Regional Background
Samoa is a volcanic archipelago running in a north-northwest direction east of Fiji, north of Tonga, and east of the Cook Islands and Tahiti.  It lies on the Pacific Plate, and consequently has never had any connection with the continental islands of Melanesia and Southeast Asia.  The archipelago is divided politically into Samoa, an independent country formerly known as Western Samoa, and American Samoa, an unincorporated territory of the United States.  To avoid confusion, the country Samoa will herein be referred to as “Independent Samoa,” and the archipelago as “Samoa.”  The archipelago, lying at a longitude of 168–173° W and a latitude of 11–15° S, comprises nine inhabited islands, plus Swains Island and uninhabited Rose Atoll, with a total area of ca. 3100 km2 (Whistler 1992a).  The main islands of independent Samoa, which comprise the western portion of the archipelago, are Savai‘i (1820 km2 area, 1860 m elevation) and ‘Upolu (1110 km2, 1100 m).  These two islands represent about 94% of the total area of the island chain.  The main islands of American Samoa are Tutuila (124 km2, 650 m) and Ta‘u (39 km2, 930 m)(Whistler 1992a).  Because the archipelago is a single natural unit, the following proposal encompasses all the islands.

The Flora and Taxonomic Breadth
Compared to Fiji just 700 miles to the west, Samoa has an attenuated flora perhaps one third as large, but which nevertheless is considerably larger than that of any other tropical Polynesian archipelago or island except Hawai‘i, which has more species but fewer genera.  The flora is estimated to be 810 species and comprises about 540 native species of flowering plants, two thirds of them dicots (Whistler, unpublished data).  An additional 250 or so species are naturalized or adventive (Whistler 1988b). The fern flora is estimated to comprise 230 species, with a much lower rate of endemism. 

The largest angiosperm families are Orchidaceae (102 native species), Rubiaceae (ca. 46), Urticaceae (ca. 25), Fabaceae (ca. 22), Myrtaceae (ca. 21), Euphorbiaceae (21), and Poaceae (ca. 22 including Polynesian introductions).  The largest genera are Psychotria with 20 species (Whistler 1986), Cyrtandra with 18 (revised from Gillett 1973), Syzygium with 16 (Whistler 1988a), Elatostema with ca. 14 (Whistler, unpublished data), Dendrobium with 12 (Cribb & Whistler 1996), and Bulbophyllum with 11 (Cribb and Whistler 1996). The rate of endemism is estimated to be about 32% at the species level, but only a single genus, Sarcopygme of the Rubiaceae, is thought to be endemic to Samoa (Whistler, unpublished data).