BACKGROUND

Extinctions following species introductions have taken place in many different settings worldwide, but islands are especially vulnerable. For instance, some islands contain unique species and therefore lost species cannot be reintroduced. On islands such as New Zealand, animals, which have evolved in the absence of predators, are especially vulnerable to being eaten by rats, mongoose, snakes and other predators. The flightless birds are classic examples of this scenario. Hawaii is one of the hardest hit areas in the world in regard to extinctions due to alien species. There were 98 species of birds native to the Hawaiian islands before Polynesians introduced rats, dogs, and pigs around 400 A.D. As a result, about 50 bird species became extinct before European arrival in 1778. Since then about 20 more bird species have become extinct (Primack, 1995). 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 realized that birds were disappearing 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 in the ‘80’s explaining this loss were pesticides left over from the war and disease. When Julie Savidge (Savidge, 1987; see student question) reported her findings about predation rates by the brown tree snake, most researchers were quite skeptical and did not believe that a snake could harm so many birds (ibid). Ecologists now understand that this is entirely plausible.

Stories about Guam’s brown tree snakes are fascinating (Rodda et al., 1997). One concerns the bizarre cases of Guam’s baby bites. Brown tree snakes are not venomous like vipers and so reports of their biting human babies were at first not believed. But the numbers of reported cases kept rising into the hundreds. Particularly surprising was the fact that most victims were infants sleeping at home, not outdoors. Apparently the snakes enter homes at night and repeatedly attempt to regrip a food source (the baby) that is much too large. Most infants were not harmed very much by the bites but a few needed assistance breathing in the hospital. In the wild, brown tree snakes can eat prey more than 70% of their mass, which is extremely unusual for non-vipers.

The snakes are 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 where they crawled out again undetected at night.

References

• Primack, R.B. 1995. A Primer of Conservation Biology. Sunderland, MA: Sinauer.
• Rodda, G. H., T. H. Fritts, and D. Chiszar. 1997. The disappearance of Guam’s wildlife. BioScience 47: 565-574.
• Savidge, J. A. 1987. Extinction of an Island Forest Avifauna by an Introduced Snake. Ecology 68: 660-668.