Evidence for biotic resistance and enemy release in coastal ecosystems

Background/Question/Methods

Over the past several decades, many theories have been developed to explain why some non-native species establish in novel ranges and others fail (e.g., enemy release, novel weapons, propagule pressure, etc.).  While the mechanisms differ, biotic resistance is a critical component of all of these ideas and provides the underlying ecological framework for the invasion process.  Currently, this question is only partially answered because our understanding of biotic resistance comes almost exclusively from formal the formal synthesis of experimental results on terrestrial plants.  In addition, synthesis efforts to date have not comprehensively addressed other factors that can mediate biotic resistance including experimental location (habitat; latitude), temporal dynamics (time since invasion; experiment duration), mechanism (competition versus consumption), and response variable (recruitment versus survivorship).   We used a meta-analysis approach with field studies in marine and estuarine species that manipulated focal invaders and native species to determine how biotic resistance varied with these factors.  We compared our results with conclusions from terrestrial plant systems regarding the relative effect sizes for native species richness and native consumers vs. competitors.  Finally, we used the same approach to compare the relative effect sizes of other mechanisms contributing to invasive success/failure including Enemy Release.

Results/Conclusions

We also found that effect sizes for marine species were generally larger and more variable than those in terrestrial systems.  We also found that effect sizes for the impacts of native consumers on invaders in marine systems were highly variable but significantly different than zero.  However, effect sizes for native competitors on invaders were also variable, but did not differ significantly from the null model (i.e., absence of biotic resistance).  According to the best fit model, this high variability is likely due to differences among marine experiments in the type of habitat, experimental latitude, time since invasion, experiment duration, and response variable (recruitment vs. survivorship).  We also found that our effect sizes differed from effect sizes measured in studies of other invasion mechanisms in marine systems including Enemy Release. Our results provide new insight into why studies examining various invasion hypotheses have produced conflicting results regarding biotic resistance.  We argue our results provide a common currency that could be used to link future studies of diverse invasion hypotheses.