Background/Question/Methods   Almost 30% of the flora in the United States is not native and such exotics make up a substantial portion of the biodiversity in old-field communities. Exotics rarely cause native extinctions and floral surveys indicate that the introduction of exotics has increased species richness overall. However, native plants have presumably coevolved regionally with each other, and with their microbial and animal communities, whereas exotic species have been introduced piecemeal from across the globe. Thus, we predicted that plant assemblages with exotic species would perform differently than assemblages comprised entirely of native species. We established plant assemblages that were either monocultures (containing only one native or one exotic species) or polycultures (containing either eight native, eight exotic or a mixture of four native and four exotic species).

Results/Conclusions   After two years, we found that increasing diversity enhanced primary productivity, particularly in exotic assemblages. Exotic polycultures were 85% more productive than the average exotic monoculture, whereas native polycultures were only enhanced by 43% and mixed native/exotic polycultures were 66% more productive than their respective monocultures. Impacts on the insect communities associated with patches followed a similar pattern of differential impacts of native vs. exotic plant diversity. While exotics showed consistently higher fruit production in polycultures (all exotics or mixed with natives) relative to monocultures, natives showed a stronger positive diversity effect with natives than when mixed with exotics. These results suggest that species that have no shared evolutionary history can be highly productive when grown together, but that native species in these communities may suffer disproportionately.