First detected in the 1950s, an invasive snake has multiplied to more than 2 million on a US territory, wiping out native birds and surprisingly causing another species to boom to levels 40 times higher than on surrounding areas

On a small US territory in the Western Pacific, an invasive snake has spent decades reshaping an ecosystem that evolved without such a predator. The brown treesnake, a nocturnal, tree-climbing reptile native to parts of Australia, Indonesia, Papua New Guinea and the Solomon Islands, arrived on Guam accidentally after World War II, most likely as a stowaway in cargo. The US National Park Service says its introduction ultimately produced an ecological disaster and the loss of almost all of the territory’s native forest birds.

What began with an accidental introduction eventually became an extraordinary population explosion. The US Department of Agriculture’s Animal and Plant Health Inspection Service says localized brown treesnake densities on Guam can reach 20 or more snakes per acre, among the highest snake densities ever recorded. With Guam covering roughly 130,000 acres, estimates cited by US authorities put the island’s snake population at more than 2 million.

But the most revealing consequence of the invasion may be found not in the number of snakes, but in what happened after the birds began disappearing.

When the birds vanished, something else flourished

The brown treesnake became an extraordinarily effective predator in an environment whose native wildlife had not evolved alongside snakes of this kind. Over the following decades, it preyed on birds, lizards and bats, steadily stripping the island of animals that once played important roles in its forests.

The National Park Service says only two native forest birds now persist on Guam, the Mariana Swiftlet and Micronesian Starling. As bird numbers collapsed, the snakes increasingly turned to other available prey, including lizards, rats, mice and bats.

Then came an unexpected consequence. With insect-eating birds largely gone, the ecological pressure they once exerted on spiders was dramatically reduced. A 2012 study published in PLOS ONE treated Guam as a rare natural experiment, comparing its forests with those of nearby Rota, Tinian and Saipan, where healthy bird populations remained.

The result was striking. During the wet season, researchers found 40 times the density of spider webs on Guam compared with the nearby islands. They recorded an average of 18.37 webs per 10 metres on Guam, versus just 0.45 on the neighboring islands.

The researchers concluded that the disappearance of birds had fundamentally altered the balance of the food web. Birds can eat spiders and compete with them for insect prey; removing those pressures gave spiders a powerful ecological advantage.

According to a 2024 BBC report cited by AS USA, estimates put the number of spiders in Guam’s forests at between 508 million and 733 million.

The invasion reaches beyond the forest

The damage is not confined to wildlife. The US Department of the Interior says brown treesnakes cause about $4.5 million in annual losses to Guam’s economy through power outages. The snakes can climb electrical infrastructure, where they cause ground faults and short circuits. Interior has described the losses as affecting the island’s power system and broader economic activity.

The US government has consequently spent millions of dollars trying to suppress the snake population and, crucially, prevent it from establishing elsewhere. In fiscal year 2023 alone, Interior distributed nearly $3.65 million for the Brown Treesnake Control Program, which involves federal, territorial and state partners.

Hawaii is a particular concern because of its transportation links with Guam. US officials estimate that a brown treesnake establishment there could eventually produce $593 million to $2.14 billion in annual economic damage.

The brown treesnake invasion therefore offers a particularly stark lesson in how invasive species operate. The snake did not merely add a new predator to an island. It altered who survived, who disappeared and who suddenly flourished.