Shortly after the end of the Second World War, migrants from New Guinea accidentally introduced the brown tree snake to the South Pacific island of Guam. By the 1980s, the long, slender, and slightly venomous snakes completely exterminated all but two of Guam’s native bird species and had a powerful impact on the mammals and lizards of the island. Indeed, the aggressive snakes were so successful as predators that they quickly eliminated all regular prey species. But the snakes also entangled themselves in power lines, resulting in blackouts throughout Guam. Soon, they began entering homes at night and attacking infants. Biologists and conservationists could find no way to control or eliminate the snakes on Guam.

    The story of Guam and the snakes that ate its wildlife is a tragedy. It is, however, even more tragic that introduction of alien species sometimes take place deliberately.

    Farmers, government officials, and scientists usually make these introductions with the best of intentions: nobody brings an organism to an area in order to cause the collapse of an ecosystem. Rather, such introductions spring from the desire to “fix” the natural world.

    A mix of selfishness and ignorance, this desire to fix things sometimes proves more dangerous than malice might be. The “selfish” component of this desire is to alter the natural world in the same way a business might optimize for profit: just as a business craves efficiency and thriftiness, species introductions involve bringing the most beautiful or most effective organisms from around the world to fix problems in a given area. The “ignorant” component signifies the attitude of species introducers, who believe that these organisms represent passive servants and fail to realize what consequences their introductions can bring.

    Cane toads provide a good example of a species people rashly introduced for selfish reasons. In the mid-1930s farmers brought these football-sized toads to Eastern Australia to control sugar cane beetles. Scientists studied the toads in outdoor pens to see if the animals could survive and breed in the climate. The toad thrived, and over several decades began to out-compete local amphibians. Numbers are high, and the toads now pester humans.

    The attractive flowering plant, purple loosestrife, provides another example of the destructive potential of species brought in from outside. Originally intended as a decorative plant, purple loosestrife now clogs waterways and wetlands across the Northeastern United States. Not only does purple loosestrife force out native species but it also changes the environment of wetlands. The plant clogs running water, and transforms watery areas into dry land. Efforts to eradicate purple loosestrife have had mediocre success. Due to this lack of success, many states have passed or are considering laws to prohibit the plant’s sale.

    Fortunately, some hope exists in the fight against alien species. Biologists have found ways to control and eradicate these unfriendly aliens. Some of these methods are chemical controls, often narrowly designed to combat a certain organism. Another alternative, perhaps a counterintuitive one in some respects, is to bring in other new species. When gypsy moths began to destroy over 13 million acres of forest a year, scientists managed to use chemical controls combined with several introduced fungi to check their populations.

    In some respects, the solution to the intentional introductions seems straightforward: convince people not to introduce potentially dangerous species in the first place, and wait for biologists to find solutions to problems that previous introductions created.

    Unfortunately, the problem isn’t always simple: sometimes alien species provide a benefit to people. This has been the case for many people living around Lake Victoria in Eastern Africa. The introduction of Nile perch in the 1980s virtually destroyed the amazingly diverse group of fish that previously inhabited the lake. In spite of the lake’s decline in biodiversity, the new fish proved a cheap source of food and the basis of an expansive fishing industry. For impoverished African families, the choice between biodiversity and food was an easy one to make.

    Even as they destroy life forms, alien species in the environment sometimes prove valuable to people. With this complication, the decisions about introductions of alien species become infinitely more difficult and lack anything approaching a clear solution.

Left: Montezuma National Park in 1968. Right: Montezuma in 1978 after the proliferation of purple loosestrife. Photo courtesy of the National Wildlife Refuge.