Non–indigenous species have many labels; aliens, non-natives, exotics, introduced, immigrants, biological pollution and an additional term for plants, noxious weeds. Non-native invasive species are defined in many ways. Non-native species can be referred to as those that have been “moved to new places by humans.” One publication defines an invasive species as a “non-native organism that causes harm to native habitats or species.”
Official U.S. definitions for invasive species provided in Executive Order 13112 signed by President William Clinton in 1999 state that “Invasive species means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.” “Alien species means, with respect to a particular ecosystem, any species, including its seeds, eggs, spores, or other biological material capable of propagating that species, that is not native to that ecosystem”
The biological and economical threat of invasive non–indigenous species has been well established over the past two decades. Almost one half (42%) of the threatened or endangered species in the United States listed under the Endangered Species Act, are in jeopardy due to competition or predation by non-indigenous species, and this proportion balloons to as much as 80% in other regions of the world. The total estimated annual cost for all invasive organisms was determined to be $136,630 billion, including the cost of losses and damages as well as control cost. Invasive non-indigenous plants are estimated to encroach upon roughly 700,000 hectares of native habitat per year. These invasives then threaten the native plants and wildlife on the site, biodiversity on a grand scale, as well as negatively impact entire ecosystems. In addition to diminishing biodiversity and disrupting ecosystem functions, invasive species seriously impact agricultural systems and can be hazardous to livestock and humans.
Many of these invasive plants are capable of forming monocultures on a given site, completely displacing native plants as well as altering the structure, productivity, fire and flooding regimes and soil nutrient properties. The results from a survey of restoration ecologists within western Washington revealed that invasive non-indigenous plants were the leading cause of failure of restoration projects. In addition to direct competition, herbivory/predation, and parasitism, additional impacts of non-indigenous species include physical or chemical alteration of the habitat and soil, and introduction of pathogens. The impacts of the introduced species are especially problematic when the impacted species are keystone species, causing disturbance of the food web structure and biodiversity functions. By removing the natural barriers between non-indigenous and native species as humans are doing at a phenomenal rate, we are altering the genetic diversity of the native species and native community.