What are Invasive Species?

Non-indigenous species are those that [[evolution|evolved]] elsewhere and have been purposely or accidentally relocated. While [[species]] can colonize new regions on their own (e.g. [[migration|migrating]] [[wildlife]], [[plant]]s and [[animal]]s 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 [[competition|competitors]] or voracious [[predation|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

[[Image:Zebra-mussels-stick.jpg|left|200px|thumb|Zebra mussels on a stick. (Photo by Simon van Mechelen, University of Amsterdam, 1990; Source: [http://www.protectyourwaters.net/hitchhikers/mollusks_zebra_mussel.php Aquatic Nuisance Species Task Force, USFWS])]]

When ships unload their cargo, they often fill their [[ballast water|ballast]] tanks with [[water]] to provide balance for their return journey. In addition to water, many aquatic [[organism]]s 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 [[port]]s to remove invasive species before they reach the destination port. Other methods being explored include using [[filtration]], [[ultraviolet irradiation|U.V. irradiation]] or [[ozone]] to [[sterilization|sterilize]] ballast water prior to discharge.

What Threats Do Invasive Species Impose?

• [[?Invasive species]] threaten [[native species]]. Exotics have contributed to the decline of 35-46% of imperiled species in the [[United States of America|United States]].
• ?In [[agriculture|agricultural]] landscapes, invasive [[plant]]s, [[herbivory|herbivores]] and [[parasite]]s 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 [[organism]]s degrade resources, such as national [[forest biome|forests]]. Current estimates of annual losses and damages from forest [[pest]]s and [[pathogen]]s in the U.S. reach $4.2 billion.
• ?Recreational and commercial [[marine fisheries|fisheries]] are also vulnerable to the threat of invasions. European green crabs negatively impact [[oyster]], [[clam]] and [[crab]] fisheries on the [[oceans#Pacific Ocean|Pacific]] [[coastal zone|coast]].
• ?Aquatic invasive species, such as the zebra mussel, clog [[freshwater biomes#ponds and lakes|lakes]] and waterways and adversely affect public [[water supply|water supplies]], [[nuclear power plant]]s, [[irrigation]], [[water treatment system]]s, 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)|genetically modified organism]]s (GMOs) provides a special case of such risk. When GMOs interbreed with wild relatives, their [[DNA]] becomes part of the [[gene pool]] of natural [[population]]s. Long-term consequences are not known.

What Can Be Done?

[[Image:2_invasion_4.JPG|left|250px|thumb|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: [http://www.esa.org/education_diversity/factsheets.php ESA])]]

Prevention is the single best way to limit impacts of [[alien species|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 [[pest]]s, but similar programs on natural areas have been slow to develop, and underfunded.

Containment (preventing further transport of existing exotics within the [[United States of America|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 [[pesticide]]s, [[herbicide]]s, or [[fungicide]]s) can be effectively used to kill [[invasive species]], but it can be problematic due to impacts on non-target [[organism]]s, including humans. Prolonged use of chemical control is expensive, and may be ineffective when target organisms evolve [[pesticide resistance|resistance]] to certain [[chemical]]s.
• 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 [[land-use and land-cover change|alteration]] of [[fire]] or [[flood]]ing regimes, is a mechanical control option when removal of individuals is infeasible.
• Biological control (introducing a natural enemy – [[predation|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.