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 (Whitten et al. 2000).
Based on the Köppen climate zone system, Sumatra falls in the tropical wet climate zone (National Geographic Society 1999). The lowland rain forests to the west of the Barisan Range receive more rainfall (~6,000 mm/year) than the lowland rain forests to the east (~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) (Whitten et al. 2000).
Sumatra's rain forests 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 rain forests. Woody climbers and epiphytes are also abundant (Whitten et al. 2000). The lowland rain forests of Sumatra support 111 dipterocarp species, including 6 endemics. The emergent trees, which can reach 70 m 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 (Whitten et al. 2000). Ground vegetation usually is sparse-mainly small trees and saplings of canopy species-and herbs are uncommon.
Figs (Moraceae) are also common in the lowland rain forest. 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 (MacKinnon 1986). Dipterocarps also use mast fruiting, perhaps to escape seed predation, by satiating the appetites of seed-predators and leaving the remaining seeds to germinate (Whitten et al. 2000). Sumatra once contained pure stands of rot- and insect-resisting ironwood (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 emergents (Whitten et al. 2000).
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 m 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 Mt. 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 (Whitten et al. 2000; MacKinnon 1997). Sumatra's lowland rain forests also are home to one of the world's tallest flower, Amorphophallus titanum, belonging to the Arum family. The flower often grows on top of a 2-m 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 (Stone 1994; Whitten et al. 2000).
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 (Whitten et al. 2000).
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 (Stone 1994). Seventeen bird species are found only north of Lake Toba, and ten are limited to the south. The white-handed gibbon (Hylobates lar) occurs only north of Lake Toba, and the dark-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 (Whitten et al. 2000). 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 (McClung 1997).
The Malayan tapir is but one of the many endangered mammals living in Sumatra's rain forests. The two-horned Sumatran rhinoceros (Dicerorhinus sumatrensis) once ranged through much of southeast Asia. Today the entire population numbers about 300-500 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 (Sukumar 1989). The Sumatran tiger (Panthera tigris) is Indonesia's largest terrestrial predator and is critically endangered (IUCN 2000). The Sumatran tiger lives in lowland and montane rain forest 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 (Franklin et al. 1999). However, the tiger is intensively hunted for skins and to supply the traditional medicine markets. There are two Level I TCUs in Sumatra that overlap this ecoregion (Dinerstein et al. 1997).
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) (Whitten et al. 2000; Stone 1994).
Table 1. Endemic and Near-Endemic Mammal Species.
Molossidae Mormopterus doriae*
An asterisk signifies that the species' range is limited to this ecoregion.
The bird fauna consists of more than 450 species, including four near-endemic species and one endemic species, the Simeulue scops-owl (Otus umbra) (table 2). The Sumatran lowland forests harbor a remarkable ten hornbill species, including the great hornbill (Buceros bicornis), absent on other Indonesian islands. These rain forests are also home to the great argus pheasant (Argusianus argus). The male argus pheasant clears patches in the forest to perform dramatic dances, flaunting his 1.5-m tail and emitting distinctive "ki-an" calls to attract females for breeding.
Table 2. Endemic and Near-Endemic Bird Species.
Family Common Name Species
Strigidae Simeulue scops-owl Otus umbra*
Irenidae Blue-masked leafbird Chloropsis venusta
Pycnonotidae Cream-striped bulbul Pycnonotus leucogrammicus
Pycnonotidae Spot-necked bulbul Pycnonotus tympanistrigus
Pycnonotidae Blue-wattled bulbul Pycnonotus nieuwenhuisii
An asterisk signifies that the species' range is limited to this ecoregion.
The 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 (Holmes, essay 1).
Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion.
Protected Area Area (km2) IUCN Category
Gunung Salawah Agam 50 PRO
Hutan Pinus/Janthoi 60 I
Gunung Leuser [IM0159] 3,220 II
Dolok Sembelin [IM0159] 110 VI
Pulau Simeulue 570 PRO
Singkil Barat 800
Pulau Bengkaru 60
Laut Tapus 40 PRO
Sibolga 240 PRO
Pulau Nias (I-IV) 710 PRO
Padang Lawas 680 PRO
Rimbo Panti 20 DE
Malampah Alahan Panjang [IM0159] 250 PRO
Bukit Rimbang-Baling-Baling 1,220 PRO
Air Sawan 880 PRO
Peranap 960 PRO
Bukit Sebelah and Batang Pangean 460 VI
Bukit Besar 1,330 PRO
Way Kambas [IM0157] 810 II
Dangku 750 IV
Bentayan 310 IV
Benakat 570 DE
Subanjeriji 1,100 DE
Rebang 120 PRO
Semidang Bukit Kabu 240 VI
Bukit Balal [IM0159] 100 VI
Bukit Raja Mandara [IM0159] 80 VIII
Gumai Pasemah [IM0159] 320 IV
Isau-Isau Pasemah [IM0159] 100 IV
Bukit Barisan Selatan [IM0159] 1,750 II
Bukit Nantiogan Hulu/Nanti Komerung Hulu [IM0159] 720 VI
Paraduan Gistana and Surroundings 730 VI
Way Waya 330 VIII
Pulau Sangiang 20 DE
Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.
Although almost 9 percent of this ecoregion is in protected areas, the habitat is not guaranteed protection. Since the fall of the Suharto government, many of these protected areas have been invaded and their natural resources mined, despite the efforts of conservation agencies and assistance from many outside conservation organizations. With the fall of the Suharto government in 1998 and the economic crises of 1998-1999, several groups exploited forests inside existing protected areas. Local people, feeling betrayed by the Suharto government, took control of the natural resources around them. A second group of people who worked for forest and oil palm concessionaires and sawmill owners also began to encroach on protected areas. Common thugs and corrupt local and military officials made up a third group and provided protection to all groups involved in illegal activities. One of numerous examples of illegal logging inside Indonesia's protected areas is the unabated logging that has occurred in and around the biologically valuable Gunung Leuser National Park since April 1999. The destruction includes prime habitat for the orangutan, siamang, and white-handed gibbon. This area also contains the only known population of tool-using orangutans (Colijn, http://www.bart.nl/~edcolijn/).
To ensure the survival of many of the top predators and wide-ranging species such as the tiger, the protected areas must be protected. Another problem is that the current protected area system is not large enough in size or distribution to protect many of the top predators and wide-ranging species over the long term. Many of the protected areas have no effective management. However, many of the largest reserves are reservoirs for biodiversity and retain the best chance for conservation in the future. These reserves include Gunung Leuser, Way Kambas, and Bukit Barisan Selatan.
Types and Severity of Threats
The remaining natural forests in this ecoregion will be completely gone within the next five years unless drastic actions are taken to halt rampant logging. The approximate forest loss in Sumatra from 1985 to 1997 has been 67,000 km2, most of it in the rich lowland forests. However, the annual rate of forest loss has been increasing across Indonesia. Country-wide, the deforestation rate in the 1980s was 8,000 km2/year. In the early 1990s this rate had increased to around 12,000 km2/year. From about 1996 to the present the rate has almost doubled to more than 20,000 km2/year. In Sumatra's lowland forests from 1985 to 1997 the average annual forest loss was about 2,800 km2/year. If the current deforestation trend continues, this ecoregion's natural forests will be gone by 2005 (Holmes, essay 1).
Logging and clearing for plantations and agriculture have been especially heavy in the lowlands east of the Barisan Range, especially since the 1998-1999 economic crisis. Extensive stands of ironwood (Eusideroxylon zwageri), a species of great commercial importance producing an exceptionally durable timber, have been almost entirely destroyed in southern Sumatra. Deforestation has been extensive in Bengkulu Province, especially along the main road running north from Bengkulu to Muko-Muko, where international involvement has been responsible for replacing tropical rain forest with oil palm and cocoa monocultures. In the absence of these forests, heavy rains in 1988 caused massive landslides and floods, which affected thousands of people.
The island of Nias presents a similar case as Sumatra's mainland forests. Almost all the natural forests are gone and exist as extensive areas of secondary forest. They still support many of the region's endemic birds, but the situation is deteriorating. Simeulue had most of its forests in the 1980s but is rapidly losing them to clove and coffee plantations.
Justification of Ecoregion Delineation
We recognized six ecoregions in Sumatra. MacKinnon (1997) placed all the biomes in Sumatra within two subunits (21a and 21b), with the subunit division based on a faunal break to the south of Lake Toba. However, according to Tony Whitten (pers. comm., 1999) the differences in floral and faunal communities across MacKinnon's subunit boundary (within a habitat type) are not great enough to warrant differentiation into distinct ecoregions. Therefore, we placed the lowland rain forests of Sumatra into a single ecoregion, the Sumatran Lowland Rain Forests [IM0158]. Although FAO (1974) and MacKinnon (1997) showed that Bangka Island has extensive heath forests, Whitmore (1984) and Whitten et al. (1984a) reported that the heath forests are less extensive, and its main forests are not heath forests; therefore, we included this island as part of the Sumatran Lowland Rain Forests [IM0158] ecoregion.
MacKinnon's biounit 21 largely corresponds to Udvardy's Sumatra biogeographic province. However, Udvardy did not include the Nicobar Islands. Eight ecoregions overlap Udvardy's Sumatra biogeographic province: Sumatran Lowland Rain Forests [IM0158], Sumatran Montane Rain Forests [IM0159], Mentawai Islands Rain Forests [IM0127], Sumatran Peat Swamp Forests [IM0160], Sumatran Freshwater Swamp Forests [IM0157], Sundaland Heath Forests [IM0161], Sumatran Tropical Pine Forests [IM0304], and Sunda Shelf Mangroves [IM1405].
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List
Prepared by: Colby Loucks and Tony Whitten