Disturbances can affect the biotic resistance of plant communities and alterations could take place in the system, such as the appearance or increase in abundance of new components. To characterize the resistance to disturbance and the aptitude to limit the establishment of alien plants, it is necessary to know the functional and specific diversity of plant communities. The objective was to evaluate the effects of grazing by feral horses on the resistance of natural grasslands of Argentina to alien plant invasion by means of specific composition and functional structure. Two grasslands under continuous grazing by feral horses and two exclosures were studied. Species composition and functional structure were characterized using species richness, Shannon's diversity index, functional richness, functional evenness and functional divergence. Alien plant richness and abundance, as well as alien:native plants ratio were determined. Specific composition and functional structure were negatively affected by feral horse grazing and the distribution of alien species in the different functional groups differed in areas with contrasting grazing histories. The activity of feral horses can modify the abundance of dominant plants and functional diversity, thus providing niche opportunities for the establishment of alien plants, in turn affecting the function and structure of the system.