History of The Brown Treesnake Invasion on Guam
Prior to the introduction of the brown Treesnake, Boiga irregularis, on Guam, the only snake present on the island was a blind snake, Ramphotyphlops braminus. This secretive wormlike snake inhabits termite nests and loose soil where it feeds on termites and ants. Its small size and lack of well-developed eyes contribute to most island residents failing to recognize it as a snake, instead thinking it is a worm. As a predator of arthropods, the blind snake poses no threat to Guam's fauna or human population. Thus, Guam effectively lacked a conspicuous snake until the brown Treesnake arrived. For a complete species account of the blind snake see "Other Snakes of the South Pacific."
The exact date and mode of transportation for the brown Treesnake's arrival on Guam cannot be conclusively identified, but it was most likely shipped to Guam as a stowaway in military cargo from the Papua New Guinea area during the post World War II years. As the U.S. Pacific Headquarters was transferred to Guam during and immediately after the war, U.S. military equipment and salvaged war material that had been in use at the military bases in the Admiralty Islands of northern Papua New Guinea were shipped back to the permanent bases and scrap metal processors on Guam. Additional support for this origination site is based on the characteristics of the snakes on Guam. Depending on the area it inhabits, the colors and scale patterns of the brown Treesnake vary. The snakes on Guam are most similar to those of the Admiralty Island group.
The snake was first detected on Guam in the 1950s near the Naval Port (central Guam), but may not have become conspicuous away from the port area until the early 1960s. By the mid 1960s, the snake had colonized over half of the island. In 1968, the snake had reached the extreme northern end of the island and was present throughout the island, although its densities varied widely from region to region.
The disappearance of birds on the island followed the advancing high densities of snakes. By 1963, several formerly abundant species of native birds had disappeared from the central part of the island where snakes were most populous. By the late 1960s, birds had begun to decline in the central and southern parts of the island and remained abundant only in isolated patches of forest on the northern end of the island. Snakes began affecting the birds in the north-central and extreme northern parts of the island in the 1970s, and most native forest species were virtually extinct when they were listed as threatened or endangered by the U.S. Fish and Wildlife Service in 1984. The species of birds remaining on Guam are extremely patchy in distribution, occurring only in special habitats where some protection from snakes exists.
At present, even small mammals are extremely rare in most forested habitats of Guam. Predation by the brown Treesnake is the most likely primary factor preventing recruitment to the single population of native Mariana fruit bats remaining on Guam. Lizard densities, particularly of introduced species with high reproductive rates, remain high, supporting the snake population. Although larger snakes are showing signs of stress, exhibited by low fat reserves, the ability to shift from birds to rodents or lizards has enabled the snake to reach and maintain extraordinarily high densities of as many as 13,000 per square mile. This is higher than snake densities in the rainforests of the Amazon Basin of Ecuador where 51 different snake species occupy the same habitat.
Colonization of Other Snake Species on Islands Around the World
Until attention was focused on the ecological and economic damages of the introduced brown Treesnake on the island of Guam, little consideration was given to assessing risks posed by the colonization of snakes on formerly snake-free islands. However, a growing mass of data, involving examples scattered throughout the world, suggests that this problem is both real and complex.
The island of Mallorca, a Spanish Island in the Mediterranean Sea is reported to contain two introduced species of snakes: Natrix maura, which poses a threat to native frogs, and Macroprotodon cucullatus, which preys on lizards. The ecological changes this island has suffered in the face of the introduced snakes is currently being studied by biologists from the Durrell Institute of Conservation and Ecology.
Christmas Island, a possession of Australia in the Indian Ocean, was colonized by the common wolf snake, Lycodon aulicus, in the last decade (Fritts, 1993, Wildlife Research 20:261-266). The snake population is currently being studied by Australian biologists, but initial examinations of the potential prey base present on the island include endemic birds and lizards that could be lost to snake predation.
The Indonesian and Philippine Archipelagos appear to have been invaded by wolf snakes, Lycodon aulicus, in previous centuries, probably due to merchant ship traffic from the Indian subcontinent where the snake is native. Due to its long-term presence on hundreds of islands in the region, it is currently impossible to predict the ecological damages that this snake colonization caused. See comments by Fritts (citation above) and Leviton, 1965, Philippine Journal of Science 94:117-140.
Islands in the Mauritius Island Group, off the coast of Madagascar in the Indian Ocean, were also colonized by the now widespread tramp species, Lycodon aulicus. In Mauritius, the snakes have probably been the major threat to native and endemic geckos and skinks. Biologists from the Mauritius Wildlife Foundation and the Jersey Trust are currently considering efforts to eradicate the snake from small satellite islands where the task is most feasible. They are contemplating use of control techniques and strategies developed within the Brown Treesnake Research Program.
Cozumel Island, off the east coast of Quintana Roo, Mexico, was recently reported to have been colonized by the boa constrictor, Boa constrictor. The snake is native to the nearby Mexican mainland but was unknown on the island prior to 1991. The presence of several species of birds on Cozumel that are lacking from mainland Mexico and mammals and lizards that are either endemic to the island or more abundant there due to special environmental conditions on the island point to the potential for serious ecological damages if the boa is allowed to reach abnormally high population levels. Cozumel previously had four species of snakes, but these species are not predators of birds and mammals, the group most likely to be exploited by the recently arrived boa constrictors.
Okinawa, an island in the Ryukyu Group in the extreme western Pacific, has been the site of an introduction of Thai cobras, Naja sp., which may have become established as snakes escaped from snake farms and amusement areas. The snakes were presumably imported for use in staged cobra/mongoose battles and are now thought to be breeding in natural habitats on Okinawa.
Tres Marias Islands and Isla Isabel off Nayarit, Mexico, may be the site of a recent arrival of the Mexican milk snake, Lampropeltis triangulum. These snakes are reported to be in extremely high abundance perhaps because of abnormally abundant prey resources furnished by seabird nesting areas and island lizard faunas.
The formerly snake-free island of Saipan in the Commonwealth of the Northern Mariana Islands has had nearly 50 brown Treesnake sightings reported. Several of these snakes have been captured or killed within the last 12 years. The increased frequency of sightings from widespread points on the island strongly suggests that this snake has managed to colonize the island as a result of movements in air and ship traffic from Guam. The rate of population increase and magnitude of problems that will ensue from this colonization will not be known for several years, but research and trapping efforts are currently underway on Saipan in an attempt to stop this population before it can become fully established. Individual snakes have also been discovered and reported on the nearby Islands of Rota and Tinian. Combined, these three islands constitute the primary source of biodiversity at present for the Mariana Archipelago and are key to any conservation efforts for this region.
Oahu, State of Hawaii, in the Central Pacific was a snake-free oceanic island until snakes began appearing in ship and air traffic from the mainland and other island areas. In recent years, 20-30 snakes per year have been discovered in Hawaii. Of particular concern are the 8 documented discoveries of brown Treesnakes on Oahu. Within one 8-week period, 2 corn snakes were captured within a single city block area, 1 boa was discovered inside a chicken coop, 2 pythons were confiscated from a hobbyist's home, and 2 other snakes were reported to have escaped capture at Hickam Air Force Base, which constitutes the primary focal point for air traffic from Guam. Emergency trapping was conducted without resulting in captures of these snakes. Whether the brown Treesnake will be the first snake to colonize the Island of Oahu, or another species arriving as a passive stowaway or smuggled pet, is unknown, but the magnitude of the traffic of snakes to Hawaii strongly contributes to the likelihood of establishment of one or more snake species in the immediate future. The presence of introduced species of birds, mammals, and lizards on the Hawaiian Islands makes it likely that such a snake population will expand rapidly, reach high population levels, and have additional ecological effects on the quality of life in the State.
One conclusion from the aggregated examples above is that knowledge, strategies, tools, and technologies appropriate to reduce or eliminate introduced snake populations will be needed in many areas of the world as dispersal events continue and species accumulate in new island situations. The ecological battles to be fought on individual islands will be important in determining what methods work and to better prepare for subsequent control programs in other areas with other species and additional priorities. Recognition of the problem and the diversity of complications that can ensue are major steps toward ultimately addressing the issue effectively. The information amassed from Guam suggests that snakes are capable of reshaping island ecosystems, and the conditions that seem to lead to problems of such magnitude appear to be widely distributed among both offshore and oceanic islands. Understanding the ecological changes that are likely to follow such introductions and modeling the resulting population trajectories are important to evaluating the severity of the problems and estimating the window for response time that will be available if permanent ecological changes are to be prevented.