Biodiversity Partnership:Invasive Species

Invasive Species

The spread of non-native or "exotic" species has emerged in recent years as one of the most serious threats to biodiversity, undermining the ecological integrity of many native habitats and pushing some rare species to the edge of extinction.

The greatest threats are posed by a relative handful of exotic species that have invasive tendencies and provide no benefit to humans. Described by some as a form of biological pollution, these species have been introduced -- sometimes by accident, but often with the best of intentions -- into systems where they thrive at the expense of native species. Some introduced species simply out-compete native plants and animals for space, food, or water. Others may also fundamentally alter natural disturbance regimes and other ecological processes, making it difficult or impossible for native species to survive.

Within their natural range, populations of most plants and animals are limited by competition, predation, and disease. Most introduced species confront the same kinds of biological and physical limitations in their new habitats, but those that have invasive tendencies and are not too limited by natural controls can spread rapidly, with dramatic, adverse ecological and economic impacts.

About 15 percent of the estimated 6,000-plus non-native plant and animal species in the United States cause severe economic or ecological impacts, according to the Congressional Office of Technology Assessment (1993).

Impacts of non-native species on biodiversity are more difficult to quantify but are clearly substantial. Exotic species have been implicated in the decline of 42 percent of the species listed for protection under the federal Endangered Species Act (The Nature Conservancy 1996).

Once established, many exotic species are virtually impossible to eradicate. Biological controls may offer hope in some cases, but many of the invasive exotics now found in the United States are probably here to stay. As a result, land managers struggle just to limit their further spread.

The best option for combating invasive exotics is to prevent them from being introduced in the first place. Early detection may allow eradication before they become fully established. However, simply getting rid of an invasive exotic may not be enough. If no action is taken to restore native species and ecosystems and to prevent future introductions, native species and ecological communities will remain vulnerable to re-invasion or to other new pests.