In search of a real definition of the biological invasion phenomenon itself

The many qualifying terms attributed to invasive species reveal the lack of precision surrounding the notion of biological invasion itself. In spite of several proposed definitions, some basic disagreements persist concerning characterization of the phenomenon. These primarily arise from the lack of pertinence of both of the main current criteria-the geographic (or biogeographic) criterion and the impact criterion-to what is really intended by invasion. Faced with this situation, it seems preferable to adopt an ontological approach allowing a return to the basic principles of the elaboration of a definition. Starting with the nature of the phenomenon itself (i.e., its essence), we try to elucidate the notion of biological invasion and we suggest a general definition compatible with most of the ideas already expressed.