Location and General Description
The Caroline Island group extends from northwest to southeast over a wide swath of the North Pacific Ocean. Moving from northwest to southeast, this ecoregion includes the outer islands of Yap State, known as the Remetau Group, and the States of Chuuk, Pohnpei, and Kosrae. The Chuuk islands are a classic example of an "almost-atoll". The huge, almost circular barrier reef has 69 islets, and is more than 60 km across at the widest point. The reef surrounds a central lagoon, which contains a mini-archipelago of 10 principal and many smaller volcanic islands. The largest island, Tol, reaches 443 m in elevation. Pohnpei, 650 km to the east of Chuuk, is an extinct volcano, composed of weathered basalt. Geologically younger than Chuuk, it stands 734 m above sea level. The center of the island is a tableland with 11 separate peaks. Kosrae has a maximum elevation of 629 m. The island is deeply dissected, and has several steep high peaks.
Located in the humid equatorial region on the southern edge of the typhoon belt, the islands at the eastern end of the Caroline chain are wet year-round, with precipitation becoming more seasonal toward the west. Rainfall in Pohnpei averages more than 4,500 mm annually, while parts of Kosrae, 550 km to the southeast, receive more than 6,400 mm per year.
Mature vegetation on the high volcanic islands is dominated by broadleaf evergreens. These high islands were probably almost completely forested at one time, with a few patches of savanna. Human activities have increased the extent of the savannas to cover large areas, especially in the western Carolines. Most of the lowland vegetation has been modified from its original forest state.
Pohnpei, at 335 km2, is the second largest island in the Carolines. It is also the highest and one of the wettest. The vegetation is less degraded than that of the other high islands to the west, and well-developed mangrove forests and freshwater swamp forests still exist in coastal areas. Some primary montane rain and cloud forest can be found on the slopes of the peaks in the center of the island. This upland forest can be divided into two major associations, with a Maesa carolinensis association at the lower altitudes (450 to 680 m), and a Cyathea ponapeana/Pandanus patina association on the upper slopes (Raynor 1993).
The endemic palm, Clinostigma ponapensis, forms the upper layer of the Maesa carolinensis association. A tree fern, Cyathea nigricans, is the dominant plant in the lower canopy, and numerous shrubs including Maesa make up the understory. Palms such as Ptychosperma and Metroxylon are also found in the montane forests. Other common broadleaf trees include Glochidion spp., Myrsine, Elaeocarpus, Syzgium, Psychotria, Timonius, Gynotroches, and Astronidium. The forest is tied together with lianas of genera such as Ipomoea, Merremia, Freycinetia, Hypserpa, and Pachygone. There are also many terrestrial and epiphytic ferns.
On the lower slopes, the vegetation is almost completely secondary, consisting of either savanna or forest. The steepest of the lower slopes are blanketed with thick, impenetrable tangles of Hibiscus tiliaceus, and approximately 33% of Pohnpei is covered with a rotating system of tree gardens.
Montane rain forest, as well as patches of cloud forest on the highest peaks, are also still present on the island of Kosrae. The principal forest genera here include Horsfieldia, Neubergia, Psychotria, Syzgium, Campnosperma, Macaranga, Cyathea, Dendrocnide, Boehmeria, and Ficus, and the only indigenous palm, Ptychosperma ledermanniana. Species diversity is high, and many different species of ferns, both terrestrial and epiphytic, are present.
Around 300 m above sea level, the montane forest changes to a dwarf cloud forest, lushly covered with mosses and hepatics (members of the buttercup family). There are many epiphytes, including Elaphoglossum carolinense, Mecodium polyanthos, Lindsaea rigida, and Peperomia kusaiensis. This forest has not been thoroughly studied, but canopy trees include Cyathea ponapeana, Elaeocarpus carolinensis, and Astronidium kusaianum. Polyscias subcapitata and Eugenia stelechanthoides dominate the lower canopy, and the undergrowth includes ferns such as Marattia fraxinea and Pteris spinescens (Fosberg 1996).
The atolls in the eastern Carolines are very flat, rising no more than a few meters above sea level. For atolls, they have an uncharacteristically diverse flora, probably because of the heavy rainfall. However, none of the known native species are particularly distinctive botanically (Fosberg 1996). Vegetation on the atolls consists primarily of pan-tropical lowland strand species.
Pohnpei and Kosrae have the only remaining patches of montane cloud forest in Micronesia. These forests are also unusual because they are among the lower elevation cloud forests in the world, starting bit higher than 450 meters (Raynor 1993). Endemism is high, in part because the islands are relatively close to the floristically rich regions of Southeast Asia and in part because of their isolation and great age.
Twenty-four species of reptiles (e.g., skinks and geckos) and amphibians, including four endemics, with one endemic genus, are found in the Caroline Islands (Dahl 1986). The island’s fruit bats (Pteropus marianas, P. molosinnus, P. insularis, P. phaeocephalus), the latter three being restricted to the Carolines, are all threatened by habitat loss and commercial hunting for export to Guam. Eighteen restricted-range species of bird occur in the Carolines (Statterfield et al. 1998). Thirteen species are endemic to the ecoregion, including the Truk monarch (Metabolus rugensis), the Pohnpei fantail (Rhipidura kubaryi), the Pohnpei mountain starling (Aplonis pelzeni), and the Pohnpei lory (Trichoglossus rubiginosus). Among the 29 recorded bird species on Pohnpei, 24 make extensive use of the upland forest habitat. The tropical moist cloud forest of Pohnpei is also home to 26 species of land snails, representing 74 percent of the total number of species recorded for that island (Raynor 1993).
Two endemic forest species (Schefflera kraemeri and Semecarpus kraemeri) are found only in small montane forest remnants on Uinipot Peak on Tol Island in Chuuk. The same forest fragments host a small endemic tree, Randia carolinensis.
For more than 2,500 years there have been human populations who have cultivated coconut, breadfruit, taro, bananas, and sugarcane. Most natural lowland forests have been altered to savanna and secondary forests dominated by non-native species.
Semi-natural montane forest in Chuuk still occurs in small patches on the steeper slopes and on the summit of Uinipot Peak on Tol. The interior highlands of Kosrae and Pohnpei still support montane and cloud forests (Stattersfield et al. 1998). Pohnpei has protected all of its mangrove forests and 51 km2 of its interior forests as a watershed reserve.
Types and Severity of Threats
A current threat to the tropical moist forests comes from the expanding commercial cultivation of sakau (Piper methysticum) on the three high islands. Sakau needs fertile organic soil so more upland forest is being cleared for its cultivation (Raynor 1993). Alien species, such as rats, continue to devastate native species and are believed to been largely responsible for the extinction of the Kosrae crake (Porzana monasa) and the Kosrae starling (Aplonis corvina). Natural habitats and species are also threatened by diverse and abundant introduced plants. Homesteading, road construction, hunting, especially of the Micronesian pigeon (Ducula oceanica) and the Caroline Islands ground dove (Gallicolumba kubaryi), and increased tourism are also endangering native forests and species in the Caroline Islands.
Justification of Ecoregion Delineation
Discussions with invertebrate specialists (e.g., D. Polhemus) suggest that a significant biogeographic division could be made between Chuuk and Pohnpei (specifically with Oruluk Atoll and Nukuoro Atoll to the west and Palakir to the east of the line). The relevance of this division to other taxa needs to be examined for future ecoregion modifications.
Dahl, A.L. 1986. Review of the protected areas system in Oceania. International Union for Conservation of Nature and Natural Resources, Commission on National Parks and Protected Areas, in collaboration with the United Nations Environment Programme.
Fosberg, F.R. 1998. Chapter 5: Micronesia. Pages 199-313 in D. Mueller-Dombois and F.R. Fosberg, editors. Vegetation of the Tropical Pacific Islands. Springer-Verlag New York, Inc.
IUCN. 1991. Directory of protected areas in Oceania. International Union for Conservation of Nature and Natural Resources.
Raynor, B. 1993. Montane cloud forests in Micronesia: Status and future management. Tropical Montane Cloud Forests: Proceedings of an International Symposium at San Juan, Puerto Rico. L.S. Hamilton, J.O. Juvik, and F.N. Scatena, editors. East-West Center Program on Environment, Honolulu, Hawaii.
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Prepared by: Sandra Zicus
Reviewed by: In process