The natural vegetation of the Mediterranean Basin is disappearing rapidly. From cultivated areas, it disappeared long ago; in the rest of the Basin it is now threatened because of increasing human demand for space, food, energy and urban settlement. Native vegetation is also disappearing because of incidents related to the misuse of land or because of natural disasters such as floods and forest fires.

The various Mediterranean environments, whether cultivated or not, have been affected to varying extents, so that some situations may be more accurately described as being ‘new’ environments. The human desire to expand cultivation of specific plants (mainly introduced) and to increase their yields, results in disturbance and a severe change in the utilised environments because of the introduction of new species, their cultivation in glasshouses and the use of irrigation and/or herbicides. In such environments, which are exploited presently or have been used formerly, some plants can either establish or increase their density to the extent that they can be termed invasive. Plants which become invasive in particular environments within the region from which they originate are termed ‘apophytes’. Plants invading after their deliberate introduction (for afforestation or for cropping), or after being introduced inadvertently (as contaminated seeds, ship ballast or in wool), are termed ‘anthropophytes’, after Quézel et al. (1990). Crop invaders, which eventually cause serious economic problems in crop management systems or by decreasing yields, I shall call ‘weeds’; this class of invasive plant is often the most aggressive.