Southeastern Asia: Mentawai Islands and Enggano Island in Indonesia

The Mentawai Islands Rain Forests [IM0127] have had a long geographic isolation that has resulted in numerous endemic mammal species, including four primates. There are seventeen endemic mammal species (39 percent), which on a per-unit area ranks it with Madagascar in endemic mammal species, notably primates. Of the four endemic primate species, these forests have the world's only exclusively monogamous leaf-monkey, the Mentawai leaf-monkey.

  • Scientific Code
  • Ecoregion Category
  • Size
    2,500 square miles
  • Status
  • Habitats

Location and General Description
This ecoregion covers the moist forests of Mentawai Islands and Enganno Island, off the west coast of central Sumatra, Indonesia. Approximately 70 million years ago, the Indian subcontinent collided with the Asian landmass, forming the Himalayas. An associated thrust formed Sumatra's Barisan Mountains, and 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. The Mentawai Islands separated from the Sumatran mainland via the Batu Islands more than half a million years ago. The rainfall on these islands is approximately 4,500 mm/year (Whitten et al. 2000). Based on the Köppen climate zone system, this ecoregion falls in the tropical wet climate zone (National Geographic Society 1999).

The natural vegetation of these islands is tropical lowland rain forest and is similar to but not as diverse as Sumatra's lowland rain forests. These forests are characterized by large buttressed trees, dominated by the Dipterocarpaceae family. There is an abundant presence of woody climbers and epiphytes. The general characteristics of these forests are canopies 24 to 36 m high, with emergents reaching more than 45 m. The emergent trees are dominated by dipterocarp species (Dipterocarpus spp., Parashorea spp., Shorea spp., and Dryobalanops spp.) and, to a lesser extent, species in the Caesalpiniaceae family (Koompasia spp., Sindora spp., and Dialium spp.). Dipterocarps also dominate much of the canopy layer, but there are many other tree families such as Burseraceae, Sapotaceae, Euphorbiacae, Rubiaceae, Annonaceae, Lauraceae, and Myristicaceae (Whitten et al. 2000). Ground vegetation is limited to small-diameter trees, and an herbaceous layer is uncommon.

Biodiversity Features
The Mentawai Islands have been separated from the mainland for more than half a million years, and the long isolation has allowed the survival of relicts of an early Indo-Malayan fauna as well as the evolution of many endemics (Whitten et al. 2000). Although this small ecoregion has a depauperate mammal fauna, consisting of just forty-four species, seventeen species are endemic and one is a near endemic (table 1).

Table 1. Endemic and Near-Endemic Mammal Species.

  Family Species
Rhinolophidae Hipposideros breviceps*
Cercopithecidae Presbytis potenziani*
Cercopithecidae Nasalis concolor*
Cercopithecidae Macaca pagensis*
Hylobatidae Hylobates klossi*
Viverridae Paradoxurus lignicolor*
Sciuridae Callosciurus melanogaster*
Sciuridae Lariscus obscurus*
Muridae Iomys sipora*
Sciuridae Hylopetes sipora*
Muridae Petinomys lugens*
Muridae Rattus lugens*
Muridae Rattus adustus*
Muridae Rattus enganus*
Muridae Chiropodomys karlkoopmani*
Muridae Leopoldamys siporanus*
Muridae Maxomys pagensis*
Muridae Rattus hoogerwerfi

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

Four endemic primates are found on the Mentawai Islands: the Mentawai gibbon (Hylobates klossii), Mentawai macaque (Macaca pagensis), Mentawai leaf-monkey (Presbytis potenziani), and snub-nosed monkey (Simias concolor). Almost all monkeys live in single- or multi-adult male groups with more females than adult males. There is only one exception: the Mentawai leaf-monkey. Like gibbons, it lives in permanent monogamous groups (one male and one female) within home ranges. The snub-nosed monkey is in an endemic primate genus but is related to Borneo's proboscis monkey (Nasalis larvatus). The primary difference is that the snub-nosed monkey has a shorter tail (Whitten et al. 2000). The Mentawai gibbon has one of the most splendid songs of any land mammal.

Numerous other mammals found on the Mentawai islands are also endemic, and these include the Mentawai palm civet (Paradoxurus lignicolor), Mentawai squirrel (Callosciurus melanogaster), Mentawai three-striped squirrel (Lariscus obscurus), and Mentawai flying squirrel (Hylopetes sipora) (Whitten et al. 2000).

Enggano Island probably was never connected to Sumatra and has extremely impoverished mammal faunas. However, Enggano is an EBA (159) containing two restricted-range bird species (Stattersfield et al. 1998). There are three endemic species (table 2).

Table 2. Endemic and Near-Endemic Bird Species.

  Family Common Name Species
Cuculidae Enggano scops-owl* Otus enganensis*
Strigidae Mentawai scops-owl* Otus mentawi*
Zosteropidae Enggano white-eye* Zosterops salvadorii*

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

Current Status
Although the forest cover data indicate that over 60 percent of the habitat is intact, in recent years larger-scale forestry operations have taken hold. There are three protected areas that cover 1,090 km2 (18 percent) of the ecoregion (table 3). Two of the three protected areas in the ecoregion are small, but one is almost 1,000 km2, although it is unknown how well protected these reserves are in light of the recent logging operations.

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

  Protected Area Area (km2) IUCN Category
Tai-tai Batti 930 IV
Muara Siberut 80 PRO
Gunung Nanu'ua 80 VI
Total 1,090  

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

Types and Severity of Threats
The primary forests of the Mentawai Islands remained essentially intact until the influx of settlers from mainland Sumatra created population pressure and disrupted traditional management practices (WWF and IUCN 1995). Extensive areas of land have been cleared for cash crops, and logging has become a serious threat (WWF and IUCN 1995), especially to the islands' four endemic primates (IUCN 1991).

Justification of Ecoregion Delineation
We placed the Mentawai Islands and Enggano Island-MacKinnon's subunit 21c-into a distinct ecoregion, the Mentawai Islands Rain Forests [IM0127].

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
Reviewed by: