The unified framework for biological invasions by Blackburn et al. (2011) on the left showing how alien species must pass through a series of barriers to establish and invade a new area. At each stage, the pool of species passing through decreases. However, in urban ecosystems, the invasion barriers are changed, resulting in different species crossing each barrier and, in general, more species moving through each of the invasion stages, i.e. the “urban effect” on invasion. The magnitude of an alien species’ impact is likely to increase along the invasion framework (introduction – naturalization – invasion continuum) in both natural and urban ecosystems. However, in urban areas, this impact will manifest in different ways (e.g. greater socio-economic impact).