What are Invasive Alien Species?

Invasive alien species (IAS) are species whose introduction and/or spread outside their natural past or present distribution threaten biological diversity.

IAS occur in all taxonomic groups including animals, plants, fungi and microorganisms, and can affect all types of ecosystems. While a small percentage of organisms transported to new environments become invasive, the negative impacts can be extensive and over time, these additions become substantial. A species introduction is usually vectored by human transportation and trade. If a species’ new habitat is similar enough to its native range, it may survive and reproduce. However, it must first subsist at low densities where it may be difficult to find mates. For a species to become invasive, it must successfully out-compete native organisms, spread through its new environment, increase in population density and harm ecosystems in its introduced range. To summarize, for an alien species to become invasive, it must arrive, survive and thrive.

Common characteristics of IAS include rapid reproduction and growth, high dispersal ability, phenotypic plasticity (ability to adapt physiologically to new conditions), and ability to survive on various food types and in a wide range of environmental conditions. A good predictor of invasiveness is whether a species has successfully or unsuccessfully invaded elsewhere.

Ecosystems that have been invaded by alien species may not have the natural predators and competitors present in its native environment that would normally control their populations. Native ecosystems that have undergone human-induced disturbance are often more prone to alien invasions because there is less competition from native species. For example imported red fire ants (Solenopsis invicta Buren) are more successful in establishing themselves in disturbed areas such as roadsides and agricultural fields and rarely colonize intact closed forests. Islands are especially vulnerable to IAS because they are naturally isolated from strong competitors and predators. Islands often have ecological niches that have not been filled because of the distance from colonizing populations, increasing the probability of successful invasions. For example in Hawaii, alien insects, disease organisms, snakes, weeds, and other pests are the greatest threat to Hawaii's economy, environment and health of the human population.

What is the Problem?

Increasing travel, trade, and tourism associated with globalization and expansion of the human population have facilitated intentional and unintentional movement of species beyond natural biogeographical barriers, and many of these alien species have become invasive. Invasive alien species (IAS) are considered to be one of the main direct drivers of biodiversity loss at the global level^{1, 2}. It is clear that IAS can produce substantial environmental and economic damage and their negative effects are exacerbated by climate change, pollution, habitat loss and human-induced disturbance. Increasing domination by a few invasive species increases global homogenization of biodiversity, reducing local diversity and distinctiveness.

IAS can change the community structure and species composition of native ecosystems directly by out-competing indigenous species for resources. IAS may also have important indirect effects through changes in nutrient cycling, ecosystem function and ecological relationships between native species. IAS can also cause cascading effects with other organisms when one species affects another via intermediate species, a shared natural enemy or a shared resource. These chain reactions can be difficult to identify and predict. Furthermore, aggregate effects of multiple invasive species can have large and complex impacts in an ecosystem.

Invasive species may also alter the evolutionary pathway of native species by competitive exclusion, niche displacement, hybridization predation, and ultimately extinction. IAS themselves may also evolve due to interactions with native species and with their new environment.

IAS can directly affect human health. Infectious diseases are often IAS imported by travelers or vectored by exotic species of birds, rodents and insects. Indirect health effects of IAS to humans are the use of pesticides and herbicides against invasive pests that infiltrate water and soil.

^{1 Millenium Ecosystem Assessment. 2005. Ecosystems and human well-being: biodiversity synthesis. World Resources Institute, Washington, DC.
2 CBD Secretariat. 2006. Global Biodiversity Outlook 2. Montreal, Canada.}

Why Does it Matter?

Invasive alien species have invaded and affected native biota in almost every ecosystem type on Earth, and have affected all major taxonomic groups¹. In economic terms, the costs of invasive alien species are significant. Total annual costs, including losses to crops, pastures and forests, as well as environmental damages and control costs, have been conservatively estimated to be in the hundreds of billions of dollars and possibly more than one trillion². This does not include valuation of species extinctions, losses in biodiversity, ecosystem services and aesthetics.

The Millennium Ecosystem Assessment concluded that the relative impact of invasive alien species on biodiversity has varied across biomes, and that for all biomes, the impact is either steady or increasing as follows³:

^{1 IUCN (The World Conservation Union). 2000. IUCN Guidelines for the Prevention of Biodiversity Loss Caused by Alien Invasive Species. Prepared by the Species Survival Commission Invasive Species Specialist Group, and approved by the 51st Meeting of the IUCN Council. Gland, Switzerland.
2 Pimentel, D., S. McNair, J. Janecka, J. Wightman, C. Simmonds, C. O’Connell, E. Wong, L. Russel, J. Zern, T. Aquino, T. Tsomondo 2001. Economic and environmental threats of alien plant, animal, and microbe invasions. Agriculture, Ecosystems and Environment 84 (2001) 1–20.
3 Millenium Ecosystem Assessment. 2005. Ecosystems and human well-being: biodiversity synthesis. World Resources Institute, Washington, DC.}

What Can be Done?

Invasive alien species (IAS) are a global issue that require international cooperation and actions. Preventing international movement of IAS and rapid detection at borders are less costly than control and eradication. Preventing the entry of IAS is carried out through inspections of international shipments, customs checks and proper quarantine regulations. Prevention requires collaboration among governments, economic sectors and non-governmental and international organizations.

The Conference of the Parties (COP) to the Convention on Biological Diversity (CBD) has recognized that there is an urgent need to address the impact of invasive alien species (IAS), and established IAS as a cross-cutting issue at its fourth meeting. The decision of COP-6 included adoption of Guiding Principles for the Prevention, Introduction and Mitigation of Impacts of Alien Species that Threaten Ecosystems, Habitats or Species.

The decision of COP-8 identified measures by which Parties, other governments, relevant organizations and the Executive Secretary should address identified pathways for invasive alien species.

The work on invasive alien species under the Convention will be reviewed in-depth at COP-9 in 2008.

For more information, consult the pages on COP decisions and CBD documents.