Ecoregions

Palawan rainforests

Introduction

Palawan represents a bridge between the Sunda Shelf and Philippine bioregions and contains faunal elements from both, as well as it own unique elements. This ecoregion, though more intact than any other region in the Philippines is under great pressure from logging interests.

Location and General Description  

caption Coron Island, Philippines. (Photograph by Sterling Zumbrunn/Conservation International)

This ecoregion includes the island Palawan plus Balabac, Ursula Island, and the Calamian Group. Palawan itself is the sixth largest of the Philippine Islands. The climate of the ecoregion is tropical wet. In northwest Palawan, a dry season lasts from November to May while the wet season lasts from June to October; the rest of the island experiences a short, one- to three-month dry season. The east coast becomes progressively drier than the west coast from north to south.

Palawan (along with the Calamianes and the island of Mindoro) was rifted (below water) from the Asian mainland approximately 32 million years ago, transported through seafloor spreading across the growing South China Sea, added to the growing Philippine Archipelago approximately 17 million years ago, and uplifted above water approximately 5-10 million years ago. Metamorphic rocks are found in the northern portion of the island north of Mt. St. Paul. Volcanic rocks are found in the vicinity of Cleopatra's Needle, just south of Mt. St. Paul. Mt. St. Paul itself and the El Nido Cliffs are karst landscapes. The southern third of the island, south of the Quezon-Aboabo Gap, is dominated by ultramafics mixed with volcanic rocks and Tertiary limestone. Tertiary sandstones and shales occur along the southwest coast.

The channel between Palawan and Borneo is about 145 meters (m) deep. During the middle Pleistocene, sea levels were 160 m lower than today, and the islands were connected. During the last ice age (late Pleistocene), sea level was approximately 120 m below current levels, and Palawan was separated from ice age Borneo by a narrow channel. Palawan has always remained separated from the rest of the Philippines. Palawan is long and narrow, consisting of a steep mountain range whose highest point is 2,085 m (Mt. Mantalingajan). More than 45 percent of Palawan consists of mountains with slopes greater than 30 percent.

Vegetation types on Palawan are diverse and include beach forest, tropical lowland evergreen dipterocarp rainforest, lowland semi-deciduous forest, montane forest, and ultramafic and imestone forest. Beach forest merges with other forest types away from the coast and includes Calophyllum inophyllum, Canarium asperum var. asperum, Pometia pinnata, Palaquium dubardii, and Ficus spp..

The lowland evergreen dipterocarp rainforest, which naturally occupies 31 percent of the island, is dominated by Agalai spp., Dipterocarpus gracilis, D. grandiflorus, Ficus spp., Tristania spp., Exocarpus latifolius, and Swintonia foxworthyi. Sygium spp., Dracontomelon dao, and Pongamia pinnata are emergent. Lianas and cycads are common. In southern Palawan, a Casuarina sp. dominates in the lowland forests.

The eastern half of the island is in a rain shadow and contains moist semi-deciduous forests. Soils are thin on the steeper slopes and support medium-sized trees (up to 15 metres tall), which shed their leaves during the March-May dry season. The rainy season is June-July. Common tree species include Pterocymbium tinctorium, Pterospermum diversifolium, Hymenodictyon spp., and Garuga floribunda.

Montane forests, found between 800 and 1,500 m, are dominated by Tristania spp., Casuarina spp., Swietenia foxworthyi, and Litsea spp. in the lower elevations. Upper montane forest trees include Agathis philippinensis, Dacrydium pectinatum, Podocarpus polystachyus, Gnetum latifolium, Cycas wadei, Cinnamomum rupestre, Nepenthes philippinensis, and Angiopteris spp.

Limestone forests are found on the islets surrounding Palawan and over large areas in the southern portions of the island. Represented are Euphorbia trigona, Aglaia argentea, and Antidesma, Drypetes, Gomphandra, Sterculia, Pleomele, and Begonia spp.

Victoria Peak, in south-central Palawan, contains the largest region of ultramafic forest on the island. Although many of the ultramafic tree species are shared with semi-deciduous forest, several species, including Scaevola micrantha, Brackenridgea palustris var. foxworthi, Exocarpus latifolius, and Phyllanthus lamprophyllus are believed to be heavy metal indicators.

Biodiversity Features

Relative to the size of Palawan, the ecoregion contains a rich fauna, including several groups that are not found in the rest of the Philippines (carnivores, pangolins, porcupines, and some insectivores).

There are many endemic mammals in Palawan, but nearly all the genera (96 percent) are also found in Borneo. Of twenty-five indigenous nonvolant mammal species, eleven (44 percent) are endemic to Palawan, and the remainder are shared with Borneo. Therefore, the greater Palawan region is rightly considered part of the Sunda Shelf bioregion rather than that of the Philippines. The large number of endemic species but few endemic genera of Palawan are consistent with a separation of Borneo and Palawan of approximately 160,000 (since the middle Pleistocene). There are fifteen endemic or near-endemic mammals in greater Palawan (Table 1).

Table 1. Endemic and Near-Endemic Mammal Species.
Family Species
Pteropodidae Acerodon leucotis*
Cervidae Axis calamianensis*
Sciuridae Sundasciurus steerii*
Sciuridae Sundasciurus moellendorfi*
Sciuridae Sundasciurus rabori*
Sciuridae Hylopetes nigripes*
Muridae Chiropodomys calamianensis*
Muridae Maxomys panglima*
Muridae Palawanomys furvus*
Hystricidae Hystrix pumila*
Sorcidae Crocidura palawanensis*
Muridae Haeromys sp. A*
Sciuridae Sundasciurus hoogstraali*
Sciuridae Sundasciurus juvencus*
Tupaiidae Tupaia palawanensis*
An asterisk signifies that the species' range is limited to this ecoregion.

The Calamian deer (Axis calamianensis) is found only in the Calamian Islands, where it survives in low densities on Busuanga, Calauit, and Culion Islands. The only protected area for this species was established to protect free-ranging African ungulates on Calauit Island.

Balabac, Palawan, and the Calamian Islands also provide habitat for an endemic subspecies of the bearded pig (Sus barbatous ahoenobarbus), another subspecies of which is widely distributed in the Greater Sundas. The World Conservation Union (IUCN) considers this species to be rare and declining. This species naturally inhabits tropical evergreen rain forest but is able to use a wide variety of habitats within forests. They are quite dependent on fruit supplies but consume a wide variety of foods. Directional large-scale population movements in scattered or condensed herds lasting days, weeks, or even months are reported for other subspecies in Borneo and Sumatra; this is generally associated with the mast fruiting of dipterocarps. Such movements have not been reported from the Philippines.

caption Palawan Island, Philippines. (Photograph by Conservation International)

Several of Palawan's endemic mammals are considered threatened. Three endemic mammal species are considered endangered, including the Calamian deer, a Sunda tree squirrel (Sundasciurus juvencus) (recommended for delisting), and the Palawan rat (Palawanomys furvus), which was collected only four times in 1962. A subspecies of mouse deer, the Balabac chevrotain (Tragulus napu nigricans), which is confined to Balabac Island, is also considered endangered. Five endemic mammal species are considered vulnerable, including Acerodon leucotis, the Palawan treeshrew (Tupaia palawanensis), the Palawan stink badger (Mydaus marchei), the Palawan binturong (Arctictis binturong whitei), and a Sunda tree squirrel (Sundasciurus rabori).

As with mammals, Philippine birds in general show a strong Bornean affinity, and it is clear that the main pathway of Asian immigration to the Philippines was through Palawan; of 395 Philippine breeding species, 137 (35 percent) also breed in Borneo. Palawan birds exhibit strong differentiation at the subspecific level when compared with its nearest Philippine neighbor, Mindoro. This is in contrast to the other partial land bridge between Borneo and the Philippines, the Sulu Islands, which have not differentiated significantly from Mindanao. Borneo and Palawan share twenty-three bird species that are not found in the rest of the Philippines. The Asian genera Polyplectron, Malacocincla, Malacopteron, Dinopium, Aegithina, Criniger, Seicercus, and Gracula are found only in Palawan within the Philippines. The island forms an important bird migration route between Borneo and the rest of the Philippines for southern migrants.

This ecoregion corresponds exactly with the Palawan EBA. The EBA contains twenty restricted-range birds, seventeen of which are found nowhere else on Earth and five of which (Palawan peacock-pheasant [Polyplectron emphanum], grey imperial-pigeon [Ducula pickeringii], blue-headed racquet-tail [Prioniturus platenae], falcated wren-babbler [Ptilocichla falcata], and Palawan flycatcher [Ficedula platenae]) are considered vulnerable. All these vulnerable birds are dependent on lowland and hill forest. There are twenty endemic or near-endemic bird species in the Palawan ecoregion (Table 2).

Table 2. Endemic and Near-Endemic Bird Species.
Family Common Name Species
Phasianidae Palawan peacock-pheasant Polyplectron emphanum*
Columbidae Grey imperial-pigeon Ducula pickeringii
Psittacidae Blue-headed racquet-tail Prioniturus platenae*
Strigidae Mantanani scops-owl Otus mantananensis
Strigidae Palawan scops-owl Otus fuliginosus*
Apodidae Palawan swiftlet Aerodramus palawanensis*
Bucconidae Palawan hornbill Anthracoceros marchei*
Monarchidae Blue paradise-flycatcher Terpsiphone cyanescens*
Irenidae Yellow-throated leafbird Chloropsis palawanensis*
Muscicapidae Palawan flycatcher Ficedula platenae*
Muscicapidae Palawan blue-flycatcher Cyornis lemprieri*
Muscicapidae White-vented shama Copsychus niger*
Pycnonotidae Sulphur-bellied bulbul Ixos palawanensis*
Timaliidae Ashy-headed babbler Malacocincla cinereiceps*
Timaliidae Palawan babbler Malacopteron palawanense*
Timaliidae Falcated wren-babbler Ptilocichla falcata*
Timaliidae Palawan striped-babbler Stachyris hypogrammica*
Paridae Palawan tit Parus amabilis*
Paridae White-fronted tit Parus semilarvatus
Dicaeidae Palawan flowerpecker Prionochilus plateni*
An asterisk signifies that the species' range is limited to this ecoregion.

The critically endangered Philippine crocodile (Crocodylus mindorensis) was historically found on the islands of Luzon, Mindoro, Masbate, Samar, Jolo, Negros, Busuanga, and Mindanao. Busuanga contains one of the only remaining populations (others are found on Mindoro, Negros, and Mindanao). Whereas the decline of the species was initially driven by overexploitation, habitat loss and human persecution are now the principal threats to the Philippine crocodile. Surveys in 1980-1982 revealed a total wild population of approximately 500 to 1000 individuals, but current wild populations may be approximately 100 nonhatchlings. Captive breeding efforts are being led by the Crocodile Farming Institute, an entity of the Philippine government.

A total of 1522 to 1672 vascular plants have been identified on Palawan, and it is estimated that more than 2000 species are present on the island. As detailed earlier, Palawan has an extremely diverse range of vegetation types for the Philippines. A small number of dipterocarps, an important timber tree group, are present on the island, as well as a variety of medicinal plants used by ethnic tribes and plants used in ceremony and as ornamentals.

Current Status

Almost all of the Philippines was once completely forested. As of 1988, Palawan contained 7,410 square kilometers (km2 )(54 percent) of total forest remaining. At the time this was the highest percentage of any of the Philippines' large islands.

Later aerial surveys indicated that significant reductions in closed-canopy forest cover had occurred since 1988 as a result of recent logging. As seen from the air, the lowlands and hillsides consist of slash-and-burn agriculture up to the edges of natural forest in the highlands. Closed-canopy forest caps only the highest areas on the island.

Palawan's forests are of low commercial value because of the small number of dipterocarps, and until the last twenty years Palawan's forests were ignored in favor of the more valuable forests of Luzon and Mindanao. Government logging regulations setting guidelines for minimum diameter, minimum rotation length, and replanting have been largely ignored.

Because of a generally high population density in other parts of the Philippines, large numbers of shifting cultivators (kaingineros) are attracted to Palawan to eke out a living on the hillsides of the island, and their cumulative impact is enormous.

All of Palawan was declared a Fauna and Flora Watershed Reserve, and this includes a variety of protected areas, including national parks, wilderness areas, experimental forests, forest research reserves, game refuges, wildlife sanctuaries, museum reservations and research sites, tourist zones, and marine reserves.

Recent reports in the international press indicate (and have been confirmed, L. Heaney, pers. comm., 2000) that the situation in Palawan has stabilized, that large-scale logging has been halted, and that a balance is being achieved between economic development and conservation; future monitoring will determine whether this is remains true.

Types and Severity of Threats

Habitatdestruction is the main threat to biodiversity in the Philippines, and Palawan, though currently in better condition, is no different. Logging and shifting cultivation (kaingin) are cited as the primary forces of habitat conversion. Logging takes many forms, from industrial scale to smaller-scale operations that use water buffalo to haul logs out of the forest. Mangroves are used locally for firewood, dyes, and tannins, and they are sometimes removed to make way for fishponds.

Hunting and the wild pet trade are also significant threats in Palawan. Leopard cats have been hunted for their pelts and are sold when kittens as pets. The Palawan binturong is hunted for meat and as pets, and the pangolin is hunted for its hide. The Palawan peacock-pheasant, blue-headed racquet-tail, Philippine cockatoos (Cacatua haematuropygia), and blue-naped parrots (Tanygnathus lucionensis) apparently are suffering greatly from the pet trade. The final destination for these birds often is the United States.

Ornamental plant collecting, especially for the orchids (Phalaenopsis amabilis and Paphiopedilum argus), pitcher plants (Nepenthes spp.), palms (Veitchia merrillii), and aroids (Amorphophallus spp. and Alocasia spp.) threatens some plant populations.

A valuable resin, known as Manila copal, is collected from Agathis dammara trees. This collection weakens the trees, and slackening production and disease combined with overexploitation are threatening the species.

Currently, Palawan's mineral wealth (chromite, copper, iron, manganese, mercury, and nickel) has not been extensively exploited, but the possible future extraction of these minerals represents a potential threat.

Justification of Ecoregion Delineation

MacKinnon identified seven subunits in the Philippines, and the Philippine Biodiversity Action Plan 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.

We placed Palawan, Calamian Islands, and Cuyo Islands into a single ecoregion, the Palawan Rain Forests. Palawan has closer zoogeographic affinities to Borneo.

Additional information on this ecoregion

  • For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
  • To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion. 

 

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. (2010). Palawan rainforests. Retrieved from http://www.eoearth.org/view/article/155119

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