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

Abstract

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.