16.8: Human activities can damage the environment: 1. Introduced non-native species may wipe out native organisms.

In the rain and sleet of a Chicago winter, the idea of colorful birds flying about is very appealing to some people, and you might see exactly that, in the form of the monk parakeet. These small green members of the parrot family are about a foot (30 cm) from head to tail tip. And if they look out of place in Chicago, it’s because they are. Monk parakeets are just one example of an exotic species (also called introduced species), which refers to species introduced by human activities, intentionally or accidentally, to areas other than the species’ native range.

Native to southern South America, monk parakeets were imported to the United States as pets in the 1960s and 1970s. Some of those pet birds escaped, survived, and bred, and free-living populations of monk parakeets are now found in 11 states across the country, including Oregon, Florida, and New York, in addition to those mentioned above, in Illinois (FIGURE 16-16).

The phrase “exotic species” suggests colorful birds and butterflies winging through a forest, but the ecological reality of such species can be much darker. Although some introduced species do not cause harm to their new habitat, in many cases they do—including economic or environmental harm, or harm to human health. When species are introduced and cause harm, they are referred to as invasive species.

The appearance of the free-living monk parakeets in the United States was greeted with alarm because they are regarded as agricultural pests in South America. Records from the Inca civilization that flourished before Pizarro’s invasion of Peru in 1532 attest to crop damage by monk parakeets. And when Charles Darwin visited Uruguay in 1833, he was told that monk parakeets attack fruit orchards and grain fields. Fortunately, the dire predictions that these parakeets would devastate crops in the United States have not been fulfilled as yet, but monk parakeets do make a nuisance of themselves in cities such as Chicago, where they build nests on utility poles, transformers, and floodlights. There is nothing delicate about a monk parakeet nest—it is a large mass of sticks and twigs. And a nest that has been soaked with rain is heavy enough to bring down wires, interrupting electrical service to entire neighborhoods.

While monk parakeets were initially introduced to the United States intentionally, in many cases the introductions of exotic species have been unintentional. This was the case with the brown tree snake, which reached Guam sometime before 1952 by stowing away in shipments of military equipment just after the Second World War. In either case—intentional or unintentional—two species characteristics, in particular, can lead to introduced species becoming harmful, invasive species (FIGURE 16-17).

• 1. Exotic species may have no predators or pathogens in their new habitat, so their populations may grow unchecked.
• 2. Native plants and animals may have no mechanisms to compete with or defend themselves against invading exotic species.

Guam, which lies in the middle of the South Pacific Ocean, had no native snakes and no predators specialized to eat snakes. But Guam did have a magnificent fauna of native birds that had evolved in isolation on the island for thousands of years. Because the species of birds on Guam had never experienced predation by snakes, they had no defense mechanisms against them—and brown tree snakes have eradicated most of the native species of birds. Extraordinary efforts are being made to prevent brown tree snakes from spreading to other Pacific islands, such as Hawaii. Like Guam, these islands are home to species of birds that occur nowhere else in the world, and the islands have no native species of snakes and no native predators of snakes.

One current promising strategy, used by the U.S. Department of Agriculture, is to airdrop dead mice with tablets of acetaminophen—the active ingredient in Tylenol—packed into their bodies. In addition to killing live animals, brown tree snakes scavenge dead animals such as mice. And it turns out that acetaminophen interferes so much with the oxygen-carrying capability of the snake’s hemoglobin that a child’s dose of Tylenol will kill a brown tree snake. While initial results are promising, it is too early to predict the long-term success of this program.

We don’t have to look beyond the borders of the continental United States, though, to find examples of invasive species that are responsible for massive ecological shifts and economic costs. Purple loosestrife is an attractive flowering plant that is native to Eurasia. It was imported to the United States in the 1800s as a garden plant. Once here, it rapidly escaped from cultivation and invaded wetlands in every state except Florida. It produces thousands of seeds a year and also spreads by sending out underground stems. This aggressive growth overwhelms native grasses, sedges, and flowering plants, replacing diverse wetland communities with monocultures of loosestrife that provide poor-quality habitats for bog-dwelling insects, birds, reptiles, amphibians, and mammals (FIGURE 16-18).

The title of “Most Destructive Invaders in North America” may belong to the zebra mussels and quagga mussels. These thumbnail-size freshwater mussels are native to eastern Europe and western Asia. In the early 1800s, the mussels spread to western Europe. They unexpectedly came to North America in the 1980s—in the ballast water of ships traveling through the Saint Lawrence Seaway on their way from the Atlantic Ocean to the Great Lakes. Female mussels produce more than a million eggs per year, and the larvae settle on any solid surface. Sometimes they settle so thickly that they clog the intake pipes of water-treatment plants and factories and the cooling systems of power plants.

The ecological threat that zebra mussels represent is far more serious than the damage they cause to industrial facilities. The Great Lakes fisheries are based on game species (lake trout, salmon, walleye) and commercial species (whitefish, perch, herring), and they produce revenues of more than $1.5 billion annually. Most of the commercially valuable fish feed on small fish species, such as smelt, which, in turn, feed on microscopic plants and animals (phytoplankton and zooplankton). Zebra and quagga mussels are exceptionally efficient at filtering phytoplankton and zooplankton from the water—so good that they are depleting the resources available to the small fish. Without enough food, the populations of small fish are crashing and eliminating the food supply for the large game and commercial fish. The Great Lakes contain nearly one-fifth of the fresh water in the world, but this enormous ecosystem is being irreversibly degraded by just two exotic species.

One legendary pest is the cane toad. Ironically, it was initially imported into Australia to control a native sugarcane pest, the cane beetle. Unfortunately, not only were the cane toads unsuccessful at controlling the pest populations, but the cane toad populations exploded—preying on numerous species of amphibians, reptiles, mammals, and even birds. They spread throughout much of the continent and now number more than 200 million. Cane toads have become a nuisance in much of the country, as conservation workers struggle to determine how to control their numbers (FIGURE 16-19). The cane toad illustrates a common problem encountered when species are intentionally introduced, say, as a method of biologically, “naturally” controlling pests: the effects of a species on an ecosystem in which it does not naturally occur are unpredictable and often terrible.

Controlling or eradicating invasive species once they have become established and spread can be difficult and costly. Consequently, the best approach when dealing with invasive species is early detection and a rapid response. Toward these goals, in the United States the National Invasive Species Council has been established and spends approximately $1.3 billion per year coordinating the efforts of 38 federal departments and agencies to prevent and control invasive species.