Details of this species in Guam
Source: Mortensen Dupont & Olesen 2008
Arrival Date: 1946-1950
Introduction: Unintentional (accidentally)
Species Notes for this Location:
The brown treesnake probably arrived on Guam as a passive stowaway in vehicles or other material transported to Guam from the Admiralty Islands in the years immediately after World War II (from 1946 to 1950) (Rodda Fritts & Conry 1992). The snake rapidly dispersed throughout the island between the 1950s and 1982 (Savidge, 1987), reaching a peak density of >100 snakes/ha. The population appears to have stabilized at an equilibrium of c. 50 snakes/ha (Savidge 1987; Rodda et al. 1992; Rodda & Savidge, 2007). B. irregularis occurs in all terrestrial habitats, especially forest and human-impacted ecosystems. B. irregularis populations in Guam have recently been found to have a low proportion of reproductively active adults and a trend in poor body condition indicating stress from overcrowding (Moore et al. 2005).
Management Notes for this Location:
Traps, visual inspections, trained dogs and specialized barriers are in use on Guam. Toxicants are in development.
Preventative measures: While it is too late to prevent the entry of B. irregularis into Guam it is possible to avert future similar disasters by maintaining strict cargo checks. As of 2007, cargo inspection certification is only voluntary in Guam (Rodda & Savage, 2007).
Small predator-free nature reserves can be created using snake barriers. Campbell (1996 in Mortensen et al. 2008) eliminated B. irregularis from two one-hectare nature reserves using this method. Lizard numbers increased rapidly in the enclosed area. Possible designs for a successful barrier include bulge barriers (low-cost but are vulnerable to damage), vinyl barriers (durable, but with a surface finish which may degrade over time allowing snakes to climb the barrier) and masonry barriers (are 100% successful in keeping snakes out, however have a high initial cost of approximately USD 300 per meter).
Physical Control: Wildlife Services maintains two to three thousand snake traps on Guam (Rodda et al. 2002). A brown tree snake Rapid Response Team of trained personnel from various organizations from the United States, Guam and other surrounding areas was created in 2002. This team was formed to respond to sightings of B. irregularis (Stanford, 2007).
Conservation actions on Guam should be directed towards an improved recruitment (artificial pollination, planting) of plant species affected by B. irregularis (see Impacts information from Mortensen et al. 2008). Restoring conditions for natural pollination or managing reproduction of vertebrate-pollinated plants is critical in the long-term conservation of native vegetation types on Guam (Mortensen et al. 2008). Efforts are now made to conserve the few remaining larger areas of uniform forest vegetation, e.g. the conservation action plan Guam Comprehensive Wildlife Conservation Strategy 2005.
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.
Last Modified: 22/12/2009 2:52:33 p.m.