Ecoregions

Sulu Archipelago rainforests

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Tawitawi Island, Sulu archipelago, Philippines (Photograph by Arvin Diesinos)

Although these islands represent transitional stepping stones from the island of Borneo to Mindanao in the Philippines, they have evolved their own distinctive faunas. The Sulu Archipelago rainforests are a principal historical ecoregion of these islands, although most of their habitat has been destroyed. The islands themselves are the dividing feature between the Sulu Sea and the Celebes Sea.

Almost no forest remains on Sulu, and only the eastern portion of Tawitawi is forested; this outcome has resulted from slash-and-burn practises of indigenous peoples. Furthermore, the islands are extremely politically unstable, which exacerbates a difficult conservation situation.

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. There are apparently short (two-week) dry seasons in January and May on Tawitawi. The Sulus are located south of the main typhoon track that so strongly influences the more northerly Philippine islands.

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. The islands are low-lying and coralline (limestone). Bongao Peak, on Bongao, reaches 300 meters (m), and Mt. Sibangkok, the highest point on the central ridge that divides Tawitawi, reaches 532 m.

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.

Vegetation types in the Sulu Archipelago originally included beach forest, lowland rain forest, scrub forest, and mangroves. 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 rainforest, or dipterocarp forest, is now mostly cleared by indigenous peoples. Representive 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).

Biodiversity Features

Unlike that of Palawan, which is located between Borneo and the Philippines, the Sulu Archipelago's fauna is not Sundaic and, though rather small, is poorly known biologically. 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. Although there are some Sulu birds with Sundaic distributions, the avifauna of the Archipelago is essentially Philippine. 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. There is one endemic mammal in the ecoregion (Table 1). The Tawitawi Island rat (Rattus taitawiensis) is considered vulnerable.

 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. 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.

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 ( 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.

 

 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).

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.

Current Status

There is almost no forest remaining on Jolo (Sulu) Island, and only the eastern and north-central portions of Tawitawi are forested. The majority of Tawitawi was selectively logged in the 1960s and early 1970s. 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. 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). 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. 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.

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. 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

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.

Justification of Ecoregion Delineation

MacKinnon identified seven subunits in the Philippines, and the Philippine BAP demarcated fifteen biogeographic units. Udvardy identified the Philippines as a single biogeographic province. We delineated nine ecoregions in the Philippine islands, including Palawan. We deviated from Udvardy, MacKinnon, Stattersfield et al., and the Philippine BAP to varying degrees and based our delineation of the Philippine ecoregions on Heaney.

The islands of the Sulu Archipelago were delineated as a separate ecoregion, the Sulu Archipelago Rain Forests. 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, are also an EBA and have been identified as a distinct biounit by MacKinnon and a biogeographic zone by the Philippine BAP.

 

Disclaimer: This article contains information that was originally published by the World Wildlife Fund. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the World Wildlife Fund should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

 

 

Glossary

Citation

Fund, W. (2014). Sulu Archipelago rainforests. Retrieved from http://www.eoearth.org/view/article/156320

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