After World War II, the brown tree snake was accidentally introduced into Guam. With no natural predators and abundant prey, the snake population steadily grew and spread throughout the island. As the snakes dispersed, forest bird and fruit bat populations plummeted. By the late 1980s, 12 species of birds and the little Mariana fruit bat had disappeared from Guam. The Refuge provides habitat for the last remaining populations of the endangered Mariana fruit bat, Mariana crow, and the Serianthes nelsonii tree. The brown tree snake is considered the primary cause for the decline of native Guam bird species.