RED EARTH Environment

Sky islands under siege

Sky islands, such as the Palani Hills (top) harbour an extraordinary number of endemic species.   | Photo Credit: Prasenjeet Yadav

It was an early morning in May, 2015. The sun was yet to rise. A cold breeze from the south wafted over us. We were sitting on a giant rock on one of the highest mountains of the Western Ghats—the Camel’s Hump mountain—overlooking the dark valley of Nilambur, waiting to catch the sunrise. And as we gazed east, the rays emerged from behind the giant peaks.

The silence was suddenly broken by a long musical call from a Sholakili from a bush, and the fluttering of a flock of mountain babblers. A giant white cloud appeared out of nowhere and gradually occupied the vacant valleys. Quite soon, the entire valley became a white ocean. Only a few mountain peaks emerged out of this vast swathe like islands, islands in the sky—the ‘sky islands’ of the Western Ghats—an extraordinary ecosystem that has always fascinated evolutionary biologists.

Mountaintops separated from each other by deep valleys are indeed much like islands of the ocean in terms of evolutionary processes because of their isolation. These mountains were formed during the uplift of the Western Ghats, starting around 150 million years ago, where the high-elevation montane habitat, the shola (a mosaic of forest and grasslands), evolved. In fact, they are cradles of evolution. The formation of species here occurred over millennia, dictated by the geographic and climatic isolation of these mountain tops. And so what we have today is high endemism, that is, a concentration of species—birds, frogs, plants, and fish—that are found nowhere else in the world.

To a scientist, mountain habitats are also natural laboratories where one can study behavioural and evolutionary changes in response to climatic and human-made pressures.

Adapting to new ways

Life forms have adapted to these sky islands much like they would to oceanic islands. Each sky island is home to distinct species. Among birds, we see huge differences—morphological and acoustic—in these islands.

Sky islands under siege

We recently uncovered two groups of birds that arrived at the Western Ghats from the Himalayan region about 11 million years ago (probably some of the earliest extant Western Ghats birds) got isolated here on different mountain-top sky islands, forming different species starting about 4.5 million years ago. These groups are very distinct from all existing birds today and so we reclassified them into two new genera with common names—Sholakili and Chilappan—which together contain seven species. Chilappans, previously known as laughing thrushes, have radiated into four different species over four sky islands within a 500 km span of the southern Western Ghats.

Some species—such as understorey birds (birds that usually move only where a forest cover exists)--are inflexibly adapted to their habitats, making them very susceptible to habitat or landscape changes.

Between 2011 and 2014, we conducted expeditions to several of these sky islands: the Bababudan Hills, the Brahmagiris, Banasura Hills, Camel’s Hump Mountains, Chembra Hills, the Nilgiris, Anamalais (Eravikulam, Munnar, and Meeshapuli mountains), the Palanis, the High Wavies, Ponmudi and Ashambu Hills. We camped for several weeks in each of these areas, spending over 300 days examining birds.

Apart from the lesser known mountains, we also spent considerable time in the Nilgiris and the Palanis, the larger urbanised areas of the Western Ghats. Finding suitable locations to study grassland birds was always difficult because human impact had converted much of the natural grasslands.

Last vestiges

The Palanis on the other hand are almost entirely overrun by wattle and eucalyptus, decreasing the densities of many grassland bird species. Some national parks such as Eravikulam, Grasshills, and Mukurthi have the last vestiges of the shola grasslands, and they harbour specialist birds.

Sky islands under siege

By examining bird diversity and distribution patterns using both ecological and genetic data, we found that bird communities in the Western Ghats are shaped by both mountain structure and ancient climate. This also implies that future climate change can further impact these habitats and birds.

But these fragile mountain islands contend with more than climate change today. While large-scale deforestation is not common in these parts of the Western Ghats, invasive species have impacted the landscape tremendously. The scotch broom (Cytisus scoparius), a shrub native to western and central Europe, is invading large parts of the Nilgiris grasslands, while the wattle (Acacia sp.) is also transforming these grasslands into wooded areas. In the shola forests, the Cestrum aurantiacum, a weed from South America, is invading the understorey.

There is also an alarming level of fragmentation—of both forest and grasslands—from agriculture, timber plantations and commercial plantations, a phenomenon that is affecting the large sky islands of the Nilgiris and Anamalai-Palani hills. And with this, bird populations are getting split into smaller populations with reduced or no gene flow between them. We uncovered that the Sholakili had a reduced gene flow on an east-west axis, between areas like Kodaikanal and Munnar. Even the songs (comparable to human language) of the Sholakili were different across this fragmented landscape, indicating cultural divergence, most likely driven by human activity.

Some small populations may well be heading towards extinction—the Nilgiri pipit, for instance, in the grasslands of Palani Hills. Landscape change—particularly from plantations and invasive species—has reduced the total available habitat for this species (globally) to less than 400 sq.km. And as climate change shrinks grasslands, these birds really are getting a raw deal.

Habitat loss threatens not just birds but all other ‘habitat specialists’ surviving in the shola sky islands—the agamid lizard Salea anamallayana, shield-tail snakes, kurinji plants, perhaps even mammals (although to a lesser extent) such as the Nilgiri tahr and Nilgiri marten.

As the sun moved up, various life forms emerged from the shrubs and tree-tops, followed by a little chorus of laughing calls, murmurs, and whispers from the hiding life forms hiding underneath. A new day had begun for life on these tiny sky islands, where adaptation and evolution that began millions and millions of years ago must continue.

C.K. Vishnudas is a biologist affiliated to Hume’s Centre for Ecology and V.V. Robin works on the ecology of birds on islands systems, and is based at IISER-Tirupati.

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Printable version | Nov 16, 2020 9:14:30 AM | https://www.thehindu.com/sci-tech/energy-and-environment/sky-islands-under-siege/article18052925.ece

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