A small Pacific island nation has become the centre of a severe ecological disaster (Image: Getty)
A tiny US island territory in the Western Pacific Ocean was once known for its vibrant forests and diverse birdlife, but now it is suffering from a population of invasive snakes so greedy that they have wiped out every bird in its forests and a spider population 40 times higher than that of its neighbouring islands. Today, Guam has become the epicentre of one of the most severe ecological disruptions in recent history, thanks to the brown tree snake (Boiga irregularis).
These invasive reptiles were accidentally introduced after World War 2, likely through military cargo shipments, and have since fundamentally altered the balance of Guam’s ecosystems. Now with numbers around two million, the brown tree snakes have spread across the island, feeding aggressively on birds, mammals, other reptiles and even human scraps left by the 168,000-strong population. 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. Today, the island’s forests suffer from an eerie lack of birdsong, and an out-of-control insect population, since there are no birds to keep it in check.
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Brown tree snakes are an invasive species in Guam, likely having arrived via military cargo shipments (Image: Getty)
In 2018, researcher Haldre Rogers, an ecologist at Virginia Tech who has studied the island’s ecology for over two decades, witnessed the brown tree snake in action first-hand. She attended a gathering in Guam, where a roasted pig was left unattended. When guests returned, a brown tree snake had wrapped itself around the meat, swallowing large chunks of meat whole.
With few predators remaining, the reptiles have shifted their diets to include small mammals, other reptile species and even each other. 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 a recent study near Andersen Air Force Base, Ms Rogers and her team tracked young Såli, a forest starling species that has managed to survive in small numbers. Shockingly, the team found that the radio transmitters they had used were inside snake stomachs. But even more unsettling was the fact that fledglings were found dead, coated in snake saliva, but uneaten. In about half of the cases, the birds were too large to swallow – the snakes had killed them and then moved on to their next target.
Many native tree species in Guam are now struggling to reproduce, as they rely on birds to disperse their seeds (Image: Getty)
Meanwhile, with birds gone, the forest itself is beginning to change, with many native tree species struggling to reproduce. Around 70% 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.
Yet, there has been one notable beneficiary of Guam’s snake infestation.
On most of the Mariana Islands, there are relatively few spiders in the rainy season, with a large spike as the climate dries out. However, on Guam, there is a near-continuous array of spider webs in its forests throughout the year, belonging to species such as yellow-bellied banana spiders, huntsman spiders (Heteropoda venatoria) and tent-web spiders.
“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,” said Henry Pollock, executive director of the Southern Plains Land Trust and former researcher in Guam.
Surveys have found the island hosts between 508 million and 733 million spiders, including the huntsman spider (Image: Getty)
Surveys have found the island hosts between 508 million and 733 million spiders, roughly 40 times more than neighbouring islands such as Rota, Tinian, and Saipan. These figures only include spiders within 6.6 feet of the ground, meaning the actual number could be significantly higher. In total, there are believed to be at least 4,064,000,000 spider eyes and an equal number of legs living in Guam’s forests.
Despite decades of control efforts – including traps, chemical repellents and even research into potential viruses that could target the species but leave others unharmed – Guam’s brown tree snake population continues to grow. The US government, which has controlled the island since 1899, currently spends about $3.8 million (£2.8 million) annually on snake control in Guam. The US Department of Agriculture uses sniffer dogs to intercept brown tree snakes at Guam’s ports and airports and prevent them from escaping to the other islands, which remain free of the snakes and still have healthy bird populations.
One of the few effective strategies at Andersen Air Force Base saw snakes baited with acetaminophen, or paracetamol, which is lethal to brown tree snakes. To prevent reinvasion, the area was surrounded by a snake-proof fence. Inside this protected zone, snake numbers have dropped significantly.
However, most scientists consider replicating this success across the island’s rugged terrain inconceivable.