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Introducing exotic grasses and legumes into North America for livestock forage has been  practiced widely for over a century.  These forage species are typically selected for traits conferring persistence under stress, potentially yielding effective invaders.    We used standardized systematic review guidelines and meta-analytical techniques to answer the following question: To what extent, if any, do exotic forage species negatively affect plant communities?  To address this question we explored a) the fundamental relationship between invasion and native plant community changes and b) transformative differences between species, photosynthetic pathway, plant functional types and structure of the plant community.  Our systematic review of nine widely distributed exotic forage species in North America (five C4 grasses, two C3 grasses, and two legumes) yielded a total of 40 papers with quantitative data from 73 case studies that met our inclusion criteria for meta-analyses.  All species that had quantitative data suitable for meta-analysis had a significantly negative effect on native plant communities except for T. repens.  The greatest negative effect was associated with E. lehmanniana.  Effect size differed by photosynthetic pathway and plant functional type with C4 grasses exerting the greatest negative effect followed by C3 grasses, and legumes.  Effect size differed among metrics, with the greatest negative effect on native plant species diversity and the least negative effect on species evenness.  Exotic forage species violate the ‘Rule of Tens' that suggests 1% of species will become a pest.  This suggests that exotic forages should be considered as a special case of invasion.  Characteristics that make exotic forages different from other invasive plants hinge on pathways of selection and dispersion.  Exotic forages present a complex socio-ecological problem exacerbated by disconnected educational disciplines, competing interests between policy and science, and well-intended efforts to increase food production.