The brown treesnake (Boiga irregularis) was recognized as a pest species by Congress in Section 1209 of the Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 (NANPCA). Like aquatic pests, tree snakes are sometimes dispersed by ships, have wide-ranging effects on natural and human communities, and threaten to spread to other areas. The problems they are causing on Guam, a formerly snake-free island, are unusual in that they involve many aspects of the biota of Guam and directly affect the human residents of the island.