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Terrestrial Ecosystems of Palau

A forested Rock Island
A forested Rock Island (Photo: Aiken/Widom Family Travel List and Photos)

Originally, Palau was almost completely forested. Currently, forest cover is only 75% of its historic numbers, as forests have been cleared and replaced by savanna and grassland.  Non-forested land in Palau is comprised of savannah, marsh, secondary vegetation, cropland, strand (shoreline) vegetation, and urban development. However, tropical broadleaf forests do still cover much of the volcanic and all of the limestone islands.  In addition, tropical moist forests, including littoral forest, upland forest (on the high volcanic islands), swamp forest, mangrove forest, atoll forest, casuarina forest, limestone forest (with a subtype in the Rock Islands), plantation forest, and palm forest can be found throughout the entire archipelago.

The interior upland forests of Palau contain several endemic species of broadleaf trees found on flat or gently sloping sites as well as river and stream banks. There are six native palm species generally found in the understory or middle canopy layers of the forest. Campnosperma brevipetiolata is a conspicuous component of the upland forests throughout the Carolines and is generally found at lower elevations on flat or gently sloping sites, close to streams. Other major upland species include Maranthes corymbosa (kayu batu), Alphitonia carolinensis (Etabob), Rhus taitensis (island sumac), Elaeocarpus carolinensis ( elaeocarpus), Serianthes kanehirae, Semecarpus venenosus, Calophyllum inophyllum, Gmelina palawensis,and Pterocarpus indicus. Common understory species include Pandanus aimiriikensis, Ixora casei, Eugenia cuminii, Osmoxylon oliveri, Manilkara udoido, Symplocos racemosa,and Cyathea lunulata.

A waterfall in a Babeldoab forest
A waterfall in a Babeldoab forest

Swamp forests are the most limited forest type in terms of area, making up only 2% of the forest and 1% of Palau’s land area. Swamp forests are found in low-lying areas, often just inland of mangroves, where the soils are inundated with fresh or slightly brackish water and above tidal influence. Swamp forests are particularly vulnerable to siltation resulting from road building activities and clearing for taro patches, which causes coastal swamp forests, in particular, to degrade and become inundated with Hibiscus tiliaceus. In the few areas of swamp forest remaining, including Peleliu, common tree species include Barringtonia racemosa and Terminalia catappa. Derris trifoliata is a common climbing vine found on trees in the swamp forest.

Palauan forest at higher elevations
Palauan forest at higher elevations (Photo: Pacific Worlds & Associates)

Mangrove forests, which border all but 20 miles (32 km) of coastline, are found along the lower portions of rivers, on coastal mudflats, and on some offshore islets. Mangroves are ecologically important because they help stabilize coastal areas by trapping and holding sediments washed down from inland areas and local watersheds. The most extensive areas of mangrove forests occur along the west coast of Babeldaob, covering approximately 80% of the shoreline. The dominant species in Palau’s mangrove forests belong to the genus Rhizophora. Well-developed mature stands can grow to 49-66 feet (15-20 m). On the seaward side, Rhizophora stylosa and Sonneratia alba dominate; at larger river mouths or bay indentations, Rhizophora apiculata and R. stylosa can become pure stands or occur with Sonneratia alba and Bruguiera gymnorrhiza; landward, Heritiera littoralis, Lumnitzera littorea and Xylocarpus granatum are included in the mix; and where the estuary becomes river-like, Bruguiera, Lumnitzera, Sonneratia and Xylocarpus species are common, wherease Rhizophora spp. becomes uncommon.

The palm Nypa fruticans is fairly common along the lower portions and mouths of rivers. Other woody species include Avicennia marina, Ceriops tagal and Scyphiphora hydrophyllacea.

Because of agroforestry, little remains of the native atoll forests, except on uninhabited atolls. Atoll forests are found toward the interior of the larger, wetter uninhabited atolls and along coasts of the high islands. They are generally located behind the strand zone but may be mixed with strand vegetation. They usually have an outer shrubby fringe of Scaevola taccada. In addition, small Pemphis acidula are common on rocky coasts, and tall Casuarina litorea trees are often found on leeward coasts.

Only some Casuarina species form forest communities. Many are too short or sparse to be classified as forest.  Because their roots can produce nitrogen through nodules containing special nitrogen-fixing bacteria, Casuarinas can grow on nutrient-poor soils and other marginal environments such as granite outcrops or sandy soils. Casuarina spp. Are evergreen shrubs and trees growing as tall as 115 feet (35 m). They are sometimes called she-oaks, ironwood, or beefwood.

Limestone forest is found on the coral islands of Peleliu, Angaur, and the Rock Islands and is composed of sharply eroded limestone with little soil cover.Although the limestone forests of Palau were heavily disturbed during World War II, they are still fairly common in patches throughout the islands. Species composition varies from island to island, and an assortment of endemic species are present; however, the habitat is similar among islands. Humus from decomposing vegetation provides a sustained cycle of nutrients. Woody species include Aidia racemosa, Badusa palauensis, Cycas circinalis, Cyrtandra todaiensis, Eugenia reinwardtiana, Flacourtia rukam var. micronesica, Garcinia matudai, G. rumiyo var. calcicola, Geniostoma sessile, Guettarda speciosa, Gulubia palauensis, Intsia bijuga, Ixora casei, Meryta senfftiana, Morinda latibracteata, Polyscias grandifolia, Premna serratifolia, Psychotria hombroniana, Rinorea bengalensis and Tarenna sambucina. The endemic Gulubia palauensis palm, now found only in Chelbacheb, was once common in the limestone forest but was decimated by introduced cockatoos.

The Rock Island forest is a subtype of limestone forest that is extremely diverse in species composition. Common species in this forest include endemic palms, such as Gulubia palauensis and Ptychosperma palauensis, and forest trees such as Semecarpus venenosus, Premna obtusifolia, Cordia spp., and Bikkia palauensis. Pandanus spp. and Dracaena multiflora are common understory plants.

Plantation forests are stands of trees created by regular placement of seeds or seedlings. Plantation forests can be composed of either native or exotic species but are often part of a diverse landscape of remnant native forests. Mahogany plantations were established during the 1930’s, but most of the older plantations have been harvested completely. However, stemming from a tree planting program established in 1970, the Forestry Department of Palau is currently planting about 10,000 mahogany seedlings and 5,000 seedlings of other species each year.

Terrestrial Biodiversity

A coconut crab
A coconut crab (Photo: Julian Sachs/Sachs Lab/University of Washington)

Palau’s flora is far richer than the rest of the Caroline Islands, with the exception of the island of Yap. Together, Yap and Palau form a distinct phytogeographic unit with the easternmost extension of several species of Indo-Malesian flora. The Palauan archipelago lies in an area influenced by the North Equatorial Counter Current (NECC) and the Mindanao Eddy. Both the NECC and the Mindanao Eddy affect overall island biodiversity by carrying coral and fish larvae, which originate in the Philippines, New Guinea, and Indonesia, to Palau. Terrestrial flora and fauna reach Palau via wind, rafting, or drifting with the NECC.

The crab-eating macaque was introduced into Palau by the Germans
The crab-eating macaque was introduced into Palau by the Germans (Photo: Sakurai Midori)

Due to its proximity to the NECC, Palau supports the highest level of terrestrial diversity in all of Micronesia and maintains a high level (25%) of species endemism among terrestrial biota. Elevated endemism is a direct result of the isolation of the islands, both from one another and from the rest of the world. There are approximately 1,260 species of plants in Palau, of which 830 species are native and minimally 194 species are endemics (although endemic plants are typically found only in Babeldaob).  In addition, more than 428 invasive plant species have been documented.

Palau's terrestrial fauna include approximately 5,000 species of insects; 141 species of birds, of which 11 species and nine subspecies are endemic; at least 40 species of  freshwater fishes, of which 4 are endemic; 46 species of terrestrial reptiles and amphibians; and three species of bat, of which one species and one subspecies are endemic.

The Palau Conservation Society logo is of a biib  the endemic Palau fruit dove
The Palau Conservation Society logo is of a biib the endemic Palau fruit dove

Palau has 50 species of resident birds, many of which are protected by local laws. The national bird is the Palauan Fruit Dove or Biib. Ten bird species, including two doves, an owl, a swiftlet, and six passerines, are restricted to the islands; however, none are threatened. In total, 16 restricted-range species are found in Palau, and one of these, the Micronesian scrubfowl (Megapodius laperouse), is considered endangered. The endangered Japanese night-heron (Gorsachius goisagi) is also present.  Because of development, habitat loss is affecting some birds on Koror and Babeldoab.

Terrestrial mammals are restricted to two extant bat species: the common Palau flying-fox (Pteropus pelewensis) and the less common insectivorous sheath-tailed bat (Emballonura semicaudata palauensis). The sheathtail bat is considered endangered while another species,the large Palau flying-fox (Pteropus pilosus), is presumed to be extinct.  The island of Angaur is the only place in Micronesia that has feral monkeys, descended, reportedly, from a pair of pet macaques brought to the island in the early 1900’s by German miners to monitor air quality in the island's phosphate mines. The crab- and bird-eating macaque monkey (Macaca fascicularis) is a pest that destroys farm crops and gardens in Angaur. Other introduced mammals include rats, mice, and the Asiatic musk shrew, Suncus murinus.

The Palau flying fox
The Palau flying fox

Palauan amphibians are an introduced marine toad (Bufo marinus) and an endemic frog, Platymantis pelewensis. The most diverse group of Palauan reptiles is the lizards with 30 species, including at least nine endemics (six gekkos and three skinks). The saltwater crocodile (Crocodylus porosus) is at the edge of its range in Palau and is under threat from habitat destruction and hunting.  The elusive mangrove monitor lizard, Varanus indicus, lives near river banks, mangrove forests, and in coastal forests.  There are also seven species of non-venemous snake, which include the endemic Palauan blind snake and the bevelnosed boa.  Sea snakes in the waters around Palau, however, are highly venomous and include the banded sea krait (Laticauda colubrina) and the yellow-bellied sea snake (Pelamis platurus). Sea kraits are oviparous and must come ashore to lay eggs. The yellow-bellied sea snake, which is less common, births its young at sea. Four species of sea turtles have been documented in Palau, although only two species, the hawksbill (Eretmochelys imbricata) and green (Chelonia mydas) turtles, maintain resident and nesting populations. The leatherback (Dermochelys coriacea) and olive Ridley turtles (Lepidochelys olivacea) occur in the islands but are much less common.

The banded sea krait
The banded sea krait is amphibious and is found in shallow Indo-Pacific tropical marine environments, coral reefs, and mangrove swamps. On land, the krait inhabits sandy beaches, coral islands, and occasionally low-hanging trees. (Photo: Jan Messersmith/Madang-Ples Belong Mi)

Terrestrial invertebrates are poorly known. Over 300 of the 1,200 known species of Palauan insects are endemics. In addition to insects, there are at least 11 families of Arachnida, which include spiders, scorpions, harvestmen, mites, and ticks. Unusual among Palau’s insects are the marine insects that inhabit coastal and offshore waters.  Included are two genera and many species of sea skaters – a type of marine insect found only in the Indian Ocean and western Pacific Ocean. The reef midge, Pontomyia oceana, is found only in the coral reefs off Palau and Queensland, Australia.