South Pacific Ocean

• Category: Marine and Oceanic Biomes.
• Geographic Location: Southern Hemisphere.
• Summary: Amid this vast oceanic biome, island ecosystems are fragmented by separation, and threatened by climate change.

The Pacific Ocean is the largest geographic feature on the Earth’s surface, with an area greater than all seven continents combined. The South Pacific is dotted with islands due to its geological structure; the divergent plate boundary of the East Pacific Rise is the formative source of the Pacific tectonic plate. In its westward movement, the plate passes over a number of volcanic hot spots; these formed the eastern Pacific volcanic island chains of Pitcairn, French Polynesia, the Cook Islands, and the Samoan island group. The Pacific plate subducts beneath the Australian plate to the east of the main islands of Tonga, Fiji, and Vanuatu, and north of Solomon Islands and Papua New Guinea, causing larger and less isolated islands to the west.

The South Pacific region includes sixteen countries and territories comprising over 8.6 million square miles (22.3 million square kilometers) of ocean area, and less than 232,000 square miles (600,000 square kilometers) of land, with 84 percent of the total land in Papua New Guinea. Most of the remaining land area is in the other larger Melanesian islands of the Solomons, Vanuatu, Fiji, and New Caledonia, with the island size generally declining from west to east. There are approximately 1,735 islands in total. Land is a primary control on ecosystem diversity, and physical conditions of geomorphology and tropical climates exert controls on habitats and ecosystems.

South Pacific ecosystems are biogeographically fragmented by physical separation of habitats across these islands, and controlled by island type: whether the island groups are characterized by high topography, limestone formation, or coral atoll structure. Of these major island groups, the high type is found mainly in Papua New Guinea, Solomon Islands, New Caledonia, Vanuatu, Fiji, Wallis and Futuna, Samoa and American Samoa, and French Polynesia. The atoll types include Tuvalu, Kiribati, and Tokelau. Nauru and Niue are limestone types; Tonga is mixed high and limestone type; and the Cook Islands are mixed high and atoll type.

Ecosystem diversity is at a maximum in Papua New Guinea to the west, and decreases toward the east as island sizes become smaller, and distances between islands become greater. Endemic species (those found nowhere else) occur particularly in terrestrial ecosystems, such as freshwater habitats, where the greatest threats are from introduced weed species. In coastal ecosystems, the greatest threats are also from direct human impacts, but becoming increasingly impacted by climate change and sea-level rise.

Papua New Guinea, Solomon Islands, New Caledonia, Vanuatu, Fiji, Wallis and Futuna, and Tonga are all west of the tectonic margin line, which influences the biogeography of island ecosystems, as larger islands to the west are older continental crust. The three geologically old, large islands of New Guinea, New Caledonia, and Viti Levu (largest of the Fiji Islands) form three diverse biogeographic sub-regions in the Pacific Islands, from which species have migrated to other islands. Southern New Guinea and New Caledonia are Gondwanan fragments, while the Fijian large islands were formed from uplifted crust over 40 million years ago. Other islands here were uplifted coral or formed volcanically in association with the subduction margin.

Island groups to the east include Nauru, Tuvalu, Kiribati, Tokelau, the Samoas, Niue, Cook Islands, and French Polynesia; these are “true islands,” mainly atoll types. This has a great influence on ecosystem biodiversity, with these being low in indigenous species, having only secured species adapted to long-distance oceanic migration.

The region's ecosystems are biogeographically fragmented by physical separation of habitats within the coastal biomes such as coral reefs, mangroves, seagrasses, and even within freshwater wetlands. South Pacific ecosystems have many analogous environmental settings, but due to the filter effect of oceanic separation, the species present in ecosystems all differ between countries (see Table 1). This results from two factors: first, the center for species diversity within all groups is on the margins of the western edge of the region; and second, island size tends to decrease and island isolation tends to increase toward the east and north.

Ecosystem habitats on islands depend on the island type. While volcanic islands can host crater lakes and river catchments, they start to erode and submerge over time, developing coastal plains, mangroves, and freshwater swamps. Fringing reefs become barrier reefs with further island submergence, increasing area and habitat variety in reef environments and introducing lagoons and seagrass habitats. Further subsidence leads to atolls, losing all freshwater wetlands—but increasing reefs and lagoons.

Four Biological Groups

The biological diversity of corals, reef fish, mangroves, and seagrasses of the region result from their biogeographic history. The Indo-Malayan archipelago to the west of the region is the center of greatest species diversity for all four groups. It is where they first appeared in the ancient Tethys Sea at equatorial latitudes. Species richness declines from this center point toward the east.

Coral reefs are the most extensive ecosystem type in the South Pacific island region, occurring offshore of nearly all coastlines that lack turbid river discharge. New Caledonia has the second-longest barrier reef in the world at an approximate length of 1,000 miles (about 1,600 kilometers), which surrounds the main island and encloses a lagoon of 9,300 square miles (about 24,000 square kilometers). Papua New Guinea has approximately 8,600 square miles (13,840 square kilometers) of coral reef, and some countries in Micronesia and Polynesia are almost entirely composed of coral atolls.

There are 5,000 square miles (13,000 square kilometers) of coral reefs in just the countries east of Samoa, compared with 2,300 square miles (6,000 square kilometers) of land. Coral reefs and freshwater mollusks are the best-studied wetland invertebrate groups in the Pacific island region; research on other groups has pointed to the need for baseline surveys and inventories. Some of the better distributional records of groups are summarized in Table 1 below.

Coral GeneraShorefish FamiliesStrombus Species (Conch)Seagrass SpeciesMangrove SpeciesBird Species (Endemics)Freshwater FishAmphibians and ReptilesPapua New Guinea70149221231168 (48)349472Solomon Islands6020917159 (59)37New Caledonia6012520111575 (20)105Vanuatu60121456 (7)7549Fiji60116146757 (22)6429Wallis & Futuna21121Tonga404721 (2)320Nauru23 (1)2Tuvalu1221Kiribati1423Tokelau1Samoa5010294333 (10)3115American Samoa5010294333 (10)2115Niue1112Cook Islands30830012 (6)6French Polynesia304771118 (11)44

Fiji’s marine environment is better studied than most other Pacific Island countries, and is facilitated by a marine research and training capability at the University of the South Pacific. In 2000, Fiji’s reefs suffered an extensive mass bleaching event, causing loss of around 80 percent of its stony corals. Following the event, the Fiji government initiated marine resource management efforts to regenerate their coral reef ecosystem. They collaborated with the University of the South Pacific, the Fiji Locally-Managed Marine Areas program, and Reef Explorer to develop and implement management plans emphasizing no-take marine protected areas, fisheries compliance, pollution reduction, and adaptive monitoring. These efforts resulted in significant reef recovery, with marine protected areas showing 500 percent more live coral cover and 50 percent greater coral species richness than adjacent fished areas.

Coral restoration efforts included cultivating heat-tolerant coral species in nurseries using rope and cement disc techniques. By 2019, more than 50 coral species had been propagated, focusing on fast-growing, thermally resilient lineages. Over 50,000 corals were transplanted back to degraded reef areas, enhancing coral cover and ecosystem resilience. Community youth were trained in propagation techniques and employed in restoration and marine ecotourism, such as snorkeling tours.

These initiatives revitalized the reef by fostering economic opportunities, strengthening local marine conservation capacity, and raising community awareness surrounding climate impacts.

As a result of mass bleaching events such as the one in Fiji, reef monitoring commenced to allow ongoing surveillance of coral reef biodiversity and condition, and provide better understanding of natural and human impacts. In some coastal areas, poor inshore water quality with nutrients, suspended sediment, and trace metals have been found to be affecting inshore reefs. American Samoan reefs are well studied and a detailed inventory is available, thanks to United States government research. The reefs of Tonga and Vanuatu are little studied compared with New Caledonia or Fiji, and other islands, such as Niue, are studied even less.

The diversity of reef-building corals in the equatorial western Pacific in Papua New Guinea is close to the highest in the world. Diversity declines to the north and the south with cooler temperatures, and from west to east across the Pacific as distance increases.

Similar regional patterns occur in the shallow water finfish, and nearshore shallow water conches, as seen in Table 1. The finfish families, which include 461 genera and approximately 1,312 species, are important sources of protein for the indigenous peoples of the South Pacific region. Reef and shore mollusks also decline in diversity from west to east across the region; the type of species present are dependent on habitat diversity. For example, different molluscan assemblages live on reef slopes, outer lagoons, and inner lagoons. Coastal sponges, polychaetes, and crustaceans also decline in diversity from west to east; however, the records of all these groups are incomplete and more research is needed.

Six of the seven known marine turtles are found in the South Pacific region; most common are the green turtle, hawksbill, and leatherback. Of these, the hawksbill is classified as critically endangered, and the green and leatherback turtles are endangered. Less common are the loggerhead, flatback, and olive ridley, of which the olive ridley is endangered, and the loggerhead and flatback turtle are vulnerable.

Marine turtles are slow to reach maturity, and migrate long distances between nesting and feeding areas. Individuals tracked from a nest at Rose Atoll, American Samoa, moved to the rich seagrass and algae beds of Fiji, spending 90 percent of their time there. Negative impacts to turtles include bycatch from long-line fisheries, over-exploitation by local communities, and human hunting when the turtles are nesting on beaches.

Mangroves occur on intertidal habitats of sheltered shorelines, which are most extensive at sedimentary estuaries. In the Pacific Islands region, the total mangrove area is about 2,196 square miles (5,687 square kilometers), with the largest areas in Papua New Guinea, Solomon Islands, Fiji, and New Caledonia. The species mixture is unique in each country or territory (see Table 1), and the mangroves provide useful ecosystem services. There are thirty-one species of mangroves of the Indo-Malayan assemblage, with highest global biodiversity in southern Papua New Guinea; diversity declines from west to east across the Pacific, reaching a natural limit in Samoa, with a possibly introduced outlier in French Polynesia.

Seagrasses occur mostly in shallow lagoons between coral reefs and shoreline mangroves in the South Pacific, with a close proximity and connectivity of these wetland types typical of island shorelines. Seagrasses evolved from terrestrial grass in the Cretaceous Tethys Sea, and spread into the Pacific. The center of biodiversity for seagrass is New Guinea, with up to thirteen species; two families and seven genera extend across the South Pacific, declining in diversity with distance (as seen in Table 1), except H. ovalis sp. bullosa, which is endemic to Fiji, Tonga, and Samoa. In the eastern Pacific, seagrass is absent from the Cook Islands, and only one species is recorded in French Polynesia. The fruits of most seagrass species are not buoyant, hence reducing their ability to disperse over long distances.

Land-Based Fauna

The tropical Pacific terrestrial mammal fauna is immensely depauperate compared with elsewhere in the world, due to island isolation and lack of landbridges, and is dominated by bats. No mammals are present in French Polynesia, Tuvalu, Kiribati, the Marshalls, or Tokelau, and one bat species is the only indigenous mammal species present in the Cook Islands and Niue. This most widespread species is the Pacific flying fox Pteropus tonganus, which roosts in tall coastal trees.

Pacific island bird distributions are shown in Table 1, which indicates that endemism is enhanced by isolation from other land masses and speciation encouraged. These totals are of bird species resident on islands recorded by Adler in 1992, although many flightless or ground nesting species became extinct as a result of human impact before that. Losses have included about 2,000 species of flightless rails, as well as petrels, shearwaters, terns, and kingfishers.

However, unusual species remain on remote islands. For instance, the endangered endemic Niuafo’ou megapode is found on the volcanic island of Niuafo’ou in Tonga. The Niuafo’ou megapode incubates its eggs by burying them in warm sands near volcanic ducts. Chicks and eggs have been released on the predator-free Late and Fonualei Tongan islands.

The estuarine crocodile occurs in the South Pacific, being fairly common in Papua New Guinea and extending to the Solomon Islands and Vanuatu, which is the limit of its Pacific range. The Solomon Islands once had substantial populations, but this animal has been decimated by human impact and cyclone damage in the twenty-first century. The crocodile is currently categorized as vulnerable in the South Pacific region.

Numbers of amphibian and reptile species decrease rapidly with distance to the east of New Guinea, also due to decrease in island size, proximity of other islands, and island age. Amphibians are intolerant of salt water and therefore have difficulty migrating in this region, a factor also influencing the prawns, shrimps, and freshwater fish (Table 1). As of 2019, studies of freshwater fish species on the large islands in Fiji found a total of sixty-four fish species from nineteen families.

Vulnerability

Pacific island ecosystems are vulnerable because of their relatively small size, poor state of protection or available knowledge, and their proximity to rapidly increasing human populations that put heavy demands on these natural resources to support their economic development. Weeds and introduced species are a great threat to these special biodiversity groups, particularly freshwater ecosystems. Climate change, sea-level rise, and cyclone damage are increasing vulnerability, particularly of coastal ecosystems. Corals are susceptible to bleaching as the temperature of ocean waters increase and reefs damaged by more-frequent extreme weather is often slow to recover. Gaps in ecosystem management in the region include weak or vague legislation for their protection in many countries, and the need for more knowledge and capacity to manage.

Wildlife, notably marine life, is at risk from pollution. Plastic in particular has become an increasing concern. An area called the South Pacific Gyre east of Australia in 2017 was confirmed to contain a garbage patch, called the Great Pacific Garbage Patch, comprised mostly of plastic particles. Microplastics are of concern because they are consumed by microorganisms and rise through the food chain. Microplastics have been found in the human body. In some cases the shores of ininhabited islands are teeming with plastic from various continents. In 2024, The Ocean Cleanup project achieved remarkable progress in tackling ocean plastic pollution, removing 11.5 million kilograms of plastic from oceans and rivers—more than the total collected in all previous years combined. Efforts included scaling operations in the Great Pacific Garbage Patch and expanding river cleanup initiatives worldwide, bringing environmental stakeholders closer to addressing this urgent environmental issue.

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