Reptiles
Brown Tree Snake
Boiga irregularis · also called brown tree snake, brown catsnake
In the forests of northern Australia and Melanesia, the brown tree snake is nothing special: a slender, night-hunting, mildly venomous tree snake that eats birds, lizards, and rodents. Put it on an island that never had a snake, though, and it becomes one of the most destructive invasive animals on record. On Guam, it ate its way through most of the native wildlife — and it is still knocking out the power.
Overview
The brown tree snake is native to coastal northern and eastern Australia, eastern Indonesia, Papua New Guinea, and the Solomon Islands, where it is common and unremarkable. It is usually 1–2 m long, slender, with a big head, vertical pupils, and reddish-brown colouring. It is rear-fanged with a weak venom — a minor bite risk to adults, more serious for infants.
The Guam disaster
The snake reached Guam as a cargo stowaway in the late 1940s or early 1950s, onto an island with no snakes and no defences. The population exploded to an estimated two million, and by the 1980s ten of Guam’s twelve native forest bird species were gone from the wild, along with lizards and both native bats.
An unravelled ecosystem
Losing the birds that pollinated and dispersed seeds has slowed the regrowth of Guam’s forests, and spider numbers surged once insect-eating birds vanished. The snakes also climb power lines, causing thousands of blackouts worth millions of dollars.
Status and biosecurity
In its native range the brown tree snake is Least Concern and secure. The concern is the reverse: stopping it reaching other islands. Guam runs an intensive programme of detector dogs, barriers, traps, and air-dropped baits, and islands like Hawai’i watch every incoming shipment.
Brown Tree Snake: common questions
How did the brown tree snake get to Guam?
It is native to a wide area of the western Pacific, and it almost certainly reached Guam as an accidental stowaway in cargo. The most likely period is the late 1940s to early 1950s, when large volumes of military equipment and supplies were being moved around the Pacific after the Second World War, some of it from areas within the snake's native range such as Papua New Guinea and the Admiralty Islands. A snake or a few snakes hidden in that freight arrived on an island that had never had a snake, and there was nothing to stop them.
How much damage did it do?
On Guam, an enormous amount. The snake population built up to an estimated two million animals, and it systematically ate its way through the island's wildlife. Ten of the twelve native forest bird species were lost from the wild, several native lizards vanished or crashed, and both of Guam's native bats were hit hard. Because those birds and bats pollinated plants and dispersed seeds, the forest itself has been affected, regenerating more slowly and with a changed mix of species, and spider numbers have surged in the absence of insect-eating birds. It is one of the most-cited cases of a single invasive predator transforming an island ecosystem.
Is the brown tree snake dangerous to people?
Only mildly. It is a rear-fanged snake with a weak venom that it delivers by chewing, effective on small prey but not a serious threat to a healthy adult, in whom a bite typically causes local swelling, redness, and soreness. The exception is very young children; there have been cases on Guam of infants bitten in their cots suffering more significant effects, because the venom dose is large relative to body size and the snake may hang on. The snake is not aggressive and bites are uncommon, but it is bold about entering houses in search of prey.
Why is it such a big concern for Hawaii and other islands?
Because Guam shows exactly what a brown tree snake introduction can do, and Hawaii and many other Pacific islands have the same vulnerability, namely rich native bird faunas that evolved without snakes. Guam is a major air and sea transport hub, so snakes regularly turn up in outbound cargo and aircraft wheel wells, and there have been several interceptions in Hawaii. Preventing establishment is far cheaper than trying to reverse it, so there is an intensive interdiction programme on Guam, with trained dogs inspecting outbound freight, and heightened vigilance at destination ports across the region.