STUDENT INSTRUCTIONS

      Read the information below and then examine Figure 3. Take your time with this graph; make sure you understand the labels on the axes, the legend, and the experimental design before you attempt to interpret the data.

      Guam is a tiny island halfway between Japan and New Guinea (which is north of Australia). Many lovely forest birds were abundant on Guam - birds with names like the White-tailed Tropicbird, the Blue-Crested Quail, the Nightingale Reed-warbler, and the Mariana Fruit-dove. But in the 1960’s and ‘70’s these and many other birds disappeared from the island. Some are now extinct, others rare.

      Two popular theories explaining this loss were pesticides left over World War II (Guam was at the center of 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. Her's was some of the first research to show that this is entirely plausible.

      Savidge examined the theory that brown tree snakes were preying on birds on Guam by baiting traps with birds (quail) and counting birds eaten. The traps contained seeds and waters for the birds inside and a hinged door that allowed a predator to enter but not escape. Because the snake and the birds lives in trees, she hung the traps about 1-3 meters above the forest floor.


After you describe and then interpret the figure, consider the following questions:

• 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 this figure, would help convince you that bird extinctions on Guam are due to predation by the introduced brown tree snake?