When a species moves into a new place, things can get a little crazy. Case in point: the brown tree snake (Boiga irregularis) on Guam.
The story begins with scrap metal. Piled high in junkyards to await recycling, the scraps, with their many crannies and spaces, make ideal hiding places for rodents, rabbits, and lizards—and the snakes that prey on them. Snakes turn up in junkyards in almost every industrial place in the world. One fateful shipment of scrap metal and wooden crates arrived on the North Pacific island of Guam via airplane about 1948. It contained at least one brown tree snake. Probably it was a single gravid female. She became the ancestor of a whole population. For decades, the growing numbers of brown snakes on the island went unremarked. It was only when other wildlife began to disappear that people noticed the danger. Originally, the island was home to 18 species of birds. But the nightingale reed-warbler, a slender bird with a mohawk of feathers, vanished; and a sea bird called the brown noddy; and the Mariana crow, with its black feathers highlighted in blue and green. In all, fifteen bird species went extinct on the island. All of them had become common prey items for the brown tree snake.
The snake is quick and so lean its eyes seem to bulge from its head. The eyes have vertical pupils, like a cat’s. Despite its name, the snake is sometimes reddish or pale yellow rather than brown. In its native range of Papua New Guinea, the Solomon Islands, Indonesia, and part of the Australian coast, it rarely grows past six and half feet long. On Guam, however, it can reach almost ten feet. Scientists believe the difference is that Guam has virtually no predators capable of eating the snake. (The few exceptions, like the cane toad, only occasionally eat snakes, and then only the youngest ones.) In Australia, there are dozens of potential predators, from owls and kookaburras to other snakes. It’s hard to be certain which of these actually eat brown tree snakes, but at least one of them, the red-bellied black snake, has been witnessed doing so. Unfortunately, the redbelly’s venom makes it dangerous to humans, so it can’t be imported to control the invaders on Guam.
The brown tree snake glides among the branches of trees, sometimes dozens of feet high, searching for birds and their eggs. It seizes an unlucky bird in its mouth. If necessary, it also wraps around its prey to constrict it. Fangs in the rear of its mouth chew in a slow, paralyzing toxin. Death happens because the snake’s tightening coils prevent the bird from taking a breath or, if the bird lives long enough, because of the toxin. Then the snake swallows its victim whole. The toxin exists mostly to fight bacteria that might otherwise multiply as the bird decays in the snake’s gut. Its power to kill the prey is a secondary benefit.
Besides birds, the snake preys on bats (it has apparently exterminated two of the three native species) and lizards. And it has left other signs of its presence. Sometimes it climbs onto power lines, causing electricity to arc. The results are a power outage and a cooked snake. Some 1200 snake-related power outages have occurred on Guam in recent years, but these accidents have made no real dent in the snake population. The native animals continue to die. Having evolved on an island with few predators, they are slow to adapt to this invasive species. The loss of the animals has had far-reaching effects. Without the birds and bats to help pollinate them, certain plants have begun to decline. For example, there’s a type of mangrove tree that grows red flowers. On nearby islands, birds visit those flowers to sip their nectar. Once the bird sticks its beak in, the flower releases a tiny explosion of pollen. Some of that pollen clings to the bird’s beak and feathers as a fine dust. The next mangrove flower the bird visits can be fertilized by the pollen. When that happens, the flower produces fruit containing fertile seeds. On Guam, birds are no longer seen feeding from the mangrove flowers. New trees appear at a much slower rate. On the neighboring island of Saipan, it’s hard to count the trees because they crowd so closely. On Guam, it’s easy.
This kind of effect—with a new organism affecting others it doesn’t directly harm—is called an ecological cascade. The trees haven’t been the only victims of the cascade on Guam. Some of the missing birds and lizards ate insects. Without those controls in place, pest insects have bred in greater numbers, damaging fruit and vegetable harvests. Mosquitoes, especially, thrive without the insect-eating kinds of birds. As a result, the people of Guam suffered an outbreak of dengue, a sometimes fatal fever spread by mosquitoes.
Because they live mostly in the trees, the snakes are hard to spot. Many people visit Guam without ever seeing a snake. This doesn’t mean the snakes avoid people, however. They’ve been known to take eggs from the nests of domestic chickens and even kill pet birds in people’s houses. And, on occasion, they try to eat human babies.
Scientists have recorded at least eleven such attempts. Generally, the snake is found in bed with the child, who has been bitten and constricted. Because the brown tree snake slowly chews its venom into a victim, it is not especially dangerous to people who are big enough to run away or fight back. Only babies are small and defenseless enough to suffer life-threatening bites. Still, the slender snake is not large enough to swallow even these smallest humans. All the babies known to have been attacked this way have been rescued, though at least two needed artificial help breathing.
[I'm indebted to John C. Murphy's Secrets of the Snake Charmer for much of the information here, and to Max C. for pointing me toward this excellent book.]