Eastern Micronesia tropical moist forests

This archipelago of low coral limestone islands and atolls, the scene of bloody conflict during WWII, contains relatively low floral and faunal diversity due to their small size, low habitat diversity, and harsh saline environment. Human development has altered much of the natural habitat of these islands. Several islands were used as nuclear weapons test sites, while others, which have favorable soil enriched by seabird guano, have been cleared for coconut plantations.

  • Scientific Code
  • Ecoregion Category
  • Size
    200 square miles
  • Status
  • Habitats

Location and General Description
This ecoregion comprises Wake Island, the Marshall Islands as well as the Gilbert Islands group and Nauru. The influence of centuries of human habitation, coconut planting, and violence associated with WWII on the existing vegetation should not be underestimated.

Wake is a small, isolated atoll between the Marshalls and the Northwest Hawaiian Islands. There are three vegetation types: a Tournefortia scrub forest with Cordia and Pisonia, a Lepturus grass cover with Tribulus cistoides and Portulaca lutea, and a Pemphis scrub margins on the lagoon side of the atoll (Herbst 1994).

The Marshall Islands archipelago is made up of two principal chains: the western Ralik, or "sunset" chain (from Eniwetok to Ebon), and the eastern Ratak, or "sunrise" chain (Taongi and Rongelap to Mili). The Marshall Islands are all low coral limestone atolls and carbonate sand islands, with a maximum elevation of 10 m on the island of Likiep. Although the archipelago consists of 30 atolls and more than 1,000 islands, and extends for roughly 1,300 km from east to west and 1,150 km north to south, the total land area is approximately the same as that of Washington, DC (Mueller-Dombois & Fosberg 1998).

The Marshall Islands border the typhoon belt, and have a wet season from May through November. There is also a north-south climatic gradient, and the climate ranges from semi-arid in the north to very humid in the south. This climatic variation is reflected in the vegetation of the islands. With the exception of the two northernmost islands, Taongi (also known as Pokak) and Bikar, the vegetation of the islands is very similar. Taongi contains the least disturbed vegetation in the Marshalls, and includes only 9 species of vascular plants (Mueller-Dombois & Fosberg 1998).

Often surviving as a windbreak, mixed broadleaf forest is the most common vegetation type in undisturbed areas of the Marshall Islands. Of low to medium stature with a closed canopy, this forest type contains only a small number of tree species, such as Tournefortia argentea, Guettarda speciosa, Pisonia grandis, Pandanus tectorius, Allophylus timoriensis, Cordia subcordata, Hernandia nymphaeifolia, and Thespesia populnea. There are also a few shrubs, and an herb layer with low species diversity that may include Lepturus repens, Thuarea involuta, Fimbristylis cymosa, and Polypodium scolopendria. A few species of epiphytes are also found in the wetter southern areas.

In the interior parts of the islands where forest still exists, there are remnants of almost pure stands of tall, clear-trunked Neisosperma oppositifolium, which possibly constitute a final successional stage. The dense canopy of N. oppositifolium creates a heavy shade, where few other species can survive. Today, these forest remnants are found only in the relatively moist northern atolls (Thomas et al. 1989).

Another monodominant community in the Marshalls is the Pisonia grandis forest, which was formerly very common and widespread throughout the Indo-Pacific. Up to 30 m tall, with trunks more than 2 m in diameter, P. grandis has a smooth pale trunk, and a soft, brittle wood. There is little or no understory or herb layer in the Pisonia forest, and the ground is covered with a thick brown spongy humus layer of semi-decomposed leaf litter. The trees are a favored roosting and nesting site for several species of seabirds, and the droppings from these birds causes a phosphate hardpan layer to develop under the humus in some of these areas. Other monodominant communities of Tournefortia argentea, Suriana maritma, and Pemphis acidula also occur in the Marshalls (Mueller-Dombois & Fosberg 1998).

The Gilbert Islands (Kiribati) also experiences a pronounced precipitation gradient, from et (3000 mm annually) to dry (1000 mm annually), as one moves from north to south. These islands are heavily overpopulated, with no large areas of unoccupied land – in fact, most of the vegetation consists of food plants. The main trees of the original forest were likely Pisonia, Hernandia, and Neisosperma (Mueller-Dombois & Fosberg 1998).

Nauru is well-known for its extensive phosphate deposits accrued over the millenia from seabird guano. These deposits have been heavily exploited. It is an elevated limestone island, with a highest elevation of 70 m. The natural semiopen forest forest on the central plateau was composed primarily of Calophyllum inophyllum and Ficus prolixa, with additional elements of Terminalia, Premna, Guettarda, Psidium, and a sparse shrub layer. Scaevola taccada dominated on low ridges. The average canopy height was 16 m. This forest has been almost completely removed in an effort to access the underlying organic phosphate deposits, leaving a stark landscape of residual limestone pinnacles. Other vegetation communities are found on a shallow depression on the south side of the island, a narrow coastal flat surrounding the island. A line of cliffs separating the plateau from the coastal flats contains the richest remaining natural vegetation on the island. While elevatated coral platform islands often contain endemic plant species, there are none known on Nauru – there is no telling how many unique species were destroyed during mining activities (Mueller-Dombois & Fosberg 1998).

Biodiversity Features
There are very few endemic species in the ecoregion due to small island areas, low habitat diversity, and harsh conditions.

Although common in temperate regions, monodominant forest communities are unusual in tropical ecosystems. They probably occur in the Marshall Islands because of the stressful environment (salt spray, periodic typhoons, etc.) and the low number of species of atoll flora (Thomas et al. 1989). Pisonia grandis monodominant stands are now one of rarest forest types left in the Marshall Islands, because of the ease of felling the trees and the fact that the fertile, organic-rich soil is ideal for growing coconuts (Mueller-Dombois & Fosberg 1998).

Four Secondary Endemic Bird Areas were delineated in the ecoregion by Birdlife International. Two endemic species occur, or occurred. One is the now extinct Wake Island rail (Gallirallus wakensis), which was last seen in 1945 and is presumed to have been eaten by the occupying Japanese forces. The other endemic is the Vulnerable Nauru reed-warbler (Acrocephalus rehsei). The purple-capped fruit-dove (Ptilonopus porphyraceus) has been extirpated from Ebon atoll in the Marshalls (and the ecoregion), but is otherwise widespread in the Pacific (Pratt et al. 1987). In addition, the restricted-range species Micronesian imperial-pigeon (Ducula oceanica) is resident and the widespread but Vulnerable bristle-thighed curlew (Numenius tahitiensis) winters in the Marshalls and Gilberts (Stattersfield et al. 1998, Hilton-Taylor 2000).

There are no native mammals in this ecoregion.

The atolls of the Marshall Islands, especially Taongi and Bikar, are extremely important as rookeries for seabirds such as great frigatebirds (Fregata minor), brown boobies (Sula leucogaster), red-footed boobies (Sula sula), wedge-tailed shearwaters (Puffinus pacificus), red-tailed tropic birds (Phaethon rubricauda), sooty terns (Sterna fuscata), white terns (Gygis alba), and brown noddies (Anous stolidis)(Thomas, et al., 1989).

Current Status
Many of these islands have been inhabited for several thousand years. The most obvious vegetation today consists of coconut plantations (Cocas nucifera), which have replaced most of the native vegetation. Significant areas of the islands have also been planted with taro. There is also limited sugarcane cultivation, and some wetlands are used for the cultivation of medicinal and food plants. The little mixed broadleaf forest that remains was spared for its value as windbreak, especially on the windward sides of the islands.

Military uses have completely altered some atolls. The atolls of Bikini and Enewetak were used as nuclear bomb test sites by the United States, and Kwajalein, a World War II battleground, is now used as a U.S. missile test range.

The Gilberts are so heavily overpopulated, with no large areas of unoccupied land, that most of the vegetation consists of food plants

Nauru’s extensive organic phosphates have been heavily exploited to the extent that little natural vegetation exists on the central plateau. With continued mining activity over the next few years, little is expected to exist on this area besides exotic plants (Mueller-Dombois & Fosberg 1998).

Types and Severity of Threats
Because it is a region of isolated atolls with harsh environmental conditions and limited land area, species diversity is low, and the islands are extremely vulnerable to the introduction of alien species and other human disturbances. Current human population growth rate is around 3 percent, which is putting additional pressure on an already fragile ecosystem.

There is a traditional system of land tenure in the Marshall Islands that is generally understood and respected by the older inhabitants. However, the system is disintegrating because the younger generation are less knowledgeable and less interested in the "old ways." This needs to be considered when developing plans for protected areas and future conservation (Thomas et al. 1988).

Justification of Ecoregion Delineation
This ecoregion comprises Wake Island, the Ratak and Ralik chains of the Marshall Islands as well as the Gilbert Islands group and Nauru. Van Balgooy (1996) groups the low coral islands of the Marshalls, Gilberts, Tuvalu, Kiribati, and Line Islands based on floristic affinities (Nauru is apparently associated with the Eastern Carolines). Birdlife International (Stattersfield et al. 1998) identifies four Secondary Endemic Bird Areas in the Eastern Micronesia: Wake Island (one extinct endemic), Marshall Islands, Nauru (one extant endemic), and the Gilberts, and these have been grouped together to form one ecoregion.

CIA World Factbook 2000 – Marshall Islands, Retrieved (2000) from http://www.odci.gov/cia/publications/factbook/geos/rm.html

Dahl, A.L. 1986. Review of the protected areas system in Oceania. International Union for Conservation of Nature and Natural Resources, Commission on National Parks and Protected Areas, in collaboration with the United Nations Environment Programme.

Fosberg, F.R. 1998. Chapter 5: Micronesia. Pages 199-313 in D. Mueller-Dombois and F.R. Fosberg, editors. Vegetation of the Tropical Pacific Islands. Springer-Verlag New York, Inc.

Herbst, D.R. 1994. Botanical survey report for Wake Atoll Outdoor Recreation Plan. Unpublished report. U.S. Army Corps of Engineers, Pacific Ocean Division. 16p.

Pratt, H.D., P.L. Bruner, and D.G. Berrett. 1987. The Birds of Hawaii and the Tropical Pacific. Princeton University Press, Princeton, NJ.

Stattersfield, A.J., M.J. Crosby, A.J. Long, and D.C. Wege. 1998. Endemic bird areas of the world: priorities for biodiversity conservation. BirdLife Conservation Series No. 7. BirdLife International. Cambridge, UK.

Thomas, P.E.J., F.R. Fosberg, L.S. Hamilton, D.R. Herbst, J.O. Juvik, J.E. Maragos, J.J. Naughton, and C.F. Streck. 1989 Report of the Northern Marshall Islands Diversity and Protected Areas Survey. Honolulu, Hawaii: East-West Center in association with South Pacific Regional Environmental Programme, Noumea, New Caledonia.

Van Balgooy, P.H. Hovenkamp, and P.C. Van Welzen. 1996. Phytogeography of the Pacific – floristic and historical distribution patterns in plants. Pages 191-213 in Keast, A. and S.E. Miller, editors. The origin and evolution of Pacific island biotas, New Guinea to Eastern Polynesia: Patterns and processes. SPB Academic Publishing, Amsterdam.

Prepared by: Sandra Zicus
Reviewed by: In process