Invasive species

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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. The term invasive species refers to a subset of those species defined as introduced species.

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Native vs. non-native invasive species

Although an invasive species is often defined as an introduced species that has become a pest, some species native to a particular area can, under the influence of natural events such as longterm rainfall changes or human modifications to the habitat, increase in numbers and become invasive pests. 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 whose nestlings they prey on. A species of wetland plant known as 'ae'ae in Hawai'i (the indigenous Bacopa monieri) is regarded as a pest species in artificially manipulated waterbird refuges because it quickly covers shallow mudflats established for endangered Hawaiian stilt (Himantopus mexicanus knudseni), making these undesirable feeding areas for the bird.

Of course all species on earth go through periods of increasing and decreasing population numbers, in many cases accompanied by expansion and contraction of range. A wide variety of circumstances are the cause of such changes, and human "alterations" on the landscape are especially significant in this regard. Anthropogenic (human derived) alteration of an environment that enables expansion of a species into a geographical area where it had not been seen before could be described as invasive because the range expansion results in the species occurring where it was not before native. In essence, one must define "native" with care, as it refers to some natural geographic range of a species, and is not coincident with human political boundaries. Whether noticed increases in population numbers is sufficient reason to regard a native species as "invasive" requires a broad definition of the term—but it seems reasonable to consider that some native species in disrupted ecosystems can become "pests" and in that sense, invasive.

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.

Multiple invasive species interaction

Ninety percent of immigrant species do no obvious harm to their new home environment. Sometimes, however, introduction of a second invasive species can allow the first invasive species to flourish and affect an environment.

One example of this is the gem clam. The gem clam had been introduced into California's Bodega Harbor from the East Coast of the United States a century ago. It had been found in small quantities in the harbor but had never displaced the native nutricola clam species.

In the mid 1990s, the European green crab arrived in the harbor and began to decimate the local clam population while largely ignoring the gem clam. With the removal of the local population, the gem clam has begun to displace the local clam and has shown accelerated growth. (Grosholz, 2005)

The threat to global biodiversity

Main article: Biodiversity

The impact on global biodiversity of human introduction of non native species that have subsequently become invasive is subjective. Climate change and the movement of the continents through the ages have created divisions and changes to species over the long history of this planet. Limited information on the circumstances and impact at the time makes it difficult to directly compare this to the advent of international travel for people and goods which has made the introduction of new species increasingly easy, and the direct ecological impacts are now far more evident and measurable.

Historically the deliberate introduction of non native species has been done with little or no consideration of the impact outside of having a favored animal, fish, or plant available locally, or perhaps an ill-conceived attempt to control a native pest. In areas with highly endemic, specialised and isolated flora and fauna such as Australia, New Zealand, Madagascar, the Hawaiian Archipelago, and the Galapagos Islands, introduced species that successfully establish themselves in habitats utilized by natives compete for limited resources or prey on the native species, some of which are unable to adapt to the more competitive environment and gradually die out.

As more adaptable and generalized species are introduced to environments impacted adversely by human activities, native species are put at a disadvantage to survive in what previously was a unique, balanced ecosystem. There are many examples of decreases in biodiversity in those areas. One of the primary threats to biodiversity is the spread of humanity into what were once isolated areas with land clearing and habitation putting significant pressure on local species. Agriculture, livestock and fishing can also introduce changes to local populations of indigenous species which may result in a previously innocuous native species becoming a pest due to a reduction of natural predators.

See also

References and external links

Further reading

  • Alan Burdick, Out of Eden: An Odyssey of Ecological Invasion, Farrar Straus and Giroux (18 May, 2005), hardcover, 336 pages, ISBN 0374219737
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