The Society Islands tropical moist forests is a tropical and subtropical moist broadleaf forests ecoregion in the Society Islands of French Polynesia.
Geography The Society Islands is a group of 14 islands, including nine high dormant volcanoes and five low coral atolls. They extend from 16º to 18º S latitude and from 148º to 154º W longitude. The islands comprise two groups, the Windward Islands (French: Îles du Vent) to the east and the Leeward Islands (French: Îles Sous-le-vent) to the west. Tahiti in the Windward group is the largest of the Society Islands, with an area of 1,042 km2. The highest elevation in the islands (2,241 m) is also on Tahiti. The islands were formed as the Pacific Plate moved slowly west-northwest over a volcanic hotspot. The islands increase in age from east to west. Mehetia at the eastern end of the chain is the youngest at less than 1 million years old, while Moorea is 1.5 million to 2 million years old. The western Leeward islands are at least 4.5 million years old, and have eroded and subsided to leave low coral atolls. The high islands' soils are mostly derived from basalt. Coralline limestone soils occur in coastal areas where ancient reefs were uplifted by geologic processes, and atoll soils are mostly of coral sand.
Climate The climate is humid and tropical. Average annual rainfall ranges from 1,700 mm near sea level to 8,000 mm or greater on windward mountain slopes. The summer months of December through February are the rainiest months, while March through November are typically cooler and drier. Easterly trade winds are consistent through much the year, and the eastern or windward sides of islands generally receive more rainfall. Mean annual temperature is 26º C.
Flora The natural vegetation is principally tropical rain forest, and includes lowland rain forests, montane rain forests, and cloud forests. Lowland rain forest was once the most widespread forest type, but has been cleared from most of the coastal lowlands and is now found in valleys and on lower mountain slopes. Typical tree species include Inocarpus fagifer, Cananga odorata, Rhus taitensis, Pisonia umbellifera, Macaranga sp., Glochidion sp., Hibiscus tiliaceus, Tarenna sambucina, and the introduced Aleurites moluccana, along with the bamboo Schizostachyum glaucifolium. Montane rain forests occur above 300 m, and extend to higher elevations on the drier western slopes. The characteristic trees include Alphitonia zizyphoides, Hernandia moerenhoutiana, Metrosideros collina, Fagraea sp., Canthium spp., and Wikstroemia sp. Cloud forests occur between 400 and 1000 meters elevation, generally on the wetter eastern (windward) slopes. The easterly winds cool as they ascend the mountain slopes, and moisture condenses as rain and nearly continual cloud cover. Average annual temperatures are much cooler than in the lowlands, ranging from 14 to 18 °C. Pterophylla parviflora and Alstonia costata are the dominant tree species. Other common trees and shrubs include tree ferns (Cyathea spp.) Fitchia spp., Myrsine spp., Fuchsia cyrtandroides, Sclerotheca spp., Cyrtandra spp., Metrosideros collina, Coprosma spp., and Psychotria spp. The trees are covered with epiphytic plants and lichens. The tree fern Angiopteris evecta on Moorea forms a canopy up to 9 meters wide. The islands are home to 623 native species of vascular plants, including 273 endemic species and one endemic genus. The trees Myrsine raiateensis and Pterophylla raiateensis are endemic to Raitaea. The Raiatean endemics Sclerotheca raiateensis and Kadua raiateensis are considered endangered.
Fauna The ancestors of the islands' native fauna arrived via long-distance dispersal from other islands, and evolved into distinct forms over millions of years. BirdLife International designates the Society Islands as an endemic bird area based on the islands' five surviving endemic species and several extinct ones. Three species recorded on Captain Cook's 1773 visit, the Tahiti rail (Gallirallus pacificus) from Tahiti, the Tahitian sandpiper (Prosobonia leucoptera) from Moorea and Tahiti, and the Raiatea parakeet (Cyanoramphus ulietanus) from Raiatea are now extinct. Other extinct endemic species include the black-fronted parakeet (Cyanoramphus zealandicus) and Maupiti monarch (Pomarea pomarea). Endangered native species include an endemic subspecies of striated heron (Butorides striatus patruelis), the Society Islands pigeon (Ducula aurorae), Tahiti reed warbler (Acrocephalus caffer), Tahiti swiftlet (Aerodramus leucophaeus), and Tahiti monarch (Pomarea nigra). Land snails in the genera Partula and Samoana are part of the islands' unique biodiversity. From ancestors that are thought to have to have arrived in a single colonization event, the Partula land snails evolved into 53 distinct species. They were once found throughout the high islands. The native land snails on Tahiti and Moorea have been decimated by introduced predatory land snails Euglandina rosea and Gonaxis spp., and 15 species are now extinct or endangered.
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