The status of explaining plant invasions: Hypothesis overload

Background/Question/Methods

The prominence of research associated with biological invasions in the evolutionary, ecological, conservation and restoration literature has skyrocketed over the past several decades resulting in a flurry of hypotheses aimed to help explain the fate of invaders.  Traditionally, two separate lines of investigation have aimed to develop generalizations regarding the success and failure of plant invaders.  The first approach (often championed by botanists or taxonomists) focuses on identification of specific plant traits associated with invasiveness (seed size, time to maturity, dispersal mode, allelopathy. etc.). More recently genetic traits, including phenotypic plasticity, overall genetic diversity in introduced populations, and evolution of reproductive systems were added to the list of plant specific traits.  These pre-adaptive traits are thought to allow introduced plants to out-compete native species however, with few exceptions reliability of trait-based predictions is poor.  The second approach (often championed by ecologists and entomologists) focuses on interactions of introduced plants with biotic and abiotic forces in recipient communities. Here the role of natural enemies (e.g., predators, parasites, herbivores, diseases, microbial communities) and established plant communities (biotic resistance) in facilitating or suppressing plant invasions is emphasized, most prominently captured by the enemy release hypothesis. Invasiveness may also evolve in response to changes in selection pressures by natural enemies yet differences in enemy pressure (not simply presence) among native and introduced relatives are surprisingly small, often suggesting no significant differences in enemy pressure for most plant species.

Results/Conclusions

These two different approaches to understanding invasive success, either focusing on plant traits or local biotic and abiotic interactions of plant invaders, have continued to live nearly entirely separate lives in the literature.  Our disciplinary focus as scientists appears to be a serious impediment for developing and testing more sophisticated and integrative predictions regarding introduction, establishment and impact or spread of invaders.  This does not necessarily imply that this “holy grail” in biological invasions can be found; idiosyncratic interactions of species with other biotic or abiotic factors and evolutionary processes of the invader or the invaded may prevent any meaningful advances beyond the myriad of currently existing hypotheses with poor predictive capabilities.  However, we should at least attempt a better integration of the different strands of investigations using more sophisticated, long-term studies to assess our ability to predict success or failure and long-term impact of biological invasions.  I will provide a number of examples to illustrate potentially successful approaches from wetland and forest plant invaders.