Introduction

New Georgia Island, Solomon Islands. (Photograph by Don Henry)

The Solomon Islands rain forests are true oceanic islands with high vertebrate endemism, including single-island endemics, restricted-range mammals, and an astounding sixty-nine bird species found nowhere else in the world. Large areas of naturally restricted lowlands below 400 meters (m) either have been or are under threat of logging or clearance for subsistence agriculture. Introduced cats have eliminated most native mammals on Guadalcanal.

Location and General Description

This ecoregion consists of tropical lowland and montane forests on Bougainville and Buka Islands in Papua New Guinea (PNG) and most of the island nation of the Solomon Islands (not including the Santa Cruz Group). The climate of the Solomon Island is tropical wet. The islands are predominantly hill forest, although only small portions of a few of the islands extend beyond 1,000 m in elevation. The mountains on Guadalcanal reach past 2,000 m. The Solomons are the result of the subduction of the Australian tectonic plate beneath the Pacific tectonic plate, and the islands are a very active tectonic area. The surface geology of the islands consists predominantly of volcanic rocks, with some metamorphic rocks, uplifted coral islands (Rennell, Bellona, and Ontong Java), and recent (Pliocene to recent) alluvium in the lowlands. The islands increase in age from northwest to southeast.

Mueller-Dombois and Fosberg outlined seven broad natural vegetation types in the ecoregion, including coastal strand vegetation, mangrove forests, freshwater swamp forests, two types of lowland rain forests, seasonally dry forest and grassland (only on Guadalcanal), and [[biomes|montane rain forest. Bougainville also contains floodplain forest, a transitional submontane rain forest, forest on ancient limestone, and vegetation on recent volcanic surfaces.

Coastal strand vegetation consists of mixed Spinifex-Canavalia containing Ipmoea, Spinifex, Canavalia, Thuarea, Cyperus, Scaevola, Hibiscus, Pandanus, Tournefortia, Cerbera, Calophyllum, Barringtonia, Terminalia, and Casuarina. Two types of mangrove vegetation are identified for the Solomons: a low forest dominated by Rhizophora apiculata and a tall forest dominated by Rhizophora spp. and Brugiera spp. Brugiera sexangula, B. parviflora, and Ceriops tagal reach their eastern limits in the Solomons. Most of the islands have large areas of freshwater swamp forest. Easily recognized subunits of freshwater swamp forest include Campnosperma brevipetiolata forests, closed-canopy Terminalia brassi forests, sago swamp (Metroxlyon solomonense), low-canopy Pandanus spp., and mixed swamp forest.

The most widespread vegetation type is lowland rain forest. The canopy is uneven as a result of frequent natural disturbance (tropical storms, landslips, treefalls). The twelve most common tree species are Calophylum kajewskii, C. vitiense, Dillenia salomonensis, Elaeocarpus sphaericus, Endospermum medullosum, Parinari salomonensis, Maranthes corymbosa, Pometia pinnata, Gmelina mollucana, Schizomeria serrata, Terminalia calamansanai, and Campnosperma brevipetiolata. Whitmore recognized six lowland rain forest types, distinguished by whether the forest was on the northern or western side of the islands, elevation, and level of disturbance. Two groups of low-diversity lowland rain forest are recognized. The first group consists of monodominant forests of Campnosperma brevipetiolata (Santa Isabel, New Georgia, Choisel) or those with co-dominant C. brevipetiolata and Dillenia or C. brevipetiolata, Pometia pinnata, and Teysmanniodendron (Verbenaceae). It is thought that these forest types are related to disturbance. The second group of low-diversity forests is associated unusual soils, including limestone (Vitex cofassus and Pometia pinnata), flooding (Pterocarpus indicus and Terminalia brassi), or ultramafic soils (Casuarina papuana, Dillenia crenata, Syzygium, or Dacrydium).

Seasonally dry forest is found only on the leeward (north) side of Guadalcanal. These forests consist of mixed deciduous forest and Themeda australis grassland. The canopy is composed of Pometia pinnata, Vitex cofassus, and Kleinhovia hospita. The deciduous species are Pterocarpus indicus, Antiais toxicaria (Moraceae), Ficus spp., and Sterculia spp. The grassland probably is related to periodic burning by humans.

The Fagaceae species that generally mark montane rain forest in the region (Castanopsis, Nothofagus, and Lithocarpus) are absent in the Solomons. Instead, a reduction in stature (from 25 to 35 m in the lowlands to 15 to 20 m in the uplands) is apparent. Syzygium, Metrosideros, Ardisia, Psychotria, Schefflera, Ficus, Rhododendron, Dacrydium, and Podcarpus pilgeri have been collected in the mountains of the Solomons.

The outlying coral atolls support depleted lowland rain forest, remnant coastal and swamp vegetation, and Pandanus thickets. The vegetation is a product of generally poor soils combined with human alteration.

Biodiversity Features

Overall richness and endemism in the Solomon Islands range from low to high when compared with those of other ecoregions in Indo-Malaysia. Bird and mammal endemism are high.

There is a clear difference between the mammalian faunas of the Solomon Islands and the Bismarck Archipelago and richer New Guinea to the west. Except for pteropodid bats, the Solomons and Bismarcks have many fewer mammals than New Guinea, and the Solomons, unlike New Britain, contain no marsupials. East beyond the Solomons there are even fewer mammal species. Almost all the mammal species have their origins in or via New Guinea.

Although the Solomon Islands contain only forty-seven mammal species, a remarkable twenty-six of those species are endemic or near endemic, including nine murid rodents (Melomys, Solomys, Uromys), fifteen pteropodid bats (Dobsonia, Melonycteris, Nyctimene, Pteralopex, Pteropus), a horseshoe bat (Anthops), and one molossid bat (Chaerephon)(Table 1). Three of the fruit bats-Bougainville monkey-faced bat (Pteralopex ancep), Guadalcanal monkey-faced bat (Pteralopex atrata), and montane monkey-faced bat (Pteralopex pulchra) are critically endangered, and three of the rodents-Specht's mosaic-tailed rat (Melomys spechti), Poncelet's giant rat (Solomys ponceleti), and emperor rat (Uromys imperator) are endangered.

Bird diversity drops off sharply from New Guinea as one moves east across the Pacific to the Solomons. Whereas New Guinea has seventy-one families and subfamilies of birds, the Solomons have forty-four. The Solomons are considered a center of bird endemism, with at least seven endemic genera. The dropoff in diversity seen in other animal groups as one moves east from New Guinea is also consistent with that seen in birds. Whereas New Guinea has seventy-one families and subfamilies of birds, and the Solomons have forty-four, Vanuatu has thirty-one.

A total of 199 bird species inhabit the Solomons. The Solomon Islands ecoregion has an almost exact correspondence with the Solomon group Endemic Bird Area (EBA). This EBA contains more restricted-range bird species (seventy-eight) than any other EBA. Several of the islands, especially Makira (San Cristobal) and the New Georgia group, have their own endemic species and would qualify as important EBAs by themselves. The unique islands of Rennell and Bellona, separated from the rest of the Solomons by a submarine trench, are also an EBA, containing a total of twelve endemic species, and seven additional species. The atolls of Ontong Java, which are also part of this ecoregion, qualify as a Secondary Area because they provide habitat for an additional species, the atoll starling (Aplonis feadensis), which is also found off of small islands in the Bismarck Archipelago. Of these ninety-one restricted-range bird species, an incredible sixty-nine species are found nowhere else in the world; thus the Solomons are a global priority for bird conservation. Ninety species are endemic or near endemic (Table 2). Three bird species are critically endangered: Makira moorhen (Gallinula silvestris), yellow-legged pigeon (Columba pallidiceps), and thick-billed ground-dove (Gallicolumba salamonis). Four additional bird species are endangered: imitator sparrowhawk (Accipiter imitator), Woodford's rail (Nesoclopeus woodfordi), chestnut-bellied imperial pigeon (Ducula rubricera), and white-eyed starling (Aplonis brunneicapilla). The Choisel pigeon (Microgoura meeki) was last reliably seen in 1904 and is presumed extinct.

Buka, Bougainville, and the rest of the Solomon Islands (excluding the Santa Cruz Group) form a distinct and rather uniform phytogeographic unit. About a third of the Solomon Islands' flora is of Malesian (southeast Asian) origin, a third has Paleotropical origins, and a third is cosmopolitan, with a small Pacific contribution. There is a distinct break in floristic compositions with the nearby Bismarck Archipelago to the west, corresponding with the New Britain Trench that separates the submarine platforms. Two important Indo-Malayan tree families, Fagacae and Dipterocarpaceae, are not present in the Solomons. There are only about a dozen common tree species in the Solomons.

Between November and April of each year the Solomon Islands are subject to tropical cyclones, which are an important source of natural disturbance to the islands' forests. Extreme droughts are also a natural event and occur irregularly at intervals of six to twenty years.

Davis et al. identified two Centres of Plant Diversity on Bougainville Island: Mount Balbi to southern coast, containing the largest stands of bamboo forest in Papuasia and remnant stands of Terminalia brassii, and Mount Takuan-Tonolei Harbour, containing natural stands of Terminalia brassii and more than 1,000 vascular plant species.

Current Status

A large Australian-run copper mine was located in Bougainville, but it was shut down because of civil unrest several years ago. Introduced species are a special concern here, and most native mammals have been eliminated from Guadalcanal by cats. Hunting native species is common. Many bird species in the Solomons are vulnerable simply because of their small natural ranges.

Only one protected area, 930 square-kilometers (km²) surrounding Mount Balbi on Bougainville, exists in the ecoregion (Table 3). A gap analysis, based on detailed vegetation and habitat type mapping, has never been performed to determine whether the existing protected area network adequately covers all habitats with protected areas that are large enough to maintain all critical ecological processes.

Types and Severity of Threats

Large areas of the naturally limited natural forest below 400 m have been logged or are planned to be logged. An adequate survey of timber resources has not been conducted.

Forest clearing for subsistence agriculture is an ongoing threat. Most households are self-sufficient (seven out of eight), and because population growth is high there is pressure to clear land. This is especially true around urban areas because the population is mobile and many people move to the outskirts of overcrowded urban centers. Satellite imagery indicates that the area under cultivation doubled between 1972 and 1992.

Justification of Ecoregion Delineation

Distinct island groups were placed in their own ecoregions: the Solomon Islands rain forests and the Vanuatu rain forests. We followed Stattersfield et al. in delineating these ecoregions. MacKinnon did not extend his assessment beyond the island of Bougainville in the Solomon Islands. However, we followed Bouchet et al. and separated the distinctive dry forests in New Caledonia from the moist forests to delineate the New Caledonia rain forests and the New Caledonia dry forests. Stattersfield et al. did not show this distinction.

Udvardy placed all the ecoregions in the New Guinea and Melanesia bioregion, with the exception of New Caledonia, into the Papuan biogeographic province of the Oceanian Realm. New Caledonia was placed in the New Caledonian biogeographic province.

Additional Information on this Ecoregion

• For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
• To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
• World Wildlife Fund Homepage