Phylogeny and community resistance to invasion

Exotic species may find it more difficult to invade communities with close native relatives if competitive interactions and shared enemies are correlated with phylogeny. Here we test the prediction that the presence of close relatives in old-field plant communities leads to increased herbivory, disease, and competition, reducing the potential invasiveness of a novel exotic species.  We hypothesized that damage (herbivory and fungal infection) would be reduced in communities with lower abundances of close relatives, and in these same communities, the absence of competitors would result in higher productivity.  To test these predictions we planted Solidago virgaurea into three community types with varying abundances of native Solidago species; we also manipulated cover.  Contrary to our prediction, herbivory was highest in intact communities that lacked native Solidago species; this was primarily the result of rodent herbivory.  Infection by a rust fungus was also similar across community types.  Finally, when competitors were removed the relative increase in productivity was highest in communities with low abundances of close native relatives. Overall, the results indicate that the aboveground impact of competition and shared natural enemies play a minor role in mediating community resistance to invasion by our study species; belowground effects may be more important.  Future work will investigate if belowground feedbacks have a stronger effect on plant growth in communities that lack close relatives.