Local nutrient addition drives plant biodiversity losses but not biotic homogenization in global grasslands

Nutrient enrichment typically causes local plant diversity decline. A common but untested expectation is that it reduces variation in nutrient conditions among localities and selects for a smaller pool of species, causing greater diversity declines at larger scales and biotic homogenization. We apply a topology linking changes in species richness across scales that characterizes changes in the numbers of spatially restricted and widespread species to a standardized experiment adding nutrients across 72 grasslands on six continents. Overall, we found similar declines of spatially restricted and widespread species, and proportional species loss at local and larger scales, and no biotic homogenization after 4 years and up to 14 years of treatment. These patterns of diversity changes were generally consistent across species groups (e.g., native species, graminoids and legume species). Thus, nutrient enrichment poses threats to plant diversity that includes widespread species that are often critical for ecosystem functions.