Definitions

A biological invasion happens when an organism arrives somewhere beyond its previous range of distribution (Williamson, 1996), and impacts upon indigenous species. Therefore, a biological invasion "produces a significant change in terms of community composition" (Cronk & Fuller 1996).

Natural expansion:
- Physical or biological barriers disappear. (e.g. through tectonic movements that connect previously separate water bodies)
Human origin (anthropogenic):
- Deliberate or accidental releases (e.g. escapes from aquaria, aquaculture activities).
- Human-mediated transport (e.g. ballast water, fouling organisms).

1. Arrival
Dispersal of individuals to a new recipient region.
2. Establishment
The non-native population persists by means of local reproduction and recruitment, may also involve local spread.
3.Integration
The new invader and the recipient region species respond to each other ecologically and evolutionarily (e.g. competition, new host, hybridisation). However, if the invader colonises a new habitat then integration may not necessarily occur.
4. Spread
The invader increases its geographical distribution within the recipient region.