This EBA includes the volcanic islands of Savai'i and 'Upolu and their offshore islands (constituting the political unit of Western Samoa), and the more easterly ones of Tutuila and Manu'a (which make up American Samoa, a US dependent territory).

The native vegetation is largely tropical rain forest which can be subdivided into lowland and montane types, with disturbed forests resulting from the destructive effects of cyclones. Cloud forest occurs above c.1,200 m on Savai'i (which reaches a maximum altitude of 1,848 m at Mt Silisili), and scrubby upland vegetation occurs on peaks and summits. Mangroves are also present and are particularly well developed along the south coast of 'Upolu (see Whistler 1992, 1993).

All the restricted-range bird species occur in forest but many are also found in plantations and gardens. This use of man-modified environments may be important for the survival of some indigenous species, given the severe loss of native habitat as a result of man's activities and cyclonic storms (see 'Threats and conservation', below; Evans et al. 1992b). Some species, however, notably Didunculus strigirostris and Gymnomyza samoensis, are dependent on remaining patches of primary forest, and conservation of sufficient areas of native habitat thus remains vital. Although many species are found in upland forest, these may not be their preferred altitudes.

Distributional patterns of species within the EBA vary (see 'Distribution patterns' table), with 10 restricted-range species (now) occurring only in Western Samoa. Two of these are confined to Savai'i: Zosterops samoensis, which has a particularly tiny distribution in the unique cloud forest and alpine scrub around Mt Silisili, and Gallinula pacifica, which was last recorded in 1873 and consequently is often listed as extinct, but two possible sightings in 1987 in upland forest west of Mt Elietoga (Bellingham and Davis 1988) indicate that it may still survive. Gallicolumba stairi also has a very restricted range within this EBA, as it was not found on Savai'i and 'Upolu during a recent lowland survey and may now be restricted to the small offshore Aleipata Islands (which are important nesting sites for many seabird species) (D. J. Butler in litt. 1993), while in American Samoa it is only known from Ofu where a single bird was sighted in 1993 (P. W. Trail in litt. 1995).

Several of the more widespread restricted-range species are shared with the Fijian Islands EBA (202) and/or occur more widely in Central Polynesian Secondary Areas (s12-s131).

Manu'a Fiji Shrikebill Clytorhynchus vitiensis powelli is endemic to the Manu'a Islands in American Samoa and is the most distinctive of the many subspecies of this widespread Polynesian bird, and a good candidate for elevation to species status (H. D. Pratt in litt. 1994). For the most complete data on American Samoan bird populations before the cyclones of 1991 and 1992, see Engbring and Ramsey (1989), also Amerson et al. (1982a,b).

In Western Samoa, the total remaining forest comprises 23% of the land area of 'Upolu and 47% of Savai'i; most of the original lowland forest has been cleared for agriculture or logged for timber, with perhaps only five tracts remaining and encroachment inland into montane areas. Following clearance, often only one or two successive crops are grown before the land is abandoned, resulting in heavy erosion and the formation of dense secondary scrub vegetation dominated by weedy species (Davis et al. 1986, Taulealo 1993, WWF/IUCN 1994-1995; see also Paulson 1994). On American Samoa about two-thirds of the native vegetation has been disturbed or cleared for settlements or agriculture (Davis et al. 1986).

In addition to the severe loss of native habitat through human activities, cyclonic storms can cause extensive damage: tree mortality from this cause was 28% in 1990 and 33% in 1991 (Elmqvist et al. 1994). Forests have the capacity to recover rapidly, but damage can be sufficient to allow cattle-grazing and taro-planting in some areas that had previously been little modified (B. D. Bell in litt. 1993, H. D. Pratt in litt. 1994), and the effects may thus be longer term. Invasion by aggressive exotic weeds is an additional hazard, and on 'Upolu the so-called mile-a-minute vine Mikania micrantha still chokes large areas of former forest and may be preventing regeneration (H. D. Pratt in litt. 1995), while on Ta'u the weed Koster's curse Clidema hirta has spread dramatically and forms nearly impenetrable tangles above c.450 m; this latter may have reduced habitat quality for Clytorhynchus vitiensis powelli (P. W. Trail in litt. 1995). Furthermore, the rate of forest regeneration may be substantially affected by the large reduction of the whole guild of vertebrate seed-dispersers including two species of flying fox Pteropus samoensis and P. tonganus, and several species of fruit-pigeons, e.g. Pacific Pigeon Ducula pacifica and Ptilinopus perousii (Elmqvist et al. 1994).

Several of Samoa's restricted-range species are classified as threatened or Near Threatened, largely as the result of deforestation, particularly combined with the severe effects of the recent cyclones, which have caused declines in bird populations (see, e.g., Trail et al. 1992). Hunting is an additional pressure which increases as the forest habitat diminishes, especially for the columbids, and notably for the distinctive Didunculus strigirostris which has been the subject of a number of studies (e.g. Beichle 1982, 1987) and has recently been chosen as a flagship species to promote conservation awareness in Western Samoa.

Introduced mammals, such as cats and rats, are thought to have contributed to the decline of Gallinula pacifica (which is believed to be almost flightless and will therefore be especially vulnerable to predation) and may also affect other species, including the threatened (Vulnerable) restricted-range Bristle-thighed Curlew Numenius tahitiensis, which breeds in Western Alaska (Secondary Area s002) and winters among Pacific islands (including the islands of this EBA) where it undergoes a flightless moult.

The state of the environment in Western Samoa is described in Taulealo (1993) and National Environment and Development Management Strategies are documented in SPREP (1993). A recent lowland forest survey in Western Samoa identified 14 key sites as critically in need of protection to conserve biodiversity, with five identified as being of international significance (Park et al. 1992, Sesega and Park 1993). This survey and a later follow-up one (Lovegrove et al. 1992) documented the serious impacts of the cyclones on birds.

There are a few protected areas in Western Samoa (see Beichle and Maelzer 1985), including the O Le Pupu Pu'e National Park (28 km²) on 'Upolu where restricted-range birds are present (Bellingham and Davis 1988), but this park was heavily damaged following recent cyclones, and logging and cattle-farming are continuing threats. Mt Vaea, a small forested ridge on the outskirts of Western Samoa's capital, Apia, is also partly protected, although surrounded by an increasingly urbanized landscape. Two areas on Savai'i-the Falealupo and Tafua Rainforest Preserves-are protected under local conservation agreements, but both were severely damaged by the recent cyclones and part of Tafua was further destroyed by a forest fire shortly thereafter (Elmqvist 1993, Elmqvist et al. 1994, D. J. Butler in litt. 1993, P. W. Trail in litt. 1993). The National Park of American Samoa was officially established in January 1994 and includes important forest areas on Tuila and Ta'u (P. W. Trail in litt. 1995).

Recommended citation
BirdLife International (2024) Endemic Bird Area factsheet: Samoan Islands. Downloaded from https://datazone.birdlife.org/eba/factsheet/202 on 23/11/2024.