Plant species responses to nutrient addition, but not to herbivore exclusion, strengthen over time in global grasslands

Changes in soil nutrients and herbivore pressure are key drivers of plant community dynamics in grasslands worldwide. Predicting how plant communities will respond to these global change factors requires understanding the responses of individual species, which can be affected by their abundance and frequency within the community, and by characteristics such as their geographical origin (native vs. exotic), life history (annual vs. perennial) and growth form (grass vs. forb). We analysed data from 2414 plant species from 98 grasslands over 13 years (739 site-years) to examine how nutrient addition (fertilised treatment vs. unfertilised control) and herbivore exclusion (fenced treatment vs. unfenced control) alter plant community composition. We quantified species-specific contributions to compositional variation and related these contributions to their dominance, commonness (whether they occurred in one or both treatments) and characteristics. Nutrient addition led to increasing plant community compositional divergence over time, whereas herbivore exclusion had weak and transient effects on plant community composition. Plant species unique to one treatment became increasingly prevalent in nutrient addition treatments but contributed surprisingly little to compositional variation. Most variation was due to common species, particularly those with similar dominance patterns in the two treatments. Indicator species contributed a disproportionately high amount of the compositional variation between treatments; these species were more likely to be dominants, common to both treatments, annuals and exotics. Synthesis. These findings highlight that global change impacts on plant community composition can be driven by shifts in a subset of species. Understanding these species-specific responses is crucial for predicting community dynamics and for biodiversity conservation efforts in the face of ongoing environmental change. Our study demonstrates the utility of identifying key drivers of compositional shifts in grasslands worldwide.