Community Structure in Grazed and Ungrazed Grassland Sites in the Flooding Pampa, Argentina

Abstract

-We compared the community structure and composition of two contiguous grasslands, one that had been continuously grazed and one that had remained ungrazed since 1972. Basal cover was measured at both sites in 1976 and 1985. The structure of the grazed area was affected by grazing during these 9 years, which was indicated by the reduction of the cover of grasses and the increase of the cover of forbs and exotic species. In the ungrazed site more than 95% of the basal cover was due to monocots, and the species that dominated this site were seldom found in the grazed site. Large tussocks replaced smaller ones in the absence of cattle. The grazed community was more diverse than the ungrazed one at a small scale (5 m), but less diverse at a larger scale (20 m). Cool season species were always dominant in the grazed site, but in the ungrazed site warm season species dominated some patches and cool season species, others. We concluded that in the absence of grazing, different competitive equilibria may be attained in different patches, probably due to subtle environmental differences. In the grazed area, the effects of cattle upon the community may override environmental heterogeneity and prevent the achievement of competitive equilibrium.