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Islands of Western Samoa and American Samoa in the Southern Pacific

The islands of Samoa comprise the last large areas of land east of Australia in the central Pacific. Their size has helped them accumulate and support a diversity of plant and animal genera not found further to the east as well as a rich, endemic flora and fauna of their own. About 80 percent of Samoa’s lowland rain forests have been lost during the 3,000 year history of human habitation. Given current trajectories, a devastating loss of native forest is predicted to occur within the next 20 years.

  • Scientific Code
    (OC0112)
  • Ecoregion Category
    Oceania
  • Size
    1,200 square miles
  • Status
    Critical/Endangered
  • Habitats

Description
Location and General Description
Samoa consists of a chain of 14 volcanic islands located 600 km east of Fiji at 13º to 14ºS latitude and 168º to 171ºW longitude (Whistler 1980). The Western Samoan islands of Savai’i and ‘Upolu make up 90 percent of the archipelagos’ land area and are separated from Tutuila and Aunu’u Islands of American Samoa by 64 km of ocean. To the east about 100 km lie Ofu, Olosega, and Ta’u Islands. Even further to the east is the tiny Rose Atoll. The islands increase in age to the west. The eastern island of Savai’i most recently erupted in 1903 and 1911. In the west, Tutuila is at least 1.6 million years old (Whistler 1980). The tallest mountains are on Savai’i (1,858 m) and ‘Upolu (1,116 m).

Soils are formed from eroded basalt and ash deposits and are generally poor. The climate is tropical with a mean daily temperature range of 23o to 30oC. Trade winds are fairly constant throughout the year. Because most mountains in the islands are oriented in an east-west direction there is no pronounced rain shadow, and all areas receive more than 2,000 mm of rain per year (Whistler 1980). The tops of the higher mountains may receive more than 8,000 mm of rain per year (Whistler 1980). December through March are the wettest months and also the time when devastating cyclones occur (Craig et al. 1994).

Tropical rain forest once covered all areas, except coastal, salty, marshy, and those that are nutrient-poor. It occurs in three broad types in Samoa: lowland forest is the most extensive, followed by montane forest, and cloud forest, with several other minor habitat types including montane scrub, Pandanus scrub, littoral scrub, montane swamp forests, and summit scrub (Whistler 1980, WWF & IUCN 1995). Lowland rain forest is dominated by Diospyros samoense and D. elliptica, Calophyllum inophyllum, Dysoxylum samoense and D. maota, Pometia pinnata, Planchonella samoense, Syzygium spp., and Myristica fatua. The wetter and cooler montane rain forest is dominated by Dysoxylum huntii, but Syzygium spp., Weinmannia spp., Canarium harveyi, Rhus taitensis, and Astronidium spp. are also found (Whistler 1992). Cloud forest occurs above 650 m in areas almost continually draped in clouds. Common species are Reynoldsia pleiosperma, Weinmannia samoense, Dysoxylum huntii, and Coprosma savaiiense. The understory contains Dicranopteris linearis, Freycinetia storckii, and Cyathea spp. tree ferns (Whistler 1978, 1980).

There are also distinct vegetation communities on relatively recent lava flows (Whistler 1992). The dominant species on lowland flows are Fagraea berteroana, Glochidion ramiflorum, Arytera brackenridgei, and Morinda citrifolia with Metrosideros collina and Weinmannia samoensis becoming more dominant at higher elevations. The highest lava flows support a scrubby community of Vaccinium whitmeei, Spiraeanthemum samoense, and Coprosma strigulosa. Littoral vegetation is typical of that of many Pacific islands with Scaevola taccada, Pandanus tectorius, Barringtonia asiatica, Calophyllum inophyllum, Pisonia grandis, and coconut (Cocos nucifera) being the most common species (Whistler 1982, Mueller-Dombois & Fosberg 1998). Mangrove vegetation occurs in several areas, particularly in the south coast of ‘Upolu (Stattersfield et al. 1998).

Biodiversity Features
There are 536 species of flowering plants with about 28% endemism. There are also 228 fern and allied species (Whistler 1992). While only one plant genus is endemic (Sacropygme), there are some spectacular groups of plants, including more than 100 species of native orchids (Whistler 1992, WWF & IUCN 1995). Many genera of plants common in western Pacific forests reach the eastern limit of their distribution in Samoa (Smith 1955). Many of Samoa’s plants are restricted to single islands and Blumea milnei, Cordia aspera, and Acronychia retusa are considered threatened or endangered (Dahl 1980).

Of Samoa’s 37 native land birds, 84 percent are endemic species or subspecies, and there are also 4 introduced species (Merlin & Juvik 1983, Stattersfield et al. 1998). Large islands in Samoa support up to 6 species of fruit-eating pigeons including the Samoan tooth-billed pigeon (Didunculus strigirostris), a monotypic genus endemic to the islands (Evans et al. 1992). The Samoan wood rail (Pareudiastes pacificus) and sooty rail (Porzana tabuensis) are probably extinct (Merlin and Juvik 1983). The Australian gray duck (Anas supercilliosia), purple swamphen (Porphyrio porphyrio samoensis), many-colored fruit-dove (Ptilinopus perousii perousii), Samoan ground-dove (Gallicolumba stairi stairi), ma’o (Gymnomyza samoensis), and island thrush (Turdus poliocephalus samoensis) are all endangered or threatened (Merlin and Juvik 1983). The Samoan flying fox (Pteropus samoensis) is a CITES-listed species that is endangered by subsistence hunting and by commercial hunting for markets in the western Pacific (Brautigam and Elmqvist 1990). There are at least nine species of terrestrial reptiles including geckos and skinks and the Pacific keel-scaled boa (Candoia bibroni).

Current Status
Since Polynesians arrived around 3,000 years ago, more than 80 percent of lowland rain forest has been lost in Western Samoa and American Samoa. The Western Samoan government predicts a total loss of forests within 20 years (Brautigam & Elmhurst 1990). Unregulated logging and clearing for subsitence and cash crops are major factors contributing to forest decline. ‘Upolu has the highest human population and, consequently, almost all accessible lowland, montane, and cloud forest has been cleared. Two recent cyclones in 1990 and 1991 killed 53 percent of forest trees, large numbers of forest birds, and facilitated the spread of a wild fire that completely destroyed large areas of forest (Elmqvist et al. 1994).

In Western Samoa, the Robert Louis Stevenson Memorial Reserve/Mount Vaea Reserve (12.9 km2) protects disturbed lowland rain forest. Falealupo Reserve (100 km2) protects one of the largest remaining areas of lowland rain forest. O Le Pupu Pu’e National Park (28 km2) protects rain forest from the top of Mt. Fito, Upolu down to sea level (Dahl 1980). In American Samoa, the National Park of American Samoa (180 km2), defined in 1988, protects tract forest on Tutuila, Ofu, and Ta’u, including a large area of cloud forest on Ta’u. All of these parks experience high levels of poaching as well as encroachment for subsistence agriculture on their borders (Carew-Reid 1990). Rose Atoll National Wildlife Refuge encompasses the whole uninhabited area of this extremely isolated atoll and protects large numbers of seabirds, but no moist forest (Dahl 1980).

There is still a need for more extensive reserves in Western Samoa to protect large areas of forest encompassing complete watersheds. In particular, Mount Silisili on Savai’i Island has been cited as a suitable reserve site because of low human population density and high mountains which support a diversity of forests. Savai’i Island is the second highest in Polynesia outside of Hawaii and New Zealand and supports large populations of many of Samoa’s endemic plant and bird species. However, widespread logging and replacement of native forest with teak and mahogany plantations are limiting future conservation options (Merlin & Juvik 1983).

Si’ osi’omaga and Ole Vaomatua are two local conservation groups active in the archipelago. Local communities have also been increasingly involved in stopping logging operations and the conservation of forests (WWF & IUCN 1998).

Types and Severity of Threats
Despite hunting restrictions, annual takes of pigeons, fruit bats, and coconut crabs (Birgus latro) are much greater than is sustainable, and hunting may result in the extirpation and extinction of species in the near future (Evans et al. 1992, Craig et al. 1994). Introduced species such as rats, marine toads, and cats take a toll on native wildlife, and alien weeds, like Mikania micrantha and Solanum torvum, are major threats to natural communities (WWF & IUCN 1995). Predatory Euglandina land snails, introduced to protect crops, have devastated a rich and endemic land snail fauna, except on a few offshore islands. The hydroelectric dam being built on ‘Upolu threatens rare swamp forest (WWF & IUCN 1995).

Justification of Ecoregion Delineation
This ecoregion includes the main Samoan islands of Savai’I and Upolu as well as the Manua Islands and Tutuila. While van Balgooy et al. (1996 ) considers Tonga and Samoa as one unit with respect to floristic affinities, Allison (1996) treats the Tongan and Samoan archipelagos as separate units with respect to herpetofauna. The Samoan Islands are delineated as an Endemic Bird Area due to the presence of 11 endemic bird species (Stattersfield et al. 1998).

References
Allison, A. 1996. Zoogeography of amphibians and reptiles of New Guinea and the Pacific region. Pages 407-436 in Keast, A. and S.E. Miller, editors. The origin and evolution of Pacific island biotas, New Guinea to Eastern Polynesia: Patterns and processes. SPB Academic Publishing, Amsterdam.

Brautigam, A. and T. Elmqvist. 1990. Conserving Pacific island flying foxes. Oryx 24:81-89.

Carew-Reid, J. 1990. Conservation and protected areas on South-Pacific islands: the importance of tradition. Environmental Conservation 17:29-38.

Craig, P., T.E. Morrell, and K. So’oto. 1994. Subsistence harvest of birds, fruit bats, and other game in American Samoa, 1990-1991. Pacific Science 48:344-352.

Craig, P., P. Trail, and T.E. Morrell. 1994. The decline of fruit bats in American Samoa due to hurricanes and over hunting. Biological Conservation 69:261-266.

Dahl, A.L. 1980. Regional ecosystems survey of the South Pacific area. South Pacific Commission, Noumea, New Caledonia.

Dhondt, A. 1976. Bird observations in Western Samoa. Notornis 23:29-43.

Elmqvist, T, W.E. Rainey, E.D. Pierson, and P.A. Cox. 1994. Effects of tropical cyclones Ofa and Val on the structure of a Samoan lowland rain forest. Biotropica 26:384-391.

Evans, S.M., F.J.C. Fletcher, P.J. Loader, and F.G. Rooksby. 1992. Habitat exploitation by landbirds in the changing Western Samoan environment. Bird Conservation International 2:123-129.

Merlin, M.D. and J.O. Juvik. 1983. Bird Protection in Western Samoa. Oryx 19:97-103.

Mueller-Dombois, D. and F.R. Fosberg. 1998. Vegetation of the Tropical Pacific Islands. Springer Press, New York.

Orenstein, R.I. 1979. Notes on the Ma’o (Gymnomyza samoensis) a rare Samoan honeyeater. Notornis 26:181-184.

Stattersfield, A.J., M.J. Crosby, A.J. Long, and D.C. Wege. 1998. Endemic Bird Areas of the World: Priorities for biodiversity conservation. BirdLife Conservation Series no. 7, BirdLife International, Cambridge, UK. 846 pp.

Van Balgooy, P.H. Hovenkamp, and P.C. Van Welzen. 1996. Phytogeography of the Pacific – floristic and historical distribution patterns in plants. Pages 191-213 in Keast, A. and S.E. Miller, editors. The origin and evolution of Pacific island biotas, New Guinea to Eastern Polynesia: Patterns and processes. SPB Academic Publishing, Amsterdam.

Webb, E.L. and S. Fa’aumu. 1999. Diversity and structure of tropical rain forest of Tutuila, American Samoa: effects of site age and substrate. Plant Ecology 144:257-274.

Whistler, W.A. 1978. Vegetation of the montane region of Savai’i, Western Samoa. Pacific Science 32:79-94.

Whistler, WA. 1980. The vegetation of eastern Samoa. Allertonia 2:45-158.

Whistler, W.A. 1992. Vegetation of Samoa and Tonga. Pacific Science 46:159-178.

WWF and IUCN. 1995. Centres of plant diversity. A guide and strategy for their conservation. Vol. 2. Asia, Australasia and the Pacific. IUCN Publication Unit, Cambridge, UK.

Prepared by: Tim Male
Reviewed by: In process

 

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