Common Name: Brown tree snake
Scientific Name: Boiga irregularis (Merrem)
Classification:Phylum: ChordataIdentification: The brown tree snake can be distinguished by: vertical pupils, rear fangs, a large head in relation to the body, and a brownish or greenish coloring with cross-band markings. Even though there is a wide variety of color variation in the brown tree snake across its range, coloration is constant at specific localities. The snake is about 18'' at hatching and may grow to about 3' in the first year. Adults can reach 8' and weight up to 5 lbs. In Australia, it is easy to confuse Boiga irregularis with Boiga fusca, a parapatric species. B. irregularis can be distinguished from B. fusca by its enlarged palatine teeth and touching preocular and frontal head shields. B. irregularis may also be known as the Eastern Brown Treesnake, Red-banded Treesnake, Pandanus Snake, Bandana Snake, Cordarilla, Night Tiger, Housesnake, Salmon Snake, Philippine Ratsnake, and Brown Catsnake. The name Philippine Ratsnake is a misnomer; the snake's simultaneous arrival on Guam with an influx of Filipino immigrants was purely coincidental.
Original Distribution: The brown tree snake is native to the Moluccas (Spice Islands), Indonesia from Wallace's Line west of Sulawesi through New Guinea, and the humid northern and eastern rims of Australia to the Santa Cruz Islands (including the Solomon Islands, but excluding the San Cristobal area).
Current Distribution: The brown tree snake is invasive to Guam. It has been sighted, but is not known to be established in Hawaii (Oahu), Texas (Corpus Christi), the Northern Mariana Islands (Saipan, Tinian, and Rota), the Marshall Islands (Kwajalein), the Caroline Islands in Micronesia (Pohnpei), other small islands in the southeast Pacific Ocean (Wake Oahu, Okinawa), and Diego Garcia Atoll in the Indian Ocean.
Site and Date of Introduction: The brown tree snake was introduced into Guam in the 1950s. The snake was first sighted inland from the seaport and became conspicuous throughout central Guam by the 1960s. By 1968, brown tree snakes had successfully dispersed throughout the island.
Mode(s) of Introduction: It is believed that the brown tree snake was introduced as a stowaway in cargo transported from the Admiralty Islands (near Papua New Guinea) by U.S. military ships during World War II. Based on their ability to hide in small, confined places, the brown tree snake may also have been dispersed by U.S. military planes, especially within plane wheel-wells.
Reason(s) Why it has Become Established: The brown tree snake has become established on Guam due to the absence of natural population controls and the abundance of vulnerable prey on the island. For example, introduced lizards are popular food items for young snakes. In the snake's native range, the presence of more diverse faunas and reduced food availability leads to higher population control. Additionally, because Guam is such a small, remote island, there are not only limited ways for prey to escape predation, but an inability for predators of the brown tree snake to enter. Furthermore, the non-seasonal climate of Guam, favorable for animal growth and reproduction, allows the brown tree snake to reproduce year-round.
Ecological Role: An active nocturnal species, the brown tree snake is most often found in densely foliated arboreal habitats. As a food generalist, the brown tree snake has been reported to prey upon lizards, introduced and domestic birds, rats, geckos, skinks, and any other available vertebrates. It can consume meals 70% of its body mass, an unusually large amount for a colubrid snake. Currently, there are up to 12,000 to 15,000 snakes per square mile on Guam. The brown tree snake begins to reproduce around age three and deposits up to twelve eggs once or twice a year in caves, hollow trees, and other areas protected from drying and overheating. The abandoned eggs hatch about 90 days later. The only known natural predators of the brown tree snake are pigs and monitor lizards. Before the arrival of the brown tree snake, the only other snake present on Guam was a tiny blind snake ( Rhamphotyphlopys braminus ). Since the blind snake lives in the soil and feeds on the eggs and young of termites and ants, it does not compete with the brown tree snake for resources and therefore cannot be an effective natural population control.
Benefit(s): Perhaps the brown tree snake's only benefits relate to the eradication of introduced species. For example, the voracious appetite of young snakes has helped to rid Guam of introduced lizards. The loss of avian seed dispersers has also caused declines in the reproductive rate of introduced shrubs, such as Lantana camara.
Threat(s): The brown tree snake is responsible for an incredible decline in Guam's biodiversity. Over the past two decades, this arboreal predator has caused the disappearance of nearly all of the native forest birds on Guam, including the extinction of the Guam rail and the Micronesian kingfisher. Nine of the eleven avifauna species present at the time of the brown tree snake's introduction have since been extirpated. Of the species that have become extinct, five were endemic at the species or subspecies level.
The abundance of the brown tree snake has also caused far-reaching secondary ecological impacts. The snake is responsible for the decline of the flying fox, a crucial species for the pollination and seed dispersal of tropical trees. Also, without the presence of certain avian insectivores, the insect population may experience a population boom and therefore negatively impact local agriculture. The cultural fabric of the island communities are negatively impacted by the brown tree snake as well. Fruit bats, an important part of indigenous rituals and celebrations on the Mariana Islands, have shown great declines since the introduction of the brown tree snake. The lower abundance of bats on the islands have not only limited this cultural practice but encouraged the exploitation of other areas in the Pacific for bat harvesting.
In addition to these negative biological impacts, the brown tree snake threatens the economy of the island through large-scale electrical power outages and damages to equipment. Since 1978, over 1200 power outages have occurred as a result of the brown tree snake crawling onto high voltage electrical lines or entering transformers or residential appliances. Moreover, continuously increasing populations of the brown tree snake are responsible for predation of farm animals, poultry, and pets, leading to further economic consequences. These snakes are mildly venomous to humans and their non-fatal bite can cause severe sickness in young children.
Control Level Diagnosis: It is recommended that the brown tree snake receive the "highest priority" diagnosis for control. Further spread of this invasive species should be prevented. More importantly, the current population should be controlled in an effort to reduce the risk of further spread and to begin to restore affected ecosystems. The significance of the ecological and economical destruction caused by the brown tree snake has already prompted legislative action in the United States. The U.S. Code: Title 16, Section 4728 states that a task force should develop a "comprehensive, environmentally sound program in coordination with regional, territorial, state and local entities to control the brown tree snake in Guam and other areas where the species is established outside of its historic range."Author: Lisa Patrick
Control Method: Various methods have been proposed for the removal of this problematic species, including barriers, traps with bait and attractants, biological controls, pathogens, and chemicals. Barriers are the most popularly used method for control and can be either temporary or permanent. Temporary barriers, which include nets and shade cloth, offer less protection and require more inspection than permanent barriers, but are less costly and easier to transport and build. Permanent barriers, on the other hand, include masonry, metal mesh, vinyl, and seawall, and are preferable in long-term protection situations. In field experiments, masonry and vinyl have provided the greatest amount of protection and durability. These barriers are therefore recommended for extra-sensitive sites, such as power stations and cargo handling facilities. For rough terrain, such as conservation areas, vinyl barriers were proven to be better adapted for predator control. In addition to barriers, researchers are in the process of developing a strain of paramyxovirus to be used for snake eradication. This deadly virus spreads between snakes through inhalation or from contact with contaminated surfaces. Methyl bromide has also been tested effectively as a fumigant, but is not currently in use because of its potentially damaging effects to the ozone layer. Currently on Guam, the Wildlife Service and the U.S. Department of Agriculture and Plant Inspection Service conducts snake trapping and nighttime spotlight searches to reduce numbers in cargo areas. Specially trained Jack Russell terriers are used to detect the hidden presence of brown tree snakes in cargo. In light of these control techniques, however, methods to prevent of snake entry into protected areas and dispersal to other geographical areas still need further research and management.
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