Impacts of competition and post-dispersal seed predation on exotic plant colonization

Background/Question/Methods

Biotic resistance refers to the joint negative effects of native competitors and enemies on exotic success.  However, what strength of biotic resistance is meaningful, and the appropriate stage of invasion where it is best examined remains unresolved.  One fruitful approach is to examine biotic resistance at the initial colonizing stages of invasion, where its significance can be tied to the degree to which biotic interactions limit colonization success, ultimate plant abundance, and impacts on community productivity.  Taking this approach, in grasslands in western Montana we staged experimental invasions to examine how strongly competition from resident vegetation and rodent consumers influenced the abundance of added species.  To contrast the effects of biotic filters on exotics versus natives, we added seeds of 19 species of exotics or natives to separate subplots that were either cleared of resident competitors or not and that were embedded in and outside of larger rodent exclosures.  Three years after seed addition, we assessed the effects of treatments on adult plant establishment and community productivity.  To gain a broader perspective, we replicated this experiment in grasslands in California and Germany to contrast how the strength of local biotic filters differed among regions. 

Results/Conclusions

In Montana, clearing plots of resident vegetation resulted in substantial gains in abundance of exotics, particularly small seeded species.  In contrast, native species did not respond as strongly as exotics to removal of competitors.   Suppression of rodent consumers enhanced both exotic and native abundance, with large-seeded species particularly favored.  In the presence of local biotic filters invasibility of Montana, California and Germany grasslands was roughly similar.  Across all regions, exotic seed addition consistently led to proportionately greater community productivity than did native seed addition.  Removing competitors enhanced the effects of seed addition on productivity, regardless of seed provenance.  Rodents reduced community productivity, but only in Germany and California.  Together these results demonstrate the importance of biotic resistance in affecting community structure and ecosystem function and suggest congruent and divergent ways that these local filters influence exotic versus native establishment, abundance and productivity.