WEEDS, INVADERS, AND COLONIZERS: WHO IS WHO AND WHY

Abstract

Weeds, invaders, and colonizers are three closely related but not identical concepts reflecting three different viewpoints: anthropocentric (weeds interfere with management goals of people), ecological (colonizers appear early in successional series), and biogeographical (invaders are spreading into areas where they are not native). Nevertheless, there is a considerable overlap between these three categories, and a large number of obnoxious species can be cross-classified as belonging to all three. It is this overlap which guarantees that generalizations made about one category can be helpful in understanding the performance of many (but not all) species belonging to the other two. We need predictive theories which can help us set priorities for the control of introduced invasive weeds and allow us to predict the risk of future invasions. Analyses of statistical relationships between native and adventive distribution ranges of species introduced from Eurasia to Americas and vice versa can provide some understanding and generate interesting hypotheses concerning the nature of successful invaders. At present, only very limited generalizations are available, based on plant physiology, genetics, or demography. However, invasiveness of pines (genus Pinus) and, very likely, other woody species of seed plants is predictable on the basis of a small number of simple biological characters. These results are promising and should encourage us to continue with this line of research.