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Datasheet

Boiga irregularis (brown tree snake)

Summary

  • Last modified
  • 27 July 2017
  • Datasheet Type(s)
  • Natural Enemy
  • Invasive Species
  • Preferred Scientific Name
  • Boiga irregularis
  • Preferred Common Name
  • brown tree snake
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Chordata
  •       Subphylum: Vertebrata
  •         Class: Reptilia
  • Summary of Invasiveness
  • Native island species are predisposed and vulnerable to local extinction by invaders. When the brown tree snake (Boiga irregularis) was accidentally introduced to Guam it caused the local e...
  • Principal Source
  • Global Invasive Species Database  

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Pictures

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PictureTitleCaptionCopyright
Boiga irregularis (brown tree snake); adult snake in typical posture. Australia. September, 2014.
TitleAdult
CaptionBoiga irregularis (brown tree snake); adult snake in typical posture. Australia. September, 2014.
Copyright©Alexandre Roux/via flickr - CC BY-NC-SA 2.0
Boiga irregularis (brown tree snake); adult snake in typical posture. Australia. September, 2014.
AdultBoiga irregularis (brown tree snake); adult snake in typical posture. Australia. September, 2014.©Alexandre Roux/via flickr - CC BY-NC-SA 2.0
Boiga irregularis (brown tree snake); habit, in characteristic coiled posture. Queensland, Australia. September, 2007.
TitleHabit
CaptionBoiga irregularis (brown tree snake); habit, in characteristic coiled posture. Queensland, Australia. September, 2007.
CopyrightPublic Domain - Released by Soulgany101 at the English Wikipedia project.
Boiga irregularis (brown tree snake); habit, in characteristic coiled posture. Queensland, Australia. September, 2007.
HabitBoiga irregularis (brown tree snake); habit, in characteristic coiled posture. Queensland, Australia. September, 2007.Public Domain - Released by Soulgany101 at the English Wikipedia project.

Identity

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Preferred Scientific Name

  • Boiga irregularis (Merrem, 1802)

Preferred Common Name

  • brown tree snake

Other Scientific Names

  • Boiga flavescens
  • Coluber irregularis Merrem in Bechstein 1802
  • Dendrophis (Ahetula) fusca Gray 1842
  • Dipsadomorphus irregularis Werner 1899
  • Dipsas boydii Macleay 1884
  • Dipsas irregularis Fischer 1884
  • Dipsas ornata Macleay 1888
  • Hurria pseudoboiga Daudin 1803
  • Pappophis flavigastra Macleay 1877
  • Pappophis laticeps Macleay 1877
  • Triglyphodon flavescens Duméril, Bibron & Duméril 1854
  • Triglyphodon irregulare Duméril, Bibron & Duméril 1854

International Common Names

  • English: brown catsnake; brown treesnake

Local Common Names

  • Germany: Braune Nachtbaumnatter
  • Guam: culepla; kulebla

Summary of Invasiveness

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Native island species are predisposed and vulnerable to local extinction by invaders. When the brown tree snake (Boiga irregularis) was accidentally introduced to Guam it caused the local extinction of most of the island’s native bird and lizard species. It also caused "cascading" ecological effects by removing native pollinators, causing the subsequent decline of native plant species. The ecosystem fragility of other Pacific islands to which cargo flows from Guam has made the potential spread of the brown tree snake from Guam a major concern.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Metazoa
  •         Phylum: Chordata
  •             Subphylum: Vertebrata
  •                 Class: Reptilia
  •                     Order: Serpentes
  •                         Family: Colubridae
  •                             Genus: Boiga
  •                                 Species: Boiga irregularis

Description

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Boiga irregularis is a slender, climbing snake with large eyes and a vertical pupil, giving it improved nocturnal vision (Fritts & Leasman-Tanner 2001). The head is considerably wider than the neck. Markings may be either vague or distinct blotches on a brownish-yellow background. In parts of Australia, blue or red banding on a white background may be seen (Rodda 1999). Black speckling may also be present on some individuals. Brown tree snakes are about 38 centimeters at hatching and may reach three meters long, but are usually one to two meters. they are adept climbers and can crawl through very small openings (USDA-APHIS 2001).

B. irregularis is rear-fanged and mildly poisonous. The snake’s venom trickles into a bite victim along grooves in the rear fangs; because of the relatively small size and position of the fangs, a brown tree snake must chew to allow the fangs to penetrate the skin (USDA-APHIS 2001). The brown tree snake will readily strike when aggravated, but it does not present a danger to adults. A bite from this snake will not penetrate most clothing, Both constriction and venom are used to help immobilize prey (USDA-APHIS 2001), and babies less than 6 months old may be at risk from both brown tree snake bites and constriction (USDA-APHIS 2001). A young victim of a brown tree snake bite should receive immediate medical attention.

Distribution

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Native range: The brown treesnake is native to eastern Indonesia, New Guinea, Solomon Islands and the coastal areas of northern and eastern Australia (Savidge 1987, Rodda et al. 1992, Fritts and Rodda 1998, in Mortensen Dupont & Olesen 2008).
Known introduced range: The population on Guam is the only confirmed breeding population outside of its native range (Stanford & Rodda 2007). Brown treesnake encounter reports come from Micronesia, US Mainland, Hawaii and the Commonwealth of the Northern Mariana Islands (Stanford & Rodda 2007). While encounters have occurred in numerous locations, they tend to be concentrated on a few high-risk islands such as Saipan, Tinian and Oahu (Fritts 1987; Fritts 1988; McCoid & Stinson 1991; McCoid et al. 1994; Fritts et al. 1999, in Stanford & Rodda 2007). The large number of encounters on Saipan has led to speculation that a breeding population may already be established (Colvin et al. 2005).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

Chagos ArchipelagoPresentIntroduced Invasive ISSG, 2011
Cocos IslandsPresentIntroduced Invasive ISSG, 2011
IndonesiaPresentNative Not invasive ISSG, 2011
Japan
-Ryukyu ArchipelagoPresentIntroduced Invasive ISSG, 2011
TaiwanPresentIntroduced Invasive ISSG, 2011

North America

USA
-AlaskaPresentIntroducedISSG, 2011
-HawaiiPresentIntroduced Invasive ISSG, 2011
-OklahomaPresentIntroducedISSG, 2011
-TexasPresentIntroducedISSG, 2011

Europe

SpainPresentIntroducedISSG, 2011

Oceania

AustraliaPresentNative Not invasive ISSG, 2011
GuamPresentIntroduced1946-1950 Invasive ISSG, 2011
Marshall IslandsPresentIntroduced Invasive ISSG, 2011
Micronesia, Federated states ofPresentIntroduced Invasive ISSG, 2011
Northern Mariana IslandsPresentIntroduced Invasive ISSG, 2011
PalauAbsent, formerly presentStanford and Rodda, 2007; ISSG, 2011Reported once, not now present
Solomon IslandsPresentNative Not invasive ISSG, 2011
Wake IslandPresentIntroduced Invasive ISSG, 2011

Habitat

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In Papua New Guinea where it is native, B. irregularis occupies a wide variety of habitats at elevations up to 1200 meters. It is most commonly found in trees, caves, and near limestone cliffs, but frequently comes down to the ground to forage at night (Fritts & Leasman-Tanner 2001). It hides during the day in the crowns of palms, hollow logs, rock crevices, caves and even the dark corners of thatched houses near the roof (Fritts & Leasman-Tanner 2001). Based on frequent mention of this snake in relation to buildings, domestic poultry and caged birds, the snake is common in human-disturbed habitats and second-growth forests (Fritts & Leasman-Tanner 2001). In part of its native range in Indonesia, it is found in tropical evergreen forests, montane forests, lowland tropical forests, mangroves, montane savanna, wet savanna, seasonal dry forests and closed shrubland. It is also found in human-modified environments, such as deforested land, grassland and croplands (coffee, rice, rubber, coconut, tea and maize cultivations). On Guam, this secretive, nocturnal, and often arboreal snake is found in all terrestrial habitats, but is especially common in forests and human-modified environments (Rodda et al. 2002). The brown tree snake spends most days coiled in a cool and dark location, such as a treetop or a rotted log; it often takes refuge in Pandanus sp. trees (Hetherington et al. 2008). Snakes sighted in the Northern Mariana Islands occured in freshwater swamp forests, herbaceous wetland vegetation, tropical montane savanna, coastal strand vegetation and mangrove forest and in human-modified environments.

Habitat List

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CategoryHabitatPresenceStatus
Terrestrial-managed
Cultivated / agricultural land Present, no further details Harmful (pest or invasive)
Disturbed areas Present, no further details Harmful (pest or invasive)
Managed forests, plantations and orchards Present, no further details Harmful (pest or invasive)
Urban / peri-urban areas Present, no further details Harmful (pest or invasive)
Terrestrial-natural/semi-natural
Natural forests Present, no further details Harmful (pest or invasive)
Natural grasslands Present, no further details Harmful (pest or invasive)
Riverbanks Present, no further details Harmful (pest or invasive)
semi-natural/Scrub / shrublands Present, no further details Harmful (pest or invasive)
Wetlands Present, no further details Harmful (pest or invasive)

Biology and Ecology

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Nutrition
The brown tree snake will eat frogs, lizards, small mammals, birds and birds' eggs. In Papua New Guinea, eggs and chicks are regularly consumed, but mammals are more frequently taken (Fritts & Leasman-Tanner 2001). Having nearly depleted the bird populations on Guam, larger snakes have been found scavenging garbage and even sneaking in to steal a hamburger off the barbeque (Fritts & Leasman-Tanner 2001).

Brown tree snakes shift their diet from smaller exothermic prey to larger endothermic prey as they grow from juveniles to adults (Savidge 1988). This is usually seen as a switch from lizards to birds and mammals. Skinks such as Emoia caeruleocauda and Carlia ailanpalai (itself an invasive species) and geckos such as Lepidodactylus lugubris and Hemidactylus frenatus (which are very abundant in human commensal areas) serve as a superabundant food source for juvenile brown tree snakes in Guam. High densities of introduced vertebrates, in particular, the gecko H. frenatus have allowed the snake to attain the high densities seen there (Rodda Fritts & Conry 1992).

Reproduction
The reproductive characteristics of the brown tree snake are poorly known. The female produces four to 12 oblong eggs, 42 to 47 mm long and 18 to 22 mm wide; they have a leathery shell and often adhere together after the shells dry (Fritts & Leasman-Tanner 2001). The female deposits the eggs in hollow logs, rock crevices and other sites where they are likely protected from drying and high temperatures (Fritts & Leasman-Tanner 2001). Females may produce two clutches per year and the timing may depend on climate and prey abundance (Fritts & Leasman-Tanner 2001). Like other snake species, the female may be able to store sperm and produce eggs over several years after mating (Fritts & Leasman-Tanner 2001).

One puzzling result of brown tree snake reproductive studies is that reproductively active males appear to be relatively rare; this is surprising, because female reproductive activity occurs at all times of year in brown treesnakes (F. J. Qualls & C. P. Qualls Unpub. Data, Aldridge 1996 1998, Rodda et al. 1999c, in Rodda et al. 2002). From an adaptive perspective, one would expect males to be able to take advantage of mating opportunities at whatever time of year they encounter a receptive female. Yet reproductively-active males are relatively rare in samples of brown tree snakes (which are collected primarily with food-baited traps). One possible explanation for this phenomenon might be that snakes that are reproductively active are refractory to trap capture. Snake breeders report that male snakes in general avoid eating while they are in reproductive condition (N. Ford Pers. Comm., in Rodda et al. 2002).

Latitude/Altitude Ranges

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Means of Movement and Dispersal

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Introduction pathways to new locations
Aircraft: The brown tree snake is an excellent climber, using minute irregularities to ascend almost any structure, is extremely efficient at entering small openings and hiding in them for protracted periods and can survive for months without food (Perry et al. 1998). This allows it to be accidentally transported in both sea and air cargo (Perry et al. 1998). For example, brown tree snakes can hide in the wheel-wells of planes.
Biological control: The rapid spread of the snake in Guam after 1960 is unexplained. It is plausible that some people might have intentionally spread the snake to suppress rat populations, which were very high on Guam before establishment of the snake (Beardsley 1964, Savidge 1986, in Rodda Fritts & Conry 1992).
Military: Brown tree snakes are associated with large-scale military exercises and cargo moving from Guam to other posts. Guam is a hub for commercial and military shipments in the tropical western Pacific. High levels of transportation with regional and external locations greatly increases the threat of the brown tree snake being transported from Guam to new locations (Stanford & Rodda 2007).
Pet/aquarium trade: Intentional importation may be a relatively minor pathway for brown tree snake dispersal as it makes a poor pet, being drab, secretive and prone to biting (Stanford & Rodda 2007). ). Nonetheless, brown treesnakes were still offered for sale on the internet as late as December 2009.
Seafreight (container/bulk): The attraction of the brown tree snake to small, dark places (Pendleton 1947, in Rodda Fritts & Conry 1992) leaves little doubt that they are potential stowaways in military and non-military cargo (Rodda Fritts & Conry 1992).

Local dispersal methods
Natural dispersal (local): After being introduced into new locations brown tree snakes may spread in an area via natural dispersal.
Road vehicles: Brown tree snakes may be moved around unintentionally in or on vehicles.
Translocation of machinery/equipment (local): Brown tree snakes may be moved around unintentionally in or on translocated machinery.

 

Pathway Causes

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CauseNotesLong DistanceLocalReferences
Biological control Yes
Military movements Yes
Pet trade Yes Yes

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Aircraft Yes
Bulk freight or cargo Yes
Land vehicles Yes Yes
Machinery and equipment Yes Yes
Ship structures above the water line Yes

Impact Summary

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CategoryImpact
Biodiversity (generally) Negative
Crop production Negative
Economic/livelihood Negative

Impact

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General Impacts

Compiled by IUCN SSC Invasive Species Specialist Group (ISSG)
 
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.

Risk and Impact Factors

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Impact mechanisms

  • Poisoning
  • Predation

Impact outcomes

  • Negatively impacts agriculture
  • Negatively impacts animal health
  • Negatively impacts human health
  • Negatively impacts livelihoods
  • Negatively impacts tourism
  • Reduced native biodiversity
  • Threat to/ loss of endangered species
  • Threat to/ loss of native species

Invasiveness

  • Benefits from human association (i.e. it is a human commensal)
  • Is a habitat generalist
  • Proved invasive outside its native range

Prevention and Control

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Management Information

Compiled by IUCN SSC Invasive Species Specialist Group (ISSG)
 
Introduction
Guam's role as a trans-Pacific shipping hub and the tendency of snakes to seek refuge in cargo makes snake dispersal to neighbouring island ecosystems a high risk possibility (Fritts 1988; Fritts et al. 1999; Vice et al. 2003; Vice Engeman and Vice 2005). In 1995 the brown tree snake was listed by the US Department of Agriculture as among the top three pests requiring control and eradication. The fragility of island ecosystems means that the establishment of the brown tree snake in Hawaii could be an ecological disaster waiting to happen.
 
Preventative measures
In 1993 Wildlife Services (WS) and the US Department of Agriculture began a programme to reduce the potential for snakes to accidentally enter Guam’s transportation system (see Engeman et al. 2002; Vice et al. 2005, in Stanford & Rodda 2007). WS has taken the primary role in this effort through trapping, oral toxicants, fence line searches and the use of BTS detection dog teams (Stanford & Rodda 2007). The protection of port facilities and the examination of outbound cargo and planes require an extensive but highly cost-effective effort; this effort has minimised the probability of snakes being transported from Guam to new locations (Stanford & Rodda 2007).
 
To reduce the probability of the snake becoming established on neighbouring islands, the highest risk islands (Saipan, Oahu and Tinian) have established inbound interdiction protocols, emphasising dog-aided inspections of cargo from Guam. Sentinel traps have been placed within port areas in the Northern Mariana Islands to assist detection and capture of arriving brown tree snakes (Stanford & Rodda 2007). Control measures are not 100% full-proof and snakes may hide in cargo only to exit at the cargo’s final destination (Stanford & Rodda 2007).
 
The Global Invasive Species Programme is fostering a cooperative effort among experts in Guam, Australia, Hawaii and USA in an effort to combat this pest. In 2002 a multi-agency Rapid Response Team was established to capture brown tree snakes introduced from Guam to neighbouring islands to prevent its spread.
 
Physical Control
A variety of modified crawfish or minnow traps have been used on Guam as snake traps (Vice Engeman & Vice 2005). Trapping snakes with live-mouse (Mus domesticus) lures is the principal control technique for this invasive species on Guam (Gragg et al. 2007). Approximately 2500 snake traps have been placed on Guam (Rodda et al. 2002) on vegetation or along security fences (Vice Engeman and Vice 2005). Longer snakes are more easily trapped and males are slightly more easily trapped than females (Boyarski et al. 2008).
 
Rodda and colleagues (2002) found it possible to create small predator-free nature reserves using snake barriers and trapping methods. Campbell (1996, in Rodda et al. 2002) eliminated brown tree snakes from two one-hectare plots and found that lizard species showed a dramatic increase in abundance compared with similar snake-inhabited plots; within a year their numbers roughly doubled. The Campbell barriers brought attention to two acute problems: typhoons and rats. Rats chew holes in all things chewable and Guam is subjected to irregular but severe cyclonic storms which may damage barriers (Rodda et al. 2002).
 
Another area of concern identified in Campbell’s (1996) study is the potential for prey abundances to sharply increase inside barriers. Prey become more numerous in areas depleted of snakes and therefore any snake present inside an otherwise snake-free area has the option of either capturing abundant prey or entering a trap to get food. Thus, high prey abundances may depress snake trapping success. This problem may also affect efforts to eradicate an incipient population on a prey-rich island such as Saipan, where numerous brown tree snake sightings have been reported (Fritts et al. 1999, in Rodda et al. 2002).
 
The brown tree snake is an excellent climber making it difficult to construct a snake-proof barrier; Rodda and colleagues (2002) describe four classes of successful designs: temporary, bulge, masonry and vinyl. Bulge barriers are vulnerable to damage by strong typhoons, though they have a low initial cost. The vinyl barrier is durable, but the surface finish may degrade over time (surface finish keeps a snake from climbing the barrier). Masonry material is a pre-stressed moulded concrete design that is 100% successful in repelling snakes, impervious to rat and typhoon damage and has a fairly high initial cost (about USD 300 per meter). A conservative life expectancy of fifty years for the concrete barrier makes the cost reasonable (USD 6 per meter per year), but it is challenging to pay for this initial cost (Rodda et al. 2002).
 
Chemical Control
One component of brown tree snake management on Guam is the use of toxic bait, consisting of acetaminophen inserted into a dead newborn mouse, which is placed in a bait station or fitted with a tiny parachute to allow it to drift into trees (Avery Tillman & Savarie 2004; Savarie et al. 2005; Westbrook and Ramos 2005). Hazards to nontarget species, especially the Mariana crow (Corvus kubaryi) which ingests mice, are of concern (Avery Tillman & Savarie 2004).
 
Efforts are being made to develop attractants and repellents. Aerosolized essential oils from cedarwood, cinnamon, sage, juniper berry, lavender and rosemary as well as derivatives of food and flavor ingredients have all been demonstrated as effective irritants to repel brown tree snakes (Clark & Shivik 2002).
 
The use of methyl bromide at an application rate of 24 gMBm-3 was found to be an effective means of fumigating cargo to exterminate the brown tree snake (Savarie et al. 2005).
 
Biological Control
Historical evidence shows that the biological control of vertebrates is fraught with unacceptable risk. Cane toads and poisonous red-bellied black snakes may prey on the young of the brown tree snake occasionally but the introduction of either could conceivably cause more problems than it could potentially solve (Caudell et al. 2002), particularly in light of the devastating impacts of the cane toad (Bufo marinus) in Australia.
 
Integrated Pest Management (IPM)
Conservation actions on Guam should be directed towards an improved recruitment of flora indirectly affected by the brown tree snake, for example, via artificial pollination and/or planting (see Ecology Page Impacts information from Mortensen Dupont & Olesen 2008). Restoring conditions for natural pollination or managing reproduction of affected plants is critical in the long-term conservation of native vegetation on Guam (Mortensen Dupont & Olesen 2008). Efforts are now underway to conserve the few remaining larger areas of uniform forest vegetation, e.g. the conservation action plan Guam Comprehensive Wildlife Conservation Strategy (2005).

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Reviewed by: Dr. Gad Perry, Associate Professor, Conservation Biology Texas Tech University, USA.

Principal sources: Rodda et al., 1999; Fritts & Leasman-Tanner, 2001; Mortensen et al., 2008

    Compiled by: Revision: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
Last Modified: Sunday, August 16, 2009

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