Invasive species are plants or animals that do not belong where humans have intentionally or accidentally brought them.

Effects: immense, insidious, irreversible

IUCN, the World Conservation Union, states that the impacts of alien invasive species are immense, insidious, and usually irreversible. They may be as damaging to native species and ecosystems on a global scale as the loss and degradation of habitats.

Hundreds of extinctions have been caused by invasive alien species. The ecological cost is the irretrievable loss of native species and ecosystems.

Species detrimental to other species

For example, the mongoose threatens endemic species on tropical cane-growing islands. They have caused the population demise or extinction of many endemic vertebrates, and continue to cause livestock damage while posing a disease risk. European red foxes introduced into Australia and temperate regions of North America have negative impacts on many native species, including smaller canids and ground nesting birds in North America, and many small and medium-sized rodent and marsupial species in Australia.

Growing global trade and communication are directly contributing to the mixing of wildlife across biogeographical boundaries.

Increasing realisation of the ecological costs of biological invasion

Species that appear in new environments may fail to survive but often they thrive, and become invasive. In fact, native species are likely to be unprepared to defend themselves against the invaders. This process, together with habitat destruction, has been a major cause of extinction of native species throughout the world in the past few hundred years.

Although in the past many of these losses have gone unrecorded, today, there is an increasing realisation of the ecological costs of biological invasion in terms of irretrievable loss of native biodiversity.

Countering the invaders

Biologists are investigating how these "invaders" succeed in establishing themselves in new environments, so that they can be controlled and eradicated where necessary.