Invasive species

Purple flowers of the highly invasive Patterson's Curse infest the Warrumbungle National Park in New South Wales.

The term invasive species usually refers to a certain subset of those species defined as introduced species. A species is regarded as invasive if it has been introduced by human action to a location, area, or region where it did not previously occur naturally (i.e., is not native), becomes capable of establishing a breeding population in the new location without further intervention by humans, and becomes a pest in the new location, threatening the local biodiversity. U.S. Executive Order 13112 (1999) defines "invasive species" as "an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health" (CEQ, 1999). Thus, the term is used to imply a sense of actual or potential harm, something that may not be true for all introduced species.

Although an invasive species is usually an introduced species, this is by no means always the case. A species that is long-established and native to a particular area can, under the influence of human modification to the environment, increase in numbers and become as invasive as an introduced species. The Pied Currawong of south-east Australia is an example: as a result of human changes to the landscape, Pied Currawongs increased greatly in range during the 20th century and have caused substantial declines in the populations of the smaller birds they prey on the nestlings of.

Table of contents

1 Historical perspective
2 See also
3 Invasive species in Europe
4 Invasive species in North America
5 References and external links

Historical perspective

Although it is assumed that invasive species have been a problem since man has been around to carry them, modern invasive species science began with the work of Charles Elton called The Ecology of Invasions, which was published in 1958. The next ground-breaking work dealing with the principles of invasions was Island Biogeography and Conservation Practice by Simberloff and Abele in 1976.

There are several classic accounts of introduced species that have been causing problems for many decades. The sea lamprey began to make its way up into the Great Lakes Region when the St. Lawrence Seaway was completed in 1959, devastating the lake trout fishing industry. It continues to be a largely-controlled problem today, but costs millions in lamprecides, traps, physical barriers, and other control methods.

Rabbits were introduced into Australia with colonists in the 1800s and their devastation is ongoing in spite of the famous rabbit fences that were built along thousands of miles of territory with the futile intention of keeping them out.

Asian Tiger Mosquito (Aedes albopictus), a species
introduced to the USA, here biting a human.

The often unsuccessful use of biological control provides another historical perspecitive of the introduced species problem. When rats overwhelmed seaports and became crop pests during the 1800s in some islands in the Pacific, mongoose were introduced to control them. The mongoose preferred to eat native species that were easier to catch than the invasive rats, and became invaders themselves.

Another early use of biological control of an invasive species was an astounding success. In 1868 cottony cushion scale (Icerya purchasi) was accidently introduced to California in a shipment of nursery plants from Australia. The scale moved into citrus groves and became a major pest, actually killing trees. Since the scale is not a serious pest in Australia, a search began to discover why. In 1888 a naturalist observed Australian lady bugs (Rodolia cardinalis) eating the scale. American lady bugs had shown little interest in the insect. The Australian species was introduced to California, and within the year, the pest was controlled to economically insignificant levels.

Modern day biological control is used only if extensive studies find that the biocontrol species will not have a negative effect on native populations.

Economically valuable crops and livestock may be introduced species, although they can have larger effects on ecosystems than even the most successful of unintentionally introduces species.