General Impact
Reduction in Native Biodiversity:The brown tree snake has decimated Guam’s birds and herpetofauna, causing the local extinction of over half of Guam’s native bird and lizard species as well as two out of three of Guam’s native bat species (Savidge 1987; Rodda & Fritts 1992). Several indigenous or endemic species of lizards have become extinct or engangered because of snake predation (Rodda & Fritts 1992). Guam's 12 forest birds were especially impacted, with 10 species eliminated and the other two severely reduced (Rodda & Savidge 2007). By eliminating native pollinators the brown tree snake has also caused "cascading" effects on Guam ecosystems, reducing pollination by lizards and birds and reducing native plant regeneration and coverage as a consequence (Perry & Morton 1999; Mortensen, Dupont & Olesen 2008).Human Health: This rear-fanged colubrid snake is mildly venomous and poses a potential health hazard to infants and young children. It is responsible for one of every thousand hospital emergency room visits on the island (United States Department of Defense 2008). Envenomation of babies has been reported as relatively frequent (Fritts et al. 1990). Besides the direct effects of brown tree snake bites, there is also the danger of increased disease carried by insects that were previously kept in check by Guam's native lizards and birds (Fritts & Leasman-Tanner 2001). Examples of this include an outbreak of dengue fever carried by mosquitoes and a high rate of infant salmonellosis for several years (Fritts & Leasman-Tanner 2001). Economic/Livelihoods: Power outages caused by snakes have been a serious problem on Guam since 1978, and the incidence of snake-caused outages continues to cause significant problems. The brown tree snake has caused thousands of power outages affecting private, commercial, and military activities, at one stage averaging once every two to three days. While most of these affect a limited area, some are widespread or island-wide blackouts. Everything from school lighting, computers used by retail outlets, traffic signals to refrigeration of perishable goods are subject to these power interruptions. The costs due to direct damages and lost productivity are conservatively estimated at $1 to 4 million dollars each year (Fritts & Leasman-Tanner 2001; Fritts 2002). A bad perception of the brown tree snake (although it is not harmful to adults) may cause tourists to avoid Guam in favour of more unspoilt locations. Since tourism is only outranked by U.S. military and government in economic importance on Guam, lost tourism dollars could cause major economic stress (Fritts & Leasman-Tanner 2001). Researchers estimate that if the brown tree snake estabishes in Hawaii tourism losses will amount to USD 0.5 to 1.5 billion (D' Evelyn et al. 2008; Rodda & Savage 2007). Agriculture: The brown tree snake is reported to be an agricultural pest (Fritts & McCoid 1991, in Engeman et al. 2002). Insect species that are no longer naturally controlled by Guam’s native birds and lizards reduce fruit and vegetable yields (Fritts & Leasman-Tanner 2001). Agriculture has continually declined in importance on Guam since 1945, around when the snake was introduced to the island (Fritts & Leasman-Tanner 2001). Agriculture has continually declined in importance on Guam since 1945, around when the snake was introduced to the island (Fritts & Leasman-Tanner 2001), although additional socio-economic factors were very important in this process.
Location Specific Impacts:Guam 
Economic/Livelihoods: The brown tree snake causes electrical problems creating short circuits by simultaneously touching live and grounded conductors. Such occurrences may happen as frequently as once a week on the island (Westbrook & Ramos 2005). Millions of dollars in damaged equipment, lost productivity, and repair costs result from snake-caused power outages in Guam (Fritts et al 1987 in Avery et al 2004).
The poultry industry is affected from loss of eggs due to the brown tree snake.
The presence of snakes may encourage tourists to chose other islands over Guam as a destination, harming the tourism industry (Rodda & Savage 2007).
Ecosystem change: The introduction of an alien top predator, the brown treesnake, has resulted in severe losses of native vertebrate populations in Guam. Among these are important pollinators and seed dispersers including lizards and birds (Mortensen et al. 2008). By reducing populations of native birds, which pollinate and disperse native plants, the brown tree snake disrupts native ecosystem functioning and theatens native plant species. These "cascading ecosystem effects" were first documented in relation to the brown tree snake in Guam by Mortensen et al (2008) who investigated seed set of the mangrove tree Bruguiera gymnorrhiza (which is in urgent need of conservation and restoration in the Mariana Islands; Vogt & Williams 2004, in Mortensen et al. 2008) and the forest tree Erythrina variegata var. orientalis on Guam (with high density of snakes) and Saipan (with nearly no snakes). Seed set of both species were significantly higher on Saipan compared to Guam. The eradication of flower-visiting birds by the invasive treesnake thus secondarily results in broken mutualistic interactions, which may, in turn, result in a lower recruitment of native plants (Mortensen et al. 2008). The loss of most insectivorous birds and many lizard species also leaves Guam vulnerable to a variety of insect pests (Fritts & Rodda 1998, Rodda et al. 1999).
Human health: On Guam the brown tree snake has been held responsible for the for loss of domestic animals and for the envenomation of human babies (Fritts et al. 1990; Fritts &McCoid In Press, in Rodda et al 1992). It is responsible for one of every thousand hospital emergency room visits on the island (United States Department of Defense 2008).
Predation: The predatory nature of the brown tree snake has caused the extinction of most of the island’s native bird species and six out of 10 to 12 native lizard species (Savidge 1987; Rodda & Fritts 1992; Fritts & Rodda 1998). Likewise, two of Guam’s three bat species have disappeared (Wiles 1987; Fritts & Rodda 1998). The brown tree snake is a major predator of the juveniles of the Marianas fruit bat (Pteropus mariannus) (Haynes and Marler 2005; Esselstyn 2006). The bat is listed as Endangered on the IUCN Red List of Threatened Species and juvenile Marinara fruit bats are preyed upon so heavily by B. irregularis that within-island recruitment has been halted for many years (Esseltyn 2006).
Threat to endangered species: Results of a study by Wiles et al (2003) indicate that 22 species have been severely affected by the brown tree snake including the Extinct Guam flycatcher (Myiagra freycineti), the Extinct in the Wild Guam rail (Gallirallus owstoni), the Critically Endangered nightingale reed-warbler (Acrocephalus luscinius), the Endangered Mariana fruit-Dove (Ptilinopus roseicapilla)), the Endangered bridled white-eye (Zosterops conspicillatus), the Near Threatened white-throated ground-dove (Gallicolumba xanthonura), the white-tailed tropicbird (Phaethon lepturus), the brown booby (Sula leucogaster), the Micronesian kingfisher (Todiramphus cinnamominus), the rufous fantail (Rhipidura rufifrons), the Micronesian honeyeater (Myzomela rubratra) and the chestnut munia (Lonchura atricapilla). Categories of species are taken from the IUCN Red List of Threatened Species. The three species which were nearly or temporarily extirpated include the brown noddy (Anous stolidus), the white tern (Gygis alba) and the Mariana crow (Corvus kubaryi). The four species which showed a decline of >90% and are 'not recovering' include the island collared-dove, the Mariana swiftlet, the black drongo and the Micronesian starling. Saipan Is. (Northern Mariana Islands)
Threat to endangered species: The introduction of the brown tree snake could potentially endanger threatened species on the island (see location notes).
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