What is an Invasive Species?

The predaceous Northern snakehead (Channa argus), native to China, is a recent alien invader of North America. Reproducing populations of snakeheads have now been discovered in Maryland, California, and Florida. Click on image for larger view and further details.

An "invasive species" is a species

• that is non-native (or alien) to the ecosystem it occupies and

• whose introduction causes or is likely to cause economic or environmental harm, or harm to human health.

Some scientists classify an alien species as an invasive species if it begins to reproduce and establish a population in its new ecosystem, which often happens very quickly after introduction. Invasive species can be plants, animals, and other organisms (such as microbes or fungi). Most alien introductions result from human activities. A well-known terrestrial example of an alien or exotic species is the red imported fire ant, a native of South America first introduced to the United States in 1930s. They entered the U.S. through Mobile, Alabama, probably in soil used for ships' ballast. The fire ant has infested more than 260 million acres of land in nine southeastern states, replacing large parts of the native ant community as it continues to spread. Another example of an invasive species is the Asian tiger mosquito, introduced to the U.S. from Japan in the mid-1980s and now spreading to many regions. This mosquito attacks more hosts than any mosquito in the world. It can transfer disease organisms from one species to another, including into humans.

Invasive species affect all regions of the United States and every nation in the world. People, too, pay a high price, measured in billions of dollars for damaged goods and equipment, a degraded environment, disease, and even death. Alien species invasions can result in the loss of native species and biological diversity at a rate that ranks second only to habitat destruction. Some of the most serious and costly alien invaders include human, animal, and plant diseases (such as West Nile virus), agricultural pests (such as the Africanized honeybee, Russian thistle, and the imported red fire ant), and a host of seemingly harmless species whose sheer numbers overwhelm native ecosystems (such as zebra mussels, purple loosestrife, European green crabs, and hydrilla).

In the United States alone, approximately 50,000 alien species are known, and the number continues to grow. By some estimates, the major environmental damages, losses, and control measures for invasive species cost the nation an average of $138 billion per year. Invasive species also threaten nearly half of the species currently protected under the Endangered Species Act.

The introduction and spread of invasive species are caused by many factors, including, but not limited to:

• expanded global trade,
• the harvesting of exotic marine species to satisfy the popular aquarium trade,
• the use of non-native species in agriculture to control unwanted pests.

Invasive species also are introduced into coastal waters when large ships exchange their ballast water. It is important to study invasive species to learn about the effects they may have on native organisms and the physical environment.

Controlling invasive species is difficult because scientists know little about when new aquatic alien invaders arrive in our waters. The introduction and viability of lionfish along the U.S. East Coast are leading to new perspectives on how fish invasions may pose threats to marine ecosystems. Many scientists, like Paula Whitfield and Jonathan Hare (2003), believe that scientists must expand their research on invasive species and develop plans to manage, mitigate and minimize their effects on ecosystems that are already stressed due to other human activities (often called stressors).

Lionfish are highly visible, recognizable, and distinctive. Many other less recognizable invasive invertebrates and freshwater fish are causing real problems, too. Recent research on lionfish raised potentially troubling questions with no clear answers, such as:

Continuing research will provide more answers to these important questions.

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