A brown tree snake

| 1st February 2006
At dead of night, unseen, a slithering stowaway disembarks from a newly docked ship… down a rope, across the quay and into concealing undergrowth. Is that what happened? Or did the snake hitch a ride on a military aircraft from Indonesia, maybe, to take up residence here on the American island of Guam in the Pacific?

No one can be sure of the details. All we know is that sometime after World War Two – in the late 1940s or early 1950s – this now notorious reptile first insinuated itself into a fresh and ideal habitat. One pregnant female, just one, would have been enough to trigger the population explosion that brought Guam massive economic and ecological headaches.

Boiga irregularis, the brown tree snake, is neither very large nor very venomous. As villains go, it seems almost ‘puny’. And yet its conquest of the island – and much of Guam’s wildlife – is complete. It has wiped out 12 bird species, some of them found nowhere else. A further dozen species of lizard look certain to follow.

Despite all human countermeasures, Boiga irregularis continues to dominate and prosper – at up to 13,000 specimens per square mile.

Guam, a key US base during the war, was utterly unprepared for such an invasion. And island creatures that had evolved thousands of miles from the nearest landmass had never encountered a snake – let alone one that climbed trees and struck in the dark. They were defenceless. A stealthy predator, whip-thin and extraordinarily elastic, the snake feeds at will on unsuspecting forest birds. Some may be nestlings, others adults incubating eggs. None can elude this relentless night hunter. Its poison, though mild, is effective and is delivered from fangs at the back of the mouth and by methodical chewing.

Once every bird has gone, Boiga irregularis will no doubt increase its intake of lizards. And should these run out, it can concentrate more on rodents such as rats. The brown tree snake is not a
fussy eater.

For years, its silently spreading colonisation was hardly noticed by the island’s human inhabitants – perhaps because it is nocturnal. But the signs were there to be read: a sudden scarcity of birds and lizards, an unprecedented boom in the numbers of their prey – spiders and other creepy-crawlies (too small a mouthful to interest a snake).

By the 1980s, however, nobody could overlook the multiplying reptiles. They appeared everywhere, indoors and out, an omnipresent pest able to do all kinds of harm. Pet rabbits, puppies, even piglets were added to the snakes’ menu.

Then came the first human casualty: a six month-old girl, bitten while she lay in her cot. She was saved by prompt hospital action. So far, no one has died from the venom of Boiga irregularis. But people have learnt to be wary, checking bathrooms, beds… The snake can be aggressive if disturbed, perhaps when scavenging for edible scraps in dustbins at night.

Furthermore, these detested reptiles bring trouble in other ways you wouldn’t dream of.

Expensive trouble. Multi-million dollar trouble. Snakes regularly scale pylons (every fourth day or so), crawl along electrical wires – and cause short circuits. Result: paralysing power cuts, some island wide. Repairs cost a fortune. And long-suffering Guamanians have to cope with inconveniences that range from computer failures to food spoilage when refrigerators are cut off.

Few feel any sympathy for the fried snake corpses hanging, as if inpublic humiliation, from fused power lines. Besides, with an estimated population of three million, the brown tree snake is unlikely to die out on Guam.

Attempts to deter or destroy them – using barriers and traps – have only limited success. Even the United States Department of Agriculture, for all its physical and financial resources, concedes it is up against a probably unbeatable foe. But USDA and related agencies cannot abandon the fight.

Herpetologists like Gordon Rodda patrol Guam’s 200 square miles in search of snakes. Not to kill them – not at first anyway – but to study them. Working by torchlight, Gordon grabs another wriggling reptile, whisks it through the air, and deftly eases it into a bulging sack. He accomplishes this with the same self-effacing skill he applies to the rest of his vocation.

What makes Boiga irregularis tick? Why is it so hardy and fertile? Which are its favourite haunts? How can it be trapped more effectively? With what sort of bait? Just some of the questions Gordon and his colleagues seek to answer in a never-ending endeavour to thwart this slipperiest of challengers.

In the meantime, government scientists are trying to develop a viral or bacterial disease that would polish off the snake while leaving Guam’s native fauna unharmed. Unlike most islanders, Gordon doesn’t hate the snakes. Indeed, he feels a kind of affection for them. ‘They’re not evil. They’re merely animals in the wrong place, opportunists that have moved in and prevailed at the expense of other species. Precisely what man has done around the globe…’

Such fair-minded views find little support here. Certainly not among dock workers; with their sniffer dogs (Jack Russell terriers), they must
monitor every cargo for hitch-hiking reptiles. The brown tree snake is despised as a parasitical ‘vermin’ – to be stopped by whatever means possible from further emigration.

If only Boiga irregularis had stayed put on its home ground – Australia, Indonesia, Papua New Guinea, the Solomon Islands. There, kept in check by competitors and other natural constraints, it is no more than an ‘average’ and rather nondescript member of the community. Nothing to worry about.

But once it gets out and ventures abroad, this travel-addicted snake leaves a lasting trail of ruin.

This article first appeared in the Ecologist February 2006


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