Sri Lanka is a continental island situated near the southern tip of the Indian peninsula, lying between altitudes 5.94’ N – 9.52’N and latitudes 79 39’ E – 81 53’E. It has an overall land area of 6,570,134 ha, but there are almost 10,000 man-made irrigation reservoirs or tanks, especially in the northern half of the island. There are three peneplains, from sea level to 2,600 m. Of the 103 rivers which start from the central hills, nine are perennial. The combined length of all the rivers amounts to 4560 km.
PROBABLE EVOLUTIONARY TRENDS IN SRI LANKAN ENDEMIC ANIMALS
Animals may have crossed the sea during the ice ages of Pleistocene from the Indian mainland distributed throughout the island. When Sri Lanka was a part of India, animals would have moved into Sri Lanka from India along the land-bridge across the Palk Straits. The presence of a variety of ecosystems and large number of habitats may have enhanced the evolution of new species. Once the island of Sri Lanka separated from the Indian mainland, newly formed species could not move back to India, and thus became unique to Sri Lanka.
The separation of Sri Lanka from the Indian mainland occurred in such a way that the southern part of the island was the rice up into three major mountain ranges. The northern part was still connected to the Indian mainland. In this way, the southern Sri Lanka became isolated from the Indian mainland for much longer time, and hence animals had more space variations to evolve into unique forms found nowhere else in the world. As the northern Sri Lanka was connected to the Indian mainland for much longer, it continually received species from India till the sea level is lower. This explains the paucity of endemics in the northern Sri Lanka vis-à-vis the wet zone of and the highlands of Sri Lanka.
Mostly ancient Sri Lankan civilization is associated within the dry zone and agriculture system was based on shifting cultivations. As a result most of natural forests were destroyed and which remaining are mostly secondary forests. The forests are therefore much younger or absence of appropriate niches to evolve new biota. Of the species unique to Sri Lanka, a number of them have became “point endemic species” that can survive only under certain micro-habitat conditions, and are therefore unable to face changing environmental conditions.
Furthermore, some species are restricted by physical barriers such as misty mountain tops that they are incapable of moving and spreading. Thus some rain forest and mountain species have been trapped in their geographical areas for along time. In addition, even the lowland dry zone may function as an effective barrier to animals sensitive to high temperatures, humidity and low rainfall. Therefore biotic interchange between Sri Lanka and the Indian mainland is effectively sealed as they are unable to cross the dry zone.
On the other hand, both India and Sri Lanka have shared the same species for a long time and the Indian races may have become extinct for several reasons such as loss of appropriate habitats, climatic variations, impact of other species, etc.
However the endemicity of Sri Lankan fauna is presumed to be the result of isolation, climate, vegetation types and physical characteristics of the island itself. Isolation from the Indian mainland was followed by speciation and dispersal of species within Sri Lanka.
Climate of Sri Lanka
The Climate of Sri Lanka is mainly tropical. Nevertheless, owing to the altitudinal differences, several climatic regimes can be observed throughout the island from sea level to the hills. Seasonal variation in rainfall is common with rain limited to two distinct periods: the northeast and southwest monsoons. The mean monthly temperatures in the low country wet zone and the low country dry zone are 27 ‘c and 30oc respectively. In the hill country, mean monthly temperatures range between 13oc-16oc and sometimes the temperature may drop to almost 0oc. Although being a small island, the soil is variable with 14 major soil types found within the island. Its significant variation in climate, topography and soil properties has given rise to a gorgeous variety of forest types, which provide habitats for an ample diversity of faunal and floral species.
FifteenFloristic Regionshave been recognized within the country. The distribution ranges of most of the fauna correspond to these particular vegetation types. Sri Lankan endemics show affinities with Indo-Malayan, Afro- Mediterranean, Filipino and Madagascan links.
TheGeological history of Sri Lankais closely related to the events of the adjacent mainland and also is in part, the history of Gondwanaland. Many geologists postulate that during Mesozoic time Sri Lanka was a small portion of a vast landmass known as Gondwanaland that had began to break up in he mid Jurassic Period, and by the Cretaceous Period the present continental outlines were formed, time, into an island(Cooray, 1967 and Jacob 1949), thus forming the narrow strip of sea, that we know today as the Palk Strait and Gulf of Mannar (Peiris, 1976).Coorary (1967)ie. Australia, Africa, India (together as one with Sri Lanka) Madagascar, S. America and Antarctica. About 20 million years ago during the Miocene an arm of the Tethys Sea stretched down the west coast of India, a long narrow gulf which gradually encroached on the land surface between India and Sri Lanka. This gulf severed the extreme portion of the India mainland and turned Sri Lanka for the first considers that the island has retained its original outline and remained more or less above the sea for the ten million years or so since the end ofMiocene. However, during subsequent periods there have been land connections and severances several times. In fact during the Pleistocene, the most recent of the geological epochs, the island was connected with India on several occasions. Moor (1960) considers there to have been four such occurrences with the last having taken place as recently as 25,000 years ago (Deraniyala 1949, Jacob 1949). Hora(1949) considers the final severance to have occurred 10,000 years ago. Although from the Miocene to the late Pleistocene there were several land connections and severances, Hora (1949a) considers that the most important land connection with Peninsular India including the continuity of hill ranges(Western Ghats of India and the wet montane zone of Sri Lanka) had been broken in the early Pliocene period. Thus the migration of specialized hill-stream forms to Sri Lanka had stopped by that period. The Satpura Hypothesis was proposed by Hora (1949) withsupporting evidence. According to this hypothesis, the Assam Himalayas, Vindhya Satpura range and the Western Ghats or Dharwarian trends were one continuous ecological belt of mountains the persisted during the Pilocene and the Pleistcene. Furthermore, Hora(1949) assumes that the hill range had a rainfall of 100 inches or more with tropical evergreen forests.
