What are Invasive Species?

Non-indigenous species are those that evolved elsewhere and have been purposely or accidentally relocated. While species can colonize new regions on their own (e.g. migrating wildlife, plants and animals rafting on debris), humans have dramatically increased the magnitude and scale of exotic species’ movements. Approximately 50,000 non-indigenous species have been introduced into the United States alone. Not all introduced species become invasive. Some introduced species appear to be relatively benign, but others are strong competitors or voracious predators with devastating effects. Invasive species are the subset of introduced species that persist, proliferate, and cause economic or environmental harm, or harm to human health.

Aquatic Hitchhikers

Zebra mussels on a stick. (Photo by Simon van Mechelen, University of Amsterdam, 1990; Source: Aquatic Nuisance Species Task Force, USFWS)

When ships unload their cargo, they often fill their ballast tanks with water to provide balance for their return journey. In addition to water, many aquatic organisms are sucked into these tanks and given transport. A ship will then empty its ballast tank (and various aquatic stowaways) at the next port where it takes on cargo. It is estimated that more than 10,000 marine species each day may travel around the globe in the ballast water of cargo ships. Many invasive species have been introduced into new areas in this manner, including the zebra mussel (Dreissena polymorpha), pictured here. One control mechanism is to exchange ballast water on the high seas between ports to remove invasive species before they reach the destination port. Other methods being explored include using filtration, U.V. irradiation or ozone to sterilize ballast water prior to discharge.

What Threats Do Invasive Species Impose?

The Asian long-horned beetle (Anoplophora glabripennis) arrived in the US in 1996. This insect infests hardwood trees, and could cause more destruction than Dutch elm disease, chestnut blight, and gypsy moths combined. (Courtesy of FWS; Source: ESA)

• •Invasive species threaten native species. Exotics have contributed to the decline of 35-46% of imperiled species in the United States.
• •In agricultural landscapes, invasive plants, herbivores and parasites outcompete crops for soil and water resources, reduce crop and forage quality, and poison some livestock species. Invasive species on US agricultural and rangelands cost over $54 billion annually.
• Introduced organisms degrade resources, such as national forests. Current estimates of annual losses and damages from forest pests and pathogens in the U.S. reach $4.2 billion.
• •Recreational and commercial fisheries are also vulnerable to the threat of invasions. European green crabs negatively impact oyster, clam and crab fisheries on the Pacific coast.
• •Aquatic invasive species, such as the zebra mussel, clog lakes and waterways and adversely affect public water supplies, nuclear power plants, irrigation, water treatment systems, recreational activities, and shipping.
• •Invasive pathogens may also directly affect human health. West Nile virus arrived in the United States as recently as 1999. In 2003, 4200 people were infected, resulting in 284 deaths.
• Hybridization with exotics can alter the genetic makeup of native species. Cultivation of genetically modified organisms (GMOs) provides a special case of such risk. When GMOs interbreed with wild relatives, their DNA becomes part of the gene pool of natural populations. Long-term consequences are not known.

What Can Be Done?

The spread of invasive species typically follows a pattern similar to the one shown above, for the colonization of jointed prickly pear (Opuntia aurantiaca). Intervention to control invasives is generally much easier during the early stages of invasion than after the species has spread over large areas. (Courtesy of Moran and Zimmerman, 1991; Source: ESA)

Prevention is the single best way to limit impacts of nonnative species. Methods include decontamination of freight, packaging material and transportation equipment that could contain unknown biotic hitchhikers, and restricting deliberate imports of potentially harmful species. Unfortunately, it is difficult to forecast which species will become a problem. However, new protocols for invasive risk assessment that consider species’ life history, habitat preferences, and history of invasive behavior have improved our predictive capacity. Once potentially invasive species are identified, it is critical to halt their intentional or unintentional spread.

Eradication may be feasible early in an invasion or in a restricted area. Early detection and rapid response is an efficient tactic for local eradication of new invaders. Regular monitoring programs to identify new exotics soon after they invade, in conjunction with any of the control methods listed below, are critical components of this strategy. Systematic early detection and rapid programs exist for agricultural pests, but similar programs on natural areas have been slow to develop, and underfunded.

Containment (preventing further transport of existing exotics within the US) is an important tool to reduce the impact of existing invaders. Strategies for containment generally combine tools used in prevention and eradication.

Control methods can include any of the following or combination of the following:

• Chemical control (using pesticides, herbicides, or fungicides) can be effectively used to kill invasive species, but it can be problematic due to impacts on non-target organisms, including humans. Prolonged use of chemical control is expensive, and may be ineffective when target organisms evolve resistance to certain chemicals.
• Mechanical control (physically removing the invasive species or changing habitat conditions) is often successful, but can be expensive and labor intensive. Modifying habitat conditions, through through alteration of fire or flooding regimes, is a mechanical control option when removal of individuals is infeasible.
• Biological control (introducing a natural enemy – predator or parasite – generally from the invader’s native range) is often the only alternative for controlling an invasive species that has established dense populations over large areas. It can be an environmentally sound way to control invasive species with minimal expense, but some control agents do not survive and others attack non-target organisms.

Restoration of native communities is an important step to minimize the chances an area will be re-invaded. Many control techniques inherently create disturbance, which may increase the vulnerability of an area to subsequent invaders.