One of the six major climate changes is related to the spread of some species outside of their native habitat: this is called biological invasions (Vitousek et al., 1997). These invasions concern both plants and animals. This issue is characterized by a scientific uncertainty that leads to various conclusions and, in fact, differentiated management choices. In this context of scientific uncertainty, perceptions are widely heterogeneous. Thus, individuals concerned about the problem will react more according to their perception of the proliferation and the representation that they have of the invasive species than according to the reality as it is described and measured by the biologists. The objective of this chapter is to illustrate this process from two examples: the proliferation of jellyfish in the Mediterranean zone and the colonization of the knotweeds (Fallopia) in the fluvial corridors of the Gard and the Rhone.