(Science Reporter, June, 1996)

ISLAND BIOGEOGRAPHY

For thousands of years, mankind has known of the existence of islands. Early explorers have commented on the differences between societies on islands and those on mainlands. The first with an ecological bent of mind found that animals and plants on islands were quite different from those on mainlands. One of the main factors was that these animals and plants were `isolated'. As it was theorised later, various animals which had been overrun by competition on the mainland survived on islands, due to the lack of predators or competition.

Charles Darwin, on his famous journey on the H.M.S. Beagle, (during which he collected much of the information that enabled him to propose his theory of natural selection) gleaned much information from the animal communities of the Galapagos islands. More recently, two Americans, Robert MacArthur and Edward Wilson proposed another theory to explain the species numbers on islands. They theorised that all the species on islands have to come from a mainland source. Some of these species will go extinct on the island and, simultaneously, others will colonise it. Therefore the number of species on an island is an equilibrium between the number of extinctions and the number of colonisations. Islands closer to a mainland will have a better chance (ie. a higher rate) of being colonised by new species than those further away. Also, on smaller islands, species will have a greater chance of going extinct; this is because smaller islands can support smaller populations of each species and the chances of their dying out by chance alone will be higher. Of course, other isolated habitats can also be treated as islands. These would include populations in lakes and habitats on top of mountain ranges.

Today, it is common knowledge that forests the world over are rapidly disappearing. A large proportion of the forests left in the world are in nature reserves and sanctuaries. With the destruction of forests, what has happened is that the remaining patches have become fragmented or, in other words, terrestrial islands. Conservationists have therefore tried to apply the theory of island biogeography to these forest fragments. This way, one can estimate the number of species that a given patch of forest might be able to support. We can also try and predict the minimum forest area that a particulare rare species might require. This has acquired particular relevance for the design of nature reserves where conservationists or the government (guided by conservationists) have a limited area of forest to protect and have to decide which of these deserve this protection. This is known as the `SLOSS' dilemma. When designing a nature reserve, due you have `Single Large' or `Several Small' reserves. The advantage of the large reserve is that one large enough reserve will support some rare species that none of the small ones may. Further, the chances of extinction are greater on the smaller reserves. However, in the case of an epidemic, the entire population will be wiped out in the case of a single large reserve.

MONTANE EVERGREEN

In the upper reaches of the Nilgiris and elsewhere in the Western Ghats, the landscape is suited to precisely this type of research. At altitudes of 1500 metres and above, the Western Ghats boast of the shola-grassland ecosystem. The "sholas" are tropical montane evergreen forests, though trees tend to be more stunted than the lowland forests. The sholas are found in patches varying from less than a hectare to several hundred hectares and they are surrounded by grasslands. Some would prefer the term `savannah' to `grassland' since they are dotted with rhododendrons which are essentially an exotic species. Thus these sholas are naturally fragmented forests or terrestrial islands where aspects of island biogeography can be researched. Of course, another way to approach this would be to view the whole of the Upper Nilgiris as an island and the shola-grassland of Anamalais and Palanis and the other ranges as islands. Since these are seperated from each other by lowland gaps, they would behave very much like islands. The shola-grassland vegetation type is the climax community at the higher altitudes of the Western Ghats. There was a great deal of debate about the origin of the grasslands in the nilgiris. While one school of thought believed that it was induced by the advent of man (ie. grazing by cattle and man induced fires), the other believed that it was a natural vegetation type of the Nilgiris. Recent work using carbon isotopes has shown that the grasslands existed 40 000 years BP, long before man came to the Nilgiris. However, the relative extents of the sholas and grasslands seem to have varied with changes in global temperatures.

Much of the the year, the climate in this area is wet. January to April is relatively dry in Upper Bhavani, and there is frost and fire at these times. Rainfall varies from 5000 mm on the western ridge to 1000 mm on the eastern slopes. However, the natural vegetation is quite similar throughout the region. The western areas get much of their rain from the South West monsoon, which lasts from June to August, while the eastern region gets most of its rain between December and March. We chose the Upper Bhavani area to carry out our research work since the sholas in this area were the least disturbed by deforestation and plantations. Upper Bhavani is about 65 km from Ooty, at an altitude of 2350 metres, and is the site of the Upper Bhavani dam. The dam is at the source of the Bhavani river and the reservoir is large, spreading across much of the region. The Bhavani river flows down to the plains, joining the Siruvani and eventually the Kaveri. The dam is also one of the entry points into the Mukurthi national park which is a sanctuary for the Nilgiri tahr, though few of these remain in this area thanks to poaching and habitat destruction. Many of the sholas that are intact lie inside the sanctuary. The southwest tip of the sanctaury which is also the southwest tip of the nilgiris is contiguous with the Silent Valley National Park. Thai shola, which is the largest shola in the Nilgiris, is about 600 hectares and lies between Upper Bhavani and Ooty.

WORK DETAILS

We decided to work on a group of animals which would be confined to individual sholas i.e., the sholas would be like islands to them. The sholas do not behave as islands to larger animals like sambar or to birds which move easily from one shola to the other. The group of animals studied had therefore to be relatively small in size. We started working on amphibians, reptiles and small mammals. There are a large number of rodents in the sholas, the grasslands and the plantations. Thai Shola, the largest shola, has predictably the largest number of species. Sholas have more species than grasslands, but surprisingly the plantations also have many spe- cies. The most common species in the sholas is the wild subspecies of the Common House rat, which demonstrates what an adaptable and hardy animal this is. Other species include the Field rat, the White Tailed Wood rat, and the Spiny Tailed dormouse. The Long Tailed Tree mouse, a soft furred, attractive animal, and highly arboreal in habit, is also found in Thai Shola. An interesting aspect of the work on rodents is it offers a chance to shed some light on the community ecology debate. Ecologists are divided over which are the important factors in structuring a community; ie. why are certain combinations of species found in certain habitats and what influences their numbers ? Some maintain that the habitat (food availability or vegetation structure) is more important and hence species adapted to a particular niche. Others maintain that direct competition (such as aggressive interactions) between species is a more important factor.

The Western Ghats, a hotspot of diversity in the world, is also considered to be the richest in amphibians in Asia. There are about 120 species of amphibians, of which 89 are endemic. Amphibians are an important group because they are said to be good indicators of environmental conditions. They are cold blooded and have very sensitive skins. Since they live on land and in water, they are vulnerable to air and water pollutants. The Western Ghats are also home to the Uropeltidae (sheildtails), a group of burrowing snakes found at altitudes of 1500 metres and above. There are 44 species of uropeltids and they are found exclusively in the western ghats and sri lanka. These snakes are highly elusive as they burrow 1 to 2 metres under the soil and venture out primarily during the monsoons and at night. They use their blunt head and spiny tail to burrow into the soft earth under logs and stones. Many of the uropeltids are very pretty snakes and are found in a vareity of colours, including bright yellow, red, blue and orange, with bright spots and patches on their sides and below. The Pit vipers, characteristic of higher altitudes, are found in this area. Many lizard species are also endemic to the Western Ghats. Amongst these is the Nilgiri Salea, an agamid ubiquitous through the Nilgiris. It shows wide colour variation, varying from brown to bright green with differing patterns on the body.

The mammals in the Nilgiris include some highly endangered species. There is the Nilgiri tahr, whose numbers have been highly depleted in the Nilgiris due to habitat destruction and poaching. The tahr are animals of the grasslands, found mainly on steep slopes and rocky cliffs. They are found in groups of 10 to 15, which may then congregate to form groups of 100 or 200. There may be fewer than 150 tahr left in the Mukurthi National Park which accounts for just about all the tahr in the Nilgiris today. Another possibly endangered animal is the Nilgiri marten, a rarely seen animal. This is a small carnivore belonging to the weasel family, and its closest relative is the Himalyan marten. Other predators in this area include tigers, leopards and wild dogs, the last being the most commonly seen. Smaller carnivores like the jungle cat, leopard cat, and the Small Indian civet are also found in this area, but jackals are the most abundant. Otters may be seen in some of the streams. The most common herbivore is the sambar, though barking deer and mouse deer are also found. Black naped hares probably form a large part of the diet for many carnivores. Elephants are believed to migrate to the upper plateau from Silent Valley and Attapadi, though there may be one or two resident herds in the upper plateau.

CONSERVATION

More work needs to be done on the animal and plant ecology of this area since the shola grassland ecosystem is rapidly disappearing. Over the past 100 years, much of this habitat in the nilgiris has been replaced by tea and other plantations. More recently, since the grasslands were not considered important, the forest department has planted wattle, eucalyptus and pine over much of the grasslands. This has reduced the habitat for a number of species of animals, and many of them are in danger of going extinct. The lesser animals, such as invertebrates, have not been studied in detail in this ecosystem, and several species that have not been studied, or even documented, may be going extinct. It is a known fact that some of the amphibian species in the Western Ghats are extremely rare. There is also a certain amount of poaching. Nilgiri langurs are killed for their blood which is believed to have medicinal value. Giant squirrels, jackals, jungle cats, and other animals are hunted for their skins, claws and teeth, which are believed to act as good luck charms. Sambar and tahr are poached for food. The shola grassland is indeed a very unusual and special ecosystem in that it is found only in a few areas in the Western Ghats. Most of it has already been lost, but the few remaining areas can be protected, if measures are taken immediately. The loss of this habitat would be a great loss to science and to mankind for it has much to offer, both in terms of diversity , and in terms of aesthetic value as very beautiful landscape.