Differentiating Successful and Failed Invaders: Species Pools and the Importance of Defining Vector, Source and Recipient Regions

Attempts to understand the dynamics of biological invasions continue to abound in aquatic and terrestrial ecosystems. Identifying the biological attributes of successful invaders, or what makes a good invader, are among the most tantalizing questions still to be answered, especially in marine ecosystems. Numerous studies across a range of taxonomic groups have examined species characteristics to determine whether certain species level factors strongly differentiate successful from failed invaders (see Rejmánek and Richardson 1996; Williamson and Fitter 1996; Reichard and Hamilton 1997; Miller 2000; Kolar and Lodge 2002; Prinzing et al. 2002; Cassey et al. 2004a,b; Miller et al. 2007). At the heart of these analyses is the comparison of successful and failed species pools, which are defined in various ways with specific consequences for the inferences that can result.

When trying to understand the effects of species characteristics on invasion outcome, most studies compare physiological tolerances, life history characteristics, and behavior of successful and failed invaders. Although it is certainly valid to compare any two groups to understand differences in their respective attributes, only a subset of such comparisons can answer questions about the invasion process. More specifically, invasions have a specific context and result from interactions among source regions, recipient regions, and transfer mechanisms (vectors). Thus, comparing invaders from one source region to non-invaders from a different source region may tell us little about attributes of successful invaders, because the latter group may not share the same opportunities for transfer and invasion, thereby introducing additional variables and confounding interpretation.