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A non-indigenous species (also known as alien, exotic, invasive, non-native) is an organism in an ecosystem other than the one in which it evolved. Because it did not evolve there, it may cause havoc in its new environment, for example, by predating on and competing with native species, and disrupting food webs and introducing diseases. Non-indigenous species enter new ecosystems by being either intentionally or accidentally transported and released by man or by extending their geographical range following natural or man-made changes in the environment — for example, the construction of the Suez Canal.

The majority of non-indigenous species in inland waters have been introduced accidentally, are for aquaculture or for angling (Figure 2.11). For many species the ecological effects are unknown but of those having a known impact on the ecosystem, the effects have mainly been adverse. France (42) and Italy (36) have the most recorded introduced freshwater species.

These human-mediated invasions, often referred to as 'ecological roulette' or 'biological pollution', represent a growing problem due to the unexpected and harmful impacts they cause to the environment, economy and human health. The introduction of non-indigenous species is ranked as the second most important threat to biodiversity by the World Conservation Union (the first being habitat destruction).

Preventing future accidental introductions is the most difficult to tackle since it involves placing restrictions on the transfer of goods and people but introductions for aquaculture and angling could be more strictly controlled.

There are numerous examples of the ecological devastation that the introduction of non-indigenous species can cause. For example, Chinese mitten crabs (Eriocheir sinensis), originally from east Asia, now have a European distribution from Finland to southern France (Clark et al., 1998). It is predominantly a freshwater species but migrates to the sea to breed. It is believed to have arrived in the Thames in the ballast water of ships. They cause riverbank erosion and destabilise unprotected engineering earthworks since they can burrow deeply into them. They can also cross dry land to invade other river systems where they cause damage to the freshwater community. In the UK, for example, they prey on the native crayfish, Austropotambius pallipes, which is already under threat from other non-native crayfish.