Background/Question/Methods Many correlative studies from natural systems indicate that exotic plants can have large impacts on both the abundance of native plants as well as on community and ecosystem processes.  However, observational studies cannot decouple observed impacts with site effects that may confound results.  Experimental studies can control or explicitly manipulate intrinsic and extrinsic factors and determine their effects on invader impact, but such studies are rare.  We examined how variation in native species diversity and resource supply individually and interactively influence invader impacts on resident native plants, resource availability, and community productivity.  We created plant assemblages that varied in native species richness and resource (water) supply and either invaded these or not with seeds of three potent exotics, spotted knapweed (Centaurea maculosa), Dalmatian toadflax (Linaria dalmatica) and sulfur cinquefoil (Potentilla recta). We also examined the competitive impacts of these exotics, in isolation, on many of the individual native species that where used to compose mixed species assemblages, as well as determining the competitive effects of individual native species on the exotics. 

Results/Conclusions

Each exotic had strong negative competitive effects on individual native species.  In contrast, native species had weak competitive effects on the individual exotic species.  These results suggest that in our system, exotics are driving invasions as opposed to acting as “passengers”.  Mixed-species assemblages with higher native species richness were less invaded and less impacted by exotics than less diverse assemblages.  Exotic impact on native species biomass increased with increasing exotic biomass in invaded assemblages.  Water addition increased invasibility (for spotted knapweed only) but had limited effects on invader impact.  Thus, diversity resisted invasion and negative impacts of exotics more than resource supply facilitated these effects.  As has been found in many diversity-ecosystem function studies, the productivity of uninvaded assemblages was highest in more diverse assemblages.  However, invasion either eliminated or reversed this relationship, depending on the identity of the invader.  Spotted knapweed was the most potent invader.  Low diversity assemblages that were highly invaded with knapweed produced up to four times the biomass compared to more diverse assemblages that were resistant to knapweed invasion.  The fact that exotics were able to “rewire” the diversity-productivity relationship suggests that they are impacting ecosystem processes in fundamentally different ways than native species.