An invasive species is an organism that is not indigenous, or native, to a particular area. Invasive species can cause great economic and environmental harm to the new area.

Not all non-native species are invasive. For example, most of the food crops grown in the United States, including popular varieties of wheat, tomatoes, and rice, are not native to the region.

To be invasive, a species must adapt to the new area easily. It must reproduce quickly. It must harm property, the economy, or the native plants and animals of the region.

Many invasive species are introduced into a new region accidentally. Zebra mussels are native to the Black Sea and the Caspian Sea in Central Asia. Zebra mussels arrived in the Great Lakes of North America accidentally, stuck to large ships that traveled between the two regions. There are now so many zebra mussels in the Great Lakes that they have threatened native species.

Introduced Species

Some species are brought to a new area on purpose. Often, these species are introduced as a form of pest control. Other times, introduced species are brought in as pets or decorative displays. People and businesses that import these species do not anticipate the consequences. Even scientists are not always sure how a species will adapt to a new environment.

Introduced species multiply too quickly and become invasive. For example, in 1949, five cats were brought to Marion Island, a part of South Africa in the southern Indian Ocean. The cats were introduced as pest control for mice. By 1977, about 3,400 cats were living on the island, endangering the local bird population.

Other invasive species descended from pets that escaped or were released into the wild. Many people have released pet Burmese pythons into the Everglades, a swampy area of south Florida. The huge snakes can grow to 6 meters (20 feet) long. Pythons, native to the jungles of southeast Asia, have few natural predators in the Everglades. They feast on many local species, including white ibis and limpkin, two types of wading birds.

Invasive Species and the Local Environment

Many invasive species thrive because they outcompete native species for food. Bighead and silver carp are two large species of fish that escaped from fish farms in the 1990s and are now common in the Missouri River of North America. These fish feed on plankton, tiny organisms floating in the water. Many native fish species, such as paddlefish, also feed on plankton. The feeding cycle of the paddlefish is slower than that of the carp. There are now so many carp in the lower Missouri River that paddlefish do not have enough food.

Invasive species sometimes thrive because there are no predators that hunt them in the new location. Brown tree snakes were accidentally brought to Guam, an island in the South Pacific, in the late 1940s or early 1950s. No animals on Guam hunted the snakes, but the island was filled with birds, rodents, and other small animals that the snakes hunt. The snakes quickly multiplied, and they are responsible for the extinction of nine of the island’s 11 forest-dwelling bird species.

Many invasive species destroy habitat, the places where other plants and animals naturally live. Nutria are large rodents native to South America. Ranchers brought them to North America in the 1900s, hoping to raise them for their fur. Some nutria were released into the wild when the ranchers failed. Today, they are a major pest in the Gulf Coast and Chesapeake Bay regions of the United States. Nutria eat tall grasses and rushes. These plants are vital to the regions' marshy wetlands. They provide food, nesting sites, and shelter for many organisms. They also help secure sediment and soil, preventing the erosion of land. Nutria destroy the area’s food web and habitat by consuming the wetland grasses.

Some invasive species do great harm to the economy. Water hyacinth is a plant native to South America that has become an invasive species in many parts of the world. People often introduce the plant, which grows in the water, because of its pretty flowers. But the plant spreads quickly, often choking out native wildlife. In Lake Victoria, Uganda, water hyacinth grew so thickly that boats could not get through it. Some ports were closed. Water hyacinth prevented sunlight from reaching underwater. Plants and algae could not grow, preventing fish from feeding and reproducing. Lake Victoria’s fishing industry declined.

Invasive species can also damage property. Small zebra mussels clog the cooling systems in boat engines, while larger ones have damaged water pipes at power plants throughout the Great Lakes region.

Eradicating Invasive Species

Officials have used a variety of methods to try to eradicate, or get rid of, invasive species. The cats on Marion Island were infected with a virus, for instance.

Sometimes other species are introduced to help control an invasive species. In Australia, prickly pear cactus, which is native to the Americas, was growing out of control. The cactus was destroying rangeland, where ranchers raised livestock. The government brought in cactus moth caterpillars to eat the cactuses. The caterpillars are natural predators of the cactus.

Introducing insects can be dangerous, however. Sometimes, the insects also damage other plant species—they can become invasive species themselves. Chemicals have also been used to control invasive species, but they can sometimes harm noninvasive plants and animals. 

Governments are working to educate the public about invasive species. For example, in the United States, international fishing vessels are warned to wash their boats before returning home. This prevents them from accidentally transporting zebra mussels or other species from one body of water to another.

Sometimes, communities approach invasive species like an invading army. Nutria in Chesapeake Bay destroy the natural habitat, as well as cost local governments and businesses millions of dollars each year. Environmental groups, business leaders, and government officials are concerned about the harm done by this invasive species.

Officials at the Blackwater National Wildlife Refuge, in the U.S. state of Maryland, worked with hunters to eradicate the 8,500 nutria in the refuge. Hunters waded into specific areas of the marsh during specific times of the year. They tracked nutria using global positioning system (GPS) equipment and set traps that would kill the rodents. The hunters moved across the refuge in a massive, coordinated, west-to-east movement. In winter, the ice on Chesapeake Bay prevented the nutria from swimming away. Hunters could shoot them on sight.

The operation took two years, but nutria were eradicated from Blackwater National Wildlife Refuge. The wetland is slowly recovering.