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Southern Asia: Southwestern part of the Island of Sri Lanka

Dutta and Manamendra-Arachchi (1996) listed fifty-three amphibian species from Sri Lanka. This was a substantial increase from the thirty-five species-groups described by Kirtisinghe (1957) in what was the definitive work on the Amphibia of Sri Lanka for almost half a century. But dedicated amphibian surveys have since increased this number to more than 250 frog species, most of them endemic to the rain forests in the southwestern quarter of the island (Pethiyagoda and Manamendra-Arachchi 1998). The globally outstanding levels of endemism in the Sri Lanka Lowland Rain Forests [IM0154] are epitomized by this assemblage of frogs.

  • Scientific Code
    (IM0154)
  • Ecoregion Category
    Indo-Malayan
  • Size
    4,900 square miles
  • Status
    Critical/Endangered
  • Habitats

Description
Location and General Description
The Sri Lanka Lowland Rain Forests [IM0154] represents the tropical rain forests below 1,000 m in elevation in the southwestern quarter of Sri Lanka.

A continental island, Sri Lanka is separated from the Indian peninsula by the shallow Palk Strait. The island was part of Gondwanaland until the Cretaceous, when as part of the Deccan Plate it became detached and drifted northward. The Deccan Plate collided with the Asian mainland-the Southern Laurasian coastline-about 55 million years later (Audley-Charles et al. 1981). Therefore, the ecoregion harbors several ancient Gondwana taxonomic groups. The island first became separated from the mainland Indian subcontinent during the late Miocene. Since then, climatic changes have interposed drier conditions between the moist forests in southwest Sri Lanka and the Western Ghats in India, the closest other moist forests. Despite the several land bridge connections with the Indian Peninsula since the initial separation, the moist forest and its wet forest-adapted biota have been ecologically isolated (Deraniyagala 1992).

The May to September southwest monsoon, extended by the intermonsoonal season before and after the true monsoon, brings more than 5,000 mm of rainfall to the ecoregion. Temperatures remain nearly constant, 27-30(C, year-round. Convectional winds from the ocean also ameliorate daily temperatures, especially along the coastal areas. Relative humidity is high (80-85 percent). The long years of isolation combined with these warm, moist conditions have resulted in the evolution of specialized species found nowhere else on Earth.

The ecoregion partially encircles the central massif, which rises to more than 2,500 m, and the detached Knuckles Mountain Range to the northeast. These mountains are placed in their own ecoregion, the Sri Lanka Montane Rain Forests [IM0155]. The ecoregion's topography is characterized by deep valleys of the major rivers that radiate out of the central mountains. The soils in this lowland wet-zone ecoregion are red-yellow podzolic soils (Survey Department 1988).

The vegetation is influenced primarily by climate, with topography and edaphic conditions contributing secondarily (de Rosayro 1950). The lowland wet evergreen forests are characterized by two floral communities: the Dipterocarpus-dominated community and the Mesua-Shorea community (Ashton and Gunatilleke 1987; Singhakumara 1995). The former is dominated by Dipterocarpus zeylanicus and D. hispidus, with Vitex altissima, Chaetocarpus castanocarpus, Dillenia retusa, D. triquetra, Myristica dactyloides, and Semecarpus gardneri (de Rosayro 1942). The canopy of the Mesua-Shorea community consists of Anisophyllea cinnamomoides, Cullenia rosayroana, Mesua ferrea, M. nagassarium, Myristica dactyloides, Palaquium petiolare, Shorea affinis, S. congestiflora, S. disticha, S. megistophylla, S. trapezifolia, S. worthingtoni, Syzygium rubicundum, and a subcanopy of Chaetocarpus castanocarpus, Garcinia hermonii, Syzygium neesianum, and Xylopia championi (Ashton et al. 1992; Gunatilleke and Gunatilleke 1981; Singhakumara 1995). Undisturbed forests have four strata, with a main canopy at 30-40 m, a sub-canopy at 15-30 m, a 5-15 m understory, and a sparse shrub layer (Singhakumara 1995). Emergent species rise above the upper canopy to 45 m (Abeywickrama 1980).

Among the distinct habitat types included within the ecoregion are patches of swamp forests closer to the coastlines, although most of these have long been converted to agriculture. Small patches of Avicennia-Rhizophora-Sonneratia-dominated riverine and fringing mangroves line the coastlines, especially near the mouths of the major rivers (Arulchelvam 1969; Spalding et al. 1997).

Biodiversity Features
Almost all of Sri Lanka's endemic flora and fauna are confined to the rain forests in the southwest quarter of the island, where the warm, moist climatic conditions and the longer period of isolation of the wet forest-adapted species have promoted the evolution of endemism and specialization (Pethiyagoda and Manamendra-Arachchi 1998; Phillips 1980; Sri Lanka National Report 1991; Wikramanayake 1990).

For instance, more than 60 percent of the 306 tree species that are endemic to Sri Lanka are found only in the lowland rain forests represented by this ecoregion, and another 61 species are shared with the montane rain forests and dry forests. Of the twelve endemic floral genera on the island, eleven are confined to the rain forests (Kendrick 1989). Ninety-eight percent of the fifty-eight species in the family Dipterocarpaceae-the dominant tree family in Asian rain forests-are endemic to the rain forests (Dassanayake and Fosberg 1980; Ashton and Gunatilleke 1987), and these include two endemic genera, Doona and Stemonoporus (Kostermans 1992). An endemic ground orchid, Anoectochilus setaceus, commonly known as the king of the forest or wanaraja, is found only in undisturbed portions of these rain forests. Several other plants have highly localized distributions. Examples include Diospyros oppositifolia, which is confined to the top of a small but species-rich peak known as Hinidumkanda, Stemonoporus moonii, and Mesua stylosa, which are known only from an inland marsh forest in Bulathsinhala, and an aquatic Cyperaceae, Mappania immersa, which is limited to some streams at Sinharaja. Floristically, the lowland and lower hill forests are the richest in Sri Lanka (Erdelen 1988; Gunatilleke and Gunatilleke 1990; Singhakumara 1995) and of all south Asia (Ashton and Gunatilleke 1987).

As a small island, Sri Lanka lacks the space to support many of the megavertebrates found on the mainland, although the fossil record indicates that ancestral forms of rhinoceroses, hippopotamuses, and lions once roamed here (Deraniyagala 1992). Despite the small number of species, the ecoregion contains several near-endemic mammals, including one strict endemic (table 1).

Table 1. Endemic and Near-Endemic Mammal Species.

  Family Species
Soricidae Suncus montanus
Soricidae Suncus zeylanicus*
Rhinolophidae Hipposideros halophyllus
Cercopithecidae Semnopithecus vetulus
Viverridae Paradoxurus zeylonensis
Sciuridae Funambulus layardi
Muridae Petinomys fuscocapillus
Muridae Mus fernandoni
Muridae Vandeleuria nolthenii

An asterisk signifies that the species' range is limited to this ecoregion.

The two endemic shrews, Suncus zeylanicus and Suncus montanus, are endangered and vulnerable, respectively (IUCN 2000). The Sri Lankan leopard (Panthera pardus kotiya) is considered a threatened genotype (IUCN 2000). The ecoregion also contains a small population of the endangered Asian elephant (Elephas maximus). Unlike the larger elephant populations in the dry-zone ecoregion, this small rain forest population is greatly threatened by habitat loss and fragmentation.

The ecoregion is completely contained within an EBA, Sri Lanka (124) (Stattersfield et al. 1998). Sixteen birds are considered near-endemic species, and two are strict endemics (table 2). The endemic green-billed coucal and Sri Lanka whistling-thrush (Myiophonus blighi) are considered threatened (IUCN 2000).

Table 2. Endemic and Near-Endemic Bird Species.

  Family Common Name Species
Columbidae Ceylon wood-pigeon Columba torringtoni
Bucconidae Ceylon grey hornbill Ocyceros gingalensis
Cuculidae Red-faced malkoha Phaenicophaeus pyrrhocephalus
Cuculidae Green-billed coucal Centropus chlororhynchus*
Phasianidae Ceylon spurfowl Galloperdix bicalcarata
Phasianidae Ceylon junglefowl Gallus lafayetii
Corvidae Ceylon magpie Urocissa ornata
Turdidae Spot-winged thrush Zoothera spiloptera
Sturnidae White-faced starling Sturnus senex
Sturnidae Ceylon myna Gracula ptilogenys
Muscicapidae Kashmir flycatcher Ficedula subrubra
Timaliidae Brown-capped babbler Pellorneum fuscocapillum
Timaliidae Orange-billed babbler Turdoides rufescens
Dicaeidae White-throated flowerpecker Dicaeum vincens*
Capitonidae Yellow-fronted barbet Megalaima flavifrons
Psittacidae Ceylon hanging-parrot Loriculus beryllinus
Psittacidae Layard's parakeet Psittacula calthropae
Strigidae Chestnut-backed owlet Glaucidium castanonotum

An asterisk signifies that the species' range is limited to this ecoregion.

Among the other vertebrate groups, two reptiles, the mugger crocodile (Crocodylus palustris) and the spineless forest lizard (Calotes liocephalus), are listed as endangered, as are eight freshwater fish species (IUCN 2000).

Several other taxonomic groups also exhibit high levels of endemism. The rhacophorid frogs in particular have undergone a remarkable radiation, and this richness places these rain forests at the apex in terms of amphibian species numbers per unit area (Pethiyagoda and Manamendra-Arachchi 1998). Many of these species have only limited range distributions, often less than 0.5 km2, and are now limited to the undisturbed habitat fragments. Thus, extinctions surely must have accompanied the widespread habitat loss in this ecoregion.

Current Status
During the past two centuries, nearly all the natural forests in this ecoregion have been cleared for tea, rubber, and coconut plantations, rice paddies, and human settlements. Only about 8 percent of the lowland wet forests now remain, as several small, isolated patches in a highly fragmented landscape. Nevertheless, because many of the endemic species in the ecoregion have small habitat needs, these patches can provide adequate refuge if effectively protected and managed to ameliorate edge effects and other external threats.

Currently, just about 2 percent of the ecoregion's intact habitat is protected within five protected areas (table 3). The most important of these are undoubtedly the Sinharaja Natural Heritage Wilderness Area and Peak Wilderness Sanctuary. The latter extends into the montane rain forest ecoregion. Together, these two protected areas represent the two largest forest patches. However, because most of Sri Lanka's endemic species and species richness lie within the rain forest ecoregions, the current level of protection is inadequate. The Forest Department has also set aside several conservation areas from its portfolio of forest reserves to address conservation gaps (IUCN 1992). The Sinharaja reserve is contiguous with several forest reserves, namely Morapitiya, Runakanda, Panagala, and Delgoda, that together form the largest forest block, accounting for about 43 percent of the remaining wet-zone forests (Zoysa and Raheem 1990). Although these reserves will contribute significantly to the overall protected areas system, it is imperative that all remaining habitat patches be conserved to safeguard the remaining beta-diversity. Unless additional protection with effective management is provided, Sri Lanka's most important elements of biodiversity will be lost forever.

Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion.

  Protected Area Area (km2) IUCN Category
Sri Jayewardenepura Bird Sanctuary 30 IV
Sinharaja 100 IV
Telwatte 20 IV
Attidiya Marsh 10 IV
Peak Wilderness [IM0155] 100 IV
Total 260  

Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.

Types and Severity of Threats
Fifty-five percent of Sri Lanka's human population lives in this small ecoregion, which represents less than 25 percent of the land area. Clearing land for agricultural expansion and settlements to support this population and illegal logging, albeit small-scale, in the remaining forests are the most serious threats to the survival of the endemic species (Gunatilleke and Gunatilleke 1990).

Justification of Ecoregion Delineation
Sri Lanka's forests have been divided into two broad climatic sub-regions, the wet zone and the dry zone. In a previous analysis of conservation units of the Indo-Malayan realm, MacKinnon (1997) placed the wet-zone forests into a single biounit, Ceylon Wet Zone 02. We used MacKinnon's regional classification as a guiding framework in delineating ecoregions across the Indo-Pacific region. But because we differentiated between lowland and montane forests in delineating ecoregions, we used the 1,000-m contour to extract and place the lowland rain forests into a distinct ecoregion, the Sri Lanka Lowland Rain Forests [IM0154]. The ecoregion also overlaps with floristic zones 5, 6, 7, and 11 identified by Ashton and Gunatilleke (1987).

References
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List


Prepared by: Eric D. Wikramanayake and Savithri Gunatilleke
Reviewed by:

 

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