The brown tree snake, Boiga irregularis, is recognized globally as one of the most destructive invasive species ever introduced to an island ecosystem. Its establishment on the Pacific island of Guam resulted in an ecological disaster, causing the rapid collapse of native bird populations. The snake’s arrival was not a deliberate act but a biological accident tied directly to military traffic and logistical operations following the end of World War II. Understanding the pathway of this accidental introduction is central to comprehending the damage that followed.
Identifying the Brown Tree Snake
The brown tree snake is a slender, arboreal species native to parts of eastern Indonesia, Papua New Guinea, the Solomon Islands, and coastal Australia. It possesses a long, agile body that can reach up to 10 feet in length, though most on Guam are between three and six feet long. The snake is nocturnal, with large eyes featuring vertical, cat-like pupils that aid in hunting under cover of darkness.
As a member of the family Colubridae, the species is rear-fanged and considered mildly venomous, posing little threat to healthy adult humans. Its coloration is typically a light brown or yellowish-brown, sometimes with darker crossbands, allowing it to camouflage effectively in trees and dense foliage. This combination of being an agile, nocturnal predator allowed it to thrive undetected in its new environment.
The Mechanism of Accidental Arrival
The brown tree snake most likely arrived on Guam sometime between the end of World War II and the early 1950s, a period of intense military demobilization and shipping activity. The probable source population is traced to the Admiralty Islands group of northern Papua New Guinea, where the United States military had established large bases during the conflict. The vast movement of equipment and supplies from these former war zones back to major transport hubs inadvertently created the vector for the snake’s introduction.
The snake’s ability to survive for long periods without food and its habit of seeking dark, confined spaces made it an ideal stowaway. It is believed that the initial invaders were passively transported in military sea cargo, such as shipping containers, construction materials, and crated goods. Major military installations, including Naval Base Guam and Andersen Air Force Base, served as the primary entry points where the cargo was offloaded.
The initial population was small and went unnoticed for decades, likely due to the snake’s nocturnal habits and the extensive amount of cargo entering the island. The snakes could have also stowed away in the wheel wells or landing gear bays of Guam-bound aircraft originating from airfields in their native range. The continuous flow of military and commercial transport following the war provided multiple opportunities for the snakes to successfully make the journey.
Ecological and Infrastructure Consequences
Once established, the brown tree snake population grew exponentially due to the complete absence of natural predators like raptors or large mammals on Guam. This unchecked reproduction led to unprecedented population densities, reaching estimates as high as 30,000 snakes per square mile in some areas. The island’s native fauna, having evolved without a snake predator, lacked defensive behaviors against the new threat, making them easy prey.
The ecological devastation was swift and profound, resulting in the extinction or extirpation of nine out of the eleven native forest bird species on Guam. These birds, which included the Guam rail and the Guam kingfisher, were unable to defend their nests or young against the agile, nighttime predator. The resulting loss of native bird species has created cascading effects on the ecosystem, particularly disrupting the natural process of seed dispersal for many native plants.
Beyond the biological impact, the snakes created a costly problem for the island’s infrastructure. The arboreal species frequently climbs utility poles and high-tension electrical wires, seeking new vantage points or hunting prey. When they bridge two electrical components, they cause short circuits that lead to frequent and widespread power outages across the island. These incidents occur approximately once every four days and are estimated to cost the island millions of dollars annually in repairs and lost economic productivity.