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Southeastern Asia: the Island of Borneo

The Borneo Montane Rain Forests [IM0103] can be likened to montane islands in a sea of lowland dipterocarp forests. This isolation has produced a unique and diverse set of montane species. Of Borneo's endemic bird species, twenty-three (73 percent) are montane. There are more than 150 mammal species in montane forests, making this ecoregion globally outstanding for mammal richness, and it is the most speciose montane rain forest found in the Indo-Pacific region. Despite this wealth of diversity, large tracts of Borneo's montane forests have not been explored to catalog the flora and fauna.

  • Scientific Code
    (IM0103)
  • Ecoregion Category
    Indo-Malayan
  • Size
    44,600 square miles
  • Status
    Relatively Stable/Intact
  • Habitats

Description
Location and General Description
This ecoregion represents the montane forests in the central region of the island of Borneo and falls within the boundaries of all three nations with territory in Borneo: Malaysia, Indonesia, and Brunei. Montane forests are much cooler and moister than lowland forests. For every 1,000 m above sea level there is an average 5°C drop in temperature, equivalent to a 10( shift in latitude. This partly explains why montane regions contain plants normally found in temperate regions. Rainfall is higher than in the lowlands, and many forests also derive moisture from clouds that bathe the region (MacKinnon 1986). The geology of the Borneo Montane Rain Forests [IM0103] is primarily old volcanic rocks and melange (rock fragments in clay) that is continental in origin. Montane soils also change with altitude, generally becoming more acidic and nutrient-poor. Montane soils are primarily inceptisols and ultisols (RePPProT 1990). Based on the Köppen climate zone system, this ecoregion falls in the highland climate zone (National Geographic Society 1999).

The montane flora of Borneo is derived from both Asian and Australian families, making it one of the most diverse montane habitats on Earth. Araucariaceae, Clethraceae, Ericaceae, Fagaceae, Lauraceae, Myrtaceae, Podocarpaceae, Symplocaceae, and Theaceae are all families commonly found in montane forests. In the lower elevations of montane forest (about 1,000-1,200 m) the dominance of dipterocarp species ends. These are replaced with highly speciose oak (Quercus and Lithocarpus spp.) and chestnut (Castanopsis spp.) forests. Myrtaceae are also important in the lower montane forests. Above 1,500 m this forest grades into a montane ericaceous belt followed by an alpine meadow on the very highest peaks. Three major and parallel changes occur in montane forests with increasing altitude. First, there is a decrease in forest height. Montane forests do not have giant emergent trees, and their overall height is much lower. The canopy typically is 10-20 m high. Second, the size and shape of the leaves change. Trees with buttresses usually are absent. Lowland forests are dominated by tree species with medium-large (mesophyll) and billowing canopy trees. Montane forests are dominated by slender trees, small leaves (microphyll), and a flattish crown surface. The third difference is the increased presence of epiphytes. Orchids, ferns, moss, lichen, and liverworts are more abundant in montane forests than in lowland rain forests (MacKinnon et al. 1996). Upper montane forests share many common species and features of structure and appearance with heath forests (Richards 1936).

Pitcher plants, rhododendron, and orchids are especially diverse in Borneo's montane habitats. More than one-half of Borneo's thirty pitcher plant (Nepenthes) species are found in this ecoregion's montane habitats. Rhododendrons are characteristic of upper montane flora, and more than twenty Vireya species are found in this ecoregion. Rhododendrons are found on acidic and peat soils and have adapted to the harsh upper montane environments (MacKinnon et al. 1996). Orchids are found at all levels of the forest but are common epiphytes in the upper montane forests. For growth, orchids need light and moisture as well as a mycorrhiza relationship with a tree or other plant to derive their nutrients. The well-lit and moist conditions of the moss forest in the upper montane zone provide ideal growing conditions for many orchid species (Lamb and Chan 1978).

Although in southeast Asia most limestone occurs in the lowlands, Borneo has important limestone forests in the montane zone. Gunung Api is highly diverse botanically, with fourteen of Borneo's fifteen Monophyllea species, disrupted altitudinal zonation compared with forest over other substrates, and montane birds occurring at atypically low altitudes. The few high-altitude swamp forests (which help regulate water supply to downstream areas) are found in the Ulu Meligan/Ulu Long Pasia montane area and the Usun Apau Plateau (WWF and IUCN 1995).

Biodiversity Features
The noticeable difference in vegetation structure and species composition also affect faunal communities found in montane forests. Animals in montane regions must face adverse climate, lack of shelter, and food shortages. For example, on Gunung Mulu in Sarawak, of the 171 bird species found on its lowland slopes, the range of most species does not exceed 900 m. By the boundary of the upper montane region at 1,300 m, only twelve species are still found. Similarly, mammal richness tends to decrease with altitude. Most primate species prefer lowland habitats, and orangutans (Pongo pygmaeus), gibbons, and langurs all show significant decrease in density between 500 m and 1,500 m in altitude. The macaque species density shows no large change between lowland and montane regions, perhaps being attributed to a greater dietary versatility (Caldecott 1980). Smaller mammals such as civets, tree shrews, squirrels, and rats dominate the montane region. Indeed, all near-endemic species fall into one of these categories (table 1). The vast tracts of montane forest still remain undisturbed and therefore still support some of the larger megafauna such as orangutan and Sumatran rhinoceros (Dicerorhinus sumatrensis).

Table 1. Endemic and Near-Endemic Mammal Species.

  Family Species
Viverridae Diplogale hosei
Sciuridae Callosciurus baluensis
Sciuridae Callosciurus orestes
Sciuridae Sundasciurus brookei

An asterisk signifies that the species' range is limited to this ecoregion.

More than 250 bird species are attributed to this ecoregion. This ecoregion overlaps with a large portion of the Bornean Mountains EBA (157) (Stattersfield et al. 1998). Many of the mountains of Borneo are ornithologically unexplored and poorly known, so the habitat needs and distributions of many species are incomplete (Stattersfield et al. 1998). Twenty-one near-endemic and two endemic bird species are found in these forests (table 2).

Table 2. Endemic and Near-Endemic Bird Species.

  Family Common Name Species
Accipitridae Mountain serpent-eagle Spilornis kinabaluensis
Phasianidae Red-breasted partridge Arborophila hyperythra
Phasianidae Crimson-headed partridge Haematortyx sanguiniceps
Podargidae Dulit frogmouth Batrachostomus harterti
Trogonidae Whitehead's trogon Harpactes whiteheadi
Capitonidae Mountain barbet Megalaima monticola
Capitonidae Golden-naped barbet Megalaima pulcherrima
Eurylaimidae Hose's broadbill* Calyptomena hosii*
Eurylaimidae Whitehead's broadbill Calyptomena whiteheadi
Pachycephalidae Bornean whistler Pachycephala hypoxantha
Oriolidae Black oriole* Oriolus hosii*
Turdidae Everett's thrush Zoothera everetti
Turdidae Fruit-hunter Chlamydochaera jefferyi
Muscicapidae Eyebrowed jungle-flycatcher Rhinomyias gularis
Zosteropidae Pygmy white-eye Oculocincta squamifrons
Zosteropidae Mountain black-eye Chlorocharis emiliae
Sylviidae Bornean stubtail Urosphena whiteheadi
Sylviidae Friendly bush-warbler Bradypterus accentor
Timaliidae Bare-headed laughingthrush Garrulax calvus
Timaliidae Mountain wren-babbler Napothera crassa
Timaliidae Chestnut-crested yuhina Yuhina everetti
Dicaeidae Black-sided flowerpecker Dicaeum monticolum
Nectariniidae Whitehead's spiderhunter Arachnothera juliae

An asterisk signifies that the species' range is limited to this ecoregion.

Current Status
The ecoregion is largely intact, and only about 8 percent of the area has been cleared or converted. There are seventeen protected areas that cover 26,380 km2 (about 25 percent) of the ecoregion (table 3). Several very large (more than 5,000 km2) reserves account for most of this protected area system. The ecoregion includes the largest protected block of rain forest in Borneo, the Kayan Mentarang National Park, which covers 140,000 km2 of lowland dipterocarp forest as well as mountain forests; this park and Gunung Bentuang both exceed 5,000 km2 (WWF-Indonesia n.d.). The Kayan Mentarang National Park and surrounding area is inhabited by several thousand indigenous people who depend on forest resources (WWF-Indonesia n.d.). However, commercial logging activities, road building, and intensive extraction of commercially valuable nontimber forest products now threaten the natural integrity of the reserve and the livelihoods of local people (WWF-Indonesia n.d.). The montane forests of Borneo have largely escaped the fires of 1997-1998, which were intentionally set throughout most of the lowland forests in Borneo. With the rapid pace of habitat loss in the lowland forests, Borneo's montane forests may serve as a final refuge for many of Borneo's species.

Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion.

  Protected Area Area (km2) IUCN Category
Bukit Tawau 290 II
Maliau Basing 450 VIII
Danum Valley [IM0123] 90 VIII
S. Kayan S. Mentarang [IM0143], [IM0123] 5,290 ?
Apo Kayan [IM0143] 480 PRO
Long Bangun [IM0143] 1,840 PRO
Batu Kristal 40 PRO
SAR (Sanctuary Reserve) 670 PRO
SAR (Sanctuary Reserve) 530 II
SAR (Sanctuary Reserve) 190 PRO
SAR (Sanctuary Reserve) [IM0114] 350 PRO
SAR (Sanctuary Reserve) [IM0114] 920 PRO
Gunung Bentuang [IM0114] 5,660 II
Bukit Batutenobang 4,650 PRO
Bukit Batikap I, II, III [IM0102] 3,100 PRO
Bukit Baka-Bukit Raya [IM0102] 1,060 II
Gunung Penrisen/Gunung Niut [IM0102] 770 IV
Total 26,380  

Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.

Types and Severity of Threats
Although little of the ecoregion has been cleared or degraded, significant threats from planned mining operations, large dams, and conversion to agriculture and high-altitude timber plantations are now increasing (WWF and IUCN 1995). Illegal collection of species for the commercial trade and shifting cultivation are also increasing, threatening the integrity of Borneo's highly distinctive montane biodiversity.

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 mammal species 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).

References
References for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.
Indo-Pacific Reference List

Prepared by: Colby Loucks
Reviewed by:

 

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