Western Ghats and Sri Lanka

Ajith Kumar⁸⁸, Rohan Pethiyagoda⁸⁹ and Divya Mudappa⁹⁰

The Western Ghats of southwestern India and the highlands of southwestern Sri Lanka, separated by 400 km (including the Palk Strait and Gulf of Mannar), are somewhat similar in their geology, climate, and evolutionary history. The montane forest ecosystems that characterize both regions are subject to monsoon weather patterns that mediate the warm, tropical climate and contribute to the high levels of biodiversity in the region.

The Western Ghats, known locally as the Sahyadri Hills, are formed by the Malabar Plains and the chain of mountains running almost parallel to India's western coast, approximately 30–50 km inland, traversing the states of Tamil Nadu, Kerala, Karnataka, Goa, Maharashtra, and Gujarat. In all, the Western Ghats stretch over a distance of 1 600 km, interrupted only by the 30-km Palghat Gap.

The Western Ghats (which have their highest point at 2 695 m; Anamudi Peak) mediate the rainfall regime of peninsular India by intercepting monsoon storm systems. Areas to the west of the higher elevations receive the greatest annual rainfall, 3 000 mm on average, with 80% of it falling during the period of the southwest monsoon (June-September) and the balance during the northeast monsoon (October–November). Annual rainfall decreases considerably along the eastern slopes of the Western Ghats; plains along the foothills and within the rainshadow, although only 10 km from the wettest areas, receive as little as 500 mm each year. Rainfall also decreases from south to north, especially north of the Palghat Gap.

The wide variation of rainfall patterns in the Western Ghats, coupled with the region's complex geography, produces a great variety of vegetation types. These include scrub forests in the low-lying rainshadow areas and the plains, deciduous and tropical rainforests up to about 1 500 m, and a unique mosaic of montane forests and rolling grasslands (shola) above 1 500 m. Tropical rainforests represent primary centers of species richness and endemism within the Western Ghats, and cover approximately 20 000 km2. Dry, moist deciduous, and scrub forests cover another 20 000 km² (Collins et al. 1991).

The most detailed study of the Western Ghats rainforests is that of Pascal (1988), who identified six major associations at low altitudes (less than 850 m), four at medium elevations (850-1 500 m), and one at elevations above 1 500 m. Of these, the Dipterocarpus bourdilloni-D. indicus-Anacolosa densiflora type (confined to altitudes below 700 m, including the coastal zone and areas south of the Palghat Gap) is believed to be the richest one, homologous to the lowland dipterocarp forests of Southeast Asia. Unfortunately, today this forest remains only as sacred groves and very small remnants in various stages of degradation, and D. bourdilloni itself has become extremely rare.

Sri Lanka is a continental island separated from the southern tip of peninsular India by the 20-m-deep Palk Strait. The island, some 67 654 km² in size, has been repeatedly connected with India between successive interglacials, the most recent one, in which sea level was about 120 m below present (Siddall et al. 2003), resulting in a 140-km-wide land bridge 20 000 years ago. Despite these repeated connections, the fauna and flora of the island show remarkably high endemism against a background of a more generalized biota representative of southern India (Davis et al. 1995; Wikramanayake et al. 2002).

Precipitation across Sri Lanka is dependent on monsoonal winds, resulting in much of the island's experiencing relatively low rainfall (less than 2 000 mm per year), except for the southwestern “wet zone” quarter, where precipitation ranges from 2 000–5 000 mm per year. While dry evergreen forests occupy almost the entirety of the “dry zone,” dipterocarp-dominated rainforests dominate the lowlands of the wet zone, and some 220 km² of tropical montane cloud forest still persist in the central hills, which rise to a maximum altitude of 2 524 m.

In terms of plant diversity, the Western Ghats harbors approximately 5 000 species of flowering plants belonging to nearly 2 200 genera and 217 families; about 1 700 species (34%) are endemic. There are also 58 endemic plant genera, 49 of which are monotypic and some highly speciose (e.g., Niligrianthus with 20 species). Some prominent genera and families are represented by large numbers of endemic species, such as Impatiens with 76 of 86 species endemic, Dipterocarpus with 12 of 13 species endemic, and Calamus with 23 of 25 species endemic. Of the 490 tree species (over 10 cm girth at breast height, or GBH) recorded from low- and mid-elevation forests, 308 (63%) species representing 58 families are endemic (Ramesh and Pascal 1991). The only gymnosperm tree, Podocarpus (=Nageia) wallichianus, is also endemic. Of the 267 species of orchids (representing 72 genera), 130 are endemic. Similarly, plant diversity and endemism in Sri Lanka is quite high, with 3 210 flowering plant species in 1 052 genera, of which 916 species and 18 genera are endemic (Dassanayake and Fossberg 1980–2002). In addition, the island's ferns (although not recently assessed) are estimated to number about 350 species. Approximately 433 plant species, and at least five genera, are confined to Sri Lanka and the Western Ghats combined. Overall, an estimated total of 3 049 species are endemic to the hotspot (out of a minimum of 5 916 species), as are at least 81 genera.

In the Western Ghats, the Agasthyamalai Hills in the extreme south are believed to harbor the highest levels of plant diversity and endemism at the species level. Nearly 87% of the region's flowering plants are found south of the Palghat Gap (37% being exclusive to this sub-region); these figures decrease to about 60% and 5%, respectively, in the Nilgiri Hills. In Sri Lanka, diversity, richness, and endemism across all taxa are much higher in the wet (including the montane) zone than in the dry zone. Indeed, the wet zone, which accounts for only a quarter of Sri Lanka's territory, contains 88% of the flowering plants occurring in the island, and 95% of its angiosperm endemics. Convincing evidence is emerging from recent phylogenetic and phytogeographic studies of Dipterocarpaceae (Gunasekera and Dayanandan 2003), and molecular dating studies of Crypteroniaceae (Conti et al. 2003) and the frog family Ranidae (Roelants et al., in press), that the wet zone biota of Sri Lanka is a unique relict of the Deccan-Gondwanic biota and, therefore, of considerable biogeographic significance.

Vertebrate diversity and endemism across the hotspot is impressive. Approximately 140 mammal species have been recorded (49 of them bats), although endemism is relatively low, with only 18 endemic species. In addition, four genera are endemic, each represented by single species. Three are confined to Sri Lanka: Pearson's long-clawed shrew (Solisorex pearsoni, EN), Kelaart's long-clawed shrew (Feroculus feroculus, EN), and the Ohiya rat (Srilankamys ohiensis). The bat Latidens salimalii (CR) is endemic to the High Wavy Mountains in the Western Ghats. Among flagship mammal species, the most prominent are the lion-tailed macaque (Macaca silenus, EN) and the endemic Nilgiri tahr (Hemitragus hylocrius, EN). One of the most threatened Indian mammals, the Malabar civet (Viverra civettina, CR), is known only from the Malabar Plains, which are densely populated and the focus of most developmental activities. The Western Ghats is also home to the largest population of Asian elephant (Elephas maximus), with about 11 000 animals, while in Sri Lanka the species has now been almost completely extirpated in the wet zone and reduced to an estimated population of around 2 500 elsewhere in the island.

A total of 457 bird species occur regularly within the Western Ghats and Sri Lanka, but only 35 are endemic. Of the endemics, seven occur in low-elevation forests, including species such as the grey-headed bulbul (Pycnonotus priocephalus), white-bellied tree pie (Dendrocitta leucogastra), and Malabar parakeet (Psittacula columboides), while seven others are associated with higher elevations, among them the white-bellied shortwing (Brachypteryx major, VU), Nilgiri flycatcher (Eumyias albicaudata), and broad-tailed grassbird (Schoenicola platyura, VU). In recognition of the significant regional avian diversity, both the Western Ghats and Sri Lanka are considered as Endemic Bird Areas by BirdLife International (Stattersfield et al. 1998); there are no fewer than 14 bird species restricted entirely to the wet zone of Sri Lanka.

As is true for a number of other biodiversity hotspots, the highest levels of vertebrate endemism within this hotspot are among amphibians and reptiles. Of the 179 species of amphibians reported, 138 are endemic. In the case of Sri Lanka, amphibian diversity is only now becoming better known, and the country's wet zone may contain as many as 140 endemic species alone (Meegaskumbura et al. 2002). Amphibian endemism is also impressive at the generic level, with 10 genera (of a total of 29) occurring only here. Recently, a new burrowing anuran family, Nasikabatrachidae, closely related to the Sooglossidae from the Seychelles, has been described following its discovery in the Idduki District in Kerala in the Western Ghats (Biju and Bossuyt 2003); this represents the only endemic family. About 265 species of reptiles have been recorded, and 175 of these are endemic; one-quarter (22) of all genera represented are endemic, and nine of these are monotypic. Families such as Uropeltidae (46 of 47 species), Gekkonidae (19 of 30), and Agamidae (20 of 26) exhibit very high endemism.

Besides terrestrial vertebrates, the Western Ghats and Sri Lanka Hotspot harbors 191 species of strictly freshwater fishes belonging to 45 genera; 139 species and eight genera are endemic (including one genus in Sri Lanka, Malpulutta). In the Western Ghats, the southern region is known to be more diverse than the northern and central regions (Daniels, 2001; Dahanukar et al. 2004).

Although knowledge of invertebrate diversity is poor, levels of endemism within certain groups in the Western Ghats and in Sri Lanka are believed to be significant; for example, among the tiger beetles, around 101 of 139 recorded species are endemic (D. Pearson, pers. comm.). This may not hold true across groups: the number of butterfly species in this region is relatively low, with only 37 endemics of 330 (Kunte et al. 1999) in the Western Ghats, and 24 of 234 species endemic in Sri Lanka (D'Abrera, 1998). The wet zone of Sri Lanka is known to contain several unique endemics such as Krombeinictus nordenae, a pollen-provisioning sphecid wasp (Krombein and Norden 1997) and Aneuretus simoni, the sole surviving representative of one of the world's 14 ant subfamilies (Jayasuriya and Traniello 1985).

The high levels of biodiversity in the Western Ghats and Sri Lanka face the same pressures as other hotspots, particularly being part of one of the hotspots with the highest population density (Cincotta et al. 2000), and it is likely that no more than 25% of the original vegetation of this hotspot remains intact. In the late 1980s, India's remaining forest cover already stood at less than 20% of its original extent. Forest loss has been driven largely by shifting agriculture; conversion to plantations of tea, coffee, teak, eucalyptus, and wattle, as well as for the creation of reservoirs; and construction of roads and railways. There are no reliable estimates for recent rates of total forest loss for the Western Ghats. A study that estimated changes in forest cover between 1973 and 1995 in the southern part of the Western Ghats using satellite data found that the area (approximately 40 000 km²) had lost 25.6% of its forest cover (Jha et al. 2000). In all, approximately 20% of the natural forest vegetation of the Western Ghats is thought to remain (Collins et al. 1991), but in a highly fragmented state, and the only tracts greater than 200 km2 are found in the Agasthyamala Hills, Cardamom Hills, Silent Valley-New Amarambalam Forests, and southern parts of the South Kannada District in Karnataka State.

On Sri Lanka, aquatic habitats are increasingly threatened by invasive alien species (more than 90% of the freshwater-fish biomass comprises exotics; Pethiyagoda 1994), and forests are threatened by encroachment and illegal produce extraction (50% of the island's energy demand is met with uncultivated firewood). While the wet zone harbors greater diversity and endemism than the dry zone, the former contains 67% of the island's 19.3 million inhabitants (Population Reference Bureau 2003) – i.e., a density of 740 people per km², which is exceptional by standards of other global hotspots (Cincotta et al. 2000). The main threats to the island's remaining rainforests are habitat fragmentation, together with invasive species, pesticide influx, and edge effects. Although 33% of Sri Lanka's territory is still forested, much of this forest is in the biodiversity-poor dry zone. Only 4.6% of the wet zone (800 km²) now contains closed-canopy rainforest, the remainder having been cleared for the cultivation of cinchona, coffee, tea, and rubber during the past 150 years. The remaining expanse of wet-zone natural forest comprises some 140 fragments, the largest three of which are Peak Wilderness (250 km²), the Knuckles Hills (175 km²), and the Sinharaja World Heritage Site (90 km²), but at least 120 of these fragments are less than 10 km² in extent.

For the purpose of addressing future threats to biodiversity conservation in this region, a major priority is the establishment of protected areas. A national protected area system has existed in India for more than a century, and was strengthened considerably with passage of the Wildlife Protection Act of 1972 and the National Wildlife Action Plan of 1983. Of India's 10 biogeographic zones (Rodgers et al. 2002), the Western Ghats has the highest percentage of protected area coverage on the mainland (15%), which is represented in 20 national parks and 68 sanctuaries (Rodgers et al. 2002). Sri Lanka prides itself in that 13% of its territory comprises protected areas, although this is an underestimate, because a further 16% of the territory is protected as forest reserves which, since an administrative ban on logging in 1989, are now in effect protected (not yet formally so designated). The challenge of biodiversity conservation in this country, therefore, lies largely in the conservation of the remaining extent of wet-zone natural forest. Overall, approximately 23% of this hotspot has some level of protection, although only 11% is represented in protected areas classified in IUCN categories I to IV.

Biodiversity conservation in the region has a long history, and one that is enshrined to some degree in religion, culture, and mythology. There is clearly an increased awareness among local people regarding natural resources and the need to conserve them, which provides an outlook for biodiversity conservation that, at least in the case of India, is perhaps much brighter in this region than elsewhere in the country. Problems do loom on the horizon, however, including government plans for new hydroelectric projects, roads, and railways, as well as possible movements by local groups in India to reclaim rights to forest lands for traditional and subsistence use.

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