Location and General Description
Located about 800 km north of the equator, 800 km east of the Philippines, and 6,000 km southwest of Hawaii, Palau is the westernmost archipelago of Micronesia's Caroline Islands. It consists of six main island groups in a chain approximately 200 km long, and oriented roughly north–south. Babeldaob, or Babelthaup, is the largest (376 km2) island in the chain, and accounts for more than 75 percent of Palau’s land area.
Palau’s climate is tropical, with a hot, humid rainy season that extends from May through November. It lies outside the main typhoon path; nevertheless, during the typhoon season of June through December, it is hit by occasional storms with high winds that damage trees and buildings. The mean annual temperature in the capital city of Koror is 27ºC, and the mean annual rainfall is about 3,730 mm. Rainfall varies little from month to month, although February, March, and April tend to be slightly drier.
Geologically, the main islands of Palau can be divided into two distinct domains: (1) the deeply weathered and leached volcanic areas of Babeldaob, parts of the island of Koror, and a few small nearby islands; and (2) the more recent, often karst-weathered, limestone of the southern islands. Because of differences in topography, drainage, soils, and soil chemistry, there are distinct differences in the vegetation communities between volcanic and limestone islands. There are also a number of outlying atolls, including Kayangel in the North and Helen and Tobi in the South.
Dense tropical broadleaf forests cover most of the volcanic and all of the limestone islands, with the exception of Babeldaob, where large areas have been cleared and replaced by grassland. The tropical moist forests of Palau can be divided into 8 main types: upland forest (found only 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 (Cole, et al. 1987).
The upland forests of Palau are the most species diverse in Micronesia and contain several endemic species. There are six native palm species, generally found in the understory or middle canopy layers of the forest. Campnosperma brevipetiolata is one of most common canopy trees in the upland forest. It is generally found at lower elevations (less than 150 m) on flat or gently sloping sites, and in riparian areas. Other major species include Parinari corymbosa, Alphitonia carolinensis, Rhus taitensis, Elaeocarpus carolinensis, 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.
Swamp forests are found in low-lying areas, just inland of mangroves and above tidal influence. These forests are generally quite disturbed, having been used extensively for taro cultivation. In the few somewhat natural areas of swamp forest remaining, such as those found on Peleliu, common species include Barringtonia racemosa and Terminalia catappa. Derris trifoliata is a common climbing vine found on trees in the swamp forest, and Hibiscus tiliaceus is often found in disturbed areas.
Mangrove forests occur along the lower portions of rivers, on coastal mudflats, and on some offshore islets. Well-developed stands may be 15 to 20 m in height. The dominant species in Palau’s mangrove forests are Rhizophora spp., but some Bruguiera gymnorrhiza are also found. On the seaward side of the forest, Sonneratia alba and Rhizophora mucronata are dominant.
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. Small, well-formed Pemphis acidula are common on rocky coasts, and tall Casuarina litorea trees are often found on leeward coasts.
Limestone forest is found on the coral islands of Peleliu, Angaur, and the Rock Islands. The species composition varies from island to island, and various endemic species are present. The habitat of all limestone forests is similar; humus from decaying vegetation provides a sustained cycling of nutrients. Common species include Intsia bijuga, Psychotria spp., and Clerodendrum inerme. This forest type once covered much of Peleliu, but much of it was destroyed or degraded during World War II. However, it is still fairly common in patches throughout the islands.
The Rock Island forest is a subtype of limestone forest that is extremely diverse in species composition. (The Rock Islands are a cluster of extremely steep karst-weathered limestone islands that extend from Koror Island south toward the island of Peleliu.) Some of more common species in this forest are 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.
Palau is floristically much richer than the rest of the Carolines, with the exception of Yap. Together, Yap and Palau form a distinct phytogeographic unit (Mueller-Dombois & Fosberg 1998). The easternmost extension of several species from Indo-Malesian flora are found here, including members of the genera Gmelina, Gulubia, Parinari, and Pterocarpus.
Ten bird species are restricted to Palau, and none of these are threatened. These include two doves, an owl, a swiftlet, and six passerines. 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), while not endemic or with a restricted range is also present in Palau (Stattersfield et al. 1998, Hilton-Taylor 2000).
Terrestrial mammals are restricted to two extant bat species: the relatively common Palau flying-fox (Pteropus pelewensis), and the rare 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 (Flannery 1995, Wiles et al. 1997, Hilton-Taylor 2000).
One ranid frog, six gekkos, three skinks, and one wormsnake are endemic to Palau (Allison 1996). The saltwater crocodile (Crocodylus porosus), while a lower risk species globally, is at the edge of its range in Palau and is under threat from habitat destruction and hunting (Allison 1996, Hilton-Taylor 2000).
Terrestrial invertebrates are poorly known, but include at least 11 arachnid families (IUCN 1991).
Originally, Palau was probably almost completely forested; today, forest cover is only about 75 percent (Cole et al. 1987). Most of the remaining land is grassland, agroforest, or secondary vegetation. Under Japanese administration during World War II, large areas of southern Babeldaub were cleared for pineapple and sugar plantations. The endemic palm Gulubia palauensis is also threatened by the feeding of an introduced cockatoo, which escaped from Oreor. In general, thanks in part to relatively low population density, much of Palau’s environment is in good condition (IUCN 1991).
Types and Severity of Threats
Loss of habitat due to development is affecting some birds, especially on Koror and Babeldaob. Guns are now outlawed, and many species of birds are protected by local laws. A growing human population and an increased push for tourism could have serious negative impacts on the environment if not carefully planned and managed.
Justification of Ecoregion Delineation
This ecoregion includes the main islands of Babelthuap and surrounding islands as well as the far flung atolls of Kayangel, Tobi, and Helen. Allison (1996) highlights Palau within the larger Micronesian region as a result of its 10 endemic reptiles and 1 endemic amphibian. Birdlife International (Stattersfield et al. 1998) also recognizes the Palauan islands as an Endemic Bird Area, with 10 endemic bird species. Van Balgooy (1996) does not explicitly characterize the Palaun floristic affinities except by omission – he does not lump Palau with the adjacent Caroline Island groups of Micronesia. Based on its bird and herpetofaunal distinctiveness, we have delineated Palau as an ecoregion.
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Wiles, G.J., J. Engbring, D. Otobed. 1997. Abundance, biology, and human exploitation of bats in the Palau Islands. Journal of the Zoologic Society of London, 241:203-227.
Prepared by: Sandra Zicus
Reviewed by: In process