Invasive Species - What are they and how do they get here?

An "invasive species" is defined as a 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 plants, animals, and other organisms (e.g., microbes). (1)  

 The various routes by which invasive species are introduced are called invasion "pathways."  Some non-native species, intentionally introduced for beneficial purposes, later turn out to be invasive.  In the U.S., examples include purple loosestrife, sold as an ornamental plant, as well as saltcedar which was introduced for erosion control.  Many invasive species are unintentionally introduced; they move as undetected stowaways or "hitchhikers" when people and products are transported by air, water, rail, and road.  Examples of invasive species unintentionally introduced into the U.S. include chestnut blight and zebra mussels. (2)  The more goods our country imports, the more opportunities exist for foreign species to reach our shores, establish, and become invasive.  We are importing increasing amounts -- U.S. imports rose by 50% between 1990 and 1997 alone. (3)

Over the past 200 years, several thousand foreign plant and animal species have become established in the United States.  About one in seven has become invasive.  Invasive plants, animals, and pathogenic organisms often reduce the economic productivity and ecological integrity of agricultural and natural ecosystems. (4) 

Invasive species effects can cause a heavy economic impact.  Cornell University researchers estimate that invasive species are costing America approximately $138 billion every year in ecosystem damages, reduced crop yields, control efforts, and lost forest products. (5)  Controlling just a single invasive can carry a price tag in the millions.  The United States and Canada are spending $14 million a year to control the sea lamprey, which caused the collapse of the lake trout and whitefish fisheries in the Great Lakes.  The list of invaders is long, and the cost of prevention and control is high and going up. (2)

Ecological and environmental costs of invasive species can be staggering as well.  Invasive species typically have high reproductive rates, disperse easily, and can tolerate a wide range of environmental conditions.  Often, they lack predators in their new environments.  As a result, invasives may out-compete native flora and fauna for biological niches carved out over the course of millennia. (6)  They will also feed upon native species, spread pathogens and parasites, or crossbreed with closely related species.  It is estimated that invasive species have contributed to the placement of 35 to 46 percent of the plants and animals on the Federal Endangered Species List.  The brown tree snake is a well-known example of this; since the 1940s, when it hitchhiked in on military planes, this aggressive predator has driven nine of Guam's eleven native land birds to extinction. (2)

Invasive species also dramatically modify habitats.  In some cases, invasive species have altered the ecology of an area to such an extent that the original ecosystem is fundamentally changed.   For example, cheatgrass has accelerated the fire cycle in some parts of the western U.S. by twenty-fold, while saltcedar has altered soil chemistry and impacted native plants and wildlife. (2)  The rate at which biological communities are reshuffled as species move in and out of them through geographic range shifts has been greatly accelerated by anthropogenic causes. (7)

The World Conservation Union (or IUCN), one of the leading scientific organizations studying threats to the world's 1.4 million-plus species, says that the impact of invasive species is second only to habitat destruction, such as the felling of rain forests, among causes of extinction.  Of 1,880 imperiled American species, 921 of them - 49 percent - are at risk of extinction at least in part because of pressure from invasives, according to a 1998 report by a team of researchers led by David Wilcove of Environmental Defense. (6)

"[Invasives are] not just a problem for the world's ecology, but [are] closely linked with global trade and public health," said Jeffrey McNeely, IUCN’s chief scientist, arguing that species invasion should be more carefully considered under world trade rules.  The harm caused by alien species is "externalized" in considerations of the costs of global trade, he said.  Because the line of responsibility is insufficiently clear to bring about the necessary changes in behavior, the general public or future generations have to pay the bill. (6)