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Sulu Archipelago rain forests

Although these islands represent transitional stepping stones from the island of Borneo to Mindanao in the Philippines, they have evolved their own distinctive faunas. Almost no forest remains on Sulu, and only the eastern portion of Tawitawi is forested. The islands are extremely politically unstable, which exacerbates a difficult conservation situation.

  • Scientific Code
    (IM0156)
  • Ecoregion Category
    Indo-Malayan
  • Size
    900 square miles
  • Status
    Critical/Endangered
  • Habitats

Description
Location and General Description
This ecoregion includes the main islands of Jolo (Sulu) and Tawitawi and the surrounding smaller islands from Sibutu up to but not including Basilan Island. The climate of the ecoregion is tropical wet (National Geographic Society 1999). There are apparently short (two-week) dry seasons in January and May on Tawitawi (Allen 1998). The Sulus are located south of the main typhoon track that so strongly influences the more northerly Philippine islands (Dickinson et al. 1991).

The Philippines are essentially accreted terrains, an accretion of previously isolated island archipelagos that were brought together during the collision and partial subduction of large oceanic tectonic plates.

The precursors of the Sulu Islands were an arc of submarine volcanoes that have existed for at least 25 million years. However, the Sulus were not clearly above-water islands until within the last 15 million years (Hall and Holloway 1998). The islands are low-lying and coralline (limestone). Bongao Peak, on Bongao, reaches 300 m, and Mt. Sibangkok, the highest point on the central ridge that divides Tawitawi, reaches 532 m (Allen 1998).

During the Pleistocene, the majority of the present Sulu Archipelago was one island, separated from Basilan-Mindanao to the north and greater Sibutu (and Borneo) to the south by deepwater channels of 205 m and 290 m depths, respectively. The distances between these ice age islands were not great, however (Heaney 1986).

Vegetation types in the Sulu Archipelago originally included beach forest, lowland rain forest, scrub forest, and mangroves (Stattersfield et al. 1998). Beach forest is composed of Barringtonia, Caesalpinia, and Terminalia. A small patch of this forest type may be found on Simunul, but this is generally an endangered habitat because of coastal development (human habitation and cultivation, coconut plantations). Formerly the most prevalent forest type on the islands (as with the rest of the Philippines), lowland rain forest, or dipterocarp forest, is now mostly cleared. Represented dipterocarp genera include Anisoptera, Dipterocarpus, Hopea, and Shorea. Little information is available about the native scrub forest, which has been extensively cleared as well. Mangroves are found on the coasts throughout the Archipelago but are especially extensive on Tawitawi; mangroves around Bongao have been cleared. The principal mangrove genera include Rhizophora, Ceriops, Brugueira, Sonneratia, Avicennia, and Nypa (palms) (Allen 1998).

Biodiversity Features
Unlike that of Palawan, which is located between Borneo and the Philippines, the Sulu Archipelago's fauna is not Sundaic (Allen 1998) and, though rather small, is poorly known biologically (L. Heaney, pers. comm., 2000). Palawan was the main pathway for immigrants from Borneo to the Philippines, and the Sulus have many taxa that are identical to or derived from taxa in Mindanao. Even Sibutu, close to Borneo and separated from the rest of the Sulus by the Sibutu Passage, contains an avifauna more closely related to the Sulus than to Borneo (Dickinson et al. 1991). Although there are some Sulu birds with Sundaic distributions, the avifauna of the Archipelago is essentially Philippine (Dutson et al. 1992). The Sulu hornbill (Anthracoceros montani) is one example of an animal whose likely closest relative, the black hornbill (Anthracoceros malayanus), is from Borneo. There is a cline of relatedness to Borneo as one moves north among the islands. Sibutu contains birds of Bornean origin that are not found on Tawitawi (Allen 1998). The Sulus (Sangasanga, Bongao, Simunul, Tawitawi) also support a population of slow loris (Nycticebus coucang), a Sundaic primate that is not found in the remainder of the Philippines (Heaney 1986). There is one endemic mammal in the ecoregion (table 1). The Tawitawi Island rat (Rattus taitawiensis) is considered vulnerable (IUCN 2000).

Table 1. Endemic and Near-Endemic Mammal Species.

  Family Species
Muridae Rattus tawitawiensis*

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

A new pig species, Sus spp. nov., is being described from the Sulus on the basis of MtDNA and skull measurements from a dead specimen (Rose and Grubb, in prep.). The same subspecies of bearded pig found on Borneo (Sus barbatus barbatus) is also found in the southwestern Sulus (Sibutu and Tawitawi), and this species can still be observed crossing open water to reach these islands from Borneo. It is unknown whether these over-water migrations are related to periodic eruptions on the Bornean mainland (Oliver 1993).

This ecoregion overlaps exactly with the Sulu Archipelago EBA. The EBA contains nine restricted-range birds, four of which are limited to the Sulus. All the restricted-range birds are forest species. Ten bird species qualify as endemic or near endemic to this ecoregion (Kennedy et al. 2000; table 2). Included in the ecoregion are the critically endangered Sulu bleeding-heart (Gallicolumba menagei), Tawitawi brown-dove (Phapitreron cinereiceps), and Sulu hornbill (Anthracoceros montani) and the endangered blue-winged racquet-tail (Prionoturus verticalis). Several endemic bird subspecies may warrant elevation to species status upon detailed review (Stattersfield et al. 1998; Collar et al. 1999).

Table 2. Endemic and Near-Endemic Bird Species.

  Family Common Name Species
Columbidae Sulu bleeding-heart Gallicolumba menagei*
Columbidae Dark-eared dove Phapitreron cinereiceps*
Columbidae Grey imperial-pigeon Ducula pickeringii
Psittacidae Blue-winged racquet-tail Prioniturus verticalis*
Strigidae Mantanani scops-owl Otus mantananensis
Apodidae Philippine needletail Mearnsia picina
Bucconidae Sulu hornbill Anthracoceros montani*
Monarchidae Celestial monarch Hypothymis coelestis
Pycnonotidae Yellowish bulbul Ixos everetti
Timaliidae Brown tit-babbler Macronous striaticeps

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

Several widespread but threatened species also occur on the islands, including the critically endangered Philippine cockatoo (Cacatua haematuropygia) and vulnerable rufous-lored kingfisher (Todirhamphus winchelli) (Collar et al. 1999; Stattersfield et al. 1998).

The critically endangered Philippine crocodile (Crocodylus mindorensis) was historically found on Jolo (as well as Luzon, Mindoro, Masbate, Samar, Negros, Busuanga, and Mindanao), but the only remaining populations are found on Mindoro, Negros, Mindanao, and Busuanga. The current wild population may be approximately 100 nonhatchlings (Ross 1998).

Current Status
There is almost no forest remaining on Jolo (Sulu) Island, and only the eastern and north-central portions of Tawitawi are forested (Stattersfield et al. 1998; Allen 1998). The majority of Tawitawi was selectively logged in the 1960s and early 1970s (Allen 1998). Apparently, there were plans to replace the remaining forests of Tawitawi with oil palm plantations. The situation on the smaller islands is mixed. Sibutu and Simunul have been largely cleared (Stattersfield et al. 1998). Simunul has some patches of forest remaining that support populations of Philippine cockatoo (Cacatua haematuropygia), blue-naped parrot (Tanygnathus lucionensis), and blue-backed parrot (T. sumatranus) (Dutson et al. 1992). Sibutu has considerable secondary forest, and the island supports numerous Sulu subspecies. The last forests of Sangasanga were cleared in 1992-1993. The island of Bongao still supports forests; an unidentified jungle flycatcher collected in 1973 has not been observed since (Dutson et al. 1992). Small islands in the Tandubas group (which also includes Tawitawi and Sangasanga) still have small forest tracts that reportedly maintain populations of the endemic Sulu bleeding-heart and Sulu hornbill (Stattersfield et al. 1998).

The main population center is on Bongao, where a busy port exists. Tawitawi is not heavily populated, but future economic development on the island is a concern (Allen 1998; Stattersfield et al. 1998). Table 3 details the existing protected area on the islands.

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

  Protected Area Area (km2) IUCN Category
Mt. Dajo 40 IV

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

Types and Severity of Threats
In general, habitat loss is the main threat to wildlife, but hunting is also a problem. Small-scale logging continues to destroy the remaining habitat (Stattersfield et al. 1998).

Justification of Ecoregion Delineation
MacKinnon (1997) identified seven subunits in the Philippines, and the Philippine BAP (Philippine DENR & UNEP 1997) demarcated fifteen biogeographic units. Udvardy (1975) identified the Philippines as a single biogeographic province. We delineated nine ecoregions in the Philippine islands, including Palawan. We deviated from Udvardy (1975), MacKinnon (1997), Stattersfield et al. (1998), and the Philippine BAP (Philippine DENR & UNEP 1997) to varying degrees and based our delineation of the Philippine ecoregions on Heaney (1993).

The islands of the Sulu Archipelago were delineated as a separate ecoregion, the Sulu Archipelago Rain Forests [IM0156]. This ecoregion includes the Tawitawi Group, Tapul Group, Jolo Group, and Samales Group of islands. These islands, with a lowland moist or semi-evergreen moist forest vegetation (Whitmore 1984), are also an EBA (Stattersfield et al. 1998) and have been identified as a distinct biounit by MacKinnon (1997) and a biogeographic zone by the Philippine BAP (Philippine DENR & UNEP 1997)

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

Prepared by: John Morrison
Reviewed by:

 

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