Invasions: the perspective of diverse plant communities

Exotic plant invasions represent a threat to natural and managed ecosystems . Understanding of the mechanisms that determine why a given species may in vade a given ecosystem, or why some biomes and regions seem more prone to i nvasions, is limited. One potential reason for this lack of progress may li e in how few studies have addressed invasion mechanisms from the point of v iew of the invaded community. On the other hand, the renewed debate about t he relationship between ecological diversity and ecosystem stability offers the opportunity to revisit existing theory and empirical evidence, and to attempt to investigate which characteristics of plant communities, includin g their diversity, contribute to their invasibility. Empirical studies have shown both positive and negative relationships between species diversity o f resident plant communities and their invasibility by external species. Ra ther than attempting to build a larger collection of case studies, research now needs to address the mechanisms underlying these relationships. Previo us knowledge about the mechanisms favouring invasion needs to be coupled wi th community theory to form the basis of these new investigations. Modern c ommunity theory offers hypotheses and techniques to analyse the invasibilit y of communities depending on their diversity and other factors, such as sp ecies' life histories and environmental variability. The body of knowledge accumulated in invasion ecology suggests that the role of disturbances, in interaction with fertility, and the importance of interactions with other t rophic levels, are specific factors for consideration. In addition, it is e ssential for future studies to explicitly tease apart the effects of specie s richness per se from the effects of other components of ecological divers ity, such as functional diversity (the number of functional groups) and tro phic diversity (the number of interactions among trophic levels).