Revisiting the use of the invasive species concept: An empirical approach

Invasion science has not been developed without controversies. Two questions that are still unsolved are: what is an invasive species?, and are invasive species an inherent conservation problem? These questions have led to discussions about effects versus origins. In contrast to the definitional problems, a unified framework describing invasion as a step-by-step process has been widely accepted. I conducted a bibliographic search with two separate databases searching for (i) evidence of less use of controversial terms over time; (ii) how many articles defined invasive species'; (iii) the criteria used to define a species as invasive; and (iv) in which stage of the invasion continuum were species labelled as invasive located. My results show that controversial terms are widely used, that authors rarely define invasive species' and, often, it is very complicated to determine which criterion they used. In addition, only a fraction of the species labelled as invasive could be classified as such according to the unified framework of invasion stages. This is not a merely semantic issue, because invasive is a strong and value-laden term that is used to guide environmental agendas. The uncritical use of a key concept could hamper research, complicate communication among peers and produce mixed results.