An accidental introduction of brown tree snake decades ago on the island of Guam has led to the near-complete disappearance of the island’s birdlife and created a chain reaction that continues to transform its natural environment.
The brown tree snake, native to Australia and Papua New Guinea, arrived in Guam shortly after World War II, likely hidden in military cargo. Since then, the snakes have rapidly multiplied, now numbering in the millions. With no natural predators on the island, the snakes have taken control of the food chain — and the consequences are becoming more visible each year.
Bird loss triggers chain reaction across island ecosystem
In the years following their arrival, brown tree snakes quickly spread across the island, feeding on native birds, small mammals, reptiles, and even food left out by residents. Their hunting has been especially devastating to bird species. Of the 12 native forest birds once commonly found in Guam, 10 have vanished entirely. The remaining two survive only in scattered urban areas and caves, far from their natural habitats.
Without birds to eat insects and disperse seeds, Guam’s forests have changed dramatically. Insects, especially spiders, now thrive. A study found that the island has up to 40 times more spiders than nearby islands Rota and Saipan — with hundreds of millions covering the landscape. Silk webs coat trees, trails, and open spaces throughout the year, creating eerie, Halloween-like scenes.
Ecologists say these changes reflect a deeper problem. It’s not just about the birds disappearing — it’s about an entire system falling apart, said Haldre Rogers, a longtime researcher who has studied Guam’s ecology for over 20 years. She has witnessed firsthand how dominant the brown tree snake has become, recounting a moment when a snake wrapped itself around a whole roasted pig at a gathering, attempting to swallow chunks of it.
Forests struggle to regrow as snakes adapt and thrive
The damage extends beyond visible changes in wildlife. About 70% of Guam’s native trees rely on birds to spread their seeds. Now, seeds drop directly beneath parent trees, where they often fail to grow due to poor conditions. As a result, empty spaces in the forest — once quickly filled by new plants — remain bare.
The brown tree snake continues to adapt in surprising ways. In one study, researchers found dead chicks covered in snake saliva but left uneaten — evidence that the snakes sometimes kill prey too large to swallow. In 2021, researchers recorded the snakes climbing metal poles to reach bird nests, using a looping technique now known as “lasso climbing.”
Despite decades of effort, controlling the snake population remains a challenge. The United States government spends nearly US$4 million a year on various control measures — including traps, toxic baits, and chemical deterrents — but the impact has been limited.
One method that has shown success involves dropping small doses of acetaminophen, a common painkiller deadly to the snakes, into protected areas. At Andersen Air Force Base, this strategy, combined with a snake-proof fence, has significantly reduced snake numbers. However, applying the same approach across Guam’s dense terrain is unlikely.
The island’s future hangs in the balance
As Guam’s forests lose their birds and tree species decline, experts warn the long-term effects could be irreversible. The changes may not only impact the island’s ecology but also affect local agriculture, tourism, and daily life for residents.
The brown tree snake now rules much of Guam’s natural landscape — a stealthy predator nearly impossible to contain. And while spiders dominate the visible picture, the real concern lies deeper: an ecosystem unraveling, one species at a time.