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).
Vectors are the means or 'stepping stones' by which non-native species are introduced to a new area. These can be natural or man-made. The Marine Aliens project is focussing on boat hulls - particularly recreational vessels - and hard surfaces in marinas as vectors of transport and transfer of invasive non-native species.
Causes of biological invasions
- 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, hull fouling).
Dispersal of individuals to a new recipient region.
The non-native population persists by means of local reproduction and recruitment, may also involve local spread.
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.
The invader increases its geographical distribution within the recipient region.