“Ecological disaster” follows the arrival of snakes and millions of spiders on a small U.S. island in the Pacific

Since the middle of the 20th century, a non-native species of snake has wrought havoc with the ecosystem on Guam, a small U.S. territory nestled in the Western Pacific Ocean.

Most notably, the snake’s invasion of the 210-square-mile, thickly-forested island has obliterated numerous populations of indigenous birds.

The brown tree snake (BTS), a three to four-foot-long species that is native to Australia, Indonesia, Papa New Guinea and the Solomon Islands, was first detected on Guam in the 1950s, says the U.S. Department of Agriculture’s (USDA) National Invasive Species Information Center.

The BTS is believed to have been imported to Guam by accident, as a stowaway in cargo.

And in the decades since its arrival, the BTS’s impact has been devastating. Indeed, the U.S.’s National Park Service describes the snake’s invasion as an “ecological disaster”.

How many bird species have the snakes eliminated?

With no natural predators to contend with, the BTS’s population on Guam has rocketed. At the same time, the snake’s predilection for feeding on avians has left the island with just two remaining native forest birds: the Mariana Swiftlet and the Micronesian Starling.

Guam previously boasted 12 native species of land birds, notes Professor Julie A. Savidge, a conservation biologist at Colorado State University, in her research paper Extinction of an Island Forest Avifauna by an Introduced Snake.

Published in 1987, Professor Savidge’s paper shed crucial light on the BTS’s central role in Guam’s rapidly dropping bird population.

How many brown tree snakes are there on Guam?

At present, it is estimated that there are an average of about 20 brown tree snakes per acre on Guam, according to the USDA’s Animal and Plant Health Inspection Service (APHIS). Given the island’s size of about 130,000 acres, that translates into a BTS population of well over two million.

“These population density estimates are among the highest snake densities ever recorded,” APHIS says.

As well as wiping out native birds, the BTS’s voracious, wide-ranging appetite has also seen it eliminate several species of lizards and bats, APHIS adds. On top of that, the snakes are known to gobble up a range of other animals, such as rats and mice.

Snakes-birds-spiders

Guam’s decimated bird population is believed to be a major reason why, along with the BTS, the island is now also overrun with spiders.

In a 2012 study carried out by biologists from a group of U.S. universities, it was found that Guam’s spider population has, amid a declining population of birds, soared to as much as 40 times that of surrounding islands.

The research showed that “birds have a strong effect on spiders”, said Dr. Haldre Rogers, a co-author of the study, who is now an associate professor at Virginia Tech’s Department of Fish and Wildlife Conservation.

This is because birds are not only known to eat spiders, but also tend to rival arachnids for insect prey. “Anytime you have a reduction in insectivorous birds, the system will probably respond with an increase in spiders.“ Dr. Rogers said.

Speaking to the BBC last year, Dr. Rogers explained that Guam’s forests are now brimming with spiders’ webs.

“It’s enough that when you’re hiking, it’s common for the person in front to pick up a spider stick and knock down the webs as they go,” she said. “Otherwise you’ll be covered in spider webs…”

Per an October 2024 report by the BBC journalist Zaria Garrett, it has been estimated that the island’s forests are now carpeted by somewhere between 508 million and 733 million spiders.

“The reptiles are winning”

Faced by the dire consequences of the brown tree snake invasion, the U.S. government spends millions of dollars a year on its BTS Control Program, a multi-agency effort to tackle the snake’s rampant proliferation on Guam.

But, as Garrett notes, “the reptiles are winning […]. The invaders have proved to be impossible to remove in large numbers.”

The U.S.’s Department of the Interior, which oversees the funding of the BTS Control Program, appears to agree that BTS elimination efforts have so far failed to deliver results on a significant scale.

“It is largely accepted that severe population suppression must be achieved before eradication can be pursued,” the DOI conceded in a program overview in July 2025.

Nevertheless, the DOI added, research is underway “to improve existing BTS control tools, such as barriers, traps, and baits, as well as to develop better control techniques such as toxins, repellents, and attractants.”

“Costs would grow exponentially”

Crucially, the BTS Control Program is also tasked with ensuring the brown tree snake does not spread from Guam to other islands in the Pacific Ocean.

“Since Guam is a major transportation hub in the Pacific, numerous opportunities exist for this invasive species to be inadvertently introduced to other areas,” the DOI says.

Of particular concern is the possibility that the BTS could make it to Hawaii, an island U.S. state that sits between Guam and the American mainland.

Were the snake to reach the Hawaiian islands, the DOI cautions, “interdiction costs would grow exponentially”. (So, too, would the danger of a subsequent invasion in the continental U.S., given the regular transportation links between Hawaii and the mainland.)

It’s not only the threat that the brown tree snake would pose to Hawaii’s abundant native wildlife; there’s also the significant impact the BTS’s arrival could have on the islands’ economic health.

On Guam, BTS have been known to cause electrical outages by interfering with power lines, leading the island’s economy to suffer estimated annual losses of around $4.5 million.

On Hawaii, however, the damage would potentially be far greater: between $593 million and $2.14 billion each year, says the USDA’s National Wildlife Research Center.

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