Plant populations have always been subject to species replacements and demographic shifts in density, both of which may lead to invasion. As agriculture developed over millennia, so the chances for plant invasion increased, especially among those plants we now call weeds. Weeds are colonising species capable of occupying otherwise unoccupied space. Only those species with several reproductive strategies and those with life cycles adapted both to summer-dry, winterwet conditions and to cultivation have become major weeds in regions of mediterranean climate. The selection forces affecting size and spread of populations of weeds have been described by Sagar (1982). After introduction, some plants may spread immediately; alternatively, the spread of some other plants may be delayed until a sizeable population of propagules has built up and only then will spread occur.

The weeds of mediterranean areas

In mediterranean-climate areas weediness comes from the biological ability of a plant species to survive a mediterranean-type climate and a particular cultivation regime associated with the development of agriculture in the Mediterranean Basin. Human effects on weed invasion have increased, not always gradually, but sometimes episodically (Holzner & Glauniger, 1982; Pignatti, 1983; Zohary, 1983; Pons & Quézel, 1985). The long and complex history of agricultural disturbance, especially in the Mediterranean Basin, has led to a large and diverse weed flora, which is still evolving.

About 11000 years bp a Neolithic culture change occurred in south-western Asia, and over the Mediterranean Basin, which was to have an irreversible effect upon human history.