Loss of Bird Populations on Guam



The island of Guam, part of the Marianas islands in the South Pacific, was strategically important to the U.S. during and after World War II. Sometime after the war as military supplies were moved, an unwelcome hitchhiker in the form of the brown tree snake was introduced to Guam. The results of that introduction have been a disaster for the birds and other small animals on Guam. Gary Wiles and his coauthors collected the counts of the island's birds from 1976 to 1998 and found that of the twenty-five bird species on Guam, twenty-two were severely affected by snakes1. Twelve species were wiped out as breeding residents and eight other species declined by more than 90 percent; and the remaining two species were kept at reduced population levels. There was a pattern of the disappearance of the birds. The brown tree snake was introduced on the southern part of the island and as it spread north, the birds disappeared.

We spoke with Thomas Fritts in the February 1998 issue of Environmental Review about the brown tree snake invasion of Guam and at that time we knew the snake was wiping out whole populations of birds2. Now the numbers are in and we know how fast this occurred and why some species of birds could hold out a little longer than others. Bigger birds lost their eggs or chicks to the snakes so there was no recruitment of new birds, but the adults survived. Smaller bird species were preyed upon at all stages of life, eggs, chicks and adults, and as soon as the snakes moved into the area the birds were rapidly eliminated.

We don't know why brown tree snakes are not such a big problem in their native habitat in other parts of the South Pacific. Their numbers may be held down by predation from other snakes, but on Guam the brown tree snake population exploded and they have eaten almost every small animal in sight, small mammals, lizards, and virtually all the birds. The story on Guam is an unfortunate demonstration of how vulnerable an island ecosystem can be to introduced species. The authorities in Hawaii and other Pacific Islands are very concerned about the brown tree snake and are taking active precautions to keep it out.

We spoke with Gary Wiles about his time on the island and his work as a biologist there during the time its wildlife was being decimated.

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