Silvia E. Burkart

Questions: Do remaining mesophytic grasslands on soils with agricultural potential respond to a latitudinal gradient? Are climatic or land‐use factors the principal drivers of regional and local diversity of these grasslands?Location: Mesophytic grasslands, Argentine Pampas, Rio de la Plata grasslands.Methods: The species presence and cover was recorded in each of 96 remnant grasslands on soils suitable for cropping, grouped into five sampling locations. In each region, CIs for γ‐diversity and for the slope of species/area curves were estimated. Unconstrained ordination was applied to detect the principal gradients in floristic composition and correlation analysis to identify their main drivers. Mantel test was used to evaluate the correlation between floristic similarity and geographic distance, and with Euclidean distance in fragmentation and climatic variables. To rank the climatic and land‐use factors that account for local species richness and percentge of exotics, we applied correlation analysis and regression models.Results: Community composition (73% native species) was mainly related to climatic variables determined by latitude, with a slight influence of fragmentation variables. Regional species richness responded to latitude: γ‐diversity decreased linearly with increasing distance from the Equator, as did the rate of species accumulation with expanding area. Alpha‐diversity did not vary in a systematic way with latitude, being associated with landscape fragmentation and mean annual precipitation. Relative importance of C₄ grasses and mean percentage exotics showed opposite latitudinal tendencies, the former decreasing and the latter increasing towards the south.Conclusions: Latitude was a strong determinant of regional diversity and community composition, but a partial driver for local species richness, mainly influenced by landscape fragmentation. While climatic and geographic drivers determined gradual latitudinal turnover in regional species pools, within regions land‐use history, stochastic processes and biotic interactions were also important. All these patterns need to be considered together when delimiting grassland nature reserves for conservation of the most diverse ecosystems of this region, currently threatened by agricultural expansion and intensification.

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.