Biodiversity of Sri Lanka

  Biodiversity, a famous term among all of us nowadays .But there isn’t a one exact definition for the term ‘biodiversity’ .Considering various definitions published by different summits ,biologists ,ecologists ,etc; most logical ,scientific & explanatory definition was published in the earth summit,1992(Rio de Janeiro).

“ ’Biological diversity’ means the variability among living organisms from all sources including ,inter alia ,terrestrial ,marine and other aquatic ecosystems and the ecological complexes of which they are part :this includes diversity within species :between species and of ecosystems”

  Sri Lanka is one of the 34 biodiversity hotspots identified in the world and has the highest biodiversity per unit area of land amongst Asian countries .The island of Sri Lanka has an area of 65 610 km²  lies between latitudes 5⁰55’ and 9⁰51’ N and longitudes 79⁰41’ and 81⁰54’ E .Three different topographic regions are found in Sri Lanka: coastal plain from mean sea level to altitude of 30 m, inland plain from an altitude of 30 m-300 m, upper central highlands from an altitude of 300 m .Central highlands can also be divided in to lower hills(from an altitude of 300-900 m) and upper hills(altitude above 900 m) .Highest point of the country is Piduruthalagala

 (2524 m from mean sea level).                                                                                                                                                                      

Topographic diversity of Sri Lanka .view from the world’s end (Horton Plains National Park)

  Sri Lanka is situated in the tropical region and close to the equator .Due to that reason Sri Lanka doesn’t have a clear seasonal division .Rain is brought by two monsoon winds .Southwest monsoon brings rain to southwest region and central highlands during the months of May to September .Northeast monsoon brings rain to the whole island from November to February .During the southwest monsoon, the water droplets that collide on the southwest face of the central highlands precipitate and fall back to the southwest region .Due to this reason ,southeast and northeast regions get no rainfall from the southwest monsoon .But ,the northeast monsoon wind travels across the central highlands and gives rain to the whole island .While wet zone has got both of the monsoon rains ,dry zone has got only the northeast monsoon rain .In between the wet zone and dry zone ,the intermediate zone is situated .Wet zone has a minimum annual rainfall of 2000 mm .The average value is 2400 mm .Dry zone has a maximum annual rainfall of 2000 mm and it’s average value is 1450 mm .Northwestern and southeastern regions are arid zones and has an annual rainfall of 1500 mm .Sri Lanka has a continuous temperature of 27-28 °C .

It has been proven by the prehistoric excavations done in the stone caves of wet zone that the temperature of the low land rainforests hasn’t been changed by even from 5 °C  during the last 31 000 years (Kennedy & Deraniyagala 1989) .

  Sri Lanka has various regions which carries different, specific ecological characteristics .These regions are rich with floral & faunal species that are oriented according to the above specific ecological characteristics . Sri Lanka can be classified in to 8 biogeographic zones.

 

Source- https://www.semanticscholar.org/paper/Molecular-species-delimitation%2C-taxonomy-and-of-Sri-Ranasinghe/c635d6a5e023b48a35ad0f3579ccc568beb0e8a6/figure/2

According to the above definition (Earth Summit-1992) biodiversity includes diversity within species ,between species and diversity of ecosystems.

 

Ecosystem Diversity of Sri Lanka

Ecosystem diversity of Sri Lanka can be mainly classified in to forest and associated ecosystems diversity ,wetland ecosystems diversity ,agro-ecosystems diversity.

 

Forest and Associated Ecosystems

 Falling under the tropical region, the Sri Lankan dense forest consists of different forest types such as lowland rain forests ,tropical dry mixed evergreen forest, montane forests, sub montane forests, tropical moist evergreen  forests and thorn scrub forests. The total natural forest cover of Sri Lanka is estimated at  19,514.73  km2,  of  which  14,382.75  km2  are  dense forests .Forest cover of Sri Lanka is 29% of the total land area.

  Tropical lowland wet evergreen forests/lowland rainforests are restricted to the southwest of the island ,up to a mean elevation of 900 m .The canopy reaches up to 30-45 m in height and they are dominated by the families Dipterocapaceae, Clusiaceae, Sapotaceae, Bombacaceae and Myrtaceae . In these forests as much as 60 – 75 % of the tree species are endemic to Sri Lanka .

   The lower/sub montane forests are confined to middle elevations (900–1500 m) of the Peak Wilderness, Knuckles (Dumbara hills), Namunukula and the Rakwana-Deniyaya ranges. Their canopies are about 20–25 m high, dominated by the families Dipterocarpaceae, Clusiaceae and Myrtaceae .

   The montane forests are restricted to the uppermost elevations of the country (> 1500 m), are dominated by the Clusiaceae, Myrtaceae, Lauraceae, Symplocaceae and Rubiaceae.The canopy reaches up to 10-11 m in height and layering cannot be observed . About 10 m tall, short-statured trees with gnarled twisted branches, an abundance of epiphytes dominated by mosses, leafy liverworts, ferns and orchids, colourful young foliage, relatively small, thick, leathery leaves and absence of drip tips are distinguishing features of its vegetation . With increase in elevation and windy conditions the canopy species become quite stunted giving way to pygmy forests. The bryophyte flora of island is the richest in the lower montane and montane ecosystems .

   Tropical dry mixed evergreen forests represent 54 % of the island’s natural forest cover and 16 % of its total land area. Typical examples of them are seen in the North, Eastern, North Central, North-Western and Southern Provinces. In the best stands they are about 25 m tall, but shorter in those towards the arid zone .The families Euphorbiaceae, Sapindaceae, Ebenaceae, Sapotaceae, and Rutaceae dominate them. In general, the latter are found in the canopy, shedding their leaves during the late dry period, allowing much light to filter to the forest floor. This enables grass and herbaceous species to flourish at the onset of the rainy season before new foliage on denuded canopy trees emerges. The appearance of these forests differs seasonally and depicts an annual cyclical variation in floristic diversity of the ground vegetation. Leaves of plant species here are small, without drip tips; many have compound leaves. The tree trunks branch lower down and show no buttresses, compared with those in rainforests. Epiphytes and lianas are relatively sparse in this forest type.

   Tropical moist evergreen forests, about 25-30 m tall, represent the transition or ecotone between forests in the aseasonal and seasonal climates. They bear similarities in species composition to both the tropical lowland wet evergreen forests and the tropical dry mixed evergreen forests and some species of their own .Fragments of this forest type are seen at Barigoda near Kurunegala and Daragoda near Moneragala, and more extensive areas in Randenigala and Samanalawewa. The dominant families here are Anacardiaceae, Sapindaceae, Euphorbiaceae and Moraceae.

   Thorn scrub forests found in the arid zone are typical of the vegetation in Yala in the southeast and Mannar region in the Northern Province. As the name implies, thorny species are abundant here and reach to about 5 m in height showing no species stratification. Its dominant families are Salvadoraceae, Mimosaceae, Euphorbiaceae and Rhamnaceae. Endemic plant species are almost absent in these forests.

 Beside above mentioned ecosystems, based on elevation, climate, plant composition and degree of disturbance, six types of grassland ecosystems are recognized in Sri Lanka.

 

1.)    The wet patana grasslands, found around and above 1800 m altitude, are best seen in Horton Plains and around Nuwara Eliya. They are dominated by the tussock grass species Chrysopogon nodulibarbis and Arundinella villosa on higher ground and Garnotia exaristata and the dwarf bamboo, Sinarundinaria densifolia, close to waterways. 

2.)    The dry patana grasslands, located between 500 – 1000 m elevation, are relatively widespread, occurring in the Uva basin, and around Gampola, Nawalapitiya, Hantana and Rakwana. These grasslands have resulted from the removal of tree vegetation for agriculture and subsequent abandonment. The dominant grass species in them are Cymbopogon nardus and Themeda tremula, both forming tussocks.

3.)    The savannas occur in the intermediate and dry zones, between 300–1000 m elevation. Those on the eastern slopes of the central massif and around the Uva basin are called upland savannas while those in Moneragala and Bibile and in the Gal Oya basin are known as lowland savannas. They resemble a parkland, with scattered, fire-tolerant trees.

4.)    Damanas, also known as dry low country parklands or grasslands, occur in the Ampara and Batticoloa districts of the eastern dry zone. They are dominated by the tussock grass.

5.)    Talawa grasslands occur in the Haldumulla area, where the topsoil is eroded and truncated to some degree.

6.)   
Villus are wetland ecosystems also known as freshwater riverine marshes found in the floodplains of the Mahaweli River in the eastern dry zone and in moist depressions with a perennially high fluctuating water table. Flooding and rain water accumulation suppress any tree growth within the villus but encourage hydrophyllic grasses, sedges and other aquatic plants including some tree species.

   A lowland rainforest                                                             

Wet patana at Horton Plains   

  

  Montane forest trees with characteristic gnarled tree crowns

                   

   Diversity of Wetland Ecosystems

 The Ramsar convention has defined wetlands as follows;

“Areas of marsh, fen, peat land or water, whether natural or artificial, permanent or temporary with

water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of

which at low tide does not exceed six metres.” 

The wetlands of Sri Lanka can be classified in to 3 broad categories.

• Inland natural fresh water wetlands (e.g. rivers, stream, marshes, swamp forests andvillus)

• Marine and salt water wetlands (e.g. lagoons, estuaries, mangroves, sea grass beds, and coral reefs)

• Man-made wetlands (e.g. tanks, reservoirs, rice fields and salterns) 

Inland Natural Fresh Water Wetlands

Streams and rivers: Sri Lanka has an extensive network of rivers and streams that drains a total of 103 distinct natural river basins. In terms of length, these flowing water bodies cover more than 4500 km.Mahaweli, Walawe and Kelani rivers originate from the central highlands and flow through all three peneplains of the island. The river basins originating in the wet highlands are perennial while many of those in the dry zone are seasonal. The Mahaweli river accounts for the largest basin, covering 16% of the island and thereby has a high socio-economic and ecological value. The streams and rivers that flow through the

high and mid altitude areas of the island have resulted in several water fall habitats.

The “Villu” wetlands: Although there are no large natural lakes in Sri Lanka, there are several flood-plain lakes, commonly referred to as “Villus”, which cover a total area of 12500 ha. Often they are cut-off former river bends. Many of the larger “Villus” are located in the Mahaweli floodplain in the East. A typical example is the inter-connected Handapan and Pendiya Villus (796 ha), which is the largest in the entire Mahaweli Villu system.

Fresh water Marshes: These are shallow inland depressions located mainly in rural areas either connected to a river or receiving water through surface run-off river floodwater and ground water seepage. Partially decomposed organic material in such marshes form peat, characterized by water logged sticky soil. A typical example is the Muthurajawela Marsh which is the largest peat bog in Sri Lanka. 

Fresh water Swamp Forest: This is a late successional stage of a freshwater marsh ecosystem, comprising of trees that are adapted to grow in shallow stagnant water. Swamp forests are seasonally inundated with river water. It is the rarest wetland type in Sri Lanka and a good example is the Walauwa-Watta Wathurana Swamp Forest (12 ha) located in the Kalu river basin.

Salt Water Wetlands

Estuaries and Mangroves: These are inter-connected coastal wetland types. Estuaries are formed in places where rivers enter the sea. The daily tidal fluctuation and the intermediate salinity between salt and freshwater (commonly termed “brackish water”) are main characteristics of this ecosystem. There are about 45 estuaries in Sri Lanka. The mangroves comprise very diverse plant communities that are adapted to grow in unstable conditions of estuarine habitat. The mangroves are a rapidly diminishing wetland type in Sri Lanka, consisting of less than 10,000 ha of discontinuously distributed patches along the coastline.Typical example of estuaries with mangrove wetlands in Sri Lanka includes Maduganga estuary, Bentota estuary and Kalaoya estuary. The latter is the largest mangrove in Sri Lanka.

Lagoons: These are salt or brackish water coastal wetlands separated from the sea by a low sand bank with one or more relatively narrow permanent or seasonal outlets to the sea. These can also harbour other coastal wetland types such as mangroves, mud flats and sea grass beds. About 42 lagoons are found around the coast in Sri Lanka. Examples include the Bundala Lagoon, Mundel Lake, and Kalametiya Lagoon.

Coral reefs and Sea grass beds: These are two important sub-tidal marine wetlands (below 6 m in depth) in Sri Lanka. Coral reefs consist of calcareous structures secreted by a group of marine invertebrates.Coral reefs are famous for their spectacular beauty. The rich biological diversity of coral reefs could be compared to that of a tropical rainforest. Extensive coral reef habitats can be found in the Gulf of Mannar region, Trincomalee to Kalmunai in the east coast and in several areas of the south and south-western coast, including Rumassala and Hikkaduwa. Sea grass beds are composed of rooted, seed bearing marine plants. These are found in shallow, sheltered marine waters, as well as in lagoons and estuaries. Most extensive sea grass beds are present in the northwest coastal waters of Sri Lanka (eg. Kalpitiya to Mannar). 

Man-made Wetlands

Tanks and Reservoirs: Although there are no natural lakes in Sri Lanka, an array of ancient irrigation tanks has substituted the former wetland type. Numbering nearly 10,000, these man-made wetlands depict the rich cultural heritage of Sri Lanka. The major irrigation reservoirs (each more than 200 ha) cover an area of 7820 ha, while the seasonal/minor irrigation tanks (each less than 200 ha) account for 52250 ha.Typical ancient irrigation tanks include the Parakrama Samudraya and the Minneriya tank.

Rice Fields: Rice fields are characterized by the presence of a standing water body, which is temporary and seasonal. Hence, flooded rice fields can be considered agronomically managed marshes. They are temporary and seasonal aquatic habitats, managed with a variable degree of intensity. The total area under rice cultivation at present is about 780,000 ha (approximately 12% of the total land area), which is distributed over all the agro-ecological regions except for areas located at very high elevation.Approximately 75% of rice lands in Sri Lanka are located within inland valley systems of varying form and size while the rest are found in alluvial plains and also on terraced uplands in the interior. 

Diversity of Agro-ecosysytems

  Sri Lanka has been depending on agriculture since ancient times. Agro-ecosystems of Sri Lanka are mainly composed of main export crops such as paddy fields ,tea ,coconut ,rubber .Besides main export crops ,coffee ,cocoa ,cinnamon ,fruits ,vegetables ,grains account for a considerable number of agro-ecosystems.

   Traditional rice cultivation of Sri Lanka nurtures on the rain water and irrigational water. Year is divided in to two seasons ‘yala’ and ‘maha’.In the dry zone ,rice cultivation depends on northeast monsoon during ‘yala’ season .During ‘maha’ season it depends upon the irrigational water supply .In the wet zone rice cultivations mainly depend on rain water .A special technique called ‘terracing’ is used for rice cultivation in upcountry .Sri Lanka has a various range of rice varieties that are resistant to harsh conditions of different eco-zones .

   Agricultural practices have led to ecosystem-level variations in many parts of the world and have provided mosaics of micro habitats that support associated plant and animal communities. These are often rich in globally significant agricultural biodiversity , not only within and between species but also at ecosystem and landscape level. The VT-based agricultural system is a unique system that sets one of the best examples for this.

   Tea plantations are distributed throughout the wet zone .Two different varieties of tea are cultivated in up country and low country wet zone .A considerable diversity of amphibians can be specially seen in the tea plantations of upcountry .

Breakdown of traditional methods of cultivation ,animal husbandry and the admittance of harmful agrochemicals ,animal food products is becoming a crucial threat for the biodiversity and mankind of the island.

 

 written by;                                                                                                 

 Rtr.Yesith Banuka Withanachchi

 Vegetable cultivation in upcountry

         A tea plantation in upcountry

References

1.)    Buddhi Marambe ,Gamini Pushpakumara and Pradeepa Silva.(2012) ‘Biodiversity  and  Agrobiodiversity in  Sri  Lanka:  Village  Tank  Systems’S-I Nakano, Yahara,T ., Nakashizuka ,T.,Yamamoto ,S.,(ed.) Biodiversity Observation Network in the Asia-Pacific Region: Towards Further Development of Monitoring .Japan:Springer ,pp. 403-430.Available from:https://www.researchgate.net/publication/236117381_Biodiversity_and_Agrobiodiversity_in_Sri_Lanka_Village_Tank_Systems [accessed Nov 20 2020].

2.)    IUCNSL and (EA ,2006 ,National Wetland Directory of Sri Lanka .IUCN Sri Lanka ,Colombo )

3.)    IUCNSL and MENR ,2012 . The National Red List 2012 of Sri Lanka; Conservation Status of the Fauna and Flora. Weerakoon, D.K. & S. Wijesundara Eds.,Ministry of Environment, Colombo, Sri Lanka.

4.)    Mac Neely ,J.A.,1992 .The sinking ark: pollution and the worldwide loss of biodiversity .Biodiversity & Conservation ,1:2-18 .

5.)    Nimal Gunatilleke,Rohan Pethiyagoda and Savitri Gunatilleke . (2008) ‘Biodiversity of Sri Lanka’, National Science Foundation of Sri Lanka ,36 special issue, p25-62.