Invasive species were officially defined in the United States on February 3, 1999, by Executive Order 13112 signed by President Clinton. The order defines an invasive species as any species that is “1) non-native (or alien) to the ecosystem under consideration and; 2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health.” Invasive species can be animals, plants, fungi, or microbes.
The 1999 definition of an invasive species is not as straightforward as might be supposed because some species considered beneficial in one ecosystem may be harmful in others. One example commonly given is the lake trout (Salvelinus namaycush), which is a desirable species in the Great Lakes, where it is native, but is regarded as an invasive species in Yellowstone Lake, where it has been introduced by humans and has displaced the native cutthroat trout (Oncorhynchus clarki bouvieri).
In addition, the perception of “causing harm” may differ from one human group to another. Some environmentalists prefer the term pest species to describe organisms of this type. One example is the Canada goose (Branta canadensis), which is technically native to North America and was considered endangered as recently as the 1980s. Since 2000, however, the expansion of geese populations along the East Coast of the United States has led to complaints about the birds' aggressive behavior, their learned habit of begging for food, their droppings, and the bacteria carried in their droppings. As a result, some states now permit hunters to trap or shoot the geese for recreation. In addition to their nuisance behavior on suburban lawns and sidewalks, Canada geese are a threat to human life because they are the species most often implicated in bird strikes at airports. They caused the engine failure of United Airways Flight 1549, which was forced to ditch in the Hudson River in 2009 with no loss of human life, and they also caused the crash of an Air Force jet at Elmendorf AFB in Alaska in 1995 with the loss of all 24 crew members.
Invasive species can originate almost anywhere on Earth and find their way to other continents and countries by air and water transport. Although the movement of plant, animal, and microbe species began with the earliest human migrations from southern Africa into Eurasia and then the Americas, the relatively primitive methods of early human transportation (foot, cart, and horseback) were slow and restricted in their geographic range.
The development of reliable seagoing vessels as early as 3000 BC added another means of transport as well as encouraging trade, which provided an important financial motivation for travel and exploration. As is well known, many plant and animal species were introduced into Europe after the discovery of the Americas in the fifteenth century, and many European species were brought to the Americas by European explorers and colonists. One example of an invasive species in this context is the pigs introduced into what is now Florida by the Spanish explorer Hernando de Soto (c. 1495–1542). De Soto had brought the pigs from Spain to feed his troops, but some escaped, turned feral, and became the ancestors of presentday feral pigs in the southeastern United States.
The invention of the railroad, automobile, and airplane in the nineteenth and twentieth century not only vastly speeded up the movement of species from one location to another but also increased the range of species movements worldwide through affordable tourism as well as trade. As of 2018, there is no place on Earth that is exempt from the introduction of invasive species. Even Antarctica, long regarded as inhospitable to most forms of life, has some species now considered invasive, such as the king crab (Neo-lithodes yaldwyni) and the chironomid midge (Belgica antarctica), a type of small fly.
According to the U.S. Environmental Protection Agency (EPA), invasive species can enter or spread within the United States by any of the following means:
The global cost of invasive species runs into billions in all the world's currencies. One British document reports that INNS cost the United Kingdom at least 1.7 billion pounds (about $2.31 billion US) per year. The Invasive Species Centre in Ontario states that the cost of invasive species to the Canadian economy is between $16.6 billion and $34.5 billion per year. According to a 2017 report to Congress, over 50,000 non-native species have been introduced into the United States, with the cost of these species having a minimum price tag of $138 billion per year.
One important measurement of the demographics of invasive species is the so-called 10% rule, which holds that only 10% of non-native species introduced into a new ecosystem survive at all; and of those 10%, only 10% (or 1% of the original number of species) become invasive. Nonetheless, even that low final percentage adds up to thousands of invasive species in North America alone.
Few people introduce a new species into an established ecosystem intending to cause harm. They may do so, however, for a number of reasons:
Human travel, trade, and recreational activities are the basic cause of the introduction of invasive species, whether intentional or inadvertent.
In addition, however, invasive species share a number of traits that enable them to compete successfully with native species:
INNS have a number of negative effects on the environment, human and animal health, international relations, infrastructure, and military readiness:
There is no single course of action against INNS that will cover all situations because there are so many different invasive species and because each threatened ecosystem is unique. Most plans, however, take one or more of the following approaches.
Early detection and rapid response (EDRR) includes monitoring of public and private lands and waters for evidence of invasive plant, animal, or insect species. Although much of EDRR monitoring is done by field biologists, veterinarians, and other specialists working for various state and federal agencies, anyone in the United States who notices signs of an infestation can take a photo or sample of the species involved and submit it to the closest APHIS office.
If complete eradication of the invasive species is feasible, it can be done by a variety of methods ranging from manual removal (digging, flooding, mulching, burning, removal of alternate hosts and destruction or removal of nests, egg masses or other life stages) to mechanical (mowing, chopping, tilling, and constructing barriers) and chemical methods (application of pesticides, herbicides, fungicides, insecticides, or sex hormones) to controlled burns and destruction of the plant or animal matter. In 2006, the government of Ecuador achieved complete success in eradicating the feral goats on the Galápagos Islands by sterilizing the females, injecting them with hormones to keep them in estrus, and then shooting the male goats when they approached to mate with the females.
In many cases, however, eradication of the INNS is not possible either because the invasive species is well established in a sizable area or because different groups in a given society disagree about the need for complete removal of the invader. People may have different opinions about the extent of harm caused by a species that may be perceived as beneficial in some respects. One example is the disputes in many communities over feral cats, defined as “cats that live outdoors and have had little or no human contact,” as distinct from strays, which are cats that lived with humans at one time but are now homeless. While bird lovers often urge shooting or euthanasia of feral cats on the grounds that they prey on songbirds, other observers favor trapping and neutering followed by returning the cats to managed colonies—a strategy known as TNR. In some areas, TNR has proved not only to be a humane approach but has proved beneficial as a pest control strategy for keeping barns, military installations, and urban buildings free of rats and mice.
As of 2018, one new method of controlling invasive species was called invasivorism, or using invasive species as a human food supply. Chefs in a number of restaurants worldwide have begun to use such INNS as zebra mussels, Asian carp (Cyprinus carpio), lionfish, rusty crayfish (Orconectes rusticus), and the Chinese mitten crab (Eriocheir sinensis) to add variety to their menus.
The fourth approach to invasive species is the restoration of land areas, forests, or waterways that have been damaged by INNS. A major reason for ecological restoration is that INNS often take advantage of natural disturbances (floods, hurricanes, forest fires, volcanic eruptions, tsunamis, etc.) to invade ecosystems that were formerly resistant to them. In addition, invasive species often degrade an ecosystem to the point that some type of human intervention is needed to restore it to a healthy and stable condition. Third, restoration projects add to scientists' understanding of the ways in which species diversity contributes to making an ecosystem resistant to invasive species.
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Rebecca J. Frey, Ph.D.