Distribution patterns of species are a consequence of long evolutionary histories. Biogeographical barriers have resulted in separate developments of biota with specific adaptations to their native ecosystems and associated environmental conditions. Especially during the past centuries, human activity has helped species to surmount these natural barriers, so that present-day patterns of alien species result from natural drivers as well as man’s history of land exploitation and construction of traffic routes. Humans created new pathways of species introductions (Chaps. 2 and 3), and also new habitats. Introduced species were thus able to invade both (semi-)natural and humanmade habitats, which differ considerably in their proportion of alien species.

With the arrival of aliens in a novel environment, interactions between resident species are disrupted, and interactions among resident and invading species have to be newly established. Though unplanned and mostly unwanted, biological invasions are considered to be an important ecological experiment, well suited for ecological studies. Because many aspects are better known in alien species than in native ones (e.g. time of isolation from the original gene pool, and we have replications by introductions into multiple localities), species invasions provide a unique opportunity to test general ecological theories as an alternative approach to focused experimental manipulations which might be more constrained by time, space, research budgets, etc.