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Questions Are there losses in species richness due to extinctions? Is the input of new species lower in environments with strong resource limitation? Is there a decrease in abundance and diversity of native species due to anthropogenic disturbance at local and regional scale? Has abundance of exotic species been favoured by grazing increased? Are floristic changes associated with homogenization processes? Location Two grassland community types of the Argentine Flooding Pampas: the mesophyte prairie and the halophyte steppe. Methods Grassland sites (n = 51) were resurveyed with the same methodology after 35 yr. At the local scale (0.25 ha), we analysed alpha diversity, site colonization and extinction rates, and changes in species and functional type composition. Site colonization rates were calculated separately for new species (not recorded in the original inventory) and old species (present in the first survey at other sample sites of the community). Correspondence analysis was applied to the presence/absence species matrix for all sites in both periods. At regional scale, we evaluated gamma diversity, heterogeneity within and between communities, changes in proportion of functional types (growth season, longevity and life form) and species origin. Results In the mesophytic prairie, average local species richness increased 46% and regional richness 28%, while in the halophytic steppe changes were not significant. The colonization rate was higher and the extinction rate lower in the richest community (mesophytic prairie). Differences in colonization rates between natives and exotics were not found. Principal variability between sites in species composition was associated with abiotic conditions explaining 20.9% of inertia, temporal changes were relegated to the second axis with 6.38%. Proportions of warm-season exotics among new arrivals were higher than expected. Biotic homogenization was evidenced in the mesophytic prairie as a significant reduction in species turnover between sampling dates. Conclusions After 35 yr under moderate grazing intensity we found increases in species richness, no biodiversity loss and no losses of native species. Instead of those species favoured by grazing, as expected, warm season species increased. Different grassland communities showed clear differences in richness changes, in degree of homogenization and in species turnover. |
