Species co-occurrence shapes spatial variability in plant diversity–biomass relationships in natural rangelands under different grazing intensities

Grazing can alter plant species interactions in natural rangelands, which in turn might influence the productivity of the ecosystem but we do not fully understand how spatial variability in plant diversity-biomass relationships are modulated by grazing intensity. Here, we hypothesized that plant species co-occurrence in rangelands is mainly driven by niche segregation due to grazing and heterogeneity in local resources, and that grazing therefore modulates diversity–biomass relationships. We tested our hypothesis across 35 rangeland sites in Iran, using a species co-occurrence index to assess plant spatial aggregation within each site. At each site, we measured aboveground biomass, plant diversity, topography, soil nutrients and three levels of grazing intensity. High spatial segregation of plant communities (low species co-occurrence) was found at heavily grazed sites, whereas greater spatial aggregation (high species co-occurrence) was found on low and moderate grazed sites, showing v...