Tropical thorn forests

Answer 1

IntroductionThe Deccan Thorn Scrub Forests harbor the last populations of the globally threatened Jerdon's courser (Rhinoptilus bitorquatus), rediscovered recently, eighty-six years since it was last recorded in 1900. Otherwise, the ecoregion is neither exceptionally species-rich nor high in endemism. Many ecologistsbelieve that the thorn scrub vegetation represents a degraded stage of the tropical dry forests, modified by human and livestock use over hundreds of years.

Location and General Description

Satellite view of southern India and northern Sri Lanka. (Photograph by National Geographic Society)

The ecoregion represents the thorn scrub vegetation in the arid parts of the Deccan Plateau. It sprawls across the Indian states of Tamil Nadu, Andhra pradesh, Karnataka, and Maharashtra and also includes part of northern Sri Lanka.

The Deccan Plateau itself was part of the ancient southern continent, Gondwanaland, that disintegrated during the Cretaceous to give rise to the Indian Subcontinent as well as Africa,Madagascar, Australia, South America, and New Guinea and some of the smaller islands such as New Caledonia and Tasmania. After the Deccan Plateau drifted northward to collide with the Eurasian continent about 50 million years ago, geological uplift gave rise to the Western Ghats Mountains along the western coast of the peninsula. This mountain range then intercepted the moisture-laden southwest monsoons and created a dry rainshadow in the vast plateau, affecting its vegetation. But in the more recent past, human influences have altered the vegetation to create vast areas of thorn scrub from what was believed to be tropical dry forests.

Annual rainfall in the ecoregion is less than 750 millimeters. All rain is received during the brief wet season, and there is practically no rainfall from November to April. Ambient temperatures can exceed a sweltering 40oC during the hotter months of the year.

The forest type in this ecoregion is mostly southern tropical thorn scrub, as defined by Champion and Seth (1968), but includes patches of tropical dry deciduous forests, which are believed to be the original vegetation. The former consists of open, low vegetation characterized by thorny trees with short trunks and low, branching crowns that rarely meet to form a closed canopy. The trees attain heights of 6-9 meters. The second story is poorly developed and consists of spiny and xerophytic species, mostly shrubs. During the brief wet season an ill-defined lower story can be discerned. The dominant vegetation is Acaciaspecies, with Balanites roxburghii, Cordia myxa, Capparisspp., Prosopis spp., Azadirachta indica, Cassia fistula, Diospyros chloroxylon, Carrisa carandas, and Phoenix sylvestris.

Changeable hawk eagle (Spizaetus cirrhatus), India. (Photograph by Rahula Perera)

Champion and Seth (1968) have also identified several habitat types within this vast thorn scrub ecoregion. In areas of particularly low rainfall and rocky soils, the thorn scrub transitions into a Euphorbia-dominated scrub (i.e., the southern Euphorbia scrub). Here the soil usually is bare, although some grassy growth may appear during the short monsoon season.

In parts of Tamil Nadu, where rainfall is even less, the vegetation is made up of open thorny forests with scattered Acacia planifrons that are characterized by umbrella-shaped crowns. This vegetation is described as Carnatic umbrella thorn forests by Champion and Seth (1968).

Scattered amid the thorn scrub are patches of dry grasslands that provide habitat for the native fauna. For example, the grasslands of southern Andhra Pradesh support a good population of the Indian bustard (Ardeotis nigriceps) and blackbuck (Antilope cervicapra). The typical grasses in this habitat include Chrysopogon fulvus, Heteropogon contortus, Eremopogon foveolatus, Aristida setacea, and Dactylocteniumspp.

Patches of dry deciduous forests, especially along the Tirupathi Hill Ranges, are known for a large number of medicinal plants and various other species of botanical interest, among which are the rare endemic cycad (Cycas beddomei) and Psilotum nudum. The latter usually is found along steep escarpments. A small patch of the dipterocarp Shorea talura exists within the Chittoor forest division, part of which is being maintained as a preservation plot by the Forest Department of Andhra Pradesh.

The Srilankamalleswara Sanctuary between the Nallamalais and Sechachalam hill ranges is known for a rare, endemic tree species, red sanders (Pterocarpus santalinus). This area is also the southern distributional limit of the nilgai (Boselaphus tragocamelus) in the Indian Peninsula.

Biodiversity FeaturesUntil the recent past, this ecoregion provided important habitat for the tiger (Panthera tigris) (in the Indian sector) and Asian elephant (Elephas maximus). But over the years, their populations have dwindled and even become locally extinct because of the adverse influences from the dense human population.

The mammal fauna in the ecoregion includes ninety-six species, including two endemic rodents and an endemic bat (Table 1).Table 1. Endemic Mammal Species. FamilySpecies Rhinolophidae Hipposideros schistaceus*Muridae Millardia kondana* MuridaeCremnomys elvira*An asterisk signifies that the species' range is limited to this ecoregion.

The endemic rodents are threatened. Other threatened species in the ecoregion include tiger, gaur (Bos gaurus), wild dog (Cuon alpinus), sloth bear (Melursus ursinus), chousingha (Tetracerus quadricornis), and blackbuck (Antilope cervicapra). The important elephant populations are now only marginally included within this ecoregion. Small wolf populations may still be left, although most have been eliminated by a combination of loss of prey and poisoning by people as retribution for livestock predation.

The ecoregion's bird fauna consists of almost 350 species, of which three are near-endemics (Table 2).Table 2. Endemic and Near-Endemic Bird Species. FamilyCommon Name Species Glareolidae Jerdon's courserRhinoptilus bitorquatusPhasianidaeCeylon junglefowlGallus lafayetiiCapitonidaeYellow-fronted barbetMegalaima flavifronsAn asterisk signifies that the species' range is limited to this ecoregion.

The Jerdon's courser (Rhinoptilus bitorquatus) is a globally threatened species that was rediscovered in this ecoregion in 1986, after the last record in 1900 (Grimmet et al. 1998). The known population of this species is limited to a small area in this and the neighboring Central Deccan Plateau Dry Deciduous Forests. The Ceylon junglefowl (Gallus lafayetii) is limited to the ecoregion's area in northern Sri Lanka. The globally threatened lesser florican (Eupodotis indica) and Indian bustard are other birds of conservation importance in this ecoregion.

Current StatusMore than 90 percent of the ecoregion's natural habitat has been degraded or cleared, but one large block of habitat remains in southern Andhra Pradesh. The eleven protected areas cover more than 4,000 kilometers 2, but this represents just about 1 percent of the ecoregion area (Table 3). The Great Indian Bustard reserve accounts for most of the protected areas system.Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion. Protected AreaArea (km2) IUCN Category Chandikulam 120 IV Vettangudi 20 IV Srivenkateswara 500 IV Nandur Madmesh War 80 IV Jaikwadi 230 IV Great Indian Bustard 2,600 IV Great Indian Bustart (extension) 250 PRO Sagareshwar 50 IV Ghataprapha 110 IV Tungabadra 90 DE Ranebennur 60 IV Total4,110 Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.

Types and Severity of ThreatsThe forests in this ecoregion have been degraded to thorn scrub solely as a result of these human activities. Among the more serious sources of degradation is pastoralism, both from heavy cattle grazing and from forest produce extracted by the pastoralists. Several village pastures have been taken over by an exotic thorny shrub, Prosopis juliflora, resulting in the loss of grazing areas for the cattle and encroachment into the reserved forests or protected areas for grazing. The conservation status of the ecoregion was changed from endangered to critical after the analysis of projected threats from the human population. There is a common perception that these dry forests are not important for conservation. Therefore, grazing and forest clearing, especially for fuelwood, are rampant