Why does a species, which doesn’t usually behave in an invasive way in its natural area of distribution, begin to proliferate in the area in which it is introduced? Is this linked to the species’ own characteristics or to the particular conditions of its new environment? The large amount of research by scientists into these questions shows that, in most cases, invasion is the result of a “combination of circumstances”, where certain of the plant’s aptitudes, such as a very high capacity for plant reproduction (via cuttings*, shoots* or suckers*) and/or sexual reproduction (via seeds), benefit from a very favourable context such as the absence of natural predators in the area of introduction or the creation by human activity of disturbed sites that are easy to colonize (wasteland or reclaimed land, wild land, cultivated land, riverbanks, etc.) which are ideal areas for primary infection. The pressure of introduction and the existence of communication routes that facilitate dissemination (canals, motorways, railway lines) also determine the success of the invasion.
This interaction between the many biological and ecological factors involved makes it very difficult to identify a future invasive exotic plant in its early stages among the plant species introduced. Furthermore, as the invasion process begins very slowly, the plant can remain unnoticed for decades; population growth then gradually shifts to a phase of rapid expansion. At this stage, all the damage begins to show and the fight against an invasive exotic species becomes much more difficult!