Human activities alter the drivers of plant diversity and ecosystem stability due to intensified grazing and nutrient enrichment. Nevertheless, the interactive impacts between herbivory pressure and eutrophication on biodiversity, productivity and ecosystem stability in grasslands remain unclear. We conducted a field experiment with four levels of nitrogen (N) addition and three rates of rotational grazing in an alpine meadow of the Qinghai–Xizang Plateau. We found that increasing N addition rates decreased plant species richness and functional native herbage (FNH) richness, while increasing stocking rates decreased community aboveground biomass (AGB) and traditional herbage (TH) AGB. Grazing tended to positively influence community AGB temporal stability due to increasing FNH richness and compositional stability, but this effect was associated with considerable uncertainty. Although N addition drove an increase in species asynchrony, the net effect of N addition on community AGB temporal stability was negative, as the destabilizing effects of reduced soil pH and FNH richness outweighed the positive contribution of enhanced species asynchrony. Furthermore, FNH richness, species asynchrony within FNH populations, and FNH–TH asynchrony were the primary mediating pathways through which community AGB temporal stability was maintained under grazing and N addition. The total biomass temporal stability was mainly mediated by belowground biomass temporal stability rather than AGB temporal stability, indicating that surveys focusing only on temporal stability of AGB provide limited evidence for the ecological consequences of grazing and nutrients. We identified an ecological threshold of 0.86 for the biomass ratio of FNH to TH in relation to community AGB temporal stability in this alpine meadow. Below the ecological threshold, community AGB temporal stability decreased as the FNH/TH ratio increased; above the threshold, further increases in FNH proportion did not produce additional stability benefits.
Li et al. (Wed,) studied this question.