Mind The Gap!

                             MIND THE GAP!

The Western Ghats in southern India is a chain of mountains running along the western coast of the Indian peninsula for about 1,600 kms. The Ghats have for long been considered as a global biodiversity hotspot (Myers et al. 2000), and also features in the list of UNESCO world heritage sites. The Ghats are non-contiguous and have a varying elevation owing to which there are numerous wide and deep valleys that form along the length of mountain range. This fragmentation is responsible for the formation of continental islands (sky islands) that can be defined as a, ‘continental or inland terrain made up of a sequence of valleys and mountains,’ (Warshall 1994). The Western Ghats Mountains in southern India harbor one such tropical sky island system (Warshall 1994) that consist a mosaic of high elevation montane temperate, evergreen forests ( OR ‘Sholas’ as they are locally called) and montane grasslands. The Shola forests have been known to host a great deal of endemic birdlife and are considered as models to study the evolution of species from an ancestral distributional range. 

The hills of Kalakad Mundunthurai Tiger reserve (Western Ghats), south of the Shencottah gap.


Mosaic of Shola forests and grasslands in the sky islands. 

The white-bellied shortwing, considered to be a rare species until recently, is a globally threatened (Vulnerable B1+2a,b,c,d,e; BirdLife International 2001), passerine, understory bird endemic to the Western Ghats of India (Collar et al. 2001). There are five species of shortwing found in the Indian Subcontinent of which three are found in the North East and two are endemic to the Ghats. The White bellied shortwing is  found uniquely in the Shola forests at elevations of 1500 msl and above on the sky island systems of  the Western Ghats (Robin & Sukumar 2002; Robin et al. 2006). These birds are specialists in terms of their habitat preferences and slight changes in their habitat can affect their populations. Once reported to be fairly common (Ali and Ripley 1987), there have been only a few records lately.

 
 Rufous bellied shortwing (©Clement Francis)

White bellied Shortwing (©Bopanna Pattada)

 The most likely causes of the decline in population can be attributed to loss of its montane evergreen habitat, continuing habitat fragmentation and degradation (Birdlife International 2001). It is estimated that the Western Ghats has lost about 25.6% of the forest cover in the last two decades (Jha C. 2010)

 Fragmentation in these mountain ranges occurs on three levels, firstly on a larger, geographical level with wide and deep valleys serving as biological barriers that prevent dispersal of individuals. Second, the mosaic of montane grasslands and high elevation shola forests and lastly, fragmentation as a result of anthropogenic effects have all contributed to the formation of allopatric populations that are genetically isolated on these ‘sky islands’.

Palaeoendemics such as Shortwings; which prefer a specialist paleoclimate and geography are affected by the deep valleys of the Palghat and Shencottah gaps (Robin et al. 2010). The Western Ghats are an ancient mountain system with the largest geographical gap, the

c. 40 km wide Palghat Gap serving as the major biological barrier, and the smaller Shencottah Gap, further south acting as a barrier on a smaller magnitude. The topography of the Western Ghats is relatively very old, having taken shape about 65 million years ago (Ma) (Gunnell et al. 2003). This indicates that the geographical conditions and paleoclimate was conducive for colonization of the mountains by passerine birds much before they actually arrived on the evolutionary scene 50-60 million years ago. The White-bellied Shortwing exhibits high population genetic divergence across islands, with populations on a single island being genetically similar, although ecologically isolated (Robin et al. 2010). Populations that inhabited islands across larger valleys such as the Palaghat Gap were found to exhibit far greater genetic divergence when compared to individuals across the smaller valleys. The evidence points to a pattern of colonization by these birds with two probable strategies of colonization. Ancestral individuals could have first arrived at the foothills and gradually moved up the mountains as paleoclimate varied as a result of which sightings have been mainly restricted to above 1500 mts. Another theory postulates that ancestral individuals initially colonized the first island in the sky island system and gradually hopped across islands to colonize them, as a result of which, hypothetically the oldest ancestors should be on the first island in the system. 


Map showing the location of the Palaghat Gap and the Shencottah Gap (Source: V.V. Robin) 

On examining the effects of genetic differentiation across populations of the white-bellied shortwing, (Brachypteryx major), on different islands of this sky island system it was observed that individuals separated by the larger, 500-million-year-old geographical genetic gap- the Palaghat Gap, were found to be genetically different from those on islands separated by smaller valleys. Genetic analysis showed significant variations in combined mitochondrial DNA genes (cytochrome b, cytochrome oxidase 1 and control region).  However populations from Grasshills and Kodaikanal showed little genetic differentiation using the same molecular markers. (Robin et al. 2010). ). It was also observed that although all three populations have similar habitat structure characterized by stunted montane evergreen Shola forests (described in Meher-Homji 1984; Shanker & Sukumar 1999), geographical isolation had lead to phylogenetic divergence over time. As a result of this divergence, two newly discovered subspecies, B. m. major and B. m. albiventris, separated by a geographical gap in the Western Ghats (Palaghat gap), with the former and latter distributed to the north and south of the gap respectively (Robin et al. 2013) were identified. Using molecular methods of sequence analysis the White-belied Shortwing was split into two subspecies, the White-bellied Shortwing, and the Rufous-bellied Shortwing. Ongoing research has provided evidence for the existence of a nest basket effect wherein deep and wide valleys serve as drivers for phylogeographic divergence, with the oldest divergences exhibited by the Palaghat Gap, while the younger divergences were seen in the Shencottah Gap and the Chaliyar valley.

However certain species failed to exhibit a pattern of divergence for which four probable patterns have been suggested. (i). Ability of species to actively disperse across these barriers, (ii). Extinction and recolonization responses to paleoclimatic changes, (iii).  Incomplete lineage sorting owing to recent divergence, (iv). recolonization during inter-glacial periods where conditions at lower altitudes were warmer compared to cooler climates at high elevations. (Robin, Vishnudas Ck, Pooja Gupta. 2015). Various inter-related factors therefore have been found to influence divergence amongst endemic birds of the sky islands. Topographical variations of the valleys, deep and wide valleys and paleoclimatic conditions have been found to serve as drivers for divergence. Further research is required to assess which of these factors impact individual species, either singly or in combination to help serve as drivers for divergence. There is a need to assess if this pattern of divergence is restricted to highly endemic specialists, or if patterns of divergence and the nest basket effect hold true for species exhibiting a broader distributional range. The outcome of such a study would be a complete phylogenetic map to the endemic birds of the Western Ghats, if not one for all the birds of the Western Ghats. This would enable detection of variations amongst inhabitants on either side of the Gaps and assess whether sub-species have been hiding in plain sight. Use of advanced molecular methods of sequencing can provide evidence for the existence of any such divergences that may not have previously been looked into. Assessment of the age of the phylogenetic divergence can serve to establish whether allopatric populations alone exhibit divergences over time or if species that have successfully colonized these sky islands before the arrival of the shortwing also exhibit divergences as a result of these geographical, genetic barriers which are ancient in their origin.

References:

i. A view from the past: shortwings and sky islands

of the Western Ghats – Robin V V, A. Sinha & U. Ramakrishnan, Indian Birds Vol. 7 No. 2 30 (Publ. 15 October 2011)

ii. Ancient Geographical Gaps and Paleo-Climate Shape the Phylogeography of an Endemic Bird in the Sky Islands of Southern India- V V Robin, A Sinha, U Ramakrishnan- PLOS One · 2013 October

iii. Deep and wide valleys drive nested phylogeographic patterns across a montane bird community, Proceedings of the Royal Society B: Biological Sciences · July 2015

iv. Determining the sex of a monomorphic threatened, endemic passerine in the sky islands of southern India using molecular and morphometric methods, V. V. Robin , Anindya Sinha,

Uma Ramakrishnan- Current Science · September 2011

v. Islands within islands: two montane palaeo-endemic birds impacted by recent anthropogenic fragmentation -ROBIN. V. V , Gupta. P et al - Molecular Ecology ∙ July 2015

vi. Singing in the sky: song variation in an endemic bird on the sky islands of southern India

V. V. Robin, Madhusudan Katti et al,  January 2013, Rare Animals of India

vii. Status and distribution of the Whitebellied Shortwing (Brachypteryx major) in the Western Ghats of Karnataka and Goa, India- Bird Conservation International · December 2006

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