Subjectivity and flexibility in invasion terminology: too much of a good thing?

Invasions biologists have frequently debated whether the definition of invasive should include ecological and economic impacts. More recent criticisms posit that objective definitions are impossible in any absolute sense, while subjectivity is desirable for its flexibility and motivational qualities. We argue that such criticisms underestimate the extent of subjectivity already present in invasion biology. Ecological questions may be methodological if they relate directly to other ecological theories and models, or motivational if they focus on issues important to society as a whole. Motivational questions are important for engaging scientists, improving public understanding of science, and often have applied benefits. In contrast, methodological questions are constrained by established scientific theories, and are therefore more efficient for the development of scientific knowledge. Contrary to recent critiques, we suggest that greater objectivity is both achievable and desirable for the discipline of invasion biology and ecology generally.