Sumatran lowland rainforests
Sumatran orangutan in captivity. Source: Greg Hume
The Sumatran lowland rainforests are one of the most diverse forests on Earth and also among the most threatened.
These forests contain comparable levels of species diversity (Biodiversity) as the richest forests in Borneo and New Guinea.The Sumatra rainforests are home to some of the world's most charismatic flowering plants: Rafflesia arnoldii, which produces the largest flower in the world (up to one metre wide), and Amorphophallus titanum, which stands more than two metres tall and produces aroid flowers. The avifauna is also exceptionally rich. More than 450 bird species are found here, more than in any other ecoregion in the Sunda Shelf and Philippines bioregion except the Borneo lowland rainforests.
In the past fifteen years rampant logging, hunting, fires, and habitat loss in the lowlands have pushed many of this ecoregion's already endangered species to the edge of extinction. These include the Sumatran rhinoceros, Malayan tapir, tiger, Asian Elephant, and Orangutan. Illegal deforestation (including slash-and-burn practises by indigenous peoples) and pervasive corruption are contributing to the more than 3000 square kilometers of forest destroyed in each year of the early twenty-first century in this ecoregion. At the current time no natural forests of mappable extent are thought to continue to exist here intact.
Location and General Description
This ecoregion represents the lowland moist forests of Sumatra, including the small islands of Simeulue, Nias, and most of Bangka. The geologic history of Sumatra provides insights into the origins of Sumatra's biodiversity. About 150 million years ago Borneo, Sumatra, and western Sulawesi split off from Gondwanaland and drifted north. Around 70 million years ago India slammed into the Asian landmass, forming the Himalayas, and an associated thrust formed Sumatra's Barisan Mountains, which run the length of Sumatra. As the Barisan Range buckled upward, it formed a deep-water channel to the west of Sumatra. During this time the islands of Simeulue and Enggano were formed. Today, to the east of the Barisan Range low hills and plains exist as a result of tectonic and volcanic events. Continued mountain building, volcanic activity, and sedimentation in the lowland occurred over the past 25 million years. Podzolic soils associated with altosols or litosols are the predominant lowland soils. Large limestone areas occur in northern Sumatra, and they are associated with brown podzolic and renzina soils.
Source: WWF
Based on the Köppen climate zone system, Sumatra falls in the tropical wet climate zone. The lowland rainforests to the west of the Barisan Range receive more rainfall (about 6000 millimeters (mm)/year) than the lowland rainforests to the east (about 2,500+ mm/year). The Barisan Range blocks much of this rainfall. However, most of Sumatra experiences less than three consecutive months of dry weather (less than 100 mm rainfall/month).
Sumatra's rainforests are quite diverse and contain levels of species diversity comparable to those of the richest forests in Borneo and New Guinea and are much richer than Java, Sulawesi, and other islands in the Indonesian Archipelago. Large, buttressed trees dominated by the Dipterocarpaceae family characterize Sumatra's lowland rainforests. Woody climbers and epiphytes are also abundant. The lowland rainforests of Sumatra support 111 dipterocarp species, including 6 endemics. The emergent trees, which can reach 70 metres tall, are also dipterocarps (Dipterocarpus spp., Parashorea spp., Shorea spp., Dryobalanops spp.) and, to a lesser extent, species in the Caesalpiniaceae family (Koompasia spp., Sindora spp., and Dialium spp.). Dipterocarps dominate the canopy layer as well. Other canopy and understory tree families that are common include Burseraceae, Sapotaceae, Euphorbiacae, Rubiaceae, Annonaceae, Lauraceae, and Myristicaceae. Ground vegetation usually is sparse, chiefly small trees and saplings of canopy species, with herbs being uncommon.
Figs (Moraceae) are also common in the lowland rainforest. There are more than 100 fig species in Sumatra, and each species usually is pollinated exclusively by a single fig-wasp (Agaonidae) species. Figs may produce (mast) from 500 to a million fruits twice a year and are important food sources for many forest animals. Dipterocarps also use mast fruiting, perhaps to escape seed predation, by satiating the appetites of seed-predators and leaving the remaining seeds to germinate. Sumatra once contained pure stands of rot- and insect-resisting the ironwood tree, Billian (Eusideroxylon zwageri) forests. Ironwood is a member of the laurel family and is distributed throughout southern Sumatra, Kalimantan, and the Philippines. Ironwood forests are dominated by Eusideroxylon zwageri but may have also contained Shorea, Koompasia, or Intsia species as canop emergents.
Biodiversity Features
Malaysian Tapir (Tapirus indicus), Sumatra, Indonesia. (Photograph by WWF-Canon / Alain Compost) Sumatra shares many of its species with peninsular Malaysia and Borneo. All three landmasses were once part of a single, larger landmass during the last ice age, when sea level was more than 100 metres lower than it is today. Consequently, these forests share many of the same flora and fauna. One of the most distinctive plant species in the region is Rafflesia. Five of the sixteen species of the parasitic Rafflesia plant are found in Sumatra and occur mainly in lowland forest, although they have been recorded as high as 1,800 m on Mount Lembuh, Aceh. Rafflesia arnoldii is found in this ecoregion and produces the largest flower in the world. Its large brown-orange and white flowers span almost 1 m in width. Rafflesia have no leaves, instead deriving all their energy from the tissues of their host, the vine Tetrastigma. Large buds emerge from the vine and have five large, flowery petals surrounding plates, which smell like rotting meat and attract pollinating insects. Sumatra's lowland rainforests also are home to one of the world's tallest flowers, Titum Arum, Amorphophallus titanum, belonging to the Arum family. The flower often grows atop of a two metre stalk and appears every few years. These plants have blotchy stems, unusual leaves, and a fetid odor, which attracts small stingless bees that act as pollinators.
Other plants common to these forests are epiphytes. Common epiphyte families found in Sumatra include Orchidaceae, Gesneriaceae, Melastomaceae, Asclepidiaceae, and Rubiaceae. Rubiaceae includes the "ant-plants," Myrmecodia and Hydnophytum. These plants harbor ant colonies in their stems, and the ants protect the plant's leaves from caterpillars and other arthropod herbivores.
Kerinci Seblat National Park, Sumatra, Indonesia. (Photograph by WWF-Canon / Mauri Rautkari) The fauna of Sumatra can be split into two regions, one to the north of Lake Toba and the other to the south.Lake Toba formed 75,000 years ago as part of a volcanic eruption that had a devastating impact on Sumatra. Seventeen bird species are found only north of Lake Toba, and ten are limited to the south. The White-headed Gibbon (Hylobates lar) occurs only north of Lake Toba, and the Black-handed Gibbon (Hylobates agilis) is found only to the south. The Tarsier (Tarsius bancansus), Banded Leaf-monkey (Presbytis melalophus), and endangered Malayan Tapir (Tapirus indicus) are found south of Lake Toba. The Malayan Tapir is the largest of the four living tapir species and the only Old World representative. The Sumatran population of the Malayan Tapir is close to extinction, with no more than fifty animals left in the wild.
The Malayan tapir is but one of the many endangered mammals living in Sumatra's rainforests. The two-horned Sumatran Rhinoceros (Dicerorhinus sumatrensis) once ranged through much of southeast Asia. Today the entire population numbers about 300 to 500 individuals scattered in several populations in Sumatra, Borneo, and peninsular Malaysia. The Asian Elephant (Elephas maximus) is found in several small populations throughout Sumatra. Only five populations number more than 200 individuals, and with rapid habitat loss the survival of many of these populations is uncertain, despite the fact that they can exploit secondary forests. The Sumatran tiger (Panthera tigris) is Indonesia's largest terrestrial predator and is critically endangered. The Sumatran tiger lives in lowland and montane rainforest and in freshwater swamp forests throughout Sumatra. There are an estimated 500 Sumatran tigers remaining in Sumatra, with approximately 100 found in Gunung Leuser National Park in northern Sumatra. However, the tiger is intensively hunted for skins and to supply the traditional medicine markets. There are two Level I Tiger Conservation Units (TCUs) in Sumatra that overlap this ecoregion.
There is only one endemic mammal in this ecoregion (Table 1). However, these forests contain numerous primate species such as several leaf-monkey species, slow loris (Nycticebus coucang), long-tailed macaque (Macaca fascicularis), pig-tailed macaque (M. nemestrina), and siamang (Hylobates syndactylus), the region's largest gibbon and found only in Malaya and Sumatra's lowland forests. Other species include the Sunda otter-civet (Cynogale bennettii), wild dog (Cuon alpinus), sun bear (Ursus malayanus), and clouded leopard (Pardofelis nebulosa).
Current StatusThe conservation status of this ecoregion's forests is critical. Before 1985 only about one-third of this ecoregion's natural forests remained. Most of this habitat had been lost to agricultural expansion and logging. However, in the past fifteen years more than 60 percent of these forests have been destroyed. The remaining areas of intact habitat are found primarily in central Sumatra. There are several protected areas in this ecoregion, which include about 9 percent of the ecoregion area (Table 3). However, encroachment, widespread illegal logging, and fires in these protected areas are severe.
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