Location and General Description
This ecoregion is made up of the peat swamp forests along the western coasts of the island of Borneo, within the Malaysian state of Sarawak and Indonesian Kalimantan. Most of the peat swamp forests are associated with coastal areas, but two large areas of peat swamp forests occur around Lake Mahakam and Lake Kapuas. Based on the Köppen climate zone system, this ecoregion falls in the tropical wet climate zone (National Geographic Society 1999).
The peat swamp forests of Borneo have vegetative and edaphic characteristics similar to those in Sumatra and peninsular Malaysia. The peat soil is predominantly organic matter, which built up behind mangrove swamps. They are ombrogenous, or rain fed, and are recent in origin (Driessen 1978; Morley 1981). Peat swamp forests form when sediments build up behind mangroves as rivers drain toward the coast. Over time, these areas build up and eventually form domes that are rarely flooded. Organic matter builds up, and peat deposits can extend up to 20 m. Because peat swamps are not drained by flooding, they are nutrient deficient and acidic (pH usually is less than 4). Compared with other moist forest ecoregions, peat swamp forests are not as species-rich or high in endemism (IUCN 1991).
Peat swamp forests encompass a sequence of forest types distributed from the perimeter to the center of each swamp. Six forest communities that have a distinct structure, physiognomy, and flora are discernible (Anderson 1983; Whitmore 1984b). The first type of forest is similar to yet less rich than lowland dipterocarp evergreen rain forests that occur on mineral soils. These forests are dominated by Gonystylus bancanus (the single most valuable timber species), Dactylocladus stenostachys, Copaifera palustris, and four Shorea species (Anderson 1983). Shorea albida plays a major role in the swamp forest communities (Whitmore 1984b; IUCN 1991) and dominates forest types two through four (Anderson 1983). Forest type four is also characterized by Calophyllum obliquinervum, Cratoxylum glaucum, and Combretocarpus rotundatus (Anderson 1983). The principal species in forest type five are Tristania obovata, Palaquium cochleariifolium, and Parastemon spicatum, and type six resembles open savanna woodland, with the most abundant species being Dactylocladus stenostachys, Garcinia cuneifolia, Litsea crassifolia, and Parastemon spicatum (Anderson 1983). Other genera of trees often found in Sarawak's peat swamp forests include Dryobalanops and Melanorrhea (IUCN 1991).
Most of the tree families of lowland dipterocarp forests are also found in peat swamp forests. Exceptions include Proteaceae, Lythraceae, Combretaceae, and Styraceae (Whitmore 1984b). Few plant species are endemic to peat swamp forests, mainly because of their recent formation. Many species found in the most acidic central portion of peat swamp forests also occur in heath forests (Sundaland Heath Forests [IM0161]). Brünig (1973) found 146 species common to both forest types. More than thirty palm species are found in peat swamp forest, including the red-stemmed sealing wax palm, Cyrtostachys lakka.
Many animal species occur in peat swamp forests, but only the bat, Hipposideros doriae, and two birds, the Javan white-eye (Zosterops flavus) and the hooked-billed bulbul (Setornis criniger), are considered near endemic (table 1, table 2). With only two exceptions, monkeys, gibbons, and orangutans are all found in Borneo's peat swamp forests, but at lower densities. Long-tailed macaques (Macaca fascicularis) and silvered langurs (Presbytis cristata) have higher densities in peat swamp forests than in lowland rain forests, but only along rivers (Wilson and Wilson 1975; Marsh and Wilson 1981; Davies and Payne 1982; MacKinnon 1983). Forest productivity is higher at the river's edge, with additional nutrient and light inputs.
Table 1. Endemic and Near-Endemic Mammal Species.
Rhinolophidae Hipposideros doriae
An asterisk signifies that the species' range is limited to this ecoregion.
Table 2. Endemic and Near-Endemic Bird Species.
Family Common Name Species
Pycnonotidae Hook-billed bulbul Setornis criniger
Zosteropidae Javan white-eye Zosterops flavus
An asterisk signifies that the species' range is limited to this ecoregion.
Peat swamp forests are key habitats for the unique proboscis monkey (Nasalis larvatus). Proboscis monkeys are found only in coastal and riverine habitats in Borneo. Proboscis monkeys are good swimmers and will swim across rivers, despite the presence of crocodilian predators. In the afternoon they are often found along the rivers, where they sleep in tall lookout trees. They eat primarily young leaves and the seeds of unripe fruit (Bennett and Sebastian 1988; Yeager 1989). Like other colobines, they have developed highly complex sacculated stomachs with specialized bacteria to digest this diet (Bauchop 1978; Bauchop and Martucci 1968).
Bird species diversity tends to be lower in peat swamp forests than in the surrounding lowland rain forests. However, in Tanjung Puting National Park, a freshwater and peat swamp reserve in Kalimantan, more than 200 bird species were recorded.
One of the most desirable and rare aquarium fish, the arowana (Scleropages formosus), is found in deep pools in peat swamp rivers. These rivers also support other typical riverine fauna such as otters, waterbirds, false gavials, crocodiles, and monitor lizards (Giesen 1987).
Peat swamp forests used to be extensive in Sarawak and also occurred in southwestern Sabah (IUCN 1991). But today about half of the area has been cleared. Brunei's peat swamp forests probably are less disturbed than those elsewhere in the region. Some of the forests in the Belait River are still in pristine condition and will soon represent the only undisturbed forests of this type in the region (WWF and IUCN 1995). The eleven protected areas cover 4,300 km2 (6 percent) of the ecoregion. Tanjung Puting, Muara Sebuku, and Kelompok Hutan Kahayan all protect more than 500 km2 of contiguous forest (table 3). The peat swamp forests are represented in Brunei's protected area network even though these forests are most intact there.
Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion.
Protected Area Area (km2) IUCN Category
Muara Sebuku 570 PRO
Muara Kayan [AA0124] 400 PRO
Unnamed 80 ?
Gunung Palung [IM0102] 250 II
Mandor 50 I
Muara Kendawangan [IM0153], [AA0124] 370 PRO
Tanjung Puting [IM0153], 90, 110] 1,120 II
Kelompok Hutan Kahayan [AA0124] 680 PRO
Danau Semayang Sungay Mahakam [IM0153] 330 PRO
Kutai (extension) [IM0153], 90, 97] 160 PRO
Muara Kaman Sedulang [IM0153], [IM0161] 290 I
Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.
In 1997-1998, drought-driven fires intentionally set to clear forests for commercial agriculture and forestry companies destroyed vast tracts of Borneo's lowland forests. More than 7,500 km2 of peat swamp forests were burned in this two-year period. Kutai and Muara Kaman suffered extensive, severe damage during the fires, with almost no area left untouched (IFFM/GTZ 1998; Yeager 1999a). Peat swamp forests are particularly vulnerable to fire and produce the most carcinogenic haze of any forest type when they are burned because of the release of large amounts of fine particulate matter. The widespread burning of these forests contributed a significant portion of the haze, which covered most of Indonesia and Malaysia and extended north to Thailand and Bangkok. Peat fires typically burn underground as well, eliminating the seedbank and destroying soil, which may take thousands of years to replace (Yeager 1999a).
The fires also had adverse effects on the wildlife populations. Unknown numbers of birds, reptiles, amphibians, primates, and other mammals died in the fires or shortly after because of the scarcity of food. Hundreds of orangutans were killed by villagers for meat, and their orphaned babies were sold to the international pet trade as they fled into villages to escape the fires. Fires are a major threat to the continued existence of the endangered orangutan (Yeager 1999b). The charismatic proboscis monkey was the primate species that lost the greatest percentage of habitat to the fires because large areas of riverine and coastal habitats were destroyed in the fires (Yeager and Russon 1998; Yeager and Fredriksson 1999).
Types and Severity of Threats
Peat swamp forests were the first formations to be logged on a commercial scale in Sarawak and for many years were the main source of timber (IUCN 1991). In Brunei, peat swamp forests are still intact but are threatened by the planned expansion of forestry operations, which may result in overexploitation of forests rich in Shorea albida (WWF and IUCN 1995). The continual threat of fires clearing forest for oil palm and other commercial agriculture crops will loom over Indonesia until stricter forest policies and protection are taken.
Justification of Ecoregion Delineation
The large island of Borneo was divided into nine ecoregions. Most of the island's lowland and submontane forests are dominated by dipterocarp species (MacKinnon et al. 1996). MacKinnon and MacKinnon (1986) divided the island's lowland forests into six subunits, with a central subunit representing the montane forests. MacKinnon (1997) revised the boundaries of these seven subunits but retained the same general configuration. These authors used the major rivers, the Kapuas and Barito, to represent zoogeographic barriers to a few species of mammals and based subunits largely on these barriers but also used climatic regimes for the drier eastern biounits (MacKinnon and MacKinnon 1986; MacKinnon 1997).
Because ecoregions are based on biomes, we first isolated the central montane ecoregion-the Borneo Montane Rain Forests [IM0103]-above the 1,000-m elevation contour using the DEM (USGS 1996). We then assigned the large patches of peat forests, heath forests, freshwater swamp forests, and mangroves, in the lowlands and along the periphery of the island, into their own ecoregions: the Borneo Peat Swamp Forests [IM0104], Sundaland Heath Forests [IM0161] (which also includes Belitung Island and the heath forests in Bangka island), Southern Borneo Freshwater Swamp Forests [IM0153], and Sunda Shelf Mangroves [IM1405], respectively.
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List
Prepared by: Colby Loucks
This text was originally published in the book Terrestrial ecoregions of the Indo-Pacific: a conservation assessment from Island Press. This assessment offers an in-depth analysis of the biodiversity and conservation status of the Indo-Pacific's ecoregions.