When are species invasions useful for addressing fundamental questions in plant biology?

Species invasions have been championed as an ideal system for understanding key processes in ecology and evolution ( Sax et al., 2002 ), including the impacts of climate change ( Moran and Alexander, 2014 ). In exploring this idea, we have noticed a striking dichotomy: whether invasions present a useful model system depends on the degree to which the question is intrinsically about a system “at equilibrium” or a system changing directionally across time or space ( Fig. 1 ). In the case of dynamic questions, such as those about rates of adaptation or range expansion, species invasions are ideal because they mimic the type of “natural experiment” necessary to test these questions. On the other hand, for questions where a stable equilibrium is implied, such as the number of species or traits an ecosystem can support, or the proximate causes of species range limits, the transient nature of species invasions can potentially lead to erroneous conclusions. Here we argue that as a fi eld, we should recognize that invasions make powerful study systems for answering certain types of questions, but are potentially misleading for others.