Interactions between global change components drive plant species richness patterns within communities in mountain grasslands independently of topography

Questions How do interactions between global change factors (climate and land use, including livestock management) shape plant species richness patterns in mountain grassland communities? Does topography interact with global change factors to modulate their effect on within-community plant diversity? Location Pyrenees, Spain. Methods We used an initial set of 20 predictors: climatic, biogeographic, livestock management and topographic. Our data set included a wide range of management and climatic conditions from mediterranean to alpine environments. After a variable selection procedure with random forest, we built GLM to explain species richness in plant communities through 100-m(2) plots (SR100), employing backward-forward selection with AIC and other techniques. Results The main filters of SR100 were the regional factors (climate and biogeography, with 23% and 17%, respectively, of the total contribution to SR100), followed by livestock management (14%) and topography (9%). Interactions between climatic and biogeographic variables were almost as important as the main effects (18%). Fragmentation effects were higher under low mean minimum temperatures and summer precipitation compared with other conditions. Connectivity interacted with most of the climatic variables. Moderately high stocking rates mitigated plant community species losses triggered by decreased connectivity. Sheep-grazed grassland communities had lower SR100 than differently managed grasslands under low fragmentation scenarios; conversely, sheep grazing enhanced SR100 when fragmentation was high. Topographic predictors accounted for lower variability in SR100 than regional and management factors and were independent from these. Conclusions Several multi-scale spatial filters determine SR100 of grassland communities in the Pyrenees, interacting through complex processes. Climate can enhance negative effects of fragmentation and lack of connectivity on SR100 under stressful conditions. Livestock management modified the impact of patch disaggregation and fragmentation on SR100.