Guam is the largest island of Micronesia and, like many islands, once provided a home
to a fantastic assemblage of native birds, reptiles, and mammals. The vast majority of
these species are now extince, and the remaining ones are threatened with extinction.
Although many factors contributed to the loss of species, the principal cause is the invasive
brown tree snake. During the Second World War, the United States Navy established a large
naval base on Guam. With the end of the war in 1925, the base proved useful for recovering
abandoned war materials from the region, in particular, vehicles, aircraft, and other
supplies from New Guinea where these items may have sat unused for some time. The brown
tree snake is native New Guinea and other regions of Australasia. It is typically nocturnal,
and, during the day, rests within crevices and holes that provide good cover. It is commonly
found in wheel wells on airplanes, under the hoods of cars, and in boxes of cargo, and
most biologists think the brown tree snake was a hitchhiker within surplus Navy equipment
brought to Guam from New Guinea in the postwar years of 1945-1950. The brown tree snake
was first noticed along Guam's southern shore near Apra Harbor. It then spread, somewhat
slowly, until it occupied the entire island by 1970. Guam has no native snakes and no native
predators that could have controlled tree snake numbers. Instead, the native vertebrates
were all small species vulnerably to predation by the generalist tree snakes. The decline
of most native forest animals was immediate and dramatic. As early as the late 1960s, native
birds were disappearing from Guam's forests. Guam's Division of Aquatic and Wildlife Resources
discontinued surveys of native birds and bats in the 1970s, as there were so few individuals
of these species to count. The three native birds and mammals that persisted the longest
were the Mariana fruit bat, Guam rail, and island swifities. The fruit bat and rail were
relatively long-lived species that likely persisted simply because some individuals escaped
predation and lived out the remainder of their lives; however, neither species had any
reproductive success in the presence of the tree snake. The island swifitiet persists to
this day but is confined to one cave on Guam. The swifitiet builds nests on the walls of
caves using its own saliva and mud to adhere the nest to the cave wall. Despite this unusual
habit, swifitiet are still vulnerable to tree snake predation, as tree snakes can easily capture
prey in total darkness and climb most surfaces. The one cave where swifitiet persist is unique
in that the cave walls are not textured enough for snakes to support themselves. This does not
inhibit the swifitiet ability to build nests, but it does prohibit tree snakes from reaching
those nests. There have been many other losses of native forest animals on Guam. Consequently,
the food web of Guam's forests(the dominant native habitat)has shifted dramatically. The most
striking change is the reorganization of the web to one in which most components are nonnative.
Beyond the brown tree snake, the Polynesian rat, Philippine turtledove, and the house mous,
among other nonnative species, have been successively established. Three native lizards survive
in Guam's forests. All the lizards are small and active during daylight. One would assume that,
with the destruction of the native food web, the brown tree snake would suffer from a lack of
food resources, but the available pool of invasive prey and native lizards maintains tree snake
densities. Consequently native species went extinct with no corresponding negative effect on
the tree snake. The tree snake even seems to be adapting to the diurnal habits of the remaining
native lizards, since biologists have documented a shift in the tree snake's previously nocturnal
activity pattern to one that increasingly includes activity during daylight hours. The reduction
in the complexity of the Guam food web has had consequences beyond the loss of native animals.
The loss of mammalian and avian insectivores presumably increased insect abundances at some cost
to agricultural crops and to crop production. And newly introduced nonnative species may find
it easy to invade Guam given the many "open niches" left by the loss of native species.