Guam National Wildlife Refuge beach. Credit: Laura Beuregard / Wikimedia Commons / CC BY 2.0
Guam, a small island in the western Pacific Ocean, has become the epicenter of one of the most severe ecological disruptions in recent history. Once known for its vibrant forests and diverse birdlife, the island is now dominated by an unlikely apex predator: the brown tree snake.
These invasive reptiles, introduced after World War II, have multiplied into the millions, fundamentally altering the balance of Guam’s ecosystems.
In the decades since their accidental arrival, likely through military cargo shipments, brown tree snakes have silently spread across the island, feeding aggressively on birds, mammals, other reptiles, and even human food scraps.
The consequences have been devastating. Today, Guam’s forests are eerily quiet, stripped of the birds that once filled the air with sound and kept insect populations in check.
Out of the 12 native bird species once common in Guam’s forests, 10 have disappeared. The two that remain survive only in caves and scattered urban areas, far from their natural habitat. The brown tree snake, a nocturnal and elusive hunter, now reigns over this fragile environment.
Snakes eat everything, even what they can’t swallow
Researchers like Haldre Rogers, an ecologist at Virginia Tech who has studied the island’s ecology for over two decades, have observed the far-reaching consequences firsthand. She recalled attending a gathering on the island where a roasted pig was left unattended.
When guests returned, a brown tree snake had wrapped itself around the meat, tearing off pieces and swallowing them whole. For Rogers, it was a startling yet familiar reminder of the snake’s dominance on the island.
The brown tree snake’s impact on Guam is not limited to the extinction of birds. With few predators remaining, the reptiles have shifted their diets to include small mammals, other reptile species, and even each other.
Brown tree snake. Credit: Pavel Kirillov / Wikimedia Commons / CC BY-SA 2.0
Henry Pollock, executive director of the Southern Plains Land Trust and former researcher in Guam, said the snakes are indiscriminate eaters and have been observed consuming animals nearly their own size.
In response to the avian collapse, a surprising new species has flourished: spiders. With their main predators gone, spider populations in Guam have exploded. Parts of the forest have turned into web-covered landscapes.
Surveys have found the island hosts between 508 million and 733 million spiders, roughly 40 times more than neighboring islands such as Rota, Tinian, and Saipan. These figures only include spiders within two meters (6.6 feet) of the ground, suggesting the actual number is significantly higher.
A web-filled forest, alive with spiders
The abundance of spiders is visible throughout the year. In contrast to the seasonal fluctuations seen on other islands, Guam’s spider numbers remain consistently high.
Rogers described the scene as something out of a Halloween movie: dense layers of silk blanketed trees, paths, and open spaces. On hikes, it’s common for the lead person to carry a stick to clear webs or risk being covered in them.
The island’s forests are home to a variety of striking spider species. Banana spiders with golden webs, huntsman spiders as large as a human hand, and tent-web spiders that create vast communal structures known as “condo webs” all thrive in Guam’s environment.
Banana spider golden silk orb-weaver. Credit: bansheed / Flickr / CC BY-NC-SA 2.0
Rogers noted that these condo webs can extend from the forest floor to the canopy, often housing dozens of spiders in a single structure.
These spiders, known locally in the Chamorro language as sånye’ye’, are not the only sign of the forest’s imbalance. The forest itself is beginning to change. With birds gone, many native trees are struggling to reproduce.
Around 70 percent of Guam’s native trees rely on birds to disperse their seeds. Now, fruits fall to the ground and rot, or seeds fail to germinate due to a lack of dispersal or poor growing conditions under parent trees.
The silent collapse of Guam’s forests
This disruption has slowed forest regeneration. In a healthy system, fallen trees create gaps that new plants quickly fill. On Guam, these gaps often remain empty. Rogers compared it to tearing down a building in a city, typically prime real estate, but now left untouched due to the absence of seed-dispersing birds.
The brown tree snake’s predatory behavior has become even more disturbing. In a recent study near Andersen Air Force Base, Rogers and her team tracked young Såli, a forest starling species that still survives in small numbers.
Using radio transmitters, the team discovered many of the devices inside snake stomachs. But even more unsettling were fledglings found dead, coated in snake saliva, but uneaten. In about half of the cases, the birds were too large to swallow. The snakes killed them anyway and moved on.
The brown tree snake is not only ruthless but also highly adaptive. To protect the remaining Såli, conservationists placed nest boxes on tall metal poles with slippery surfaces, believed to be unclimbable.
But in 2021, Julie Savidge, a professor at Colorado State University, and her team documented an entirely new behavior: lasso climbing. The snakes were able to wrap themselves around the poles, looping their tails and inching upward, mimicking the motion of climbing a coconut tree.
Snake control is expensive and often ineffective
Despite decades of control efforts, Guam’s brown tree snake population continues to grow. Authorities have tried nearly every method: visual surveys, traps, chemical repellents, toxic baits, and even research into potential viruses that could target the species without harming others.
This approach is similar to the controversial use of myxomatosis in rabbits, which caused widespread suffering and raised ethical concerns.
The U.S. government currently spends about $3.8 million annually on snake control in Guam. But success has been limited. One of the few effective strategies has been implemented at Andersen Air Force Base.
There, snakes were baited with acetaminophen, common in over-the-counter pain relievers but lethal to brown tree snakes. Just 80 milligrams, a fraction of a standard tablet for humans, is enough to kill even large individuals.
To prevent reinvasion, the area was surrounded by a snake-proof fence. Inside this protected zone, snake numbers have dropped significantly. But replicating this success across the island’s rugged terrain is considered unfeasible by most scientists.
A future held hostage by a predator
As the forest continues to lose its birds, the long-term consequences remain uncertain. Many tree species are already declining, and the structural integrity of the forest is weakening. Without intervention, the damage may be irreversible soon.
For now, Guam remains an island quietly ruled by snakes; stealthy, persistent, and nearly impossible to control.
The spiders that now blanket its forests are only the most visible sign of a deeper ecological collapse, one that may take generations to repair; if repair is even possible.