Are alien plant species different from native species? This simple question has taxed plant ecologists for several decades and has led to heated debates between those who believe aliens are different and those who think they are the same. Chances are these two groups have been arguing at cross purposes since outcomes of comparisons between native and alien species may depend very much on the methods used. This is unfortunate, since understanding how and why alien species establish and have impacts in native plant communities is incredibly important from a conservation perspective. To move the discussion forward, we identified seven aspects of current research in this field that need prompt improvement if we are ever going to generate more light than heat in this debate.
Our seven steps to success address conceptual and methodological limitations in the studies to date. A major problem is that studies are undertaken at different spatial scales, from national floras to vegetation plots, and the expectations and value of such comparisons differ considerably. Often the traits chosen appear relatively arbitrary and focus on those aspects most easily measured rather than those most important in the dynamics of plant communities. Plants interact with each other on multiple levels, competing for light, soil resources, pollinators and avoiding grazing so it is logical to look at multiple traits as a related suite rather than independently, but the methods used to date are not ideal. We also need to know what we would expect if aliens and natives were no different and if the patterns we observe are consistent with such a null view. Since plant traits can vary as the environment changes and over time, it is sensible to look at these relationships along environmental and temporal gradients rather than at a single site and time.
We point out how best to address these limitations with the expectation that studies will, in the future, become more rigorous and consistent. This should help move the debate forward so that we can better assess where and when differences in aliens and native matter, in order to have an informed approach to the management of plant invasions.
Photo caption: Alien lupins (Lupinus polyphyllus) invading native grasslands in New Zealand where native nitrogen fixers are rare.