The brown tree snake, Boiga irregularis, is an infamous invasive species that profoundly altered the ecology of Guam. This slender, nocturnal reptile is native to coastal Australia, Papua New Guinea, the Solomon Islands, and eastern Indonesia. Its notoriety stems from its accidental introduction to Guam shortly after World War II, likely as a stowaway in military cargo or aircraft landing gear. Its arrival set the stage for an ecological disaster that continues to define the island’s landscape and challenge its infrastructure. The snake’s ability to thrive, combined with the extreme vulnerability of Guam’s native wildlife, allowed its population to explode.
Biological Factors Driving Invasiveness
The inherent biological traits of the brown tree snake make it a highly effective invasive predator. This species is highly arboreal, spending most of its active time at night hunting in the trees and canopy. Its specialized locomotion, including the ability to form a “lasso” around cylindrical objects, allows it to climb utility poles and other structures with ease.
The snake is a non-selective, opportunistic feeder with a broad diet that includes birds, eggs, lizards, and small mammals. This flexibility means that as one food source is depleted, the snake can quickly switch to another, which helped it survive and spread across Guam. Although the snake is mildly venomous and rear-fanged, its venom aids significantly in quickly subduing its prey.
Reproductively, the brown tree snake is well-suited for colonization. The population on Guam reproduces year-round, unlike its seasonal cycle in its native range. Females lay small clutches of eggs, typically averaging around four to five eggs per clutch, in dark, sheltered locations. The ability to lay multiple clutches annually, coupled with a lack of population control mechanisms, fueled the rapid increase in snake density.
Ecological Vulnerability of Invaded Habitats
The primary reason the brown tree snake became such a devastating invader is the “ecological naiveté” among Guam’s native fauna. The island’s birds and other vertebrates evolved for millennia without the presence of an arboreal, nocturnal snake predator. Consequently, these native species never developed defensive behaviors, such as recognizing the snake as a danger or building snake-proof nests.
Many of Guam’s native bird species were either flightless or ground-nesting, making them exceptionally easy prey. They did not possess the protective instincts or physical adaptations seen in animals that co-evolved with arboreal snakes. The absence of native predators capable of controlling the snake’s population on Guam also contributed significantly to the problem. Guam lacked any equivalent natural enemy, allowing the snake to grow to unprecedented population densities. This combination of an effective predator and defenseless prey created a perfect storm for the island’s biodiversity.
Devastating Impact on Guam’s Ecosystem and Infrastructure
The consequences of the brown tree snake’s invasion have been catastrophic, affecting both Guam’s natural environment and its infrastructure. Ecologically, the snake is responsible for an extinction cascade that has nearly silenced the island’s forests. It has caused the extirpation or extinction of approximately 10 of Guam’s 12 native forest bird species, including the Guam rail and the Guam kingfisher, which now only survive in captive breeding programs.
The removal of these native birds has caused significant secondary effects throughout the ecosystem, particularly concerning plant life. Many of Guam’s native trees rely on frugivorous birds to eat their fruit and disperse their seeds. Without these flying seed dispersers, tree regeneration has been drastically reduced, with some studies estimating a drop in new tree growth by as much as 92% in certain areas.
The snake has also become a severe economic and infrastructural burden. Its habit of climbing utility poles and electrical infrastructure leads to frequent short circuits and power outages. Guam suffers hundreds of these outages annually, causing costly damage to the electrical grid and impacting military, commercial, and residential operations. The direct costs associated with repairing infrastructure damage and lost revenue are estimated to exceed one million dollars each year.
Current Mitigation and Prevention Efforts
Control and prevention efforts focus on two main strategies: reducing the existing snake population on Guam and preventing its accidental spread to other vulnerable Pacific islands. The most innovative control method involves the aerial delivery of toxic baits to the snake-infested areas. This method uses dead, thumb-sized mice as bait, each containing an 80-milligram dose of the common human pain reliever acetaminophen.
Acetaminophen is highly toxic to the brown tree snake, disrupting the oxygen-carrying function of its hemoglobin, leading to a quick death. This specific bait is designed to be delivered into the forest canopy, where the nocturnal snakes are most likely to encounter it, while minimizing the risk to non-target species. Trapping is also used extensively, employing specialized traps with live mouse lures placed around high-risk areas like ports and airfields.
Interdiction efforts are a major part of the strategy to protect other islands. These measures include rigorous inspection of all outgoing cargo, aircraft, and military equipment leaving the island. Specially trained teams of detector dogs are employed at ports to sniff out stowaway snakes hiding within cargo containers or in the wheel wells of planes. These multi-faceted efforts are designed to contain the existing threat and prevent the brown tree snake from establishing itself elsewhere in the Pacific.