What Are Invasive Species, and Why Should We Be Concerned About Them?

An invasive species is a nonnative species (in any reproductive stage, including seed, egg, spore, or other propagule) whose introduction causes or is likely to cause economic harm, environmental harm, or harm to human health. These species grow and reproduce rapidly, causing major disturbance to the areas in which they are present.

Yellow Starthistle
Steve Dewey, Utah State University, bugwood.org

The majority of nonnative species, including most of our sources of food and fiber, are not harmful, and many are highly beneficial. Only a small percentage of nonnative species are invasive, but that small percentage causes a great deal of damage.

  • If left uncontrolled, invasive species can limit land use.
  • Invasive species can decrease outdoor enthusiasts’ ability to enjoy hunting, fishing, camping, hiking, boating, and other outdoor recreational activities. For example, invasive aquatic plants overtaking a lake can make boating or swimming in the lake impossible.
  • Conservative estimates suggest that exotic, invasive species cause $120 billion per year in economic losses in the United States. Both direct costs, such as loss of a crop, and indirect costs, such as the cost associated with managing an infestation to prevent further spread, contribute to these economic losses.
  • Almost half of the native species in the United States are endangered because of invasive species. For example, the Asian chestnut blight fungus virtually eliminated American chestnut from more than 180 million acres of the Eastern United States. Many animals were adversely affected, including the ten species of moths and butterflies that became extinct because they could live only on American chestnut trees.
  • Compared to other threats to biodiversity, introduction of invasive species ranks second only to habitat destruction, such as clearing of land for building.
  • Pollen from invasive plants increases the severity of respiratory allergies.
  • West Nile Virus (WNV) is an invasive pathogen in humans and animals. In July 2008, the Centers for Disease Control and Prevention (CDC) reported that 1,086 deaths in the United States were caused by WNV.
  • Invasive species alter ecosystem processes. One example of such alteration is the change to the ecosystem of the Great Lakes caused by zebra mussels. Another example is a change in the frequency or severity of a fire regime caused by an invasive plant. Such changes make it difficult or impossible for native plants and animals to survive in the affected ecosystem.

Karan A. Rawlins, University of Georgia, bugwood.org

Often human activities (both intentional and inadvertent) are primary culprits in the importation and spread of nonnative invasive or potentially invasive species. Human activities such as trade, travel, and tourism have greatly increased. As a result, species are being moved to new areas more quickly and in greater numbers than ever before. Also, we continually look for better, easier ways to do things; this tendency is one of those human qualities that have a good side and a bad side.

Fortunately, we are learning. We have learned that we need to be much more cautious when introducing something nonnative to the ecosystems in which we live, and we pay closer attention to normal activities that may inadvertently give organisms the means to be moved from one location to another. These nonnative species are living organisms that do not recognize our human-made boundaries. They don’t stop at backyard fences or county, state, or national boundaries. Also, we have learned to take action quickly when needed. The longer we ignore an invasive species, the harder and more expensive the battle for control becomes. Early Detection and Rapid Response refers to a plan to find and remove nonnative species with invasive tendencies that have been introduced to new habitats before they become overwhelming problems. 

We can do more, however. We all can help stop the introduction and spread of invasive species by starting with informed, responsible stewardship of our own little pieces of America, however large or small they may be. Most states have an exotic or invasive species council. Join your local council, and ask about ways you can help and for a list of noninvasive alternatives to use for landscaping.

1. Pimentel, D., R. Zuniga, and D. Morrison. 2005. Update on the environmental and economic cost associated with alien-invasive species in the United States. Ecological Economics 52: 273–288.
2. OTA. 1993. Harmful non-indigenous species in the United States. Office of Technology and Assessment, United States Congress, Washington, DC.
3. Introduced Species: The Threat to Biodiversity & What Can Be Done. Daniel Simberloff: http://www.actionbioscience.org/biodiversity/simberloff.html
4. National Invasive Species Council. 2008. 2008–2012 National Invasive Species Management Plan. 35 pp.
5. Centers for Disease Control and Prevention: http://www.cdc.gov/ncidod/dvbid/westnile/surv&controlCaseCount11_detailed.htm