The Sumba deciduous forests are found on the single island of Sumba and are part of the region known as Wallacea, which contains a distinctive fauna representing a mix of Asian and Australasian species.

Sumba Island, Indonesia. (Photograph by WWF-Canon/Edward Parker)

Although vertebrate diversity is low, the ecoregion contains seven bird species found nowhere else in the world and several other birds with very limited ranges.

As a result of forest clearance and repeated burning for grazing and agriculture, the forested area of Sumba has declined significantly over the last century.

Location and General Description

This ecoregion represents the semi-evergreen forests on the island of Sumba, in the eastern Indonesian Archipelago. The surface geology of Sumba is composed primarily of sandstone and mudstone, with some igneous intrusions overlain by recent limestone. Sumba is believed to be a fragment of the Australian continental crust that was separated some 20 million years ago, well before the neighboring outer arc island of Timor. The island is quite rugged, consisting of deeply dissected plateaus. There is very little area above 1,000 meters (m), and the highest point on the island is 1,225 m. Precipitation in Sumba is seasonal, and based on the Köppen climate zone system, this ecoregion falls in the tropical dry climate zone.

The naturally dominant vegetation of the island was deciduous monsoon forest. However, the southern hill slopes along the southern coasts, which remain moist during the dry season, are covered with lowland evergreen rain forest. The most extensive and important of these rain forest areas is the Mt. Wanggameti-Laiwanga forest complex in East Sumba, a major water catchment. In East Sumba there are extensive gallery forests in ravines and along rivers that form riparian corridors across open grasslands or savannas. The savanna understory includes an endemic insectivorous sundew (Drosera indica).

Biodiversity Features

The ecoregion harbors seventeen mammal species, but none are considered to be endemic or even near endemic.

The avifauna of this ecoregion is highly distinctive, with both Asian and Australian influences, although the total diversity is low. There are approximately 180 bird species on the island, and 12 of these species are endemic or near endemic (Table 1). The ecoregion corresponds to the Sumba Endemic Bird Area (EBA). The Sumba EBA contains twelve restricted-range bird species, seven of which are found nowhere else on Earth. Four of these species are considered vulnerable: Sumba buttonquail (Turnix everetti), red-naped fruit-dove (Ptilinopus dohertyi), Sumba boobook (Ninox rudolfi), and Sumba hornbill (Aceros everetti). These threatened species have specific habitat needs that make them susceptible to forest clearance.

Current Status

Almost three quarters of the ecoregion area has been burnt for hunting or cleared, mostly for agriculture or firewood extraction. A few small, intact patches exist but are scattered in isolated fragments. Most of the original monsoon forests have been replaced by savanna and grassland. The four small (average size 83 square-kilometers [km²) protected areas include about 3 percent (330 km2) of the ecoregion area (Table 2).

Pressures from the rapidly increasing, poor population are intense in this ecoregion, and nearly three-quarters of this ecoregion has been deforested, with only isolated fragments of natural habitat remaining.

Types and Severity of Threats

Threats include deforestation, burning of grasslands to establish agricultural fields, livestock grazing, and poaching. Much of the forest has already been replaced by fire-resistant casuarinas or eucalypts and extensive deciduous scrub. For instance, the ecoregion's dry thorny forest, which is especially vulnerable to clearance by fire, has almost completely disappeared.

Justification of Ecoregion Delineation

The drier forests in Nusa Tenggara were placed in three ecoregions that corresponded to the biogeographic units identified in Monk et al: Lesser Sundas deciduous forests, which includes the chain of islands extending from Lombok, Sumbawa, Komodo, Flores, and the smaller satellite islands corresponding to the Flores biogeographic unit; Timor and Wetar deciduous forests, corresponding to the Timor biogeographic unit; and the Sumba deciduous forests, corresponding to the Sumba biogeographic unit. All three ecoregions belong to the tropical dry forests biome.

Additional information on this ecoregion

• For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
• To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
• World Wildlife Fund Homepage