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Terrestrial Fauna of Guam

A brown tree snake
The brown tree snake is responsible for extensive economic and ecological damage. (Photo: U.S. Fish and Wildlife Service)

Guam never supported a rich terrestrial vertebrate fauna.  Of the limited number of vertebrates which once occurred there, many species were extirpated by the introduction of the invasive brown tree snake (Boiga irregularis) shortly after World War II. The brown tree snake was transported to Guam from its native range in Indonesia, Papua New Guinea, Australia, and northwestern Melanesia, probably as a stowaway in ship cargo. The snake, which is a member of the family Colubridae, possesses grooved fangs in the rear portion of its mouth, along which mildly neurotoxic venom is directed when the snake bites its prey. It is aggressive if threatened but does not pose a health hazard for adult humans. Infants and small children, however, may experience severe medical consequences if bitten.

Due to an abundance of prey species and lack of major predators, the brown tree snake’s population exploded shortly after being introduced to approximate densities of nearly 13,000 per square mile. Not only does the snake inhabit grasslands, heavily forested areas, and sparsely forested areas, caves, and cliffs, but it also has invaded residential areas, resulting in the loss of poultry, pets, and caged birds.

The Micronesian kingfisher
The Micronesian kingfisher

Overall, the brown tree snake is responsible for extensive economic and ecological damage in Guam. Of 10 of 12 native bird species, 10 native lizards, and two native bats (flying foxes) now extinct, all but four of the bird species (which vanished due to habitat loss and alteration), were exterminated by the brown tree snake. With the loss of most seed-eating birds on the island, Guam’s forest ecology has undergone substantial change. It has been suggested that these changes may ultimately increase the severity of bush fires, already a major problem, and increase run-off from storm-caused erosion, which adversely affects delicate coral ecosystems of the island.

The Guam fruit bat
The Guam fruit bat

Aside from the brown tree snake, Guam is home to a number of native lizards (skinks, geckos, and monitor lizards) and introduced anoles, skinks, and geckos - all of which are preyed upon by the brown tree snake. (Interestingly, the eggs and young of the monitor lizards are preyed upon by the brown tree snake, but the adult monitors prey upon the snakes themselves, though not in high enough numbers to significantly reduce the brown tree snake population.) Additionally, the Spaniards introduced dogs, cats, chickens, pigs, the Philippine deer, and the Asiatic water buffalo (carabao) to the island. Other introduced species include the marine toad (Bufo marinus), the giant African land snail (Achatina sp.), and a species of frog, which not only is a noisy pest, but also is prey for the brown tree snake. A damaging pest of coconut palms, the coconut rhinoceros beetle (Oryctes rhinoceros), also was detected on Guam as recently as 2007.

Guam’s forest ecology has also been altered substantially as a result of the damage to the indigenous flora during World War II, subsequent introduction of the tangan tangan tree and other non-native flora, and arson by poachers. Flourishing grasslands and barren areas have also replaced previously forested areas, leading to greater erosion and siltation in coral reef areas. Besides loss of the pollinating and seed-dispersing birds and bats, Guam’s forests are also threatened by grazing feral carabao (water buffalo), wild pigs, and Asian deer, all of which cause extensive damage to trees.