Alien plant species: A real fear for urban ecosystems in Europe?

Urbanization has led to a concentration of people in urban areas in Europe and worldwide. Thereby, a typical urban nature has co-adapted to human disturbed urban settlements. Recently, non-native species have become into focus as being a serious threat to urban biodiversity. This paper analyses the rational background of the fear of so-called "alien" species for citizens and urban ecosystems. The urban flora of vascular plants in Germany is investigated to the extent to which it contents neophytes, i.e. plant species introduced after 1492, and what are their ecological species traits. The EU DAISIE research project lists two non-native urban plant species among "100 of the Worst" Invasive Alien Species in Europe, namely Common Ragweed (Ambrosia artemisiifolia) and the Tree of Heaven (Ailanthus altissima). Their impacts on mainly biodiversity and human health are analysed in more detail in comparison to native urban species with a similar ecological niche. Conclusions are drawn for the evaluation and treatment of non-native plant species in urban areas.