The largest individuals of brown tree snakes, Boiga irregularis, can grow up to 3 metres in length. Due to the availability of prey on the island of Guam brown tree snakes commonly grow larger than within their native range.
PHOTOGRAPH BY Matt Cornish
After accidentally being brought to the Pacific Island of Guam post World War II, the brown tree snake has uncontrollably exploded in numbers decimating native naive bird and lizard populations. Now locals, scientists, and border control officials are working together to eliminate the snakes and save Guam’s rich rainforests with their last slither of hope.
The U.S. owned Island of Guam is a fragment of land disproportionately teeming with tropical rainforests and species seldom seen in other reaches of the planet. It’s a modestly small place at 30km long and 10km wide. Native flying foxes and endemic birds such as the Guam Flycatcher and Guam Kingfisher glide and flutter between the trees and streams that enrich the land. Forests serenade with a crescendo of harmonised chirps, whistles, and croaks but year after year the singing has grown dimmer as the forests’ residents have drastically changed.
Guam is a lively transport hub within the Pacific and has been a renowned US military base since World War II. With the constant trade of arms and supplies comes a lack of examination of each cargo shipment, the perfect passage for Guam’s reckoning: the brown tree snake. Their native home lies in the reaches of Australia, Papua New Guinea, and Indonesia before inadvertently being transported in the early 1950s to the island of Guam. The brown tree snake’s extradition to Guam made it the first species of snake on the island but it was by no means a death sentence. Surrounded by a myriad of naive species which have evolved in the absence of a snake predator, the species was decisively unchallenged. The snakes spread relentlessly across the island reaching densities of over 100 snakes per hectare growing to an uncontrollable total population of 1-2 million across the whole island.
Brown tree snakes have a voracious appetite for small mammals, lizards, bats, but predominately birds in particular. Using their venom, which isn’t fatal to adult humans, the snakes asphyxiate the naive defenceless species resulting in dire plunges in population numbers. Guam was a bustling ecosystem, but no longer. The forest’s former chorus of diverse bird chimes has disappeared, falling silent to the mercy of the snake’s successes.
But what can be done to revitalise the native species populations and remove the snakes before they collapse Guam’s ecosystem and fortuitously colonise other islands within the Pacific region?
Brown tree snakes are commonly found in trees, caves and near limestone cliffs but frequently come down to the forest floor at night to forage. PHOTOGRAPH BY Janelle Lugge Across the island, the brown tree snake has exerted numerous negative impacts on the native species as well as the locals. Direct predation upon Guam’s forest bird species is perhaps the worst. Since the snake’s introduction in the 1950s, they have insatiably fed upon adults, juveniles, and eggs driving 10 out of 12 bird species to extinction. Revered species known locally as the Ko'Ko' (Guam rail) along with the Guam kingfisher have vanished, stripping Guam’s natural character, and replacing it with the plundering legacy of the brown tree snake. Now, these species have been reduced to captive breeding programmes with the only chance of seeing them being behind the bars of a zoo.
The Guam kingfisher, Todiramphus cinnamominus, is native to the island of Guam. The species is now extinct in the wild due to the voracious predation by the introduced brown tree snake. Today the species is restricted to a captive breeding programme that maintains its existence. PHOTOGRAPH BY Michael Fitzsimmons Without its native bird species, the Guam forest ecosystem is struggling as tree species aren’t receiving the previous affection they once received. Compared to the neighbouring island called Saipan, 77% of seeds in degraded forests were ingested and dropped by frugivorous birds compared to a resounding 0% on Guam. To germinate successfully and survive seeds commonly need to pass through the gut of an animal and be transported away from their parent tree. Seed dispersal has been lost as the majority of seed-eating birds have become extinct, resulting in new tree growth declining in Guam by up to 92%. Not only have snake numbers boomed, but birds’ previous prey has also flourished with the lack of predation, creating an imbalance in food chains. For Locals, strolls through Guam’s forests have now been replaced with blindly swatting at spider webs with a stick in hand. Spider densities on Guam are 40 times greater compared to adjacent islands in the wet season. Not only is Guam overrun with snakes, but consequently spiders now too.
Spider web density in the wet season in Guam is 40 times higher compared to neighbouring islands. The lack of predation from birds has allowed spider populations to flourish. PHOTOGRAPH BY Gabriela Beres Brown tree snakes aren’t just consequential for the natural world, in Guam, they interfere with how all island residents live their lives. As the snakes slither throughout every crack and crevice of the island they incidentally wriggle and climb onto the island’s power lines cutting off electricity to Guam’s residents. These constant outages cost the island’s economy over $4.5 million yearly with the locals now referring to power cuts as ‘brown-outs’. The snakes’ adventurous habits have also led them to explore the houses of residents. Multiple cases have been reported of children getting bitten whilst they peacefully sleep at night, being hospitalised due to the breathing problems that arise from the venom of the snake. Ultimately, how do we control the snakes to revive the forest ecosystem and keep people safe? A U.S. government-funded project has developed a bizarre but inventive way to get back at the snakes. Brown tree snakes actively hunt as well as scavenge carrion and the authorities have set their trap to exploit this behaviour. Tiny dead mice attached to parachutes have been dropped from helicopters across the most inaccessible rainforests landing in the forest canopy, the snake’s preferred habitat. The idea is that curious hungry snakes will unknowingly eat the mice laced with a substance toxic to the species. In this case it’s paracetamol. Commonly used by us to treat aches, pains, and fevers, however a small dose in this species leads to disruption in the snake’s haemoglobin blood proteins, resulting in death. Killing snakes using this method is substantially more efficient than the labour-intensive snake trapping used prior. Due to the deterioration of Guam's ecosystem by the brown tree snake, there are fears that the snake could move to other islands within the Pacific. Of most concern is Hawaii. The island state shares Guam’s abundance of prized endemic species which have evolved in the absence of snakes. Therefore, cargo checks have been stringent with the help of man’s best friend. Detector dogs form the last line of defence for preventing the dispersal of the brown tree snake to neighbouring islands. If they fail, a similar story could unfold. Researchers have predicted that the economic damage of brown tree snakes in Hawaii could reach $2.14 billion annually just for dealing with medical costs, power outages and tourism losses alone.
A sniffer dog and handler assess cargo for a border check. PHOTOGRAPH BY Shutterstock Reducing snake presence is the initial and primary aim, but when and how do we try to reintroduce Guam’s captive birds back into their wild home? Insight into how to prevent the snakes from reaching the birds lies in their recently discovered movement ability, called lasso locomotion. In essence, the snakes encircle their lower body around a tree trunk or pole creating a knot with their tail before proceeding to push their looped body up the wide object in wave-like motions. Researchers suggest that this climbing technique has facilitated the snake’s success by being able to reach more prey. However, now this locomotion has been discovered, we can implement specially designed nest boxes to keep the snakes at bay. Cone shaped nests with a small bottom and incredibly wide top could prevent the snakes from climbing up them as the lasso grip would loosen as the cone becomes too wide, leading them to fall off.
Ultimately, Guam’s travesty highlights the problem of when invasive species decimate naive endemic species totally unadapted to the new methods of predation. The long-term goal of Guam is to fully eradicate the snake. However, if these solutions don’t work, we may remain in a situation where the only chance to see Guam’s native birds is behind cages, hiding away from the snake’s reaches.
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