An “invasive species” is a species that has been introduced by humans into a new ecosystem, in which it had never naturally occurred, and due to that introduction has caused economic or environmental damage. Invasive species can include plants, animals, and even microbes. While many invasive species may seem innocuous at first glance, invasive species are one of the main causes of extinctions around the world, and the annual economic losses from invasive species is estimated to be over $120 billion each year.
Background – Causes of Invasive Species
Invasive species can be introduced intentionally for seemingly beneficial purposes, such as for livestock feed, flood control, and pest management. Invasive species can also be accidentally introduced into the environment, often as an undetected passenger on a ship, plane, or vehicle. The brown tree snake was introduced to Guam this way, and as a result, wiped out most of the native bird life on the island. Invasive species on islands are extremely damaging because many island species have evolved without the presence of predators, and have no defenses against them. As an extreme example, the Stephens Island wren in New Zealand was driven to extinction by one individual cat, owned by the lighthouse keeper on the island. More recently, invasive species have been introduced when pet owners decide to release exotic pets, such as the Burmese Python, into the wild because their owners cannot take care of them anymore.
Estimates on the number of non-native species in the United States vary and are likely underestimated, but the USGS estimates that there are now more than 650 invasive species that poses risks to native ecosystems, human health, and economic activities in the United States. With the globalization of trade, the increased speed of travel, the massive volume of cargo shipments, and rising tourism, the rate of introduction, establishment, and cost of mitigation will continue to rise.
Invasive, non-native species cause harm to the economy, human health, and the health of other animal species. This damage varies ranging from diminished farmland property values, increased disease, and higher operating costs, to irrigation water losses, collapse of buildings, competition with native animals, sport, game, and endangered species losses, ecosystem disturbances, and much more.
Terrestrial Invasive Species
Invasive species now occur in virtually every habitat in the United States. Invasive plants and animals can cause severe damage to natural ecosystems and can damage agricultural interests, ranchland, and forests.
- Mediterranean fruit flies were introduced to Hawaii in 1910 and to the mainland in 1929. These flies can infest over 400 species of plants, including valuable citrus and vegetable crops, and annually cause millions of dollars in damage to agricultural interests.
- Kudzu is one of the more famous non-native, invasive species. Originally introduced for erosion control in 1876, its prolific ability to spread has lead to landscapes where Kudzu has spread over all other plants, and even human structures.
- The European Starling was introduced to the United States in 1890. It is a very aggressive bird and out-competes native cavity-nesting species for nest sites. In addition, it is a significant pest for agricultural, especially dairy and livestock facilities where they consume and contaminate feed. Starlings also pose threats to safe aircraft operations, and may cause property damage due to accumulations of feces.
- The Cane toad was introduced to the United States from South America in 1955. Originally, it was introduced to act as a pest control agent in sugar cane fields in Florida and Hawaii. Unfortunately, the cane toad has persisted in the environment, even though sugar cane production has declined in the United States. Cane toads are voracious predators of native species, and are highly toxic to predators. Each year, many household pets become sick after interacting with cane toads.
Freshwater Invasive Species
Invasive species can spread just as easily in the water as they do on land. Given the difficulties of working in the water, they can be even harder to remove than terrestrial invasive species. Each year, they cause millions of dollars in damage.
- The quagga mussel and the zebra mussel were introduced in the late 1980s from Eastern Europe through ballast water to the Great Lakes. Both species are prodigious water filterers and can remove substantial quantities of suspended particulate matter and phyto-plankton from the water. They can quickly blanket almost any structure that is submerged in the water, and quickly clog water intake structures. Cleaning and removing mussels from water intakes and other structures costs hundreds of millions of dollars each year. The total impact of mussel infestations has been estimated at over $3.1 billion.
- Due to their behavior of jumping out of the water, Asian Carps are one of the most famous invasive species. Asian carps were first documented in the Mississippi river in 1971 and have spread through most Midwestern rivers. The carp are in direct competition with native aquatic species for food and habitat. Their rapid population increase is disrupting the ecology and food web of the large rivers of the Midwest, including the Missouri River and Mississippi Rivers. Today, Asian Carp are threatening to invade the Great Lakes. In response, the Obama Administration announced a series of new measures to protect the Great Lakes from Asian carp and to prevent this invasive species from developing self-sustaining populations in the Great Lakes in its 2011 Asian Carp Control Strategy Framework.
Ocean Invasive Species
Invasive species in marine environments can have devastating impacts because it is nearly impossible to fully eradicate them once they have become established in a new location. Invasive species can impact fisheries, recreational interests, and ecosystem function.
- Lionfish were most likely introduced via the aquarium trade in the 1990s in Florida. Sine then, they have spread along the Atlantic coast of the United States, the Caribbean coasts of Central and South America, the Gulf of Mexico, throughout the Greater Antilles and through the Leeward Island. Lionfish are highly predatory and can eliminate native fish populations in areas where they have spread. Their introduction has decimated fisheries and coral reef systems, threatening the viability of eco-tourism operations as well as regional fisheries.
- Ballast water is water that is taken onboard a ship and stored in tanks to add weight, thereby maintaining the ship’s trim and stability. Depending on where the ballast water is taken onboard, it may be freshwater, brackish, or saltwater, and might contain organisms that are not native to the port area where ballast water will be discharged. Aquatic invasive species constitute a significant threat to biodiversity in the world’s coastal waters because they often have no natural predators and may out-compete native species for food in their new environment. Once established, invasive species can cause major environmental and economic harm as they multiply and spread.
Microbial Invasive Species
The smallest of all invasive species are among the hardest to control and the most dangerous to human health and the environment.
- Bubonic plague was introduced to the United States from Asia in the late 1800’s, and has since spread throughout the United States. It was spread by non-native black rats carrying disease-infected fleas. It has decimated native squirrel and prairie dog populations throughout the country. While it is easily treatable, if treatment is not undertaken promptly, can still lead to illness or death.
- West Nile Virus first arrived in the United States in 1999 inside another invasive species, the Asian tiger mosquito. Within a decade, West Nile virus has spread to nearly every state in the country. And, for people who are exposed, one in 150 infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis, and even death. West Nile Virus also has reduced wildlife populations throughout the United States.
- White nose syndrome first arrived in the United States in 2005 from Europe. Since its introduction it has killed at least one million bats and has spread to 11 states. Bats are some of the most important natural pest control agents, consuming half their weight in insects each night.
- White Pine Blister Rust was introduced from Asia around 1900, and has killed millions of acres of pine trees throughout the west. One tree impacted by blister rust is the whitebark pine, which is considered a “keystone” species because of its importance to other organisms, including the grizzly bear and the Clark’s Nutcracker. In Glacier National Park, whitebark pine communities have been disappearing and their loss could have a profound effect of many other species. The white bark pine is now so rare that the U.S. Fish and Wildlife Service is considering listing the plant as a threatened species.