Spotlight On An Invasive Species: The Brown Tree Snake and Its Devastation of Guam

There is quite a bit of controversy regarding both the exact meaning of the term “invasive species” as well as what should be done about invasive species. Many scientists consider “invasive species”, “exotic species”, “non-native species” and “introduced species” as meaning virtually the same thing, while others believe that a species should be considered “invasive” only if it dramatically alters the ecosystem that it invades.

Whatever the meaning, invasions of species abound on our planet. Some scientists view all invasions as potentially devastating to the ecosystems they invade; invasive species monopolize niches and compete for resources with native species, often driving endangered species to extinction, or decimating healthy populations of animal species. For this reason, conservation biologists focus a great deal of effort on both preventing the arrival and spread of invasive species, as well as their eradication.

Others believe that expending resources for invasive species management programs is somewhat of a waste, given the fact that, ultimately, all species can be considered invasive. For example, wheat, corn, honeybees, cattle and chicken are all “invasive” to North America. Furthermore, there are a few scientists who believe that the arrival of an invader can actually be beneficial to the life history of a native species. The thought is this: an invasive species will compete with native species for resources and this competition will increase the rate of evolution and diversification of native species. In other words, some scientists see invasion as leading to a decrease in biodiversity, while others believe it could lead to an increase in species biodiversity.

There are some very clear-cut illustrations of an invasion being harmful, however. One example of the potentially devastating effects of an invasion involves the island of Guam, in the South Pacific, located about 3800 miles west of Hawaii. After WWII, the US began using Guam as an important naval base, shipping materials to and from the island. It is believed that around 1949, a member of the snake species, Boiga irregularis, known as the brown tree snake, stowed away on a freighter carrying military cargo from Papua New Guinea to Guam. At this point in time, nobody was aware of the kind of havoc that could be wrought by a single introduced species. Within 20 years of the brown tree snake’s introduction to Guam, it established itself throughout the island in dense populations.

The damage caused by the brown tree snake can be divided into three categories:

• Ecological damage: It took the brown tree snake all of about 40 years to devastate Guam’s natural environment. In that time, the snake drove 12 of Guam’s bird species to extinction, and also devastated populations of other animal species on the island, such as bats and native lizards. Guam’s forest birds, some endemic to the island (not found anywhere else on the planet), were the most susceptible to the attack of the brown tree snake. This is because Boiga irregularis is an arboreal animal, meaning it spends a great deal of time in trees, where it also likes to hunt. The dense forest vegetation provides the snake with the perfect way of concealing itself from its prey. The bird species that have survived the invasion have actually changed their behaviors in response to the attack. Birds that would nest and roost in the forest are now spending more and more time in urbanized areas, away from dense vegetation. With the elimination or severe reduction of organisms such as birds and bats, which aid in seed dispersal and pollination, the composition of plants on Guam has also been changing. Because birds and bats are also responsible for keeping the insect population in check, the insects that do arrive on Guam threaten to destroy agricultural and wild plant species.

• Effects on Human Health: Not only do the insects that arrive on Guam threaten agriculture and natural vegetation, they also threaten human health. Since the decline in the birds and bats on Guam, there has been an outbreak of Dengue Fever on the island, caused by a parasite carried by the Aedes mosquito. Furthermore, though the brown tree snake bite is not fatal to humans and not very harmful to healthy, adult humans, it is mildly venomous and it can cause painful reactions in children, the elderly and in people with weak immune systems.

• Electrical Problems: It is estimated that brown tree snakes cause nearly 200 power outages per year on the island. How does this happen? This arboreal snake loves to slither up guy wires leading to power poles; when it comes into contact with grounded and live conductors at the same time, short circuits are caused. These short circuits lead to power failures in hospitals, schools, businesses and popular tourist destinations. It is estimated that the damage caused by these electrical problems totals between 1 and 4 billion dollars a year.

In the case of the brown tree snake, the solution is obvious, though difficult to implement. The brown tree snake, or at least its effect, must be eliminated on Guam in order to attempt restoration of Guam’s ecosystem and to alleviate all the financial burdens caused by the snake. Because the population density is so high, in the thousands per square mile, completely eradicating the snake may be impossible. The population needs to be kept in check, and, more importantly, extreme care must be taken so as to not allow the snake to leave the island and wreak the same havoc in other areas.

That said, however, complete invasive species removal is an approach that must be undertaken with caution. The New York Times reported the case of an intervention gone wrong not so long ago, on Macquarie Island. What seems to be clear is that not all cases of “invasions” are as black and white as that of the brown tree snake and so each situation must be studied and understood in all of its complexity as much as possible before a decision is made.

http://www.fort.usgs.gov/Resources/Education/BTS/

http://tiee.ecoed.net/vol/v1/figure_sets/species/species_overview.html

http://alic.arid.arizona.edu/invasive/sub1/index.shtml

Principles of Conservation Biology (Groom et al 2006)

Ecology (Ricklefs and Miller 2000)

K. Burton. "An Invader Worse than a 'B' Movie." Endangered Species Bulletin. 2000

G.H. Rodda and J.A. Savidge. "Biology and Impacts of Pacific Island Invasive Species. 2. Boiga irregularis, the Brown Tree Snake ( Reptilia: Colubridae)." Pacific Science. 2007.

G.J. Wiles et al. "Impacts of the Brown Tree Snake: Patterns of Decline and Species Persistence in Guam’s Avifauna." Conservation Biology. 2003.

Photo taken by Gordon H. Rodda, courtesy of U.S. Fish and Wildlife Service