Invasive species are animals, plants, and other living things that spread rapidly in new environments where there are few or no natural controls on their growth. People transport large numbers of species from one region to another. Only a small portion of these introduced species become invasive, but invasive pests can cause great environmental damage. They crowd out native species, putting them at risk of extinction. They also cause billions of dollars in damage each year to agriculture, fisheries, forestry, and public health.
How species become invasive.
People have carried species from one environment to another for thousands of years. But increases in travel since 1800 have greatly expanded the number and variety of introduced species. Travelers and cargo on ships, airplanes, or other vessels carry some creatures accidentally. Others are imported deliberately. People may introduce animals and plants to combat native species they consider harmful to agriculture. They may transport species as farm animals or as ornamental plants. People may even dump species into new habitats simply to get rid of them.
Some invasive pests spread quickly after they arrive in new environments. The zebra mussel, a kind of shellfish, arrived in North America from Russia in the late 1980’s, carried in ballast water in the deep holds of ships. It quickly spread out of control in North American lakes, threatening native aquatic life. Some living things become invasive only after another invasive species arrives. In Florida, landscapers have used introduced ornamental fig trees for over 100 years. Until the 1980’s, the trees grew only in yards and parks because they could not reproduce without the wasps that pollinated them. But in the 1980’s, some of these wasps invaded the region, and now fig species are spreading into natural areas.
How invasive species affect habitats.
Invasive species can prey on native ones, compete with them for resources, or alter them through mating. Some pests may change an entire habitat and endanger a wide array of native life.
Endangering native species.
Invasive animals can directly threaten native species by preying on them. The brown tree snake traveled to the island of Guam in ship and airplane cargo from other South Pacific Islands. Since arriving on Guam, brown tree snakes have eliminated many of the island’s native forest birds.
Invasive species may also drive out native life by competing for food, light, and other natural resources. In the United Kingdom, competition between invasive North American gray squirrels and native red squirrels has caused the red squirrel population to decline. In warm North American waters, the South American water hyacinth has become so abundant that it keeps sunlight from reaching submerged native plants, threatening these plants’ survival.
Invasive species can gradually change and even extinguish native species by mating with them. The rainbow trout, a popular food and sport fish from western North America, has been introduced to other areas to boost local fisheries and sport fishing industries. Some of these trout mate with native trout species, producing hybrids. In certain cases, the hybrids are replacing native trout.
Altering habitats.
An invasive pest can alter an entire habitat in ways that threaten many native species. In parts of Florida, Australian paperbark trees with highly flammable leaves and twigs have increased the number and intensity of forest fires. This environmental change has driven out native plants not adapted to frequent or intense fires. The loss of native vegetation has, in turn, harmed native animals that relied on the old plants for food and shelter.
In the Mediterranean region, a variety of Caulerpa seaweed from Australia has become a major pest. In 1984, an aquarium in Monaco accidentally released it into the Mediterranean Sea. Since then, the seaweed has grown out of control on seabed habitats throughout the Mediterranean region. Overabundance of Caulerpa has led to the decline of fish and other sea animals in those areas.
Invasive pests can also damage habitats by spreading disease. For example, the Asian fungus that causes chestnut blight disease was introduced to North America in the late 1800’s. Chestnut blight not only killed most North American chestnut trees, but it also affected moths, birds, and mammals that relied on these trees.
Managing invasive species.
The most effective way to manage invasive species is to keep them out. Many nations restrict the importation of species that might become damaging or of products that can carry them—such as untreated wooden packing material.
To combat invasive species after they arrive, people can use one or more of four basic approaches. These approaches are mechanical control, chemical control, biological control, and ecosystem management.
Mechanical control
includes trapping animals and uprooting plants. Such methods are often effective. People in the United Kingdom completely eliminated the nutria, an invasive South American rodent, through trapping. But mechanical control usually involves intensive human labor. The United States has combated the Asian long-horned beetle, a pest imported from China, by cutting down and burning hundreds of beetle-infested trees.
Chemical control.
People employ chemical pesticides to manage many introduced species. Pesticides have helped control such pests as the Anopheles mosquito, which spreads the disease malaria. But chemicals can prove expensive, particularly if used over large areas. Moreover, many pesticides become ineffective if species develop resistance to them.
Biological control
involves introducing a natural enemy of an introduced pest to reduce the pest’s numbers. In Africa, for example, the South American cassava mealybug once devastated cassava plant crops. This problem was controlled by an imported South American wasp that preyed on the mealybug.
Biological control agents can also cause harmful side effects. Pacific islanders introduced the North American rosy wolf snail to many islands to control the previously introduced giant African snail. The rosy wolf snails not only failed to control the African snails but also attacked native snails, eventually driving some of them to extinction. The Russian wheat aphid, a native of southeastern Europe and southwestern Asia, reached the United States in 1986. It quickly spread through western North America and nearly eliminated wheat and barley crops in certain areas. To combat the aphid, farmers distributed the Eurasian sevenspotted lady beetle. But this beetle competed fiercely with native ladybugs for natural resources, threatening native populations.
Ecosystem management.
Management of an entire ecosystem—that is, all living and nonliving things in a particular place—is one of the newest methods of controlling invaders. For instance, many North American farmers and ranchers prevent livestock from overgrazing on native grasses. This policy helps keep an invasive plant called the Eurasian musk thistle from becoming a damaging weed. If native grasses remain plentiful, they can successfully compete with the thistle and keep its numbers low.
