Alien flora of the Czech Republic, its composition, structure and history

information on the alien flora of the Czech Republic is presented. The first com- plete list of non-native taxa reported from the country since the beginning of agriculture 7,300 year ago was developed from available knowledge. There are 1378 alien taxa, 33.4% of the total plant diversity of the country. The composition and structure of the alien flora is classified using the cat- egories of Richardson et al. 2000 (there are 64.7% casuals, 28.8% naturalized, and 6.5% invasive taxa), by residence time (24.1% of archeophytes. i.e. introduced before 1500, and 75.9% neophytes introduced after then), by introduction mode (53.4% of accidental arrivals, 46.6% deliberate intro- ductions), and by habitat type invaded (62.8% confined to human-made habitats, 11.0% recorded in seminatural and natural habitats, and 26.2% recorded in both). Archaeophytes differ from neophytes in the distribution of particular categories of invasive status, abundance in the landscape, vegetation type invaded, Grime's life strategies, and representation in families. Most "neophytic genera/fami- lies" contain very low number of species whereas in archaeophytes the distribution is shifted towards more species-rich genera/families. The available data suggest that the most intense immi- gration of archaeophytes occured during Neolithic/Chalcolithic period, then proceeded at a steady pace until the Medieval when another peak of immigration occurred. Problems associated with acquiring reliable quantitative data on alien floras and with comparisons between regions are dis- cussed.