The shortwings on sky islands
Bangalore/Akkamalai (Tamil Nadu): At first the term “sky island” sounds like a space station, a place right out of the realms of science fiction. But sky islands are on earth, on a sequence of mountains separated by valleys. The valleys here play the role of the water that surrounds an island. Like the volcanic islands of the Galapagos, where isolation by sea acted as a barrier for species migration and helped retain its unique characteristics, as explained by Charles Darwin in his book On the Origin of Species, sky islands across the world have been cradles of evolution. These elevated terrestrial islands on mountain tops are home to a unique set of species, living in a time warp and having the highest ecological and evolutionary significance in our natural world.
There are 20-40 sky island complexes on earth, according to different groups of scientists. One among them is part of the Western Ghats, a 1,600km-long chain of mountains running all along the west coast of India. The southern Western Ghats, around 670km long, from Baba Budan to Peparra range, is one of the most isolated sky island systems in the world. The sky islands are situated on the upper elevations of these mountains, above 1,500 metres (m), which are home to the unique stunted Shola forests, a mosaic of montane evergreen forests and grasslands. Shola is a Tamil word which means evergreen dense thicket. Identified as a biodiversity hotspot, the Western Ghats have the highest species endemism in the Indian subcontinent—meaning species that are found only in this region. The term “biodiversity hotspot” specifically refers to 25 biologically rich areas in the world that are significant reservoirs of biodiversity threatened by human destruction.
“Sky islands are different from oceanic islands, here intervening valleys act as barriers or may turn into bridges for dispersal, depending on the ecology of the species and the conditions in the valley,” says V.V. Robin, ornithologist and fellow at the National Centre for Biological Sciences (NCBS).
The Shola ecosystem on the Western Ghats sky islands is home to a diminutive bird called shortwing, an insectivorous blue bird weighing barely 25g. Since 2000, Robin has been conducting research on avian species in the Western Ghats sky islands, focusing specifically on the shortwing. As its name suggests, the shortwing has a very short wingspan, which restricts its flight over long distances. The bird, found above 1,200-1,500m in the Shola forests, is known by four names—the white-bellied shortwing, the white-bellied blue robin, the Nilgiri blue robin and the rufous-bellied blue robin. The one in the Western Ghats is among the six species of shortwings found in the world. The rest are found in the forests of north-east India and other parts of South Asia. The Western Ghats shortwing, an endangered species, is the only one found in southern India and remains restricted to the Shola forests.
The taxonomy, or classification, of the shortwing in the Western Ghats has always been debated—by noted ornithologists including Thomas C. Jerdon, Edward Blyth, Eugene Oates, E.C. Stuart Baker, Salim Ali and Sidney Dillon Ripley— dating back to the colonial times. In 2005, Pamela Rasmussen, a research associate of Ripley, split the species into two by its distribution on either side of the Palghat Gap, the widest and lowest in elevation in the Western Ghats. According to Rasmussen, the Nilgiri blue robin is found north of the Palghat Gap and the white-bellied blue robin is found south of it. Robin of NCBS confirms an ancient divergence in these two populations and their elevation to full species through DNA (deoxyribonucleic acid) sequencing studies, but the genus position is not settled yet. Whether these races interbred remains a mystery. A single museum specimen at the Natural History Museum in Tring, the UK, shows plumage characteristics of both races.
In terms of age, the Western Ghats are much older than the Himalayan range. The Ghats are not purely mountains, but a series of undulating domal uplifts of an upraised plateau. The Western Ghats have three major geographical breaks, the youngest and northernmost is the Goa Gap, which was formed 65-80 million years ago; the other two are the Palghat Gap, which is the widest, and the Shencottah Gap, the narrowest, both of which were formed about 500 million years ago. “There have been very few studies examining the impact of these gaps on the distribution or population, genetic structure of different species in the Western Ghats,” says Robin. These gaps were formed in successive Ice Ages when forests contracted and grasslands expanded because of aridity in the tropics. The world over, forests expanded after the last Ice Age.
This is why sky islands pose numerous puzzles about vertical migration strategies used by plants and animals annually and over glacial time periods.
The investigation on the evolutionary history of the shortwing has led Robin to conclude: “All shortwing populations were a single population that split into two, on either side of the Palghat Gap, about five million years ago.”
During the British era, the Shola grassland ecosystem was seen as unproductive and was replaced with widespread plantations of tea, coffee and timber. This dealt a severe blow to the shortwings. These birds are known to be very sensitive to habitat disturbance and avoid crossing inhospitable habitats. The Nilgiri pipit, another endemic bird, was wiped out from the Kodaikanal range with the loss of its grassland habitat.
At present, Robin, with support from different organizations, has been trying to understand the songs of the shortwing and other endangered birds endemic to the area, such as the Nilgiri pipit, the black-and-orange flycatcher, the broad-tailed grassbird, the grey-breasted laughingthrush, the Nilgiri laughingthrush and the Nilgiri flycatcher. The “song project” is a collaborative effort of bird watchers interested in avian acoustics like Anil Prabhakar from the department of electrical engineering, Indian Institute of Technology, Madras, who comes with experience in “signal processing” to examine differences in bird songs across sky island species. “We realized the books were totally inaccessible to people with visual impairments who could be strong partners in our conservation efforts. It began with algorithm developments, and my association with the bioengineering research initiative at NCBS, borrowing on ideas typically used for human speaker identification. The bird song project has relied on support from interns and students,” says Prabhakar.
Bird songs are known to vary across scales of geographical separation. When Robin compared songs from three shortwing populations, one was genetically distinct from the other two populations on another island. These two populations were genetically similar, but were separated by recent deforestation. “The research revealed that songs across genetically different populations were completely different, while songs of populations separated by deforested lands were also different. We are in the process of examining the song syntax for cultural transmission of song types between geographically isolated, but genetically similar populations,” says Robin. “Scientists are still not clear about the difference between the robin and the shortwing. It is a cryptic bird, which remains hidden on the forest floor, except during the breeding season when its characteristic song, a series of shrill whistles and buzzing, makes it easy to locate. Very little is known about the relationship between species distribution patterns and major geographical barriers in the Western Ghats. There is no information on the influence of topography on the phylogeography of a Western Ghats species,” says Robin.
A trek through the Shola forests in the Akkamalai Hills in Tamil Nadu gives a sense of how difficult it is to locate the shortwing. In the eerie silence, birds scurried past like bullets, there were dozens in the undergrowth, yet no sightings of the shortwing. The forest itself is a dense canopy filled with life—innumerous insect species, birds, elephants, tigers and leopards. The Shola forests are under the government’s protected area network and one cannot enter the landscape without special permission from the state forest department.
During the last Ice Age, much of the northern hemisphere turned into ice and sea levels fell. Around this time, the tropics faced a severe dry spell, causing the wet forests to retreat to the higher reaches of the mountains. “Our data indicate that during this time all the shortwing populations on different sky islands shrank. Instead of forming a single refugium on a mountain top, the shortwings formed several independent refugiums on each mountain top. When the Ice Age receded, the forests expanded and shortwing populations increased at an evolutionary scale. But the recent anthropogenic fragmentation and deforestation could be causing a decline in the population of this species,” says Robin.
According to a new research paper, Reassessment of the distribution and threat status of the Western Ghats endemic bird, Nilgiri Pipit, published in Current Science on 25 August, the Nilgiri pipit is possibly India’s most restricted range species, found only in an area of 400 sq. km in the Western Ghats, and nowhere else on the planet.
The paper also highlights extensive loss of grasslands as having a hand in local extinction of the Nilgiri pipit in places such as Kodaikanal and Ooty, and historic climate change for the present distribution pattern.
Based on current research data, Robin and his colleagues C.K. Vishnudas and Uma Ramakrishnan believe the distribution of the Nilgiri pipit in general is shaped by mountain physiography and palaeo-climatic events, while at a smaller scale (within each mountain) the loss of habitat has impacted the distribution of the species. “It’s fascinating to explore the Shola community in the Western Ghats and the evolution of the species that live there. It’s also amazing to realize that while these systems are supposedly well studied, more systematic research suggests we still know very little about the distribution of these species,” says Ramakrishnan, who is a scientist at NCBS.
In the last few decades, India has lost about 30% of its forests. The conversion of the Shola forests into tea and coffee plantations, cultivation land, reservoirs and human settlements has led to extensive damage and fragmentation of forests in the Western Ghats. There is an increasing demand to open more areas for development in these Ghats in opposition to a Western Ghats Ecology Expert Panel report, which proposed a series of conservation measures to protect this sensitive eco-zone.
The new government is in a wait-and-watch mode and environment minister Prakash Javadekar remains noncommittal on the issue. In a recent statement, Javadekar just said that the Western Ghats are not only home to rich biodiversity, but also support a population of approximately 50 million people and include areas of high human population density. However, the transformation of the landscape may have affected the ecosystems of the Western Ghats.
The shortwing and sky islands have evolved and survived through many millennia. Can they survive man’s ruthless developmental agenda?
This is the fourth part in a series in which Mint looks at species that are less talked about and struggling for survival. Read the first part of the series about dholes, the second part about wolves and the third part about pangolins .
Mint’s wildlife writer Ananda Banerjee received a fellowship from the Forum of Environmental Journalists of India and the Ashoka Trust for Research in Ecology and the Environment to study these species.