shola forest mosaics. The elevation and bioclimate create thermal profiles that are likely to be unsuitable for several species. For instance, on some mountaintops with shola forest mosaics, the onset of frost limits the distribution and colonisation of plants.

Science of Diversity

Given that evolution is often a slow process spanning several million years, and the shola forest mosaics have a history of climate and habitat fluctuation, they are a perfect laboratory to study speciation. In fact, numerous species are being discovered here in the Western Ghats even today.

In the case of frogs, for example, a surprising number of discoveries have come from the Western Ghats. In 2014, researchers from the Indian Institute of Science described nine new species of bush frogs belonging to the genus Raorchestes, within the tree frog family Rhacophoridae. Of these, three were collected from shola-grasslands. These researchers further compared the distribution of bush frogs along the length of the Western Ghats. They started by creating a phylogenetic tree. Phylogenetic trees are a means of analysing relatedness of a species wherein two closely related species share a common ancestor. When the genetic relatedness is plotted as a figure, it resembles a tree. Their work indicated that around 32 million years ago, a group of bush frogs separated from their ancestors in the Western Ghats. While one population got isolated north of the Palghat Gap, a 60km wide flat region in the Western Ghats, the other was isolated south of it. Since then, both groups continued to diversify separately.

This incredible discovery prompted another hypothesis—if this relatively narrow, flat gap in the Western Ghats resulted in isolation and subsequent speciation, could the gaps between different mountain peaks too cause similar evolutionary events? Indeed, yes. The researchers found that adjacent mountain peaks have different bush frogs, but they are closely related. For example, in the Agasthyamalai region, one can find R. chalazodes, but in Meghamalai, just north of it and across the Shencottah Gap, one can find R. flaviocularis. The two species turned out to share a common ancestor. They are called sister species and are indicative of the fact that the two mountaintops once had similar habitats that were connected. At some point in time, as the mountain-tops got isolated, likely because the habitat connecting them changed, and transitioned from suitable to unsuitable habitats, the two isolated species also evolved to adapt accordingly.

A case of speciation across different mountain peaks. The Raorchestes chalazodes is found in the wet evergreen forests in and around the Agasthyamalai region.
The R. flaviocularis can be seen just north of this region, in Meghamalai, across the Shencottah Gap. (Photos: David V Raju/Wikimedia Commons)

Further studies brought up a question: could species have evolved differently on each mountain, along the elevation gradient? After all, the habitat changes considerably with elevation. Turns out, they have! Different but closely related bush frog species were found on the same mountain at different heights. This pattern was common across other mountains too, suggesting that species diversified independently on each mountain.

From bushes to Grass

The ancestors of all these bush frogs were forestdwelling species. Over time, a greater number of species transitioned to living in grasslands to the south of Palghat. The wet forests had likely served as a refugium when the landscape was changing from forests to drier grasslands and back because of climate fluctuations.

Life in the grass posed different challenges. The species restricted to the shola grasslands evolved shorter limbs compared to their relatives in the forest patches, as grass blades are not as complex a habitat to navigate as forests. This anatomical transition coincides with their habitat transition into grasslands—a phenomenon referred to as adaptive radiation.

Our shola forests have witnessed the transition of frogs from forested landscapes to the grasslands, which formed a new ecological habitat for species to adapt to and colonise. Researchers highlight that this is one of the largest adaptive radiations of a vertebrate in South Asia.

Two frogs of the same genus, Raorchestes, evolved different limbs according to their habitat. R. resplendens (left) lives among grasslands and has shorter limbs, while R. bobingeri (right) lives on tree canopies and has longer limbs. These specimens are from the Bombay Natural History Society collections. (Photos: Seshadri K S)

The value of the sholas

The shola forest mosaics, perhaps among the most geologically and biologically unique habitats, continue to be threatened. Grasslands are natural ecosystems and support unique forms of life. Minor changes in temperature and precipitation could drastically alter the habitat and affect species that depend on it, especially frogs. Climate change predictions often indicate that species move higher in altitude with rise in temperatures. For the shola-grassland mosaic, however, there is no farther elevation to go up to. In the short term, the threat to these unique ecosystems appears to come from unregulated tourism. Grasslands also have exotic trees planted in them, are cut by roads and are burnt.

Not all hope is lost, though. Shola-grassland mosaics are gaining increasing attention both among civil society as well as academicians. Many are located inside protected areas. The least we could do is shy away from our largely utilitarian perspective of these habitats and recognise the myriad diversity of life that makes this place whole. That these forest-grassland ecosystems have a fascinating and dynamic history spanning millions of years should enthral us.

If we look carefully, every organism that calls this habitat home has a story to tell—a story that implores us to conserve what is left of our sholas.

Seshadri K S

Seshadri KS studies the evolutionary ecology of amphibians at the Centre for Ecological Sciences, Indian Institute of Science, Bengaluru. He has discovered new amphibian behaviour as well as species, and is also an avid birdwatcher.

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