The Case of the Brown Tree Snake and the Sad Silence

Boiga irregularis is the scientific name of the Brown tree snake, one species of a group of reptiles known as tree snakes or cat snakes. The brown tree snake case is a classic in the literature of the ecological consequences of an  invasive species, due to its devastating impact on the remote Pacific island of Guam.

The Misplaced

This species of snake is native to parts of Papua-New Guinea, Indonesia, a number of islands in Melanesia, and north-east coastal regions of Australia.

Description

This snake is slender and has a head noticeably wider than its body. Its eyes are prominent and have vertically oriented pupils, a property associated with improved night vision. Its colour can be variable with brown being the most prominent hue.

Length and mass are quite variable. Adults can reach 3 m in length if sufficient food is available. Weight can vary between 100g and 2,000g.

It is nocturnal and generally arboreal. It is also an expert climber of surfaces such as cave walls and cliff faces.

It is venomous but its fangs are at the rear of its mouth and so it has to actually chew on prey species to poison them rather than "just" strike. It is only considered to be a threat to very young humans.

Reproduction

The Brown Tree Snake's lifespan is reported to range from 10 to 15 years. It is sexually mature and able to breed by about 3 years of age.

Females produce, on average, 2 clutches per year, with each clutch consisting of 4 to 12 eggs. No parental care has been reported other than placement of the eggs in sheltered positions, where they are less likely to desiccate or get eaten.

The eggs hatch after about 90 days and offspring are miniatures of their parents. The average length of a hatchling is approximately 45cm, although younger snakes may produce smaller hatchlings at first.

Ecology and Feeding

Brown tree snakes are not able to occupy areas which are subject to hard frosts and also are excluded from arid areas which do not meet their precipitation/humidity needs.

Prey animals for B. irregularis include birds, bird eggs, frogs, lizards, bats, rats, and small rodents.

They can greatly expand their gut and body wall to ingest creatures such as birds and rodents whole. This is visible in the images 1 and 4 below. They can eat meals which represent up to 70% of their body weight.

The Displaced

Brown tree snakes have a well documented history of invasion on the island of Guam, so further comments will reference its status in that location.

The island of Guam is a territory of the United States of America, located in the Pacific Ocean in the Micronesia region. It has an area of 540 square kilometres. It is classified as tropical rainforest and is generally hot and humid throughout the year with little seasonal temperature variation, although there is a wet season and a dry season. The island is remote from other sizeable islands and is home to a large US military base. It is also a tourist, shipping and communications hub.

The island has both inland and coastal environments. Soils are either volcanic in origin supporting grasslands, or based on limestone which supports forests with upper and under storeys.

Guam lacks native large mammals, but birds and lizards are well represented in the native macrofauna.

In the 1950s, brown tree snakes were accidentally transported to Guam, probably as stowaways on cargo ships. By the late 1960s, they were found all over the island.

As stated above, birds, frogs, lizards and small mammals are all threatened by the introduction of this new predator into their system. And because ecosystems are dynamic, interdependent assemblages, significant damage to one part of the system has a flow on effect.

The Consequences

Where is the invasion?

When the brown tree snake was introduced to the island of Guam, it had no local predators and an abundance of food. So its numbers exploded.

4,600–5,800 per square km, peaking in the 1970s and 80s

2,000–5,000 per square km long term average

The impact on Guam's avifauna has been severe. The Guam kingfisher and Guam rail are extinct in the wild as a result of the invasion.

According to Wiles et al, writing in 2003,

Our results indicate that 22 species, including 17 of 18 native species, were severely affected by snakes. Twelve species were likely extirpated as breeding residents on the main island, 8 others experienced declines of ≥90% throughout the island or at least in the north, and 2 were kept at reduced population levels during all or much of the study. Declines of ≥90% occurred rapidly...¹

There is a knock on effect from the removal of birds. In forests,  birds are important for pollination, spreading seeds around the forest and controlling insects that feed on plants.

According to ecology researcher, Haldre Rogers,

some plant species need birds to handle their seeds to ensure effective germination. In addition, seed predators and fungi that kill seeds are often found in high density directly beneath a parent tree, so the trees rely on birds to disperse seeds beyond the range of those negative effects. If native birds performed those functions on Guam, tree populations could suffer from the loss of birds. It appears 60 percent to 70 percent of tree species in the native forests are dispersed, at least in part, by birds, she said. ...
But the biggest indirect impact, she said, could be altered seed scattering that in turn, might in the near future, transform the remaining forest from a diverse mixture of tree species to clumps of trees of the same species, separated by open space. That could have serious consequences, including extinction, for plant and animal species that still live in the forests.²

Another consequence is the danger of increased disease carried by insects that were previously kept in check by Guam's native lizards and birds.

Remediation and prevention

Public education about Boiga irregularis was largely absent in the early days of the invasion and it took over 10 years for Guam residents and tourists to become aware of the problem. This issue has now been addressed and information is freely available for anyone present on the island to report sightings to a dedicated government team.

Since 1993, APHIS-Wildlife Services (WS) has conducted ongoing operations on Guam to reduce damage caused by the brown tree snake. Scientists at WS’ National Wildlife Research Center conduct research to identify effective toxicants, repellants, and trapping methods. Using specially designed traps, hand capture, and oral toxicants, WS field personnel remove an average of 7,000 brown tree snakes from Guam each year.

Although this invader is still present on the island, their population has been reduced. Off island breeding of remnants of bird species absent from Guam has occurred on both small uninfected nearby islands and also in zoos and research stations. It is hard to know how successful these program will be in the long term.

Awareness of the problems on Guam has brought about the implementation of much improved prevention of accidental spread of these problematic reptiles to other vulnerable environments.

Guam has also suffered from the introduced presence of Cane toads, Giant African snails, Yellow Crazy Ants and Wild boars, plus the Coconut rhinoceros beetle, and various types of cattle and deer.

Dig Deeper

• Practical concerns in the eradication of island snakes
• With fewer animals to spread their seeds, plants could have trouble adapting to climate change
• Only three native species left in the wild
• Predation thresholds for reintroduction of native avifauna following suppression of invasive Brown Treesnakes on Guam
• Island of No Bird Song
• On Guam there is no birdsong, you cannot imagine the trauma of a silent island