“Why do some introduced species become invasive?” has been a fundamental question in ecology for decades. Efforts to better understand invasions arguably started by examining traits, but more recently, doubts have been cast on the ability of traits to explain or predict invasiveness, as the importance of certain traits is often context-dependent. Thus there have been calls to abandon the search for plant traits related to invasiveness. However, I argue that such calls are premature for three reasons.
First, context-dependency and the lack of a ‘one-size fits all’ suite of invader traits does not mean that traits are not important for species success. Recent research on plant functional traits has identified major axes in trait variation that reflect markedly different growth and survival strategies that are optimal in some but not all environments that plants find themselves in. Outside invasion ecology, it is well-understood that no one trait-related strategy is universally optimal, yet there has been no call to reject the study of traits.
Second, invasion ecology has taken a ‘shot-gun’ or ‘lottery’ approach to the study of plant traits that might confer invasion success, without considering mechanistically how the trait in question promotes invasion. I reiterate the proposal of MacDougall et al (2009) to consider how, when and which traits actually result in a fitness advantage for an introduced species over natives, or a niche difference that allows invasion. This requires recognition of different types of invasive species: those that ‘fit in’ to existing communities, those that dominate communities, and those that are simply passengers of human-induced disturbance of plant communities.
Third, we should no longer consider traits in isolation from other proposed factors that lead to invasion. There are many ways in which traits and biotic interactions can be interrelated drivers of invasions, but the importance of such interrelatedness remains little investigated. This is especially the case for interactions between soil biota and plants, and belowground traits related to resource acquisition and competition.
Results/Conclusions
If we accept that traits are generally relevant to the ecology and evolution of organisms, then they should also play a role in the ecology of invasive species. Rather than abandon traits entirely, invasion ecologists should take a more sophisticated approach informed by the wider field of ecology to identify how, when and which traits are likely to confer an advantage for introduced species.