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 varied associational patterns of species with grazing intensity. Soil nutrients increased with grazing intensity and spatial segregation of plant communities was greater at sites with high soil nutrient concentrations, indicating that grazing intensity influences the spatial heterogeneity of plant communities via nutrients deposited in urine and feces. Declining plant biomass with grazing intensity was related to a strong decline in graminoid species diversity, which suggests that the diversity-biomass relationship is influenced by selective grazing of palatable species. The relationships between species co-occurrence and biomass or plant diversity suggest non-random patterns in species co-occurrences with grazing intensity, which could be the result of competition driven by high livestock grazing intensity. We, therefore, suggest that rangeland stocking rates should be managed properly to maintain rangeland production while promoting plant diversity.

中文翻译：

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不同放牧强度下自然牧场的物种共存塑造了植物多样性-生物量关系的空间变异性

放牧可以改变天然牧场中植物物种的相互作用，进而可能影响生态系统的生产力，但我们并不完全了解放牧强度如何调节植物多样性-生物量关系的空间变异性。在这里，我们假设牧场中的植物物种共存主要是由于放牧和当地资源的异质性导致的生态位隔离，因此放牧调节了多样性-生物量的关系。我们在伊朗的 35 个牧场站点上测试了我们的假设，使用物种共现指数来评估每个站点内的植物空间聚集。在每个地点，我们测量了地上生物量、植物多样性、地形、土壤养分和三个级别的放牧强度。在重度放牧地发现植物群落的高度空间隔离（低物种共生），而在低度和中度放牧地发现更大的空间聚集（高物种共生），显示出具有不同放牧强度的物种关联模式。土壤养分随着放牧强度的增加而增加，并且在土壤养分浓度高的地点，植物群落的空间隔离更大，表明放牧强度通过尿液和粪便中沉积的养分影响植物群落的空间异质性。随着放牧强度的增加，植物生物量下降与禾本科物种多样性的强烈下降有关，这表明多样性-生物量关系受到可口物种选择性放牧的影响。物种共现与生物量或植物多样性之间的关系表明，物种共现与放牧强度之间存在非随机模式，这可能是高牲畜放牧强度驱动的竞争结果。因此，我们建议应妥善管理牧场放养率，以维持牧场生产，同时促进植物多样性。