Brown Tree Snakes and Bird Extinctions on the Island of Guam
(Exercise adapted from D’Avanzo, C. and Musante, S., What are the Impacts of Introduced
Species? Issues to Teach Ecology, Volume 1 in Teaching Issues and Experiments in Ecology)
Background
Extinctions following species introductions have taken place in many different settings
worldwide, but islands are especially vulnerable. Some islands host organisms not found
elsewhere and therefore lost species cannot be reintroduced. On islands such as New Zealand,
animals which have evolved in the absence of predators are literally sitting ducks for rats,
mongoose, snakes, and other predators. The flightless birds are classic examples of this
scenario. Hawaii is one of the hardest hit areas of the world in regard to extinctions due to alien
species; of 140 known native bird species, 70 have become extinct since human arrival on the
island and 30 are on the endangered species list (Stone 1999). In addition to habitat loss,
introduced species are a main reason for this decline.
The disappearance of wildlife on the island of Guam is an intriguing example of damage due to
an introduced species. On Guam, like Hawaii and many other islands, about half of the native
birds were exterminated by prehistoric humans. After that, bird species numbers remained fairly
stable – even through World War II – until the 1960’s when biologists noticed missing birds from
the south end of the island. By 1985, most of the bird species were either completely gone, or
living in small, isolated pockets (Rodda et al. 1997).
Two popular theories explaining this loss were pesticides left over from World War II (Guam was
at the center of the fighting in the Pacific) and disease. When Julie Savidge reported her
findings about predation rates by the introduced brown tree snake (see Figure 3), most
researchers were quite skeptical and did not believe that a snake could harm so many birds.
The snakes were thought to have been introduced to Guam during postwar salvage operations.
Brown tree snakes are native in eastern Indonesia, New Guinea and in Australia. They are
nocturnal and likely crawled into vehicles and equipment at night and were carried to Guam
when they crawled out again undetected at night. Ms. Savidge’s research was some of the first
to show that this is entirely plausible.
Savidge examined the hypothesis that brown tree snakes were preying on birds on Guam by
baiting traps with birds (quail – a native bird to Guam) and counting birds eaten. The traps
contained seeds and water for the birds inside and a hinged door that allow a predator to enter
but not escape. Because the snake and the birds live in trees, she hung the traps about 1-3
meters above the forest floor.
Directions: Study Figure 3, and interpret the data it presents
In the next class, we will be discussing the following:
What observations lead to Savidge’s hypothesis?
Why did Savidge use 3 different types of sites (birds extinct, declining, stable)? What
question was she asking?
Why might predation rates be high in one location and low in another? How would you
test your ideas?
What other evidence, in addition to the data in the figure, would help convince you that
bird extinctions on Guam are due to predation by the introduced brown tree snake?
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