In 1940, a Creature Snuck Onto an Island in the U.S. and Devoured Everything. Today, Two of Man’s Most Feared Species Coexist Alone

History is filled with stories of islands where nature doesn’t always come together in predictable ways. Some species are so incompatible that others vanish in their presence. A notable example is Japan’s human-facilitated changes in biodiversity. But in Guam, the “stowaway” arrived unnoticed.

A devastated ecosystem. In the jungles of Guam, one of the 14 unincorporated U.S. territories, an ecological paradox is unfolding. The island’s forests now have up to 40 times more spiders than the neighboring Pacific islands—not because of its original habitat, but because of an invasive species that arrived in the 1940s.

The island is in a cycle of ecological upheaval, with brown tree snakes and a rampant spider population almost entirely displacing native wildlife. These invasive species have dramatically altered Guam’s landscape, creating a disturbing environment and irreversibly changing its biodiversity.

An accident. Haldre Rogers, an associate professor in Virginia Tech who has studied Guam’s ecology for 22 years, shared a scene he witnessed at a gathering on the island five years ago with the BBC. While visitors roasted a pig outside the venue, an unexpected guest appeared: a brown, shiny, scaly creature with vertical eyes and a wide mouth. Wrapping itself around the pig, the creature tore off chunks of meat and swallowed them whole.

The surprise “visitor” was a brown tree snake, an invasive species believed to have arrived on Guam in the 1940s, likely after stowing away on a cargo ship. Since then, the snakes have triggered an ecological disaster unlike anything seen before on the island. Within a few decades, these predators nearly wiped out Guam’s native bird species, which had no defenses against the newcomers.

Without birds, Guam’s forests have lost not only their song but also a crucial ecological function: seed dispersal, which once supported the regeneration of the island’s lush trees.

Spider webs. The absence of birds has led to another unique phenomenon: A boom in spider populations that has transformed the forest into a landscape dominated by webs stretching in all directions. The spiders, facing no natural predators, have multiplied, blanketing the vegetation in dense webs that drape over the forest like a veil.

In every clearing and corner of the island, spiders, especially those of the Argyrodes species, have woven complex communal structures. This is a visible manifestation of the ecosystem change quietly unfolding in Guam.

A forest structure in danger. The loss of birds has altered the dynamics of Guam’s forests. Without birds to disperse seeds, many trees are no longer reproducing, threatening the forest’s natural regeneration and placing the ecosystem at serious risk.

No solution. Despite tireless efforts by researchers to control the snake population, from deploying poisoned bait to developing “snake-safe” climbing methods, their success has been limited. An estimated two million brown tree snakes now inhabit the island.

The brown tree snake has proven to be a formidable adversary. Its adaptability and resistance to eradication methods have thwarted even the most well-funded projects. A few controlled areas, such as Andersen Air Force Base, have managed to keep the snake population in check, but much of the island remains dominated by this invasive species, which arrived stealthily and decimated nearly every native species.

Uncertain future. With a forest losing its diversity and an unbalanced food web, Guam’s future faces an irreversible transformation, sparking study and debate. Once a vibrant ecosystem, the island has become a natural laboratory where species interactions have collapsed, and new dynamics, controlled by invaders, have emerged.

Ultimately, Guam’s plight raises serious questions about the fragility of island ecosystems and the enduring impact of invasive species. It stands as a stark reminder of the power of invasive forces to reshape nature’s boundaries, leaving behind an ecosystem marked by silence and a dense tangle of cobwebs.

Image | Philip Davis (Unsplash) | Animalia

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