The propagules of some plant species are dispersed naturally over long distances by agents such as birds, water or wind. We usually refer to these species collectively as the so-called ‘cosmopolitan flora’ of a region (Kloot, this volume). Such species often occur along coasts or around lakes of more than one region. Kloot estimates that from 50 to 60 plant species – about 5 per cent – occurring presently in the flora of South Australia may have arrived naturally by the process of longdistance dispersal. Of the 134 species that comprise the major weeds of the western Mediterranean Basin, Guillerm et al. (1990) estimate 24 – 18 per cent – to be cosmopolitan, of which 12 (9 per cent) species are grasses. But most species which migrate from their native region to other parts of the world appear to have done so by human means.

From the earliest times of documented records, plants have been moved from one region to another for human purposes. Records of human settlements in the Mediterranean Basin provide evidence for the deliberate movement of plants in cases where trees or cereals were of direct benefit to humans. For instance, Darius (530–522 bc) wrote to his steward Gadatar, ‘…you are taking trouble over my estates in that you are transferring trees and plants from beyond the Euphrates to Asia Minor’ (as quoted in Thirgood, 1981, p. 45).