Songbirds in the Western Ghats driven to high altitude “sky island” refuges by ancient climate upheavals now face further genetic isolation, as human action continues to partition the habitats of birds with nowhere else to go.
We don’t tend to think of the sky as having limits. In our imaginations, the flight of birds acts as a metaphor for freedom without boundaries. But in truth the ranges where birds are found can be highly sensitive to environmental change, and barometers of our present impact on the planet. So-called “sky islands” in the Western Ghats carry songbirds restricted to high elevations by their need for the cool, moist climate of Tropical Montane Cloud Forests, locally known as shola forests. Many such fragmented songbird species were driven to their high altitude refuges by climate fluctuations millions of years ago, according to recent work by V. V. Robin and colleagues at the National Centre for Biological Sciences (NCBS), Bangalore. Additional findings from the same group suggest that deforestation within the last two centuries has isolated these island populations still further, with no sign of recent migration between fragments. Without adequate conservation measures, future habitat loss could push these fragile populations over the brink.
Mountain ranges such as the Western Ghats and the Himalayas can harbour an astonishing variety of life, partly because they support habitat types across a wide range of altitudes. Extreme dips in the terrain can also constrain migration, isolating species into pockets that diverge with time. The NCBS researchers looked at three such extremities in the landscape that might have inhibited songbird dispersal: the Palghat Gap, the Shencottah Gap and the Chaliyar Valley. The Palghat Gap in particular, separating the Nilgiris in the north from the Anamalais to the south, has been highlighted as a historical barrier for organisms from elephants to tiny frogs. These researchers looked beyond individual species, hoping that a collective study of 24 bird species might highlight larger patterns in the genetic landscape. They used the genetic distance between populations split by ecological barriers in order to reconstruct when isolation might have happened and how.
What they found is that some species of songbird have avoided crossing the 40 km-wide Palghat Gap for millions of years, since before humans walked upright. A little under half of the species studied showed genetically distinct breaks at the Palghat Gap – with progressively smaller subsets of that group impacted by the younger, narrower Shencottah Gap and Chaliyar Valley. This nested pattern might be due to the structure of the Western Ghats, which lack montane shortcuts around deep valleys. “Because the Western Ghats are so linear, the breaks are very clean and very clear,” says Robin.
But what kind of barrier is a valley for a creature with wings? The key, says Robin, is climate. Divergence for many of the songbird species coincided with climate upheavals in the past, particularly the turbulent period of the Pleistocene. “Topography provides the canvas for climate-mediated habitat shifts to occur,” the authors suggest. One possibility is that as the climate in the valleys became drier, the cool, wet habitats that the birds favoured moved to higher altitudes, and the birds followed in their wake.
Today, environmental change is accelerating again, but this time much of it is human driven. In the past two centuries, the habitat of high altitude birds has been fragmented by both human habitations and agricultural plantations. Songbirds adapted to montane landscapes have been reduced to islands within islands—but how much could further patchiness affect species adapted to fragmentation? The researchers asked this question about two Shortwing species that live on sky islands in the Western Ghats fragmented by human activity. In order to decouple the isolation caused by recent human-driven change from older climate-mediated change, they compared two statistics with different levels of sensitivity to recent change. One measure changes at a much slower rate over time and can pick up historic change quite well, while the other compares populations at the level of the frequency of specific alleles and might more easily detect a recent drop in migration.
The result was clear-cut: deforestation is isolating the birds as effectively as climate oscillations during the last Ice Age. Just as songbirds cannot simply fly across a deep valley, Shortwings have ceased to intermingle between fragments cut off by tea estates. Even their songs are growing distinct. “Lack of gene flow in the historically connected Shortwing populations is an alarm call for policy makers as the species populations are already small and isolated,” says Pooja Gupta, one of the ecologists involved with the study. “Continued fragmentation in the landscape may perhaps lead to its extinction,” she says.
Fragmentation leads to unstable populations because it limits genetic diversity, which is highest when all pockets of a species can freely intermix. Genetic diversity is essential in turn for weathering inevitable environmental change. Variety once lost can be regained, but only with the accretion of beneficial mutations over millions of years. Deforestation is happening at a much smaller time scale. Thus, all we can do is physically reconnect the populations that our actions have splintered.
Uma Ramakrishnan, professor at NCBS
and an author on the study, emphasizes that habitat fragmentation has become a widespread problem for all wildlife. “Animals in today’s world are almost like those in zoos,” she says, referring to their dwindling, isolated populations. Many potential solutions exist in the natural world, from allowing contiguous patches of habitat to grow back, to native-tree wind breaks, to human-facilitated migration. But more research along the lines of this study is essential. “Without understanding the effects of fragmentation, you cannot begin to plan for mitigation,” she says.
More information on sky islands and the project at www.skyisland.in