Invasive alien species:
Invasive alien species are a major threat to native biodiversity, natural ecosystems and ecosystem services. They are particularly devastating to island ecosystems, which harbour much of the world's threatened biodiversity. Amongst these isolated populations, extinction rates are especially high. The invasive species problem is growing in severity and geographic extent as volumes of international trade and travel increase. In spite of these effects, national and international leaders remain under-informed regarding the scope and gravity of the invasive species problem and options to fight back. Useful initiatives, which contribute to better management practices and a reduced incidence of biological invasion, are being taken by communities all over the world. Invasive alien species are now a major focus of international conservation concern and the subject of cooperative international efforts, such as the Global Invasive Species Programme (GISP), the IUCN SSC Invasive Species Specialist Group (ISSG) and the Cooperative Islands Initiative (CII). Recommended reading  

The Invasive Species Specialist Group (ISSG):
The Invasive Species Specialist Group (ISSG) is a network of expert volunteers, organised under the auspices of the Species Survival Commission of IUCN-World Conservation Union. The ISSG was established in 1994. It currently has more than 170 voluntary members from over 40 countries an d is chaired by Dr Mick Clout at the University of Auckland, New Zealand. The mission of the ISSG is to reduce threats to natural ecosystems and the native species they contain, by increasing awareness of alien invasions and of ways to prevent, control or eradicate them.

ISSG provides technical and policy advice, facilitates the exchange of information and expertise, and contributes to capacity building to manage invasive alien species to achieve and sustain positive biodiversity outcomes. ISSG publishes the Aliens newsletter, conference proceedings and awareness raising material. It coordinates the Cooperative Initiative on Invasive Alien Species on Islands (www.issg.org/cii) and manages the Aliens-L listserver and the Global Invasive Species Database (GISD). For more information, see www.issg.org

Definitions:
The Invasive Species Specialist Group and the Global Invasive Species Database (GISD) use definitions 1 and 2 (below) to define the scope of their work.

1. From the World Conservation Union - IUCN (2000) IUCN Guidelines for the Prevention of Biodiversity Loss caused by Alien Invasive Species
"Alien invasive species" means an alien species which becomes established in natural or semi-natural ecosystems or habitat, is an agent of change, and threatens native biological diversity.

"Alien species" (non-native, non-indigenous, foreign, exotic) means a species, subspecies, or lower taxon occurring outside of its natural range (past or present) and dispersal potential (i.e. outside the range it occupies naturally or could not occupy without direct or indirect introduction or care by humans) and includes any part, gametes or propagule of such species that might survive and subsequently reproduce.

2. From Decision VI/23 of the Sixth Conference of the Parties (COP6) of the Convention on Biological Diversity (CBD) in 2002
“Invasive alien species" means an alien species whose introduction and/or spread threaten biological diversity.

3. The Global Invasive Species Programme (GISP) uses a broader (1999) definition of invasive alien species.
Invasive alien species” are non-native organisms that cause, or have the potential to cause, harm to the environment, economies, or human health.

Some examples:
People worldwide are witnessing the ruinous impacts of harmful invasive pests:

• The introduced brown tree snake has devastated Guam's terrestrial birds.
• The prolific tree Miconia calvescens has overrun Tahiti's native forests.
• Philippine rice farmers have lost nearly US$1 billion in crops to the invasive golden apple snail.
• African nations alone spend an estimated US$60 million annually on the control of alien water weeds, like water hyacinth Eichhornia Crassipes and water lettuce Pistia stratiotes.
• Avian malaria, through its mosquito vector has contributed to the extinction of at least 10 native bird species in Hawaii and threatens many more.  

All invasive alien species are native somewhere:
An alien species is one that occurs outside of its natural range as a direct or indirect result of human activities. Useful species have been carried by humans to new locations throughout our history and most of these introductions (see definition) have caused little no damage to the environment. However, in a small percentage of cases, introduced species take advantage of favourable conditions in new locations and wreak ecological and economic havoc in their new environments.

Note: Native species can become problematic in their native environment, but this usually occurs only when that environment has been disturbed (e.g. for agricultural purposes).