In ecosystems with a history of disturbance, plant community assemblages that include nonnative species are remarkably common.  How these invaders interact with each other and native plants remains an open question in community ecology. One common hypothesis is that the active colonization of a nonnative plant species reduces overall fitness and diversity of surrounding vegetation as a result of resource competition and leads to eventual competitive exclusion.  Less traditional hypotheses include more complex indirect interactions, such as differential pollinator visits and herbivory levels; a nonnative plant may thus have an insignificant or even a positive effect on resident diversity and community productivity. In this study, we evaluated potential community effects of invasion by a nonnative thistle, Carduus acanthoides. We planted communities of annual plant species that are native to or naturalized in Pennsylvania in a randomized block design, each with equal levels of initial diversity.  C. acanthoides was present or absent in the plots, and the communities were monitored throughout the growing season.  We describe aggregate community level responses to the addition of the invader by comparing measures of community attributes.

Results/Conclusions

Preliminary results suggest that the presence of C. acanthoides did not drastically alter the community growth or diversity.  Initial measures of biomass in each plot indicate insignificant productivity differences between invaded and uninvaded communities.  Among invaded plots, we found species-specific variation in plant height and abundance, suggesting that attributes of individual species are potentially important.  On a community level, we expected the thistle to reduce the amount of space available to the other species.  However, surprisingly, several invaded plots showed a greater number of flowering individuals (excluding thistles) in comparison to uninvaded plots.  In addition, preliminary analyses of biological diversity and evenness did not indicate biologically significant differences in diversity or evenness associated with thistle invasion.  Therefore, our study shows that interactions between invaders and the invaded community can lead to a complex relationship between plant invader success and community diversity.