Exotic Species

A state without snakes keeps a wary eye out for an alien invader

06-01-1998 // Anne Rillero

Fred Kraus lifts a piece of corrugated metal, inspecting the ground beneath it. "You have to think like a snake," he says, while considering places where a reptile might hide. In the middle of a Honolulu suburb, this wooded ravine offers unlimited
possibilities: waist-high grass, tangled brush, rodent burrows, illegally dumped trash, even clumps of houseplants growing wild.

As the alien species coordinator for Hawaii´s Department of Land and Natural Resources, Kraus is investigating a report of a snake seen by a boy playing in these woods. The brown-colored snake climbed a tree until it was level with the eyes of the boy, who turned and ran.

Throughout much of the United States, such a snake sighting would attract little attention. But in Hawaii--except for the harmless, earthwormlike Brahminy blind snake, which was introduced to the state from Asia--there are no terrestrial snakes. So any sighting suggests frightening possibilities. Given the Aloha State´s subtropical climate and lack of predators, some of the world´s most venomous snakes could easily become established there.

Kraus is particularly concerned about the threat of an infestation by Boiga irregularis, the brown tree snake. The cause of a devastating ecological catastrophe on the island of Guam, it is considered one of the most frightening alien species threatening Hawaii today.

Biologists believe the first brown tree snakes arrived in Guam after World War II as stowaways on cargo ships from one of their homelands: northern Australia, Indonesia, New Guinea or the Solomon Islands. Free of the natural controls of their native habitat and finding abundant prey for every stage of their lives, the snakes multiplied at an astonishing rate. Some areas of Guam are now infested with as many as 12,000 brown tree snakes per square mile and bird life of any kind is rare on the Pacific island.

An extraordinary climber, the snake devours eggs, hatchlings and adult birds. It has decimated 9 of Guam´s 12 native forest birds, pushing 3 species into extinction. Another imperiled creature, the Marianas fruit bat, is also close to extinction because of snake predation.

"The brown tree snake is capable of adapting to many tropical areas," says Thomas Fritts, a biologist with the U.S. Geological Survey who has been studying the species since 1984. Sightings of the reptiles have been reported in Okinawa, Wake Island, the Marshall Islands, Tinian, Rota, Diego Garcia, Texas and Spain. Biologists believe that most or all of these snakes originated on Guam.

Known for its bulging eyes and large head, an adult brown tree snake averages four to seven feet in length. It is a voracious nocturnal hunter that kills by constricting its prey with a mild venom injected with its back teeth. On Guam, it has snatched chickens and pets from yards and has even attacked babies asleep in cribs. Guam´s public health records indicate that 74 toddlers were treated for snake bites between 1989 and 1995.

To date, the reptile has not caused any human fatalities. But it has caused other problems for people. According to authorities at Guam´s electric companies, the climbing snakes frequently short out power lines, causing an average of one electric outage every three days.

Because the brown tree snake is light sensitive, it coils into concealed hiding places by day. When it selects an airplane wheel well or outgoing cargo as its hiding place, the reptile can hitchhike to new habitats.

Since 1981, seven brown tree snakes have been found in Hawaii. All were captured or dead upon discovery, but some other reported snake sightings have not been resolved and authorities worry whether they can effectively stop the reptile from slithering into the state.

As home to 41 percent of all endangered birds in the nation, Hawaii has a lot to lose. The state imposes fines as high as $25,000 for importing or owning snakes of any type--a penalty that has not stopped people from smuggling in pythons and other snakes for pets. But the brown tree snake threat is different.

"Never in history has a snake done as much ecological damage as this snake," says Mike Pitzler, a biologist with the U.S. Department of Agriculture in Guam. Pitzler leads a team of federal, state, military and private individuals struggling to keep snakes from leaving Guam aboard outgoing flights and ships. The team maintains 1,400 snake traps in airports and other targeted sites around the island. It also relies on 14 Jack Russell terriers, which work in shifts around the clock, sniffing aircraft and cargo for snakes before departure.

Pitzler´s staff captures 3,000 to 5,000 snakes per year, but he acknowledges his program´s limitations. "Our canine teams are not 100 percent effective all the time," he says. "There are also cargo items that are difficult to inspect."

On the Hawaiian island of Oahu, meanwhile, five beagles put their noses to work sniffing out snakes on arriving commercial and military flights from Guam. For most flights, one of the dogs and an inspector are waiting at the gate to examine the aircraft. The pair then hurries to a nearby warehouse to inspect cargo from the flight. But because of a shortage of funds for the program, not all military flights are inspected and that worries state authorities.

"Is there an acceptable risk? The answer for Hawaii is no," says Mike Wilson, chairperson of Hawaii´s Department of Land and Natural Resources. "Every brown tree snake that we don´t stop now will turn into tens of thousands of snakes over the next 10 or 20 years." The species has a clutch size of 4 to 12 young and females may produce more than one clutch per year.

Newly hatched snakes immediately begin to forage for food. On Guam, small skinks are readily available prey for the young snakes. An introduced alien initially thought to be harmless, one skink species is largely responsible for the population explosion of brown tree snakes on the island by allowing greater numbers of the snakes to survive into adulthood. "The relationship between skinks and the brown tree snake´s population is an example of what happens when you introduce nonnative plants and animals to a place," says Kraus. "You can get a synergistic effect, things that you never expected."

If one of the reptiles should slither off into Hawaii´s landscape, Kraus usually oversees efforts to find the reptile. "In some habitats in Hawaii," he notes, "you could be standing right next to a snake and not know it." To search for the snake that chased off the boy in the suburban Honolulu ravine, Kraus brought in eight volunteers. The reptile was never found, though he concluded that it was not a brown tree snake because it was sighted during daylight.

While Kraus continues his exhaustive searches, other experts are pursuing new methods to eradicate the reptile. But so far no such method has been found. "We continue looking for solutions," says Thomas Fritts. "We´re not ready to give up."

Hawaii writer Anne Rillero went snake hunting with authorities on Oahu for this article.

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