Introduction of an Alien:
In 1947, Roswell, New Mexico was famously rumored to be the site of an unintended landing. Strange reptilian beings were reported to have been removed from the debris of a metallic vessel. The area has since become synonymous with all things “extra-terrestrial”. Around the same time on Guam, an island in the south Pacific, an almost identical albeit far more earthbound story was beginning to unfold.
The aliens on Guam were also reptilian and they too are believed to have arrived in the remains of metallic vessels but, unlike the little green men of Roswell, these aliens did not disappear into the murky realms of myth and urban legend. These aliens were the pioneers in an invasion that seems more at home in the pages of an Orson Welles screenplay than in the annals of natural history.
The invader was the brown tree snake (Boiga irregularis), and it didn’t arrive here from the far reaches of the galaxy, but from the Admiralty Islands of Papua New Guinea. The ecological and socio-economic chaos that ensued has led to Guam becoming the definitive case study for invasive species ecology.
The snake is believed to have arrived on Guam as a stowaway in salvaged wartime material which was collected from the Admiralty Islands bases and then transported to Guam for storage at the military bases or processing at its scrap metal facilities. Sites of disturbance are known facilitators of invasions by non-indigenous species and the disturbance caused by wartime activities on Guam would have allowed rats and other prey species to thrive, which in turn allowed the snake to gain a strong foothold on the island.
Initial reports of sightings were irregular, but by the late 1960s the snakes were being found throughout the island and the population continued to grow at an exponential rate.
Guam’s native wildlife suffered greatly; almost all of the native forest dwelling species of bird have been wiped out and those that remain do so almost exclusively in captive breeding programs. Guam’s lizard population has suffered an equally devastating demise though this is not exclusively as a result of the BTS invasion. Additional pressure has been put on the lizards through competition from introduced lizard species. Introduced birds and lizards have also provided a stable food supply for the brown tree snake population. This food supply allows the snake population to persist at high densities (as high as 3000 per square mile) further adding to the plight of the island’s native species.
Guam was home to three endemic species of bats. The subsequent disappearance of two of those species cannot be attributed entirely to the Brown Tree Snake, but its proliferation on the island seems to have been the nail in the coffin of the Pacific Sheath-tailed Bat and the Little Marianas Fruit Bat, and it continues to pose a threat to the dwindling populations of the Marianas Fruit Bat.
The loss of species is the most obvious and visible consequence of the snake’s introduction to Guam. Less obvious are the indirect effects this had on ecosystems across the island. The loss of insectivorious species saw a boom in pest numbers. Mosquitoes and agricultural pests went practically unchecked, causing human health issues and economic damage.
Additionally, many of Guam’s plants require birds or bats for pollination and seed dispersal. A recent study has shown that 60-70% of Guam’s trees are in some way dependent on birds for seed dispersal. Without birds ingesting the seeds and removing the tough outer coats, fewer seeds are successfully germinating. Furthermore, because the seeds are not being transported very far from the parent plant (most do not disperse any further than 6 feet) , they are at the mercy of seed killing fungi and seed predators which tend to occur in higher densities beneath the parent plant. Coupled with direct competition from the parent plant, seedlings stand a very low chance of survival. Researchers speculate that the problems of seed dispersal are going to reshape the island’s vegetation structure drastically. They anticipate that diversity-poor pockets of vegetation will remain, with increasingly large stretches relatively barren habitat emerging between them.
Non-indigenous species also suffered following the invasion. Poultry were regularly taken as prey and the difficulty in fighting the snake invasion led to many abandoning poultry farming. As a result,imports of eggs and meat have risen causing additional economic strain.
Attacks on domestic pets are commonplace and worryingly attacks on young children are also quite common. While the snake is not considered to be a serious threat to humans, it is mildly venomous and bites can cause some medical complications.
The damage caused does not end with the environment. Infrastructure has been affected too. The snakes cause electrical failures when they climb on power transmission lines and transformers. These outages are an almost daily occurrence and while they are usually restricted to a few blocks they can affect a much wider area. To counteract the damage, many electrical grids must be shut down during hours of darkness, when the snakes are most active.
Medical services on Guam are under increasing pressure from the snakes; snakebites are increasing in occurrence and despite their non-lethal status they can cause problems particularly for young children. Difficulty in breathing has been reported in some instances.The regular power outages also affect the stability of medical services.
To prevent the snakes spread to neighbouring islands, Guam’s exports are subject to rigorous searches and associated protocol that limit its efficiency as a hub for cargo shipping.
Infrastructure damage coupled with the bad press associated with the snake are likely to adversely affect tourism revenue which is of great importance to the Guam economy.
Control efforts utilised on the island also costs several million dollars annually.
A wide array of control efforts have been utilised on Guam. While eradication is virtually impossible, controlling the snakes in key areas may prevent the snakes from spreading to neighbouring islands and allow preservation of habitats for conservation efforts.
Canine sniffers can be used to search outgoing cargo for stowaway snakes and on some nearby islands they are used to inspect incoming material . The dogs are quite efficient at locating infested cargo but there are drawbacks. The efforts are very labour intensive, expensive and strong odours can interfere with successful identification.
High risk areas such as cargo stores can be secured within enclosures to prevent snakes escaping into vegetated habitats until such time as the cargo can be properly inspected.
Exclosures can be used to essentially generate snake free zones. These areas are particularly useful for cargo storage and habitiat protection. By storing cargo in these areas, the risk of introducing snakes to neighbouring islands is significantly reduced.
These enclosures can be purpose built or in some cases existing fences and other physical barriers are fitted with heavy duty mesh or cloth to create exclusion zones. The protected Guam Rail habitats utilise the latter type of barrier.
Keeping snakes out of sensitive areas can be aided by modifying habitats in such a way that they do not provide snakes with refugia. Rubble and waste around warehouses and other high risk areas can harbour snakes and pest species on which they prey. Removing this material makes the area less enticing for the snakes and should help reduce its area of influence.
Other more labour-intensive control methods include visual searches and baited traps. While these methods are not going to limit the snakes density island wide, they are a useful tool in keeping snakes from entering priority areas such as Guam Rail habitats. Patrolling the perimeter of such areas should help capture snakes attempting to penetrate the physical barriers enclosing them.
One of the more elaborate efforts, undertaken recently, involved air-dropping mice laced with acetaminophen into infested habitats. Acetaminophen is found in generic painkillers such as Tylenol, but in Brown Tree Snakes it inhibits haemoglobin’s oxygen carrying capabilities causing the snake to slip into a coma and die. Each mouse was fitted with a piece of card that would make it more likely to get snagged in the treetops, increasing the likelihood that the bait will be ingested by the arboreal snake rather than ground dwelling species such as monitors.
How successful this exercise will be in controlling the snake’s numbers remains to be seen.
Why fight back?:
It is highly unlikely that the brown tree snake will ever be removed from Guam. In terms of the loss of native species, most of the damage can never be undone. So why bother trying to control the snake? Is there any benefit to fighting a battle we can never win?
Controlling the snake population on Guam may prevent the snake reaching nearby islands such as Hawaii. Hawaii has no native snakes and should brown tree snakes infiltrate the island it is not unreasonable to assume that the consequences would be dire. The history of Guam also serves as a case study which we can use to predict and prevent similar problems arising elsewhere.
For now it appears that there can be no happy ending for Guam. To return to the science fiction analogy it appears that this particular alien will continue to “Live long and prosper”.
This article originally appeared in The Herpetological Society of Ireland’s publication, Lacerta.
About the Author: Rob is a zoologist specialising in invasive species of freshwater bivalves. He is the current Editor for The Herpetological Society of Ireland. Find him on Twitter here.