Location and General Description
Situated between Fiji to the west and Samoa to the northeast, the Kingdom of Tonga comprises a scattered distribution of 170 islands running north to south between 15º to 23ºS latitude and173º to 177ºW longitude. Niue is 400 km east at 19ºS latitude and 169ºW longitude. The Tongan Islands lie over the tectonic zone where the Pacific plate is being subducted under the Asia-Australia plate (Steadman 1993). The uplifting of the Asia-Australia plate in this area is responsible for the raised coral reef and limestone which form the basis for the island groups of Tongatapu, Ha’apai, and Vava’u. Niue is a 259 km2 uplifted coral atoll with cliffs on all sides and no fringing coral reef (Worthy et al. 1998). Islands along a fracture zone from southwest to northeast, including Niuafo’ou, Tafahi, Niuatoputapu, Late, Kao, Tofua, Hunga Ha’apai, Hunga Tonga, and ‘Eua, were created by active volcanoes that continue to erupt on an average of every 4 years (Bryan et al. 1972). 'Eua, however, also is believed to be partly comprised of an ancient fragment of Gondwanaland that has moved close to Tonga after being much nearer to Fiji and New Caledonia millions of years ago (Mueller-Dombois & Fosberg 1998). The youngest volcanic island is probably 1 million year old Niuafo’ou Island in the far north (Rinke 1986a). The climate is tropical with mean daily temperatures ranging from 24º to 30oC in February and from 20º to 26oC in August (Steadman 1998). The islands receive an average annual rainfall of more than 2,000 mm although rainfall increases with elevation up to the 1,000+ m summits of the higher mountains. December to April have greater rainfalls, and infrequent cyclones can bring torrential rains and forest-destroying winds during this time (Sykes 1981). Coastal leeward areas often experience droughts from May through November.
Tropical moist forest is usually subdivided between lowland broadleaf rain forest below 500 m and a subtropical rain forest above 500 m. Lowland broadleaf rain forest, with a canopy to 30 m is dominated by Diospyros spp., Rhus taitensis, Alphitonia zizyphoides, Calophyllum neo-ebudicum, Elattostachys falcata, Litsea mellifera, Pisonia grandis, Pittosporum arborescens, Garuga floribunda, Ficus obliqua, Pleiogynium timoriense, Cryptocarya turbinata, and Maniltoa grandiflora (Sykes 1981, Whistler 1992). Dense areas of staghorn fern (Dicranopteris linearis) often occur in the understory with the shrubs Macropiper puberulum and Psychotria insularum. Mangrove (Rhizophora spp.) forest still exists on many small atoll islands and along lagoon shores of the larger islands. Casuarina equisetifolia dominates communities on newer ash deposits and lava flows, behind beaches, and generally on thin soils, often with Pandanus tectorius, Syzygium dealatum, Hibiscus tiliaceus, and Scaevola taccada (Sykes 1981).
Tonga supports 419 angiosperm plants and fern species with approximately 3 percent endemic, including some spectacular Hibiscus spp. (Whistler 1992). Plant diversity on individual islands ranges from 340 species on Tongatapu and 300 species on ‘Eua, to 145 species on Late, and 107 on Vava’u (Sykes 1981).
The herpetofauna of Tonga, consisting of 20 known species, is considered depauperate. There is one iguanid, nine geckos, nine skinks, and the Pacific boa (Candoia bibroni). The iguanid, the South Pacific banded iguana (Brachylophus fasciatus), is also found in Fiji and is believed to have rafted from the Americas (Allison 1996). The species is Endangered (Hilton-Taylor 2000). The skink Tachygia microlepsis is considered Extinct (Hilton-Taylor 2000).
Tonga, Niue, and Niuafo’ou are considered secondary Endemic Bird Areas because of two island group endemics and a suite of species restricted to central Polynesia (Stattersfield et al. 1998). Tonga currently supports 20 species of land and freshwater birds with one island group endemic, the Tongan whistler (Pachycephala jacquinoti) in the Vava’u Group (Rinke 1986b, Stattersfield et al. 1998). More than 100,000 sooty terns (Sterna juscata) are thought to breed in the volcanic crater on Fonualei Island in the Vava’u Group (Jenkins 1980). Niue has 6 seabird and 12 land bird species, including two endemic subspecies (Kinsky & Yaldwyn 1981). Large seabird colonies are present on uninhabited and rat-free islands, including particularly large numbers of brown noddy (Anous stolidus). The Niuafo’ou megapode (Megapodius pritchardii) is restricted to the island of Niuafo’ou where it buries its eggs in the warm sands near volcanic ducts (Rinke 1986b, Stattersfield et al. 1998). All other species of megapode in Polynesia have been extirpated, and the nearest extant species is in Vanuatu, 1,600 km west (Goth & Vogel 1995). The Phoenix petrel (Pterodroma alba) and bristle-thighed curlew (Numenius tahitiensis), also found in the ecoregion, are considered Vulnerable (Hilton-Taylor 2000).
Archaeological evidence suggests that Tonga and Niue supported a much more spectacular vertebrate fauna before Polynesians arrived approximately 3,000 years ago (Steadman 1989). At least 3 land bird species disappeared from Niue and 23 disappeared from Tonga after Polynesians arrived, including 2 parrot species, 4 pigeons and 2 megapodes. A giant land iguana and several other reptiles and bats have also been found (Steadman 1989, 1993, Worthy et al. 1998).
Only 45 of Tonga’s 170 islands are currently inhabited, but the population of nearly 100,000 is growing rapidly. The population increase has resulted in more forest clearing on settled islands as well as the establishment of crop plantations on many uninhabited islands (Rinke 1986b). The volcanic islands of Late (17 km2) and Tofua (55.4 km2) have some of the best remaining high diversity native forest and still support large populations of birds and reptiles (Steadman 1998).
Types and Severity of Threats
Habitat destruction, poaching for food and feathers, and introduced species are the principal threats to remaining Tongan and Niue biodiversity. Introduced black rats (Rattus rattus) and Norway rats (R. norwegicus) can have catastrophic impacts on breeding seabirds and passerines. They are currently found on most inhabited islands, and preventing their spread to other islands is essential for the protection of bird populations and perhaps some plant populations. A number of native species have been extirpated from most inhabited islands by a combination of poaching and rat predation. Captive breeding of the Niuafo’ou megapode, red shining parrot (Prosopeia tabuensis), and blue-crowned lorikeet (Vini australis) has begun as have efforts to establish populations of these species on uninhabited islands.
There are no national parks in Tonga or Niue. Perhaps the greatest potential for conservation lies in the protection of uninhabited, forested, and predator-free islands that are stocked with threatened flora and fauna from inhabited islands (Rinke 1986b). Paleoecology studies suggest many of the target species once occurred on these refuge islands and this approach may offer the best chance for conservation of many threatened species.
Justification of Ecoregion Delineation
This ecoregion includes Tongatapu, the Vava’u, and Ha’apai Groups, as well as Niue. 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. Tonga, Niue, and Niuafo’ou are delineated as an Endemic Bird Secondary Areas due to the presence of two endemic and a number of restricted-range bird species in those areas (Stattersfield et al. 1998). These have been lumped to form the Tongan Tropical Moist Forest ecoregion.
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Prepared by: Tim Male
Reviewed by: In process