Origin and Population Growth of the Brown Tree Snake, Boiga irregularis, on Guam

After the accidental introduction of the Brown Tree Snake, Boiga irregularis, to the island of Guam after World War II, the snake became exceedingly numerous, and most of Guam's native vertebrates either became endangered or disappeared from the island. In this paper we summarize what is known about populations of this snake on Guam and the likely origin of the Guam population. Scale counts and transportation records suggest that the Guam population originated in the Admiralty Islands, about 1500 km south of Guam. It was probably transported to Guam in ships that transported salvaged war materiel after World War II. For ca. 35 yr after its introduction, the presence of the snake on Guam was documented only by popular accounts, occasional photographs, and a few museum specimens, indicating that the snake's distribution was fairly limited initially, but ultimately a period of sharp population growth and wide dispersal occurred, with the snake reaching all parts of the island by the late 1960s. Peak population levels were attained about a decade or more after each area was colonized. Mark-recapture and removal data indicate that the capture of 50 snakes per ha at one site in northern Guam during 1985 probably represented a population density of around 100 snakes per ha, but by 1988 this population had declined to around 30% of the 1985 density. However, this reduction may not be permanent. In central Guam, where the snake irrupted decades ago, the snake's numbers have continued to fluctuate, and in some cases it has attained densities in excess of 50 per hectare. THE INTRODUCTION OF A SPECIES provides an opportunity to evaluate ecological relationships within a community by monitoring the trajectories of the component populations as a new equilibrium is established. This process is most observable in communities with few species and major perturbations, conditions that facilitate interpretation ofcausation. The 1 Funding for this research was provided by the U.S. Department of the Interior, Technical Assistance Program, the U.S. Navy, and the U.S. Air Force. Manuscript accepted 5 April 1991. 2 Arizona Cooperative Fish and Wildlife Research Unit, Biological Sciences East Room 210, University of Arizona, Tucson, Arizona 85721. 3U.S. Fish and Wildlife Service, National Museum of Natural History, Washington, D.C. 20560. 4Guam Division of Aquatic and Wildlife Resources, P.O. Box 2950, Agana, Guam 96910. Present address: Hawaii Division of Forestry and Wildlife, 1151 Punchbowl Street, Honolulu, Hawaii 96813. accidental introduction of the Brown Tree Snake, Boiga irregularis, on Guam is believed responsible for the nearly complete extirpation of native bird species (Savidge 1987, Engbring and Fritts 1988), for hundreds of power outages (Fritts et al. 1987), for loss of domestic and pet animals (Fritts and McCoid in press), for envenomation of human babies (Fritts et al. 1990), and for the probable extirpation of native bat and lizard populations (Wiles 1987, Fritts 1988). Thus, Guam's vertebrate population ecology is of great interest not only to researchers and island biologists but also to managers responsible for the control and containment of the snake. Only recently has any attempt been made to document the numbers of the snake. Population size measurements were not undertaken before 1985, but some information exists on the date of initial colonization and the chronology of the spread of the snake across the