Climatic and land-use drivers along a latitudinal gradient: species diversity in temperate grasslands on agricultural soils

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 c-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: c-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 C-4 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.