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The species composition of the flora in disturbed and natural sites along a Mediterranean to desert aridity gradient was studied. The chorological and ecological data (i.e. phytogeographical composition, growth form spectra, primary production) show that the main impact of man is the levelling of zonal differences. Thus, while local species diversity increased in most regions, an overall decrease in diversity occurred along the gradient, with the sharply distinct natural vegetation of the different zones giving way to a similar ruderal flora in the disturbed sites. These generally lacked endemic species and showed a rise in annuals, in mesic, Mediterranean elements as well as in introduced and wind-pollinated species.