Invasive species are common occurrences on many landscapes and pose serious threats to biodiversity. This study investigated the relationships among plant diversity, nutrient addition, and susceptibility to invasion by smooth bromegrass (Bromus inermis) and crested wheatgrass (Agropyron cristatum) at two different locations in North Dakota, one representing the tallgrass prairie ecoregion and one representing the mixed-grass prairie ecoregion. Results indicate ecoregional differences in invasion by the two species. At the tallgrass prairie site, smooth bromegrass was the major invasive species and was inversely related to species diversity. At the mixed-grass prairie site, crested wheatgrass was the major invasive species and was also inversely related to species diversity. Nitrogen fertilization significantly increased smooth bromegrass biomass at the tallgrass prairie site while phosphorus fertilization had no effect. At the mixed-grass prairie site, there was a significant increase in smooth bromegrass biomass only when crested wheatgrass was used as a covariate, indicating that managers must identify which specific invasive species poses the greatest threat to grassland communities in the ecoregion. The results of this study indicate that increasing species richness can be a useful management tool to reduce invasive species biomass in grassland restorations.