Saturday, December 11, 2010

Life on a raft

A friend recently approached me for some mapping help and this led me to rediscover some old tools (DIVA-GIS is one of the best free map tools that I know of) and forced me to look back at some recent thoughts. Generating a false-colour altitude map of the Indian Subcontinent heightened my appreciation of some observations and recent readings.
Elevations of India (click to see detail and legend)

Most people, even birdwatchers, often do not appreciate the peculiarities of species distribution and those that do not have the fortune of having a training in biology miss out entirely on the joy of mental stimulation that one gets when one tries to ask more questions.

On the border between northern Bengal (Jaigaon) and Bhutan (Phuntsholing) one can see an interesting phenomenon. Just walk into Bhutan and you see (apart from fewer humans and orderly traffic) that there is a lot of grass and vegetation and after walking up the first bend of the road you will find the commonest sparrow to be the Tree Sparrow, a species that simply refuses to accept life in India a few 100 metres away ! Along the busy roads of Jaigaon, only the House Sparrow may be seen, although it is also found on the Bhutan side. There is a difference in their habitat preferences and the Tree Sparrow seems to be the more picky species. Tree Sparrow do not have the marked sexual dimorphism that is found in the House Sparrow. When the two species occur in the same place, there is little confusion, but hybrids are known although they perhaps need further study. For instance, the only place where they are said to hybridize in India is not in the main distribution area but in the Eastern Ghats of Andhra Pradesh. The hill forests here are perhaps the least studied of the numerous biogeographical islands within India. The evidence for this supposed hybridization is not even well known - P. C. Rasmussen in her Birds of South Asia (2005) mentions this (presumably based on a specimen that shows hybrid characters) while Clement et al. (1993) (full references can be found on the Wikipedia articles) suggests that the species was introduced by ship to this part of India !

A painting by Bhawani Das (c. 1777)

Looking at the false-colour altitude map one can see a sharply delineated region in the Gangetic plains of Bihar in the altitude range of 50-100 m which corresponds to the region with the most records of the most-likely-extinct Pink-headed Duck. Now we do not really know much about that species, it may have lived in dense swampy wetlands and may even have been crepuscular in habit. Now we do not know about the habitat choice, behavioural or ecological adaptations evolved by this species and sadly perhaps, we will never know.

Considering this, one would imagine that we are better off when it comes to species that are not endangered but this just is not the case. Indeed, few have even really looked at the information available, not so much because they cannot, but because much of the information is scattered and it involves considerable work and trouble to bring together the data in a single location to even begin to examine for patterns. Often the tools required can only be handled by specialists and getting them to work across disciplines can be  daunting given the splintered scholarly ecosystem where academics carve niches outside of which they fear to step. Like mixed-species flocks of birds, interdisciplinary associations must be a tricky balance. A pleasant surprise however comes in the recent work on the  isolation and speciation of two high altitude birds from Southern India - the Rufous-bellied Shortwing and the White-bellied Shortwing. - these birds, whose exact higher level relationships are still shrouded (the genus placement remains questionable with scientists merely guessing that it should be close to the Himalayan Myiomela),  appears to have had ancestors that were widespread when the climate was a lot cooler. With changes in climate, populations were pushed up into the higher reaches of the hills of Peninsular India. Not being strong fliers , these birds were restricted into breeding locally within their own little pools (or beanbags )  and over time, these populations diverged in form, shaped by accident and selective forces into forms that are very different in plumage.

Disjunct distribution of Nephenthes

Now this is not an isolated incident, it is the norm although the isolation mechanisms and forces are harder to identify in other cases. Looking at the islands formed by just plotting altitudes, it is clear that that numerous other studies of this kind could be made within India and naturally "islands" can be created by any kind of barrier. The effects depend on the mobility and the evolutionary history of the species under consideration. Look at this distribution of pitcher plants in the genus Nephenthes for instance. It is well accepted that the Indian Plate rafted away from Gondwana into Asia but lots of debate exist on the timing and presence of bridges. These debates are largely raised by fossil evidence or surprising discoveries like the Purple frog. A recent talk (and paper) at IISc by Prof. Ashok Sahni was particularly interesting - working in an isolated island of scholarship in northern India, his team has looked at insects in Indian amber dating to the Eocene  - apparently there are tons of these fossils in lignite mines and they are usually just destroyed. And the wealth of information trapped there is being looked at - by the worlds leading  paleo-entomologists including David Grimaldi . Apparently the endemism levels in insect are high for India as most of  the insect fauna got onto the raft from Gondwanaland. These fossils come from after the K-T incident (65 million years ago).

Coming back to thoughts of Bhutan - the White-bellied Herons, 36 or so individuals - that live on the edge in the lower valleys of the Himalayan rivers surely have an interesting specialization, found only on the  lower elevations on rivers running southwards along the edge of the Himalayas  they are seriously threatened by dams that are needed mainly to power the growing populations of India. The world population has been optimistically estimated at 200 and as the bird-folks in Bhutan say - one hopes that the supposed 150 more birds in Burma are safer. And Burma is also the the last hope for the Pink-headed Duck.

Further reading

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