Location and General Description
The Society Islands comprise a chain of high volcanic islands extending from 16º to 18ºS latitude and 148º to 154ºW longitude. Of the 14 islands in the group, 9 are high dormant volcanoes while the remainder are atolls with volcanic substrata deep beneath (Mueller-Dombois & Fosberg 1998). Soils are derived from basalt except in areas with uplifted reefs and on atolls where soils are limestone-derived. The Society Islands range in age from Mehetia which is less than 1 million years old, to Moorea at 1.5 million to 2 million years old, to the leeward atolls which are at least 4.5 million years old (Meyer 1996). The maximum elevation of 2,241 m is on the largest island, 1,042 km2 Tahiti. The climate is tropical with a mean annual temperature of 26ºC. December through February is wetter with torrential rains, and March through November is drier and cooler (Meyer 1996). Annual rainfall ranges from 1,700 mm near sea level to at least 8,000 mm on mountain peaks (Mueller-Dombois & Fosberg 1998). Easterly trade winds are consistent throughout half the year, and hurricanes are uncommon.
Tropical moist forest occurs in three general forms: lowland rain forest, montane rain forest, and cloud forest (Fosberg 1992). Lowland forest remains only in valleys and on scattered low mountain slopes where Inocarpus fagifer, bamboo (Schizostachyum glaucifolium), Cananga odorata, Rhus taitensis, Pisonia umbellifera, introduced Aleurites moluccana, Macaranga sp., Glochidion sp., Hibiscus tiliaceus, and Tarenna sambucina are the main tree species. Medium stature montane rain forest is found on slopes above 300 m and on less wet western slopes of high islands. Common species include Alphitonia zizyphoides, Hernandia moerenhoutiana, Metrosideros collina, Fagraea sp., Canthium spp., and Wikstroemia sp. (Fosberg 1992). Epiphyte-laden cloud forest occurs between 400 to 1,000 m where mean annual temperature ranges from 14º to 18oC and photosynthesis is restricted by continual cloud cover (Mueller-Dombois & Fosberg 1998). Weinmannia parviflora and Alstonia costata are the dominant tree species, but Cyathea spp. tree ferns, Fitchia spp., Myrsine spp., Fuchusa spp., Sclerotheca spp., Cyrtandra spp., Metrosideros collina, Coprosma spp., and Psychotria spp. are also common (Fosberg 1992).
Isolated in the middle of the Pacific Ocean and evolving on its own for millions of years, the fauna of the Society Island is perhaps best known for its spectacular radiation of 53 Partula land snail species thought to have derived from a single colonization event (Cowie 1992). Partula species were once found on almost all the high islands in every habitat, their beautiful shells hidden among the canopy foliage.
About 200 years ago, bird communities in moist forest included at least 14 land bird species, including 6 endemic species and 5 regionally endemic species (Pratt et al. 1987, Stattersfield et al. 1998). The islands support a vascular flora of 623 native species including 273 endemic species and one endemic genus (Florence 1987). Notable plant species include tree ferns (Angiopteris evecta) with 9 m wide canopies in Moorean valleys and a scandent cliff-dwelling Metrosideros sp. (Mueller-Dombois & Fosberg 1998).
Since the arrival of Polynesians 1,000 to 2,000 years ago, humans have had a massive impact on the flora and fauna of the Society Islands (Lepofsky et al. 1996). These islands are among the most altered, with lowland and atoll habitats largely replaced by coconut groves (Stattersfield et al. 1998). Archeological sites have yielded bones of 10 seabird species, including an extinct Larus gull, and 14 land bird species no longer present in the Society Islands including 2 parrots (Vini vidivici and V. sinotoi), 3 pigeons, and a starling (Aplonis sp.) found nowhere else (Steadman 1989a, 1989b). Additionally, two species of Prosobonia sandpiper, 3 parrots, and 1 pigeon have become extinct since European arrival (Pratt et al. 1987, Walters 1991). Predation by introduced rats (Rattus rattus, R. norwegicus, and R. exulans) is the most probable cause of many of these extinctions (Seitre & Seitre 1992).
A number of extant bird species are considered endangered including the green-backed heron (Butorides striatus patruelis), Society Islands pigeon (Ducula aurorae), Tahiti reed warbler (Acrocephalus caffer), Tahiti swiftlet (Aerodramus leucophaeus), and the Tahiti monarch (Pomarea nigra) (Dahl 1980, Thibault 1988, Seitre & Seitre 1992). Only the plants Apetahia raiateensis and Kadua sp., endemic to the central plateau of Raiatea, are considered endangered (Dahl 1980), but many more species are probably highly restricted in their distribution.
Humans have also directly impacted forests by clearing almost all areas below 500 m and burning areas frequently. This causes a transition to fire-tolerant savanna and grassland on western slopes and lowland areas (Fosberg 1992). The deliberate introduction of predatory snails (Euglandina rosea and Gonaxis spp.) has resulted in the extinction or endangerment of almost all 15 endemic land snails (Samoana and Partula spp.) in the wild on Moorea and Tahiti (Cowie 1992). The only formal reserves in the Society Islands is the 10 km2 conservation area on Mt. Mara’u, Tahiti and the Fa’aiti Natural Park on Tahiti (Dahl 1980). Other areas in need of protection are lowland Inocarpus forest on Moorea, vallée de Vaiote on Tahiti, the island of Manuae, the vallée d’Avera and montane forest of Raiatea, lowland forests of Oponohu on Moorea, and seabird colonies in Tetiaroa, Fenuaura, Motuone, and Mopiihaa Atolls (Dahl 1980, Thibault 1988).
Types and Severity of Threats
Currently the greatest threat to the forests of the Society Islands is that they will all be replaced with monotypic stands of the neotropical melastome Miconia calvescens. This species, introduced in 1937, already covers 65 percent (700 km2) of Tahiti with additional populations on Moorea and Raiatea (Meyer 1996). The Miconia tree grows up to 15 m high and forms a dense canopy that shades out all but its own seedlings (Meyer 1996). The seeds are dispersed by birds, and only continuous manual control has slowed Miconia’s spread on smaller islands. It is hoped that the introduction of biological control agents from its native range might reduce its rate of spread and level of dominance (Meyer 1996). Other problematic invasive species include coffee (Coffea arabica), guava (Psidium guajava), Lantana camara, mango (Mangifera indica), African tulip tree (Spathodea campanulata), and Cecropia palmate. In total, more than 1,500 introduced plant species are present (Meyer 1996).
Justification of Ecoregion Delineation
This ecoregion includes the Society Islands of Motu One, Manuae, Maupihaa, Bora-Bora, Huahine, Raiatea, Maiao, Moorea, Mehetia, and Tahiti). Allison treats the Cooks, Societies, Tuamotus, and Marquesas as a unit herpetologically as they share a similar reptile assemblage. Van Balgooy similarly lumps the Cooks, Niue, Societies, Tuamotus, Tubaui, and Marquesas based on floristic affinities. However, Birdlife International (Stattersfield et al. 1998) separates the Societies from the neighboring island groups on the basis of 5 extant and 2 extinct endemic bird species. We have followed Birdlife’s lead and delineated the Society Islands as a distinct ecoregion.
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Prepared by: Tim Male
Reviewed by: In process