Interest in biological invasions has quickened recently (see Groves & Burdon, 1986; Kornberg & Williamson, 1986;Macdonald et al., 1986; Mooney & Drake, 1986; di Castri et al., 1990), both in terms of revising the theoretical framework for the subject and the addition of supplementary data. Two main questions underlie this awakened interest: firstly, what are the biological features that determine whether a species will become an invader or not, and secondly, what are the site properties that determine whether an ecological system will be relatively prone to or resistant to invasion? The answers are many and relate to biological characteristics such as a high reproductive rate, high dispersal ability, inbreeding capacity, disturbance frequency, lack of predators or disease, climatic matching, vacant niche, etc. Disturbance attributable to human actions is a factor in many cases, but the very multiplicity of interpretations indicates that there is neither a simple cause nor a few obvious characteristics which are able to explain biological invasions (Elton, 1958). As Crawley (1987) said, we are ‘unable to predict whether a particular introduction will be successful’, but we guess that many communities are invasible and that a number of common and widespread plants may behave as invaders.

In the Mediterranean Basin, invaders are comparatively few and they generally have little economic impact, contrary to the situations in other areas of mediterranean climate (Guillerm, this volume).