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.
FULL SPEED AHEAD: INVASIONS ARE IDEAL FOR QUESTIONS INVOLVING RATES OF CHANGEIn most natural systems, change happens at temporal and spatial scales that are diffi cult or impossible to study. For several questions at the core of ecology and evolution, such as the rate and mode by which species adapt to new environments, invasive species have proved to be invaluable models because invasives, having left their home environment and coevolved ecological interactions behind, are expected to be under strong selection to adapt to new conditions. For example, Dlugosch et al. (2015) provided compelling evidence that since its arrival in North America, invasive yellow star thistle has rapidly evolved increased competitiveness through larger size and increased reproduction. Th is rapid evolution has allowed it to displace (nearly) all competitively similar natives at disturbed sites, even though it tends to be competitively excluded in intact grasslands. More generally, studies examining evolution in invasive species have contributed some of the central evidence that evolution can operate on ecological time scales ( Carroll et al., 2007 ).Studying invasives can also help us to predict how fast species can spread across a landscape. One active area of research is determining how quickly native species can expand their ranges in response to climate change and which native species are most at risk for extinction through a failure to adapt or expand their range ( Moran and Alexander, 2014 ). Since invasive species spreading in their new range will likely encounter novel biotic interactions, they also provide a model for how range expansion of native species will be impacted by changes in biotic interactions. For example, the enemy release hypothesis, originally conceived to explain how a loss of natural enemies could lead to some species becoming invasive upon introduction, may equally apply to native spe...