Möglichkeiten der Prognose synanthroper Pflanzenausbreitungen )

Abstract

Summary The established aliens in regional floras share actually at about 5—25 % of the total number of higher plant species, in some urban and island floras more than 50 %. Most of the weeds could spread with trade and travel already during the last century. The influx of newcomers is now decreasing in some highly developed countries. This is to be seen from the years of the first record of all established aliens in Central Europe. A census of the neophytic Asteraceae of Europe and North America reveals that most of the species with a large native distribution have been already introduced into the other continent, whereas the species with a narrow native distribution generally don’t tend to synanthropic range expansion. With respect to the zonal position and the oceanity degrees the potential range in other continents resembles the native distribution. The limits of the potential range are usually reached by the newcomers within a time of some 100 or 200 years. The rapid spread often follows only after a lag-phase of some decennia. It is impossible to predict with certainty the degree of the anthropogenous enlargement of the potential range, but the general distribution character is usually preserved during the spread into the adjoining zones or oceanity degrees (as in the case of “equiformal progressive ranges”). The accelerated evolution of weeds by man’s impact on selection pressure and on possibilities of hybrididization is a further warning of an incautious prediction of range expansion of aliens.