Guam (Guahan)


Location: 13.33N, 144.75E

Elevation: 406m (1332 ft.)

Area: 541 sq. km

Island type: raised coral in north (152 m); old weathered volcanic in south

The island of Guam is approximately 30 miles in length with a variable width, ranging from 12 miles to 4 miles at its narrowest point. The largest island in Micronesia, Guam has a total landmass of 212 square miles, excluding reef formations. The island has two basic geological compositions. Two-thirds of Guam, the central and northern features, are primarily raised limestone with several volcanic formations at Mount Santa Rosa and Mount Mataguak. The northern cliff lines drop precipitately into the sea with an elevation ranging from 300 to 600 feet. The southern features are basically volcanic with an elongated mountain ridge dividing the inland valleys and coastline. The highest point is Mount Lamlam with an elevation of 1,332 feet. The Peak of a submerged mountain, Guam, rises 37,820 feet above the floor of the Marianas Trench, the greatest ocean depth in the world. A metal object would take 64 minutes to fall through the Marianas Trench, just east of Guam, with a depth of 6.79 miles, where pressure is over 18,000 pounds per square inch. The British Survey Ship Challenger pinpointed the Marianas Trench in 1951, and on January 23, 1960, the manned U.S.N. bathyscaph Trieste descended to the bottom. On March 24, 1995, the unmanned Japanese probe Kaiko also reached the bottom and recorded a depth of 35,797 feet. Guam straddles the edge of the Philippine Plate with the Pacific Plate thrusting below it. The Western shoreline faces the Philippine Sea while just a few miles away the Eastern beaches faces the Pacific Ocean. Ancient perpendicular fault lines that collect water now determine paths of existing tributaries. Guam is the "Isle of Orthogonal Rivers", and westernmost U.S. territory. It is west of the International Dateline and is 1 day ahead of the U.S.: Hence the slogan "Where America's Day Begins".

Natural threats: cyclones

Human impact: heavy damage during WWII; extensive urban development around Hagatna and elsewhere; major military bases; some agriculture.

Ecosystems: lowland rainforest on limestone with considerable endemism (remaining fragment along extreme northern rim); some ravine and beach forests; possible cloud forest on Mt. Lamlam; savanna and grasslands, scrub; poor mangroves; marsh, reed swamp and other aquatic habitats; fringing reef, lagoons and barrier reef (CRD).

Special Features: Lake Fena (manmade); seabird rookeries at Anae islet and Orote Point; former sea turtle nesting beaches; caves in south with endangered swiftlet; feral pigs and other domestic animals; introduced giant African snail (Achatina fulica), elk, monitor lizard (Varanus indicus) and Philippine Rat Snake (Boiga irrgularis, 1947) with effects on native flora and fauna.


TTL Species

# Endemic

% Endemic















Other Invertebrates






Reptiles & Amphibians



















Species of conservation interest:

931 [or 530] species of flowering plants and ferns, of which at least 330 are native, 69 are endemic to the Mariana Islands; 20 endemics and more than 30 non-endemics threatened on Guam, including:
        Serianthes nelsonii (Leguminosae) endemic to Guam and Rota (4 remaining trees on Guam), Endangered (RDB)
        Heritiera longipetiolata (Sterculiaceae) group endemic, Endangered (RDB)
        Tabernaemontana rotensis (Apocynaceae) one remaining tree on Guam
       Hernandia ovigera (Hernandiaceae)
        Merrilliodendron megacarpum (Icacinaceae) known from two areas
        Xylosoma nelsonii (Flacourtaceae)
        Fagraea galilai (Loganiaceae) two sites including Mt. Lamlam
        Solanum guamense few plants known
        Ceratopteris gaudichaudii from freshwater areas
        Potamogeton mariannensis from freshwater, Coral area

15 species of butterfly, 1 only on Guam
         Neptis guamensis endemic, not seen since type collection in 1916

Other invertebrates: 9 native land snails, including 4 Partulidae (tree snails):
        Partula gibba
        Partula salifana
        Partula radiolata
        Samoana fragilis

        Ramphotyphlops (?) pseudosaurus (Typhlopidae) endemic?

95 species, 17resident, 7 introduced, 12 indigenous terrestrial bird species, 2 endemic
        Rallus owstoni (Guam Rail) endemic, Vulnerable (RDB), fewer than 100 (1984), declining.
        Ptilinopus roseicapilla (Marianas Fruit Dove) group endemic, northwest forest, Vulnerable (RDB), estimate of 241 on                  Guam (1981)
        Myiagra freycineti (Guam Flycatcher of Broadbill) endemic, endangered, fewer than 100 (1983).
        Gallinula chloropus guami (Marianas Gallinule or Common Moorhen)  group endemic subspecies, Rare (RDB),                  100-200 (1983).
        Gallicolumba xanthonura xanthonura (White-throated Ground Dove), 150-200 (1983).
       Aerodramus vankorensis bartshi (Vanikoro Swiftlet), less than 50 (1983)
        Halcyon cinnamomina cinnamomina   (Guam Micronesian Kingfisher) endemic subspecies, Endangered (RDB) or                  vulnerable, conflicting reports of 3000 or 250-300 remaining  (1982-83)
       Rhipodura rufifrons uraniae (Rufous-fronted Fantail), less than 200 (1983).
        Zosterops conspicillata conspicillata (Bridled White-eye), vulnerable, less than 50 (1984)
       Myzomela cardinalis saffordi (Cardinal Honeyeater), estimated 250-300 (1983)
       Apolinis opaca guami (Micronesian Starling), 1000 birds (1983)
        Corvus kubaryi (Marianas Crow) Endangered (RDB), 150-200 birds (1983). At least 4 bird species or subspecies are                  now extinct on Guam.
       Pteropus marianus marianus (Mariana Fruit Bat) endangered by poaching.
        Pteropus tokudae (Little Marianas Fruit Bat) only in northern mature forest, may be extinct.
        Emballonura semicaudata (Sheath-tailed Bat) may be extinct.
        Dugong dugong (Dugong), vulnerable (RDB), very rare locally

Protected areas:
        Anao Conservation Reserve..................................263 ha
        Pati Point Natural Area.........................................112 ha
        Haputo Ecological Reserve Area.............................73 ha     (102 ha including reef and coastal waters)
        Orote Peninsula Ecological Reserve Area................12 ha     (55 ha including water area)
        Masso River Reservoir Area...................................67 ha
        Guam Territorial Seashore Park (V)...................3,645 ha     ( 6,075 ha including reef and coastal waters)
        War in the Pacific National Historical Park (V)......374 ha     (779 ha including water and coral reef areas)

Natural conservation status 0
Ecosystem richness 2
Species richness 2
Economic pressure 2
Human threat 0
Natural vulnerability 1
Practicality of conservation action 1
Reliability of data 3
Human Impact 9
Conservation Importance 27


Uracas  Maug   Ascuncion  Agrigan   Pagan  Alamagan   Guguan  Sarigan   Anatahan  Medinilla

Saipan  Tinian   Aguihan  Rota   Guam

Back to Marianas Islands