Do biological invasions pose a threat to ecosystems? One of the current global environmental problems is the alarming decrease in biodiversity. Biological diversity is affected not only by the extinction of species but also by the spread of organisms into eco5}'Stems in which they were previously absent. The latter phenomenon is of special ecological importance if species establish populations beyond the boundaries of their previous range. This biogeographical process of range expansion is termed 'biological invasion ·. Although the distribution of species has always been dynamic, human activity accelerated this biogeographical change by (I) bridging dispersal barriers, (2) changing environmental conditions, and (3) altering biological traits of species by breeding. For instance, the 1,607 plant species native in north-eastern and central Germany are faced by 6ol alien species naturalized in this region, accounting for ca. 27% of the flora.

Scientific interest in biological invasions has two main reasons: Firstly, the damage caused by some invasive species calls for applied research in order to assess their ecological and economic impact and to the develop appropriate management strategies. And secondly, biological invasions can be regarded as natural experiments for basic ecological research, enabling general biogeographical processes to be analysed.