Like the serpent in the biblical Garden of Eden, the arrival of the brown tree snake (Boiga irregularis) brought the end of paradise for the birds of Guam. Of Guam's twelve native forest bird species, nine have been driven to extinction within the last fifty years, including five endemic species and subspecies. The primary cause of these extinctions was the unintentionally introduced brown tree snake. As the bird species disappeared before the eyes of the world's ornithologists, the culprit initially went unidentified and was extremely successful in its expansion across the island.
The island of Guam is the largest and southernmost island of the Mariana archipelago, located to the north of New Guinea. Following World War II, Guam became an important U.S. naval base and a repository for a large amount of military equipment used throughout the Pacific during the war. It is believed that the brown tree snake, a fearsome arboreal predator, first arrived in Guam as a stowaway on one of these interisland shipments. Because of similarities in markings and coloration, it is thought that the brown tree snakes of Guam probably originated in the Admiralty Islands, to the south.
The first reports of brown tree snakes on Guam were made in the 1950s, but because the snakes were initially misidentified as a predator of rodents, the threat to Guam's birds was not appreciated. The native fauna of Guam was unprepared for an invasion of predatory, arboreal snakes because they had evolved in an island sanctuary free of such dangers. The only snake native to Guam (Ramphotyphlops braminus) is blind and so small that it is commonly mistaken for a worm.
With an ample food supply and an absence of predators, the snake population grew exponentially, and their range expanded across the island. All species of forest birds suffered a similar pattern of decline—disappearing first in the south in the 1960s and then in a wave of decline moving north until 1986, when the last forest dwellers disappeared. Small mammals and reptiles also suffered severe population declines, and two bat species and at least three lizard species have also been lost. Bird species were particularly vulnerable because the snakes are voracious egg-eaters as well as preying on adults and nestlings. Small songbirds such as the rufous fantail (Rhipidura rufifrons), bridled white-eye (Zos-terops conspicillatus), and the Guam flycatcher (Myiagra freycineti) were particularly vulnerable and are among the extinct. Even the white tern (Gygis alba), which feeds on ocean fish, has suffered drastic declines, because its nests in trees are accessible to the snakes. The only species to escape the carnage were the marsh-living native yellow bittern (Ixobrychus chi-nensis) and introduced game birds inhabiting treeless areas. Most of the native forest species were all but extinct when they were listed as threatened or endangered by the U.S. Fish and Wildlife Service in 1984.
The species extinctions on Guam are a tragic loss of biodiversity that is compounded by secondary effects we are only beginning to understand. The decline of many insectivorous species has allowed insect populations to grow unchecked, resulting in defoliation and crop damage. Birds and bats commonly play important roles as pollinators and seed dispersers, and Guam has undoubtedly been affected by the loss of these services. Unfortunately, little was known about the ecology of many of the extinct species and their roles in Guam's ecosystem. Brown tree snakes have also caused problems for humans in more direct ways. The snakes often prey on domestic poultry and have been known to attack sleeping infants. Electrical outages are frequently caused by the snakes, which are notorious for short-circuiting power lines. Interruptions in power are costly and inconvenient.
Two of Guam's bird species have survived, thanks to heroic efforts that brought the last surviving individuals into captivity in order to establish captive breeding programs. These are the Guam rail (Gallliraillus owstoni), a flightless bird endemic to Guam, and the Micronesian kingfisher (Halcyon cinnamom-ina). Several zoos have participated in the scheme of carefully planned matings intended to preserve the genetic diversity of the species and to create healthy captive populations. The breeding program for the Guam rail has been quite successful, and an attempt has been made to establish a free-living colony of rails on the nearby island of Rota. Although Rota does not have brown tree snakes, feral cats were responsible for the deaths of many of the introduced rails. Bird reintroductions in Guam are also being attempted, but only in very limited areas because of the effort required to evict the snakes.
The Micronesian kingfisher has not enjoyed the same success as the Guam rail. The cur rent number of birds in captivity is not much larger than the founding population. The primary challenge has come from the dearth of knowledge about the species' habits and nutritional requirements. Without sound information about the ecology of the species, zoos have had to rely on techniques extrapolated from knowledge about similar species. Early reproductive failures were likely associated with lack of acceptable nesting places and dietary problems.
Control of the snakes on Guam and prevention of snake introduction to other islands is an issue of high priority. The Hawaiian islands, where bird populations have already suffered from the introduction of predators such as the cat, mongoose, and pig, as well as introduced diseases, are of particular concern. Various methods have been employed to control or eliminate the snakes on Guam. Trapping and hand catching snakes are widely practiced. Researchers are investigating possible ways of poisoning the snakes or introducing a snake-specific disease, but both of those options may pose risks to other species as well. In some areas electric barriers are placed on nesting trees, and the tree branches may be trimmed to eliminate the snakes' "highway in the sky," but both of those methods are labor intensive and costly, making them impractical over large areas. Terrestrial barriers around areas cleared of snakes have also shown promise during initial trials, but constant vigilance is required to detect breaches.
See also: Birds; Endangered Species; Extinction,
Direct Causes of; Reptiles
Amand, Alexandria. "Boiga irregularis (Brown Tree Snakes) on Guam and Its Effect on Fauna." http://www.hort.agri.umn.edu (accessed January 16, 2002); National Research Council, Committee on the Scientific Bases for the Preservation of the Mariana Crow. 1997. The Scientific Bases for the Preservation of the Mariana Crow. Georgetown: National Academy; Pimm, Stuart L. 1987. "The Snake that Ate Guam." Trends in Ecology and Evolution 2: 293-295; Pimm, Stuart L. 1991. The Balance of Nature? Chicago: University of Chicago Press; USGS. "History of the Brown Tree Snake Invasion on Guam." http://www.mesc.usgs.gov (accessed January 16, 2002).
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